Testing the equivalency of ultraviolet light and chlorine for disinfection of wastewater to reclamation standards

Full-scale ultraviolet (UV) light and chlorine disinfection systems were operated in parallel using nitrified and partially denitrified tertian, treated wastewater effluent. A UV dose of 75 mW . s/cm(2) reduced the concentration of fecal coliforms, enterococci, fecal streptococci, MS2 bacteriophage, and poliovirus by four logs. A higher dose was needed to reduce the concentration of heterotrophic plate count (HPC) by four logs and to achieve a total coliform value less than or equal to 2.2 MPN per 100 mL within a consecutive 7-day period as required by the most restrictive California Wastewater Reclamation Criteria (CWRC). The cleaning frequency needed to maintain a minimum operational UV dose was assessed by constructing a lamp fouling curve describing reduction in UV intensity as a function of elapsed time since lamp cleaning, Substitution of UV light for chlorine disinfection eliminated formation of trihalomethanes, reduced formation of aldehydes, and formed a mid-polarity unidentified peak at a bench-scale dose of 2 800 mW . s/cm(2). The unidentified UV peak was not detected at a full-scale dose of 188 mW . s/cm(2). No chronic toxicity was observed for the full-scale UV irradiated effluent; a decline in the reproductive rate of Ceriodaphnia dubia was observed for the full-scale chlorinated effluent.