Modelling the Impacts of Rainwater Tanks on Sanitary Sewer Overflows

Theme of Conference was: Partnering with industry and the community forr innovation and impact through modelling. Abstract: Recent studies have demonstrated that the Water Sensitive Urban Design (WSUD) strategies are sustainable, innovative and cost-effective approaches for managing stormwater runoff in urban developments. Such runoff from excessive rainfall can cause sewer system overflows. It is well established that intense rainfall increases flow into the sewer system and this increased flow is called rainfall-derived infiltration and inflow (RDII). RDII is the increased portion of sewer flows which enter the sewer network as inflow due to flooding of yard gully taps, roof downpipes illegally connected to the sanitary sewers, broken manhole covers and cross-connections between stormwater and sewer pipes. These excessive flows can also enter the sewer network as infiltration, where stormwater runoff filters through the soil and then enters the sewer pipes via damaged pipes, leaky joints and defective manhole walls. Intense rainfall events (especially of short duration) are becoming more frequent and intense in recent years. Increasing intensity will lead to more RDII. This in turn will lead to more frequent occurrences of sanitary sewer overflows (SSOs). According to various studies, there are different types of WSUD approaches available, which include rainwater tanks, rain gardens, bio-retention cells, infiltration trenches, porous pavements and vegetative swales. These WSUD approaches can reduce sewer overflows by managing excess stormwater runoff that enters the sewer network during intense storm events. However, few studies have analyzed the role of WSUD approaches on reducing sewer overflows. Therefore, rainwater tank as one of the popular WSUD approaches has been used for this study. This paper presents a detailed hydraulic modelling with rainwater tanks to quantify the impacts of WSUD approaches for SSO reduction. Rainwater tank captures roof runoff which reduces surface stormwater runoff during a storm event. Thus, it reduces the amount of RDII into the sewer networks. In this study, four different rainwater tank capacities are considered: 500L, 1000L, 1200L and 1500L. In addition, the impact of drain time on the sewer overflow volumes and peak overflow rates is also investigated. Drain time (T) is the time required to drain out the depth of the stored water in the rainwater tank. A residential area in Glenroy, Melbourne, has been taken as the case study to illustrate the rainwater tank modelling results. The analysis indicates that rainwater tanks can reduce SSO volumes by a maximum of 31% and can reduce peak overflow rates by 30% (for the 1500L tank with a drain time of 36 hours). This study has also evaluated the cost efficiency of the rainwater tanks based only on the capital cost of rainwater tanks. Thus, the approach used in this study will help water professionals to consider rainwater tanks as a mitigation strategy for controlling SSOs