City of Dubuque Water Treatment Facilities Plan of Action Study January 1991 by Strand Associates IncZ
363,1
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Iowa
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CITY OF DUBUQUE
WASTEWATER TREATMENT FACILITIES
PLAN OF ACTION STUDY
r
lief MICHAEL Zz
zra DORAN z
s ?�,S 1055 6
a
4,4144)11111110
/6WA #'
STRAND ASSOCIATES, INC.
910 W. WINGRA DRIVE
MADISON, WISCONSIN 53715
JANUARY, 1991
3 1825 00260 2798
i
i
i
R 363.7284 CIT Ia Bks
City of Dubuque wastewater
treatment facilities
ABSTRACT
This report evaluates existing conditions at the City of Dubuque's wastewater
treatment facilities, reviews anticipated effluent limitation requirements,
projects loading and flow capacity needs to the year 2013, screens and evaluates
alternatives for biological wastewater treatment and sludge processing, evaluates
needs in other plant areas such as preliminary -primary treatment and
disinfection, and makes recommendations concerning the best course of action to
meet long-term needs at the plant.
The plant is equipment, labor, and energy intensive, requiring operating costs
far higher than for similarly -loaded plants having more conventional unit
processes. In reviewing plant needs, particular attention was given to those
processes where operating labor and expenses are high, and an effort was made to
consider alternatives which would reduce operating costs at the plant.
In the case of biological secondary wastewater treatment, final recommendations
are deferred pending the results of pilot -scale studies of the biotower process
currently underway at the plant. These studies are anticipated to be completed
in the next few months.
In the case of disinfection, plant -scale studies are recommended during high
flows this coming spring, to determine if pathogen destruction can be
accomplished at the relatively low residual chlorine concentration anticipated
to be required by the NPDES Permit following its reissuance in 1992.
Major identified needs at the plant include:
• "Zimpro" System This system was installed to provide thermal
conditioning of the plant secondary sludge to
allow its dewatering on vacuum filter equipment.
The Zimpro system causes odors, involves very
high operating costs, has deficient gravity
thickening capacity, lacks standby for key
elements of the process, and results in high
process return loadings to the biological
wastewater treatment process. It is recommended
that this process be removed from service.
• Vacuum Filters Two vacuum filters were installed with the
original plant construction. These units are
over 20 years old, and their performance has been
relatively poor. They are unserviceable, and
have been removed from operation. It is
recommended that new sludge dewatering equipment
be provided at the plant to replace the vacuum
filters. Centrifuges have been tentatively
selected, based on successful testing at Dubuque,
but a pilot study of high performance belt press
units has been recommended for comparative
purposes as part of final design.
•
•
•
•
•
•
Bar Screens
Grit Removal
Structural Repairs
Primary Sludge Pumps
Incinerators
Activated Sludge
The present mechanically -cleaned bar screens have
exceeded their useful life. It is recommended
that these units be replaced with new units.
The present grit removal process is of the
aerated type, with chain and scraper type of
solids removal equipment. The system was
designed for high gross solids loadings from
industry. These loadings no longer occur, and
are not anticipated to be experienced in the
future. The present units release odors at the
plant headworks due to the aeration provided, and
operation, maintenance and replacement costs are
high for the solids removal equipment. It is
recommended that these units be replaced with new
units of the "vortex" type that will reduce odor
potential as well as operating costs.
Structural repairs are needed in the primary
treatment area, to rectify deterioration of the
existing facilities.
Replacement of the present manual sludge drawoff
and transfer system with an automated system is
recommended, to reduce operating labor costs.
A heat exchange system is recommended to be
installed (recuperator) which would recover heat
from the exhaust gas, for preheating the outside
air provided to the unit. This will
significantly reduce the requirements for
supplemental fuel oil to maintain proper
temperatures in the units. Other identified
improvements include structural repair of the
building and updating of the air pollution
control system.
A high purity oxygen activated sludge system is
utilized at the plant for secondary biological
treatment. This system involves high operating
costs for energy, supplies, labor and
maintenance. It appears that a more conventional
process (biotower-air activated sludge) may have
promise for significantly reducing annual costs.
Pilot -scale studies of the biotower are currently
underway to provide information to make final
recommendations. If good biotower performance
can be demonstrated, and if concerns relative to
potential odor generation can be answered, it may
prove attractive to convert the present system to
biotower-activated sludge to provide long-term
cost savings.
EOM
•
Final Clarifiers
If the decision is made to retain the oxygen -
activated sludge system, additional automatic
control equipment is recommended to reduce labor
costs, and minor structural and other repairs
will be needed.
The final clarifiers provide inadequate control
of solids during high plant flows, and
replacement and modification of the in -tank weir
troughs and solids removal equipment is
necessary.
Recommended improvements have been divided into two phases:
Phase I
Phase II
• screening and grit facilities
• primary area structural improvements
• new primary sludge pumps
• new sludge dewatering equipment
• incinerator modifications
•
•
secondary treatment modifications
disinfection improvements
The recommended phase II improvements will be refined based on the results of
pilot -scale studies (biotower) currently underway, and based on full-scale
testing of the disinfection system at low chlorine residual and high plant flow.
Phase I improvements have been estimated to require an initial capital cost of
approximately $5 million, and could be constructed utilizing current capital
reserves of the wastewater department. Implementation of the Phase I
improvements would result in a decrease in plant annual operating costs of about
$460,000, or about 15% of the department's annual budget.
Phase II improvements have been estimated to require an initial capital cost of
approximately 51.7 to $5.3 million, depending upon final needs and
recommendations. Depending upon the range of Phase II costs and the source of
funds, principal and interest payments would range from about $160,000 to about
$589,000 per year. Total savings in annual plant operating costs could increase
to about $670,000.
Note that the potential initial annual operating cost savings from Phase II
(about $210,000) cannot be directly compared with amortized capital costs for the
Phase II work. Long-term costs for equipment replacement, etc., must also be
considered. See Section 5 for a discussion of the life -cycle cost approach to
comparing alternatives.
A preliminary implementation schedule is presented in Section 7 (Table 7.01-1).
The schedule calls for completion of the Phase I improvements by the end of 1993,
with Phase II being completed (depending upon the scope of the improvements) by
mid-1995. An approximate schedule of capital outlay requirements is presented
in Table 8.02-2.
iii
SECTION 3-
TABLE OF CONTENTS
SECTION 1- INTRODUCTION
Page No.
1.01 BACKGROUND AND SCOPE OF WORK 1-1
1.02 ABBREVIATIONS 1-2
SECTION 2- EXISTING CONDITIONS
2.01 EXISTING FACILITIES 2-1
2.02 EFFLUENT LIMITATIONS 2-2
2.03 CURRENT LOADINGS AND PLANT PERFORMANCE 2-2
2.04 DESCRIPTION AND RATED CAPACITY OF MAJOR TREATMENT UNITS 2-3
WASTELOAD AND FLOW FORECAST
3.01 POPULATION GROWTH 3-1
3.02 INDUSTRIAL LOADINGS 3-1
3.03 DOMESTIC LOADINGS
-1
3.04 • INFILTRATION/INFLOW (I/I) 33-2
3.05 SUMMARY OF WASTELOADS AND FLOW PROJECTIONS 3-3
SECTION 4- SCREENING OF ALTERNATIVES
4.01 BIOLOGICAL WASTEWATER TREATMENT
4-1
A. POTENTIALLY APPLICABLE TECHNOLOGY 4-1
B. APPROXIMATE COSTS FOR POTENTIALLY APPLICABLE
TECHNOLOGY 4-4
C. NON -MONETARY FACTORS 4-5
D. SELECTION OF ALTERNATIVES FOR DETAILED ANALYSIS 4-5
4.02 SLUDGE THICKENING 4-6
A. POTENTIALLY APPLICABLE TECHNOLOGY 4-6
B. SELECTION OF ALTERNATIVES FOR DETAILED ANALYSIS 4-8
4.03 SLUDGE STABILIZATION AND DISPOSAL 4-8
A. POTENTIALLY APPLICABLE TECHNOLOGY 4-8
B. APPROXIMATE COSTS FOR POTENTIALLY APPLICABLE
TECHNOLOGY 4-12
C. NON -MONETARY FACTORS 4-12
D. SELECTION OF ALTERNATIVES FOR DETAILED ANALYSIS 4-14
SECTION 5- ANALYSIS OF MOST FEASIBLE ALTERNATIVES
5.01 BIOLOGICAL WASTEWATER TREATMENT 5-1
A. DESCRIPTION OF ALTERNATIVES 5-1
B. CAPITAL COSTS 5-4
C. OPERATION AND MAINTENANCE COSTS 5-4
D. PRESENT WORTH COSTS 5-4
E. NON -MONETARY FACTORS 5-6
5.02 SLUDGE THICKENING 5-8
A. PRIMARY SLUDGE THICKENING 5-8
B. SECONDARY SLUDGE THICKENING 5-9
iv
Table of Contents (Continued)
5.03 SLUDGE TREATMENT AND DISPOSAL/REUSE 5-10
A. DESCRIPTION OF ALTERNATIVES 5-11
B. CAPITAL COSTS 5-12
C. OPERATION, MAINTENANCE AND REPLACEMENT COSTS 5-12
D. PRESENT WORTH COSTS 5-13
E. NON -MONETARY COSTS 5-13
5.04 DISCUSSION AND SELECTION OF BEST OVERALL APPROACH 5-15
A. BIOLOGICAL WASTEWATER TREATMENT 5-15
B. SLUDGE THICKENING 5-16
C. SLUDGE STABILIZATION AND DISPOSAL/REUSE 5-17
D. RESERVE CAPACITY 5-17
SECTION 6- OTHER PLANT NEEDS
6.01 ODOR CONTROL 6-1
A. HYDROGEN SULFIDE FORMATION 6-1
B. ODOR CONTROL MEASURES 6-3
C. RECOMMENDATIONS 6-4
6.02 SCREENING 6-4
6.03 GRIT REMOVAL 6-5
6.04 PRIMARY CLARIFIERS 6-6
6.05 DISINFECTION 6-6
6.06 SUMMARY OF COSTS 6-8
SECTION 7- TIMING OF IMPROVEMENTS
7.01 IDENTIFIED IMPROVEMENTS 7-1
A. HIGH PRIORITY IMPROVEMENTS 7-1
B. MEDIUM PRIORITY IMPROVEMENTS 7-2
C. LOW PRIORITY IMPROVEMENTS 7-2
7.02 RECOMMENDED TIMING OF IMPROVEMENTS 7-3
SECTION 8- SUMMARY OF RECOMMENDED PLANT IMPROVEMENTS
8.01 DESCRIPTION OF PROPOSED IMPROVEMENTS 8-1
A. PHASE I IMPROVEMENTS 8-1
B. PHASE II IMPROVEMENTS 8-2
8.02 FISCAL IMPACT OF RECOMMENDATIONS 8-2
APPENDIX A- PLANT LOADINGS AND PERFORMANCE
List of Tables
Page Following
2.01-1 Design of Criteria for Major Units 2-1
2.02-1 Current Wastewater Effluent Limits 2-2
2.03-1 Summary of 1989 Plant Loadings 2-2
2.03-2 1989 Monthly Plant Performance Data 2-2
2.04-1 Rated Capacity of Major Wastewater Treatment Processes 2-3
v
List of Tables (Continued)
3.02-1
3.03-1
3.03-2
3.05-1
4.01-1
4.01-2
4.01-3
4.02-1
4.02-2
4.03-1
4.03-2
4.03-3
5.01-1
5.01-2
5.01-3
5.01-4
5.01-5
5.01-6
5.01-7
5.01-8
5.01-9
5.01-10
5.01-11
5.01-11A
5.03-1
5.03-2
5.03-3
5.03-4
5.03-5
5.03-6
5.03-7
5.03-8
Operating Day Loadings - Major Industries
Domestic base Wasteload and Flow Projection - Year 2013
Wasteload and Dry Weather Flow Projections - Year 2013
Summary of Wasteload and Flow Forecasts - Year 2013
Potentially Applicable Technologies -
Biological Wastewater Treatment
Approximate Present Worth Costs - Biological Wastewater
Treatment - Alternative Screening
Overall Comparison of Alternatives Screened -
Biological Wastewater Treatment
Potentially Applicable Technologies - Sludge Thickening
Screening of Biological Sludge Thickening Alternatives -
Approximate Costs
Potentially 1pplicable Technologies -
Sludge Stabilization and Disposal
Approximate Present Worth Costs -
Sludge Management Alternatives
Overall Comparison of Alternatives Screened
Sludge Management Alternatives
Projected Primary Effluent Loadings -
Biological Wastewater Treatment Evaluation
Preliminary Design Criteria - Trickling Filter
Activated Sludge Alternative
Preliminary Design Criteria - Biotower - Activated
Sludge Alternative
Preliminary Design Criteria - Oxygen Activated
Sludge Alternative
TF - Activated Sludge
BT - Activated Sludge
0 - Activated Sludge -
- Activated Sludge
BT - Activated Sludge
OZ - Activated Sludge -
- Estimated Capital Costs
- Estimated Capital Costs
Estimated Capital Costs
- Estimated 0, M & R Costs
- Estimated 0, M & R Costs
Estimated 0, M & R Costs
Present Worth Cost Estimates -
Biological Wastewater Treatment Alternatives
Present Worth Cost Estimates -
Biological Wastewater Treatment Alternatives -
Staged Construction of Biotowers 5-5
Preliminary Design Criteria - Anaerobic Digestion -
Liquid Agricultural Reuse 5-11
Preliminary Design Criteria - Anaerobic Digestion
Dewatered Agricultural Reuse 5-11
Preliminary Design Criteria -
Dewatered Lime Stabilization 5-11
Preliminary Design Criteria - Incineration 5-11
Digestion = Liquid Ag Reuse - Estimated Capital Costs 5-12
Digestion - Dry Ag Reuse - Estimated Capital Costs 5-12
Lime Stabilization - Dry Ag Reuse -
Estimated Capital Costs 5-12
Incineration - Estimated Capital Costs 5-12
vi
3-1
3-2
3-2
3-3
4-2
4-4
4-5
4-6
4-7
4-9
4-12
4-14
5-1
5-1
5-1
5-1
5-4
5-4
5-4
5-4
5-4
5-4
5-5
List of Tables (Continued)
5.03-9 Digestion - Liquid Ag Reuse - Estimated 0, M & R Costs 5-13
5.03-10 Digestion - Dry Ag Reuse - Estimated 0, M & R Costs 5-13
5.03-11 Lime Stabilization - Dry Ag Reuse -
Estimated 0, M & R Costs
5.03-12 Incineration- Estimated 0,M & R Costs 5-13
5.03-13 Present Worth Cost Estimates -
Sludge Handling Alternatives 5-13
5.04-1 Overall Comparison- Biological Wastewater Treatment
Alternatives
5.04-2 Overall Comparison -
Reuse
Sulfide Analysis
Sludge Stabilization and
Disposal/
5-13
5-16
6.01-1 5-17
6.01-2 6-4
6-4
6.02-1 Bar Screen Replacement Costs 6-5
6.03-1 Grit Removal Costs 6-5
6.04-1 Primary Clarifier Improvements 6-6
6.05-1 Chlorination System Improvements 6-8
6.05-2 Dechlorination with Sodium Bisulfite 6-8
6.06-1 Summary of Costs for Other Recommended Plant
Improvements 6-8
7.01-1 Summary of Identified Improvements 7-1
7.01-2 Preliminary Implementation Schedule- Recommended
Plant Improvements
8.02-1 Projected Initial Capital Costs- Phase I and Phase II
Plant Improvements
8.02-2 Approximate Capital Outlay Projection- Phase I and
Phase II Improvements 8-3
A-1 1989 Oxygen Activated Sludge Performance A-3
Hydrogen Peroxide Addition Costs
List of Figures
7-3
8-2
Page Following
2.01-1 Schematic Diagram - Wastewater Treatment
2.01-2 Schematic Diagram - Primary Sludge Processing
2.01-3 Schematic Diagram - Waste Activated Sludge Processing
2.03-1 1989 Plant Influent Flow
2.03-2 1989 Plant Influent BOD5
2.03-3 1989 Plant Influent TSS
2.03-4 1989 Plant Influent NH3N
2.03-5 Plant Influent Loading Trends
2.03-6 Material Balance for 1989 Data
3.01-1 Dubuque Population Projections
3.04-1 Wet Weather 1985-1986
4.01-1 Trickling Filter - Activated Sludge
4.01-2 Biotower - Activated Sludge
4.01-3 Air Activated Sludge
4.01-4 Sequencing Batch Reactor
4.01-5 High Purity Oxygen Activated Sludge
4.03-1 Liquid Ag Reuse
4.03-2 Anaerobic Digestion/Dry Ag Reuse
vii
2-1
2-1
2-1
2-2
2-2
2-2
2-2
2-2
2-3
3-1
3-2
4-2
4-3
4-3
4-3
4-4
4-10
4-10
List of Figures (Continued)
4.03-3 Liquid Lime Stabilization 4-10
4.03-4 Dry Lime Stabilization/Agricultural Reuse
4.03-5 Dry Lime Stabilization/Sanitary Landfill 4-11
4.03-6 Incineration 4-11
4.03-7 Composting 4-11
5.01-1 Trickling Filter - Activated Sludge 4-11
5.01-2 Biotower - Activated Sludge 5-1
5.01-3 Oxygen Activated Sludge 5-1
5.03-1 Anaerobic Digestion - Liquid Ag Reuse -11
5.03-2 Anaerobic Digestion - Dry Ag Reuse 511
5-11
5.03-3 Dry Lime Stabilization
5.03-4 Incineration 5-11
8.01-1 Preliminary Site Plan 5-11
8-1
viii
1-1
SECTION 1
INTRODUCTION
This section provides details of the background and scope of work. A list of
abbreviations used in the text is also included as an aid to the reader.
1.01 BACKGROUND AND SCOPE OF WORK
The City of Dubuque Utilities operates a wastewater treatment plant providing
secondary treatment for residential, commercial and industrial wastewaters. At
times, the plant receives a sizeable infiltration/inflow (I/I) flow component
during storm events or high river levels.
The treatment facilities are highly complex, energy and maintenance intensive,
and involve high replacement costs as units reach the end of their useful lives.
Several key unit processes do not have adequate standby capacity, which could
lead to failure of the system to meet effluent limitations should the units
malfunction. The original facilities at the site were constructed over 20 years
ago, and certain plant components are reaching the end of their useful lives.
The treatment facilities were designed to accept biochemical oxygen demand (BOD5)
and suspended solids (TSS) loadings substantially higher than current loadings.
Loadings are lower than original design levels due to reduction in industrial
wastewater flows and also due to industrial wastewater pretreatment. Unit costs
($/lb) for BOD and TSS are relatively high for the plant, and this is in part
due to the reduced BOD5 and TSS loadings. Another factor is the selection of
unit operations that require relatively high energy use. At the time of plant
design, energy costs were relatively low. Energy costs have escalated
substantially, however, such that expenditures for energy are now a very
significant portion of the wastewater treatment budget. Past studies have
concluded that significant annual cost savings could be realized by making
process changes, particularly in the sludge handling area of the plant.
The need to replace certain plant components, and the desire to reduce plant
operating costs prompted the City to prepare this Plan of Action Study. The
scope of this study is to identify the best alternatives for sludge handling and
biological wastewater treatment for use at the facilities, and to also review and
make recommendations concerning other plant needs including preliminary and
primary treatment, disinfection, and general plant facilities. The year 2013 was
used as the planning year for this study, which would represent about 20 years
of operation of the updated facilities.
The report is arranged in the following sections:
SECTION 1
SECTION 2
- - introduction, background, and scope of work
- current conditions, effluent limits, loadings and
performance, and rated capacities of major plant units
SECTION 3 -- wasteload and flow forecasts
SECTION 4 -
1-2
screening of biological wastewater treatment, sludge
thickening, and sludge stabilization and disposal
alternatives
SECTION 5 -- analysis of most feasible biological wastewater
treatment and sludge processing alternatives
SECTION 6 - review of other plant needs
SECTION 7 - timing of plant improvements
SECTION 8 -- summary of recommended plant improvements and analysis
of fiscal impact
APPENDIX -- supporting documentation
The recommendations made in Section 5, for biological wastewater treatment, are
based on estimated performance of biotower units, based on full-scale results
from similar facilities at Muscatine, IA, and at other locations. On -site
biotower pilot tests are currently being conducted to better determine process
performance, and appropriate design criteria. The recommendations made in
Section 5 regarding biological wastewater treatment, therefore, should be
considered preliminary, and it is anticipated that they will be updated following
completion of the biotower pilot tests in early 1991.
1.02 ABBREVIATIONS
The following abbreviations are used:
AS
ASCE
AW
BOD5
BT
BTU
CBOD5
cfh
cfm
cfs
col/100 mL
cu ft
cu yd
deg F
DIA
DNR
F:M
gal
gcd
gpd
gpm
hp
hr
activated sludge
American Society of Civil Engineers
average wet weather
five day biochemical oxygen demand
biotower
British thermal unit
carbonaceous BOD5
cubic feet per hour
cubic feet per minute
cubic feet per second
colonies (bacteria) per 100 milliliters
cubic feet
cubic yard
degree Fahrenheit
diameter
Iowa Department of Natural Resources
food to microorganism ratio
gallons
gallons per capita per day
gallons per day
gallons per minute
horsepower
hour
PIN
HRT hydraulic retention time
HVAC heating, ventilating and air conditioning
I/I infiltration/inflow
JWPCF Journal Water Pollution Control Federation
kw kilowatts
kwh kilowatt-hours
lb pounds
lb/d pounds per day
lb/hr pounds per hour
MCC motor control center
min minutes
mil gal million gallons
mgd million gallons per day
mg/L milligrams per liter
MLSS mixed liquor (aeration tank) suspended solids
MLVSS mixed liquor volatile suspended solids
NEC National Elejtrical Code
O&M operation and maintenance
OM&R operation maintenance and replacement
pcd pounds per capita per day
pH hydrogen ion activity (standard units)
PHWW peak hourly wet weather flow
RAS return activated sludge
SBOD5 soluble five day biochemical oxygen demand
scfm standard cubic feet per minute
sq ft square feet
SRT solids retention time
SWO side water depth
TDH total dynamic head
TF trickling filter
TKN total Kjeldahl nitrogen
TS total solids
TSS total suspended solids
TWAS thickened waste activated sludge
WAS waste activated sludge
1-3
2-1
SECTION 2
EXISTING CONDITIONS
This section provides a discussion of the existing wastewater treatment and
sludge processing facilities and current loading conditions. The present units
are rated with respect to their treatment capacities.
2.01 EXISTING FACILITIES
The existing wastewater treatment facilities include screening, aerated grit and
scum removal, primary clarification, trickling filtration (currently bypassed),
pure oxygen activated sludge, final clarification, and chlorine disinfection of
the plant effluent. Final clarifier effluent is recycled to the head end of the
biological wastewater treatment system to maintain a minimum forward flow through
the activated sludge system of about 10 mgd. A schematic of the wastewater
treatment system is presented in Figure 2.01-1. Major design criteria are listed
in Table 2.01-1.
Waste biological sludge from the pure oxygen system is centrifugally thickened,
heat treated by low pressure oxidation (Zimpro), and further thickened by
gravity. The heat treated and thickened waste activated sludge is then blended
with raw primary sludge and is dewatered on a belt filter press. The dewatered
sludge is incinerated. Schematics of the WAS processing and primary sludge
processing facilities are shown in Figures 2.01-2 and 2.01-3, respectively. The
sludge vacuum filters, provided with the original plant construction for
dewatering of the sludge, are no longer serviceable and are not in use.
The existing wastewater treatment facilities were constructed in 1969. At that
time, the facilities consisted of preliminary and primary treatment, trickling
filtration, vacuum filtration and sludge incineration. Clarifiers at the City's
old plant were utilized for clarifying the trickling filter effluent, with sludge
pumped from the old plant site to the head end of the new facilities.
In 1973, the grit removal tanks, primary tanks, and trickling filters were
covered, and an exhaust air odor scrubbing system was installed. In 1975, the
biological treatment facilities were expanded to include pure oxygen activated
sludge and final clarifiers. At that time, waste activated sludge thickening and
heat treatment (Zimpro) facilities were also provided. A belt filter press was
installed in 1983 to improve the plant's solids dewatering capability. Four new
WAS centrifuges were installed this past year, to replace the original units.
At the time of the 1975 addition, the plant was designed for the following
influent loadings:
Flow: Average Weekday
Peak Rate
BOD5: Average
Weekday
TSS: Average
Weekday
15 mgd
40 mgd
47,600 lb/d
55,400 lb/d
49,000 lb/d
55,000 lb/d
RAW WASTEWATER
CATFISH TERMINAL -CEDAR
ALTERNATE ROUTE
METER
OVERFLOW ` VAULT -INPLANT RECYCLE
PRIMARY
TANKS
TRICKLING
FILTERS
( OUT OF
SERVICE )
1:;140
OXYGEN
ACTIVATE
SLUDGE
r
1 GRIT/GREASE
TANKS
LIFT 4-
TAT10N I
SPLIT #3
if
'
n
u
U
u
FINAL CLARIFIERS
ASH PONC,
DECANT
TO RIVER
CHLORINE MIX
BASE LOADING
BAR SCREENS 1. VACUUM FILTER FILTRATE & OVERFLOW
2. PRIMARY SLUDGE HOLDING DECANT
3. GRIT / GREASE BEACHING WATER
4. FLOOR DRAINS
5. ZIMPRO & INCINERATOR COOUNG
CENTRATE RECYCLE
ZIMPRO DECANT
❑PUMP
FIGURE 2.01-
CITY OF DUBUQUE
SCHEMATIC DIAGRAM
WASTEWATER TREATMENT
STRAND
ASSOCIATES. INC
E N O
Unit
1. Bar screens
2. Grit/Grease Removal
Number of units
Size/bay
Volume/Unit
Aeration
Blowers
Firm air/unit
Grit/grease
removal
3. Primary Clarifiers
Number of units
Size/Unit
Total area
4. Trickling Filters
Number of Units
Size/unit
Media type
Distributor
Total Media Volume
5. Intermediate Lift
Station
TABLE 2.01-1
DESIGN CRITERIA FOR MAJOR UNITS
DUBUQUE, IOWA WWTP
Sizes/Design Capacities
2 - 3'6" wide x 3/4"
openings, mechanically
cleaned
1 - 3'6" wide x 1-5/8"
openings
1 - 24" belt conveyor
for screenings/grit
2 - Four bay trains
20 ft x 55 ft x 15 ft
SWD
494,000 gal
Transverse diffused air
3 - 1,100 cfm
33 cfm/1,000 cu ft
chain and flight
3 - circular
90 ft dia x 9 ft SWD
19,100 sq ft
2 - circular
195 ft dia x 7 ft media
Rock
Rotary
418,000 cu ft
3 - 14,000 gpm trickling
filter effluent pumps
2 - 250 gpm WAS pumps
Remarks
20 mgd capacity each
hand cleaned for
standby
18 min. HRT @ 40 mgd
2,100 gpd/sq ft @ 40
mgd
Presently out of
service
40 mgd firm capacity
0.36 mgd firm capacity
Table 2.01-1 (Continued)
6. Aeration Tanks
Type
Number of Units
Size/Bay
Total volume
Mixers/aerators
Total hp
7. Oxygen Generation
Type
Capacity
Compressors
Liquid Storage
Vaporization
Capacity
8. Final Clarifiers
Number of units
Size/Unit
Total Area
9. Final Pumping
Station
10. Chlorination
Type
Feeders
Evaporator
Chlorine Storage
Chlorine Contact
11. WAS Thickening
Type
Number of Units
Capacity
Covered pure oxygen
3 - three bay trains
26' x 90' x 12' SWD
1.89 mil gal
Mechanical
One 30 hp, one 10 hp,
one 7-1/2 hp
three 5 hp
one 7-1/2 hp, one 10 hp,
one 15 hp
285 hp
Cryogenic
26 tons/day
3 - 300 hp reciprocating
44 tons
22.8 tons/day
4 - circular
105 ft dia x 12 ft SWD
34,600 sq ft
6 - 2,000 gpm RAS pumps
3 - 3,125 gpm base
loading pumps
Chlorine gas dissolution
2 - 8,000 lb/day
1 - 8,000 lb/day
Ton cylinders
Mix chamber/outfall
sewer
Horizontal
centrifuge
4 (new units)
60 gpm/unit
scroll
Feed Pumps 6 - Progressive cavity
3 hr HRT @ 15 mgd
First bay
Second bay
Third bay
1.13 hp/1,000 cu ft
433 gpd/sq ft @ 15 mgd
14.4 mgd firm capacity
9.0 mgd firm capacity
26,000 lb/day dry
solids @ 1% feed and 4
units in operation
Table 2.01-1 (Continued)
12. WAS Heat Treatment
Type
Capacity
Feed Pumps
Heat Exchangers
Reactor
Air Compressor
Heat Source
Decant tank
Thickened Sludge
Pumps
13. Primary Sludge
Holding Tanks
Number of Units
Size/Unit
Total Volume
Sludge Pumps
14. Vacuum Filters
Number of Units
Size/Unit
Design Capacity
Chemical
conditioning
15. Belt Press
Number of Units
Size/Unit
Design Capacity
16. Incineration
Number of Units
Type
Design Capacity
Stack Emission
Control
Preheat
Auxiliary Heat
Heat Recovery
Sludge Feed
Low pressure oxidation
50 gpm
2 - 50 gpm each
2 - in series
1
1 - 97 scfm, 40 hp
1 - 100 hp boiler
1 - incinerator waste
heat boiler system
27' x 27' x 8' SWD
3 - Progressive cavity
2
35' x 35' x 16' SWD
293,000 gal
3 - 175 gpm progressive
cavity
3 - 175 gpm progressive
cavity
2
11.5' dia x 12'
8,700 lb/hr/unit
Polymer Feed
1
2 meter
1,400 lb/hr
2
Fluidized Bed
2,600 lb dry solids per
hr/unit
Wet scrubbers
Natural gas
Fuel oil
Waste heat boiler
2 - pugmill
progressive cavity units
19.5 dry tons/day
15 min. HRT
Approximate
Variable speed
for clarifier sludge
withdrawal
for feeding vacuum
filters
Presently out of
service
NOTE: Capacities listed are design capacities. Actual rated capacities of units
are listed in Table 2.04-1.
PRIMARY
SLUDGE
DECANT TO PLANT RECYCLE
HOLDING TANKS
BLEND
TANK
Q MACERATER FILTRATE TO
STORAGE TANK
6::)
VACUUM
FILTERS
BELT
PRESS
) CAKE
SOLIDS
OVERFLOW
TO RECYCLE
CAKE
FILTRATE
TO RECYCLE
ASH TO
NCNERATORS
PONDS
FIGURE 2.01-2
CITY OF DUBUQUE
SCHEMATIC DIAGRAM
PRIMARY SLUDGE PROCESSING
STRAND
ASSOCIATES. O.0
ENGINEERS
AIR MIX
TANK
BLEND
TANK
CENTRATE
TO RECYCLE
CENTRFUGES
TO RECYCLE
QUPERNATE
CA
TANK
ZIMPRO
THICKENED SLUDGE
VACUUM
PETERS( 1 N
CAKE TO NCQVERATTON � ) FILTRATE
BELT
PRESS
FILTRATE
TO RECYCLE
!MUDS OVERFLOW
TO RECYCLE
CAKE TO INCINERATION
LEGEND
STORAGE
TANK
Q PUMP
® FLOW METER
RAS RETURN ACTIVATED SLUDGE
WAS WASTE ACTIVATED SLUDGE
FIGURE 2.01-3
CITY OF DUBUQUE
SCHEMATIC DIAGRAM
WASTE ACTIVATED SLUDGE
PROCESSING
STRAND
sseociatEe. ;NC
E N O; N E E A B
2-2
Because of problems with the design of the facilities, and due to the impact of
recycle loadings from sludge processing, the plant cannot adequately treat
loadings up to the above original design values. The rated capacities of the
present units are presented in Section 2.03.
2.02 EFFLUENT LIMITATIONS
The Mississippi River at Dubuque has been classified by the DNR as equivalent to
a high quality resource warm water stream. The current NPDES Permit, due to
expire in 1992, establishes conventional secondary effluent limits, as well as
seasonal disinfection for recreational use. The current permit limits are shown
in Table 2.02-1.
Recent changes to Chapter 61 of the Iowa Administrative Code (Water Quality
Standards), include specific methods for determining effluent limitations for
toxic substances, including ammonia and residual chlorine. Requirements of the
new code will be placed into effect when the current permit is reissued in 1992,
and a schedule of compliance will be included for any changed limit conditions.
In reviewing the requirements of the new code, it appears that ammonia
requirements may be achievable (using statewide average chemical data). It also
appears that the chlorine residual limitation will be reduced to about 0.3 mg/L,
concurrent with a fecal coliform bacteria limit of 200 col/100 mL.
2.03 CURRENT LOADINGS AND PLANT PERFORMANCE
This section summarizes information on current plant loadings and overall plant
performance. Since the time of original plant design, substantial reductions in
plant organic loadings have occurred, due to reduced industrial flows to the
plant, and due to implementation of industrial wastewater pretreatment systems
at FDL Foods, Inc. FDL Foods currently operates an anaerobic biological
treatment lagoon which provides a substantial reduction in organic loadings in
their discharge.
A. Plant Loadings
Table 2.03-1 summarizes plant loadings for calendar year 1989. Daily
loading values for calendar year 1989 are shown graphically in Figures
2.03-1 through 2.03-4 for flow, BOD5, TSS and NH3N respectively. Figure
2.03-5 is a graph of monthly average flow, BOD5 and TSS for 1988, 1989 and
1990 (through November). All figures are based on summing of the
Terminal -Cedar and Catfish forcemain data.
B. Overall Plant Performance
Table 2.03-2 compares 1989 monthly influent and effluent concentrations
for BOD5 (CB005 for effluent) and TSS on a monthly average and maximum
weekly average basis. The monthly percent removal is also indicated in
the table. As indicated, plant effluent concentrations were consistently
below the effluent requirements of the Iowa NPDES Permit.
TABLE 2.02-1
CURRENT WASTEWATER EFFLUENT LIMITS
DUBUQUE, IOWA WWTP
Parameter
30 Day Average Daily Maximum
Flow, mgd 15 40
CBOD5a:
lb/day 3,128 5,129
mg/L 25 41
TSSa:
lb/day 3,781
mg/L 30
6,057
48
pH, range 6 - 9
Fecal Coliform, col/100 mlb
Notes:
a Minimum percent removal for CBOD and TSS is 85%
b Disinfection required only from April 1 through October 31
200
FLOW, mgd
BOD5, lb/d
TSS, lb/d
NH3N, lb/d
TABLE 2.03-1
SUMMARY OF 1989 PLANT LOADINGS
DUBUQUE, IOWA WWTP
Minimum Average Maximum Maximum
Monthly Monthly Monthly Weekly Maximum
Average Average Average Average Daily
7.90 8.58 9.00 9.82 12.01
9,975 13,450 15,180 16,930 26,040
11,000 12,960 13,930 15,110 39,700
1,400 1,910 2,290 4,250 5,460
kf F
4611.111 iiYlik4 tJs[ IlB►tli� if;T3' '
tftii 4 ttlfli Uttt 1411 13414x tgitm
�eNi fv trivt* 'F711#. - - !R•. I:/01/ IMF/ O Li"$7. I!/'At.
FIGURE 2.03-1
1989 PLANT INFLUENT FLOW
M J J
DAY
30
111i“ ;ttt' ;3tti nut' Etitti.' tF:Ui +1.3U110 111111' fatlii
arts R2if . i+�l a ifff t-: i ri3.S:- Ring; • 91P) Rrtti rtfti
FIGURE 2.03-2
1989 PLANT INFLUENT BOD5
25-
20-r
f
10
5-
4'
0
J FM AMJ J A SOND
DAY
I II
J.I
YRfSD
7q ? I' s
410111 %!ilC
IXi
#}}[ Sit , 99'.i l' IIS[ti't 4.:44kit N111 ' llf Kilt'
14111.1 1.4411
Htli:, Cliff
FIGURE 2.03-3
1989 PLANT INFLUENT TSS
NI 01,0
DAYS
6000
5000 -
4000 -
-a
z 3000 -
N)
z
2000 -
1 000
0
fi f4
ekgitt
111111 Iiitly IriatS1 1/141 4Hvpnt *1111, Vint
FIGURE 2.03-4
1989 PLANT INFLUENT NH3
J
F
v
A
J
J A
DAY
0
z
0
0
"•••4'..t. 111101
- 141-41 111.41*.* ,1111,A
17
16-
15
14
13
12
11-
10
9
8
AMA'
any
Its :iv'
riot trithu Wit e km: in
-AN Wu .1 if 4C1-, RUM"'ID 4.11-tt Polo irssto
FIGURE 2.03-5
PLANT INFLUENT LOADING TRENDS
IWO
• •
* g
• •
r•
\ • • •
I . :
e ••‘ ,
e •
•
II .•
11
i : 1
t • .
1..••••Te
i 'it.....
f•
•
iss.. • s ..
A -
: • A,
,
I N 1 •
• • 6
• • .•
0.. • 0
1 I
I • • • ..„ .... % ,* - 0 41'..
11
.• • • ..- •
• e /
l'
I , o 't
• r
r ,
g
.• •.. *.
• .„. I r . •
• • .... • ...
•
• I V • •
7.• • ..
• 1 ? •
• 0
. 1 . • 1
• •
••
• , . • s .• 1 •
• , • . I • .
• . , :I ....... • . ' . .• : g • •
•
• , • ... . •
• , • • ; • •
'• - ...s... \ : I , ., • • :
... •
11.
i/ ... . ....... ••,..... •..
• .........
• •
• I
• •
• :
• • •
•
• i• :rt
• • :
•
•
7111111-11-111
1 /88 5/88 9/88 1/89 11 5/89 9/89 1/94 5/90 9/90
MONTH
FLOW, mgd BOD5, Ib/d(X1000) TSS, Ib/d(X1000)
MO"
TABLE 2.03-2
1989 MONTHLY PLANT PERFORMANCE DATA
DUBUQUE, IOWA WWTP
OXYGEN DEMAND TOTAL SUSPENDED SOLIDS
WEEKLY WEEKLY
INF EFF PERCENT EFF INF EFF PERCENT EFF
BOD5 CBODS REMOVAL MAX TSS TSS REMOVAL MAX
MONTH mg/1 mg/1 % mg/1 mg/1 mg/1 % mg/1
JAN 179 10 94.3 11 178 13 92.8 14
FEB 207 12 94.4 14 183 15 92.1 17
MAR 208 11 94.6 13 190 14 92.7 19
APR 197 12 94.0 13 176 15 91.5 15
MAY 208 8 96.0 11 191 11 94.0 16
JUN 193 8 95.6 16 195 10 95.1 12
JUL 200 12 94.2 15 182 23 87.4 32
AUG 185 12 93.6 14 179 13 92.6 30
SEP 177 6 96.4 20 183 8 95.7 11
OCT 142 8 94.6 9 157 9 94.4 11
NOV 159 8 95.0 9 171 9 94.5 12
DEC 171 8 95.1 10 173 9 94.9 10
AVERAGE 186 10 94.8 13 180 12 93.1 17
LIMIT
- 25(MAX) 85(MIN) 41(MAX)
30(MAX) 85(MIN) 48(MAX)
an
Figure 2.03-6 is a material balance for the plant, based on 1989 data.
The proceedures used for developing the mass balance are discussed in
Appendix A.
The plant data from 1989 indicate the following (see Appendix A):
• Approximately 18% of the BOD5 and 22% of the TSS loading to the
primary treatment facilities was due to recycle streams from sludge
processing.
• About 20% of the BOD5 and 18% of the TSS loading to the biological
wastewater treatment process was due to recycle streams from WAS
processing and sludge dewatering.
• Due to process return flows entering the system downstream of the
primary clarifiers, the BOD5 loading to the activated sludge system
was about 120% of the plant influent BOD5.
• The activated sludge facilities were loaded at about 63 lb/d/1,000
ft3 BOD5, and at an F:M of 0.3/day, which is somewhat lower than
typical for pure oxygen systems. BOD5 removal averaged 96%, and WAS
production averaged 0.66 lb/lb BOD5 removed.
• the average effluent CBOD5 and TSS were 10 and 13 mg/L respectively,
well below the NPDES Permit limits.
Please refer to the discussions and data presented in the 1985 and 1988
reports for additional information on past process performance. Solids
capture in the sludge processing units has increased significantly in the
last year, due to the installation of the new WAS centrifuges, and to
discontinuing the use of the vacuum filters for sludge dewatering.
2.04 DESCRIPTION AND RATED CAPACITY OF MAJOR TREATMENT UNITS
Table 2.04-1 lists rated capacities of the present treatment units. Major
process deficiencies and limitations of the present treatment units are discussed
below. See Section 6 for further discussion of plant maintenance/rehabilitation
needs.
A. Preliminary Treatment
The preliminary treatment facilities were constructed in 1969. Flow
enters the plant via two force mains. The 24-inch force main carries
wastewater from the Catfish Cr. pumping station. The 42-inch force main
carries wastewater from the Terminal and Cedar pumping stations. Both
force mains are monitored by magnetic flow meters installed in a meter
vault at the plant. The preliminary treatment facilities consist of bar
screens, designed to remove rags and other similar material from the
wastewater, and aerated grit removal units, designed to remove sand and
similar inorganic materials.
8.47
13.400 (190)
14 200 (200)
12.700(180)
PLANT
INFLUENT
8.88
16.300(220)
19.200(280)
16 , 200 (220)
T
BAR
SCREENS
(a)
GRIT
REMOVAL
8.83
12,500(170)
8100(110)
5900 (80)
10.38
15,800(180)
14,100(180)
7400 (85)
3.10
TS- 1.3%
344,000 b/d
0.09
TS-1.3%
10,000 G/d
RAS
WAS
COOUNO NPW
0.22
18(10)
31(17)
23(13)
1
8.62
700 (10)
1200 (17)
900 (12)
PRIMARY
CLARIFIERS
(b)
PRS
0.48
3700 (940)
9900 (2500)
8000 (2050)
0.09
OM" 800(1200)
4900 (8500)
4500(6000)
(c)
0.03
380o(15,200)
® 11 , 100 (44 , 400)
10,300(41.200)
AERATION
TANKS
MSS- 3700 M03/1.
T
FINAL
CLARIFIERS
T
CHLORINE
BASE
FLOW
SP.0.04
2400 (7200)
4800 (14 , 500)
850 (2550)
0 0.06
750(1500)
1000(2000)
500(1000)
1.45
120(10)
200 (17)
160 (13)
LEGEND:
FLOW . m9d
6005. ib/d (mg/L)
TS. D/d (mg/L)
TSS. b/d (mg/L)
RECYCLE FLOWS
ECPWUIO Flows
0). N EI.N'T WASTE
p). 3CLM BEJCNNO
N). WAS A0CE35EMs EE-3M3
FIGURE 2.03-6
MATERIAL BALANCE FOR 1989 DATA
DUBUQUE, IOWA WWTP
STRAND
A SSCCIA TES EC.
ENOINEEn3
1. Mechanical Screens
2. Aerated Grit Removal
3. Primary Clarifiers
4. Trickling Filters:
Low Rate
Roughing
Hydraulic max
5. Intermediate Pumping
Station:
Wastewater Pumping
WAS Pumping
6. Aeration Tanks
(Secondary treatment
without nitrification)
7. Final Clarifiers
Hydraulic Loading
Solids Loading
(MLSS - 4,000 mg/L,
Recycle - 55%)
TABLE 2.04-1
RATED CAPACITY OF MAJOR WASTEWATER TREATMENT PROCESSES
Capacity
Criteria
3 fps. max. through openings°
15 min. for preaeration°
1,500 gpd/sq ft (PHWW)b
800 gpd/sq ft (AWW)
15 lb BOD5/day/1,000 cu ft"
100 lb BOD5/day/1,000 cu ft'
Distributor capacity
Pump Capacities
Pump Capacities
F:M - 0.75 at 3,000 mg/L
MLVSS°
600 gpd/sq ft (AWW)b
1,200 gpd/sq ft (PHWW)b
30 lb/day/sq ftb
50 lb/day/sq ftb
All Units
in Service
40 mgd (PHWW)
29 mgd (PHWW)
15 mgd (AWW)
6,300 lb BOD5/day
41,800 lb BOD5/day
30 mgd (PHWW)
60 mgd
0.72 mgd
35,500 lb BOD5/day
21 mgd (AWW)
42 mgd (PHWW)
20 mgd (AWW)
33 mgd (PHWW)
One Unit
Out of Service
40 mgd (use hand -cleaned
screen) (PHWW)
47 mgd (PHWW)
19 mgd (PHWW)
10 mgd (AWW)
3,100 lb BOD5/day
20,900 lb BOD5/day
15 mgd (PHWW)
40 mgd
0.36 mgd
23,600 lb BOD5/day
16 mgd (AWW)
31 mgd (PHWW)
15 mgd (AWW)
25 mgd (PHWW)
8. Final Pumping Station
100% RAS Recycleb
17 mgd (AWW)
14 mgd (AWW)
Table 2.04-1 (Continued)
Process
9. Oxygen Generation
10. Centrifuges
with Polymer
without Polymer
11. WAS Heat Treatment
12. Vacuum Filters
13. Belt Press:
100 % PRS
60% PRS/40% TZS
[elver 4 j4 C 0 +Ht!?411-3'l
Criteria
1.2 lb 02/1 b BOD5 applied
7,200 lb/day/unit, 90% recovery`
7,200 lb/day/unit, 75% recovery`
50 gpm @ 2.2% TS
900 lb cake/hr/unitd
c
1,300 lb/hr
1,500 lb/hr
14. Incinerators 2,600 lb dry solids/hr/unit
NOTES:
Typical Design Value
b DNR Standard
Plant -experience Based
d These are at end of useful life; now out of service
PHWW - peak hourly wet weather flow
AWW - average daily wet weather flow
Capacity
All Units One Unit
in Service Out of Service
41,700 lb BOD5/day
25,900 lb/day/recovery 19,400 lb/day recovery
21,600 lb/day/recovery 16,200 lb/day recovery
13,200 lb/day
1,800 lb cake/hr
36,000 lb/day
31,200 lb/day
900 lb cake/hr
5,200 lb dry solids/hr 2,600 lb dry solids/hr
154-211\JMC:td
2-4
1) Bar Screens
Raw wastewater discharges from the force mains to the bar screen
facilities. Each of the three bar screens are 3-feet 6-inches wide. The
two mechanically cleaned screens have 3/4 inch bar openings. The third
hand cleaned (standby) screen has 1 5/8-inch bar openings. The
mechanically cleaned screens discharge to a belt conveyor which unloads
the screenings to a dump truck for eventual landfilling. Each screen is
installed in a separate channel, which can be gated off and drained for
service of the bar screen.
The mechanical screens have reached the end of their useful life, are no
longer serviceable, and are in need of replacement. Although the screens
have a nominal capacity of 40 mgd, with all screens in service, the rake
mechanisms cannot keep up with debris buildup during storm flow
conditions, and significant operator attention is needed at these times to
prevent hydraulic backups.
2) Grit Removal
Following screening, the wastewater flows to the aerated grit removal
basins. The basins are designed to allow sand and other gritty materials
to settle by gravity, but to provide sufficient turbulence by aeration to
keep finer organic material in suspension.
The basins were originally designed to remove heavy loadings of grease and
paunch manure (from meatpacking) in addition to grit. Aeration is
provided by three 1,100 cfm centrifugal blowers, and a system of air
piping and submerged diffusers. There are two tankage trains, each train
having four series tanks 20-feet wide by 15-feet deep by 55-feet long.
One or both trains may be placed into service, or the entire system may be
bypassed by way of a central bypass channel. Normal practice is to
operate one train.
Each grit tank is equipped with a longitudinal scraper assembly (chain and
flight), and the fourth tank in each train is equipped with a skimmer
assembly. Each train has a separate cross collector mechanism for
collecting settled material from the grit tanks, and discharging this
material to the belt conveyor in the bar screen building for disposal
along with screenings material. Skimmings are conveyed by a tip trough in
the 1st bay of each train to scum pits located in the solids processing
building. Grease and scum is lifted by beaching mechanisms (one per
train) and discharged to the dewatered sludge conveyors for incineration
along with dewatered sludge.
Maintenance costs are high for repair and replacement of the chain and
flight mechanical scraping equipment. The aerated design of the unit
promotes the release of sulfide gas, which may cause odors at times and
results in corrosive damage to the plant facilities in the vicinity of the
grit tanks. Operation of the grit aeration blowers also results in
significant operation and maintenance costs.
2-5
B. Primary Clarifiers
Primary clarification provides a quiescent settling process, designed to
allow settleable solids to be removed (as primary sludge) from the
wastewater flow. The primary clarifiers were constructed with the
original plant in 1969. Following grit removal, wastewater flows to the
primary influent splitter box for diversion to the three 90-foot DIA by 9-
foot SWD units. Flows are split on the basis of the vertical setting on
the 6-foot wide downward opening gate serving each tank. Provision was
made in the original design of the plant to add a fourth clarifier.
Sludge is scraped to a center well in each unit by means of a rotating
mechanism. Primary sludge withdrawal is accomplished by manual operation
of telescopic valves at each tank, which transfer sludge to a sludge well
adjacent to each tank. Primary sludge is pumped from the sludge well to
the primary sludge holding tanks. The rotating mechanism also removes
scum (floatables) from the tank surface, which is conveyed to a scum well
adjacent to each tank.
At the time the units were covered (1973) for odor control, an off -air
chemical scrubbing system was provided. The chemical scrubbing system is
no longer operative.
C. Trickling Filters
Trickling filtration is a process wherein wastewater is trickled through
rock media, and oxidation of the organics present in the wastewater is
accomplished by aerobic microorganisms present in the biological slime
which builds up on the surface of the rock media.
The trickling filters were constructed in 1969. The units are 195-feet in
diameter, with 7 feet of rock media. Although domes were installed in
1973, the domes have been removed due to structural failure.
The units are not currently in use. The capacity of the activated sludge
system is sufficient to meet current plant needs, and the use of the
trickling filters would increase operation and maintenance costs since the
oxygen generation system is operating near its minimum capacity at the
present time. Past operation of the trickling filters resulted in odor
problems at times. A review of the hydraulic design of the units
indicates that the design of the underdrain system is not adequate for the
intended loadings, resulting in surcharging of the underdrains and the
inability for the units to "breathe". This is verified by observations
made by the plant staff (water well above drains in the unit's effluent
channel, and smoke testing indicating that only a small area of the
filters was "breathing" properly). Resolution of underdrain and aerating
problems would be required prior to returning the units to service.
D. Intermediate Pumping Station
The intermediate lift station was constructed as part of the 1975 plant
additions. The pump station houses the trickling filter effluent pumps,
which are required to lift the wastewater from the elevation of the
iro
trickling filter effluent up to the aeration tanks when the trickling
filters are in operation. The pump station also houses the waste
activated sludge (WAS) pumps, which are used to waste excess biological
sludge (that grows up processing the wastewater) to the sludge management
facilities.
The station houses three large capacity (14,000 gpm) trickling filter
effluent pumps, and two 250 gpm WAS pumps. Provision is made for metering
trickling filter effluent by means of two Cipolletti weirs. WAS is
metered by a magnetic flow meter.
The wastewater lift pumps are not currently in service (trickling filters
are bypassed). Although these pumps were designed to provide 40 mgd
capacity with one pump out of service, operating problems with the pumps
prevent their operation at full capacity.
E. Activated Sludge System
Activated sludge is a process where wastewater is placed in contact with
a suspended culture of aerobic microorganisms, which accomplish oxidation
of the organic matter present in the wastewater. The microorganisms are
collected (as secondary sludge) in the final clarifiers. The major
portion of the secondary sludge is recycled to the aeration tanks as
return activated sludge (RAS) to maintain an adequate microorganism
concentration. Excess sludge is wasted as waste activated sludge (WAS).
Oxygen must be provided to maintain aerobic conditions in the aeration
system, and mixing energy must also be provided to maintain the
microorganisms in suspension.
The activated sludge facilities were constructed in 1975. The activated
sludge system is of the high purity oxygen type, with oxygen cryogenically
manufactured on site, and is delivered to the aeration tanks under a
concrete deck cover. Mechanical mixers provide for mixing and oxygen
transfer to the mixed liquor (blend of wastewater and RAS in aeration
tanks). The aeration tanks are arranged in three trains, each having
three tanks in series. Each tank is 26-feet wide, by 12-feet deep, by 90-
feet long. Each train has 9 mixers with a total horsepower of 95 hp for
each train. The cryogenic system has a nominal capacity of 26 tons per
day of oxygen. Air is provided to the system by three 300 hp
reciprocating compressors.
The only major problem experienced with aeration tanks has been concrete
corrosion, probably caused by the low pH and high carbon dioxide
environment in the tanks. One of the three aeration tank trains has been
recently rehabilitated.
Cryogenic facilities have a limited capability for "turn down". The
practical lower limit of the existing facilities is about 13 tons/day,
which at 90% utilization would provide about 12 tons per day of oxygen to
the activated sludge system. This is over double the theoretical oxygen
requirement under normal operating conditions at the present time.
2-7
Operating costs, therefore, are substantially higher than would be the
case with a system better able to match the demands of the system.
F. Final Clarifiers
There are four final clarifiers, 105-feet in diameter, with a 12-foot side
water depth. The clarifiers were constructed in 1975.
The design of the clarifier in -tank equipment is inadequate during periods
of high plant flow. Solids carryover results, due to the inability of the
sludge siphon tubes to effectively remove solids from the unit. Leakage
of the joints in the precast concrete effluent launder sections has had to
be resolved by damming the launder discharges to reduce water differential
between the launders and the tank level.
G. Final Pumping Station
The final pumping station was constructed in 1975. The station houses six
2,000 gpm return sludge pumps (RAS) for recycling secondary sludge to the
activated sludge system. Also provided are three 3,125 gpm base loading
pumps for recirculating final effluent to the head end of the biological
treatment system, two 100 gpm secondary scum pumps, and three 600 gpm
plant process water pumps utilizing plant effluent.
H. Chlorination
The chlorination facilities, located in the bar screen building, consist
of two 8,000 lb/d feed units and an 8,000 lb/d evaporator. Also provided
are two 100 lb/d cylinder mounted chlorinators for disinfecting plant
effluent water used•for process water supply. The chlorination units were
installed as part of the original 1969 plant construction.
The 8,000 lb/d feed units meter and dissolve gaseous chlorine to a
solution water flow, which is discharged to the plant effluent in a mixing
chamber at the plant site. Contact time for disinfection is provided in
the effluent sewer from the plant site to the outfall at the river.
I. Waste Activated Sludge Thickening
As part of the 1975 plant additions, waste activated sludge thickening
facilities were installed in an addition to the solids processing
building. The units are scroll centrifuges of the continuous decant
design. The bowl of the centrifuge rotates at high .rpm, causing
centrifugal forces many times that of gravity which causes concentration
of the WAS solids. The thickened or concentrated waste activated sludge
is removed from the unit as thickened waste activated sludge (TWAS) by the
scroll conveyor. The clarified material (decant) is discharged from the
unit as a second flow stream.
Originally, six centrifugal WAS thickening units were installed. These
units, however, exhibited poor solids capture and high operating and
maintenance costs. Solids capture as low as 50% was common for the
2-8
original units, which have recently been replaced by four larger units of
newer design. The new units perform far better than the units originally
installed.
From pumps located in the intermediate pumping station, WAS is pumped to
an aerated holding tank. From the holding tank, the WAS is pumped to the
centrifuges by positive displacement pumps located in the solids
processing building basement. Thickened sludge is discharged to a well
located beneath the centrifuges. Decant is collected and recirculated to
the activated sludge system.
J. Waste Activated Sludge Heat Treatment
The 1975 plant additions included a low pressure oxidation system (Zimpro)
for the heat treatment of WAS.. The purpose of the heat treatment was to
improve the dewatering characteristics of the WAS so that it could be
effectively dewatered on vacuum filters prior to incineration. High
pressure and temperature are employed to break down the cells of the waste
activated sludge. The dense cell wall material is recovered by the
process, and the remaining cell material is solubilized and recycled to
the activated sludge system. This is a significant source of the total
loading to the activated sludge system. The heat treatment system has a
maximum rated capacity of about 50 gpm of thickened sludge.
From the thickened waste activated sludge well, the thickened sludge is
pumped to the heat treatment system by one of two high pressure
reciprocating pumps. The pumps are equipped with macerators (grinders) on
their suction side. The high pressure pumps convey the sludge through two
pre -heat heat exchangers in series, and then into a reactor vessel.
Effluent from the reactor vessel flows back through the pre -heat heat
exchangers, where heat is transferred to the feed sludge. Air and steam
are added to the reactor. A significant heat input as steam is needed to
maintain process operating temperature.
Effluent from the reactor flows to a gravity thickening unit, where the
heat treated solids are recovered by gravity, and the supernatant is
returned to the wastewater flow stream. Heat treated and thickened WAS
solids are pumped to the sludge dewatering system by progressive cavity
pumps. Supernate is returned to the biological wastewater system by means
of a separate supernatant pump station.
A major limitation with the existing treatment plant is the ability to
process waste activated sludge. Of particular concern is that the WAS
thermal sludge conditioning system lacks standby capability. With no
duplicate units provided for the key reactor and heat exchanger
components, any extended down -time would seriously disrupt the sludge
handling program and would ultimately impair performance of the wastewater
treatment system.
Past studies have recommended elimination of the "Zimpro" WAS heat
treatment system. This change would reduce loadings to the activated
2-9
sludge due to the high strength supernatant, and would significantly
reduce plant operating costs.
K. Primary Sludge Holding Tanks
As part of the original plant construction in 1969, two primary sludge
holding tanks were constructed adjacent to the solids processing building.
Each tank is 35-ft by 35-feet by 16-feet deep, with a concrete deck cover.
The units are equipped with diffused air aeration systems, and valved
drawoffs for withdrawing and recycling supernatant. Each primary
clarifier is served by a 175 gpm progressive cavity pump, which pumps
primary sludge from the sludge well adjacent to each clarifier to the
holding tanks.
L. Vacuum Filter Dewatering Units
The two vacuum filters were installed in 1969, at have reached the end of
their useful lives. The units have been removed from service, and are
available for standby use only.
M. Belt Press Dewatering Unit
To supplement the vacuum filters, a two meter belt press was installed in
1983. The unit was sized for 1,440 lb/hr through put (dry weight basis)
and to produce a 30% cake. A serpentine conveyor was installed to deliver
the press cake to the incinerator feed pumps. A skid mounted polymer
system was also installed, including a batch tank, a feed tank, and
associated mixers and controls. Two new progressive cavity sludge pumps
were installed to feed a holding/blending tank serving the belt press.
One pump transfers primary sludge from the primary sludge holding tanks,
and the second pump is available to pump unthickened WAS from the aerated
WAS holding tank.
Following initial installation and startup, the belt press failed to meet
operating expectations. Cake solids were significantly below the
specified 30%. Subsequent improvements, including improved filtrate
capture and, more recently, use of a new polymer with superior sludge
conditioning characteristics, have resulted in improved performance. At
present, the single beltpress serves as the only reliable means of sludge
dewatering. Provision of additional dewatering equipment is needed to
assure system reliability.
N. Incineration System
The incinerator system was provided with the original 1969 plant
construction. An energy recovery system, including stack gas cyclone and
waste heat boiler, was added in 1975. The incinerators effectively
incinerate the volatile solids present in the raw sludge, resulting in ash
having about 1/4 the dry weight mass of the original sludge.
Two incinerators are provided, each having a design capacity of about
2,600 lb dry solids per hour (62,400 lb/d). The units were originally
2-10
designed to handle up to 500 lb/hr (dry weight) of grease having a BTU
content of 16,000 BTU/lb (dry weight), and 2,100 lb/hr of sludge cake (dry
weight), at an average of 13,000 BTU/lb (dry weight).
Dewatered sludge is conveyed to one of two pugmills, each of which
discharges to a mechanically variable speed progressive cavity incinerator
feed pump. Feed pumps discharge to either incinerator through a feed gun.
The incinerators are of the fluidized bed type, using outside air for
fluidizing the sand bed media in a cold wind box design. Each reactor is
equipped with a natural gas preheat burner, and a fuel oil auxiliary fuel
burner. Reactor off -gases are discharged through a cyclone assembly, a
waste heat recovery heat exchanger, and wet scrubbers, to the atmosphere.
The wet scrubbers are served with non potable (plant effluent) water.
Potential improvements to the incinerator include modifications to the air
supply system, to allow pre -heating the fluidizing air by heat transfer
from the stack gases. This change would reduce fuel auxiliary fuel
consumption by recovering some of the energy in the incinerator exhaust.
Improvements are also needed to the air pollution control equipment, to
reduce carryover of water added in the wet scrubber system.
3-1
SECTION 3
WASTELOAD AND FLOW FORECASTS
This section provides estimated plant capacity requirements to the year 2013.
This would represent 20 years of operation following the completion of any major
plant improvements.
3.01 POPULATION GROWTH
Projections of future Dubuque populations have been made by the Dubuque City
Planning Department. The information was available through the year 2000 and is
shown in Figure 3.01-1. The population for the year 2013 was extrapolated by
assuming that the growth rate projected for the years 1990 to 2000 would remain
constant to the year 2013. Using this information, the current (1990)
population is estimated to be 62,500 and the future 2013 population is estimated
as 66,000.
3.02 INDUSTRIAL LOADINGS
Based on 1987-1989 wastewater treatment plant data, operating day loadings from
major industries are given in Table 3.02-1. Also shown in Table 3.02-1 are
projected future loadings for each major industry. These values were based on
daily maximum loadings that are currently allowed under agreements between the
City of Dubuque and each industry.
In addition to current and projected industrial loadings, it is recommended that
the projection of plant needs include some reserve capacity that would be
available for new industry or for unforeseen expansion of existing industry. For
this purpose, we have included values of 10% and 25% of domestic and industrial
loadings, as reserve, to provide alternate levels of design loadings for further
consideration and analysis.
3.03 DOMESTIC LOADINGS
Based on 1987 through 1989 data, loadings from domestic, commercial, and small
industrial sources are as follows:
Flow - Average 6.98 mgd
Dry Weather 6.02 mgd
BOD5 8,730 lb/day
TSS 9,340 lb/day
NH3 1,487 lb/day
The values for ammonia shown above were based on 1989 data, and assume that 85%
of FDL Food's TKN is ammonia and 75% of the Sanofi Bio Industries' TKN is
ammonia. Dry weather flow contributions are based on data for December 1989.
The City of Dubuque population served during 1990 is estimated to be about
62,500. Based on this population, base domestic loadings are estimated to be:
POPULATION
II 1
100
90
-- 80
tn
cn 70
0
60
50
tt;
I _F.
a PIYIL
%FHI
ftl3: I' lilt
4;I
9dl,lu E#tt,
FIGURE 3.01-1
DU3UQUE POPULATION PROJECTIONS
Nrik
h
tip
40 ( I ( 1
1940 1950 1960 1970 1980 1990 2000 2010 2020
YEAR
DUBUQUE COUNTY DUBUQUE CITY
TABLE 3.02-1
OPERATING DAY LOADINGS
MAJOR INDUSTRIES
DUBUQUE, IOWA WWTP
Current Loadings:1 Flow BO05 TSS
(mall (lb/day) (lb/day)
FDL Foods 2.09 2,700 2,600
Sanofi Bio Industries 0.18 2,600 1,700
Inland Protein 0.09 1,700 300
TOTAL 2.36 7,000 4,600
Projected Loadings:2 Flow BOD5 TSS
m d (lb/day) (lb/day)
FDL Foods 2.80 4,300 4,000
Sanofi Bio Industries 0.30 2,000 2,000
Inland Protein 0.10 2,000 500
TOTAL 3.20 8,300 6,500
Notes:
Based on data from 1987, 1988, and 1989.
2 Based on 1989 agreements between industries and City of Dubuque for maximum
daily discharge.
3-2
Flow (6.02 mgd)/62,500 = 96 gcd
BOD5 (8730 lb/day)/62,500 = 0.140 pcd
TSS (9340 lb/day)/62,500 = 0.149 pcd
NH3 (1487 lb/day)/62,500 = 0.024 pcd
The BOD and TSS values shown above are somewhat lower than typical domestic
wastewater loadings. It is possible that the values appear low because the major
industrial discharges, which were subtracted out from the total plant loadings,
may have a somewhat higher measured strength at the point where they are sampled
compared with their effect at the point where the plant influent is sampled.
The characteristics of the discharges may change over the length of time it takes
to reach the treatment plant.
In the Dubuque WWTP Phase I Report (February 1985) domestic loadings were
estimated to be 106 gcd, 0.206 pcd BOD5, and 0.242 pcd TSS. The population at
the time of the report was estimated as 64,000, including the City of Asbury.
Asbury is no longer served by the Dubuque WWTP.
According to the Iowa Administrative Code, whenever loading rates are less than
0.17 pcd BOD or 0.2.pcd TSS, values of 100 gallons flow, 0.17 pounds BOD5 and
0.20 pounds TSS per capita per day may be used. Therefore values of 100 gcd,
0.17 pcd BOD5 and 0.20 pcd suspended solids will be used to project future
loadings. This provides a conservative approach, and is in closer agreement with
historical data.
Projections of future Dubuque populations have been made by the City Planning
Department. Based on these projections, the year 2013 population is estimated
to be about 66,000.
The year 2013 domestic base loadings are projected as shown in Table 3.03-1.
Projected year 2013 domestic and industrial loadings, including reserve capacity,
are listed in Table 3.03-2.
3.04 INFILTRATION/INFLOW (I/I)
During wet weather flow periods, inflow of surface water and infiltration of
groundwater into the sewer system occurs. In certain portions of Dubuque's sewer
system infiltration is significantly increased when the Mississippi River
elevation is high. This I/I results in a significant increase in plant flow.
To estimate rates of I/I, flow records for 1985 and 1986 were reviewed. More
recent data were not used due to the unusually dry weather conditions observed
in the midwest over the past two to three years. The average daily flow, peak
24 hour flow, and total rainfall for each month during 1985 and 1986 are shown
in Figure 3.04-1. Peak hourly flow rates are also shown where this data was
available; the plant records containing this information for late 1985 and all
of 1986 are missing.
Peak hourly flow rates for months which had no available records were generally
estimated from peak 24 hour flow rates by using a peaking factor of 1.3. This
is the average "peak hour" to "peak day" factor for the nine months for which
peak hourly data were available.
TABLE 3.03-1
DOMESTIC BASE WASTELOAD AND FLOW PROJECTIONS
YEAR 2013
DUBUQUE, IOWA WWTP
Per -Capita
Loading
Loading
Projection
Flow: 100 gcd 6.60 mgd
BOD5: 0.17 pcd 11,200 lb/day
TSS: 0.20 pcd 13,200 lb/day
Notes: Based on projected 66,000 population.
Values are average daily loadings, exclusive of infiltration/inflow.
Per capita values based on Iowa Administrative Code.
1M.
TABLE 3.03-2
WASTELOAD AND DRY WEATHER FLOW PROJECTIONS
YEAR 2013
DUBUQUE, IOWA WWTP
Flow BOD5 TSS
(mgd) (lb/day) (lb/day)
Domestic: 6.60 11,200 13,200
Industrial: 3.20 8,300 6,500
SUBTOTAL 9.80 19,500 19,700
10% Reserve: 0.98 2,000 2,000
TOTAL 10.78 21,500 21,700
25% Reserve: 2.45 4,900 4,900
TOTAL 12.25 24,400 24,600
Notes: Total flows shown are average dry weather and do not include
infiltration/inflow.
FIGURE 3.04-1
WET WEATHER 1985-1986- DUBUQUE WWTP
J F M A M J J A S O N D J F M A M J J A S O N D
MONTH
AVE. FLOW PEAK DAY =+c PEAK HOU IN. RAIN
3-3
Maximum peak plant flows occurred in the spring and fall of 1986, with the
maximum 24 hour flow of 24.8 occurring in October of 1986. This flow and the
rainfall observed that month is very unusual; therefore, this value was not used
in further calculations. The peak hourly wet weather flow was obtained from the
March 1986 data; a peak hour/peak day factor of 1.4 was applied to the peak day
flow of 20.0 mgd to obtain the peak hourly flow. The average dry weather flow
for this period had been previously estimated at 8.94 mgd, not including I/I
(Phase 1 Report, 1985), and the peaking factor of 1.4 was applied to this value
to estimate the peak hourly dry weather flow. The peak I/I reaching the plant
was therefore estimated as:
Peak flow at plant
Peak dry weather flow
Peak I/I
28.05 mgd
-12.52 mgd
15.53 mgd
The peak 24 hour wet weather infiltration was calculated by using the values
described in the previous paragraph without the 1.4 peaking factor as follows:
Peak day at plant
Base flow
Peak 24 hour I/I
20.04 mgd
-8.94 mgd
11.10 mgd
The average wet weather I/I was estimated in accordance with the Iowa
Administrative Code methods. The May, 1986 average flow rate was used a the
highest monthly flow for the 1985-1986 period, and the 8.94 mgd base flow
(domestic plus industrial) was subtracted as follows:
Wet weather month
Average Base Flow
Average wet weather I/I
14.02 mgd
-8.94 mgd
5.08 mgd
The average dry weather infiltration was estimated by obtaining the 1985-1986
average dry weather flow and subtracting the base flow. The average dry weather
flow was obtained averaging the flow for months which had less than two inches
of total rainfall (January and June of 1985 and December, 1986). The average dry
weather infiltration was estimated as follows:
Average dry weather flow
Base flow
Average dry weather I/I
10.09 mgd
-8.94 mgd
1.15 mgd
3.05 SUMMARY OF WASTELOADS AND FLOW PROJECTIONS
Design flow rates were obtained by using the above infiltration estimates along
with the projected 2013 flow rates shown in Table 3.03-2. Table 3.05-1
summarizes the flow and loading projections for the year 2013 based on the
information presented in Sections 3.01 through 3.04.
TABLE 3.05-1
SUMMARY OF WASTELOAD AND
FLOW FORECASTS
YEAR 2013
DUBUQUE, IOWA WWTP
Loading Parameter
Average Dry Weather Flow (ADW, mgd)
Domestic
Industrial
Reserve
I/I
10% 25%
Reserve Reserve
6.60
3.20
0.98
1.15
6.60
3.20
2.44
1.15
Total 11.93 13.39
Average Wet Weather Flow (AWW, mgd)
Average dry weather flow 11.93 13.39
Average I/I 5.08 5.08
Total 17.01 18.47
Maximum Wet Weather Flow (MWW, mgd)
Average dry weather flow 11.93 13.39
Maximum I/I 11.10 11.10
Total 23.03 24.49
Peak Hourly Flow (PHWW, mgd)
Peak hourly dry weater flow
Peak I/I
16.70 18.75
15.53 15.53
Total 32.23 34.28
BOD5 (lb/day)
Domestic 11,200 11,200
Industrial 8,300 8,300
Reserve 2,000 4,900
Total 21,500 24,400
Table 3.05-1 (Continued)
TSS (lb/day)
Domestic 13,200 13,200
Industrial 6,500 6,500
Reserve 2,000 4,900
Total 21,700 24,600
Nil
4-1
SECTION 4
SCREENING OF ALTERNATIVES
Numerous technical alternatives are available for biological wastewater
treatment, sludge thickening, and sludge treatment, dewatering and disposal. The
purpose of this section is to identify those alternatives which may be
potentially applicable at the Dubuque plant, and to perform a screening of those
alternatives to identify those which are most attractive and merit detailed
evaluation. Approximate costs and non -monetary factors such as reliability,
expandability, and applicability to anticipated future regulatory requirements
will be considered.
4.01 BIOLOGICAL WASTEWATER TREATMENT
Biological wastewater treatment alternatives will be reviewed and screened in
this section. Other wastewater treatment processes including preliminary
treatment, primary treatment, and disinfection are addressed in Section 6 of this
report.
At the present time, biological wastewater treatment is provided by a high rate
pure oxygen activated sludge process. Rock trickling filters, provided with the
original plant construction, are not currently utilized. See Section 2 for a
description of the present facilities.
A. Potentially Applicable Technology
Theoretically, any physical -chemical or biological wastewater treatment process
for secondary treatment would be applicable at Dubuque. Certain alternatives,
however, can be screened out initially:
• lagoon processes these processes would be too land 4ntensive, would
not take advantage of existing facilities, would
provide a lower level of perforrance than other
available systems, and would not be adaptable to
higher levels of treatment that —.ay be required
(e.g. nitrification);
• physical -chemical processes employing chemical coag,iation-settling,
filtration and activated carbon treatment would
involve high capital and operating cost, and would
be complex to operate and maintain compared to other
alternatives available;
• RBC systems the rotating biological contactor process has proven
to be quite unreliable and uneconomical, compared to
other processes available, for all but possibly
small domestic wastewater flows;
• Biotower systems biotower systems, which would e -ploy stationary
media (without coupling with a dow-stream activated
sludge process) are not considered a practical
alternative for Dubuque because of cost, odor
4-2
potential, reliability in terms of achieving
treatment standards, and lack of adaptability to
higher levels of treatment (e.g. nitrification); and
anaerobic systems anaerobic processes are more applicable to higher
strength wastes (such as industrial wastes), would
involve high costs to achieve treatment standards,
and would result in an increased level of concern
for odor, corrosion and deterioration of plant
equipment and structures, and safety concerns.
The most practical systems to provide biological wastewater treatment at Dubuque
are the mechanical aerobic systems of the activated sludge and hybrid trickling
filter (or packed tower) -activated sludge type. These systems have generally
proven to be the most reliable, economical, and adaptable processes available to
meet treatment requirements in settings similar to Dubuque's. In addition, these
processes would have the potential of making maximum economical use of present
plant facilities.
The use of trickling filters or biotowers upstream of the present pure oxygen
activated sludge facilities is not considered practical. The need for pure
oxygen in this configuration would be much lower than the minimum oxygen plant
capacity, and the resulting costs for oxygen generation would be excessive. The
extended aeration activated sludge process is also not considered practical at
Dubuque. Extended aeration is appropriate for consideration for small
communities where a stable process is desired, or for a facility where advanced
levels of BOD5 or ammonia control is necessary. For Dubuque, the process would
involve high capital and 0 & M costs, and would require a large area of land for
aeration tankage.
Table 4.01-1 lists those biological wastewater treatment alternatives which will
be evaluated on an initial screening basis.
A description of each of the alternatives is provided in the following sections.
Design assumptions used were for preliminary screening purposes only. Section
5 presents design criteria used for the detailed evaluation of those alternatives
selected for further study.
1. Trickling Filter -Air Activated Sludge
Figure 4.01-1 is a schematic diagram of this alternative, which would
provide improvements to the present rock trickling filters, and utilize
them in conjunction with a downstream air activated sludge process. The
present activated sludge process would be converted to an air activated
sludge system. For screening purposes, it was assumed that the present
mechanical aerators would be utilized, with modifications as may be
necessary. The present pure oxygen production facilities would be removed
from service.
The intermediate pumping station, which is now off line, would be returned
to service. New covers, rotary distributors, forced ventilation, and odor
control facilities would be provided for the trickling filters. Due to the
TABLE 4.01-1
POTENTIALLY APPLICABLE TECHNOLOGIES
BIOLOGICAL WASTEWATER TREATMENT
DUBUQUE, IOWA WWTP
ALTERNATIVE DESCRIPTION
TF-Air AS Rehabilitation/modifications to present rock trickling
filters for roughing treatment, followed by air activated
sludge incorporating modifications to present activated
sludge system with the use of diffused air for aeration
BT-Air AS As for TF-Air AS, except for modification or replacement
of present trickling filters to provide synthetic media
biotowers to replace rock media trickling filters
Air AS Conventional multi -pass flow -through air activated sludge,
to be implemented by modifications and expansion to
present aeration tankage and by providing diffused air
aeration system
SBR Sequencing batch reactor air activated sludge system,
which would incorporate new fill and draw tankage
02-AS High rate oxygen activated sludge, which would basically
be an update of the present process utilized at the plant
Primary Effluent
Return Sludge
1
AERATION TANKS
Fin.' Eiiluont
TRICKLING
FILTERS
FINAL
TANKS
FIGURE 4.01-1
TRICKLING FILTER -
ACTIVATED SLUDGE
DUBUQUE, IOWA WWTP
STRAND
•9510Ci•rt5 tiC
E N G I N E E n e
4-3
low hydraulic loading rates to the trickling filters, because of their
large surface area, covers would be necessary to prevent excessive cooling
of the wastewater during cold weather. Forced ventilation would be
necessary to provide for adequate air flow through the media. Odor control
is included for purposes of this analysis, to mitigate potential problems
with odors which have been a problem with trickling filter operations at
the plant in the past. The final clarifiers would be rehabilitated to
include new in -tank equipment, influent wells, effluent launders and
baffles.
2. Biotower-Air Activated Sludge
Figure 4.01-2 is a schematic diagram of this alternative. The present rock
media trickling filters would not be returned to service. New biotowers
would be constructed with deep synthetic media, which is a more modern
design approach than the present shallow rock media trickling filters, and
would allow for a higher organic loading rate.
The biotowers would be covered, and would be provided with mechanical
forced ventilation and odor control equipment to control odors. The
present intermediate pumping station pumps would be replaced with pumps of
higher head capability to serve the new biotowers. Activated sludge
aeration tank, aeration, and final clarification improvements would be the
same as for the previous alternative.
3. Air Activated Sludge
Figure 4.01-3 is a schematic diagram for this alternative. With this
alternative, attached growth roughing treatment upstream of activated
sludge would not be provided, and all of the biological wastewater
treatment would be provided by the suspended growth activated sludge
process. Use of the present intermediate pumping station would not be
required. The process would be designed at a conventional moderate organic
loading rate, and would utilize diffused air for aeration. A significant
increase to the present aeration tank volume (present volume is 253,000 cu
ft) would be required, and aeration would be provided by a fine bubble
diffused air system. The pure oxygen generating facility would not be
utilized. Improvements to the final clarifiers would be the same as for
the previous alternatives.
4. Sequencing Batch Reactor (SBR) Activated Sludge
This is a relatively new modification of the activated sludge process where
contact of the wastewater with the activated biomass, and settling of the
mixed liquor to produce an effluent, occur in the same tankage. A separate
clarification step is not necessary, and return sludge pumping facilities
are not required either. Wastewater is directed to the tankage, where
mechanical mixing and diffused air aeration is typically provided to
maintain aerobic conditions, and periodically an individual tank is
isolated, mixing and aeration turned off, and effluent decanted after a
period of time. Automatic controls regulate the sequential operation of
the tankage. The SBR process normally employs longer sludge ages, and
Return Sludge
,f,
Primary Effluent
AERATION TANKS
B IOTOWERS
1
-
---------- -
--:
Final Sffluant
FINAL
TANKS
FIGURE 4.01-2
BIOTO WER-
ACTIVATED SLUDGE
DUBUQUE, IOWA WWTP
STRAND
AEEOCLATEE iNC
E N G I N E E R E
Return Sludge
r-
Primary Effluent
AERATION TANKS
1
Final Effluent
Diffused Air
FINAL
TANKS
7
FIGURE 4.01-3
AIR ACTIVATED SLUDGE
DUBUQUE, IOWA WWTP
S3,
STRAND
•990C'ATE9. ANC
ENGINEEPS
B.
4-4
lower food -to -microorganism ratios than employed in other forms of
activated sludge processes for secondary treatment.
With this alternative, the present oxygen plant, aeration tanks, final
clarifiers, and return activated sludge pumps would be removed from
service, and new SBR tankage and equipment would be installed. The present
trickling filters and intermediate pumping station would not be utilized.
Figure 4.01-4 is a schematic for this alternative.
5. Oxygen Activated Sludge
This alternative would continue to employ the present high purity oxygen
activated sludge system, with upgrading to provide improved performance and
reliability. The present trickling filters and intermediate pumping
station would not be utilized.
Improvements would include: new variable capacity centrifugal compressor;
new supervisory controls and monitoring to allow improved operation and to
facilitate restarting of the unit after a power failure; repairs to the
aeration tank covers to make them more gas -tight; and new final clarifier
equipment (sludge removal mechanisms, center baffles, and effluent launders
and baffles).
Air Products, Inc., has developed a new process (the A/0 Process) that has
the potential to reduce power requirements compared to the present system.
Our initial calculations indicate that the costs for implementing this
change may be relatively high compared to anticipated savings. We
therefore have not included these modifications as part of this screening
analysis. Additional consideration of this modification will be given in
Section 5 in the detailed evaluation of alternatives.
Figure 4.01-5 is a schematic of the process.
Approximate Costs for Potentially Applicable Technologies
Preliminary estimates for capital, operation and maintenance, and replacement
costs were made for the five alternatives considered most practical (TF-Activated
Sludge, BT-Activated Sludge, Air Activated Sludge, SBR, 02 Activated Sludge).
The estimates were made on the basis of historical costs for similar facilities,
information obtained from equipment manufacturer's representatives, and published
data. All cost information given herein is on a 3rd quarter 1990 basis.
Estimated costs are to provide facilities needed for the 25% reserve capacity
loading level.
Table 4.01-2 lists approximate present worth costs for the alternatives. The
present worth cost can be considered as the total sum of money required today to
construct, operate, and maintain the facilities (including equipment replacement)
during the twenty year project life, less any salvage value remaining at the end
of the twenty year period. The overall monetary cost of alternatives are most
commonly compared on a present worth cost basis.
Primary Effluent
J
REACTOR REACTOR
1
2
REACTOR
3
.l
REACTOR
4
Final Effluent
FIGURE 4.01-4
SEQUENCING BATCH REACTORS
DUBUQUE, IO W A WWTP
Sa
STRAND
S88OCJATES lNC
E N G, N E E. 8
Return Sludge
r
Primary Effluent
AEBAT ION TANKS
1�1
High Purity
c _ .
Oxygen
FINAL
TANKS
Final Effluent
FIGURE 4.01-5
HIGH PURITY
OXYGEN ACTIVATED SLUDGE
DUBUQUE, IOWA WWTP
SEL
STRAND
ASHOC,TE® ANC
E N G i N E E A B
.116
TABLE 4.01-2
APPROXIMATE PRESENT WORTH COSTS
BIOLOGICAL WASTEWATER TREATMENT ALTERNATIVE SCREENING
DUBUQUE, IOWA WWTP.
Alternative
Trickling Filter -Activated Sludge
Biotower-Activated Sludge
Air Activated Sludge
Sequencing Batch Reactors
02 Activated Sludge
Note:
Approximate
Present Worth Cost
$ 6,000,000
$ 9,000,000
$10,000,000
$17,000,000
$ 8,000,000
Costs are 3rd quarter 1990 dollars basis.
Costs are for facilities for 25% reserve capacity level.
Present worth costs are for 20 years at 8 7/8% discount rate.
MN
�n
4-5
C. Non -Monetary Factors
Several non -monetary factors are of importance, including: reliability,
constructibility, expansion potential, and the ability to comply with potentially
more stringent future treatment requirements.
1. Reliability
The TF-Activated Sludge, BT-Activated Sludge, and SBR processes are
considered to be the most reliable. The air activated sludge process is
judged to be somewhat less reliable. The 02-Activated Sludge processes is
felt to be somewhat less reliable also, due to reliance on the 15 year old
oxygen generating plant, certain key components of which have no standby.
The TF-Activated Sludge and BT-Activated Sludge would provide improved
reliability due to the hybrid attached -suspended growth system. The
trickling filters or biotowers would have the capacity to handle peaks in
organic loading, such as may result from an industrial discharge, and would
dampen the loading to the downstream activated sludge system. Hybrid
systems of this type have typically provided an excellent quality effluent.
The SBR process would provide improved reliability due to its relatively
long sludge age and lower F:Mv ratio.
2. Constructibility
The TF-Activated Sludge, BT-Activated Sludge, and 02-Activated Sludge
processes are considered to pose the least difficulty for construction.
The other processes would be more difficult to construct while maintaining
effective treatment.
3. Expansion Potential
The TF-Activated Sludge, BT-Activated Sludge, and 02-Activated Sludge
processes could be readily expanded by providing additional process
tankage. The SBR and Air -Activated Sludge processes would utilize a
significant area of the site and may prove difficult to expand.
4. Future Limits
Because of their ability to be readily expanded, the TF-Activated Sludge,
BT-Activated Sludge, and 02-Activated Sludge processes are judged most
easily incorporated into a future upgraded system as may be required to
meet more stringent future effluent limits (e.g. nitrification).
D. Selection of Alternatives for Detailed Analysis
Table 4.01-3 provides a relative comparison of the monetary and non -monetary
factors considered in this screening of alternatives.
TABLE 4.01-3
OVERALL COMPARISON OF ALTERNATIVES SCREENED
BIOLOGICAL WASTEWATER TREATMENT
DUBUQUE, IOWA WWTP
Factor TF-AS BT-AS Air -AS SBR OZ-AS
Monetary Cost + +/- +/- +
Reliability + + +/- + +/-
Constructibility + + +
Expansion Potential + + +/- +
Future Limits + + - +/ +
SCREENING * * *
Note:
* means selected for further detailed evaluation
+ means favorable
+/- means neutral
means unfavorable
4-6
Due to their relatively favorable comparison to the other alternatives, it is
recommended that the TF-Activated Sludge, BT-Activated Sludge, and 02-Activated
Sludge processes be retained for detailed evaluation.
4.02 SLUDGE THICKENING
Sludge thickening processes are employed to increase the solids concentration,
thereby improving the efficiency and economy of further processes for sludge
handling, treatment and disposal.
A. Potentially Applicable Technologies
Potentially applicable technologies for primary and waste biological sludge
thickening are discussed in the following sections. Table 4.02-1 lists
technologies for sludge thickening that are considered most practical for
application at Dubuque.
1. Primary Sludge Thickening
At the present time, primary sludges are withdrawn by telescopic valve from
the clarifiers, and pumped to a storage tank for holding prior to further
processing. Supernatant withdrawal is provided at the holding tank. A
sludge concentration in the range of 4 to 5% solids has been achievable
with the present system. This is a relatively high sludge concentration,
and reduces the requirement for subsequent thickening.
As the primary sludge is capable of being thickened to a relatively high
concentration by gravity alone, mechanical thickening of primary sludge
will not be considered. Mechanical means of thickening would include
centrifugation, mechanical belt dewatering, and dissolved air floatation.
These processes would involve high capital and operating costs, compared
to gravity settling, and odor control in the processing area would be a
concern.
Although functional and capable of achieving a relatively high solids
concentration, the present system requires significant operator attention.
Other potentially feasible means of primary sludge thickening include:
a. Air -Operated Pumps
Air -operated pumping would replace the current telescoping valve and
progressive cavity pumping arrangement with air -operated diaphragm
pumps. The diaphragm pumps would pump more continuously, at a
relatively low rate, and would withdraw sludge from the clarifiers at
approximately the rate of sludge accumulation. Automatic controls
would be provided to control the frequency of pump operation.
Installations of this type have demonstrated the capability to
produce a solids concentration in the 5% to 6% range.
TABLE 4.02-1
POTENTIALLY APPLICABLE TECHNOLOGIES
SLUDGE THICKENING
DUBUQUE, IOWA WWTP
ALTERNATIVE DESCRIPTION
PRIMARY SLUDGE:
Optimization Optimization of the withdrawal of primary sludge
from the primary clarifiers, so as to maximize
solids concentration (current method)
Air -Operated Pumps Install air operated diaphragm pumps to maximize
withdrawn sludge solids concentration
Gravity Settling Install gravity settling units for primary sludge
WASTE BIOLOGICAL SLUDGE:
Centrifugation Continue with present use of centrifuges for
thickening waste biological sludge
Replace centrifuges with mechanical gravity belt
system
Mechanical Belt
Dissolved Air
Replace centrifuges with dissolved air floatation
system
4-7
b. Gravity Thickening
Separate gravity thickening units are sometimes employed in
wastewater treatment to maximize the solids concentration of sludge
withdrawn from the primary clarifiers.
Gravity thickeners are designed much like circular primaries, except
that the rotating mechanism generally includes a "picket" which
provides for improved concentration of the thickened sludge. As the
hydraulic loading rate, based on sludge flow, would be so low as to
potentially cause odorous gas generation from units of this type, a
secondary source of flow (normally plant effluent) is provided. This
increases plant recycle flows and loadings when gravity thickening is
employed.
The potential need for primary sludge thickening improvements can be better
evaluated following a determination of the most feasible means of further
sludge stabilization and disposal. A detailed evaluation of each of the
alternatives is presented in Section 5.
2. Secondary Sludge Thickening
Secondary (biological) sludges are generally far less concentrated than are
primary sludges upon withdrawal from the clarification units. It is
therefore desirable to provide thickening of waste biological sludge to
improve the economy of further sludge processing. At the present time,
waste activated sludge is directed to a storage tank and is thickened by
centrifugation prior to further processing. Four of the six centrifuges
were recently installed.
Of the technologies that could be applied for secondary sludge thickening,
the following are not considered practical for application at Dubuque:
• primary cosettling cosettling of waste biological sludges in
primary clarification may cause significant
interference with the performance of primary
clarification, generally does not provide
optimum thickening of the sludges, and is not
normally practiced at larger facilities
• gravity thickening gravity thickening of aerobic waste biological
sludges is generally a poor choice, due to the
generation of gasses in the waste sludge which
interfere with thickening and compaction of the
sludge and may cause poor solids recovery in
the gravity thickening units
Other feasible means of secondary sludge thickening which will be given
consideration are listed in Table 4.02-1.
Table 4.02-2 summarizes estimated capital, operating, and present worth
costs for the alternatives. As indicated, the present centrifuges would
Note:
TABLE 4.02-2
SCREENING OF BIOLOGICAL SLUDGE THICKENING ALTERNATIVES
APPROXIMATE COSTS
DUBUQUE, IOWA WWTP
Alternative Approximate
Present Worth Cost
Centrifuges $ 900,000
Gravity Belt Thickeners $1,100,000
Dissolved Air Floatation $1,500,000
All costs expressed in 3rd quarter 1990 dollars.
Costs are for facilities for 25% reserve capacity level.
present worth costs are for 20 years at 8 7/8 % discount rate.
4-8
have the lowest approximate cost, with gravity belt units being of a
similar or very slightly higher cost. Dissolved air floatation, because
of the relatively high capital investment, would have the highest cost.
B. Selection of Alternatives for Detailed Analysis
It is recommended that the following sludge thickening alternatives be given
detailed consideration:
Primary Sludge Thickening
• present scheme of optimizing sludge withdrawals
• air -operated diaphragm pumps
• gravity thickening
Waste Biological Sludge Thickening
• present centrifuges
• gravity belt units
4.03 SLUDGE STABILIZATION AND DISPOSAL
Following thickening, as may be appropriate, sludges generated in wastewater
treatment require stabilization and proper disposal (or reuse) to provide an
effective overall environmentally sound wastewater treatment program. The
present method of sludge stabilization and disposal is incineration with ultimate
disposal of incinerator ash. This present method of sludge stabilization and
disposal, as well as feasible alternative methods, will be discussed in this
section.
A. Potentially Applicable Technology
0f the sludge stabilization/disposal technologies potentially available for
application at Dubuque, the following are not considered practical and will
not be given further consideration:
• Heat Treatment Thermal sludge conditioning was developed
primarily as a means to improve sludge
dewatering characteristics prior to dewatering
on vacuum filters. The present "Zimpro" system
involves high costs for operation and
maintenance, lacks standby capacity for key
components, has limited capacity in it's
thickening unit, results in high recycle
loadings of BOD5 to biological treatment, and
is not needed with current sludge dewatering
technologies. Previous investigations have
concluded that the use of this system at the
plant should be discontinued.
• Chlorination Chlorine oxidation (purifax) has been
demonstrated to have the potential for
• Aerobic Digestion
4-9
producing chlorinated hydrocarbon compounds and
significant odor potential.
Aerobic digestion would involve high operating
costs (for aeration energy) and, during cold
weather, would appear unlikely to comply with
new EPA pathogen reduction requirements without
further treatment.
• Raw Landfilling Landfilling of raw or unstabilized sludge is
viewed as having public health and safety
concerns as well as very poor public
acceptance.
0f the processes available for dewatering of sludge, the following
processes are not considered practical for application at Dubuque, and will
not be given further consideration:
• Drying Beds
Sludge drying, beds (with or without vacuum
assist) are seen as too land intensive and not
practical for a plant the size of Dubuque.
Site area would make implementation of this
alternative impractical.
• Vacuum Filtration Vacuum filtration is seen as outdated
technology, with relatively high cost and poor
performance, compared to other processes
currently available
Table 4.03-1 lists sludge stabilization and treatment technologies that
will be screened for potential application at Dubuque. For purposes of
screening of the alternatives, the technologies were combined to form the
following overall sludge handling systems:
1. Anaerobic Sludge Digestion,
Thickened Digested Sludge;
2. Anaerobic Sludge Digestion,
Dewatered Digested Sludge;
with Agricultural Reuse of
with Agricultural Reuse of
3. Lime Stabilization of Thickened Sludge, with Agricultural
Reuse of the Stabilized Liquid Sludge;
4. Dewatering of Raw Sludge, followed by Lime Stabilization and
Agricultural Reuse;
5. Dewatering of Raw Sludge, followed by Lime Stabilization
and Sanitary Landfill;
6. Dewatering of Raw Sludge, followed by Incineration and Sanitary
Landfill of Incinerator Ash; and
TABLE 4.03-1
POTENTIALLY APPLICABLE TECHNOLOGIES
SLUDGE STABILIZATION AND DISPOSAL
DUBUQE, IOWA WWTP
ALTERNATIVE DESCRIPTION
SLUDGE STABILIZATION:
Incineration Continued use of incineration of dewatered sludge,
with consideration of energy recovery from stack gas
for preheating of incinerator bed fluidizing air
supply
Anaer. Digestion
Liquid Lime
Dry Lime
Compost
DISPOSAL/REUSE:
Liquid Land
Dry Land
Landfill
Sale
DEWATERING:
Centrifuge Centrifugal dewatering of raw or stabilized sludge
Belt Press Upgrading and expansion of dewatering operation for
dewatering of raw or stabilized sludge
Windrow drying in conjunction with composting or to
increase sludge dryness prior to disposal or reuse
Anaerobic digestion of thickened primary and waste
secondary sludge
Addition of lime and/or kiln dust to stabilize
sludge in liquid process prior to liquid or
dewatered sludge disposal
Addition of lime and/or kiln dust to dewatered
sludge for stabilization prior to sludge disposal
Composting of dewatered sludge with wood waste or
solid waste for stabilization prior to disposal or
reuse
Application of thickened stabilized sludge to
agricultural land for reuse and recycling of organic
matter and nutrients
Application of dewatered stabilized sludge to
agricultural land for reuse and recycling of organic
matter and nutrients
Landfilling of incinerator ash or dewatered
stabilized sludge
Windrow
Sale or giveaway program for sludge stabilized by
composting, for landscaping or other beneficial use
4-10
7. Dewatering of Raw Sludge, followed by mixing with Wood Waste and
Composting, with Compost Recycled for Landscaping or as an
Agricultural Soil Amendment.
1. Anaerobic Digestion -- Liquid Agricultural Reuse
Anaerobic digestion provides a mixed anaerobic (no oxygen) environment, at
elevated temperature (95 deg. F) to provide an environment suitable for
bacterial populations that convert complex organics to organic acids,
carbon dioxide and methane, and other breakdown products. Anaerobic
digestion is the most common process employed in wastewater treatment for
sludge stabilization.
Figure 4.03-1 is a schematic diagram for this alternative. With this
process, thickened waste biological sludge and primary sludge would be
pumped to one of two primary digesters which would be heated and mixed.
Heating would be provided by methane -fueled boilers and external heat
exchange units, and mixing would be provided by compressing digester gas
and releasing it at the bottom of the primary digesters through mixing
devices. One secondary digester would be provided for digested sludge and
digester gas storage. The digesters would have floating covers to provide
operating flexibility and for improved safety. Gas safety equipment and
an excess gas burner would be provided, as well as transfer pumps,
recirculating pumps, and other ancillary equipment. It was assumed that
the digesters would be built as part of a new complex, with a central
control building, with provisions for adding a future fourth digester.
Following digestion, it was assumed that gravity belt thickeners would be
employed to produce a 6% to 8% sludge, and that the sludge would be hauled
to a remote site for lagoon storage (180 days) and ultimate application to
agricultural land.
2. Anaerobic Digestion -- Dewatered Agricultural Reuse
A schematic of this alternative is provided as Figure 4.03-2. This
alternative would be similar to the previous alternative, except that the
digested sludge would be dewatered, stored off -site in cake form, and
applied to agricultural land with cake spreader type vehicles.
3. Liquid Lime Stabilization -- Agricultural Reuse
This alternative would provide quicklime storage hoppers and feed units,
and mixed reaction vessels, to blend lime with raw (combined primary and
secondary) thickened sludge. Raising the pH of the sludge to 12, and
holding for two hours would provide pathogen reduction and provide a stable
product. About 25 lb of quicklime would be added for each 100 lb of raw
sludge (dry solids basis). With sufficient lime dose, pH would remain
elevated for an extended time. Following reaction with the lime, gravity
thickening would be provided and the sludge would be pumped to on -site
temporary storage. The treated sludge would be hauled to off -site lagoons
for storage and eventual application to agricultural land. Figure 4.03-3
is a schematic diagram for this alternative.
PHI
PIM
PHI
RIM
SECONDARY
DIGESTER
I
Raw Sludge
(Th PRIMARY
DIGESTERS
Digested Sludge
THICKENINC
STORAGE
HIGHWAY TRANSPORT
I
I 1STORAGE
I LAGOONS
1
1
AGRICULTURAL
LAND APPLICATION
FIGURE 4.03-1
ANAEROBIC DIGESTION/
LIQUID AG REUSE
DUBUQUE IOWA WWTP
Sal
STRAND
AEBOC ATES. NC
E N G I N E E. 9
SECONDARY
DIGESTER
Raw Sludge
Digested Sludge
Cake
PRIMARY
DIGESTERS
DEWATERING
STORAGE
HIGHWAY TRANSPORT
COCD
0
BUNKER
STORAGE
CAKE LAND
APPLICATION
FIGURE 4.03-2
ANAEROBIC DIGESTION/
DRY AG REUSE
DUBUQUE, IOWA WWTP
Sa
BTRAND
Assoc,.
E N G i N E E n 9
FIGURE 4.03-3
Rain Sludge
LIME STORAGE
AND FEED UNITS
stabilized Sludge
THICKENING
STORAGE
HIGHWAY TRANSPORT
STORAGE
LAGOONS
AGRICULTURAL
LAND APPLICATION
LIQUID LIME STABILIZATION/
AGRICULTURAL REUSE
DUBUQUE, IOWA WWTP
SEiel
STRAND
♦BBOCiaT ES ANC
E N Q I N E E . g
4-11
4. Dry Lime Sludge -- Agricultural Reuse
Figure 4.03-4 is a schematic for this alternative. Raw sludges would be
blended and dewatered. Following dewatering, quicklime would be blended
with the dewatered sludge in a pugmi l l at a mass ratio of about 50 lb
quicklime per 100 lb dry sludge solids. The process of quicklime addition
to dewatered sludge causes pathogen destruction due to the combination of
elevated pH and elevated temperature. Following blending with lime, the
material would be hauled to a remote storage site for ultimate application
to agricultural land.
For purposes of this screening of alternatives, it was assumed that belt
filter presses would be utilized for sludge dewatering, and that building
modifications would be made to install the belt filter presses in the area
now occupied by the vacuum filters. Additional building modifications
would be made to provide for a truck loading area and for the lime feed and
blending equipment.
5. Dry Lime -- Sanitary Landfill of Sludge
A schematic of this alternative is presented in Figure 4.03-5. This
alternative would be similar to the previous alternative, except that the
sludge would be landfilled following blending with lime.
6. Incineration
A schematic of this alternative is provided as Figure 4.03-6. This
alternative would continue the use of incineration of dewatered raw sludge
solids. Landfilling of ash is assumed for purposes of this screening
analysis. It is also assumed that a recuperator would be installed to
recover incinerator stack gas heat to preheat incinerator air supply, as
a means of saving fuel. Installation of belt filter presses to replace the
present vacuum filters is assumed. Certain other projected improvements
are also assumed, as reflected in the capital improvement budget.
7. Composting
A schematic of this alternative is included as Figure 4.03-7. With this
alternative, raw sludge would be dewatered at the plant, and hauled to an
off -site composting facility. The composting facility would provide a
mixing building, where dewatered sludge would be blended with wood waste;
a composting building , where the sludge/wood waste mixture would be placed
in piles underlain with perforated aeration piping; a screening facility
where wood waste would be separated from the composted sludge, compost
curing and storage areas; blower facilities; and office and support
facilities. The elevated temperatures reached during active composting
would provide for disinfection of the sludge, and the degradation which
would occur would provide for a stable product. It is assumed that about
half of the material would be utilized for landscaping, and that the
remainder of the material would be utilized as an agricultural soil
amendment. For purposes of this screening of alternatives, it is assumed
Rau Sludge
DEWATERING
FIGURE 4.03-4
LIME STORAGE
AND FEED UNITS
t,+
LIME BLENDING
Stabilized
Cake
STORAGE
HIGHWAY TRANSPORI
1
BUNKER
STORAGE
AGRICULTURAL
LAND APPL I CA T ION
DRY LIME STABILIZATION/
AGRICULTURAL REUSE
DUBUQUE. IO W A WWTP
Sa
STRAND
A99OCiATE9
ENGINEERS
Cake
Rau Sludge
J
i SLUDGE
j DEWATERING
LIME STORAGE
AND FEED UNITS
LIME BLENDING
Stabilized j
Cake
'u1
7
CAFE TO SANITARY
LANDFILL
FIGURE 4.03-5
DRY LIME STABILIZATION/
SANITARY LANDFILL
DUBUQUE, IO W A WWTP
STRAND
.aaoGaES. iNo
E N p i N E E R
INCINERATOR
FIGURE 4.03-6
1MM
FIGURE 4.03-6
INCINERATION
Raw Sludge
Cake
Ash
DEWATERING
Heat
ASH TO
LANDFILL
INCINERATION
DUBUQUE, IO W A W W TP
t J
RECUPERATOR
STRAND
A40OCI4TES IMC
E N O 1 .4 E E n a
Rau
Sludge
Wood Chips
BULF I NG
AGENT
STORAGE
COMPOST
CELLS
r
DELIA TER INC
H I GHWAY
TRANSPORT
j CAFE STORAGE
BLENDING
kJLJ
1
AIR
SUPPLY
COMPOST CURING
ButiRin5 Agent RF.scover4'
FIGURE 4.03-7
I [1 BENEFICIAL USE
(2 (2 L)
COMPOSTING
DUBUQUE, IOWA WWTP
Sa
STRAND
,ssoc,ATe. ,,c
E NOINE EM•
MN
4 12
that belt press facilities would be installed at the plant for sludge
dewatering.
8. Approximate Costs for Potentially Applicable Technologies
Preliminary estimates of costs for capital, operation and maintenance, and
replacement costs were made for the sludge management alternatives. The cost
estimates were made on the basis of historical costs for similar facilities,
information obtained from manufacturer's representatives, and other available
data. All cost information contained herein is presented on a 3rd quarter 1990
dollars basis. Estimated costs are to provide facilities necessary for the 25%
reserve capacity loading level.
Table 4.03-2 lists approximate present worth costs (20 years, 8 7/8% discount
rate) for each alternative. The present worth cost can be considered as the sum
of money required today to build and operate the facilities for the 20 year
project life, including the costs of replacements, less any salvage value
remaining. The monetary costs of alternatives are most commonly compared on a
present worth cost basis.
C. Non -Monetary Factors
Several non -monetary factors are of importance, including: reliability,
constructibility, expansion potential, beneficial reuse of the sludge material,
and the ability to comply with potentially more stringent future regulatory
requirements.
1. Reliability
Reliability factors with the alternatives include the reliability of sludge
dewatering or thickening operations, sludge stabilization, and ultimate
sludge reuse or disposal.
a. Treatment
With respect to treatment technologies, digestion and
incineration are viewed as being the most reliable. Anaerobic
digestion is a common process, employed with success at many
facilities. Duplicate equipment would be provided for key
components of the digestion system. Sludge dewaterability is
a process variable, and may effect reliability, but it is common
to all alternatives.
Incineration is a complex process, but it has proven to be quite
successful at the Dubuque plant. The incineration process
provides standby for key plant components.
The lime systems are viewed as being somewhat less reliable than
digestion. Lime systems involve storage and feed systems that
are somewhat difficult to operate due to the nature of the lime
which causes a high wear factor for equipment as well as
problems with dust and housekeeping. With the lime systems, the
TABLE 4.03-2
APPROXIMATE PRESENT WORTH COSTS
SLUDGE MANAGEMENT ALTERNATIVES
DUBUQUE, IOWA WWTP
Alternative Approximate
Present Worth Cost
Digestion -Liquid Agricultural Reuse $13,500,000
Digestion -Dry Agricultural Reuse $14,000,000
Liquid Lime Stabilization-Ag Reuse $12,000,000
Dry Lime Stabilization-Ag Reuse $14,000,000
Dry Lime Stabilization -Landfill $12,500,000
Incineration $ 9,000,000
Composting $14,500,000
Note:
All costs expressed in 3rd quarter 1990 dollars.
Costs are for facilities for 25% reserve capacity level.
Present worth costs are for 20 years at 8 7/8% discount rate.
4-13
proper dose of lime to provide stability and pathogen
destruction would be an important process parameter and would
be somewhat difficult to control. The dry lime system is judged
to be somewhat more reliable than would be the liquid system.
Composting is viewed as being the least reliable of the
treatment processes considered. The static pile system would
require the construction of about one pile every working day at
design conditions. Each pile would require monitoring for
temperature, proper aeration rate, etc. Cold weather operation
would be problematic. The process would be labor-intensive.
There is little full-scale operating experience in the midwest.
b. Reuse or Disposal
All of the alternatives are somewhat problematic with respect
to reuse or disposal reliability, due to the uncertainty of the
requirements of future regulations. The incineration process
is viewed as the most reliable in this regard, with the
agricultural reuse and composting alternatives viewed as
somewhat less reliable. Of the reuse options, digestion -dry
agricultural reuse is seen as somewhat more advantageous due to
the smaller volume of sludge produced.
Agricultural reuse would be dependent upon regulations, public
acceptance, and the development of an effective program that
would be matched to the needs of the agricultural community.
Topography and land features in the Dubuque area would make
implementation difficult.
Composting is viewed as potentially even more problematic than
agricultural reuse from the standpoint of reliability of the
reuse portion of the program, as it would also depend upon the
public's perception of the utility and quality of the product,
and their demand for it for landscaping purposes.
2. Constructibility
The incineration alternative is judged to be the easiest to construct, as
only updating of the present system would be required. The dry lime -
landfill alternative is also viewed as being relatively easy to construct,
as no off -site facilities would be required and since the modifications to
the plant would be relatively minor. Of the remaining alternatives,
composting is viewed as being the most favorable, as most of the
construction would take place at a new site, with minimal potential
disruption of present operations.
3. Expansion Potential
For a significant plant expansion, the digestion and incineration options
are seen as being the most favorable. Provisions would be provided in the
design of the digestion facilities to incorporate expansion. The
�a
4-14
incinerators have significant capacity above that required for the year
2013.
The lime alternatives would be expandable, but operating costs would become
a significant disadvantage due to the cost of lime and the cost of
transportation and reuse or disposal of the relatively large quantity of
finished product. Composting would also be problematic for a significant
expansion due to increased labor requirements, and likely limits on demand
for the finished product.
4. Beneficial Reuse
If successful, composting would provide the highest level of beneficial
reuse. Incineration and landfilling would provide the lowest level of
beneficial reuse. 0f the remaining alternatives, the digestion -dry
agricultural reuse alternative is viewed as somewhat less advantageous with
respect to beneficial reuse, as a lower proportion of the nitrogen
originally present in the raw sludge would be recycled.
5. Regulatory Requirements
Dry lime-ag reuse and composting are viewed as slightly more favorable with
respect to anticipated future regulatory requirements. The dry lime and
composting processes have the capability to produce a product that could
be exempted from many regulatory requirements. It should be noted that,
in general, sludge reuse and disposal regulations are presently undergoing
significant development and change.
D. Selection of Alternatives for Detailed Analysis
Table 4.03-3 shows the relative ranking of the alternatives with respect to the
factors considered. Because of their overall better ranking, compared with the
other alternatives, digestion-ag reuse, lime -dry ag reuse, and incineration are
recommended to be selected for detailed evaluation. Because of their similarity,
and because of the potential advantages of providing both liquid and dewatered
reuse capability, it is recommended that both liquid and dry agricultural reuse
(or a combination) be considered following anaerobic digestion. For alternatives
employing sludge dewatering, dewatering by belt filter press or centrifuge
(centripress) will be considered in the detailed evaluation.
Factor
TABLE 4.03-3
OVERALL COMPARISON OF ALTERNATIVES SCREENED
SLUDGE MANAGEMENT ALTERNATIVES
DUBUQE, IOWA WWTP
Digest Digest Lime Lime Lime Incin Compost
Liq-Ag Dry-Aq Liq-Aq Dry-Aq Landfl
Monetary Cost +/- +/- +/- +/- +/- +
Reliability:
Treatment + + +/- +/- +/- +
Reuse/
Disposal +/ ++/- +/ ++/- +/ + +/--
Constructi-
bility +/- +/- +/- + + + ++/-
Expansion
Potential + + +/- +/- +/ + +/-
Beneficial
Reuse +/- +/-- +/- +/ - +
Regulations +/- +/ +/- + +/ +/- +
SCREENING * * * *
Note:
* means selected for further detailed evaluation
+ means favorable
++/- means intermediate
+/- means neutral
+/-- means intermediate
means unfavorable
5-1
SECTION 5
ANALYSIS OF MOST FEASIBLE ALTERNATIVES
In Section 4, a number of alternatives for biological wastewater treatment,
sludge thickening, and sludge stabilization and disposal were screened, and the
most feasible alternatives for the City of Dubuque were identified. These most
feasible alternatives are evaluated in detail in this section. Comparisons are
made on the basis of capital cost, operation and maintenance costs, overall
economic cost, and non -monetary factors. Overall recommendations are made as to
the best approach for long-term wastewater management. The evaluation in this
section of biological wastewater treatment alternatives is preliminary. Pilot
scale biotower studies are currently underway, and the results of these studies
may impact anticipated performance, costs, and other factors. Recommendations
concerning biological wastewater treatment will be updated, based on the pilot
study results, following conclusion of the pilot studies in early 1991.
5.01 BIOLOGICAL WASTEWATER TREATMENT
Following preliminary and primary treatment to remove coarse material and
settleable solids, wastewaters receive biological treatment. The present plant
was designed to provide biological wastewater treatment employing trickling
filtration followed by high purity oxygen activated sludge. Due to odors and
other operational problems with the trickling filters, and due to plant BODE
loadings substantially lower than the original plant design, the trickling
filters are not in service at the present time, and the high purity oxygen
activated sludge facilities receive the effluent from the primary clarifiers.
Three overall options for biological wastewater treatment are evaluated in this
section:
• Trickling Filtration -Air Activated Sludge
• Biotower-Air Activated Sludge
• High Purity Oxygen Activated Sludge
These options were evaluated based on the loading criteria listed in Table 5.01-1
and based on a required secondary level of treatment (30 mg/L average effluent
BOD5 and TSS). Wasteload forecasts are developed in Section 3. Effluent
limitations are discussed in Section 2.02.
The TF-Activated Sludge and BT-Activated sludge options were evaluated at 10% and
25% alternate levels of reserve capacity, whereas cost figures were developed for
10%, 25% and 50% reserve capacity for the existing high purity oxygen activated
sludge system. See Section 3.05 for a discussion of reserve capacity.
A. Description of Alternatives
Figures 5.01-1 through 5.01-3 provide a schematic diagram of each of the
biological wastewater treatment alternatives. Major design criteria for each
alternative are listed in Tables 5.01-2 through 5.01-4.
TABLE 5.01-1
PROJECTED PRIMARY EFFLUENT LOADINGS
BIOLOGICAL WASTEWATER TREATMENT EVALUATION
DUBUQUE, IOWA WWTP
Item 10% Reserve 25% Reserve 50% Reserve
Average Flow, mgd
Influent 11.93 13.39 15.85
Sidestreams (7.5%) 0.89 1.00 1.19
Total 12.82 14.39 17.04
Peak Flow, mgd
Peak Hourly Flow 32.20 34.30 36.11
Sidestreams (7.5% avg) 0.89 1.00 1.19
Total 33.09 35.30 37.30
Average BOD5, lb/d
Influent
Sidestreams (10%)
Subtotal
Primary Effluent (70%)
Peak Hourly BOD, lb/d (300%)
Influent
Primary Effluent
21,500
2,150
23,650
16,550
64,500
49,660
24,400
2,440
26,840
18,790
73,200
56,360
29,300
2,930
32,230
22,560
87,900
67,680
Average Total Suspended Solids, lb/d
Influent 21,700 24,600 29,500
Sidestreams (15%) 3,250 3,690 4,420
Subtotal 24,950 28,290 33,920
Primary Effluent (40%) 9,980 11,320 13,570
Note: 30% BOD5 and 60% TSS removal in primary treatment assumed.
Sidestreams are estimated recycle quantities from sludge
processing.
FIGURE 5.01-1 TRICKLING FILTER - ACTIVATED SLUDGE
TRICKLING FILTER
Primary
Effluent
Ventilation
-e
PUMP
STATION
Recycle
0 Ventilation
Aeration
AERATION
BASIN
Retum Sludge
FINAL
CLARIFIER
Waste Sludge
Effluent
Primary
Effluent
BIOTOWER
r
PUMP
STATION
Recycle
FIGURE 5.01-2
BIOTOWER - ACTIVATED SLUDGE
Ventilation
AERATION
BASIN
Retum Sludge
Aeration
FINAL
CLARIFIER
Waste Sludge
Effluent
n
FIGURE 5.01-3 OXYGEN ACTIVATED SLUDGE
Primary Effluent
r
High Purity Oxygen
HIGH PURITY OXYGEN
GENERATING PLANT
Aeration
AERATION
BASIN
Retum Sludge
FINAL
CLARIFIER
Waste Sludge
Effluent
TABLE 5.01-2
PRELIMINARY DESIGN CRITERIA
TRICKLING FILTER -ACTIVATED SLUDGE ALTERNATIVE
DUBUQUE, IOWA WWTP
Unit Process or System
Trickling Filters:
(no.) and size
total media volume
organic loading
10% reserve
25% reserve
TF Recirculation Pumping:
type
pumps
firm capacity
Intermediate Pump Station:
type
pumps
firm capacity
Aeration Tanks:
(no.) and size
total volume
organic loading
10% reserve
25% reserve
Aeration:
type
oxygen transfer capacity
10% reserve
24% reserve
Return Activated Sludge
type
firm capacity
@10% reserve
@25% reserve
Final Clarification:
(no.) and size
total area
hydraulic loading
10% reserve
25% reserve
Sludge Removal
Design Criteria of Loading
(2) units, 195 ft dia, 6 ft media depth
358,400 cu ft
46 lb BOD5/d/1000 cu ft
52 lb BOD5/d/1000 cu ft
submersible
(3) 5 mgd capacity pumps
10 mgd
existing wetwell/drywell
(3) 7.2 mgd capacity pumps
35.5 mgd
(3) trains, 84,200 cu ft each
1.89 mil gal (252,600 cu ft)
29 lb BOD5/d/1000 cu ft (winter)
34 lb BOD5/d/1000 cu ft (winter)
low speed surface aerators (upgrade existing)
575 lb/hr (peak)
700 lb/hr (peak)
Pumping:
centrifugal (existing)
11.5 mgd
90% avg flow
80% avg flow
(4) units, 105 ft dia, 12 ft depth
34,600 sq ft
370 gpd/sq ft (avg), 956 gpd/sq ft (peak)
416 gpd/sq ft (avg), 1020 gpd/sq ft (peak)
combined scraper/hydraulic suction
TABLE 5.01-3
PRELIMINARY DESIGN CRITERIA
BIOTOWER-ACTIVATED SLUDGE ALTERNATIVE
DUBUQUE, IOWA WWTP
Unit Process or System
Biotowers:
(no.) and size
10% reserve
25% reserve
total media volume
10% reserve
25% reserve
organic loading
10% reserve
25% reserve
Biotower Feed Pumping:
type
pumps
10% reserve
25% reserve
firm capacity
10% reserve
25% reserve
Aeration Tanks:
(no.) and size
total volume
organic loading
10% reserve
25% reserve
Aeration:
type
oxygen transfer
capacity
10% reserve
25% reserve
Return Activated Sludge
Pumping:
type
firm capacity
@10% reserve
@25% reserve
Design Criteria of Loading
(2) units, 90 ft dia, 18 ft media depth
(2) units, 100 ft dia, 18 ft media depth
229,000 cu ft
282,700 cu ft
72 lb BOD5/d/1000 cu ft
66 lb BOD5/d/1000 cu ft
nodify existing intermediate lift station
(wetwell/drywell)
(3) 16.6 mgd capacity pumps
(3) 17.7 mgd capacity pumps
33.1 mgd
35.3 mgd
(3) trains, 84,200 cu ft each
1.89 mil gal (252,600 cu ft)
29 lb BOD5/d/1000 cu ft (winter)
32 lb BOD5/d/1000 cu ft (winter)
low speed surface aerators (upgrade existing)
575 lb/hr (peak)
575 lb/hr (peak)
centrifugal (existing)
11.5 mgd
90% avg flow
80% avg flow
Table 5.01-3 (Continued)
Unit Process or System
Final Clarification:
(no.) and size
total area
hydraulic loading
10% reserve
25% reserve
Sludge Removal
Note:
Design Criteria of Loading
(4) units, 105 ft dia, 12 ft depth
34,600 sq ft
370 gpd/sq ft (avg), 956 gpd/sq ft (peak)
416 gpd/sq ft (avg), 1020 gpd/sq ft (peak)
combined scraper/hydraulic suction
Criteria subject to change, based on results of biotower
pilot studies to be completed in early 1991.
TABLE 5.01-4
PRELIMINARY DESIGN CRITERIA
OXYGEN -ACTIVATED SLUDGE ALTERNATIVE
DUBUQUE, IOWA WWTP
Unit Process or System
Aeration Tanks:
(no.) and size
total volume
organic loading
10% reserve
25% reserve
50% reserve
Aeration:
type
oxygen transfer
capacity
Return Activated Sludge
Pumping:
type
firm capacity
@10% reserve
@25% reserve
Final Clarification:
(no.) and size
total area
hydraulic loading
10% reserve
25% reserve
Sludge Removal
Design Criteria of Loading
(3) trains, 84,200 cu ft each
1.89 mil gal (252,600 cu ft)
65 lb BOD5/d/1000 cu ft
74 lb BOD5/d/1000 cu ft
89 lb BOD5/d/1000 cu ft
high purity oxygen - low speed
(existing)
1,750 lb/hr (existing)
surface aerators
centrifugal (existing)
11.5 mgd (existing)
90% avg flow
80% avg flow
(4) units, 105 ft dia, 12 ft depth
34,600 sq ft
370 gpd/sq ft (avg), 956 gpd/sq ft (peak)
416 gpd/sq ft (avg), 1020 gpd/sq ft (peak)
combined scraper/hydraulic suction
5-2
1) Trickling Filtration -Air Activated Sludge
This alternative would entail removal of the rock media from the present
trickling filters, construction of a new media underdrain system,
installation of new synthetic media, installation of new trickling filter
covers, installation of new trickling filter rotary distributors and forced
ventilation, construction of a trickling filter recirculation pumping
station, modifications to the present intermediate pump station pumps to
resolve cavitation problems, minor structural repairs and modifications to
the present activated sludge tankage, modifications to the present low -
speed activated sludge aerators to increase their oxygen transfer capacity
for use with air rather than high purity oxygen, installation of fans to
blow air under the aeration tank covers, and upgrading of the final
clarifiers with new rotating mechanisms, weir troughs, weirs and baffles.
Other work would include associated piping and mechanical work, electrical,
and site work.
The high purity oxygen plant would be removed from service with this
alternative. It is assumed that the oxygen plant would be purged with
nitrogen gas and placed in a standby condition, with minimal continue.d
maintenance, to allow its future use if necessary.
An evaluation of the potential for using the present rock media and clay
tile underdrains was performed, and it was determined that the underdrain
system does not have adequate hydraulic capacity for the design conditions.
The hydraulic limitations would result in flooding of the underdrains, the
inability to provide adequate ventilation of the rock media, odors, and
poor process performance with the use of the units in their present
configuration. Because of this, it was concluded that the trickling
filters would have to be provided with new media and underdrains if they
were to be utilized in an upgraded process.
Much of the BOD5 removal provided with this process would be achieved in
the upgraded trickling filter. Under average design loading conditions and
at average temperature, it is estimated that about 75% of the primary
effluent BOD5 would be removed in the trickling filters. Average design
primary effluent BOD5 in the trickling filters would be about 73% at the
25% reserve capacity level. These figures would fall to an estimated
removal of about 56% (10% reserve) and 54% (25% reserve) during the coldest
winter months. Because of the substantial removal of B0D5 during the
moderate and warm periods of the year, the activated sludge system would
be utilized under most conditions primarily for "solids contact" purposes,
to coagulate fine suspended solids leaving the trickling filters and to
improve the clarity of the effluent. This would require use of only one
of the three aeration trains, a minimal MLSS level, and a low level of
aeration energy.
To further minimize energy use in activated sludge aeration, it is proposed
that a mechanically adjustable weir be installed at the effluent end of
each aeration train. The weir would be automatically adjusted in response
to monitored in -tank dissolved oxygen (DO) levels. For in -tank DO levels
higher than required, the weir would be lowered which would result in
n
5-3
lowering of the liquid level and reduced submergence on the aeration
equipment. This would reduce power consumption and the rate of oxygen
transfer into the system to match the demand. Should the in -tank DO level
fall below the desired level, the weir would be automatically raised which
would increase aerator power and oxygen transfer. Aeration capacity of the
existing aeration equipment would be increased by providing larger
horsepower motors and larger capacity gearboxes to selected units. The
upgraded units would be sized to accommodate an oxygen demand three times
the anticipated average rate in cold weather, to meet peaking requirements.
Covers would be provided to reduce operating problems in freezing weather,
as freezing problems with the large area trickling filters would be
anticipated. Adjustable rate forced air ventilation would be employed,
with the capability to reduce ventilation rates in cold weather to control
temperature loss.
Down -draft forced air ventilation, and adequate hydraulic flushing rates
would be used for improved odor control. It is believed that the major
reason for historic problems with odors with trickling filter operations
was due to the inadequate underdrain system which caused anaerobic
conditions to develop due to poor ventilation. Additional odor control of
the TF off -gases could be accomplished by conveying the off -gases to the
activated sludge tanks to ventilate the cover, where exposure to the MLSS
during aeration would provide an additional measure of odor control.
2) Biotower-Air Activated Sludge
The Biotower-Air Activated Sludge alternative would in concept be similar
to the previous alternative, except that new biotowers would be constructed
having a deeper media than could be provided in the trickling filters.
Performance similar to that expected with the upgraded trickling filter
could be achieved with less media, The higher hydraulic loading rate
provided would reduce concerns relative to cold weather operating problems,
such that covering for freeze protection would not be necessary.
Improvements would be: upgrading the existing intermediate pumping station
with new pumps to serve the new biotowers; new biotowers with synthetic
media, and forced ventilation; minor aeration tank structural
modifications; upgrading of the activated sludge aeration equipment to
increase its oxygen transfer capacity; new final clarifier equipment;
installation of automatic adjustable effluent weirs on the aeration tanks,
and miscellaneous piping, mechanical, electrical and site work. As with.
the TF-Activated sludge alternative, it is assumed that the oxygen plant
would be purged with nitrogen gas and maintained in a standby mode.
With the 25% reserve capacity level, with this alternative, the sizing of
the biotower was increased to maintain the activated sludge aeration
requirement at the same level as with the 10% reserve capacity option. Due
to economies of scale in construction of the biotower, this would be a more
economical approach.
5-4
Down -draft mechanical ventilation and adequate hydraulic flushing rates
would be used to maximize process performance and odor control. Down -draft
ventilation would expose the off -gases to bio-slime in the BT, which has
been reported to reduce odor potential. Additional odor control could be
accomplished by routing the air exiting the BT to the activated sludge
tankage for ventilation of the aeration tank covers.
3) Oxygen Activated Sludge Alternative
This alternative would continue the use of the present cryogenic plant to
produce high purity oxygen, and would continue to rely solely on the
activated sludge system for biological wastewater treatment.
Improvements would be made to the system, including installation of a new
centrifugal compressor, repairs to the aeration tanks, new final clarifier
equipment, and associated electrical and instrumentation improvements.
B. Capital Costs
Capital costs for the alternatives were estimated based on unit prices for items
of work from past similar projects, and based on the estimated installed cost of
equipment. Pricing for major items of equipment was obtained from manufacturer's
representatives to assist in developing the cost estimates. A 30% factor was
included to provide a contingency and to account for technical service costs.
Capital cost estimates for the alternatives are presented in Tables 5.01-5
through 5.01-7. All costs are on a third quarter 1990 basis.
C. Operation and Maintenance Costs
Operation and maintenance costs for labor, supplies and chemicals, energy, and
miscellaneous repair were estimated for each alternative on the basis of present
costs with adjustments for changes that would occur with the particular
alternative. The timing and cost of replacement of major items of equipment, as
necessary to maintain the function and performance of each alternative system
were also estimated.
Operation, maintenance, and replacement cost estimates are presented in Tables
5.01-8 through 5.01-10. All costs are on a third quarter 1990 basis.
D. Present Worth Costs
Economic comparisons of alternatives are most commonly compared on a present
worth cost or equivalent annual cost basis. Present worth cost may be thought
of as an amount of money needed today to build and operate a system over its
planning life. A planned life of each alternative was taken as 20 years, from
1993 to 2013. Moneys in excess of the original capital cost would be invested
at the "discount rate", with the present worth concept" and the accumulating
funds used to pay annual costs and future replacement costs. A credit is also
given for any remaining salvage value at the end of the project life, discounted
by the discount rate over the 20 year period.
TABLE 5.01-5
TF-ACTIVATED SLUDGE
ESTIMATED CAPITAL COSTS
ITEM
TF Covers
TF Rotary Distributors
TF Media
TF Mechanical Ventilation
TF Removals
TF Structural Modifications
TF Recycle Pump Station:
Structures
Equipment
Intermed. Pump Station Mods.
Aeration Tank Structural Mods.
Aeration Tank Motorized Weirs
Aeration Tank Equipment Mods.
Final Clar. Equip. Replacement
02 Plant Expense
Piping and Mechanical
Electrical
Site Work
Subtotals
Contingencies, Tech. Services.
At 30%
Totals
10% 25%
RES. RES.
$1,340,000
$260,000
$1,470,000
$205,000
$90,000
$45,000
$35,000
$175,000
$100,000
$105,000
$75,000
$175,000
$510,000
$50,000
$175,000
$125,000
$50,000
$1,340,000
$260,000
$1,470,000
$205,000
$90,000
$45,000
$35,000
$175,000
$100,000
$105,000
$75,000
$250,000
$510,000
$50,000
$175,000
$125,000
$50,000
$4,985,000 $5,060,000
$1,496,000
$6,481,000
Note: All costs are third quarter 1990 dollars.
$1,518,000
$6,578,000
ITEM
TABLE 5.01-6
BT-ACTIVATED SLUDGE
ESTIMATED CAPITAL COSTS
10% 25%
RES. RES.
BT Rotary Distributors $195,000
BT Structures $725,000
BT Media $705,000
BT Mechanical Ventilation $100,000
Intermed. Pump Station Mods:
Structural $50,000
Equipment $310,000
Aeration Tank Structural Mods. $105,000
Aeration Tank Motorized Weirs $75,000
Aeration Tank Equipment Mods. $175,000
Final Clar. Equip. Replacement $510,000
02 Plant Expense $50,000
Piping and Mechanical $225,000
Electrical $125,000
Site Work $100,000
Subtotals
Contingencies, Tech. Services.
At 30%
Totals
$220,000
$835,000
$870,000
$110,000
$50,000
$310,000
$105,000
$75,000
$175,000
$510,000
$50,000
$225,000
$125,000
$100,000
$3,450,000 $3,760,000
$1,035,000 $1,128,000
$4,485,000 $4,888,000
Note: All costs are third quarter 1990 dollars.
ITEM
Aeration Tank Repairs
02 Plant Compressor
Final Clar. Equip. Replacement
Electrical/Instrumentation
Subtotals
Contingencies, Tech Services.
At 30%
Totals
aLzi
a Ali- NA
19Fi t
TABLE 5.01-7
02—ACTIVATED SLUDGE
ESTIMATED CAPITAL COSTS
0{{SI
10%
RES
m 04..}r45
25%
RES
nrr
144, 4-5417
50%
RES.
Note; All costs are third quarter 1990 dollars.
$80, 000
$310, 000
$510, 000
$275, 000
$1, 175, 000
$352, 000
$1, 527, 000
$80, 000
$340, 000
$510, 000
$275, 000
$80, 000
$380, 000
$510, 000
$275, 000
$1, 205, 000 $1, 245, 000
$362, 000
$1, 567, 000
$374, 000
$1, 619, 000
ltT4 4811 ‘)H 4110
TABLE 5.01-8
TF-ACTIVATED SLUDGE
ESTIMATED O,M & R COSTS
OPERATION AND MAINTENANCE
Initial Year:
Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
Design Year (2013):
Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
EQUIP. REPLACEMENT
Yr 2003:
Aeration Equip.
Pumps
Yr 2008:
Instrumentation
10% 25%
RES. RES.
$110,000 $110,000
$45,000 $50,000
$110,000 $110,000
$60,000 $60,000
$325,000 $330,000
$115,000 $120,000
$45,000 $50,000
$125,000 $145,000
$70,000 $75,000
$355,000 $390,000
10% 25%
RES. RES.
$200,000 $200,000
$600,000 $600,000
$100,000 $100,000
Note: All costs are third quarter 1990 dollars.
1114
TABLE 5.01-9
BT-ACTIVATED SLUDGE
ESTIMATED O,M & R COSTS
OPERATION AND MAINTENANCE
Initial Year:
Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
Design Year (2013):
Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
EQUIP. REPLACEMENT
Yr 2003:
Aeration Equip.
Pumps
Yr 2008:
Instrumentation
10% 25%
RES. RES.
$110,000 $110,000
$45,000 $45,000
$105,000 $115,000
$60,000 $60,000
$320,000 $330,000
$115,000 $115,000
$45,000 $45,000
$130,000 $155,000
$70,000 $70,000
$360,000 $385,000
10% 25%
RES. RES.
$200,000 $200,000
$250,000 $250,000
$100,000 $100,000
Note: All costs are third quarter 1990 dollars.
I
TABLE 5.01-10
02-ACTIVATED SLUDGE
ESTIMATED O,M & R COSTS
OPERATION AND MAINTENANCE
Initial Year:
Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
Design Year (2013):
Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
EQUIP. REPLACEMENT
Yr 2003:
Yr 2008:
Aeration Equip.
Pumps
Compressor.
Deck Repair
02 Plant Maint.
Instrumentation
10%
RES.
25%
RES.
50%
RES.
$175,000
$80,000
$170,000
$100,000
$525,000
$185,000
$90,000
$200,000
$110,000
$585,000
10%
RES.
$175,000
$80,000
$170,000
$100,000
$525,000
$190,000
$100,000
$255,000
$120,000
$665,000
25%
$175,000
$80,000
$170,000
$100,000
$525,000
$195,000
$120,000
$290,000
$140,000
$745,000
50%
RES. RES.
$200,000 $200,000 $200,000
$250,000 $250,000 $250,000
$310,000 $340,000 $380,000
$120,000 $120,000 $120,000
$1,000,000 $1,000,000 $1,000,000
$150,000 $150,000 $150,000
Note: All costs are third quarter 1990 dollars.
5-5
The equivalent annual cost can be considered the annual amount that would be
needed to meet all capital obligations, with principal and interest retired at
the "discount rate", and as needed to meet annual costs. Credit is also given
for any future salvage value of the facilities at the end of the project life.
With either present worth or equivalent annual cost comparisons of alternatives,
future inflation of costs is generally not considered, with the discount rate
being adjusted to account for anticipated inflation. Future costs are evaluated
on a "constant dollars" basis, using their value at the time that the analysis
is made. The selection of the proper discount rate is of importance, as a
relatively high discount rate will tend to favor alternatives with low initial
costs and high annual costs, whereas a relatively low discount rate will tend to
favor alternatives having higher initial costs and lower annual costs. The
current discount rate used for U.S. Environmental Protection Agency Water
Programs is 8.875%, and present worth and equivalent annual cost calculations
were performed using this rate. This discount rate was established about one
year ago. Based on current economic conditions, it appears that we may be
entering a period of low economic growth, such that a lower discount rate may be
appropriate. For this reason, present worth and equivalent annual cost
calculations were also performed using a discount rate of 5%. A 5% discount rate
has been used on a number of "Superfund" projects that we have been involved in
recently, for comparing the economic costs of alternatives. In real terms, the
discount rate can be considered to be the earnings that could be obtained on a
conservative investment, minus the inflation rate. This is the real rate of
growth of funds. 0n this basis, the 5% discount rate is probably more
representative of current economic conditions than would be the 8.875% rate.
Table 5.01-11 summarizes the capital, 0&M and replacement costs for the
alternatives, and lists the computed present worth and equivalent annual costs
for each alternative for both 5% and 8.875% discount rates. Table 5.01-11A
presents the same information as Table 5.01-11, but with the construction of the
biotowers staged. One biotower would be constructed initially, and the second
constructed after 10 years (year 2003). Using the 02-Activated Sludge
Alternative as a basis, the economic costs of the TF-Activated Sludge and BT-
Activated sludge would compare as follows:
Alternative Fraction of Oxygen -Activated Sludge Cost
10% Reserve 25% Reserve
5% Discount Rate:
TF-AS 1.09 1.07
BT-AS 0.89 0.91
Staged BT-AS 0.83 0.84
02-AS 1.00 1.00
8.875% Discount Rate:
TF-AS 1.29 1.27
BT-AS 1.03 1.05
Staged BT-AS 0.92 0.94
02-AS 1.00 1.00
MI1
ITEM
TABLE 5.01-11
PRESENT WORTH COST ESTIMATES
BIOLOGICAL WASTEWATER TREATMENT ALTERNATIVES
TF-ACT SLG TF-ACT SLG BT-ACT SLG BT ACT SLG 02-ACT SLG 02-ACT SLG 02-ACT SIG
10% RES 25% RES 10% RES 25% RES 10% RES 25% RES 50% RES
Initial Capital Cost
Yr 2003 Replacements
Yr 2008 Replacements
Est. Yr 2013 Salvage Val
Present Vat (5%, 20yr)
Present Val (8.875%, 20yr)
Initial 0&M/yr
Yr 2013 0&M/yr
Present Val (5%, 20yr)
Present Val (8.875%, 20yr)
Total Present Val:
5%, 20yr
8.875%, 20yr
Equiv. Annual Cost:
5%, 20yr
8.875%, 20yr
$6,481,000
$800,000
$100,000
$2,338,000
$6,139,065
$6,423,903
$325,000
$355,000
$4,197,951
$3,087,349
$6,578,000
$800,000
$100,000
$2,338,000
$6,236,065
$6,520,903
$330,000
$390,000
$4,407,995
$3,227,356
$4,485,000
$450,000
$100,000
$1,353,000
$4,299,431
$4,458,188
$320,000
$360,000
$4,184,884
$3,072,615
$4,888,000
$450,000
$100,000
$1,496,000
$4,648,536
$4,835,080
$330,000
$385,000
$4,383,373
$3,211,697
$1,527,000
$1,880,000
$150,000
$267,000
$2,652,680
$2,323,446
$1,567,000
$1,910,000
$150,000
$267,000
$2,711,097
$2,376,264
$1,619,000
$1,950,000
$150,000
$267,000
$2,787,654
$2,445,356
$525,000 $525,000 $525,000
$585,000 $665,000 $745,000
$6,838,126 $7,232,079 $7,626,033
$5,023,393 $5,273,939 $5,524,484
$10,337,016 $10,644,059 $8,484,315 $9,031,909 $9,490,806 $9,943,177 $10,413,687
$9,511,252 $9,748,259 $7,530,803 $8,046,777 $7,346,838 $7,650,203 $7,969,840
$829,469 $854,107 $680,803 $724,744 $761,567 $797,866 S835,621
$1,032,659 $1,058,392 $817,637 $873,658 $797,664 $830,601 $865,305
Note: All costs are third quarter 1990 dollars.
TF-ACT SLG and BT-ACT SLG costs assume "passive" odor control and do not include
potential chemical scrubbing of TF or BT off -gases.
TABLE 5.01-11A
PRESENT WORTH COST ESTIMATES
BIOLOGICAL WASTEWATER TREATMENT ALTERNATIVES
STAGED CONSTRUCTION OF BIOTOWERS
TF-ACT SLG TF-ACT SLG BT-ACT SLG HT ACT SLG 02-ACT SLG 02-ACT SLG 02-ACT SLG
10% RES 257 RES 10% RES 25% RES 10% RES 25% RES 50% RES
Initial Capital Cost $6,481,000 $6,578,000 $3,265,000 $3,468,000 $1,527,000 $1,567,000 $1,619,000
Yr 2003 Replacements/Constr. $800,000 $800,000 $1,670,000 $1,870,000 $1,880,000 $1,910,000 $1,950,000
Yr 2008 Replacements $100,000 $100,000 $100,000 $100,000 $150,000 $150,000 $150,000
Est. Yr 2013 Salvage Val $2,338,000 $2,338,000 $1,722,000 $1,932,000 $267,000 $267,000 $267,000
Present Val (5%, 20yr) $6,139,065 $6,236,065 $3,689,333 $3,935,969 $2,652,680 $2,711,097 $2,787,654
Present Val (8.875%, 20yr) $6,423,903 $6,520,903 $3,692,107 $3,942,224 $2,323,446 $2,376,264 $2,445,356
Initial O&M/yr $325,000 $330,000 $320,000 $330,000 $525,000 $525,000 $525,000
Yr 2013 O&M/yr $355,000 $390,000 $360,000 $385,000 $585,000 $665,000 $745,000
Present Val (5%, 20yr) $4,197,951 $4,407,995 $4,184,884 $4,383,373 $6,838,126 $7,232,079 $7,626,033
Present Vat (8.875%, 20yr) $3,087,349 $3,227,356 $3,072,615 $3,211,697 $5,023,393 $5,273,939 $5,524,484
Total Present Val:
5%, 20yr
8.875%, 20yr
Equiv. Annual Cost:
5%, 20yr
8.875%, 20yr
$10,337,016 $10,644,059 $7,874,217 $8,319,342 $9,490,806 $9,943,177 $10,413,687
$9,511,252 $9,748,259 $6,764,722 $7,153,920 $7,346,838 $7,650,203 $7,969,840
$829,469 $854,107 $631,848 $667,565 $761,567 $797,866 $835,621
$1,032,659 $1,058,392 $734,462 $776,718 $797,664 $830,601 $865,305
Note: All costs are third quarter 1990 dollars.
TF-ACT SLG and BT-ACT SLG costs assume "passive" odor control and do not include
potential chemical scrubbing of TF or 8T off -gases.
5-6
The 50% reserve capacity 0 -Activated Sludge alternative would involve costs of
about 10% and 8% higher, than 10% reserve 02-Activated Sludge, at the 5% and at
the 8.875% discount rates, respectively.
On an economic cost basis, the TF-Activated Sludge alternative would appear to
offer no advantage over the present 02-Activated Sludge system. Although the
annual costs with this alternative are estimated to be significantly lower than
with the present system, there would be insufficient savings to offset the
relatively large initial investment.
Without staging, the BT-Activated Sludge alternative Would offer some economic
savings over the present system, for both the 10% and 25% reserve capacity levels
at the 5% discount rate. At the 8.875% discount rate, costs would be slightly
higher with the BT-Activated Sludge system, as compared with the 02-Activated
Sludge system. We have calculated that at a discount rate of 8.1%, the 10%
reserve BT-Activated Sludge alternative (without staging) and the 10% reserve 0 -
Activated Sludge alternative would have equal present worth costs. 8.1% would
therefore be the internal rate of return in investing in the 10% reserve BT-
Activated Sludge Alternative (without staging), as compared with the 10% 02-
Activated Sludge Alternative. If it is assumed that energy costs will escalate
at a rate of 3% per year higher than the general inflation rate, the internal
rate of return would increase to 8.8%. This assumes that chemical scrubbing of
the BT off -gas is not necessary for odor control.
With staging, the BT-Activated Sludge alternative would provide some economic
savings over the present system, for both the 10% and 25% reserve capacity levels
at both the 5% and 8.875% discount rates. We have calculated that at a discount
rate of 12.5%, the 10% reserve Staged BT-Activated Sludge alternative and the 10%
reserve 0 -Activated Sludge alternative would have equal present worth costs.
12.5% would therefore be the internal rate of return in investing in the 10%
reserve BT-Activated Sludge Alternative (with staging), as compared with the 10%
02-Activated Sludge Alternative. If it is assumed that energy costs will
escalate at a rate of 3% per year higher than the general inflation rate, the
internal rate of return would increase to 13.4%. As with the BT-Activated Sludge
option, without staging, this assumes that chemical odor scrubbing equipment
would not be required.
Note that staging of the biotower construction in this fashion may be a very
practical consideration, as current actual plant loadings are less than half of
the 10% design loading level. Economic advantage would be gained by delaying
construction until the time it would actually be required. See Section 7 for
additional discussion concerning staging of plant improvements.
E. Non -Monetary Factors
Other factors that should be considered in the comparison of alternatives
include: expansion potential, reliability, ease of operation, and other factors.
The alternatives were compared with respect to these factors as follows:
5-7
1) Expansion Potential
The BT-Activated Sludge and 02-Activated Sludge alternatives are judged to
be about equal with respect to this criterion, and somewhat more
advantageous than the TF-Activated Sludge alternative. The BT-Activated
Sludge alternative could be readily expanded by construction of parallel
treatment units. The 02-Activated Sludge alternative could be expanded by
construction of upstream roughing towers. The TF-Activated Sludge system
would involve a commitment to the shallow depth, large surface area,
trickling filter configuration which would involve higher costs for a
significant future capacity expansion.
2) Reliability
All of the alternatives are considered to be relatively reliable. Two
factors, however, would give the TF-Activated Sludge and BT-Activated
Sludge a slight advantage in reliability over the 02-Activated Sludge
System: a) reliance on newer equipment; and b) when operated in its most
energy efficient configuration, the 02-Activated Sludge process would be
more prone to upsets and pass -through of high organic loadings. This is
due to the fact that to conserve power with the 02-Activated Sludge
process, the process is run with aeration tankage out of service, or at a
reduced MLSS level. This reduces the ability of the process to handle high
loading peaks. The TF-Activated Sludge and BT-Activated Sludge processes,
by comparison, would provide a significant dampening of peak loadings in
the attached growth BT or TF processes, such that they would be somewhat
more resistant to upset or pass -through.
If staged construction were employed to implement the BT-AS alternative,
reliability would be less during the initial years of the life of the
project, as less treatment capacity would be provided.
3) Ease of Operation
The TF-Activated Sludge and BT-Activated Sludge Alternatives are considered
to be more easily operated than the 02-Activated Sludge System. The unit
processes involved are relatively simple and less equipment intensive.
Less time would be required on the part of operation and maintenance
personnel. These advantages would be partially offset by the need to
closely monitor the process to prevent significant warm weather
nitrification (conversion of ammonia to nitrate) that would significantly
increase oxygen requirements. This would need to be controlled by reducing
the volume of aeration tanks in service during moderate and warm weather,
and would involve additional sampling and analysis (BT or TF effluent) to
properly gauge process performance. The present 02-Activated Sludge system
operates at a relatively low pH, due to the high partial pressure of carbon
dioxide gas in the system, and this inhibits significant nitrification.
5-8
4) Other Factors
The BT-Activated Sludge and TF-Activated Sludge alternatives would produce
about 25% less waste biological sludge, which would reduce costs for sludge
handling.
All three alternatives would be capable of producing an effluent of
excellent quality. All three alternatives would make use of existing
facilities having significant remaining useful life.
The BT-Activated Sludge and TF-Activated Sludge alternatives would be
designed to reduce odor potential, but odors would be possible at times.
The 02-Activated Sludge alternative is considered somewhat more
advantageous than the other alternatives from the standpoint of odor
potential. Should odors prove to be a problem with the BT and TF
alternatives, scrubbing of the TF or BT off -gases could be provided. If
necessary, such odor control would not be expected to be continuously
required. The potential need for off -gas treatment, however, entails some
risk. If chemical scrubbing were necessary with the BT or TF-Activated
Sludge Alternatives, capital and operating costs would significantly
increase. This could potentially reduce or eliminate the monetary cost
advantage shown for the BT-Activated Sludge alternative in Tables 5.01-11
and 5.01-11A.
5.02 SLUDGE THICKENING
Thickening of raw sludges is advantageous, as it reduces sludge volume and costs
associated with downstream sludge processing. At the present time, manual
operation of telescoping valves on the primary sludge withdrawal lines is
performed to maximize the concentration of sludge withdrawn from the units. This
has been effective, but is labor intensive. As an alternative, installation of
air -operated diaphragm pumps is considered in this section which would allow
maximum concentration of primary sludge withdrawn from the clarifiers in an
automatic mode of operation. An additional alternative, separate gravity
thickening, will also be considered.
Waste biological sludges are currently thickened by centrifugation. The
centrifuges utilized for this purpose are new units, recently installed as part
of the settlement of litigation. As an alternative, gravity belt thickening will
be considered.
A. Primary Sludge Thickening
Alternatives to the present manual primary sludge withdrawal method would include
air -operated diaphragm pumps and separate gravity thickening.
5-9
1) Diaphragm Pumps
This would entail replacement of the present progressive cavity pumps with
air operated diaphragm pumps, modification of the sludge withdrawal piping,
and associated electrical and mechanical work. Four pumps would be
installed, with interconnecting piping to allow back-up operation, and a
duplex air compressor would be provided for backup of the air supply. The
capital cost (including 30% for contingencies and technical services) is
estimated to be about $125,000. We have estimated that about $35,000 per
year in operating costs (primarily operating labor) would be saved with
this alternative, which would provide a "payback" of about 3.6 years.
2) Gravity thickening
This would entail manual setting of the telescopic valves to provide a
continuous withdrawal of primary sludge of low concentration to the gravity
thickening unit. Supplemental effluent water would also need to be
provided to the thickener to provide adequate hydraulic loading. We have
estimated that the capital cost (including 30% for contingencies and
technical services) of installation of a gravity thickener would be about
$450,000. Annual cost savings would be small. Labor would be saved in
operation of the telescopic valves at the primaries, but the thickening
equipment would need to be operated and maintained. The sidestream loading
to wastewater treatment would be significant with this alternative, which
would tend to increase treatment costs. Installing a gravity thickening
unit would not be economical.
3) Recommended Approach
Installation of air -operated diaphragm pumps is recommended, as the capital
cost would be relatively small, compared to the anticipated savings in
labor costs for manual operation of the telescopic valves at the primary
tanks.
B. Secondary Sludge Thickening
Thickening of waste biological solids is currently accomplished by centrifugation
using new horizontal scroll continuous flow units. These units have a firm
capacity of about 19,400 lb/d of sludge, with polymer use, and have provided a
solids recovery of over 90%. They are a substantial improvement over the units
originally installed with the plant, which at times operated with a solids
recovery of 50% or less.
The gravity belt thickening alternative would involve removal of the centrifuges
and installation of two gravity belt units. The gravity belt units would be
single -belt machines, and would function much like the gravity zone of the
present belt press. Washwater pumps, associated piping, and improved heating and
ventilating would be required.
It is estimated that about $25,000 per year in annual operating costs could be
saved with the gravity belt units. Installation of two 1 1/2 meter units,
however, would involve capital costs (including 30% for contingencies and
5-10
technical services) of about $500,000. There would not be sufficient savings to
justify this expense.
Continued reliance on the new centrifuges for waste biological sludge thickening
is recommended.
5.03 SLUDGE TREATMENT AND DISPOSAL/REUSE
Solids which settle to the bottom of the primary clarifiers are periodically
removed from the units, at a consistency of 4 to 6% solids. This stream is
referred to as primary sludge. Excess biomass which develops in the biological
wastewater treatment system must be wasted regularly from the process to maintain
a proper solids inventory. This material is referred to as waste activated
sludge, and is thickened in centrifuges to about 3 to 4% solids prior to further
handling. These raw sludges are unstabilized, and require further treatment
prior to disposal or reuse due to their high organic content and the presence of
pathogenic organisms. The present method of sludge stabilization is incineration
in fluidized bed incinerators.
Four overall methods of sludge stabilization and ultimate disposal/reuse are
evaluated in this section:
• anaerobic sludge digestion with liquid agricultural reuse of stabilized
sludge;
• anaerobic sludge digestion with agricultural reuse of stabilized and
dewatered sludge cake;
• dewatering of raw sludge, lime addition, and agricultural reuse of lime -
stabilized sludge; and
• incineration and landfill disposal of incinerator ash.
The alternatives to incineration were evaluated primarily for their ability to
reduce operating costs for labor, energy, and other commodities. Anaerobic
digestion and agricultural reuse is the preferred method of sludge handling
throughout the midwest, due to its simplicity, economy, and because it provides
the benefit of returning organic matter and nutrients to the farmland.
The digestion and lime -stabilization options were evaluated on the basis of 10%
and 25% reserve capacity. Costs were developed for the incineration option for
10%, 25% and 50% levels of alternative reserve capacity. Please refer to Section
3 for wasteload forecasts and a discussion of alternate levels of reserve
capacity.
The sludge management alternatives were evaluated, on the basis of the following
average sludge production rates for the various loading alternatives:
Alternative
TF-AS and BT-AS
10% Reserve
25% Reserve
02-Activated Sludge
10% Reserve
25% Reserve
50% Reserve
Primary Sludge
15,000
17,000
15,000
17,000
20,400
A. Description of Alternatives
Biological Total
Sludge, lb/d Sludge, lb/d
9,700 24,700
11,000 28,000
13,800
15,800
19,000
28,800
32,800
39,400
5-11
Figures 5.03-1 through 5.03-4 provide a schematic diagram of each of the sludge
stabilization and disposal/reuse alternatives. Major design criteria for each
alternative are listed in Tables 5.03-1 through 5.03-4.
1) Anaerobic Digestion -Liquid Agricultural Reuse
With this alternative, the incinerators would be removed from service, and
thickened primary and waste activated sludge would be pumped to two mixed
and heated primary anaerobic digestion tanks, where the organic matter
would be stabilized and digester gas would be produced. The digester gas
would be utilized as a fuel source to heat the process. A secondary
digester would be provided for sludge storage. The digesters would have
floating covers, and the secondary digester cover would be of the gas
holding type to provide gas storage.
Following digestion, the stabilized sludge would be thickened to 6 to 8%
total solids on gravity belt thickeners, and would be hauled to off -site
lagoons for storage. Sludge holding tanks (existing) at the plant would
be utilized for temporary storage. It is anticipated that the off -site
location could be up to 20 miles distant from the plant, in an area having
soils suitable for land application of sludges (adequate depth to bedrock
and water table, gentle slopes, remote from development, etc.). 180 days
of off -site storage would be provided, consistent with current sludge
management guidelines, to provide for periods of the year (e.g. winter and
growing season) where it is not possible to apply sludge.
Implementation of this alternative would require identification, testing,
and DNR approval of suitable application sites, as well as on -going
monitoring and recordkeeping relative to the sludge reuse program. Long-
term agreements with the farmers for use of their lands would be desirable.
It is anticipated that the City would purchase the lagoon site.
2) Anaerobic Digestion-Dewatered Agricultural Reuse
This alternative would be similar to the previous alternative, except that
the sludge would be dewatered at the plant to about 25% solids prior to
hauling to off -site storage, where 180 days of storage would be provided.
Dewatering would reduce hauling and application costs because of the
reduced volume, but capital costs would be higher than for the liquid
handling option.
1.1,4 INV
101
311
MI II
f
vi
1414 1/1,11
vilt I IZ,P
441 4131 0111)
OW, tiWtr kit FP HMO, F411t4
FIGURE 5.03-1 ANAEROBIC DIGESTION - LIQUID AG REUSE
PRIMARY
OP -
PRIMARY DIG.
THICKENING
SECONDARY
-01" "S-E-CO-ND-AR-Y
DIG.
THICKENING
0 oo'
TRANSPORT
cc
STORAGE
HEM APPUCATION
FIGURE 5.03-2 ANAEROBIC DIGESTION - DRY AG REUSE
PRIMARY
PRIMARY DIG.
THICKENING
SECONDARY
-11111." 'SECONDARY
DIG.
DE WATERING
TRANSPORT
C)
ier0,4_, STORAGE
Lo
FIELD APPLICATION
1
0-1
FIGURE 5.0 3 -3
PRIMARY
STORAGE
THICKENING
t
SECONDARY
7
DRY LIME STABILIZATION - AG REUSE
LIME
;--0°'
DEWATERING
I.
UME BLENDING
TRANSPORT
0 0
lir
Acir- STORAGE
FIELD APPllCATION
1 �
1
/_
FIGURE 5.0 3 -4 INCINERATION
DEWATERING
YARD WASTE
- PREHEATED
AIR SUPPLY
TABLE 5.03-1
PRELIMINARY DESIGN CRITERIA
ANAEROBIC DIGESTION - LIQUID AGRICULTURAL REUSE
DUBUQUE, IOWA WWTP
Unit Process or System Design Criteria of Loading
Primary Anaerobic Digesters:
(no.) and size
10% Reserve (2) units, 75 ft dia, 27 ft depth
25% Reserve (2) units, 80 ft dia, 27 ft depth
Covers floating, scum submergence type
Type mixed and heated to 95 deg F
Organic Loading 80 lb Volatile Solids/d/1000 cu ft
Secondary Anaerobic Digester:
(no.) and size
10% Reserve (1) unit, 75 ft dia, 27 ft depth
25% Reserve (1) unit, 80 ft dia, 27 ft depth
Cover floating, gas holding
Sludge Thickening (2) 1 1/2 meter gravity belt units
Sludge Hauling (2) 5,500 gal tank trailer vehicles
Off -Site Storage 180 day capacity in lagoons
Sludge Application (2) 2,000 gal injector vehicles
(1) 12,000 gal trailerable nurse tank
TABLE 5.03-2
PRELIMINARY DESIGN CRITERIA
ANAEROBIC DIGESTION - DEWATERED AGRICULTURAL REUSE
DUBUQUE, IOWA WWTP
Unit Process or System Design Criteria of Loading
Primary Anaerobic Digesters:
(no.) and size
10% Reserve
25% Reserve
Covers
(2) units, 75 ft dia, 27 ft depth
(2) units, 80 ft dia, 27 ft depth
floating, scum submergence type
Type mixed and heated to 95 deg F
Organic Loading 80 lb Volatile Solids/d/1000
Secondary Anaerobic Digester:
(no.) and size
10% Reserve
25% Reserve
Cover
Sludge Dewatering
Sludge Hauling
Off -Site Storage
Sludge Application
cu
ft
(1) unit, 75 ft dia, 27 ft depth
(1) unit, 80 ft dia, 27 ft depth
floating, gas holding
(2) 2 meter belt press units
(2) 30 cu yd semi -trailer vehicles
180 day capacity in covered bunkers
(1) 1.5 cu yd endloader
(2) dry spreader vehicles
TABLE 5.03-3
PRELIMINARY DESIGN CRITERIA
DEWATERED LIME STABILIZATION - AGRICULTURAL REUSE
DUBUQUE, IOWA WWTP
Unit Process or System
Sludge Dewatering
Lime Equipment
Lime/Sludge Blending
Sludge Hauling
Off -Site Storage
Sludge Application
IIIP
Design Criteria of Loading
(3) 2 meter belt press units
(2) quicklime storage silo and feed systems
(2) blending pugmills
(2) 30 cu yd semi -trailer vehicles
180 day capacity in covered bunkers
(1) 1.5 cu yd endloader
(2) dry spreader vehicles
Unit Process or System
Sludge Dewatering:
Type
(no.) and size
10% Reserve
25% Reserve
50% Reserve
Energy Recovery
Note:
TABLE 5.03-4
PRELIMINARY DESIGN CRITERIA
INCINERATION
DUBUQUE, IOWA WWTP
Design Criteria of Loading
Centrifuge
100 gpm firm capacity
100 gpm firm capacity
100 gpm firm capacity
recuperator to preheat fluidizing air to 500 deg F
12,000 scfm exhaust flow @ 1,600 deg F max
6,000 scfm heated air flow
Incinerator fuel requirements control firm capacity
needs for the dewatering equipment.
5-12
3) Dewatering and Lime Stabilization -Agricultural Reuse
With this alternative, the incinerators would be removed from service, and
lime addition would be employed to stabilize the lime prior to agricultural
reuse. Lime addition would significantly raise the pH of the sludge (>12),
and the temperature as well. The combination of high pH and temperature
would destroy pathogenic microorganisms, and a stable product would be
generated due to the elevated pH which would inhibit decomposition. About
1/2 pound of quicklime would be added for each pound of dry sludge solids.
Following lime stabilization, the sludge would be hauled to off -site bunker
storage and would be applied to agricultural lands in a manner similar to
the previous alternative.
An advantage of this alternative is its relatively low initial capital
cost. Disadvantages are the relatively large volume of sludge generated,
and high annual 0&M costs.
4) Incineration
This option would continue use of the present fluidized bed incinerators.
A recuperator would be installed to extract heat from the incinerator stack
gases, for preheating the air supply to the fluidized bed. This would
reduce costs for supplemental fuel. New sludge dewatering centrifuges and
material handling conveyors would be provided. Other miscellaneous
improvements would include improvements to the pollution control equipment
to reduce liquid carry-over. The present vacuum filters would be removed
from service, and the space renovated for installation of the new
dewatering equipment. It has been assumed that, in future, incinerator ash
would be landfilled.
One potential benefit of continued use of the incinerators would be the
potential for incinerating lawn and garden wastes. Properly prepared, and
free of glass and metals, brush, lawn clippings, etc., could be co -
incinerated with the treatment plant sludge. In addition to providing a
means of disposal of these wastes, co -disposal could reduce costs
associated with supplemental fuel for the incinerators.
B. Capital Costs
Estimated capital costs of the four alternatives are included in Tables 5.03-5
through 5.03-8. Costs were estimated on the basis of unit prices for similar
completed projects. Equipment manufacturer's were contacted for pricing for
major pieces of equipment as an aid in developing the cost estimates. Included
is a 30% factor for contingencies, and for technical services. All costs are on
a third quarter 1990 basis.
C. Operation, Maintenance and Replacement Costs
Annual costs for operation and maintenance were estimated on the basis of current
costs, with adjustments made as appropriate for each alternative. Estimates were
made for labor, supplies and chemicals, energy, and miscellaneous maintenance
ITEM
TABLE 5.03-5
DIGESTION - LIQUID AG REUSE
ESTIMATED CAPITAL COSTS
10% 25%
RES. RES.
Digester Structures $1,505,000 $1,640,000
Digester Covers $760,000 $840,000
Gas Mixing/Safety Equip. $315,000 $355,000
Digester Heating Equip. $310,000 $355,000
Digester Transfer Pumps $35,000 $35,000
Building Modifications/Removals $150,000 $150,000
Gravity Belt Thickeners $408,000 $408,000
Polymer Equipment $100,000 $100,000
Odor Control Chem. Feed Equip. $150,000 $150,000
Thickened Sludge Pumps $95,000 $95,000
Piping/Mechanical $1,150,000 $1,200,000
Electrical $775,000 $825,000
Off -Site Storage Lagoons $300,000 $315,000
Vehicle Barn and Service Bldg. $150,000 $150,000
Transport Vehicles $225,000 $225,000
Injection Equipment $300,000 $300,000
Land $200,000 $200,000
Site Work $250,000 $275,000
Subtotal
Contingencies, Tech. Services
At 30%
Total
$7,178,000 $7,618,000
$2,153,000
$9,331,000
Note: All costs are third quarter 1990 dollars.
$2,285,000
$9,903,000
ITEM
TABLE 5.03-6
DIGESTION - DRY AG REUSE
ESTIMATED CAPITAL COSTS
10% 25%
RES. RES.
Digester Structures $1,505,000 $1,640,000
Digester Covers $760,000 $840,000
Gas Mixing/Safety Equip. $315,000 $355,000
Digester Heating Equip. $310,000 $355,000
Digester Transfer Pumps $35,000 $35,000
Building Modifications/Removals $150,000 $150,000
Belt Presses $736,000 $1,105,000
Polymer Equipment $150,000 $150,000
Odor Control Chem. Feed Equip. $150,000 $150,000
Conveyor Equipment $75,000 $75,000
Piping/Mechanical $1,250,000 $1,325,000
Electrical $800,000 $850,000
Off -Site Sludge Cake Storage $695,000 $760,000
Vehicle Barn and Service Bldg. $150,000 $150,000
Transport Vehicles $220,000 $220,000
Sludge Application Equipment $370,000 $370,000
Land
$200,000 $200,000
Site Work $300,000 $325,000
Subtotal
Contingencies, Tech. Services
At 30%
Total
$8,171,000 $9,055,000
$2,451,000
$10,622,000
Note: All costs are third quarter 1990 dollars.
$2,716,000
$11,771,000
ITEM
TABLE 5.03-7
LIME STABILIZATION - DRY AG REUSE
ESTIMATED CAPITAL COSTS
10% 25%
RES. RES.
Sludge Transfer Pumps $35,000
$35,000
Belt Presses $1,105,000 $1,105,000
Material Handling Conveyors $100,000 $100,000
Building Modifications/Removals $150,000 $150,000
Odor Control Chem Feed Equipment $175,000 $175,000
Dry Lime Feed and Storage Equip. $125,000 $150,000
Lime Pugmills $80,000 $80,000
On -Site Sludge Storage $50,000 $50,000
Piping/Mechanical $450,000 $475,000
Electrical $400,000 $425,000
Off -Site Sludge Cake Storage $765,000 $835,000
Vehicle Barn and Service Bldg. $150,000 $150,000
Transport Vehicles $220,000 $220,000
Sludge Application Equipment $370,000 $370,000
Land $200,000
Site Work $350,000 $200,000
$375,000
Subtotal
Contingencies, Tech. Services
At 30%
Total
$4,725,000 $4,895,000
$1,418,000
$6,143,000
Note: All costs are third quarter 1990 dollars.
$1,468,000
$6,363,000
TABLE 5.03-8
INCINERATION
ESTIMATED CAPITAL COSTS
ITEM
Sludge Transfer Pumps
Centrifuges
Material Handling Conveyors
Building Modifications/Removals
Odor Control Chem Feed Equip.
Recuperator
Scrubber Improvements
Piping/Mechanical
Electrical
Subtotal
Contingencies, Tech. Services
At 30%
Total
Note:
10%
RES.
25%
RES.
50%
RES.
$135,000
$1,240,000
$125,000
$150,000
$75,000
$250,000
$200,000
$375,000
$275,000
$2,825,000
$135,000
$1,240,000
$125,000
$150,000
$75,000
$250,000
$200,000
$375,000
$275,000
$2,825,000
$135,000
$1,240,000
$125,000
$150,000
$75,000
$250,000
$200,000
$375,000
$275,000
$2,825,000
$848,000 $848,000 $848,000
$3,673,000 $3,673,000 $3,673,000
All costs are third quarter 1990 dollars.
Incinerator feed requirements control
dewatering equipment sizing.
5-13
with each alternative. Timing and costs associated with future replacement of
major items of equipment were also made. Estimated costs for operation,
maintenance and replacement are presented in Tables 5.03-9 through 5.03-12. All
costs are on a third quarter 1990 basis.
D. Present Worth Costs
The monetary cost of the alternatives was compared using the same methodology as
was used for comparing biological wastewater treatment alternatives (see Section
5.01 D). Table 5.03-13 presents the results of present worth and equivalent
annual cost analyses for each alternative. Calculations were made for two
discount rates (5% and 8.875%). The 8.875% rate is the rate currently used by
the U.S. EPA for Water Programs. The 5% rate may be more representative of
current economic conditions and has also been widely used.
Using the Incineration alternative as a basis, the monetary costs of the
alternatives would compare as follows:
Alternative
5% Discount Rate
Digestion -Liquid Ag
Digestion -Dry Ag
Dry Lime Stab-Ag
Incineration
8.875% Discount Rate
Digestion -Liquid Ag
Digestion -Dry Ag
Dry Lime Stab-Ag
Incineration
Fraction of Incineration Monetary Cost
10% Reserve 25% Reserve
1.03
1.11
1.29
1.00
1.19
1.30
1.33
1.00
1.08
1.19
1.33
1.00
1.24
1.40
1.37
1.00
The 50% reserve capacity incineration alternative would involve monetary costs
of about 3 to 4% higher than the 10% incineration alternative.
The incineration alternative is the lowest in monetary cost on the basis of this
analysis. Although the digestion alternatives would offer a very significant
annual cost savings over incineration, the savings would not justify the
relatively high capital investment required.
E. Non -Monetary Factors
In addition to monetary costs, other factors are of importance to the selection
of alternatives, including: expansion potential, reliability, ease of operation,
and other factors.
1) Expansion Potential
The digestion and incineration alternatives are viewed to be the most
expandable. The incinerators have significant capacity beyond that
required for year 2013 loadings, and the digester facilities would be
TABLE 5.03-9
DIGESTION - LIQ AG REUSE
ESTIMATED O,M & R COSTS
OPERATION AND MAINTENANCE
10% 25%
RES. RES.
Initial Year:
Super./Admin.
O&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
Design Year (2013):
Super./Admin.
0&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
EQUIP. REPLACEMENT
$20,000 $20,000
$125,000 $125,000
$90,000 $90,000
$80,000 $80,000
$60,000 $60,000
$375,000 $375,000
$20,000 $20,000
$155,000 $170,000
$130,000 $150,000
$95,000 $110,000
$70,000 $80,000
$470,000 $530,000
10% 25%
RES. RES.
Yr 1998:
Yr 2003:
Yr 2008:
Vehicles
Vehicles
Misc Equip
Vehicles
Misc Equip
$110,000 $110,000
$525,000 $525,000
$200,000 $200,000
$55,000 $55,000
$200,000 $200,000
Note: All costs are third quarter 1990 dollars.
TABLE 5.03-10
DIGESTION - DRY AG REUSE
ESTIMATED 0,M & R COSTS
OPERATION AND MAINTENANCE
10% 25%
RES. RES.
Initial Year:
Super./Admin.
O&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
Design Year (2013):
Super./Admin.
O&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
EQUIP. REPLACEMENT
$20,000 $20,000
$110,000 $110,000
$100,000 $100,000
$65,000 $65,000
$70,000 $70,000
$365,000 $365,000
$20,000 $20,000
$125,000 $140,000
$140,000 $160,000
$70,000 $80,000
$80,000 $90,000
$435,000 $490,000
10% 25%
RES. RES.
Yr 1998:
Vehicles
Yr 2003:
Vehicles
Misc Equip
Yr 2008:
$110,000 $110,000
$590,000 $590,000
$300,000 $300,000
Vehicles $55,000 $55,000
Misc Equip $300,000 $300,000
Note: All costs are third quarter 1990 dollars.
TABLE 5.03-11
LIME STABILIZATION - DRY AG REUSE
ESTIMATED 0,M & R COSTS
OPERATION AND MAINTENANCE
10%
RES.
25%
RES.
Initial Year:
Super./Admin.
O&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
Design Year (2013):
Super./Admin.
O&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Totals
EQUIP. REPLACEMENT
$20,000
$175,000
$410,000
$60,000
$100,000
$765,000
$20,000
$240,000
$645,000
$80,000
$120,000
$1,105,000
10%
RES.
$20,000
$175,000
$410,000
$60,000
$100,000
$765,000
$20,000
$255,000
$735,000
$95,000
$130,000
$1,235,000
25%
RES.
Yr 1998:
Yr 2003:
Yr 2008:
Vehicles
Vehicles
Misc Equip
$110,000 $110,000
$590,000 $590,000
$300,000 $300,000
$55,000 $55,000
$300,000 $300,000
Note: All costs are third quarter 1990 dollars.
Vehicles
Misc Equip
OPERATION AND MAINTENANCE
TABLE 5.03-12
INCINERATION
ESTIMATED O,M & R COSTS
10%
RES.
25%
RES.
50%
RES.
Initial Year:
Super./Admin.
O&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Ash Disposal
Totals
Design Year (2013):
Super./Admin.
O&M Labor
Supplies/Chemicals
Energy
Misc. Repair
Ash Disposal
Totals
EQUIP. REPLACEMENT
Yr 1998:
Incinerator
Yr 2003:
Incinerator
Misc Equip
Yr 2008:
Incinerator
Misc Equip
Major Maint
Major Maint
Major Maint
Note: All costs are third
$20,000
$180,000
$125,000
$200,000
$115,000
$30,000
$670,000
$20,000
$190,000
$135,000
$210,000
$125,000
$45,000
$725,000
10%
RES.
$20,000
$180,000
$125,000
$200,000
$115,000
$30,000
$670,000
$20,000
$200,000
$140,000
$230,000
$130,000
$50,000
$770,000
25%
RES.
$20,000
$180,000
$125,000
$200,000
$115,000
$30,000
$670,000
$20,000
$220,000
$150,000
$240,000
$140,000
$60,000
$830,000
50%
RES.
$250,000 $250,000 $250,000
$1,150,000 $1,150,000 $1,150,000
$200,000 $200,000 $200,000
$250,000
$300,000
$250,000
$300,000
quarter 1990 dollars.
$250,000
$300,000
ITEM
TABLE 5.03-13
PRESENT WORTH COST ESTIMATES
SLUDGE HANDLING ALTERNATIVES
DIG-LIQ AG D►G-LIQ AG DIG -DRY AG DIG DRY AG LIME -DRY AG LIME -DRY AG INCIN
10% RES 25% RES 10% RES 25% RES 10% RES 25% RES 10% RES
INCIN
25% RES
INCIN
50% RES
Initial Capital Cost
Yr 1998 Replacements
Yr 2003 Replacements
Yr 2008 Replacements
Est. Yr 2013 Salvage Val
Present Val (5%, 20yr)
Present Val (8.875%, 20yr)
Initial O&M/yr
Yr 2013 0&M/yr
Present Val (5%, 20yr)
Present Val (8.875%, 20yr)
Total Present Val:
5%, 20yr
8.8757, 20yr
Equiv. Annual Cost:
5%, 20yr
8.875%, 20yr
$9,331,000
$110,000
$725,000
$255,000
$2,868,000
$8,904,015
$9,260,289
$375,000
$470,000
$5,141,149
$3,751,440
$9,903,000 $10,622,000 $11,771,000
$110,000 $110,000 $110,000
$725,000 $890,000 $890,000
$255,000 $355,000 $355,000
$3,048,000 $3,222,000 $3,452,000
$9,408,175 $10,210,994 $11,273,309
$9,799,426 $10,585,091 $11,692,099
$375,000
$530,000
$5,436,614
$3,939,349
$365,000
$435,000
$4,893,416
$3,581,040
$6,143,000
$110,000
$890,000
$355,000
$1,505,000
$6,379,113
$6,419,569
$6,363,000 $3,673,000 $3,673,000 $3,673,000
$110,000 $250,000 S250,000 $250,000
$890,000 $1,350,000 $1,350,000 $1,350,000
$355,000 $550,000 $550,000 $550,000
$1,583,000
$6,569,716 $4,962,224 $4,962,224 $4,962,224
$6,625,328 $4,566,871 $4,566,871 $4,566,871
$365,000 $765,000 $765,000
$490,000 $1,105,000 $1,235,000
$5,164,259 $11,207,894 $11,848,069
$3,753,290 $8,110,810 $8,517,946
$670,000
$725,000
$8,620,524
$6,343,248
$670,000
$770,000
$8,842,123
$6,484,180
$670,000
$830,000
$9,137,588
$6,672,089
$14,045,164 $14,844,789 $15,104,410 $16,437,569 $17,587,007 $18,417,784 $13,582,748 $13,804,347 $14,099,812
$13,011,729 $13,738,775 $14,166,131 $15,445,389 $14,530,378 $15,143,274 $10,910,119 $11,051,051 $11,238,960
$1,127,020 $1,191,184 $1,212,017 $1,318,993 $1,411,227 $1,477,891 $1,089,915 $1,107,697 $1,131,405
$1,412,715 $1,491,652 $1,538,051 $1,676,943 $1,577,598 $1,644,142 $1,184,538 $1,199,839 $1,220,241
Note: All costs are third quarter 1990 dollars.
5-14
constructed to facilitate future expansion. Annual operation and
maintenance costs with the lime alternative would limit the practicality
of a significant expansion, due to the high cost of the lime and the
relatively large volume of sludge requiring disposal.
2) Reliability
Reliability factors with the alternatives include the reliability of sludge
dewatering, sludge stabilization, and ultimate sludge reuse or disposal.
With respect to treatment technologies, digestion and incineration are
viewed as being the most reliable. Anaerobic digestion is a common
process, employed with success at many facilities. Duplicate equipment
would be provided for all key components of equipment. Sludge
dewaterability is a major variable, but it is common to all alternatives.
Incineration is a complex process, but has proven to be quite successful
at Dubuque. The incineration process provides standby for key system
components.
Lime stabilization is viewed as being less reliable than the other
alternatives. Lime systems involve storage and feed equipmentthat are
somewhat difficult to operate due to the nature of the lime which causes
a high wear factor for equipment, as well as problems with dust and
housekeeping. With lime treatment, the proper dose for pathogen kill and
long-term sludge stability would be a key process parameter and would be
difficult to control at an economical lime dose.
With respect to reuse or disposal, all of the alternatives are somewhat
problematic owing to the developing nature of the sludge regulations. The
incineration alternative is viewed as being the most reliable in this
regard, with the digestion-dewatered sludge reuse option also being favored
because of the relatively small volume of sludge generated. Agricultural
reuse would be dependent upon regulations, public acceptance, and the
development of an effective program that could be matched to the needs of
the agricultural community. Topography and land features in the Dubuque
area would make implementation somewhat difficult.
3) Ease of Operation
The digestion and incineration alternatives are judged to be similar with
respect to ease of operation. Although the incineration process is more
complex than the relatively simple processes employed with the digestion
alternatives, sludge disposal/reuse with the digestion alternatives would
be somewhat difficult to implement. The lime stabilization alternative is
judged more difficult to operate because of the difficult lime process, and
due to the relatively large volume of sludge generated.
5-15
4) Other factors
The agricultural reuse alternatives would return valuable nutrients and
organic material to the land. These alternatives are therefore favored
from the standpoint of resource recovery and reuse.
The incineration alternative would make the best use of existing facilities
the still have significant useful life remaining. This alternative would
also offer the advantage of allowing for potential co -disposal of shredded
brush, lawn clippings and other similar municipal solid waste.
On an overall basis, the alternatives are considered to be neutral with
respect to these factors.
5.04 DISCUSSION AND SELECTION OF BEST OVERALL APPROACH
Alternatives for biological wastewater treatment were evaluated in Section 5.01,
alternatives for sludge thickening in Section 5.02, and alternatives for sludge
stabilization and disposal/reuse in Section 5.03. The merits of the alternatives
evaluated are summarized in this section, and recommendations are made as to the
best approach.
A. Biological Wastewater Treatment
Monetary costs favor the BT-Activated Sludge alternative, if plant loadings
remain below the 10% reserve level; or if plant loadings remain below the 25%
reserve level and long-term discount rates are closer to 5% than 8.875%, or if
energy costs escalate at a more rapid rate than the general inflation rate. It
would appear likely that one or more of the above factors will be satisfied in
the long run. For example:
1) BO05 loadings are the most critical parameter controlling plant
performance. At 10% reserve, plant loadings would be about 225% of
current levels. At 25% reserve, plant loadings would be about 260%
of current loadings. Plant BOD5 loadings would have to increase by
a factor of about 2.5 for the BT-Activated Sludge Alternative to be
clearly less economical than the present 02-Activated Sludge System.
It is likely that any future industrial user of the facility would
provide pretreatment, rather than discharge a strong waste to the
facility, due to business economic factors. This is almost
invariably the trend across the country, particularly in communities
where an equitable system of user charges is in force which charges
all customers in proportion to the cost of providing sewer service.
2) As discussed earlier, the current EPA discount rate of 8.875% may not
be representative of current economic conditions, and a likely long-
term view is that discount rates will fall.
3) Given the world energy situation, it can be expected that there will
be continuing inflationary pressures tending to increase costs of all
forms of energy. It is likely that energy costs will increase at a
greater rate in the future than other goods and services. Assuming
5-16
energy prices increase 3% per year faster than other costs, the rate
of return for the BT-Activated Sludge Alternative would be about
8.3%.
With staging of construction of the BT-AS alternative (build one tower initially,
the second in ten years -depending upon actual loadings experienced), the
economics of the BT-AS alternative appear favorable using either discount rate,
so long as loadings remain below the 25% reserve level.
The above assumes that the "passive" approach (see Section 5.01 A.3.) to odor
control is successful, and that chemical odor control of the BT off -gases is not
required. If chemical oxidation of the BT off -gases would prove necessary,
capital and operating costs may prove to be substantial for covering the units,
providing odor control chemical scrubber units, and for chemical, labor, power
and other 0&M costs. Potentially, this could reduce or eliminate any savings for
the BT-Activated Sludge alternative, compared with the 02-Activated Sludge
alternative.
The BT-Activated Sludge Alternative has good expansion potential, is reliable and
easily operated. Maintaining the oxygen plant in a standby mode would reduce any
risk in selecting this alternative, that there would be inadequate future
treatment capacity. In fact, with the oxygen plant on standby and with
construction of the biotowers, the plant would be able to accept a very
substantial loading, well above even the 50% reserve level, by returning the
oxygen plant to service.
Table 5.04-1 provides an overall comparison of the factors considered in
evaluating the biological wastewater treatment alternatives. Based on the cost
data and assumptions presented, the BT-Activated Sludge Alternative appears to
be the most favorable on an overall basis, and it also appears that it may be the
most advantageous to stage the construction of the system. It is recommended
that cost projections be finalized, and that a determination as to the potential
need for BT off -gas chemical odor scrubbing be made, following completion of the
BT pilot study currently underway.
B. Sludge Thickening
It was concluded in Section 5.02 that the best approach to handling primary
sludges would be to install air operated diaphragm pumps, that could operate
automatically and significantly reduce labor costs. The "payback" for
installation of this equipment would be about 3.6 years.
It was also concluded in Section 5.02 that it would not be economical to install
gravity belt thickening equipment for waste biological sludge processing, and
that the City should continue to rely on the new centrifuges which were recently
installed and are performing well.
It has also been concluded that the present Zimpro process for conditioning the
waste biological sludge should be removed from service. This will save
significant operating dollars, as well as reduce loadings to the wastewater
treatment units due to the high strength supernatant stream from Zimpro.
TABLE 5.04-1
OVERALL COMPARISON
BIOLOGICAL WASTEWATER TREATMENT ALTERNATIVES
DUBUQUE, IOWA WWTP
Alternative
Factor TF-AS BT-AS Staged O2-AS
BT-AS
Monetary Cost + ++ + to +/-
Expansion Capability +/- + + +
Reliability + + +/- +/-
Ease of Operation + + + +/-
Other Factors:
Sludge Production + + + +/-
Effluent Quality + + + +
Odor Potential
Note:
++ Means most favorable
+ Means favorable
+/- Means neutral
- Means unfavorable
Means most unfavorable
+
Monetary cost does not include the potential need for chemical
scrubbing of TF and BT off -gases for odor control.
5-17
C. Sludge Stabilization and Disposal/Reuse
Monetary costs favor continuing to utilize the incineration process at the
present time. New sludge dewatering equipment would be provided for improved
performance and reliability, and the incinerators would be upgraded to reduce
their energy consumption.
Non -monetary factors also tend to favor the incineration process at the present
time. The process had significant reserve capacity beyond that which can be
anticipated to be required. The process had demonstrated its reliability, and
key components of the system have duplicate units. At the present time there is
one belt press available for sludge dewatering, and this piece of equipment is
nearing the end of its useful life. Installation of new dewatering equipment
will provide reliable sludge dewatering.
Table 5.04-2 provides an overall comparison of the sludge stabilization and
disposal/reuse alternatives. Continued use of incineration has the most
favorable overall ranking at this time. In the future, at the time that the
incinerators are no longer serviceable or should energy costs escalate rapidly,
anaerobic digestion and land application should be again evaluated.
D. Reserve Capacity
In continuing to evaluate the BT-Activated Sludge alternative, it is recommended
that the 10% reserve capacity level be used. This level would allow for more
than twice the current B005 loading level to the plant, and is the level where
the most potential savings could be realized. Should greater reserve capacity
be required in the future, it could be readily provided up to the 25% level by
increasing the capacity of the aerators in the activated sludge basins. Further
increases in capacity could be provided by the construction of additional
parallel units or, ultimately, by returning the oxygen plant to service.
For incineration, selection of a design reserve capacity level is not required
due to the existing reserve capacity of the system. With conservative selection
of the sludge dewatering equipment, the solids processing area of the plant
should not be the critical element of overall plant capacity.
TABLE 5.04-2
OVERALL COMPARISON
SLUDGE STABILIZATION AND DISPOSAL/REUSE ALTERNATIVES
DUBUQUE, IOWA WWTP
Factor Alternative
Dig-Lig Ag
Dig -Dry Aq Lime-Aq
Incineration
Monetary Cost +/- +
Expansion
Capability +/- +/- +/-
Reliability +/- +/- +/-
Ease of Operation +/- +/- +/-
Resource Utilization + + +
Other Munic. Waste -
-
Note:
+ Means favorable
+/- Means neutral
Means unfavorable
6-1
SECTION 6
OTHER PLANT NEEDS
Section 5 reviewed plant needs for biological wastewater treatment and for sludge
management. This section considers other plant needs, including odor control,
influent screening and grit removal, primary clarification, and disinfection.
6.01 ODOR CONTROL
Odors at the plant may give rise to citizen complaints and components of plant
odor may also cause premature deterioration of plant structures and equipment.
Causes of odor generation and potential means of odor control are addressed in
this section.
A. Hydrogen Sulfide Formation
The Dubuque wastewater treatment plant has had odor problems in the past which
appear to be associated with elevated concentrations of hydrogen sulfide (H2S).
Corrosion of aluminum gates and duct work has been noted at the plant headworks.
The concrete covers on the grit basins are corroded, apparently also due to the
sulfide. The plant was originally equipped with odor control systems including
covers for the grit and primary sedimentation basins and the trickling filters.
The original design allowed for ventilation air from under the covers to be
treated using scrubbers. The scrubber system was not totally effective. Nearby
residents complained of strong odors from the wastewater treatment plant with the
trickling filters in service. These complaints, combined with lower loadings to
the plant, have led to the bypassing of the trickling filters. Although it is
believed that anaerobic conditions in the trickling filters, due to the
inadequate design of the underdrains, was the major cause of these odors,
hydrogen sulfide in the wastewater influent was also a probable factor. Other
odor problems at the plant are associated with the Zimpro process, which is
planned to be removed from service.
Hydrogen sulfide may reach the plant influent by either being present in the
wastewater originally, or through conversion of organic sulfur, sulfates, or
sulfites to hydrogen sulfide by anaerobic bacteria. Conversion of more oxidized
forms of sulfur to hydrogen sulfide is common in long force mains due to the low
oxygen conditions. At neutral pH, dissolved hydrogen sulfide (H2S) is in
equilibrium with the hydrosulfide ion, HS., at about equal concentrations
("Sulfide in Wastewater Collection and Treatment Systems", ASCE Manuals and
Reports on Engineering Practice No. 69, 1989). Hydrogen sulfide gas is primarily
in the dissolved form in a force main flowing full, since there is not enough
head space above the sewage for H2S gas to be released.
Once hydrogen sulfide reaches the influent of a wastewater treatment plant,
hydrogen sulfide gas is given off. The amount of H2S gas given off from the
solution depends on the pH and temperature of the wastewater, the solubility and
partial pressure of H2S and the degree of agitation provided to the wastewater.
At 20 degrees celsius, a 0.5 mg/L solution of H2S could result in a concentration
of approximately 100 to 150 ppm H2S in the air ("Odor and Corrosion Control in
Sanitary Sewerage Systems and Treatment Plants", Robert P.G. Bowker, et. al.,
EPA/625/1-85/018, 1985). Hydrogen sulfide is noticeable as a strong odor at air
concentrations between 0.5 to 30 ppm, and can cause eye and respiratory injury
at concentrations between 50 and 300 ppm (ASCE, 1989).
Hydrogen sulfide data collected at the Dubuque sewage lift stations in August and
September, 1984 are presented in Table 6.01-1. This table indicates that
concentrations at the pump stations average between 1.10 and 1.79 mg/L. Since
the 1984 H2S data were collected, FDL foods constructed an anaerobic lagoon for
wastewater pretreatment. FDL foods strips hydrogen sulfide from their lagoon
effluent prior to discharging it to the sanitary sewer at the Cedar Street force
main. However, it is possible that not all of the sulfide in the lagoon effluent
is removed and the lagoon may contribute to the total H2S entering the treatment
plant. Limited data collected in the spring of 1990 indicate that the sulfide
levels at the Cedar Street lift station may be as low as 0.2 mg/L.
The amount of hydrogen sulfide formed in a force main due to conversion of other
sulfur compounds may be approximated by the following equation ("Generation and
Control of Sulfide in Filled Pipes", Richard Pomeroy, JWPCF, Vol. 31, No. 9,
1959):
CS = K x t x CEBOD((1+0.01 d)/d)
where: CS = increase in H2S concentration, mg/L
K = 0.0020 for retention times of 10 to 60 minutes
0.0026 for retention times of 1 to 5 hours
t = retention time in force main, minutes
CEBOD = effective BOD5, mg/L
d = pipe diameter, inches
The effective BOD5 is defined as the five day BOD as determined at 20 degrees C,
with the temperature correction factor applied:
CEBo = BOD5 x 1.07 x exp(T-20)
where T is the wastewater temperature, degrees celsius.
This equation was used along with average wastewater flow rates and BOD5
concentrations from 1989 plant data to estimate the H2S production in the force
mains feeding the plant. The wastewater temperature was assumed to be 20 degrees
C. The retention time in the Cedar Street force main upstream from the Terminal
lift station was estimated to be about 50 minutes, and the retention time in the
Cedar plus Terminal Street force main was estimated to be approximately 170
minutes at average flow rates. The retention time in the Catfish force main was
estimated to be 8 minutes at average flow rates. The H2S production using the
above equation is estimated to be 0.82 mg/L from the Cedar lift station to the
Terminal lift station, 2.65 mg/L from the Terminal lift station to the plant, and
about 0.2 mg/L in the Catfish force main. This gives a total of 2.7 mg/L
generated sulfide in the combined plant influent. Adding this amount to the
values given in Table 6.01-1 for the H2S concentrations at the pumping stations
would yield an approximate influent concentration of 4 mg/L at the wastewater
treatment plant.
TABLE 6.01-1
SULFIDE ANALYSES
DUBUQUE, IOWA WWTP
Date of Sampling Catfish Pumping Cedar Pumping Terminal Pumping _
Station Station Station
8-27-84 1.79
8-28-84 1.27
8-29-84 1.25
8-30-84 1.42
1.57
8-31-84 1.08
1.03
9-01-84 0.73
0.83
9-02-84 0.93
Average 1.19
3.66
0.09
0.24
1.37
1.18
1.32
1.47
0.68
0.68
0.34
1.10
3.15
1.32
1.18
1.27
1.27
2.75
2.75
1.22
1.22
No Sample
1.79
6-3
B. Odor Control Measures
Hydrogen sulfide may be removed from the wastewater by precipitation or oxidation
reactions. Chemical precipitation may be effectively accomplished by the
addition of metals such as iron or zinc. Zinc would be a concern with respect
to toxicity. Ferrous and ferric iron salts are typically added to wastewaters
and sludges for this purpose. The disadvantages of iron addition are it can be
costly and would result in higher metal concentrations in sludge and potential
scaling of the sewers. Biological oxidation of sulfide can also be quite
effective at reducing H2S concentrations. However, biological oxidation
typically occurs on tank walls, covers, and pipes, and the sulfuric acid that is
formed as a product corrodes concrete and metal surfaces.
Chemical oxidation is most typically used to treat odors in wastewater. Common
oxidizing agents include oxygen, chlorine, and hydrogen peroxide (H202), as
discussed below.
1. Oxidation with Oxygen
Chemical oxidation of H2S with oxygen can occur over a relatively long
period of time. Certain elements that may be present in wastewater such
as glycerol are known to inhibit the oxidation process (ASCE, 1989). The
oxygen may limit the production of hydrogen sulfide gas by either
inhibiting the anaerobic bacteria which form H2S or by oxidizing the H2S
to form elemental sulfur. Oxygen only oxidizes the dissolved hydrogen
sulfide, not the hydrosulfide ion or the gaseous H2S.
2. Oxidation with Chlorine
Sulfide in solution in the H2S form may be oxidized by chlorine through the
following reactions:
C12 + H2S = 2H+ + 2C1- + S°
4C12 + H2S + 4H20 = 10 H+ + 18C1- + SO4=
The former reaction is believed to be more predominant at higher pH
values. The latter reaction occurs at neutral pH. This latter reaction
indicates that about 8 mg/L Cl are required to oxidize 1 mg/L H?S. The
hydrosulfide ion is not oxidized by chlorine. Thiosulfate, sulfite, and
other oxidation products may be formed if mixing is not complete or not
enough chlorine is added (ASCE, 1989). Addition of large amounts of
chlorine to plant influents have given rise to concerns relative to
toxicity and wastewater treatability. Safe handling and feeding of
chlorine is also a concern.
3. Oxidation with Hydrogen Peroxide
Hydrogen Peroxide may be added to wastewater for H2S oxidation. The
reactions which are believed to occur around neutral pH are:
6-4
H2S + H202 = 2H20 + S°
HS- + H202 + H` = 2H20 + S°
These reactions indicate that approximately 1 mg/L H202 is required to
oxidize 1 mg/L H2S or HS. In practice, a dose of 2 to 8 mg/L H202 per mg/L
H2S or HS-is generally required. Hydrogen peroxide also has an advantage
in that it decomposes to oxygen and water. The oxygen left over after
oxidation of the sulfides is complete may inhibit the formation of
additional sulfide (ASCE, 1989).
The above discussion indicates that hydrogen peroxide added to the Terminal
Street force main may be the most feasible way to reduce H2S concentrations, and
associated odors, at the wastewater treatment plant. The addition of H202 could
be accomplished by installing metering pumps and hydrogen peroxide feed tanks at
the Terminal Street lift station. Experience has indicated that thorough mixing
and a detention time between 10 and 60 minutes maximizes the effect of H202
addition.
Estimated costs for the addition of a hydrogen peroxide odor control system to
the Cedar -Terminal force main are shown in Table 6.01-2. The costs shown assume
that additional building space would required for housing the pumps and controls.
Yearly chemical costs, assuming a 7 mg/L dose of 35% hydrogen peroxide, would be
approximately $225,000. The total capital cost for the system including pumps,
controls, and a storage tank would be approximately $177,000.
C. Recommendations
Section 6.03 recommends new grit removal equipment, for improved performance and
reduced odor generation. Specific design features are proposed in conjunction
with the biotower - activated sludge process to minimize odor generation. With
these improvements, the potential for nuisance odor generation at the plant will
be lessened.
At this time it is recommended that hydrogen peroxide feed equipment not be
installed at the Terminal Street Lift Station, until the completion of other
planned improvements. If plant odors continue to be a problem hydrogen peroxide
feed facilities at Terminal Street appear to be the most feasible solution, and
could be installed at that time.
6.02 SCREENING
The existing preliminary screening system consists of three bar screens 3-feet,
6-inches wide with 3/4-inch openings. Two of the screens are mechanically
cleaned while the third is manually cleaned. The bar screens were installed with
the original primary treatment plant in 1969. The bar screen rakes and the
screenings conveyor are approaching the end of their design life. Furthermore,
the bar screens are becoming obsolete, and obtaining replacement parts is
difficult. The current timer controls are inadequate during high flow events,
resulting in wastewater backups.
It is recommended that the two mechanical bar screens and the screenings conveyor
be replaced. New differential level controls would help reduce operating
TABLE 6.01-2
HYDROGEN PEROXIDE ADDITION COSTS
DUBUQUE, IOWA WWTP
Item Capital Cost
Building Modifications $ 100,000
Pumps, Piping and Safety Equipment 16,000
Storage Tank 6,000
Electrical 7,000
Heating and Ventilating 7,000
Subtotal $ 136,000
Engineering and Contingencies 41,000
Total Capital Costs $ 177,000
Yearly Chemical Costs* $ 225,000
*Average flow of 12.5 mgd; H202 dose of 7 mg/L; nine months per year of odor
control.
All costs are in third quarter, 1990 dollars.
6-5
problems during high flows. Corrosion resistant materials would be used for the
new equipment. Other improvements which are recommended for the bar screen
building include roof replacement; rehabilitation of the heating and ventilation
system including new duct work in the bar screen room; and interior painting.
The total estimated cost for the bar screen replacement and headworks
improvements would be $383,000 (Table 6.02-1).
6.03 GRIT REMOVAL
The existing grit removal system consists of two aerated grit basins and three
1,100 cfm centrifugal blowers. The grit removal system was installed in 1969.
The two basins have four flow passes each and are equipped with scrapers to
collect grit and skimmers which direct skimmings to tip troughs and then to scum
pits. The grit basins are covered with concrete roof planking. Deterioration
of the planking has been a recurring problem. In addition the chain and flight
grit removal equipment has involved high maintenance costs. Odors are generated
when elevated hydrogen sulfide is present, due to stripping of the sulfide as a
result of aeration.
The existing aerated grit system is inefficient, with a high volume sidestream
returned to the wastewater treatment plant. The tip trough on one of the basins
leaks badly, which results in a large volume of wastewater entering the scum
pits. The concrete covers have corroded due to high sulfide levels in the
wastewater. The collector equipment, air piping, diffusers and blowers are near
the end of their useful lives.
Replacement of the existing grit removal system with a conventional "vortex" grit
removal system is recommended. The vortex system would be more simple to operate
and maintain, since there are no blowers, diffusers or tankage associated with
it. The system would also be less of a concern than the present system with
respect to odor generation, and less costly to operate since the blowers would
not be needed.
The two new vortex channels would be built within the existing basins. The
existing grit dragouts would be removed. A new control structure and gates would
be installed to direct flow to either channel. The existing covers would be
removed and new hand rails placed around the channels. New covers would not be
required for odor control due to the relatively small wastewater surface area and
since aeration would not be provided.
The vortex grit system would also include two grit pumps and a grit washer. The
grit pumps would be located in a small tunnel or dry well located lower than the
vortex channels to provide flooded suction. The grit washer would be located
inside the bar screen building. Grit from the washer would be conveyed along
with screenings to a storage hopper or dump truck for eventual hauling to the
landfill.
The estimated capital costs for the grit removal system improvements are shown
in Table 6.03-1. The total capital cost would be approximately $579,000.
TABLE 6.02-1
BAR SCREEN REPLACEMENT COSTS
DUBUQUE, IOWA WWTP
Item Capital Cost
Mechanical Bar Screens and Controls $160,000
Screenings Conveyor 40,000
Removals 10,000
Building Roofing and Painting 30,000
Electrical 15,000
Heating and Ventilating 40,000
Subtotal $295,000
Engineering and Contingencies 88,000
Total Capital Cost $383,000
All costs are in third quarter, 1990 dollars
TABLE 6.03-1
GRIT REMOVAL COSTS
DUBUQUE, IOWA WWTP
Item Capital Cost
Vortex Grit Removal Equipment and Pumps $205,000
Grit Washers 55,000
Basin Structural Modifications 50,000
Control Structures and Gates 25,000
Removals 15,000
Pump Tunnel 50,000
Piping 15,000
Electrical 25,000
Heating and Ventilating 5,000
Subtotal $445,000
Engineering and Contingency 134,000
Total Capital Cost $579,000
All costs are in third quarter, 1990 dollars.
6-6
6.04 PRIMARY CLARIFIERS
There are currently three primary clarifiers at the Dubuque WWTP, constructed in
1969, with a total surface area of 19,100 sq ft. Sludge draw off is accomplished
by means of telescoping valves. Sludge flows to a wet well from which it is
pumped to holding tanks. The clarifiers are covered.
The Iowa Code recommends overflow rates for primary clarifiers of 1,500 gpd/sq
ft and 1,000 gpd/sq ft at peak hourly wet weather flow and average wet weather
flow, respectively. The actual overflow rates at design conditions with 10%
reserve capacity would be 1,690 gpd/sq ft and 891 gpd/sq ft at peak hourly and
average wet weather flows, respectively. Constructing a fourth clarifier would
reduce these peak and average loadings to 1,270 and 668 gpd/sq ft.
The primary clarifiers have functioned well in the past, even though peak flow
rates have approached an estimated 32 mgd. Peak daily wet weather flow rates are
typically no more than 28 mgd, which would result in an overflow rate of less
than 1,500 gpd/sq ft. During a storm event the wastewater would be relatively
dilute from the large quantity of I/I entering the sewers; because of this, a
lesser degree of clarifier performance could be tolerated during the few events
when design peak hourly flow conditions are attained. For the reasons discussed
above, and because the Iowa Code criteria are met at average wet weather
conditions, construction of a fourth clarifier is not considered to be justified.
Several modifications and improvements to the existing primary treatment
facilities are recommended. The pipe tunnel walls and roof require repairs to
prevent leakage. This includes replacement of the asphalt roof surface with a
new membrane roof, and improving storm water runoff and collection. The
clarifier dome covers need, repair and waterproofing. This would involve the
application of a latex -based coating to the domes to fill the small cracks which
are present throughout the concrete and provide water proofing for the concrete
on the domes. The covers on the primary sludge holding tanks also need
replacement, since the existing concrete covers show signs of corrosion which
extends to the reinforcing steel in some areas. The covers also show excessive
deflection possibly due to a softening effect on the concrete by hydrogen sulfide
in the sludge. It is also proposed that the roof on the sludge pumping station
be removed and replaced due to its age and condition. The costs for these
improvements are shown in Table 6.04-1. The total capital cost would be
approximately $254,000.
6.05 DISINFECTION
The existing disinfection system consists of two v-notch chlorinators, a chlorine
evaporator, a chlorine detector, a ton cylinder scale and hoist, and two chlorine
injection water pumps. Chlorine solution may be fed to the headworks, the
trickling filter influent, or the effluent chlorine flash mixing tank. Contact
time is provided in the mixing tank and effluent outfall pipe, which would
provide approximately 16 minutes of detention time at design average wet weather
flow and 8.5 minutes at peak hourly wet weather flow (assuming 10% reserve
capacity). The permit limit for fecal coliform is 200 organisms per 100 ml. It
is expected that the residual chlorine will be limited to about 0.3 mg/L when the
discharge permit is renewed in 1992 (Section 2).
TABLE 6.04-1
PRIMARY CLARIFIER IMPROVEMENTS
DUBUQUE, IOWA WWTP
Item Capital Cost
Repair and Improve Pipe Tunnel Roof;
Repair Leaks in Tunnel
Repair Clarifier Dome Covers
Repair Pump Station Roof
Repair Primary Sludge Holding Tank Covers
Improve Area Drainage
Subtotal
Engineering and Contingencies
Total Capital Cost
All costs are in third quarter, 1990 dollars.
$ 20,000
70,000
20,000
75,000
10,000
$195,000
59,000
$254,000
6-7
The chlorination equipment is currently functioning adequately and replacement
parts are available for the chlorinators. However, the equipment is over 20
years old and may require replacement within a few years. Improvements will also
be required for the chlorine storage area to bring it up to current code
requirements by sealing it off from the rest of the bar screen building and
adding new access doors and a new ton cylinder hoist. The chlorine feed room
door needs to be replaced in order to provide outward acting "panic" hardware.
Construction of a chlorine contact tank may also be required to provide a contact
time of 15 minutes at peak flow rates and 30 minutes at average flow rates. The
effluent pipe currently functions as a plug flow contact tank, and has provided
adequate contact time for the necessary fecal coliform kill thus far.
A dechlorination system may be required when the permit is reissued, in order to
meet the residual chlorine requirement of 0.3 mg/L. Dechlorination would
typically be accomplished by the addition of gaseous sulfur dioxide or liquid
sodium bisulfite. The gaseous system would be similar to the existing
chlorination system, and would require two sulfonators, a ton cylinder scale, a
hoist, injection water pumps, and separate rooms for chemical storage and feed.
The feed and storage rooms would require special ventilation and separate
doorways to the outside of the building, similar to a chlorination system.
The liquid dechlorination system would be more simple, requiring the installation
of two chemical feed pumps and one or two sodium bisulfate storage tanks.
Dilution water would be provided from the existing non -potable water system. The
ventilation requirements would not be as stringent as for sulfur dioxide gas, but
improved ventilation over normal building space ventilation would be desirable
due to the odor emitted from sodium bisulfite. Chemical storage tanks would be
vented to the outside. Safety equipment such as an emergency shower and eyewash
station would also be required. The sodium bisulfite system could be installed
in the existing blower room of the bar screen building, since the blowers would
no longer be needed for aerated grit removal (see Section 6.03).
Prior to the installation of a dechlorination system, it is recommended that a
study be conducted to determine whether the residual chlorine limit can be met
at effective coliform kill, by chlorine dosage control. Plant personnel have
noted that it is difficult to meet a 200/100 mL limit at the outfall with a low
mg/L chlorine residual. The residual which is achievable at the 200/100iL limit
is typically somewhere between 0.1 and 1.0 mg/L, according to plant personnel.
Therefore, it is possible that a 0.3 mg/L chlorine residual could be met without
dechlorination.
A study should also be conducted to address whether an adequate fecal coliform
kill can be achieved with the existing outfall sewer used as a contact tank at
peak flow rates. It may be necessary to use a higher dosage of chlorine followed
by dechlorination at peak conditions to achieve the required fecal coliform kill
using the outfall sewer for contact. The other option would be to construct a
chlorine contact tank to provide better contact and a better kill at peak flow
rates.
If chlorine contact tankage is required, two tanks would be provided with length
to width ratios of 40 to 1 and a retention time at PHWW flow of 15 minutes. The
tanks would have a total volume of about 0.36 million gallons and a surface area
of about 3,000 sq ft each. Gates would be provided to remove one tank at a time
from service for cleaning or repairs. The existing flash mixing tank would be
utilized as the chlorine addition point and as a control cham'er to split flow
to the two tanks. The new tanks could be constructed east of the existing final
clarifiers.
Approximate capital costs for improvements to the chlorination system, assuming
a contact tank is required, are estimated to be $728,000 (Table 6.05-1).
Estimated capital costs for dechlorination with liquid sodium bisulfite are shown
in Table 6.05-2. The dechlorination costs assume that the existing blower room
in the bar screen building can be modified to house dechlorination chemicals and
equipment; however, it is possible that a small enclosure located closer to the
chemical application point will be required. The total estimated capital cost
for dechlorination is $85,000, and the annual chemical costs would be
approximately $15,000.
6.06 SUMMARY OF COSTS
A summary of the capital costs for other recommended plant needs including
influent screening and grit removal, and primary clarification,, is provided in
Table 6.06-1. The total estimated cost is $1,216,000. Ccsts for hydrogen
peroxide odor control, chlorination, and dechlorination are lot included, as
these alternatives are not recommended for immediate imple-antation. These
latter alternatives will be further evaluated at the time c= the new permit
issuance.
TABLE 6.05-1
CHLORINATION SYSTEM IMPROVEMENTS
DUBUQUE, IOWA WWTP
Item Capital Cost
Building Upgrades $ 50,000
Chlorinators and Accessories 35,000
Injection Water Pumps 10,000
New Chlorine Contact Tanks 350,000
Piping Modifications 100,000
Electrical 10,000
Heating and Ventilating 5,000
Subtotal $ 560,000
Engineering and Contingencies 168,000
Total Capital Cost $ 728,000
All costs are in third quarter, 1990 dollars.
TABLE 6.05-2
DECHLORINATION WITH SODIUM BISULFITE
DUBUQUE, IOWA WWTP
Item Capital Cost
Building Modifications $ 20,000
Pumps, Piping and Safety Equipment 20,000
Storage Tank 5,000
Electrical 5,000
Heating and Ventilating 15.000
Subtotal $ 65,000
Engineering and Contingencies 20.000
Total Capital Cost $ 85,000
Yearly Chemical Costs* $ 15,000
*Average flow of 14 mgd; dose of 1.5 mg/L; six months per year disinfection.
All costs are in third quarter, 1990 dollars.
TABLE 6.06-1
SUMMARY OF COSTS FOR
OTHER RECOMMEND PLANT IMPROVEMENTS
DUBUQUE, IOWA WWTP
Item
Bar Screen Replacement and
Building Upgrades
Grit Removal
Primary Clarification
Notes:
Capital Cost
383,000
579,000
254,000
Total $1,216,000
1. All costs are in third quarter, 1990 dollars.
2. Costs include 30% for contingencies and technical services.
3. Costs for hydrogen peroxide odor control at the Terminal Street pumping
station, chlorination, and dechlorination are not included, as these are
not recommended for immediate implementation.
7-1
SECTION 7
TIMING OF IMPROVEMENTS
Sections 5 and 6 of this report have recommended improvements to the preliminary
treatment units, primary treatment units, secondary treatment process, sludge
processing system, and miscellaneous plant facilities. Certain of these
improvements should be implemented soon, because the present systems lack
reliability or to remedy a problem requiring priority attention. Other
improvements are not so urgent from a timing standpoint, and can be delayed until
required by the NPDES Permit, or as financial resources allow. The purpose of
this section is to discuss the timing of the recommended improvements, and to
recommend a course of action which would give the most critical and/or
advantageous improvements priority in the schedule of implementation.
As discussed in Section 5, a pilot study is currently underway to help define
process design parameters for the biotower (BT) process at the Dubuque plant.
Performance data from the pilot units will be collected at several loading and
temperature conditions, with the actual Dubuque primary effluent. The results
are expected to be available in late winter or early spring of 1991. Based on
these results, a much better picture will be available as to the cost and
practicality of the BT-Activated Sludge process for Dubuque. As a result, the
present focus will be primarily upon the other identified improvements, and
discussion relative to modifications to the secondary treatment system will be
preliminary. Following completion of the pilot study, the long-range capital
needs will be identified in an update to this report.
The costs presented in Section 5 for the selected incineration process train, are
based on new solids dewatering centrifuges (centripresses) for sludge dewatering.
This equipment has been successfully pilot -tested at the Dubuque plant. It is
possible that similar performance could be achieved with high performance belt
presses available from some of the higher quality machines currently available.
If so, it is possible that initial capital costs could be slightly lower, and
annual costs for power may also be somewhat reduced. Prior to final design of
the dewatering system, it is recommended that pilot tests be conducted with high
performance belt press equipment, for comparison with the centripress results.
7.01 IDENTIFIED IMPROVEMENTS
Table 7.01-1 lists the identified improvements, from Sections 5 and 6, together
with cost and other pertinent information. Additional discussion is provided as
follows:
A. High Priority Improvements
As indicated in Table 7.01-1, certain of the identified projects are viewed as
having relatively high priority:
• Screening This equipment is at the end of its useful
life, and is no longer serviceable. High
operating labor is needed at times due to
inadequate controls.
TABLE 7.01-1
SUMMARY OF IDENTIFIED IMPROVEMENTS
DUBUQUE, IOWA WWTP
Unit or System
Preliminary Treatment:
Est. Initial Notes Reference Priority
Capital Cost Report Section
Odor Control $ 177,000 1 6.01 Low
Screening $ 383,000 2,3 6.02 High
Grit Removal $ 579,000 3 6.03 High
Primary Treatment
Repairs $ 254,000 4 6.04 High
Primary Sludge Pumps $ 125,000 3 5.02 High
Secondary Treatment $ 1,527,000 to
4,485,000 3,5,6 5.01 Medium
Disinfection $ 175,000 to
813,000 6,7 6.05 Low
Sludge Processing $ 3,673,000 3,6,8 5.03 High
Notes: 1 Would involve high operating costs. Not recommended for
implementation until need established following implementation
of other improvements.
2 Equipment is at end of useful life.
3 Improvements would significantly reduce operation and
maintenance costs.
4 Structural repairs to existing system.
5 Final clarifier improvements required to rectify problems with
solids carry-over at high flow.
6 Pilot testing recommended (or now underway) 'o verify final
design, cost and performance factors, or need.
7 Will need to be addressed when NPDES Permit is re -issued in
1992. Low range of cost assumes minor upgrading to chlorination
building, and selective equipment replacement.
8 Vacuum filters no longer serviceable. No standby for belt
press. Structural repairs needed. Recuperator will save on
fuel oil use by recovering heat from the incinerator stack gases
for pre -heating fluidizing air.
All costs are third quarter 1990 basis.
7-2
• Grit Removal This system was designed to accommodate past
heavy industrial loadings (gross solids) which
are not currently received and would not be
anticipated in future. Equipment
repair/replacement involves very high costs.
Odors released due to aeration. Conversion to
"vortex" type system will reduce operation and
replacement costs, and will reduce odor release
in headworks area.
• Primary Repairs This work is needed to provide structural
repairs to the present system. Delay would
cause increased deterioration.
• Primary Pumps Primary sludge pumping operation currently
involves high labor cost. This modification
would provide automation and reduced costs.
• Sludge Processing Elimination of "Zimpro" system will
significantly reduce operating costs. The
present vacuum filters are no longer
operational. The belt press lacks standby
capacity. Incinerator improvements will reduce
fuel oil costs.
B. Medium Priority Improvements
Table 7.01-1 indicates the following improvements as having medium priority:
• Secondary Treatment The present final clarifiers suffer from poor
control of settled solids, during high flows,
resulting in solids carry-over to the plant
effluent. The present high -purity oxygen
system involves high operation and maintenance
costs. Significant cost savings may be
possible by providing more conventional
treatment system. Biotower-Activated Sludge
(air) shows promise, and a pilot study of the
biotower process is currently underway at the
plant with results expected in the next several
months.
C. Low Priority Improvements
Table 7.01-1 lists the following projects as having low priority:
• Odor Control
This would entail hydrogen peroxide addition to
the influent wastewater (to Terminal -Cedar
force main). This would eliminate septicity in
the plant influent wastewater, but would result
in high operating costs, primarily for required
chemicals. It has been recommended that action
7-3
be delayed on these potential improvements,
pending the installation of the proposed new
grit removal system which should reduce
problems with sulfide release in the plant
headworks area.
• Disinfection The current chlorination system consists of
gaseous chlorine dissolution and feed, a mixing
chamber at the plant site, and utilization of
the outfall sewer to provide contact time. The
present system meets current NPDES
requirements. In 1992 the NPDES Permit will be
reissued, and more stringent limits on residual
chlorine are anticipated. This may require
improvements to the system to provide chlorine
contact tankage and dechlorination (sodium
bisulfite) facilities at the plant site.
Building improvements would also be needed.
Full-scale studies have been recommended to see
if the present system is capable of meeting the
anticipated permit requirements.
7.02 RECOMMENDED TIMING OF IMPROVEMENTS
Based on the prioritization of identified improvements, as discussed in the above
Section 7.01, it is recommended that the plant improvements be phased into two
projects:
Phase I High Priority Improvements
• Screening
• Grit Removal
• Primary Treatment Repairs
• Primary Sludge Pumps
• Sludge Processing
Phase II Lower Priority Improvements
• Secondary Treatment (to be further defined
based on ongoing pilot study)
• Disinfection (scope to be defined based on
recommended full-scale testing)
Follow-up work could include the plant influent odor control (peroxide feed)
system, if additional headworks odor control is desired, following installation
of the proposed new grit removal system.
Table 7.01-2 presents a preliminary implementation schedule for the recommended
improvements.
Item
Phase I:
•
•
•
•
•
•
•
•
Phase II:
•
•
•
•
•
•
TABLE 7.01-2
PRELIMINARY IMPLEMENTATION SCHEDULE
RECOMMENDED PLANT IMPROVEMENTS
DUBUQUE, IOWA WWTP
Meeting with DNR to discuss Planning,
Loan Document, and Design Document
submittal requirements
Submit Planning Documents
DNR Approves Planning Documents
Begin Design
Complete Design
DNR Approves Design Documents
Receive Construction Bids
Complete Construction
Complete Pilot Studies
Submit Planning Documents
DNR Approves Planning Documents
Begin Design
Complete Design
DNR Approves Design Documents
Receive Construction Bids
Complete Construction
Timing
February, 1991
March, 1991
May, 1991
May, 1991
December, 1991
March, 1992
June, 1992
December, 1993
April, 1991
May, 1991
August, 1991
September, 1991
July, 1992
November, 1992
March, 1993
June, 1995
8-1
SECTION 8
SUMMARY OF RECOMMENDED PLANT IMPROVEMENTS
Previous sections of this report have evaluated current plant conditions,
projected wasteloads and flows, screened potential technologies for biological
wastewater treatment and sludge processing and evaluated the most attractive
technologies, reviewed other plant needs, and considered the appropriate timing
of recommended actions. This section summarizes the conclusions reached and
recommendations made, and provides an analysis of their fiscal impact.
8.01 DESCRIPTION OF PROPOSED IMPROVEMENTS
Figure 8.01-1 shows a site plan of the plant, with the proposed or potential
improvements noted. The recommended improvements have been summarized in Table
7.01-1, and have been divided into two phases of work (see discussion in Section
7):
• Phase I High Priority Needs
• Phase :: Lower Priority Needs
A. Phase I Impr:iements
Recommended Phase improvements include:
• new influent mechanically -cleaned bar screens;
• new "vortex" grit removal system, to replace the present aerated grit
system;
• structural repairs to the primary treatment facilities;
• new air -operated diaphragm primary sludge pumps; and
• sludge processing system improvements, including new dewatering
equipment and incinerator improvements.
The new screens are necessary to replace the present screens which are at the end
of their useful life, and to resolve control problems during storm flow events.
The new grit removal system will substantially reduce present high maintenance
costs, and operating costs for the aeration system, and will also reduce hydrogen
sulfide odor release in the plant headworks area.
The structural repairs in the primary clarifier area are needed to resolve
structural problems and deterioration of these existing facilities, which have
been in service for over twenty years.
The new air -operated primary sludge pumps will produce a thicker sludge, and will
require significantly less labor cost than the current method of primary sludge
withdrawal and transfer.
8-2
Sludge processing improvements involve removing the present "Zimpro" system from
service, which has a very high operating cost, providing new sludge dewatering
equipment, to replace the vacuum filters which are no longer serviceable, and
improvements to the incinerator system to improve its performance and fuel
efficiency.
A preliminary implementation schedule for the Phase I improvements has been
presented in Table 7.01-2. Please refer to Sections 4, 5, 6 and 7 for more
detailed discussion concerning the recommended improvements, and alternatives
which were considered.
B. Phase II Improvements
The following actions are being contemplated as part of Phase II of the plant
improvements:
• improvements to the plant secondary biological wastewater treatment
system, including potential conversion of the process from a high
purity oxygen activated sludge system to a biotc4er-air activated
sludge system.
• disinfection improvements, including chlorination system upgrading
and, potentially, a sodiu- bisulfite de -chlorination system to meet
anticipated more stringent future residual chlorine limitations.
Pilot biotower studies are currently underway at the plant, that will provide a
better picture of anticipated performance and cost of the process at Dubuque.
Recommendations concerning the secondary treatment improvements will be refined
within the next few months, utilizing the data obtained from the pilot study.
Full-scale plant testing of the present disinfection system is recommended, to
determine if the present system is capable of meeting the anticipated future
limitations for fecal coliform bacteria (200 col/ 100 mL) and residual chlorine
(0.3 mg/L)t The future limits are anticipated to be set by DNR as part of the
reissuance of the City's NPDES Permit in 1992. If the present system cannot
reliably meet the future limits, significant upgrading to the present system may
be required, including new chlorine contact tankage and a de-ch'orination system.
8.02 FISCAL IMPACT OF RECOMMENDATIONS
Projected initial capital costs for the Phase I and Phase II work are listed in
Table 8.02-1.
Based on the figures of Table 8.02-1, and the projected timing of the
improvements as presented in Table 7.01-2, we have developed an approximate
capital cost outlay projection through the year 1995. This projection is
included as Table 8.02-2.
The ranges indicated in Tables 8.02-1 and 8.02-2 represent the minimum to maximum
range of costs for secondary treatment and disinfection as indicated in Table
7.01-1. The allocation of cost to ;ears 1991 through 1995 is approximate, and
is based on experience on past similar projects.
TABLE 8.02-1
PROJECTED INITIAL CAPITAL COSTS
PHASE I AND PHASE II PLANT IMPROVEMENTS
CITY OF DUBUQUE, IOWA WWTP
Item
Phase I:
Screening
Grit Removal
Primary Clarifier Repairs
Primary Sludge Pumps
Sludge Processing
Total
Phase II:
Secondary Treatment
Disinfection
Total Range
Projected
Initial Capital
Cost
$ 383,000
$ 579,000
$ 254,000
$ 125,000
$ 3,673,000
$ 5,014,000
$ 1,527,000 to
4,485,000
$ 175,000 to
813,000
$ 1,702,000 to
5,298,000
Projected Cost Timeline
Start Complete
2/91
2/91
Note: All costs are third quarter, 1990, dollars.
12/93
6/95
TABLE 8.02-2
APPROXIMATE CAPITAL OUTLAY PROJECTION
PHASE I AND PHASE II IMPROVEMENTS
CITY OF DUBUQUE, IOWA WWTP
Year Phase I Work Phase II Work Total
1991 $ 300,000 $ 40,000 $ 340,000
to 120,000 to 420,000
1992 $ 1,470,000 $ 60,000 $ 1,530,000
to 180,000 to 1,650,000
1993 $ 2,944,000 $ 400,000 $ 3,344,000
to 1,250,000 to 4,194,000
1994 $ 300,000 $ 802,000 $ 1,102,000
to 2,500,000 to 2,800,000
1995 $ 400,000 $ 400,000
to 1,248,000 to 1,248,000
Totals S 5,014,000 $ 1,702,000 $ 6,716,000
tc 5,298,000 to 10,312,000
Note: All costs are third quarter, 1990 dollars.
8-3
The Wastewater Division has approximately $6,000,000 in capital reserves, from
accumulated depreciation and the proceeds of settlement of disputes relative to
plant design and plant equipment. Current reserves could fund much of the
project, with the remainder potentially borrowed on the revenue bond market, or
from the Iowa "State Revolving Fund".
The "State Revolving Fund" is a program established by the state to help finance
improvements to water pollution control works, and replaces the old EPA grant
program which has been discontinued. Rather than a grant of funds, the program
provides loans at a subsidized interest rate. Currently, such loans are offered
for a term of up to 20 years, at an interest rate of approximately 5 per cent.
This compares with a commercially available rate, from the revenue bond market,
of about 7.5 percent.
Based on funding $5,000,000 of the project from current reserves, and borrowing
the remainder, annual principal and interest payments would be about:
$2,000,000 borrowing
Revenue Bonds $196,200/year
State Revolving Fund $160,500/year
$6,000,000 borrowing
Revenue Bonds $588,600/year
State Revolving Fund $481,400/year
The approximate net impact on current cash basis revenue requirements can be
determined by considering savings in annual operating costs following
implementation of proposed improvements. An estimate of these savings is as
follows:
Item Estimated Annual Savings
Eliminate Zimpro System
Primary Sludge Pumping
Incinerator Fuel Savings
Subtotal -Phase I
Secondary Treatment
Dechiorination Expense
$gt5o,000
$ 35„ 000
$ i4c1,000
$ 585,000
$ 240,000
$-30,000
Total $ 795,000
Note that the figure given for secondary treatment compares the BT-Activated
Sludge alternative projected costs with current costs for the 02-Activated Sludge
system, not the figures from Tables 5.01-11 and 5.01-11A which reflect reduced
costs due to improvements to the system. Note also that the above figures do not
include the potential cost of chemical scrubbing of the BT off -gases should this
prove to be necessary.
As indicated above, the projected operating cost savings as a result of
implementing the Phase I improvements are about $585,000 per year. As cash
reserves could be utilized to implement Phase I, these improvements would result
in a total annual cost reduction equivalent to about 19% of the FY 1991 operating
budget.
The impact of implementation of both Phase I and Phase II improvements would be
dependent upon the extent of the improvements (i.e. whether the BT-Activated
Sludge alternative were implemented and whether chlorination/dechlorination
improvements were necessary). Assuming the $795,000 annual operating cost
savings from above, the net impact would range from a savings in annual revenue
requirements of about $196,000 per year to a savings of about $314,000 per year
depending upon whether the State Revolving Fund or revenue bond financing was
utilized. The $314,000 figure would be equivalent to about 10 per cent of the
FY 1991 department budget.
8-1
SECTION 8
SUMMARY OF RECOMMENDED PLANT IMPROVEMENTS
Previous sections of this report have evaluated current plant conditions,
projected wasteloads and flows, screened potential technologies for biological
wastewater treatment and sludge processing and evaluated the most attractive
technologies, reviewed other plant needs, and considered the appropriate timing
of recommended actions. This section summarizes the conclusions reached and
recommendations made, and provides an analysis of their fiscal impact.
8.01 DESCRIPTION OF PROPOSED IMPROVEMENTS
Figure 8.01-1 shows a site plan of the plant, with the proposed or potential
improvements noted. The recommended improvements have been summarized in Table
7.01-1, and have been divided into two phases of work (see discussion in Section
7):
• Phase I
High Priority Needs
• Phase II Lower Priority Needs
A. Phase I Improvements
Recommended Phase I improvements include:
• new influent mechanically -cleaned bar screens;
• new "vortex" grit removal system, to replace the present aerated grit
system;
• structural repairs to the primary treatment facilities;
• new air -operated diaphragm primary sludge pumps; and
• sludge processing system improvements, including new dewatering
equipment and incinerator improvements.
The new screens are necessary to replace the present screens which are at the end
of their useful life, and to resolve control problems during storm flow events.
The new grit removal system will substantially reduce present high maintenance
costs, and operating costs for the aeration system, and will also reduce hydrogen
sulfide odor release in the plant headworks area.
The structural repairs in the primary clarifier area are needed to resolve
structural problems and deterioration of these existing facilities, which have
been in service for over twenty years.
The new air -operated primary sludge pumps will produce a thicker sludge, and will
require significantly less labor cost than the current method of primary sludge
withdrawal and transfer.
8-2
Sludge processing improvements involve removing the present "Zimpro" system from
service, providing new sludge dewatering equipment to replace the vacuum filters
which are no longer serviceable, and improvements to the incinerator system to
improve its performance and fuel efficiency.
A preliminary implementation schedule for the Phase I improvements has been
presented in Table 7.01-2. Please refer to Sections 4, 5, 6 and 7 for more
detailed discussion concerning the recommended improvements, and alternatives
which were considered.
B. Phase II Improvements
The following actions are being contemplated as part of Phase II of the plant
improvements:
• improvements to the plant secondary biological wastewater treatment
system, including potential conversion of the process from a high
purity oxygen activated sludge system to a biotower-air activated
sludge system.
• disinfection improvements, including chlorination system upgrading
and, potentially. a sodium bisulfite de -chlorination system to meet
anticipated more stringent future residual chlorine limitations.
Pilot biotower studies are currently underway at the plant, that will provide a
better picture of anticipated performance and cost of the process at Dubuque.
Recommendations concerning the secondary treatment improvements will be refined
within the next few months, utilizing the data obtained from the pilot study.
Full-scale plant testing of the present disinfection system is recommended, to
determine if the present system is capable of meeting the limitations for fecal
coliform bacteria (200 col/ 100 mL) at anticipated future residual chlorine (0.3
mg/L) limits. The future limits are anticipated to be set by DNR as part of the
reissuance of the City's NPDES Permit in 1992. If the present system cannot
reliably meet the future limits, significant upgrading to the present system may
be required, including new chlorine contact tankage and a de -chlorination system.
8.02 FISCAL IMPACT OF RECOMMENDATIONS
Projected initial capital costs for the Phase I and Phase II work are listed in
Table 8.02-1.
Based on the figures of Table 8.02-1, and the projected timing of the
improvements as presented in Table 7.01-2, an approximate capital cost outlay
projection through the year 1995 was developed. This projection is included as
Table 8.02-2.
The ranges indicated in Tables 8.02-1 and 8.02-2 represent the minimum to maximum
range of costs for secondary treatment and disinfection as indicated in Table
7.01-1. The allocation of cost to years 1991 through 1995 is approximate, and
is based on experience on past similar projects.
TABLE 8.02-1
PROJECTED INITIAL CAPITAL COSTS
PHASE I AND PHASE II PLANT IMPROVEMENTS
CITY OF DUBUQUE, IOWA WWTP
Item
Phase I:
Screening
Grit Removal
Primary Clarifier Repairs
Primary Sludge Pumps
Sludge Processing
Total
Phase II:
Secondary Treatment
Disinfection
Total Range
Projected
Initial Capital
Cost
$ 383,000
$ 579,000
$ 254,000
$ 125,000
$ 3,673,000
$ 5,014,000
$ 1,527,000 to
4,485,000
$ 175,000 to
813,000
$ 1,702,000 to
5,298,000
Note: All costs are third quarter, 1990 dollars.
Projected Cost Timeline
Start Complete
2/91
2/91
12/93
6/95
TABLE 8.02-2
APPROXIMATE CAPITAL OUTLAY PROJECTION
PHASE I AND PHASE II IMPROVEMENTS
CITY OF DUBUQUE, IOWA WWTP
Year Phase I Work Phase II Work Total
1991 $ 300,000 $ 40,000 $ 340,000
to 120,000 to 420,000
1992 $ 1,470,000 $ 60,000 $ 1,530,000
to 180,000 to 1,650,000
1993 $ 2,944,000 $ 400,000
to 1,250,000
1994 $ 300,000 $ 802,000
to 2,500,000
$ 3,344,000
to 4,194,000
$ 1,102,000
to 2,800,000
1995 $ 400,000 $ 400,000
to 1,248,000 to 1,248,000
Totals $ 5,014,000 $ 1,702,000 $ 6,716,000
to 5,298,000 to 10,312,000
Note: All costs are third quarter, 1990 dollars.
8-3
The Wastewater Division has approximately $6,000,000 in capital reserves, from
accumulated depreciation and the proceeds of settlement of disputes relative to
plant design and plant equipment. Current reserves could fund much of the
project, with the remainder potentially borrowed on the revenue bond market, or
from the Iowa "State Revolving Fund".
The "State Revolving Fund" is a program established by the state to help finance
improvements to water pollution control works, and replaces the old EPA grant
program which has been discontinued. Rather than a grant of funds, the program
provides loans at a subsidized interest rate. Currently, such loans are offered
for a term of up to 20 years, at an interest rate of approximately 5 per cent.
This compares with a commercially available rate, from the revenue bond market,
of about 7.5 percent.
Based on funding $5,000,000 of the project from current reserves, and borrowing
the remainder, annual principal and interest payments would be about:
$2,000,000 borrowing
Revenue Bonds $196,200/year
State Revolving Fund $160,500/year
$6,000,000 borrowing
Revenue Bonds
$588,600/year
State Revolving Fund $481,400/year
The approximate net impact on current cash basis revenue requirements can be
determined by considering savings in annual operating costs following
implementation of proposed improvements. An estimate of these savings is as
follows:
Item Estimated Annual Savings
Eliminate Zimpro System
Primary Sludge Pumping
Incinerator Fuel Savings
Subtotal -Phase I
Secondary Treatment
Dechlorination Expense
$ 350,000
$ 35,000
$ 75.000
$ 460,000
$ 240,000
$-30,000
Total $ 670,000
Note that the figure given for secondary treatment compares the BT-Activated
Sludge alternative projected costs with current costs for the 02-Activated Sludge
system, not the figures from Tables 5.01-11 and 5.01-11A which reflect reduced
costs due to improvements to the system. Note also that the above figures do not
include the potential cost of chemical scrubbing of the BT off -gases should this
prove to be necessary.
8-4
As indicated above, the projected operating cost savings as a result of
implementing the Phase I improvements are about $460,000 per year. As cash
reserves could be utilized to implement Phase I, these improvements would result
in a total annual cost reduction equivalent to about 15% of the FY 1991 operating
budget.
The impact of implementation of both Phase I and Phase II improvements would be
dependent upon the extent of the improvements (i.e. whether the BT-Activated
Sludge alternative were implemented and whether chlorination/dechlorination
improvements were necessary). Assuming the $670,000 annual operating cost
savings from above, the net impact would range from a savings in annual revenue
requirements of about $81,000 per year to a savings of about $189,000 per year
depending upon whether the State Revolving Fund or revenue bond financing was
utilized. The $189,000 figure would be equivalent to about 6.5 per cent of the
FY 1991 department budget.
It is evident that the most significant net positive impact on the department's
revenue requirements would be provided by the implementation of the Phase I
improvements.
A-1
APPENDIX A
PLANT LOADINGS AND PERFORMANCE
DUBUQUE, IOWA WWTP
This appendix describes the procedures and results of the analysis of 1989 data
and data from the 1985 Phase 1 and 1988 Phase 2 reports prepared by Strand
Associates. A list of abbreviations used in this appendix is shown in Section
A 4
A.1 DATA COLLECTION POINTS
The following is a description of the data collection points and type of data
reported on the 1989 plant data sheets:
1. In -Plant Waste Pumping Station. The in -plant waste (IPW) pumping
station received wastes including scum beaching water (SBW), vacuum filter
overflow (VFO), vacuum filter wash water (VFW), cooling water and scrubber
water, sludge storage tank decant, and plant drains. The flow rate was
generally recorded daily while BOD5 and TSS data were recorded less
frequently.
2. Supernatant Pumping Station. The supernatant holding tank contained
wastes including Zimpro decant supernatant (ZDS), a portion of the vacuum
filter filtrate (VFF), and the belt filter press filtrate (BFPF). Flow
data were recorded from the pumping station daily, and BOD5 and TSS data
were recorded less frequently.
3. Raw Sludge. Flow and percent total solids data were collected daily
for the raw waste activated sludge (WAS) and raw primary sludge (PRS).
4. WAS Processing. Flow and percent solids entering the Zimpro process
were generally recorded daily, as well as total sludge to the incinerators.
5. Flow. Influent plant flow and base flow (effluent returned to the
biological treatment process) were recorded daily.
6. Plant BOD and TSS. Data for influent and effluent BOD5 and TSS were
obtained from the WWTP monthly reports to the DNR for 1987 and 1988. Data
for 1989 was obtained from the plant daily records, which were recorded on
Lotus 123 data spreadsheets.
A.2 PROCEDURES
The following are procedures that were used for determining values for the 1989
WWTP material balance:
1. General. Data from 1989 were generally reported as either mg/L of TSS
or %TS, and not both. Numerical values for TSS were therefore calculated
from TS data, and vice -versa. It was assumed that the ratio of TSS:TS was
A-2
similar for particular sample types during 1989 as they were during the
short-term study reported the 1985 Phase 1 report.
2. Scum Beaching Water. Data for scum beaching water were estimated based
on information in the Phase 1 and 2 reports, and based on a material
balance around the preliminary and primary treatment units.
3. Centrifuges. The centrate and centrifuge output were calculated by
performing a mass balance around the unit, since the feed rate (WAS) flow
and %TS were known. The BOD5 in the centrate was estimated from the data
in the 1985 Phase 1 report. It was assumed that the percent solids
recovery was 90%, and that the insoluble BOD5 followed the same trend as
TSS for percent recovery in the centrifuges.
4. Cooling Water. The cooling water flow rate was estimated based on
conversations with plant personnel for non -potable water and based on plant
water use billings for city potable water.
5. Belt Filter Press. Data on percent recoveries for the belt filter
press were obtained through conversations with plant personnel. Values for
BOD5 were estimated based on the assumed soluble percent of BOD in the
sludge and by assuming that insoluble BOD followed the same trend as TSS.
A.3 RESULTS
Flow and mass balances for the plant were developed based on the 1985 Phase 1
report, the 1988 Phase 2 report, discussions with plant personnel, and data
reported in the 1989 monthly plant records as described above. The balances were
made based on average conditions for each month, then averaged for the year. The
resulting material balance is presented in Figure 2.03-6. See report Section 2.
It should be noted that information such as centrate and scum beaching water
could not be accurately determined due to lack of data. However, since data for
IPW and SP were gathered on a fairly regular basis, results for recycle streams
are fairly accurate, as is the overall performance of the sludge processing
equipment.
The following observations were made based on the mass balance resulting from the
1989 data:
1. Primary Clarifiers.
During 1989, the overflow rate for the primary clarifiers was 463 gpd/ft2
and the BOD5 and TSS loadings were 16,300 lb/day and 16,200 lb/day,
respectively. 0f these loadings, approximately 18% of the BOD and 22% of
the TSS came from the IPW return stream. The clarifiers removed 23% of the
influent BOD5 and 64% of the influent TSS. Typical percent removals for
BOD and TSS during primary clarification are 25-40% and 50-60%,
respectively. The BOD percent removal was less than typical during the
year. This is most likely due to the high percentage of soluble BOD that
is returned to the IPW pumping station from sludge processing. The overall
A-3
percent BOD removal would most likely increase if the IPW return stream was
reduced.
2. Pure Oxygen Activated Sludge
The average loadings to the activated sludge process during 1986 were
15,800 lb BOD5/day and 7,400 lb TSS/day, and the average flow was 10.38
mgd. Approximately 20% of the BOD5 and 18% of the TSS loadings were due
to recycle streams from the Zimpro process and the centrifuges.
Loading parameters experienced during 1989 are compared to typical design
values in Table A-1. Also shown are predicted design loading rates, based
on a design average wet weather flow of 18.47 mgd and influent BOD of
24,400 lb/day (25% reserve capacity). During 1989, the organic loading,
MLSS and SRT were lower than typical values for pure oxygen activated
sludge systems while the HRT was higher than typical values. The
concentration of MLSS that may be achieved is limited by the secondary
clarifiers, which should not be loaded above 30 lb solids/day/ft2 at
average flow (DNR standard). During 1989, the recycled activated sludge
flow was 30% of the secondary influent, which resulted in a solids loading
to the final clarifiers of 12 lb/day/ft2. Therefore, even though the MLSS
and SRT were below recommended values, performance of the final clarifiers
was good and 96% BOD removal was achieved through the biological treatment
system.
At design conditions of 25% reserve capacity, the BOD5 load to the
activated sludge system is expected to be 29,300 lb/day. If the MLSS were
maintained at 3700 mg/L, the SRT would be outside of the recommended range
and treatment performance could be adversely affected. If the MLSS were
increased to 6,000 mg/L, the SRT would be closer to the recommended range.
However, the clarifiers would then be somewhat overloaded at 36 lb/day/ft2
(assuming a 30% recycle rate). The maximum MLSS that could be achieved at
30% recycle and 30 lb/day/ft2 loading to the final clarifiers would be
about 5,100 mg/L. The SRT at this MLSS would be 4.4 days. Based on past
performance of the pure oxygen system, it is expected that 85 to 90% BOD5
removal could be achieved at this SRT and the predicted organic loading.
The permit limits should be achievable at these removal rates.
It was determined during the week-long study that the oxygen requirement
during the activated sludge process is approximately 1.2 lb 02/1b BOD
applied. At this rate, the oxygen generation plant has the capacity for
BOD5 loadings up to 41,700 lb/day. Thus, the plant has enough capacity for
the predicted design BOD loadings at average flow and peak hourly flow
rates.
3. Sludge Processing
The centrifuge percent total solids recovery is typically 90% and the
produced thickened WAS solids concentration is about 4.5%, according to
plant personnel.
OHM
r�1
A-4
The supernatant from the Zimpro picket thickener, along with the belt press
filtrate and a portion of the vacuum filter filtrate, accounted for about
15% of the influent BOD load to the biological treatment process during
1989. This was lower than during previous years primarily because plant
operators allowed the thickened solids to settle longer before decanting.
The Zimpro reactor will have enough capacity to process thickened WAS at
design loading rates; however, it is possible that the picket thickener
will not have enough capacity, and percent solids recovery could drop off
further. It has been recommended that the Zimpro process be removed from
service.
The overall sludge processing percent solids recovery, based on the mass
balance shown in Figure 2.03-6, was approximately 50% during 1989, even
though the centrifuges and belt press were reported to have good solids
recovery. The relatively low overall percent recovery could have partially
resulted from poor vacuum filter operation during the beginning of the
year. Based on the material balance analysis, the total amount of BOD5
returned from sludge processing was 6,050 lb/day, which is 45% of the plant
influent BOD5, and the total amount of TSS returned was 4,850 lb/day, or
38% of the influent.
A.4 ABBREVIATIONS
BFPF
C
I PW
PI
PRI
PRS
RAS
SBW
SP
TWAS
VFF
VFO
VFW
ZDS
Belt Filter Press Filtrate
Centrate
Inplant Waste recycle
Plant Influent
Primary Clarifier Influent
Raw Primary Sludge
Return Activated Sludge
Scum Beaching Water
Supernatant from Zimpro Decant Tank
Thickened Waste Activated Sludge
Vacuum Filter Filtrate
Vacuum Filter Overflow
Vacuum Filter Wash Water
Zimpro Decant Solids
1
TABLE A-1
1989 OXYGEN ACTIVATED SLUDGE PERFORMANCE
DUBUQUE, IOWA
Predicted Typical
Parameter 1989 Design Range
Organic Load
lb/day/1,000 ft3
MLSS, mg/L
F:M, d
HRT, hours
SRT, days
Recycle
BOD5 Removal
63
3,700
0.32
4.4
6.0
116
4,500 - 6,000
0.55 @ 4,500
0.40 @ 6,000
2.5
3.9 @ 4,500
5.2 @ 6,000
100 - 200
6,000 - 8.000
0.19 - 0.562
1-3
8-20
30% 25-50% 25-50`
96% 85-90% 85-95`_
1 Based on Metcalf & Eddy Wastewater Engineering, 1979, for secondary treatment
by pure oxygen activated sludge.
2 Assumes 75% volatile