Characterisation of the performance of an off line storage tank

A stormwater facility consisting of a hydrodynamic separator as an overflow device, an open concrete storage tank and two subsequent open earth retention basins was investigated. The results are used to characterise the qualitative performance of the storage tank. By evaluating the cumulative runoff volume and settleable solids load it is shown that the qualitative performance can be characterised by two effects: pollutants are retained because runoff is stored until the tank starts to overflow; pollutants are retained because the tank operates as a clarifier during the overflow period. The influencing factors on these two processes are discussed.