Biological air-pollution control technology* has been used extensively in Europe for decades and, to a lesser extent, in North America for the past two decades. Most installations in the U.S. have been on air-emission vent streams that were not subject to numerical limits (i.e., odors). Several factors influenced the limited use of biooxidation equipment in the U.S. First, the relatively large size of the units made applications in existing industrial areas virtually impossible because there simply wasn’t enough room. (A typical footprint for a single, 1-m deep, compost-bed biofilter to treat an 85,000-m3/h stream would be approximately 25 m × 30 m.) Second, the weight of these biofilters precluded their placement on a building roof, as is sometimes done with thermal oxidizers. Also, the compost-based media (used exclusively in the 1970s and 80s, and still in use today) are subject to channeling, compaction, bed collapse and short-circuiting of air flow (2). For this reason, as well as the relatively fragile nature of these biological systems and the often variable concentrations of contaminants in the emission streams, regulatory agencies believed that biofiltration was not a reliable technology. Industry was reluctant to install a control device that was looked upon by the regulatory agencies with skepticism. Finally, the compliance assurance monitoring (CAM) regulations of the 1990 Clean Air Act Amendments require verification of control equipment performance on a continuous basis, which is difficult to accomplish for biofilters because of the inherent biological variability (metabolically and by species of microbes).