While there are several methods for controlling foul odors in municipal wastewater collection systems, vapor phase treatment solutions utilizing biological odor control are increasingly being adopted as the system of choice. There are several reasons for this trend. The first of which is their effectiveness in treating H2S, the most common and abundant odorous gas produced in wastewater conveyance systems. Other reasons include the fact that utilizing biological treatments provides a sustainable alternative to other options, as well as the long-term costs savings biological systems provide municipalities when compared to other treatment options.
What Are We Treating?
Our previous blog How Odors Form in Municipal Wastewater Collection Systems details the most common odorous gases produced during wastewater conveyance. H2S is the main contaminant that needs to be accounted for when designing an odor treatment system for a collections application, but systems must also be designed to handle the handful of other contaminants that lead to odor complaints.
How do Biological Odor Control Systems Work?
Naturally occurring microorganisms utilize the available odorous contaminants in the foul airstream as their food source (either a source carbon or energy). Any biological odor system is designed to create the optimal environment for these bacteria to colonize, grow, and thrive. To do this, the systems must include a media for the bacteria to colonize and also have moisture and proper temperature (50-105F).
The bacterial colony forms a biofilm layer that adheres to treatment media surfaces. For the odorous contaminants to become available as a source of energy or carbon the gas must first be dissolved into water molecules. Once dissolved, mass transfer into the biofilm can occur. At this point biological degradation of the odorous compound occurs and odors are treated.
There are two different microbial colonies that are relied upon in biological odor control:
Autotrophic microorganisms use hydrogen sulfide as an energy source and carbon dioxide as a source of carbon. Specifically, Acidithiobacillus is the autotrophic microorganism that degrades hydrogen sulfide.
Heterotrophic microorganisms rely on available organic compounds as both an energy and carbon source. These microorganisms are best suited for the treatment of volatile organic compounds (VOC’s) and other reduced sulfur compounds.
Why Two-Stage Designs Suit Collection Systems
H2S is the primary contaminant to be treated in wastewater conveyance systems, but there will always be lower levels of other reduced sulfur compounds and/or VOC’s present. Any of these compounds can create odor complaints if not properly treated.
H2S and the other odorous compounds mentioned above are best treated in separate treatment stages, as they rely on different microorganisms to break them down.
Because Acidithiobacillus is a very aggressive bacteria and always migrates towards the food source (foul air inlet), the first treatment stage should be designed for H2S removal. The volume of media required and empty bed residence time (EBRT) is directly proportionate to the inlet H2S loading. Common EBRT’s for this application are typically between 8-15 seconds. The airflow face velocity through this stage is also relatively fast at 20-70 feet per minute (FPM). Acidithiobacillus will naturally create a low pH environment because its waste stream is comprised of elemental sulfur and sulfate. The bacteria thrive at pH levels between 1.5-3.0. No other bacteria type can survive at these levels.
The second stage should then be designed to polish any remaining odorous compounds. It will require longer EBRT’s (15-25 seconds), and the airflow velocity is reduced to 5-15FPM. Heterotrophs will thrive in this stage, as long as the pH remains relatively neutral. The second stage is generally larger than the first as more time is needed to degrade the more complex compounds.
To learn more about two-stage biological odor control systems with long-life media options that treat a wide range of applications CLICK HERE
Biological Odor Control: Pro’s & Con’s
Extremely reliable and strong performance
Low life-cycle costs
Long-life media options
Few, if any, consumables
No continual chemical or nutrient feed required
Minimal maintenance requirements/O&M
Larger footprint due to lower face velocity requirements
Potential for musty or earthy type odors as a byproduct of the biological process
Above ground systems will need to factor in aesthetics at some higher profile lift/pump stations where landscaping may be required
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