Performance of Wet Weather Treatment Facility for Control of Combined Sewer Overflows: Case Study in Cincinnati, Ohio

Efforts aimed at reducing pollutant loads from combined sewer overflows (CSOs) on the Muddy Creek receiving waters in Cincinnati, Ohio 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 (disinfection was inactive during this study). Calculations using hydrographs and water quality samples collected at the WWTF during rain events established the mass of biochemical oxygen demand (BOD)5, chemical oxygen demand, and total suspended solids (TSS) removed. Ten storm events sampled from January to September 2002 helped characterize pollutant removal efficiencies for flow-through treatment. Pollutant removal was classified into four components: flow to the wastewater treatment plant (WWTP), sedimentation, storage, and screening. Most pollutant removal was achieved through settling 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 load in the early portion of the CSO hydrograph, 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 investigated.