AWWA MTC53912

For over 25 years, brackish water from wells along south San Francisco Bay hasbeen pumped to tidal sloughs as part of a basin wide aquifer reclamation program(ARP) operated by the Alameda County Water District (District) of Fremont,California. The water was high in chlorides, hardness and overall total dissolvedsolids (TDS) as a result of saltwater intrusion, but otherwise met all potablewater criteria. Beginning in 1990, several brackish groundwater projects werecompleted in California demonstrating the cost effectiveness of using reverseosmosis (RO) for producing potable water from brackish water. The District neededto increase the water supply due to continued increases in residential andindustrial demand, and an integrated resource plan (IRP) recommended building a10 million gallon per day (mgd) brackish water RO system. The source water wouldbe the 8,000-12,000 acre-feet of ARP well water discharged to the San FranciscoBay each year. The brackish water RO system is expected to operate at 75-85%recovery and produce 1.3 mgd of concentrate with a TDS between 12,000 to 15,000mg/l. The TDS concentration in the sloughs varies from 2,000 mg/l to 35,000 mg/ldepending on the location and the tide status, and the EIR identified thelocation of salt tolerant plants compatible with the concentrate discharge. Afterthe general EIR evaluation, the District proceeded with the application for asurface water NPDES discharge permit, and prepared a technical submittalincluding acute toxicity testing of the concentrate from the pilot test, analysisof discharge alternatives and mass balance for selenium and other trace metals.Five suppliers of high capacity membranes for municipal water treatment plantswere identified based on the ability to achieve high rejection of dissolved saltsand minerals at pressures less than 200 psi. In general, quality control duringmembrane manufacturing results in predictable operating pressures and rejectioncharacteristics. However, membrane layers are susceptible to fouling fromcontaminants suspended in the feedwater and scaling from the precipitation ofdissolved minerals. Also, the aesthetic qualities of the product water cannot beprojected using the computer programs supplied by the membrane manufacturers.Therefore, the initial phase of RO system design was to verify that the ARP water didnot result in excessive fouling or scaling, and that the water quality associatedwith the proposed membrane design was acceptable. This was determined byoperation of a small pilot unit for several months. The results of this studywere used to select a cost-effective RO system design by providing accurateestimates of operational costs for pumping, chemicals, concentrate disposal andmembrane replacement. The selected RO system design was then used to preparedetailed construction plans for the ancillary facilities and procurementspecifications for the RO equipment and membranes. Includes 2 references, tables.