Water & Resource Recovery Center Odor Abatement Efforts UpdateCity of Dubuque
City Council Meeting
Consent Items # 09.
Copyrighted
March 18, 2024
ITEM TITLE: Water & Resource Recovery Center Odor Abatement Efforts Update
SUMMARY: Water & Resource Recovery Center Director Deron Muehring is
providing an update on odor abatement efforts at the Water & Resource
Recovery Center.
SUGGESTED Suggested Disposition: Receive and File
DISPOSITION:
ATTACHMENTS:
Description
MVM Memo
Type
City Manager Memo
Staff memo, Water & Resource Recovery Center Odor Staff Memo
Abatement Efforts Update
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TO: The Honorable Mayor and City Council Members
FROM: Michael C. Van Milligen, City Manager
SUBJECT: Water & Resource Recovery Center Odor Abatement Efforts Update
DATE: March 14, 2024
Water & Resource Recovery Center Director Deron Muehring is providing an update on
odor abatement efforts at the Water & Resource Recovery Center.
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Mic ael C. Van Milligen
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Attachment
CC' Crenna Brumwell, City Attorney
Cori Burbach, Assistant City Manager
Deron Muehring, Water & Resource Recovery Center Director
William O'Brien, Water & Resource Recovery Center Manager
Dubuque
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TO: Michael C. Van Milligen, City Manager
FROM: Deron Muehring, Water & Resource Recovery Center Director
SUBJECT: Water & Resource Recovery Center Odor Abatement Efforts Update
DATE: March 13, 2024
INTRODUCTION
The purpose of this memo is to provide an update on odor abatement efforts at the
Water & Resource Recovery Center.
BACKGROUND
The Water & Resource Recovery Center (WRRC) uses physical, biological, and chemical
processes to remove up to 98% of incoming organic pollutants. This process does result in
the creation of gases and compounds that can create unpleasant odors. The nature of the
odors is a function of the chemical characteristics of the influent wastewater received at the
WRRC. The wastewater generated in Dubuque has higher concentrations of pollutants than
typical domestic waste. Therefore, it has a higher propensity to produce odors.
As wastewater with high organic content undergoes decomposition, it releases gases such
as hydrogen sulfide (112S) and mercaptans. 112S is also called "sewer" gas known for its
pungent "rotten egg" odor even at low concentrations. Mercaptans are known for their
pungent "smelly sock" odor. These gases can be produced and released at multiple
locations within the treatment system. In fact, these gases can be present to some degree in
the wastewater when it first reaches the WRRC.
The most likely sources for the odors at the WRRC, listed from highest contributor to
lowest, are the primary clarification process, raw influent from forcemain/pressurized
sewers, blended sludge storage, waste activated sludge storage, and anaerobic digestion.
Even though odor producing compounds will always be present at the WRRC, there are
steps that can be taken to minimize the release of the gases and odors into the
atmosphere.
In June of 2023, the City hired US Peroxide, LLC (USP) to assist WRRC staff with
performing an odor control evaluation at the WRRC. USP is a leading provider of
peroxygen-based technologies and full -service chemical treatment programs for municipal
and industrial water and wastewater treatment applications. They have extensive
experience in liquid phase treatment for odor control. USP provides temporary chemical
feeds equipment to dose the system and monitoring equipment to measure the
effectiveness of the chemical dosing. The odor control evaluation began with discussing
treatment objectives, suspected locations of odor release, treatment facility design and
wastewater characteristics. The initial consultation was followed by bench testing of
wastewater from several process locations to determine the extent of possible sulfide
removal at varying reaction times and dose rates. Bench testing was conducted, with
multiple chemicals, to identify which chemicals would likely provide the most efficient and
cost-effective odor reduction. In August of 2023, the WRRC began dosing hydrogen
peroxide between where the wastewater reaches the WRRC (the headworks) and the
primary clarifiers where the highest levels of hydrogen sulfide (H2S) can be expected.
USP determined that a 50% reduction in H2S (most prominent odor producing compound)
can be achieved by adding between 50 and 60 gallons of peroxide per day into the waste
stream between the headworks and the primary clarifiers. That would cost between
$130,000 and $150,000 per year based on current chemical pricing. Based on these results,
an improvement package will be included as part of the Fiscal Year 2025 budget to increase
the WRRC operating budget to fund this odor reduction effort.
An error committed by a private company hired to assist with a required equipment
inspection resulted in equipment failure necessitated temporary operational changes —
changes necessary to continue to receive and treat the city's wastewater in accordance with
the federal Clean Water Act. As the manufacturer of the equipment, the company was hired
because they should possess unparalleled insight into the intricacies of the equipment.
Utilizing the manufacturer for servicing minimizes the learning curve often associated with
third -party service providers. Their familiarity with the WRRC system should translate to
streamlined processes and quicker responses when issues arise. By choosing them, the City
was entrusting the equipment to a contractor with years of experience in designing,
manufacturing, and servicing similar equipment. Their expertise should have ensured that
the WRRC equipment would be kept in peak operational condition. Unfortunately, that was
not the reality.
Inspection of the equipment required the contractor to remove the electrical service and
controls to allow access to the interior of the equipment. Following inspection, the contractor
re -wired the unit to put it back into service. After WRRC personnel finished up additional,
unrelated maintenance of the unit, the unit was restarted by the contractor who then left the
premises. Approximately five (5) hours later, a power interruption caused the unit to
shutdown and WRRC personnel were unable to restart the unit. The contractor returned later
in the week to try and get the equipment to operate but left after several hours without any
success. Believing that it wasn't related to the equipment but had something to do with the
WRRC computer system that controls the equipment, WRRC staff and a second contractor,
a computer program logic controller (PLC) contractor, were left to try and troubleshoot the
issue. They systematically worked through the process of ruling out potential causes of the
failure. Finally, a week later the WRRC team, along with the PLC contractor discovered that
the service contractor had re -wired the equipment incorrectly. Once corrected, the unit
started up.
The WRRC employs an anaerobic digestor system to break down organic material in
system solids. This process involves microorganisms that thrive in anaerobic (oxygen -free)
conditions. The breakdown of volatile organic material in an anaerobic digester occurs
through a series of microbial activities, primarily carried out by bacteria and other
microorganisms. The equipment that was inoperable for two weeks helps to maintain the
delicate balance and relative population of the microorganisms in the anerobic digesters.
As a result of the inoperable equipment, two of the four digester units became dormant and
could no longer accept and treat the waste stream. The other two remained active. To try
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and remain compliant with permit discharge limitations, WRRC staff had to limit the waste
stream sent to the two functional digesters. To accommodate this, waste has been
temporarily stored in tanks where they normally are not. As a result, the odor producing
gases can readily escape into the atmosphere. And this has resulted in what might be
unprecedented odors coming from the WRRC.
DISCUSSION
The Water & Resource Recovery Center (WRRC) has been working to address both a
temporary, short term odor issue and more typical, long-term odors at the wastewater
treatment facility.
Short-term Odor Abatement
The short-term odor issue is related to a wiring error by a private contractor during a routine
inspection that resulted in inoperable equipment for an extended period. Without the use of
the equipment, two of the four anaerobic digesters at the Water & Resource Recovery
Center (WRRC) became dormant and could no longer accept and treat waste in a normal
fashion. To adhere to permit limitations, waste was diverted into uncovered excess flow
tanks that allow odor -producing gases to readily escape into the atmosphere. It is the odor
from these tanks that has resulted in unprecedented odors coming from the WRRC starting
late in 2023.
The first step to address the short term, unprecedented odor issue, is to bring the anaerobic
digesters back within normal operational parameters. Despite best efforts, building back the
health of the digesters has taken longer than expected. To grow the methane forming
bacteria as required for adequate wastewater treatment, WRRC operators have been
carefully feeding the waste to the digesters while monitoring volatile acid production and
alkalinity. That has proven to be two steps forward, one step back process. Efforts have
included hauling in "healthy" bacteria from Iowa City and feeding the system with medicine in
the form of caustic soda. By avoiding significant setbacks, three of the four digesters are now
fully functional with characteristics (e.g., acid levels, alkalinity, pH, methane gas production)
at normal operational values. While the fourth digester continues to improve, it has not fully
recovered.
To fully eliminate the short-term odor issue, the waste temporarily stored in the excess flow
tanks must be handled. The north tank has been emptied. The waste in the south tank is
now slowly draining back into the WRRC wastewater treatment system. Because the stored
waste is no longer being disturbed by the addition of new waste, the surface has solidified
resulting in a barrier that has reduced the free release of odorous gaseous discharge from
the waste. While the waste in the tank continues to emit odors, it may be less now than when
waste was initially redirected into the tanks back in November of 2023.
WRRC staff continue efforts to identify disposal alternatives. It was reported with the January
update that steps were being taken to have a private contractor collect and haul the waste
from the WRRC and dispose of it in accordance with environmental regulations. To that end,
WRRC staff have been collecting and having samples tested and analyzed to characterize
the waste in accordance with state regulations. The final testing results are expected by the
end of the month which will enable identification of all viable options to dispose of the waste
in accordance with state environmental regulations. Based on the results, the WRRC will
take steps to remove the waste from the storage tank as soon as possible.
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Recognizing the contractor's failure to securely reattach wiring removed as part of their
services, they have been notified that as a result of the negligence of their employees, the
City has sustained significant costs and that the City is in the process of gathering
information about those costs.
Long-term Odor Abatement
The investigation into reducing long-term odor emissions at the WRRC continues. The City's
consultant (USP) hired to assist WRRC staff with performing the odor control evaluation has
been onsite to complete the installation of the temporary storage tank and system to dose
the waste stream between the digesters and the centrifuge with ferric chloride. Dosing with
ferric chloride can both reduce hydrogen sulfide and the formation of struvite.
When struvite producing substances (i.e., magnesium, ammonium, and phosphate) come
together in the right concentrations and conditions, they can precipitate out of solution and
form struvite, a mineral deposit that clogs pipes and equipment. Struvite formation in the
anaerobic digesters results in the release of more hydrogen sulfide, results in more odors.
Therefore, dosing with ferric chloride is planned at another location upstream of the
anaerobic digesters. To determine the best location, USP is onsite performing an analysis to
determine the levels of the struvite producing substances throughout the treatment process.
Based on the study findings to date, an improvement package request has been prepared
for City Council consideration as part of the Fiscal Year 2025 budget approval process which
would provide for $462,000 to purchase chemicals to be used to reduce odor emissions.
This will provide for dosing the waste stream with hydrogen peroxide just before the primary
clarifiers which was shown to provide a 50% reduction in hydrogen sulfide (112S), the most
prevalent odor -causing compound in wastewater. It will also provide for dosing the waste
stream with ferric chloride at two locations such as between the digesters and the centrifuge
discussed previously and a second location such as between the aerobic biological
treatment and anaerobic digesters.
In addition, a second improvement package request has been prepared for City Council
consideration as part of the Fiscal Year 2025 budget that would help address odors at the
WRRC. It would provide $75,000 per year to clean one of the four digesters each year.
Cleaning the digesters will prevent the buildup of inert materials such as grit, sand, and non -
biodegradable solids that can hinder the digestion process, reduce the effective volume of
the digester, and lead to operational issues. Regular cleaning will reduce undesirable side
effects such as odors.
Periodic updates on the progress towards addressing both the short-term odor issue and the
comprehensive odor reduction evaluation are being posted on the City of Dubuque website
at www.cityofdubuque.org/odorcontrol.
ACTION REQUIRED
This memorandum is intended for informational purposes.
Cc: Crenna Brumwell, City Attorney
William O'Brien, W&RRC Plant Manager
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