Disinfecting Unfiltered Secondary Effluent Meeting The Challenge With Low Pressure – High Intensity UV Systems

The City of Manteca discharges secondary effluent to the San Joaquin River near Stockton in Northern California. Chlorination and dechlorination of secondary effluent has been accomplished historically with chlorine and sulfur dioxide gas utilizing a converted sedimentation basin as a chlorine contact tank. Because of poor hydraulics and a partially-nitrified effluent, satisfying NPDES coliform requirements consistently with conventional chlorination practices has proven difficult. As part of an expansion and upgrade of the 7 mgd plant, construction of a UV disinfection system was considered. To determine the feasibility of disinfecting unfiltered secondary effluent with UV light, a survey of treatment plants in the United States and the United Kingdom was undertaken in conjunction with an equipment supplier prequalification process. Pilot testing of a low pressure-high intensity UV disinfection system was subsequently initiated to confirm design dosages and operational parameters required to achieve a total coliform standard of 23 MPN/100 mL with low transmittance secondary effluent. The results of the pilot study, treatment plant survey, and preliminary design for the proposed 10 mgd disinfection facility are presented in this paper along with projected capital and operation and maintenance costs.