In 2001, a new three-barrel siphon was constructed under Arizona Highway 101 on the western boundary of the City of Mesa. This area is at the intersection of a major east-west thoroughfare and loop highway in an area of dense commercial development. The siphon handles an average 30 million gallons per day (MGD) through a 54-inch gravity interceptor. A 24-inch air jumper is provided within the Baseline Road overpass structure. Shortly after commissioning the siphon, strong odors and numerous complaints were received in the vicinity of the siphon. A packaged biofilter was installed to treat odors extracted from the siphon tail box. Testing showed it achieved greater than 98% hydrogen sulfide (H2S) removal. However, poor treatment of reduced organic sulfur compounds caused continued odor complaints. A new multistage bioscrubberbiofilter system with added airflow capacity was the best option for improved odor control, given the extraordinarily high H2S concentrations of greater than 100 parts per million (ppm) in this part of the interceptor system. The new, higher airflow system was installed on the head end of the siphon to achieve improved fugitive emissions control in the upstream interceptors. The new system design incorporated a new inorganic media biofilter, preceded by a bioscrubber, and located on the upstream side of the siphon. Performance testing of the bioscrubber, operating at seven second residence time and pH 2 circulating solution, indicated greater than eighty percent removal efficiency for H2S. Removal of reduced organic sulfur compounds such as mercaptans, was negligible in the bioscrubber because of the low operating pH, but over 90% effective in the inorganic media biofilter. Overall H2S removal exceeded 99.9%. This project included several notable design features, including a cover system for the abovegrade biofilter, enabling the treated exhaust gases to be discharged through a stack or drawn through third stage activated carbon treatment. This enables improved vertical plume dispersion and greater dilution of treated exhaust gases, or polishing of the exhaust. The project achieved several goals including achieving extremely high odor removal in a robust multistage treatment system with minimum available space, and blending the architectural features of the system into the immediate surrounding area.