Odor Control Efforts

Why do we sometimes smell odors near the wastewater treatment plant?

Odors are a natural part of the substances handled and treated at any wastewater treatment plant. 

Odors are typically contained to the wastewater treatment plant site; but occasionally odors drift from the plant site depending on weather conditions and wind direction. 

Routine treatment operations are designed to reduce the amount of odors present; however, certain weather conditions and equipment maintenance may lessen the effectiveness of these routine odor control operations.

What causes these odors?

Most of the odors detected in and around wastewater treatment plants are signals that nature’s treatment process is working; organic matter is decomposing and pollutants are being removed from the wastewater. 

As the table Odorous Compounds In Wastewater shows, three major odorous compounds naturally occurring in the treatment process, hydrogen sulfide, amines and mercaptans, are detectable by the human nose at extremely low concentrations.   

Odorous Compounds In Wastewater

Compound Name Recognition Threshold parts per million Odor Description

Allyl mercaptan


Disagreeable, garlic



Pungent, irritating

Amyl mercaptan


Unpleasant, putrid

Diisopropyl amine



Dimethyl amine


Putrid, fishy

Ethyl amine



Ethyl mercaptan


Decayed cabbage

Hydrogen sulfide


Rotten eggs



Fecal, nauseating

Methyl amine


Putrid, fishy

Methyl mercaptan


Rotten cabbage

From Table 2.1, Odor Control in Wastewater Treatment Plants, 1995, WEF & American Society of Civil Engineers

Were it not for odor control measures, all wastewater treatment processes are capable of emitting odors.

Several steps in the wastewater treatment process are notorious for emitting odors.  At the Columbia Regional Wastewater Treatment Plant (WWTP) these areas are described and their general locations shown on the aerial photo of the treatment plant below:

  1. The raw sewage influent pump station wet well is where wastewater first enters the plant after traveling many miles in the sanitary sewer mains.  At this site raw wastewater is exposed to the air on its way to treatment sites.
  2. Raw wastewater is transferred to the primary clarifiers where most solids are separated from the liquid portion of wastewater in the treatment process.  At this site odors are volatized by the  turbulence in the center wells and as the wastewater cascades over the effluent weirs and through the effluent channel.
  3. Wastewater undergoing aerobic digestion (decomposition with free oxygen) in the aeration basins emits a characteristically musty odor due to the particular type of biogases released in the process.
  4. In a similar fashion as in the primary clarifiers, odors from the partially treated organic solid portion of the wastewater in the sludge thickeners are volatized by the turbulence in the center wells and effluent weirs and channel.
  5. Digested biosolids rich in nitrogen emit an ammonialike odor (amines) while in the sludge storage lagoon awaiting final reuse as agricultural fertilizer.
  6. The WWTP burns biogases created in the anaerobic digesters to produce electricity and heat used on site.  If more biogas is produced than can be burned in the engine generator or boiler, it is burned in the waste gas burner.   If the waste gas burner fails or is impaired, a sewer gas odor from the escaping biogas will be present.

WWPT Odor Map

What is being done at the WWTP to reduce odors?

  1.  A ferrous chloride solution is added to the raw wastewater before it enters the primary clarifiers to reduce odors at that treatment stage.  Ferrous chloride molecules capture hydrogen sulfide molecules thereby forming insoluble compounds which precipitate out of the waste stream.
  2. Misting systems with odor neutralizing liquids are located at eight locations in the WWTP.  Odor neutralizing liquids break down the foul smelling chemical compounds in the biogases at each site.   These odor neutralizers are atomized into the air via high pressure fogging systems and fogging fans.  The wastewater treatment operators’ daily routine includes keeping all of the misting system components working properly.  On days when local weather conditions contribute to enhancing odors, WWTP staff increase the concentration of the odor neutralizing liquids in the misters.
  3. Chlorine gas is used to disinfect the non-potable water which is used daily to wash down all areas of the plant and control floating odor causing scum in the various areas where it can accumulate.
  4. Wastewater operators routinely check the digester pressure relief valves to make sure they are not venting to the outdoors and that the waste gas burner is performing optimally.

What can I do if I smell an odor I think is coming from the plant?

You may call our service number, 445-9426, to inform the staff you detect an odor.   The plant staff will ask you for the following information:

  • Your name, address, and phone number; 
  • Information about the odor; such as, what time you noticed it, is it still noticeable, a description of the odor (refer to the Odorous Compounds In Wastewater table) and how strong is the odor.

What happens when the WWTP receives an odor complaint?

Plant personnel receiving the call will record all of the complainant’s information described above; as well as,  temperature, humidity, weather conditions, wind velocity, and wind direction.  

All of the information combined helps us determine if the odor is from the WWTP or from elsewhere.  

If it is determined to be coming from the plant site,  this information will help direct the operators’ investigation to a likely location.   Operators will check all of the odor control equipment for proper operation.   These response tasks are recorded in the WWTP Operations Daily Log and the information is passed on to the next shift and supervisor.  Whenever possible we send a staff person to the complainant’s site to see if the odor is still present.