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COMBINED-SEWER OVERFLOW CONTROL AND TREATMENT
Wojtenko**, I, C Fan*, R Field*, F Lai*, M K. Stinson*, AND T P. O'Connor*. COMBINED-SEWER OVERFLOW CONTROL AND TREATMENT. Presented at 5th Int Conf: Diffuse?nonpoint Pollution and Watershed Management, Milwaukee, Wisc, 6/1-15/2001.
Combined-sewer overflow (CSO), along with sanitary-sewer overflow and stormwater are significant contributors of contamination to surface waters. During a rain event, the flow in a combined sewer system may exceed the capacity of the intercepting sewer leading to the wastewater treatment plant, releasing a mixture of stormwater and raw sanitary wastewater into the receiving water. This paper highlights the USEPA's NRMRL's recommendations for CSO contro9l and treatment. Source control and collection system control are important aspects of CSO management. Source control includes flow as well as pollutant control, since dissolved pollutants and pollutants associated with particulate matter are also abated. Collection system control includes overflow and sediment reduction approaches such as improved sewer design, modifications to catchbasins and improved regulator performance. In addition, CSO management involves improved storage and treatment practices. The paper discusses inline and offline storage systems, as well as in-receiving water systems. Three major types of treatment approaches are discussed: 1) physical (screening nd sedimentation), where the benefits of high-rate vs. conventional processes are emphasized; 2) physical/chemical (filtration and high-rate sedimentation): (3) biological (trickling filters, activated sludge, aerated lagoons, and rotating biological contactors). The EPA National CSO Control Policy requires CSO disinfection after primary treatment in areas where required by local authorities. The most commonly used disinfecting agent is chlorine (Cl2). Because of the high flowrates and volumes associated with CSO, proper treatment often requires high CL2 concentrations, and thus results in high levels of toxic by products and Cl2 residuals in the receiving waters. Accordingly, alternative disinfectants (e.g., ultraviolet light, chlorine dioxide, ozone and peracetic acid) are being evaluated. In addition, due to the high flow characteristics of CSO, high-rate disinfection processes with increased mixing intensity are being recommended. Furthermore, high levels of suspended solids (SS) in CSO was found to decrease disinfection effectiveness, raising the importance of proper treatment to remove SS prior to disinfection.