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.
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.
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.
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.
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.).
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.
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.
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.