Effect of Treatment on the Quality of Harvested Rainwater for Residential Systems

Rainwater harvested from four pilot‐scale roofs (concrete tile, green, metal, and asphalt–fiberglass shingle) was batch‐chlorinated to a target total chlorine 10 min residual of 2 mg/L as Cl 2 and passed through an activated carbon filter after 24 h to simulate treatment in a residential system. Total coliforms (TCs) were not detected, and the total trihalomethane (TTHM) concentration was typically (>85% of samples) below the US Environmental Protection Agency maximum contaminant level (MCL) of 80 μg/L in chlorinated–unfiltered rainwater harvested from conventional roofing materials. In contrast, TCs were always detected, and the TTHM concentration was 3–5 times higher than the MCL in chlorinated–unfiltered rainwater harvested from the green roof. The filter lowered the turbidity and TTHM concentration, but it also shed coliforms. Batch chlorination and filtration are more suitable potable treatment techniques for rainwater harvested from conventional roofs than from a green roof, but concerns regarding disinfection and turbidity remain.