Combined Sewer Overflow Control: Performance of a Wet Weather Treatment Facility

Efforts aimed at reducing pollutant loads from combined sewer overflows (CSOs) on the Muddy Creek receiving waters in Cincinnati, OH have been underway in recent years. This includes an investigation of the treatment performance of a flow-through wet weather treatment facility (WWTF) using off-line sedimentation tanks, fine screening and chemical disinfection (which was inactive during this study). Calculations using hydrographs and water quality samples collected at the WWTF during rain events established the mass of BOD5, COD and TSS removed. Ten storm events sampled from January-September 2002 helped characterize pollutant removal efficiencies for flow-through treatment during various runoff events. Pollutant removal was classified into four components: flow to the wastewater treatment plant (WWTP), treatment (sedimentation), storage and screening. Most pollutant removal was achieved through sedimentation and storage in the treatment tanks, with removal efficiencies of 20-50% for BOD5 and 25-70% for TSS commonly observed. Owing to the high pollutant concentration in the early stages of the rain event, first-flush containment, or capturing and conveying the early portion of the runoff event to the WWTP, was the most efficient treatment method for every storm.