Biofilters and Biotowers For Treating Odors—Nceexperience at Various Empty Bed Contact Times and Resulting Odor Elimination Capacity Curves

The objective of this paper is to provide information related to odor control design criteria used in sizing and selecting biotechnology-based odor control systems and to provide odor removal performance data obtained from a variety of different types of biotechnology-based odor control systems applied in full scale at waste water treatment plants.Biofilters are typically sized to provide sufficient contact time for the odorous compounds to be absorbed, adsorbed and bio-degraded. This contact time is known as the empty bed residence time (EBRT). This paper will include data for specific odor causing compounds and overall odor reduction in terms of odor concentrations as measured by odor panels. Performance will be presented in terms of both Empty Bed Retention Times (EBRTs) and field experienced elimination capacity plots for several key odor compounds.Typically the odorous compounds of concern in WWTP applications include relatively high levels of hydrogen sulfide and lower levels of reduced sulfur organic species such as methyl mercaptan, dimethyl sulfide, and carbonyl sulfide. EBRTs for odorous compounds typically found in wastewater collection and treatment systems vary with the type of system and the media selection. For in-ground, open vessel biofilters EBRTs of 30 to 120 seconds are common, with a nominal EBRT of 60 seconds for organic based media systems and 60 to 120 seconds for soil systems. For vendor supplied enclosed vessels with propriety “high rate” media, EBRTs of 30 to 45 seconds are fairly common. Biotower systems are typically operated at significantly lower EBRTs than Biofilters with EBRTs in the range of 10 to 30 seconds being common, but field experience does exist for even lower EBRTs which will be presented in this paper.Performance data have been collected from over 55 operating biotechnology-based odor control systems including soil-based biofilters, organic-based biofilters and inorganic/inert media biotowers. EBRTs for the various systems range from 1.8 to 177 seconds. All biotechnology-based odor control systems tested were treating foul air collected from wastewater treatment plant process units. Sources of odorous air loads included wastewater collection system lift station wet wells, plant headworks facilities, primary clarifiers, trickling filters, sludge dewatering and sludge or sludge cake storage facilities. Performance data collected include both total odor and compound-specific removal efficiencies.The following conclusions are drawn from the available data.Biotechnologies are a proven approach to Odor control at WWTPs.Biofilters are routinely selected as technology of choice due to ease of operation and proven performance as well as the lack of complexity and the fact that no chemicals are needed.All systems tested achieved > 84% H2S removal efficiency. This included an inert media biotower with a contact time of 1.8 seconds. Average removal rates were between 96 – 99% for the media types tested. In general terms, odor and TRS removal rates were highest for organic-based systems, followed very closely in performance by soil biofilter systems, and lowest for the inorganic/inert media biotower systems.Bioscrubber and Biotower systems are developing an increasingly proven track record and are considered a field proven technology.Full scale Biotower results show promise in dramatically reducing Biotechnology system footprints while still maintaining high removal rates for H2S as high as 99% removal levels with EBRTs in the range of 10 to 15 seconds.Overall odor unit removal with Biotowers may require higher EBRTs approaching 20 to 25 seconds.