DESIGN CHALLENGES IN SEWER FOUL AIR EXTRACTION AND TREATMENT

Several large diameter interceptors in the metropolitan Phoenix area produce significant fugitiveodor emissions. Wastewater velocity is relatively fast, causing liquid-induced airflow and air pressurization in the sewer headspace and out-leakage of foul air with H2S concentrations routinely exceeding 100 parts per million (ppm). Chemical odor control treatments and sealing manhole covers has only been partially effective in controlling these odor emissions.The Sub-Regional Operating Group (SROG – representing five cities in the Phoenix area) initiated a project to test air extraction from the sewer headspace with the long-range goal of installing several air treatment systems along odorous interceptors. The foul air extraction tests were conducted with a mobile, trailer-mounted fan and variable speed motor, with a flexible duct hose connected to a sheetmetal manhole adapter. Tests were conducted on three sections of interceptor ranging from 42- to 90-inch diameter. Airflow extracted from the sewers ranged from 2,700 cubic feet per minute (cfm) to 5,200 cfm.The sewers tested typically exhibit pressures between +0.10 and +0.25 inches water column (w.c). The extraction tests successfully reversed this pressure so that sewer headspaces were under negative pressures ranging from −0.03 to -.80 inches w.c. The magnitude of negative pressure diminished as distance from the foul air extraction point increased. The distance from the extraction point in which sewer headspace pressure was influenced was typically about 5 miles. Headspace H2S concentrations were routinely between 50 and 200 ppm and were not reduced significantly by forced ventilation of the sewer headspace.The study concluded with a comparative evaluation of odor control performance of a full scale bulk media biofilter and chemical wet scrubber in service on the interceptor system. Total reduced sulfur (TRS) and odor removal were tested on each system. Both systems achieved greater than 99 percent H2S removal. Odor and TRS emissions were better controlled by the bulk media biofilter.