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

Abstract

Full‐scale ultraviolet (UV) light and chlorine disinfection systems were operated in parallel using nitrified and partially denitrified tertiary 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 ≤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.