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The City of Boca Raton, Florida will be constructing one of the world's largest(40 million gallons per day permeate capacity) membrane softening(nanofiltration) plants. The source water for the membrane plant will include upto 56 existing wells from 5 different wellfields. As with many other utilities,some of the City's older wells present raw water quality concerns for membranesoftening. This paper discusses well water quality and pretreatment testingcarried out during a two-phase pretreatment pilot testing program. The resultsdemonstrate that typical groundwater pretreatment is not always adequate toproduce suitable membrane feedwater. Preliminary membrane pilot studies conductedby the City linked cartridge filter and NF membrane fouling to sand and silt,typically found in the groundwater source used by the City. The addition of amulti-media pressure filter upstream of the cartridge filters appeared to extendcartridge filter life and reduce membrane cleanings. However, incorporatingfull-scale pressure filtration as part of the membrane pretreatment systemsignificantly increases construction and operational costs. In an effort toidentify cost-effective pretreatment alternatives, the City approved amulti-phased pretreatment pilot testing program. Phase 1 pretreatment pilottesting, completed in the summer of 1999, determined that biological fouling hadmore impact on cartridge filter life than sand and silt. As a result of Phase 1testing, pretreatment processes were tested for Phase 2 primarily for theirability to resist cartridge filter fouling and also their ability to remove sandand silt. Results from Phase 2 testing indicated the dual media pressure filterand backwashable cartridge filter technologies produced the highest qualitymembrane feed water. Use of the dual media pressure filter extended cartridgefilter life to approximately 3 months, more than 6 times the cartridge filterlife associated with any of the other pretreatment technologies. Two dual-mediafiltration rates (11 gpm/sf and 20 gpm/sf) were tested and little difference inperformance was observed. Biological testing results indicated that iron andsulfur-related bacteria are present in the membrane feedwater and some of thebacteria strains were identified as slime forming. Testing also indicated thepresence of inorganic precipitates, such as iron sulfide. Dissolved oxygenanalysis indicated that air is entering some wells and the raw water transmissionsystem. This air can oxidize and precipitate dissolved iron and sulfur species aswell as support aerobic bacteria growth, which can increase membrane operatingcosts. With the current membrane source water, dual-media pressure filtrationappears to be the most cost-effective pretreatment process. The results of thiswork will be of interest to utilities who are considering implementing membranetreatment on groundwater supplies. Includes table, figures.

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Edition: Vol. - No. Published: 05/01/2001 Number of Pages: 17 File Size: 1 file , 290 KB