AWWA WQTC60716

Due to reductions in supply, population growth, water supply system reliability and other factors, several utilities in California are aggressively pursuing the development of potable water supplies derived from seawater. Given a current understanding of the required technology for the economical pursuit of this new potable water supply, single pass reverse osmosis (RO) membrane treatment is the consensus choice. However, some key water quality (WQ) concerns require a good understanding of the capabilities of single pass RO treatment systems using current seawater RO membranes. These WQ concerns include: permeate WQ with respect to State of California (CA) Action Levels (AL); concentrate WQ with respect to NPDES discharge limits and the CA Ocean Plan; permeate concentrations of sodium and chloride and other constituents as they relate to concentration effects in reclaimed water systems; and, the presence of radionuclides in some source waters. As part of an AwwaRF-sponsored research project and in collaboration with other research partners, West Basin Municipal Water District (WBMWD) constructed a MF/RO seawater desalination pilot plant. This 30-gpm pilot plant was composed of a USFilter CMF-S microfiltration (MF) unit and dual RO trains containing Dow Filmtec and Hydranautics seawater RO membranes. A monitoring plan was developed, in part based on operating parameters and in part based on regulatory limits, to evaluate the effectiveness of the process. In general, weekly monitoring was performed on the raw seawater, MF filtrate, MF backwash, RO permeate, and RO concentrate for regulated inorganic 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-derived potable supplies containing relatively high concentrations of bromide with surface water supplies containing relatively high concentration of organic matter. Initial attempts to use raw seawater chlorination to control biofouling and improve flux in the MF process, followed by ammonia addition to protect the RO membranes, resulted in rapid membrane oxidation and increases in permeate total dissolved solids (TDS) concentrations. This phenomenon, in conjunction with the SDS-DBP results, highlighted the importance of bromide in the seawater desalination process. Permeate boron concentrations were found to potentially meet the CA AL without the need for advanced, 2nd pass treatment. Concentrate metal concentrations generally met CA Ocean Plan discharge standards, but require special attention so that proper analytical techniques are employed for accurate measurement. Seawater and concentrate metal concentrations also require careful consideration of site-specific details, such as co-located powerplant configuration 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 that require low sodium and chloride concentrations for blending purposes. Finally, while high concentrations of gross beta activity were detected (greater than 300 pCi/L), the RO permeate effectively removed these constituents and complied with all radiological standards. Includes tables, figures.