ULTRAVIOLET LIGHT DISINFECTION OF COMBINED SEWER OVERFLOW

The objective of this state-of-the-art review is to examine the performance and effectiveness of ultraviolet (UV) light disinfection for combined sewer overflow (CSO) applications. Topics presented include the use of UV light as an disinfecting agent, its practical applications, doseresponse relationships, and factors affecting its performance, including influent quality and type of UV system used. The efficiency of light disinfection is strongly dependent on the quality of the influent and the occluding effects of solids. This dependency is very important for treatment of low quality water such as CSO, and it is being investigated in pilot-scale studies.Disinfection of CSO significantly decreases the discharge of pathogens into receiving waters. The national CSO Control Policy requires disinfection in areas where it is required by state and local authorities after primary clarification (EPA, Combined Sewer Overflow Control Policy, 59 Federal Register 18688, April 19,1994). The high flowrates and volumes of CSO together with its high suspended solids content, variable temperature, and disinfectant-resistant microorganisms require use of high-rate disinfection techniques with powerful microbe-killing capabilities (Field, 1996). Conventional disinfectants, e.g., chlorine gas and sodium hypochlorite having rapid oxidation capabilities and relatively low cost are suitable for use in high-rate processes and are effective for CSO. However, due to the high flowrates, volumes, and chlorine demand of CSO, effective treatment requires a relatively high chlorine concentration potentially resulting in a high level of toxic byproducts and chlorine residuals in receiving waters. Because chlorine and its byproducts have a negative impact on aquatic life, alternative disinfection processes are been investigated. Based on our investigations, UV irradiation has a potential to be used in high-rate processes.UV light disinfection is a physical procedure that does not alter the smell or chemical composition of water. Its use eliminates the need for chemicals, their associated transportation, handling, and storage, as well as use of expensive dechlorination facilities (Stinson et al., 1999). It eliminates large disinfection facility buildings, large contact tanks, and associated real estate. Thus, the capital and operating costs of this technology as well as costly liability insurance premiums are greatly decreased. All of these benefits make UV light an attractive alternative to chlorination. UV is already being used for secondary and tertiary quality waters (Ashok et al., 1997). Laboratory and pilot-scale research indicates that UV light is a promising disinfectant for CSO applications. However, full-scale verification of its disinfection efficiency in CSO influent is still needed. Presently, there are no full-scale UV facilities in use today for the disinfection of CSO.