Removal of enteric viruses and selected microbial indicators by UV irradiation of secondary effluent

The Watercare Mangere Wastewater Treatment Facility, which treats wastewater from the greater Auckland New Zealand region, is undergoing a major expansion/upgrading to add advanced treatment and disinfection prior to discharge into a harbor. One important goal of this project is to protect the receiving water from microbial contamination. Since sufficient information on the fate of various microorganisms through wastewater treatment plants in New Zealand was not readily available, extensive pilot- and bench-scale studies were undertaken to develop specific design criteria for the treatment and disinfection systems. The specific objective of this study was to evaluate the removal and inactivation of enteric pathogens and other microbial indicators through treatment processes that employs UV irradiation as a final disinfection process. The removal of indicator organisms through secondary treatment was typically between 2.5-log (99.7% removal) and 2.8-log (99.8% removal) for fecal coliforms and enterococci, respectively. Indigenous F-specific bacteriophage exhibited a mean removal of 1.6-log (i.e. 97.7% removal) and Clostridium perfringens spores showed a mean removal of 1.3-log (i.e. 95% removal). The UV dose required to achieve a one log reduction in the concentration of indigenous F-specific bacteriophage was found to be approximately 20 mWs/cm2 per log removal. The concentration of enterovirus and adenovirus were consistently reduced to the limit of detection (1 TCID50/100L) at UV doses of 35 to 40 mWs/cm2 and 40 to 45 mWs/cm2, respectively. Clostridium perfringens spores were the most resistant indicator organisms, being reduced to less than 200 MPN/100 mL at a UV dose of 75 mWs/cm2.