Biofiltration is an established economic air pollution control technology for removing organic contaminants in air. An engineered biofilter using synthetic media, such as activated carbon, has been developed that shows improvements in removal efficiency, biodegradation and space requirements over the existing filters. This carbon filter has been operated for > 11 months in a bench-top unit, accomplishing > or = 90 percent removal efficiency consistently in a stream containing 10 to 20 ppm of toluene. The mass transfer zone remained stationary and extremely short (1 to 4 seconds of empty bed contact time) during the entire operating period. Compared with existing filters, this engineered filter showed 40 to 80 times greater biodegradation efficiency under the selected conditions, resulting in a decreased space requirement and other advantages in equipment sizing. (Copyright (c) 1994 - Air & Water Management Association.)