An approach to managing water quality impacted by treated sewage

As competing pressures from residential development and commercial shellfish-growing increase, it is necessary to maintain water quality as new knowledge about pathogens and their prevalence emerges. Viruses in sewage discharges are a particular hazard as, unlike bacteria, they may survive for weeks or months in the environment and can be infective at low doses. Viruses causing gastroenteritis are typically transmitted by the faecal-oral route, therefore may be present in sewage. Sewage discharges can lead to a public health risk from consumption of contaminated shellfish or ingestion of recreational water. Current approaches for controlling receiving water quality include an assessment of the impact on the receiving environment and/or managing the quality of the wastewater at the treatment plant. In New Zealand, there are no microbiological guidelines for wastewater quality, as the guidelines relate to the receiving environment. These guidelines warn that the relationship between indicator bacteria and viruses needs to be established for each point source discharge of treated sewage. This would require a risk assessment for every sewage discharge where there is a likelihood of human exposure. An alternative approach is to have a guideline which requires wastewater treatment plants to remove viruses before discharge. This paper summarises the derivation of a water quality criterion developed for the Mangere Wastewater Treatment Plant discharge in Auckland, New Zealand through application of the principles of risk management. It reviews the applicability of that virus removal criterion as a management tool in other locations. Given national and international water quality management strategies and recognising the difficulties of efficiently measuring viruses in water, an efficacious approach would be to prepare guidelines based on a tiered risk assessment approach.