Disinfection Methods for Treating Low TOC, Light Graywater to California Title 22 Water Reuse Standards

This research was performed as part of the San Francisco Public Utilities Commission's search for more sustainable water resources. Disinfection methods were evaluated for treating light graywater to California drinking water standards for unrestricted, direct nonpotable water use, indoors. Graywater originating from sinks and showers in a commercial office building in southern California was collected in grab samples over 4 weeks. Samples were filtered and tested for disinfection efficacy toward indigenous total coliforms and Enterococci as well as seeded male-specific 2 (MS2) coliphage. The filtered samples were exposed to monochromatic (253.7nm) ultraviolet (UV) light, chlorination with sodium hypochlorite, and ozonation. A UV dose of completely inactivated Enterococci and inactivated total coliforms to California drinking water standards required levels and achieved 5-log reduction of MS2 in half of the samples. Sodium hypochlorite met bacteria and virus standards for the graywater at a free-chlorine concentrationxtime (CT) well below the 450 required by California drinking water standards. An ozone CT of 0.4 met bacteriological standards for reuse in two of the four samples. Ozone virus inactivation criteria was inconclusive due to the high-ozone demand of the water matrix when seeded with the test organism; however, an applied ozone dose of would be expected to satisfy the ozone demand and meet the virus criteria. Of the disinfection options discussed, UV light was the least expensive, followed in order of lowest to highest cost by sodium hypochlorite delivered in bulk, sodium hypochlorite generated on-site, and ozone. An important and novel component of this research was that graywater treatment was performed only with tertiary processes and thus, biological treatment was not used. These results are applicable to other light graywaters with similar total organic carbon (TOC) concentrations (less than as carbon).