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EBCT was required to remove H 2 S (hydrogen sulfide), while the other 40 seconds were required to remove the volatile organic compounds (VOCs) in the inlet air. Even flawed, the EBCT of 60 seconds seemed to work. However, it is easy to see that the different sizes of media (from fine compost to large sticks and branches) have vastly different surface areas, porosities, unit weights, and head losses. Over the ensuing years, engineers have been able to sample more and more inlet and outlet odor concentrations, drainage water constituents, and other operating parameters to assist with design criteria, but the lack of specific testing protocols (such as ASTM) places major design emphasis on the old EBCT criteria, which is used to this day. Although the EBCT design criteria is seriously flawed for organic media biofilters, it continues to be a point of contention in the design of these very effective and important odor control technologies. Some vendors of packaged biofilters advertised lower EBCTs for their products because they are "better" and deserve a lower EBCT design criteria. In fact, with no other standardized guidance criteria, a lower EBCT results in a smaller biofilter, which translates into lower cost. Some package system suppliers advertised extremely low EBCTs for their systems without any basis in fact that they would work at that rate, simply to be lower cost and win the job. It is often claimed in the literature that a certain packaged biofilter "only requires 20 seconds of EBCT to remove the odor" when, in reality, the fine print says "odor is H 2 S," not VOCs. Of course, it is known that VOCs require twice the time to remove as H 2 S, but the ploy works. Biotrickling Filters The same situation has happened with biotrickling filters. The basic process of a biotrickling filter is illustrated in Figure 4. Biotrickling filters (BTFs) are specialized biological odor scrubbers in that the biology is grown in special plastic media with very high surface areas, permeabilities, and odor removal rates. The odorous air is collected and forced through the specialized plastic media where certain species of biology grow to consume and remove the odor compounds. Irrigation water is sprayed on the top of the media and nutrients are added in some cases. It is important to note that some researchers and manufac- turers have invested heavily to test and progressively improve their biotrickling filter media to make it more effective and cost-efficient. This investment has allowed some structured biotrickling filter media providers to greatly outperform older-style media. Testing has shown that some new-generation biotrickling-filter-structured media have greatly surpassed the performance of earlier and more common media. Naturally, this investment is protected with patents and a certain amount of old-fashioned secrecy to prevent copying and ensure their market advantage. Figure 4: Common Components Of A Biotrickling Filter Odor Control Process While some manufacturers can claim total odor removal with much lower EBCTs (based upon higher-performing media), the engineering community is still skeptical of these claims in the absence of additional supporting design criteria. Summary It is clear that biological odor control technologies are in need of additional standardized design. It is not that the sampling and testing methods necessary to calculate the specific odor elimination rate (or specific odor compound elimination rate) in grams/day/cubic foot of media are not available. Nor is it that we lack the knowledge of how to establish standardized testing for biotrickling filter media so that we can consistently provide high- quality odor controls. We simply lack the organization, the will, and the determination to prepare the standardized criteria and persuade ASTM to include these criteria as part of its program. We stand on the brink of some breakthroughs in biological odor control processes, and we only have to reach out and grasp them. We must move biological odor control from "black box science" into the daylight of science. 12 wateronline.com ■ Water Innovations James Joyce, PE, has more than 36 years of experience in wastewater odor and corrosion control, and has written six manuals of practice and more than 50 peer-reviewed technical papers. Joyce has managed more than 500 odor and corrosion remediation projects worldwide and has conducted numerous technical workshops, seminars, and lectures. He holds bachelor's degrees in biology and civil engineering and a master's degree in environmental engineering from Virginia Tech. About The Author ODORANDCORROSIONCONTROL