DYNAMICS OF SANITARY SEWER VENTILATION AND ODOR RELEASE IN THE HEIGHTS/HILLTOP AND SOUTHWEST/WEST LEG INTERCEPTOR SEWER SYSTEMS

Odor control in deep tunnel sanitary sewers rarely, if ever, consists of a single-step solution. Long-term increases in population naturally bring about increases in flow levels in sanitary sewer systems. These flow increases commonly result in changes in airflow dynamics and subsequent increases in deep tunnel pressurization and odor release. A successful deep tunnel odor control program should therefore consist of periodic monitoring of the deep tunnel sewer system to determine the odor-causing effects of both newly constructed sewers and increases in flow in existing sewers.In 1995, the Northeast Ohio Regional Sewer District (NEORSD) experienced odor complaints in the vicinity of the District's then recently constructed deep sewer tunnels, namely the Heights/Hilltop (HHI) and the Southwest/West Leg (SWI) Interceptors. Based upon an evaluation of the tunnel air pressures, the District constructed five biofilters at strategic locations on the deep tunnel system. These biofilters have been operating since 1996 and have successfully relieved the tunnel pressure and odor complaints.As would be expected, flows increased in these sewer systems over the five year period from 1995 to 2000. Given this and the fact that seven interceptor sewers are planned for construction over the next four years, an updated evaluation of the depressurization effect that the biofilters were having on the tunnel pressures was deemed necessary in the Spring of 2000. As a result of these tests much was learned about how the ventilation and pressurization phenomena have changed over the past five years in each of the two sewer systems.This paper will describe, in detail, the causes and effects of the deep tunnel pressurization phenomena both in the HHI and SWI sewers over the six-year period from 1995 to 2001. Data gathered during the recent tests will also be presented to substantiate the described pressurization effects of pipe diameter constrictions and drop structures in the system.