The Littleton/Englewood Wastewater Treatment Plant serves over 200,000 people in a mixed commercial and residential zone in the southern Denver metropolitan area. Light and medium industries are located west of the plant while commercial activities surround the plant on all sides. In some cases, warehouses and service centers are within 30 meters (100 feet) of wastewater treatment facilities. Residential areas are located to the east, extending up to the plant boundary, as well as being interspersed in the industrial and commercial area. A major shopping mall, Cinderella City, is within 1.6 kilometers (1 mile) of the plant, as is Englewood’s City Hall. Odor sources were targeted for systematic incremental improvements according to the relative intensity of the odor, pervasiveness, and potential for control and treatment. A simple premise guided development of a control strategy for each priority order source. Biological processes used for wastewater treatment can either produce foul air emissions or metabolize odorous compounds. By incorporating appropriate features into the design of these systems, existing odor sources could be mitigated and some processes could be used to treat foul air biologically. Conventional foul air scrubbing was considered a method of last resort with limited use for severe odor problems.