Assessment of a rainwater harvesting system for pollutant mitigation at a commercial location in Raleigh, NC, USA

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Low Impact Development (LID) and Water Sensitive Urban Design have as one of their tenets the use of rainwater harvesting (RWH) systems to provide water for use on site. Historically implemented in arid or semi-arid regions, RWH has recently surged in popularity in more humid regions, such as the southeastern USA, due to increased interest in water conservation during severe drought conditions. An LID commercial site in Raleigh, NC, incorporated RWH with other stormwater control measures to mitigate runoff quantity and improve runoff quality. A 57,900-liter RWH tank used for landscape irrigation was monitored to determine influent and effluent water quality. Samples were analyzed for total nitrogen, total phosphorus, total Kjeldahl nitrogen (TKN), total ammoniacal nitrogen (TAN), nitrite-nitrate (NOX), orthophosphate (Ortho-P) and total suspended solids (TSS). Low concentrations were observed for all pollutants monitored; for example, influent and effluent TP concentrations were 0.02 and 0.03 mg/L, respectively. Statistical testing showed significant increases in TAN and organic nitrogen (ON) concentrations by 33 and 38%, respectively, from inflow to outflow. NOX and TSS concentrations decreased significantly by 23 and 55%, respectively. Concentrations of all other pollutants were not significantly different between the inflow and outflow. Influent concentrations to the RWH tank were less than previously published rainfall pollutant concentrations, indicating potentially irreducible concentrations onsite. While a single case study, this RWH system appears to offer some pollutant mitigation, especially for TSS.

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