The quality and safety of reverse osmosis (RO)-treated secondary wastewater (WW), for indirect potable re-use, was assessed using a dataset of 375 chemicals measured in RO-treated WW. A screening health risk assessment indicated that four N-nitrosamines were of potential concern, although median concentrations of these chemicals were always below health values. The most frequently detected chemicals in RO-treated water were disinfection by-products, volatile organic compounds, metals and complexing agents, in contrast to many monitoring programmes that focus on pharmaceuticals, personal care products and hormones. Frequent detections in RO-treated WW were most related to high concentrations in secondary WW, relative to limit of reporting, and the potential for chemicals to form or be added during the treatment process, rather than poor rejection by RO membranes. Between 3.7 and 10.7 μg/L of dissolved organic carbon (DOC) in RO permeate could be attributed from chemicals detected on at least one occasion, with the majority of this total attributed to chemicals detected in less than 25% of samples. While chemicals below detection may contribute a significant component of DOC, it is likely that natural organic matter and soluble microbial products still contribute the majority of DOC to RO permeate. A high degree of safety is demonstrated for the use of RO-treated WW as an indirect source of potable water.

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