Effectof roof type and harvesting time on the quality of water harvested from impervious surfaces in urban residential areas

Over recent decades, fresh water resources have been exposed to pollution meaning that in some areas, available supplies are now scarce. It is necessary, therefore, to harvest precipitation water in order to help controland manage flood and supply waters in urban areas. Roof material type and the timing of water harvesting are two main controls on harvested water. The main objectives of this study were therefore to investigate rainwater quality and to identify the effectof roof material type and harvesting time (first flush and posterior roof runoff)on harvested water quality in district 8, Tehran city, Iran.To address these objectives, 12 water quality parameters (pH, electrical conductivity, carbonate, bicarbonate, calcium carbonate, total dissolved solid, total suspended solids, nitrogen, nitrate, phosphate, phosphorous and sulfate)were measured in 30 runoff samples collected from two mosaic tile and bitumen roofs during2013-14. T-tests and the Mann Whitney U-testwere used to assess the effect of roof material type and harvesting timeon the harvested water quality. Correlation and cluster analysis were used tothe relationships among, and clusters of, roof runoff water quality parameters, respectively.The most important parameters affecting water quality were nitrogen and nitrate.The results also showed that the roof material types change theroof runoff water quality parameters. The bitumen roof compared to the mosaic tile roof had the most substantial effect on water quality. All roof runoff water quality parameters, except pH, were affected by harvesting time for the runoff (first flush and posterior roof runoff). The results of cluster analysis showed that the roof runoff water quality controls classified into four clear clusters. The results of this case study suggest that first flush runoff cannot be stored for water supply because of its unsuitable quality, but that posterior roof runoffcan provide a means to supply water of acceptable quality in arid regions and in areas with water scarcity issues.