Quantitative microbial risk assessment: a catchment management tool to delineate buffer distances for on-site sewage treatment and disposal systems in Sydney’s drinking water catchments

On-site sewage systems, such as septic tank-absorption trenches, are used by approximately 20 000 people who live within the catchments that supply Sydney’s drinking water. These systems discharge sewage, treated to varying degrees depending on the system type and level of maintenance, to the environment. This can result in contamination of drinking water supplies if systems are not designed or managed appropriately. The aim of the project was to develop a methodology to define appropriate buffer distances between on-site sewage systems and waterways in Sydney’s drinking water catchments, to ensure the protection of drinking water quality. Specific objectives included: identifying the current status of on-site sewage management; assessing the effluent quality and treatment performance of septic tanks, aerated wastewater treatment systems (AWTS) with disinfection and an amended material sand mound; and development of an appropriate methodology for delineating buffer distances and assessing development applications. Viruses were used as a focus for delineating the buffer distances due to their mobility and robustness in the environment, and the potential health consequences of their presence in drinking water. A Quantitative Microbial Risk Assessment (QMRA) model was developed to calculate the cumulative impact of the on-site sewage systems in the Warragamba catchment based on data from literature and experiments, with consideration of virus loads from sewage treatment plants within the catchments. The model enabled consideration of what was a tolerable impact in terms of the resulting infections within the community. The QMRA the tolerable loads of viruses from the Warragamba catchment were 108 viruses per year in raw water and 104 viruses per year in treated water. A log reduction method was developed to facilitate individual site development assessments. This method was compared to other management approaches to development assessment: fixed minimum buffer distances of 100m, reducing failure rates to zero, and ...