CITY OF UNALASKA ULTRAVIOLET DISINFECTION OF A PRIMARY EFFLUENT

摘要

The City of Unalaska, Alaska was required by the EPA to upgrade their wastewaterrntreatment facilities to meet water quality standards. Treatment at the existing plantrnconsisted solely of fine screening. Outfall dilution models indicated that the plant couldrnnot comply with water quality standards for fecal coliform. To meet water qualityrnstandards, EPA set an effluent limit of 10,000 fecal coliforms per 100 milliliters.rnPreviously, an engineering report was completed to identify the most attractiverndisinfection alternative. This study concluded: (1) disinfection should be by on-siternhypochlorite generation and (2) Ultraviolet (UV) disinfection could not reliably disinfectrnthe primary effluent flow. Detailed construction and operating costs for thernhypochlorite alternative were developed as part of the engineering report. Thesernestimates, $5 million for construction and $148,000/year O&M, indicated UV shouldrnbe re-evaluated.rnThe initial phase of UV re-evaluation included four collimated beam tests. This wasrnfollowed by an on-site three-day pilot test using a medium pressure UV lamp. Thernresults, combined in a dose response curve, showed close correlation between therntwo types of test data. The curve indicated that a dose of 25 mWsec/cm?wasrnnecessary to reduce the effluent fecal coliforms to 1,000 fc/100 ml which was set asrnthe design goal to assure compliance with the 10,000 fc/100 ml effluent standard.rnThe City bid the UV equipment for prepurchase during July 1998 and plans on arnstartup date of October 1999. Bid documents required manufacturers to provide arnsystem that achieved 3 log removal at 2 mgd for a wastewater with a transmittance ofrn11% and 240 mg/l TSS. Bidders were required to provide a 2 year bonded processrnguarantee. The selected UV equipment, PCI's high intensity low pressure system,rncost $332,000. The UV O&M costs are projected to be $36,000/year.