A study to quantify the impacts of implementing rainwater tanks in terms of minimising SSOs
T Nasrin, N Muttil, AK Sharma
Publication Date (Web): 30 May 2016

Urban drainage systems are frequently unable to cope with increasingly intense storm events, mainly due to non-stationary climate and rapid urbanisation. As drainage systems become less efficient, the incidence of urban flooding and sanitary sewer overflows (SSOs) increases. This, in turn, has various detrimental impacts, including on human health and the environment.

SSOs are caused by rainfall-derived infiltration and inflow (RDII), which is the increased portion of flows that enter the ageing sewer network in terms of inflow as well as infiltration. Inflow is the stormwater that enters the sewer pipes through direct connections. Sources of inflow include illegal connections of roof downpipes to the sewer pipes and broken manhole covers, etc. On the other hand, infiltration denotes stormwater runoff, which enters the sewer pipes after percolating through the soil. Sources of infiltration include cracked sewer pipes and defective joints. Sanitary sewers are designed to accommodate a certain amount of RDII flows. However, during intense rainfall events this amount of inflow and infiltration is exceeded and may lead to SSOs.

Recently, there has been an increase in the implementation of water-sensitive urban design (WSUD) strategies to manage the urban water cycle in a more sustainable way. These strategies include rainwater tanks, rain gardens, bio-retention cells, porous pavements, green roofs and vegetative swales. If adopted, either alone or in combination, they can reduce urban flooding and SSOs by controlling the excess stormwater runoff that enters the drainage system.

This study aims to quantify the impacts of implementing a commonly used WSUD approach, rainwater tanks, in terms of minimising SSOs. Rainwater tanks can capture roof runoff, which may reduce excess stormwater runoff entering the sanitary sewer network in terms of RDII. This study did not consider the rainwater tank as an alternative source of water supply for non-potable uses in households.

For a case study residential catchment in Melbourne, Victoria, a detailed hydraulic modelling of implementing rainwater tanks using PCSWMM (a commercial version of US EPA’s Stormwater Management Model, SWMM) has been presented and results compared with the base case (without implementing rainwater tanks). Various rainwater tank parameters were analysed in the modelling for assessing the reduction in SSO volumes. These parameters included: tank size, drain time, drain delay and number of households with rainwater tanks.

The study considered four different tank sizes: 500L, 1,000L, 1,200L and 1,500L; four different drain times (time required to empty the tank): 12, 24, 36 and 48 hours; four different drain delays (time elapsed before opening the underdrain outlet pipe): 0, 12, 24 and 36 hours and two different values of the number of households with rainwater tanks: 100% and 50% of the households. It was observed from the analysis that the rainwater tanks could lead to a reduction in SSO volume by a maximum of 33% when compared to the base case. A much larger reduction in SSO volumes could be expected when other WSUD strategies would be implemented in conjunction with rainwater tanks.


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