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How Odors form in Municipal Wastewater Collection Systems

Updated: Aug 24, 2020

One of the most important components of municipal wastewater treatment infrastructure in the United States is the wastewater collection system. Wastewater collection systems carry domestic, commercial, and industrial waste to a centralized wastewater treatment plant for proper treatment. Once the wastewater is delivered to the treatment facility, it must be treated per the standards set by local regulatory agencies. After treatment, the remaining water is discharged into a local river, lake, reservoir, or reused. The remaining solids are either land applied, disposed of, and sold as fertilizer. Without effective collection systems, centralized wastewater treatment would be impossible.

While this may sound like a simple system of moving waste from one point to another, it is an intricate process with many factors to consider. An important consideration are systems that can treat the odorous gasses produced from the collection system. Odors generated are problematic, and will cause complaints from nearby residents, reduction in property values, illnesses, and/or corrosion issues to existing infrastructure.

How are these Gases Produced?

Nearly every wastewater collection system functions under anaerobic conditions. This means that there is no free oxygen present in the system. There is a special type of bacteria, known as sulfate reducing bacteria, that thrives in this kind of environment.

Sulfate reducing bacteria thrive in these otherwise deadly conditions because they require no oxygen to survive. Instead, they utilize readily available sulfate ions (SO4-) to power their biological processes. Sulfate ions (SO4-) are abundant in wastewater. As these sulfate reducing bacteria biodegrade and break down organic materials present in the wastewater, they produce odorous hydrogen sulfide (H2S) gas as a byproduct.

You can learn more about the production of hydrogen sulfide (H2S) gas by sulfate reducing bacteria here and in the chart below.

There are many other potential odorous compounds that can be generated in the collection and treatment of wastewater. Four common odorous compounds produced during wastewater treatment and collection are listed below. Detection limit refers to the level of detectability in air by the human olfactory system measured in parts per million.

While all of the above gasses are byproducts of wastewater treatment, hydrogen sulfide (H2S) is by far the most common odorous contaminant found in wastewater collection systems. It has a very, very low odor threshold, and in most cases is the largest contributor to odor complaints. High levels of H2S can also create dangerous conditions, causing injury and/or death.

Factors that can affect the level of odorous compounds generated are the length of the collection system, the detention time of the wastestream in the collection system, ambient temperatures, areas that create agitation or turbulence of the wastewater (which creates off-gassing), and the amount of water versus solids.

The volume of water and the volume of hydrogen sulfide (H2S) are directly correlated. Indeed, the higher the volume of water present in the collection system, the higher the level of hydrogen sulfide (H2S) that can be produced.

How are these Gases Released?

These odorous gasses and other contaminants are typically released into the air at locations along the collection system where turbulence (i.e. violent or unsteady movement of air, water, or of some other fluid) occurs in the wastewater stream.

Sewage lift stations, pump stations and wet wells are common areas of disrupted or turbulent flow. As a result of turbulence at these points in the collection system, hydrogen sulfide (H2S) is stirred up and released into the surrounding air. If these stations lack proper odor treatment systems, municipalities will likely receive complaints of foul odors in the area. Corrosion of infrastructure will also likely occur.

Odor emissions from pumping stations, lift stations and wet wells can be extremely noxious. These foul odors can cause distress to the community and in high enough concentration, cause illness,, injury to the respiratory system, or even death. It is a public health necessity for municipalities to install effective odor control systems at the appropriate locations along their wastewater collection infrastructure.

What’s the Solution?

Controlling odors is no small task, and municipalities can employ multiple processes to treat the odors. Odor control systems mitigate and/or treat these gasses. Certain types of odor control systems can also greatly reduce corrosion within the collection system.

Collection systems are built to fit the needs of each community and as such, vary greatly in their design. As a result, each requires a unique odor control solution designed specifically for the application.


Odor control systems need to be designed to treat levels of high hydrogen sulfide (H2S), while also handling lower levels of other odorous contaminants present in the foul air stream. Collecting and testing samples of the foul air stream identifies and quantifies the odorous contaminants present. One can then design an odor control system that will deliver a high level of odor treatment, eliminating odor complaints, reducing corrosion, and creating safer environments.

To learn more about foul air stream testing, factors in designing an odor control system, and choosing the correct odor control technology for your application, please read our other blogs (coming soon):

Please click here to schedule a free consultation on any odor control challenges you’re facing.


Image: Jiang, Guangming & Bond, Philip & Li, Xuan & Kappler, Ulrike. (2017). The Ecology of Acidophilic Microorganisms in the Corroding Concrete Sewer Environment. Frontiers in Microbiology. 8. 10.3389/fmicb.2017.00683. 

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