Investigation of potential surrogate organisms and public health risk in UV irradiated secondary effluent

Watercare's Mangere Wastewater Treatment Plant in Auckland, New Zealand treats sewage from a population equivalent of approximately 1,000,000. The treatment plant is currently undergoing a major upgrade, and as a part of this upgrade the largest UV disinfection plant in the world (at the time of award of the contract) is being constructed. Pilot scale investigations were undertaken at a purpose built facility. The pilot plant employed secondary treatment, and filtration, UV disinfection and a number of low pressure membrane systems. Investigations at the facility focussed on attempting to identify relationships between potential surrogate indicator organisms (including enterococci, faecal coliforms, Clostridium perfringens spores and F-specific bacteriophage) and pathogenic organisms (including culturable human enteric viruses, bacterial pathogens and parasites). The aim of the study was to identify a suitable indicator organism and an associated effluent concentration that would ensure that an acceptable level of public health risk was maintained in the environment. The results showed that no suitable surrogate indicator organism could be found. However the results did indicate that a two tiered operating strategy, based on the concentration of enteroviruses present in raw sewage and an appropriate UV dose, would ensure that an acceptable level of public health risk was maintained in the environment.