The City of Gloucester, Massachusetts owns and operates a primary wastewater treatment facility (WWTF) that serves over 30,000 people and a significant commercial/industrial customer base. After odor complaints increased, an odor control study was performed in 2001 by a local engineering consulting firm. This study was comprised of a review of wastewater loadings and processes at the WWTF. As part of the study odor sampling and analysis was conducted in the summer of 2001. Results obtained from this analysis were used to rank the process units in odor producing capability by odor mass, intensity, and character. The study recommended that a biofilter be used to treat foul air from several of the process systems and that the construction be phased over several years. In 2003, Brown and Caldwell (BC), Boston, Massachusetts, was selected by the City of Gloucester to confirm the recommendation of the 2001 study and to design the odor control improvements for the WWTF. A preliminary design report was performed by Brown and Caldwell in conjunction with Bowker & Associates, Portland, Maine, during the summer of 2003 that quantified odor sources and confirmed the selection of a biofilter as the preferred treatment technology. The work comprised a review of historical information, sampling and analysis at the WWTF in August and September 2003, and a preliminary design of the odor control system. Both SEA and BC sampling results confirmed the appropriateness of a biofilter. The BC report identified additional sources to be treated; determined air flow rates for all sources; presented a layout configuration for the ductwork and biofilter; and suggested operational and process changes that would assist in short term odor control at the facility. Design and construction of the proposed improvements was initiated in late 2003. Construction of the improvements was substantially complete in December 2005.