ASSESSING THE EFFECTIVENESS OF SEWER HEADSPACE FOUL AIR EXTRACTION

Several large diameter interceptors in the metropolitan Phoenix area exhibit hydrogen sulfide concentrations in excess of 100 parts per million (ppm) in the headspace of the sewer. Additionally, wastewater velocity is relatively fast, causing liquid-induced airflow and air pressurization in the sewer headspace. Chemical odor control treatments and sealing manhole covers has only been partially effective in controlling fugitive odor emissions into commercial and residential areas near these interceptors.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. However, major sewer junction structures limited the extent of pressure influence.Hydrogen sulfide (H2S) concentrations were continuously monitored and data-logged before, during, and after the extraction tests. This data indicated that H2S concentrations were routinely between 50 and 200 parts per million (ppm) and were not reduced significantly by forced ventilation of the sewer headspace.