While the issue of odor control has been an important factor in wastewater treatment plant design in the past, new regulations require that engineers designing new or expanded plants employ even more stringent methods to contain or eliminate the release of both odors and toxic air contaminants (TACs). Meeting the demands of ever-increasing environmental legislation has become a major consideration for municipalities operating wastewater treatment plants and design engineers charged with altering an established process. The Clean Air Act of 1990 (CAA) presented a new set of challenges to the wastewater industry. As existing treatment plants expand, the issue of air toxic releases must be addressed. In addition to the federal regulations, new regulations at the state level must be considered. Regulated air emissions include smog-forming pollutants that result from combustion, reactive organic gases, and air toxics.