Investigation of pathogen disinfection and regrowth in a simple graywater recycling system for toilet flushing

Graywater treatment systems must inactivate pathogens, prevent regrowth, be low cost, and be simple to operate to support their widespread adoption for alleviating water stress. A treatment system comprised only of filtration and disinfection could meet these constraints. To investigate pathogen disinfection and regrowth in such a system with minimal organic matter removal, herein three disinfectants (chlorine, ultraviolet irradiation, and ozone) were tested in combination with three filter types (coarse, sand, and cartridge) for inactivation of pathogens in graywater from the showers and hand washbasins of 14 student residences. Graywater was spiked with bacterial and viral pathogens or surrogates post-filtration. Chlorination post-filtration achieved log reductions greater than 7.1, 8.0, and 7.4 for Escherichia coli, Salmonella enterica, and Pseudomonas aeruginosa, respectively, and 3.8 for MS2 bacteriophage. UV was similarly effective, but would not prevent regrowth without a disinfectant residual. Ozonation generally was ineffective at the doses tested, with the exception that MS2 log removal was 3.7. Pathogen regrowth could be prevented for 4 d with a chlorine residual of 2.75 mg/L even for a simulated high-contamination event (6 log each pathogen). When chlorine residual was maintained, regrowth of indicators and pathogens was prevented for the light graywater investigated.