Stormwater as an alternative water source: quality changes with rainfall duration and implications for treatment approaches

The pressure on the world's water resources is rapidly increasing due to population growth and climatic changes. Valorisation of stormwater as a water resource for non-potable reuse can reduce high-quality water demands and save it for potable uses. In this context, not only roof runoff but also drained stormwater outflow, representing considerably higher pollution levels, should be considered as a potential resource. We analysed the quality of stormwater runoff from the municipal separate sewer system in a residential catchment located in a medium-sized town in Poland. The changes in concentrations of TSS, COD, BOD5, and E. coli with rainfall duration were assessed during 7 torrential rainfalls with an intensity exceeding 15 L/s.ha. The concentrations of contaminants in the "first flush" of stormwater varied from 93 to 1598 mg/L TSS, from 112 to 815 mg O-2/L for COD, and from 7 to 48 mg O-2/L for BOD5 and significantly dropped with rainfall duration. The number of E.coli in stormwater outflow fluctuated from 2.5.10(3) to 8.1.10(5) MPN/100 mL. Rapid filtration on sand filters was applied in laboratory-scale for the treatment of raw stormwater outflow, providing removal efficiencies of 87-88% for COD, 50-90% for TN, and 88-96% for TP. The quality of raw and treated stormwater was discussed with regard to the existing and developing European standards for water reuse. The results from our study show that treated stormwater outflows can be applied for landscape irrigation. Moreover, rapid filtration is appropriate for stormwater treatment and can be applied either with the support of pre-sedimentation or even as the only separation process.