Odor and air emissions control using biotechnology for both collection and wastewater treatment systems
Introduction
Wastewater collection and treatment systems often generate offensive odors, resulting in complaints from neighbors. Collection systems transport wastewater from residences, commercial and industrial facilities to wastewater treatment plants (WWTPs) for processing. Collection systems can include force mains, through which wastewater is pumped, and gravity sewers, through which the wastewater flows without pumping. Odors from collection systems generally come from anaerobic decomposition.
Both collection and treatment systems typically emit trace amounts of air emissions. Air emissions from the wastewater treatment plant are well understood, while air emissions from the collection system are not. Two general types of air emissions are regulated by federal, state, and regional air quality agencies: criteria pollutants and hazardous or toxic pollutants. A third type of air quality indicator – odor – is not specifically addressed under air quality regulations, other than by public nuisance requirements. Most of the odors generated within the collection system are sulfur-based compounds, the predominant compound often being hydrogen sulfide (H2S). Other sulfur-based compounds found in lower concentrations can also cause odor complaints because they are detectable at very low levels and tend to disperse below nuisance concentration levels relatively slowly. These compounds can also be difficult to treat. They are the family of total reduced sulfur (TRS) compounds that include methyl mercaptan, dimethyl sulfide, dimethyl disulfide, ethyl mercaptan, carbon disulfide, and carbonyl sulfide. Air emissions from the collection system results from air stripping, wastewater turbulence, and some byproducts of biologically mediated processes seen in the collection system and breakdown of volatile organic compounds. Odors and air emissions from collection systems can be released from manholes or transported in the wastewater to the WWTP, where they are released at the headworks.
In addition to sulfur-based odorous compounds, nitrogen-based compounds also can cause odors. Nitrogen-based odorous compounds include ammonia, the amine family of compounds (such as ethyl amine, trimethyl amine), indole, and skatole.
Odors are generated in WWTPs when the wastewater turns septic because of excess time spent in clarifiers, because of anaerobic decomposition of solids, or because of decomposition of nitrogen compounds, such as proteins, releasing ammonia and other nitrogen-based odorants. The same biological mechanisms and physical conditions inside the collection system can release air emissions.
Other sources of odor in a WWTP include dewatering, solids handling, and further processing, such as alkaline stabilization or composting. Each of these unit processes may release different odorous compounds; nonetheless, nuisance odors are generated. The focus of this paper is the performance of full-scale biotechnology-based odor and air emissions control systems operating at WWTPs and collection systems to treat sources of nuisance odors. Typically biotechnology has not purposefully been used to treat other air emissions, but incidental air emissions removal often occurs in these same devices as they treat odors. Some researchers have seen as great as 90% air emission reductions in units specifically designed for air emission biotechnology applications [1], [2], [3]. A bioscrubber vendor also has data that shows both high odor (>95% odor removal) and lower air emissions removals (in the range of 40–83% removal that is VOC compound-dependent) [4].
Section snippets
Background
Biofiltration has become a popular choice for treating odorous air streams [4], [5]. Others believe it can be used to treat air emissions, as well. Biofiltration is becoming popular because of improving reliability of these systems, and because it is a “green” technology that uses no chemicals and creates no issues of potentially hazardous media disposal. As odorous air and air emissions pass through the biofiltration system, odorous and air emission compounds are removed and then oxidized by
Biotechnology-based odor and air emissions control design criteria
Biofilters are traditional solid media systems that use organic-based media such as compost, bark or woodchips or other proprietary vendor-supplied media. In both biofilters and biotowers, the media provides the home for the microbes that consume the odorous compounds. In a biofilter system, the media itself provides the trace nutrients such as organics, nitrogen, potassium, and phosphorous that the microbes need to thrive. In biotower systems, these nutrients are provided in the humidification
Performance data
Performance data have been collected from a total of 34 operating biotechnology-based odor control systems including six soil-based biofilters, 11 organic-based biofilters and 17 inorganic/inert media biotowers. Several of these sites also looked at air emissions reductions, as well. But, none of the test sites specifically were designed for air emissions removal only. All biotechnology-based odor control systems tested were treating foul air collected from wastewater treatment plant process
Summary and conclusions
The following conclusions are drawn from the data presented in this paper:
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Biofilters, if designed and operated properly, are a reliable and often preferred approach to odor control at collection system and wastewater treatment plant applications.
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Biotowers are showing a rapid growth in full-scale system experience and appear to provide very high removal rates for H2S at empty bed contact times well below those typically required for biofilters.
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Biotowers do not appear to provide overall odor
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