SODIUM HYPOCHLORITE AND MEDIUM PRESSURE UV IN SERIES: A VALLEJO CASE STUDY

摘要

In 1996, the Vallejo Sanitation and Flood Control District (VSFCD) replaced anrnoutdated chlorine gas disinfection system with a state-of-the art medium pressure UVrnsystem, in combination with a liquid chemical system for use as backup during wetrnweather periods. The new $2.5 million system is capable of disinfecting up to 60rnmillion gallons per day (mgd) of effluent discharged to the San Francisco Bay. Therndesign intent was to use the UV system during normal dry weather flow periods and tornswitch to sodium hypochlorite/bisulfite during wet weather, when the UV transmittancerndropped to unacceptable levels. The new facilities were designed to meet a 30 day logrnmean limit of 200 fecal coliform per 100 ml, as stated in VSFCD抯 draft NPDES permit.rnSince the startup of the facilities, several issues have caused VSFCD to operate bothrnthe UV and sodium hypochlorite systems in series for all flow conditions: ThernCalifornia Regional Water Quality Control Board has not finalized VSFCD抯 permit forrnthe 200 fecal coliform limit, and the District has been disinfecting to meet a morernrestrictive limit of 240 total coliform; problems with the operation of the UV systemrnhave interrupted its performance; and continuous operation of the sodiumrnhypochlorite and bisulfite systems has proven to be more reliable than switchingrnbetween disinfection methods.rnAlthough the system was not originally intended to have both the hypochlorite and UVrnsystems operating concurrently, VSFCD has found this to be a very effective mode ofrndisinfection. Low dosages of hypochlorite are used to supplement the disinfectionrnprovided by the UV system and also to prevent biological growth on the UVrnequipment. A comparison shows that to meet the higher 240 total coliform limit, usingrnthe combination of UV and hypochlorite together is more cost effective and morernreliable than using hypochlorite alone.rnThis paper reviews VSFCD抯 three year operating history, and presents a summaryrnof startup and post-startup problems and their resulting solutions. Typical operatingrndata are presented and approximate costs for operation and maintenance of the UVrnsystem and sodium hypochlorite/bisulfite system are reported.