Effective first flush volumes in experimental household-scale rainwater catchment systems

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Abstract
Rainwater collection systems often include a first flush system to divert contaminants away from collected and stored rainwater. These have traditionally been designed for a set volume, to capture and divert the first 1–2 mL of rain deposited onto a roof. However, environmental and collection system parameters can vary the volume of the first flush necessary to effectively divert contaminants. Using a test-scale rainwater collection system in Amherst, Massachusetts (USA), a series of experiments were conducted to assess the quality of rainwater in the system per collection volume and time. This included a tracer study of an ideal contaminant, and water quality measurements of atmospheric rain, fractionated first flush, and the collection tank during rain events. First flush samples contained elevated dissolved organic carbon (DOC) concentrations up to 40 mg/L, with high variability between the rain events. UV 254, DOC, and conductivity all trended together indicating a uniform wash-off of contaminants. Higher intensity storms increased roof wash-off deposition and environmental conditions affected the necessary first flush volumes. The majority of contaminants likely originated from roof wet and dry deposition. The design of first flush in rainwater harvesting systems needs to account for local precipitation patterns, storm intensity, and canopy conditions.

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