Quantifying the Performance of Storm Tanks at Wastewater Treatment Works

Storm tanks at Wastewater Treatment Works (WwTW) are an essential but often neglected component of the sewerage system and in combined systems they provide relief to the WwTW at the time of storm events. Historically in the UK approximately 6 times mean daily dry weather flow (DWF) is discharged to the works with approximately half the inflow — circa 3 times DWF — being discharged to the storm tanks. Once the tanks are full the excess flows are spilled, untreated, to the nearest watercourse or ocean. After the storm event has subsided and the DWF returns to normal, the storm tanks are emptied to the works with the effluents given full treatment. It may be argued that in recent years, the design philosophy for storm tanks has been static, certainly in the UK, and has not moved forward in the same way as other developments associated with integrated quantitative and qualitative modelling of sewer systems and the performance of other ancillary structures. It is common practice for the pollution retention performance of storm tanks to be omitted from regulatory guidelines and hence there is little need to understand what is actually spilled to the receiving water. The design of tanks is usually based on a volume of effluent to be stored but little is known about how the tanks retain pollution or of the subsequent impact that the spilled pollution has on the receiving waters. New legislation in Europe, in the form of the Water Framework Directive, will require all water utility companies to better understand the magnitude, volume and quality of all intermittent discharges that issue from sewerage systems into receiving waters. This approach is recommended at the integrated catchment scale and hence the contribution to receiving waters from storm tanks may form one of the major elements of pollution. This paper describes a review of current practice and presents results of an experimental programme which improves understanding of the hydraulic and pollutant retention performance of storm tanks. This is based on UKWIR research project WW22B.