Evaluation of UV disinfection systems for large-scale secondary effluent

A study at the Southeast Water Pollution Control Plant, San Francisco, California, was undertaken to evaluate the feasibility of replacing its current secondary effluent chlorination system with a 6.57 m 3 /s ultraviolet (UV) disinfection system. Two vertical lamp units and one horizontal lamp unit were pilot tested in parallel. The effective UV dose for coliform removal and the effects of feedwater characteristics and reactor hydraulics were examined. The horizontal pilot unit gave a more consistent performance than the vertical pilot units. Based on the test results, a UV dose of 65 mW s/cm 2 would be required for the plant to achieve the target effluent total coliform level (240 CFU/100 mL) 95% of the time. The large fluctuation of UV disinfection results could be attributed to the wide range of feedwater quality inherent to a combined sewer system. High suspended solids, characteristic of the plant's secondary effluent during storm events, significantly increased the UV dose required to achieve the target coliform level. Existing UV inactivation models were evaluated. The hydraulic behavior of the pilot units was found to significantly affect their test results. Higher virus removal efficiency was observed with the UV systems than the full-scale chlorination system.