An analysis of the potential human health and ecological risks associated with wet weather sewer overflows discharging into an urban freshwater stream

Overflows from sanitary sewers during wet weather, which occur when the hydraulic capacity of the sewer system is exceeded, are considered a potential threat to the ecological and public health of the waterways which receive these overflows. As a result, water retailers in Australia and internationally commit significant resources in order to manage and abate sewer overflows. However, whilst some studies have contributed to an increased understanding of the impacts and risks associated with these events, they are relatively few in number and there still is a general lack of knowledge in this area. <br><br>City West Water (CWW), a water retailer in Melbourne, in conjunction with RMIT University commissioned the present research to discover the level of human health and ecological risk associated with wet weather sewer overflows discharging to waterways. The research aimed to provide this information in a manner which enhances responsible authorities’ decision-making processes with regard to the management of these overflows. <br><br>To assess the level of risk a tiered risk assessment approach was adopted involving an initial screening of contaminants based on a conservative estimate of risk. Of all the pollutants considered in the risk screening phase only microbiological contaminants were identified as posing a high risk from wet weather sewer overflow. Therefore, a Bayesian network (BN) model was developed to undertake a more rigorous assessment of the public health risk caused by microbiological contaminants. <br><br>BN risk outcomes based on the current conditions at the study site showed that the public health risk to important waterway values of the waterway following sewer overflow are predominantly medium to low. Probabilistic inference showed ‘hotspot’ sites are indicated where there is a high probability of frequent wet weather sewer overflow, waterway use and high levels of microbiological contaminants in the waterway. Probabilistic inference also showed that treatment of sewer overflow reduced the risk most to many waterway values as opposed to sewer augmentation and restricting waterway use. Overall, management options to reduce risks at 'hotspot' sites were shown to be more effective than for the study site. <br><br>The BN developed in this research can be used to represent other sites or situations, and in particular to identify priority sites for attention, thus aiding decision-making in relation to the management of wet weather sewer overflows. Furthermore, the BN will be useful to water retail companies and other responsible authorities in prioritising management options to minimise public health risks from sewer overflow.