AWWA WQTC60716

Due to reductions in supply, population growth, water supply system reliability and otherfactors, several utilities in California are aggressively pursuing the development ofpotable water supplies derived from seawater. Given a current understanding of therequired technology for the economical pursuit of this new potable water supply, singlepass reverse osmosis (RO) membrane treatment is the consensus choice. However, somekey water quality (WQ) concerns require a good understanding of the capabilities ofsingle pass RO treatment systems using current seawater RO membranes. These WQconcerns include: permeate WQ with respect to State of California (CA) Action Levels(AL); concentrate WQ with respect to NPDES discharge limits and the CA OceanPlan; permeate concentrations of sodium and chloride and other constituents as theyrelate to concentration effects in reclaimed water systems; and, the presence ofradionuclides in some source waters.As part of an AwwaRF-sponsored research project and in collaboration with otherresearch partners, West Basin Municipal Water District (WBMWD) constructed aMF/RO seawater desalination pilot plant. This 30-gpm pilot plant was composed of aUSFilter CMF-S microfiltration (MF) unit and dual RO trains containing Dow Filmtecand Hydranautics seawater RO membranes. A monitoring plan was developed, in partbased on operating parameters and in part based on regulatory limits, to evaluate theeffectiveness of the process. In general, weekly monitoring was performed on the rawseawater, MF filtrate, MF backwash, RO permeate, and RO concentrate for regulatedinorganic and bacteriological constituents, as well as selected UCMR-type constituents.In addition, limited simulated distribution system (SDS) disinfection byproduct (DBP)experiments were performed to assess the importance of blending seawater-derivedpotable supplies containing relatively high concentrations of bromide with surface watersupplies containing relatively high concentration of organic matter.Initial attempts to use raw seawater chlorination to control biofouling and improve flux inthe MF process, followed by ammonia addition to protect the RO membranes, resulted inrapid membrane oxidation and increases in permeate total dissolved solids (TDS)concentrations. This phenomenon, in conjunction with the SDS-DBP results, highlightedthe importance of bromide in the seawater desalination process. Permeate boronconcentrations were found to potentially meet the CA AL without the need for advanced,2nd pass treatment. Concentrate metal concentrations generally met CA Ocean Plandischarge standards, but require special attention so that proper analytical techniques areemployed for accurate measurement. Seawater and concentrate metal concentrations alsorequire careful consideration of site-specific details, such as co-located powerplantconfiguration and material specification with the seawater desalination treatment plant.Relatively high permeate sodium and chloride concentrations (greater than 100 mg/L)remain a concern if a particular site will impact recycled water systems or systems thatrequire low sodium and chloride concentrations for blending purposes. Finally, whilehigh concentrations of gross beta activity were detected (greater than 300 pCi/L), the ROpermeate effectively removed these constituents and complied with all radiologicalstandards. Includes tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/15/2004

Number of Pages:

17

File Size:

1 file , 1 MB