Factors Affecting the Quality of Roof-Harvested Rainwater

Abstract

Rooftop rainwater harvesting (RWH) is receiving increased attention as an alternative source of drinking water. In urban areas, RWH can bring many benefits and may serve to cope with current water shortages, urban stream degradation, and flooding. The present study examined the effect of some of the factors that affect the quality of roof-harvested rainwater, namely, type of the roofing material, age of the roof and characteristics of rainfall event, on the quality of roof-harvested rainwater. More than 100 roof-harvested rainwater samples collected from newly constructed pilot-scale roofs and existing full-scale roofs of different materials were analysed for different physico-chemical and microbiological water quality parameters. Enrichment factor, correlation analysis and other statistical techniques were used to arrive at conclusions. Results showed that, in general, the physico-chemical characteristics except pH, turbidity and some heavy metals conformed to the drinking-water quality standards. Bacteriological contamination was found for most of the runoff samples from roofs. The results showed high variability in the concentration of different water quality parameters between different roofing materials. Enrichment factor (that is the ratio of concentration in roof runoff to that in free fall) was found to be different for different roofing materials and its value was >1 for most of the parameters tested indicating the significant effect of roofing material on the concentration of various parameters by weathering and/or by deposition. The enrichment factor was high for old roofs compared to new roofs. Correlation analysis revealed a positive correlation between the concentration of different water quality parameters and the dry period, and a negative correlation between some water quality parameters and the amount of rainfall. Thus, roofing material and characteristics of rainfall event were found to have a significant influence on the quality of roof-harvested rainwater.