EXTREME SIPHON ODORS CONTROLLED WITH MULTISTAGE BIOLOGICAL SYSTEM

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.