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carbon type, and empty bed air velocities. Media bed air velocities between 50 and 100 feet per minute (fpm) are recommended, with 60 fpm being used in most municipal odor control cases. Combined with the maximum recommended media depth of 3 feet (based upon head losses), this results in a 3-second EBCT for the media. Although EBCT is used by engineers as part of the design process, it is based on the results of common scientific principles plus standardized carbon testing using prescribed testing procedures (e.g., ASTM D-6646 in the case of activated carbon). ASTM stands for the American Society for Testing and Materials, which has published standard test procedures for most everything related to engineering, materials, and the environment. The testing is performed in strict accordance with approved written testing procedures. Since the EBCT design criteria for activated carbon odor control systems is based in large part on science and standardized testing, the inaccuracy impact is low. Chemical Scrubbers Figure 2 illustrates the basic components of a standard chemical-based odor control scrubber. The vessel commonly contains a plastic chemical-resistant packing, which provides a large surface area for the chemical-laden water pumped up through spray nozzles. The high surface area allows direct oxidation of odor compounds by the chemicals in the water. A small volume of the scrubbing liquid is allowed to drain out the oxidized byproducts. This volume of water is replaced by makeup water and fresh chemicals. Chemical scrubbers are designed based upon the surface area of the plastic packing in the vessel, water and chemical concentrations and flowrates, and media contact time. The typical media EBCT for a chemical scrubber is between 2 to 4 seconds, with 3 seconds being common. Figure 2: Typical Chemical Scrubber Vessel And Media Configuration However, the EBCT for chemical scrubbers is heavily dependent upon the inlet odor concentration, the rate of chemical addition (chemical concentration), and the chemicals used. Higher inlet odor concentrations require higher chemical concentration, which is provided based upon chemical addition pumps controlled by sensors. Different chemicals also require different EBCT/chemical concentrations as the rate of oxidation of odor compounds is not uniform. Once again, EBCT is found to be a common design criteria but the reliance on EBCT is low, balanced by higher reliance on the inlet odor chemistry and the stoichiometry of the well-known chemical reactions taking place in the packing. Biological Odor Control Biological odor control processes are still considered a new entry into the odor control technology field, despite a 25-year history of excellent performance in the U.S. Biological odor control started with the development of "biofilters," which force odorous air through different types of organic and inorganic media designed to grow biological organisms. As the air is forced through the media, naturally present soil bacteria acclimate and grow on the media while consuming and removing the odor compounds before releasing clean air. Figure 3 is an illustration of a early in-bed, organic media biofilter. Figure 3: Common In-Ground, Organic Media Biofilter These biofilters used compost, crushed wood products, large tree bark, and many different types of media, all with different shapes, sizes, tree types, and densities. While engineers learned how to control and operate these biofilters through trial and error, there was no standardized media testing. Many mistakes were made using media that were too fine, too coarse, too weak, too large, etc. The biofilters that worked well were scrutinized just as much as the biofilters that failed, but there was no concensus on the reasons to adopt any of them, so the only common criteria that was adopted by most designers was EBCT. Since EBCT for an in-ground biofilter is directly related to media depth and foul air flowrate, these criteria were, and still are, major design criteria. The early biofilters that worked well had widely varying EBCT values. Evaluation of the EBCT data discovered that EBCTs below 60 seconds usually resulted in poor performance, while those above 60 seconds generally performed well. As a result, 60 seconds became the de facto EBCT for organic media biofilters. Further sampling and measurement of the exhaust from these biofilters revealed that approximately 20 seconds of wateronline.com ■ Water Innovations ODORANDCORROSIONCONTROL 11