Assessment of tertiary treatment technology for water reclamation in San Francisco, California

Abstract

Tertiary treatment processes were tested to demonstrate their ability to produce effluents that meet the most stringent standards in the California Wastewater Reclamation Criteria. The processes studied were in-line, granular media filtration and microfiltration for particulate removal, and chlorination and UV light for disinfection. In-line filtration met CWRC turbidity requirements, although filter performance was dependent on feed water turbidity, and alum and polymer doses. Microfiltration produced an effluent virtually free of suspended solids and coliforms, and a 1.9-log removal of MS2 phage. This decreased the level of microorganism inactivation required of the UV system. MS2 phage sensitivity to UV light was not constant, which may question its suitability as an indicator organism to assess virus inactivation. Minimum chlorine Ct value of 340 mg.min/l, and UV dose of 800 J/m2 were identified for in-line filtration effluent, while a dose of 450 J/m2 was sufficient for microfiltration effluent. In-line filtration followed by UV disinfection is less costly for water reclamation than any other combination of treatment units tested, and, therefore, these processes were selected for a 53.000 m3/d water reclamation plant in San Francisco.