Trihalomethane (THM) occurrence in reclaimed water and its exposure risk to human health during non-potable reuse was evaluated for a coastal Arabian city. Recycled water was monitored at two representative wastewater treatment facilities: a city-wide activated sludge treatment (AST) plant and a neighborhood membrane bioreactor (MBR). Actual total THM concentrations in the post-chlorinated effluents varied with effluent temperature, peaking at 69.28 ± 2.54 μg/L and 41.15 ± 2.34 μg/L in AST and MBR effluents, respectively, in summer. Higher bromide concentration and favorable Br/Cl2 ratio during disinfection in the AST resulted in stronger brominated THM speciation. Conversely, the MBR effluent was dominated by chloroform. Salinity fingerprinting indicated the presence of saline water in the AST effluent. Individual excess lifetime cancer risk (IELCR) was calculated using the outdoor box exposure model based on annual peak THM concentrations and peak THM formation potential (THMFP) for each species. Inhalation exposure to chloroform in the MBR effluent using peak THMFP values presented the highest cancer risks of 1.28 × 10−6 and 1.12 × 10−6 to residential adult and child receptors, respectively, exceeding the USEPA IELCR guidance threshold of 1 × 10−6 for carcinogens. Chlorinated THM species are important in determining the health risk from reclaimed water during non-potable water reuse.

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