Impact of rainwater harvesting systems on nutrient and sediment concentrations in roof runoff

Rainwater harvesting (RWH) systems have the unique ability to contribute to stormwater management goals via mitigation of runoff volumes and peak flow rates. Additionally, collecting and storing runoff via RWH systems can potentially provide water quality benefits due to physical and chemical processes that occur within the storage tank. This study quantified the water quality improvement provided by storing rooftop runoff via RWH systems at four sites in Raleigh, North Carolina, USA. Roof runoff and extraction spigot samples were analyzed for total suspended solids (TSS), nitrogen species and total phosphorus. Roof concentrations were significantly greater than spigot concentrations for all constituents except TSS, indicating the ability of RWH systems to significantly lower nutrient concentrations of incoming roof runoff. Lack of significant TSS reduction was likely attributable to low, 'irreducible' concentrations of TSS in the roof runoff. The use of additional filtration components prior to the extraction spigot could aid in lowering spigot TSS concentrations. The findings presented herein contend that stormwater benefits associated with RWH are not only limited to hydrologic mitigation, but also include reductions in concentrations of nitrogen and phosphorus species. Thus, it is recommended that pollutant removal credit be assigned to these systems when used as stormwater control measures.