Odor Control
Long-Term Odor Abatement
Wastewater Odor Reduction Analysis
Due to ongoing odor concerns, the City is conducting a wastewater odor reduction analysis. The odor reduction analysis follows the process of identifying where hydrogen sulfide (H2S) is most prevalent in the treatment system, chemical dosing at locations that can effectively reduce H2S, and adjusting dosing rates that balance the effectiveness of H2S removal and preventing the dosing from hindering the overall effectiveness of the treatment process.
There are four steps to the process:
- Identify dosing locations.
- Set up temporary dosing stations (tanks to store chemicals and equipment that controls measured dosing rates).
- Determine the effectiveness of dosing (optimum chemical and dosing rate).
- Identify the cost to establish permanent dosing (i.e., cost of storage tanks, chemical feed skids, chemicals, etc.).
We have been dealing with a process issue at the WRRC over the past few weeks that we have learned is leading to higher-than-normal hydrogen sulfide (H2S) emissions — the gas most often associated with strong wastewater odors. Something happened at the end of August that impacted the system. We continue to investigate the cause. But the primary focus has been looking into how we restore the system to eliminate the heightened H2S, eliminate the heightened odors.
WRRC staff, working closely with engineering consultants, have started making adjustments to bring the system back into balance. One of the main strategies involves carefully managing how much of the solids from the activated sludge process is removed each day to maintain an appropriate sludge volume and sludge age using new calculations and monitoring tools to guide those adjustments. These targeted changes are starting to improve conditions, though it will take time for the system to fully stabilize.
The way our treatment system is designed, solids are separated from the liquid portion of the wastewater. The liquid receives biological treatment and ultraviolet (UV) disinfection before it is released to the river, while the solids go through a separate treatment process that includes anaerobic digesters. Normally, hydrogen sulfide does not form in the liquid side of the process. What we discovered is that oxygen is being used up too quickly in our activated sludge basins. This leaves much of the system without oxygen, which allows hydrogen sulfide to form in areas where we do not usually see it. And when we remove the solids from the liquid process – send them to the solids processing systems for further treatment, the hydrogen sulfide is being carried with them. As a result, odors have been noticeable not only around our aeration and clarification tanks, but also in our solids storage and handling systems.
Generally speaking, we have made progress with odor reduction, things like chemical dosing to reduce hydrogen sulfide levels to improvements to drying pad operations and maintenance. Those gains are often hidden when a larger upset occurs. Given the odor issues in late 2023, early 2025, and now again this summer, frustration would be understandable. One of the initiatives underway will help avoid upsets like the current issue.
We are currently working on identifying and implementing enhanced process monitoring and controls that will provide real-time, actionable data when the system begins to move away from its normal balance. With this information, our operators will be able to make immediate adjustments to keep processes stable and prevent the type of imbalance we are experiencing today. At their recent Goal Setting, the City Council identified improvements and odor control at the WRRC as a high priority. The WRRC will continue to prepare budget recommendations that support the City’s odor reduction efforts and the City’s stated goals and priorities.
Another update will be provided by October 10 describing progress towards resolving this recent odor issue.
This is an update on our efforts to eliminate the temporarily stored waste in the south excess flow tank at the Water & Resource Recovery Center (WRRC), which has contributed to recent stronger odors.
Over the past two weeks, we have been slowly pumping the waste into the treatment system here at the WRRC. Earlier this week we started adding water to dilute the material and make it more workable. And as of a little after 10:00 a.m. this morning (June 25), WRRC staff began mixing the remaining contents of the tank to mobilize the material for removal. Mixing is typically the step most associated with the displacement and transmission of odors. This mixing process is currently underway and is expected to conclude by approximately 11:30 a.m. today. This will be the final time the tank will need to be mixed. Later today, we will begin pumping the remaining diluted waste back into the treatment process. This work will continue over the next few days, and we anticipate the tank will be mostly emptied by the end of the week. Early next week, we will proceed with rinsing and cleaning the interior of the tank to remove any remaining residue. Once this final material has been removed and the tank has been cleaned, we expect the odor-producing potential of the stored waste to be eliminated. We will continue to implement other odor mitigation strategies already in place to manage additional odor sources and improve long-term reliability.
We appreciate your patience and continued engagement as we work to mitigate/minimize odors from the WRRC.
Odor Update for April 14, 2025
Understanding Odors at the WRRC
The City of Dubuque recognizes that odors from the Water & Resource Recovery Center (WRRC) can be unpleasant and is committed to being transparent about their source. Wastewater treatment, by its nature, produces some level of odor. The raw wastewater entering the WRRC through the sanitary sewer system carries naturally occurring odors. As the wastewater is treated, it breaks down and releases gases—such as hydrogen sulfide—which is commonly associated with a “rotten egg” smell.
Environmental conditions also play a role. Factors such as temperature, humidity, and wind influence whether odors dissipate quickly or linger. On calm or humid days, odors are more likely to remain concentrated near the facility.
While there is no switch to eliminate odors, the City understands the impact they can have on nearby neighborhoods. In response, the City continues to invest in odor control strategies, evaluate WRRC operations, and actively plan for long-term improvements. These efforts reflect the City’s ongoing commitment to balancing its environmental responsibilities with the quality of life that Dubuque residents expect and deserve.
Facility Incident in December 2024
On December 2, 2024, a gas leak caused an explosion in the WRRC’s solids handling building. Thankfully, no one was injured. However, the incident significantly disrupted our solids treatment systems—key infrastructure responsible for processing the material left over after wastewater is cleaned.
As a result of this interruption, solids began to accumulate within the treatment system. With our primary equipment offline, WRRC staff had to take extraordinary steps to maintain operations and avoid violations of the federal Clean Water Act. This included storing solids in alternative locations throughout the facility and modifying standard treatment processes to maintain regulatory compliance.
Unfortunately, some of these temporary operational changes increased the potential for odor emissions. The strategies we adopted—while necessary—required us to balance the need for environmental compliance with the reality of diminished treatment capacity. These adjustments were made carefully, with the goal of protecting public health and the Mississippi River, even while they may have contributed to short-term odor challenges.
All major systems have since been restored, and the WRRC continues working toward restoring full treatment capacity. We expect that all remaining temporary, odor-producing approaches will be eliminated by the end of April 2025, returning the facility to more typical operating conditions and reducing odor impacts.
Odor Mitigation Progress
Over the past two years, the City has made significant investments in odor control:
- Hydrogen Peroxide Dosing: In 2023, we began adding hydrogen peroxide to the wastewater at the headworks (the start of the treatment process). This reduced hydrogen sulfide levels by approximately 50%, leading to noticeable air quality improvements around the facility.
- Ferric Chloride Evaluation: In 2024, ferric chloride was tested to reduce both odors and mineral buildup (struvite) in part of the treatment system. While it helped with odors, it was not effective for mineral control at that location. As a result, we’ve discontinued ferric dosing there and will instead use hydrogen peroxide at that location.
- New Treatment Study Underway: In February 2025, a new chemical dosing system was installed to apply ferric chloride directly into the facility’s anaerobic digesters. This approach will target odor-causing compounds and helps reduce struvite. The dosing is expected to begin in April to determine the most effective dosing strategy, with results expected later this year.
- Anaerobic Digester Cleaning Program: The City has funded a long-term maintenance program to clean each of the WRRC’s four digesters on a four-year rotation. Cleaning these systems reduces odor potential and improves treatment efficiency. The first cleaning, originally planned for the spring of 2025, was postponed due to the December 2024 explosion in the solids handling building. We expect to move forward with cleaning Digester #4 in the summer of 2025.
- Odor Monitoring Proposal: As part of the Fiscal Year 2026 Budget, we have proposed installing electronic odor sensors—also known as “e-nose” devices—around the perimeter of the WRRC. These sensors would provide real-time odor data, allowing us to better understand where and when odors may be leaving the facility.
Currently, our odor control efforts are evaluated by measuring reductions in hydrogen sulfide (H₂S) concentrations at specific locations within the plant. While this provides useful information, it does not directly measure whether odors are leaving the facility or how they are perceived in nearby neighborhoods. The relationship between H₂S levels and offsite odor impact is not always one-to-one due to weather conditions, compound interactions, and other variables.
The e-nose system will give us a way to measure the effectiveness of our odor reduction efforts more directly. You can’t manage what you can’t measure. These sensors will allow us to detect patterns, identify potential problem areas, and make more informed, targeted operational decisions. By integrating this data into our facility’s SCADA system, staff will be able to respond proactively to changing conditions and continuously improve our odor mitigation approach. - Drying Pad Evaluation: The WRRC’s drying pad plays a key role in protecting the treatment process by keeping inorganic debris—such as sand, grit, and sludge from car washes and sewer cleanouts—out of the sanitary sewer system. This material, which is not biodegradable, can damage pumps and clog pipes if allowed into the treatment process. The drying pad provides a controlled space where this material can be deposited, allowed to dewater, and then hauled to the landfill for proper disposal.
While the drying pad serves an essential function, it can produce odors—particularly when fresh material is placed near partially dried waste. Staff recently conducted an operational and structural evaluation of the facility and found that, although it was not originally designed for its current use, it remains the only site of its kind in the tri-state area and fills a critical regional need.
As part of the evaluation, the City also explored the feasibility of relocating the drying pad to the regional landfill. While this may remain a consideration for the future, several challenges currently limit its viability. The landfill does not have sufficient space available for a decanting facility. If space were provided, additional equipment would be needed for WRRC staff to operate the site, or the landfill would need to hire and train personnel to perform drying pad maintenance, including the movement and removal of dried material. These operational and logistical hurdles make on-site improvements at the WRRC the more practical option at this time.
To improve current operations, several functional enhancements have already been made:- A hoop building that previously occupied a portion of the drying pad was removed, increasing the available space for better staging and separation of material.
- A compact track loader, funded in the FY25 budget, has been acquired and is now used to manage the pad more effectively—separating freshly deposited waste from older, drier material to reduce cross-contamination and minimize odors.
- The maintenance schedule has been adjusted to allow for more frequent material movement and improved drying conditions.
Looking ahead, a concept-level redesign of the drying pad is being considered as part of the WRRC’s broader Facility Plan Update. The goal is to improve drainage, handling efficiency, and odor control, while maintaining this critical service to the community and surrounding region.
Facility Plan Update
To guide future investments and ensure regulatory and operational readiness, the City Council funded a comprehensive Facility Plan Update currently underway at the WRRC. A facility plan typically examines:
- Projected wastewater flow and treatment needs
- Anticipated regulatory changes
- New or emerging technologies
- System capacity, performance, and resiliency
- Opportunities for energy and resource recovery
Importantly, odor potential and odor mitigation are also key elements of this study. Every process area—from preliminary treatment to final solids handling—will be reviewed for its odor-generating potential. Odor control will be factored into any recommendations for improvements, whether through design modifications, operational changes, or technology upgrades.
For existing systems that are expected to remain in operation for the next 20 years, we are also exploring strategies to reduce their odor impact over the long term. The drying pad, for example, is already under review as part of this planning effort.
Continued Commitment
The WRRC team remains committed to continuous improvement and community responsiveness. We recognize that odors can be disruptive, and we are working diligently to reduce their occurrence while maintaining safe, reliable wastewater treatment services for all Dubuque residents.
In December 2024, a gas leak caused an explosion in the WRRC's solids handling building. Thankfully, no one was injured, but the event severely impacted the solids treatment process. Since then, our priority has been restoring this system to full functionality. While the system is operational again, we are still in the process of bringing it back to full treatment capacity. And unfortunately, during that process, the facility may emit more odors than normal.
Even with these challenges, odor reduction remains a top priority. We continue to dose with hydrogen peroxide and ferric chloride to reduce emissions from the primary clarifiers. However, some planned odor reduction efforts have been delayed by the December incident. For example, the City Council provided funding for regular cleaning of our four anaerobic digesters on a rolling basis, ensuring each is cleaned once every four years. Over time, grit, sand, and other non-biodegradable materials can accumulate in the digesters, reducing their efficiency and potentially contributing to odors. Regular maintenance not only extends the lifespan of the digesters but also helps ensure consistent biogas production and reduces the likelihood of odor issues. We had planned to hire a contractor this winter to begin cleaning in late spring. However, given the impact of the incident, we’ve had to postpone this effort until the digesters return to full health.
To further improve our odor management, we’ve proposed a capital improvement project for the FY26 budget (beginning July 1, 2025) to install an odor monitoring system. This system would include electronic nose (e-nose) units placed around the facility’s perimeter to provide real-time data on odor emissions. Integrated with our Supervisory Control and Data Acquisition (SCADA) system, the e-nose units would allow us to make targeted, data-driven decisions to address odors more strategically.
The City Council has allocated funding to clean one of the four digesters each year and to increase chemical dosing operations. The dosing alone will likely reduce odor emissions to some effect. The cleaning of the digester in most need will be done in the next six months, followed by cleaning a second digester next summer. The planned dosing with ferric chloride to address struvite issues and cleaning the digesters will ultimately help operate the digesters as designed and minimize hydrogen sulfide emissions.
The Water & Resource Recovery Center (WRRC) has made significant progress in its long-term odor abatement efforts. Multiple odor-producing locations have been identified, with the primary clarifiers being the most prominent source. To date, the facility has implemented a two-pronged chemical treatment approach to combat odors. Hydrogen peroxide is being dosed at the headworks, which initially resulted in a 50% reduction of hydrogen sulfide in the primary clarifiers. Additionally, ferric chloride is being introduced between the anaerobic digesters and the centrifuge, serving the dual purpose of reducing hydrogen sulfide and preventing struvite buildup. The combined effect of hydrogen peroxide and ferric chloride treatment has been remarkable, reducing hydrogen sulfide levels in the primary clarifiers by nearly 90%. The success of the dosing was further evidenced when a mixer pulled from an anaerobic digester for maintenance showed surprisingly little struvite buildup. The WRRC is planning to expand its odor control measures in September by dosing the waste stream at a third location, before it enters the anaerobic digesters.
Later in 2024, one of the four digesters will be cleaned. Over time, inert materials such as grit, sand, and non-biodegradable solids can accumulate in the digesters. These materials can hinder the digestion process, reduce the effective volume of the digester, and lead to operational issues and increased odors. The budget now includes funding to clean each digester every four years.
The drying pad on the eastern end of the property is another source of potential odors. Earlier this month, that facility was cleaned and is now functioning efficiently. That eliminated the chronic buildup of standing septic water that produced odors near the softball fields to the east.
There will always be a need to mitigating odors at the WRRC. And it will continue to be a consideration when making short-term operational decisions and long-term facility improvements. The combined efforts to date reflect the WRRC's commitment to addressing odor issues comprehensively and effectively, with promising results already observed and further improvements anticipated.
Dosing the waste stream with ferric chloride between the digesters and the centrifuge with ferric chloride continues without issues. The data (e.g., dosing rate and hydrogen sulfide levels) is being collected to be used to determine the optimum dosing rates for both odor control and struvite control.
Dosing the waste stream with ferric chloride between the digesters and the centrifuge with ferric chloride continues without issues. And data is being collected to determine the effectiveness and appropriate dosing rates to reduce the formation of hydrogen sulfide and struvite.
The City Council adopted the Fiscal Year 2025 budget which includes $462,000 for chemical dosing with ferric chloride to reduce WRRC odor emissions. The recurring funding will allow for odor and struvite reduction within the waste stream based on the findings of the chemical dosing analysis.
Odor control dosing to start this week! The City entered into a contract with USP to start dosing the WRRC waste stream with ferric chloride between the digesters and the centrifuge to reduce the formation of hydrogen sulfide and struvite. The $80,000 analysis will determine the optimum dosing rate to address both odors and struvite production.
The Fiscal Year 2025 budget recommendation includes $462,000 for chemical dosing to reduce WRRC odor emissions. The recurring funding will allow for odor and struvite reduction within the waste stream based on the findings of the chemical dosing analysis.
The City’s consultant (USP) is on site this week to make final preparations necessary before starting to dose the WRRC waste stream with ferric chloride between the digesters and the centrifuge. Dosing with ferric chloride can reduce hydrogen sulfide and the formation of struvite. They are also performing an analysis to determine where the struvite producing substances (i.e., magnesium, ammonium, and phosphate) are within the treatment system. When these components come together in the right concentrations and conditions, they can precipitate out of solution and form struvite which clogs pipes and equipment. Struvite in the anaerobic digesters results in the release of more hydrogen sulfide, results in more odors.
As mentioned previously, 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 in funding to purchase chemicals to be used at the WRRC to reduce odor emissions. That request includes funding to dose the waste stream with ferric chloride at two locations to both reduce H2S and struvite within the waste stream between the aerobic biological treatment and anaerobic digesters.
In the past two weeks, the City’s consultant (USP) has been on site to complete the installation of the temporary storage tank and system to dose the WRRC waste stream with ferric chloride between the digesters and the centrifuge. Installation is expected to be complete in the next week with dosing to follow in early March as previously planned.
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 in funding to purchase chemicals to be used at the WRRC to reduce odor emissions. An ongoing analysis determined that dosing the waste stream with hydrogen peroxide just before the primary clarifiers achieved a 50% reduction in hydrogen sulfide (H2S), the most prevalent odor-causing compound in wastewater. It was also determined that dosing the waste stream with ferric chloride at two locations could both reduce H2S and struvite within the waste stream between the aerobic biological treatment and anaerobic digesters.
Chemical dosing equipment was delivered to the site along with a chemical storage tank. This equipment will be utilized to analyze the effectiveness of chemical dosing at two additional locations within the waste stream.
WRRC staff began sampling and testing struvite-producing nutrients and hydrogen sulfide (H2S) between the digesters and the centrifuge to establish baseline levels prior to chemical dosing. This will allow for determining the reduction in H2S and in struvite-forming nutrients that can be achieved by chemical dosing. H2S is the most prevalent odor-causing compound in wastewater. Struvite is a mineral compound primarily composed of magnesium, ammonium, and phosphate. Struvite crystals can accumulate on surfaces within the treatment system, including pipes, pumps, and heat exchangers. This accumulation can result in clogging and scaling issues, reducing the flow capacity of the system.
How is struvite related to odor abatement efforts? When struvite builds up in wastewater treatment systems, it can trap and hold onto odorous compounds produced during the treatment process. When these deposits get disturbed, they release odors. Additionally, the struvite can affect the organisms in the system, affecting how organic matter breaks down and potentially creating more odors. Struvite can also interact with certain compounds, like those containing sulfur, leading to the release of gases like H2S. To deal with struvite, treatment facilities use methods like adding chemicals and regular cleaning to prevent or get rid of struvite deposits. This helps keep the treatment process running smoothly and reduces the chances of unwanted odors.
The City’s consultant (USP) will be on site in early February to complete the installation of the temporary dosing system and initiate the analysis at the site between the digesters and the centrifuge.
Identify Dosing Locations
Since reporting last month, the City’s consultant (USP) has identified two additional locations to dose the waste stream to further reduce H2S emissions from the WRRC. Their scope of services involves dosing the system with ferric chloride (iron salt). In addition to binding up sulfur and reducing the production of H2S, iron salts can bind with orthophosphates and minimize the production of struvite within the system. Struvite is a mineral composed of magnesium, ammonium, and phosphate. It often forms as a crystalline precipitate in anaerobic digesters and pipes causing scaling and clogging of pipes. Dosing with iron salts will both reduce H2S emissions and improve the efficiency and effectiveness of the treatment process. Dosing of the waste stream between the digesters and the centrifuge is expected to begin in early February.
During the week of February 18, USP will be on site to quantify nutrient loading in the WRRC treatment train to help identify the optimum locations for additional ferric chloride dosing. One of the anticipated dosing locations is at the aeration tank overflow. Dosing with ferric chloride at a second location is anticipated to begin in March. The anticipated dosing rate will be calculated prior to the initial dosing such that the benefits will be immediate. However, it will take some time to adjust the dosing rate to where it achieves the desired H2S and struvite mitigation without negatively impacting the treatment process. Once the optimal dosing rate for irons salts is determined, dosing with additional hydrogen peroxide will be evaluated to further reduce H2S emissions.
Short-Term Odor Abatement (Resolved October 2024)
The short-term odor issue is related to a wiring error by a private contractor during a routine inspection that resulted in inoperable equipment. 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 the waste stream. To adhere to permit limitations, waste must be temporarily stored in the 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.
There are three main steps to eliminate the short-term odor issue:
- Nurse the anaerobic digesters back to health and return to normal waste handling.
- Remove the waste being temporarily stored in the excess flow tanks.
- Clean the excess flow tanks.
Staff have addressed the shorter-term odor issue associated with the waste stored in open tanks at the WRRC. Any current odor issues are likely associated with the facility's anaerobic digesters. The City Council has allocated funding to clean one of the four digesters each year and to increase chemical dosing operations. Dosing at a second location with ferric chloride will be underway in the next few weeks as staff work to line up a contractor to install the dosing port. The dosing alone will likely reduce odor emissions to some effect. The cleaning of the digester in most need will be done in the next six months, followed by cleaning a second digester next summer. The planned dosing with ferric chloride to address struvite issues and cleaning the digesters will ultimately help operate the digesters as designed and minimize hydrogen sulfide emissions.
As of Sept. 27, the excess flow tank is clean. Only a thin, dusty film remains. The plan is to rinse it out the week of Sept. 30 - Oct. 4. Any recent odors would have likely originated from the eastern end of the WRRC property where a contractor has been mixing wetter material from the tank with drier material before loading and hauling the waste material to the landfill. As we were getting to the last bit of waste in the tank, it became wetter. This process likely gave off more odor, but was necessary. There is still some wetter material on the drying pad that will be dealt with this week. We will then deal with the dryer, less odorous material. It will take some time to load, haul, and dispose of the dry material as the landfill has a limited capacity to accept loads each day. But again, that material is much less odorous than the wet material. While we have been making progress over the past few weeks, we know we have more work to do. We know there are odor issues to address, and we remain committed to addressing them in a timely manner.
This week, the Water & Resource Recovery Center (WRRC) successfully completed the process of removing the waste temporarily stored in an excess flow tank. It was this waste that was causing the shorter-term extra ordinary odors originating from the WRRC. The three-month process involved mixing the waste within the tank and then pumping into trucks to be hauled away to other treatment/disposal facilities. Disposal of the waste was limited to facilities that approved it, with one facility taking nearly two months to complete its review. The effort was further complicated by the addition of approximately 280,000 gallons of rainwater which increased the volume of the waste mixture to manage. In total, 376 truckloads were required to dispose of 1.9 million gallons of waste. The mixing process involved pumping waste from the bottom of the tank and spraying it across the surface. This agitation released trapped gases and odorous compounds into the air, allowing them to disperse quickly to surrounding areas. The spraying action further facilitated the spread of these odors, making them more noticeable in the vicinity. That process has been completed.
Currently, about 100,000 gallons of the waste (mixture of waste, water, and debris) remains in the tank, but the odor potential has significantly diminished. Moving forward, contracts are being finalizing as necessary to deal with the remaining waste mixture and clean the tank. The goal is to eliminate all remaining odors from the tank by the end of September.
Next week the City will start removing the waste that is causing the short-term odor issue at the Water & Resource Recovery Center (WRRC).
As presented previously, the anaerobic digesters needed to be brought back “online” to function within normal operational parameters. That was achieved on April 2, and waste handling returned to normal operations at the WRRC at that time. Now, to eliminate the odor for good, the 1.7 million gallons of waste must be removed from the temporary storage tanks.
Identifying potential disposal options required testing, analyzing, characterizing, and quantifying the waste. Potential disposal options considered involved either treating the waste at the WRRC, hauling it to an offsite treatment facility, or a combination of the two. While the WRRC can treat some of the waste, it can only accommodate a limited amount which would require several months to deal with all the excess waste. Therefore, efforts focused on identifying offsite disposal alternatives in accordance with applicable environmental regulations.
Elimination of the waste and associated odor will involve mixing the waste, pumping it into trucks, and hauling it to other treatment facilities in the region. The estimated time to remove all the waste and clean the temporary storage tanks is 6-8 weeks. It will require over 300 truckloads to haul the waste away for disposal.
Restoring the anaerobic digesters was the essential first step to address the significant odors emanating from the WRRC. Initial schedules for mitigating the waste and odors were based on early projections indicated that the digesters would resume normal function by early January. However, the restoration process extended beyond initial estimates and wasn’t achieved until early April. Because the restoration process took longer, it necessitated the temporary diversion of more waste to the excess flow tanks, also affecting the timing and full characterization of waste disposal.
The second step was to characterize the waste that had been diverted to these tanks. This characterization could commence only after the digesters returned to full functionality and additional diversions ceased. This involves sampling and testing the waste at the source and in the excess flow tanks. We obtained the final test results last week.
The third step involves evaluating effective strategies for the treatment or disposal of the waste in the excess flow tanks. This includes compiling the water quality data, reviewing Iowa Code, and assessing the time and financial implications of each method. Initiated in January, this comprehensive evaluation is now progressing with the completion of testing, allowing for data analysis and comparison against disposal standards.
Currently, five disposal options are under consideration, ranging from on-site treatment to off-site hauling. Given the atypical use of the excess flow tanks for storage, this phase includes consultations with the Iowa Department of Natural Resources. While budget constraints are acknowledged, financial flexibility in the FY24 and FY25 budgets will accommodate necessary adjustments to implement the optimal disposal strategy. We anticipate concluding this phase by the end of May.
The fourth step is to implement the selected waste treatment or disposal method or methods. The approach will vary; options include seeking bids to hire a contractor for waste removal for land application or negotiations to lease equipment to partially treat the waste at the WRRC. Although the physical removal of the waste is expected to be brief, preliminary activities such as contract negotiations or equipment acquisition could delay the start of operations until July.
Simultaneously, we are exploring short-term solutions to mitigate odors from the excess flow tanks. The formation of a semisolid grease cap on the waste surface has notably decreased emissions. To further reduce odors, we are considering a temporary cover enhanced with chemical treatments to adjust the pH level. This intervention will be expedited once deemed safe and effective.
In adherence to the IDNR-issued NPDES permit, and recognizing the critical nature of the situation, the complete elimination of the waste and its associated odors remains a top priority.
The City Council adopted the Fiscal Year 2025 budget which includes $75,000 per year to clean each of the four digesters once every four years. More frequent cleaning 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 also reduce hydrogen sulfide emissions.
Health of the Digesters
All four digesters are now fully functional!
Even though the odors from the temporarily stored waste are now noticeably less, it remains a priority to remove the waste from the excess flow tanks. WRRC staff has identified multiple disposal alternatives, from reintroducing the waste into the WRRC treatment process to hiring a private contractor to collect and haul the waste from the WRRC and dispose of it in accordance with environmental regulations. While waiting for the results of a 20-day test, the benefits of each disposal option are being assessed (e.g., regulatory constraints/requirements, time required, cost, etc.). Pending the outstanding test results and input from the Iowa Department of Natural Resources, the City will pursue the optimal option to minimize odors from handling the stored waste.
The Fiscal Year 2025 budget recommendation includes $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 odors.
Health of the Digesters
Having avoided 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 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 from the south tank is slowly draining back into the WRRC wastewater treatment system. The surface of the waste is no longer being disturbed as a result of adding additional waste to the tank. Instead, the surface of the waste has solidified creating a barrier on the surface of the waste that reduces the free release of odorous gaseous discharge from the waste. Therefore, the odors emanating from the WRRC are noticeably less now than they were when waste was initially redirected into the excess flow tanks back in November of 2023.
But WRRC staff continue efforts to identify disposal alternatives such as having 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. Additional sampling is planned for this week as we try to establish all viable methods to dispose of the waste in accordance with state environmental regulations. Once all options are identified, we will take steps to remove the waste from the storage tank as soon as possible.
Health of the Digesters
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 can be a two steps forward, one step back process. In addition to hauling in “healthy” bacteria from Iowa City and feeding the system with medicine in the form of caustic soda, growing the bacteria population has been slow. However, by avoiding significant setbacks, the digesters are getting closer to full functionality and the system characteristics (i.e., acid levels, alkalinity, pH, methane gas production) are nearing normal operational values. If the current trend continues, three of the four digesters are expected to be back operating at full capacity within the next few weeks. During this process, the amount of waste fed to the digesters will continually increase.
To eliminate the short-term odor issue, the waste diverted from the normal treatment process must be removed from where it has been stored in the excess flow tanks. As previously reported, the north tank has been emptied. Waste from the south tank is now slowly draining back into the WRRC wastewater treatment system. It could take months to reintroduce the stored waste back into the WRRC treatment system. For this reason, WRRC staff sought quotes from private contractors to collect the waste, haul it from the WRRC, and dispose of it in accordance with environmental regulations. Before that can happen, the waste must be sampled, tested/analyzed, and characterized as disposal options depend on the waste characteristics. Sampling and testing were initiated in early February with initial results reported the week of February 12. The full test results will dictate the disposal plan to be followed by the contractor. The goal remains to remove the waste from the storage tank as soon as possible.
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 $75,000 per year to clean each of the four digesters once every four years. 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 maintenance helps extend the lifespan of the digesters, promotes consistent biogas production, and reduces the likelihood of undesirable side effects such as foaming and odors.
Health of the Digesters
The health of the digesters continues to improve. WRRC staff has been able to feed more waste to the digesters. To assist with the rehabilitation, sodium hydroxide is being added to increase the alkalinity which will prevent a drop in pH that would harm the microorganisms within the system that break down the waste material.
WRRC staff are working through the process of characterizing the waste temporarily stored in the excess flow tank. That work entails sampling the various sources of the incoming waste and within the tank itself. Based on the test results, the waste will be characterized and the options for disposal will be firmly established.
Health of the Digesters
WRRC staff have continued to take steps to return the anaerobic digesters back to good health. Approximately 90,000 gallons of anaerobic seed sludge were transported from Iowa City and pumped into the dormant, ailing WRRC anaerobic digesters. This introduces healthy bacteria into the system, raises the alkalinity, and dilutes whatever is afflicting the WRRC digesters. To increase alkalinity, roughly 12,000 lbs. of sodium bicarbonate was added to the system.
Two of the four digesters have been brought to within normal parameters. More improvement is required in the other two before returning to normal waste-handling operations. WRRC staff will continue to take steps to address the struggling digesters.
Disposal of Waste in Excess Flow Tanks
The waste temporarily stored in the north excess flow tank has been reintroduced into the waste stream. WRRC staff sought and received quotes to have the waste in the south digester removed and disposed of when it is no longer needed to store waste. Removal of the waste and cleaning by a private contractor could be accomplished in a 7-10 day period. Funding has been identified to fund this approach and the City will proceed when the tank storage is no longer needed.
Cleaning of the Excess Flow Tanks
With the waste removed from the north excess flow tank, cleaning will proceed when the weather allows. Cleaning of the south tank will be done at the same time the waste is removed. This could be accomplished by later February or early March, again weather permitting.
December 14, 2023, Update
While progress is being made to identify the location/source, and reduce the presence of, odor-producing gases and compounds at the WRRC, those improvements are currently being masked by odors created because of temporary operational changes – changes necessary to continue to receive and treat the city’s wastewater in accordance with the federal Clean Water Act. The operational changes were necessary due to an unfortunate error by a private contractor hired to assist with the annual inspection of equipment that helps ensure that the WRRC effectively treats the wastewater generated in the Dubuque community.
Error By Private Service Contractor Results in Inoperable Equipment
As the equipment manufacturer, the private contractor was hired because they would 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 rewired the unit to put it back into service. After finishing up unrelated maintenance of the unit, the equipment was restarted, and the contractor left the facility. A few hours later, there was an interruption of power on the public grid which caused the equipment to shut down. WRRC staff could not restart the equipment when power returned a few minutes later.
The contractor returned to try and help troubleshoot the equipment but left without having resolved the issue, believing that it wasn’t related to the equipment but had something to do with the WRRC computer system that controls the equipment. That left WRRC staff and a second contractor, a computer program logic controller (PLC) contractor, to try and troubleshoot the issue. They tried a variety of things to rule out various elements of the controller system. 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.
Inoperable Equipment Results in Unprecedented Odors
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 a week helps to maintain the delicate balance and relative population of the microorganisms in the anaerobic digesters. As a result of the inoperable equipment, two of the four digester units became dormant. They could no longer accept and treat the waste stream. The other two remained active. However, to ensure that the discharge from the WRRC remains within permit discharge limitations, WRRC staff had to limit the waste stream sent to the two functional digesters. To accommodate this, waste is being temporarily stored in tanks where they normally are not. As a result, the odor-producing gases can readily escape into the atmosphere. This has resulted in what might be unprecedented odors coming from the WRRC.
Efforts to Reduce Odor-Producing Gasses & Compounds
WRRC staff have been doing what they can to bring the dormant systems back to life. That has included collecting digester seed sludge from Iowa City’s wastewater treatment facility and introducing it into the dormant digester units to try and jump-start the regrowth of the microorganism ecosystem.
The effectiveness of the steps taken to date will be assessed during the week of December 18, 2023. If the system shows clear signs of recovery, then more of the solids waste stream will be sent to the digesters for treatment. This will be done gradually to ensure that the system can handle the waste. If the system reacts well to the additional waste, the WRRC might be able to return to normal operations by early January. Unfortunately, that will not eliminate the odors as it will take some time to reintroduce the waste that is being temporarily stored in the tanks exposed to the air. Again, that material will have to be slowly reintroduced and metered back into the treatment process. But if the system comes back as described, the temporary tanks will be emptied and cleaned in February.
Recognizing the failure of the contractor and the City’s costs that have resulted, not to mention the intangible effect the odors have on citizens in the community, discussions are underway with the City Attorney's Office to explore possible legal remedies due to the contractor’s error.
As stated previously, progress has been made to identify the location/source and reduce the presence of odor-producing gases and compounds at the WRRC as measured by the reduction in hydrogen sulfide. It has been determined that a 50% reduction in H2S can be achieved if between 50 and 60 gallons of peroxide per day is added 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. The odor reduction evaluation doesn’t end there.
Next Steps for Odor Reduction
The next step for the odor reduction evaluation to start in early January is to evaluate the odor reduction benefits of adding a combination of peroxide and ferric salts (iron salts) to the waste stream at other strategic locations. In addition to binding up sulfur and reducing the production of H2S, iron salts can minimize the production of struvite within the system. Struvite is a mineral composed of magnesium, ammonium, and phosphate. It often forms as a crystalline precipitate in anaerobic digesters and pipes causing scaling and clogging of pipes. Therefore, dosing the waste stream before it enters the anaerobic digesters will be evaluated. It may take anywhere from eight to fourteen weeks to determine the optimum dosing rates, but odor reduction benefits could be realized right away. The evaluation will also include determining the effective dosing rate and combination of adding peroxide and iron salts in the waste stream before it enters the centrifuge. Determining the effective dosing rate at this location should only take about four weeks once dosing is initiated.
Background Information
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 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 (H2S) and mercaptans. H2S is also called “sewer” gas and is 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 at the WRRC. 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 force main / pressurized sewers,
- blended sludge storage,
- waste-activated sludge storage,
- and anaerobic digestion.
Even though odor-producing compounds will always be present at the WRRC, some steps can be taken to minimize the release of 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 feed 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.
For additional information, please email staff or call 563-589-4176.