Odor Control

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. 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:

  1. Nurse the anaerobic digesters back to health and return to normal waste handling.
  2. Remove the waste being temporarily stored in the excess flow tanks.
  3. Clean the excess flow tanks.    

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.

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: 

  1. Identify dosing locations.
  2. Set up temporary dosing stations (tanks to store chemicals and equipment that controls measured dosing rates).
  3. Determine the effectiveness of dosing (optimum chemical and dosing rate).
  4. 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.

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.

For additional information, please email staff or call 563-589-4176.