Bioscrubbers, also known as biotrickling filters, have been used to control odors at many North American wastewater treatment plants since the 1970s. The primary objective of this study was to examine the long-term performance of some of these existing bioscrubbers. The paper describes several of these North American bioscrubber installations, discusses specific data from three long-term operating facilities and one new facility, and provides design criteria for new facilities. A bioscrubber is physically similar to a chemical scrubber, but chemicals are replaced by a fixed-film biomass formed on lava rock, plastic media, or foam. Typically, secondary effluent is passed over the filter media, and soluble, odorous gases and oxygen are continuously absorbed into the fixed film of the bioscrubber (sometimes called the slime layer), where microorganisms induce biological oxidation of the absorbed pollutants. Custom designed bioscrubbers have been operating for at least 20 years at the California agencies of Eureka, Dublin San Ramon, Aliso, Encina, and Sacramento, and at Manukau WWTP in Sydney, Australia. Four bioscrubber facilities are discussed in greater detail. These bioscrubbers range in air flow capacity from 12,000 to 94,000 cubic feet per minute (cfm) and have varying detention times, media types, and liquid flow rates. They were all installed to handle air streams with relatively high hydrogen sulfide loads and to act as a first stage, roughing scrubber.