Investigation of stormwater quality from successive first flush diverted from roof for different roofing materials

There are uncertainties on when and for what purposes roof-collected rainwater can be safely used since such rainwater contains different pollutants. Moreover, there should be some established relationships on the wash-off behaviour of such pollutants, which stakeholders can easy use to device associated policies. This paper presents qualitative behaviour of first flush in improving the quality of roof-collected rainwater. In addition, mathematical relationships were developed to estimate water turbidity, pH and electrical conductivity (EC). Successive stormwater quality data was collected from two different buildings (having different roof materials; isogum and cement sheet) within Tarbiat Modares University, Tehran, Iran. A total of 93 samples (from 10 rainstorms) were collected and studied for the mentioned pollutants. It is found that in regard to turbidity, the runoff from the isogum roof is having better quality compared to the runoff from cement sheet roof. However, the levels of turbidity in runoff from both the roofs were more than the potable water standard. The ranges of EC in the collected samples were 58.64–1335.18 µs/cm for the isogum roof and 209.92–1929.60 µs/cm for the cement sheet roof. For both the roofs, pH values were within the acceptable range as defined by the US EPA. The turbidity values of the rainwater samples collected from the isogum roof were much higher than the National Organization Standard and was in the range of 21–596.4 NTU. Based on the findings, collected water can be applied for irrigation of all types of plants after diversion of the first 1.5 mm of runoff for the isogum roof and the first 1 mm of runoff for the cement sheet roof. The proposed models’ performances were satisfactory, except for the pH due to the non-exponential changes in data.