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Sweetwater Authority (SWA) owns and operates the Robert A. Perdue Water TreatmentPlant (WTP), a 30 mgd conventional filtration plant located in San Diego County, CA.After testing ozone, enhanced coagulation, chlorine dioxide (ClO2), MIEX andsubmerged microfiltration/ultrafiltration (MF/UF), SWA has, determined that chlorinedioxide and membrane filtration should be in the Authority's future. This paperdocuments the results of SWA's five-month pilot testing of two submerged MF/UFmembrane systems, presents an overview of SWA's decision-making process, andhighlights some important lessons learned in the preliminary cost estimating and designphases of this project.The Perdue WTP has the ability to treat two source waters, local surface water fromSweetwater Reservoir (SWR) (average total organic carbon = 6.5 mg/L, bromide up to0.5 mg/L) and San Diego County Water Authority (CWA) aqueduct water (average TOC= 2.5 mg/L, low bromide). SWA currently applies free chlorine and potassiumpermanganate to the raw SWR water and uses chloramines in the distribution system.However, excessive trihalomethanes (THMs) formation is a potential concern for futurecompliance with the Stage 2 Disinfection Byproducts (DBP) Rule using the LocationalRunning Annual Average. When treating water from the local reservoir during periods ofhigh runoff, THM formation could potentially exceed 120 ppb in the distribution system,with typical distribution system values in the 60 to 80 ppb range.The testing investigated various combinations of ClO2, dissolved air flotation (DAF), andconventional flocculation/sedimentation (floc/sed) as pretreatment processes to theMF/UF membranes. In addition to monitoring the performance of the DAF, floc/sedprocess, and the membrane systems; simulated distribution system (SDS) testing wasperformed on the membrane filtrate on a weekly basis to estimate disinfection by-product(DBP) formation.With DAF as pretreatment, both of the submerged membrane systems tested providedreliable water quality at fluxes above 35 U.S. gallons per square foot per day (gfd), withwater recoveries around 95%, for at least 30 days before the need for membrane clean-inplace(CIP). Based on these initial results, the flux rates were increased to above 40 gfdand higher with conventional flocculation/sedimentation pretreatment. Membraneperformance under the higher flux conditions was mixed but provided valuable design information. SWA also held a series of peer review meetings to analyze the results of this and previoustesting. The consensus was that chlorine dioxide, DAF or floc/sed, and submergedMF/UF membranes represent the best process combination for SWA and potentially forother agencies with similar water quality concerns. SWA has begun design of a 40 mgdupgrade to ClO2 and submerged MF/UF membranes. Includes 3 references, figures.

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Edition: Vol. - No. Published: 03/01/2005 Number of Pages: 16File Size: 1 file , 820 KB