Updated: Aug 24, 2020
Wastewater collection systems are a vital element of municipal wastewater treatment infrastructure, but the nature of their construction leads to the production of odorous gases that must be treated as a matter of public health, safety, and livability. Collection systems transport domestic, commercial and industrial wastes from their source to the central wastewater treatment plant, prior to discharge into streams, rivers, lakes, or larger bodies of water.
How Odors Are Formed (Pt. 1 of this blog series)
The majority of wastewater collection systems function under anaerobic conditions. This means there is no free oxygen available in the system, which creates an environment for sulfate reducing bacteria, which do not require oxygen, to thrive in these otherwise deadly conditions. 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. The more water you have in a collection system the higher the concentrations of H2S you will have.
While there are other odorous compounds produced in collection systems, such as dimethyl sulfide (DMS), dimethyl disulfide (DMDS) & methyl mercaptan (MM), their concentrations are not as high as that of hydrogen sulfide. Nevertheless, they still need to be treated in order to stop odor complaints from the surrounding public.
Treatment of Odors
There are several methods of odor control that can be deployed to treat the airstream. They fall into two categories, liquid phase methods which treat the water to alter or stop the chemical reactions that produce odors; or vapor phase, methods which are treating the foul airstream after the gases have been released into the air following a point of turbulence in the collection systems’ flow.
The focus of this post is on the vapor phase odor control application utilizing carbon adsorption.
Carbon adsorption uses an activated carbon pellet media. Odor control is achieved when the contaminants adhere to the surface area of the carbon media, while clean air pass through and is released to the atmosphere.
As with any odor control technology, carbon adsorption has its pro’s & con’s. To properly design an odor control system, it is recommended the airstream first be tested to determine the various contaminants and their respective concentrations. That said, there are different types of activated carbon media designed for treating different applications.
Advantages of Activated Carbon
The main advantages of an activated carbon odor control system are:
Excellent odor removal capabilities across a wide range of contaminants
No acclimation period. The system will begin to treat odors immediately
Relatively low initial capital cost when compared to chemical scrubbers or biological systems
Simplicity of operation
Passive technology. Can be dormant for extended periods of time but work when needed.
Disadvantages of Activated Carbon
The main disadvantages of an activated carbon system are:
Not well suited for higher H2S concentrations (>20ppm average)
Media changeouts are expensive
Unplanned media changeouts, due to shortened lifespan, can negatively affect annual maintenance budgets
Media changeouts may involve confined space entry
There may be difficulty disposing of the spent media
When media is depleted there is immediate odor breakthrough leading to odor complaints, which may lead to odor complaints
Systems must be taken off-line during media changeout
Activated carbon systems are an effective solution for wastewater collections applications with low H2S concentrations. Carbon adsorption systems are the most popular overall type of odor control for wastewater applications. This is in large part driven by the low upfront costs to install the systems and the relative ease of operation. When considering which type of odor control solution to deploy for any given application, municipalities must make sure the application design is well suited to handle the contaminants in the airstream. They also need to consider the long term operating and maintenance costs associated with owning and operating the system. Applications that may require frequent and costly media changes are not a good fit for carbon adsorption technologies.
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):
Odor Control System Design Considerations for Municipal Wastewater Collection Systems
Foul Airstream Testing in Municipal Wastewater Applications