A Novel Approach to Seawater Desalination Using Dual-Staged Nanofiltration

As the quantity and quality of inland water sources decline, more coastal municipalities are looking at seawater desalination as a potential source of drinking water. The Long Beach Water Department (LBWD) developed an alternative technology to desalt seawater by using dual-staged nanofiltration (NF2). This novel NF system treats the first-stage permeate through a second stage in order to produce finished water with salinity levels that meet drinking water standards. Three commercially-available NF membranes were selected for this study based upon their designation as NF membranes by the manufacturers and their salt rejection characteristics. Results from the bench-scale evaluation were integrated in a performance-predicting model, which was subsequently calibrated against the results obtained with an 8-gpm pilot unit. The pilot-test plan considered the impact of temperature, pressure, and array configuration on permeate water quantity and quality. The percentage of desalinated water that could be blended into LBWD's distribution system was determined by taking into account the issues of disinfection by-product (DBP) formation, disinfectant residual, and corrosivity. Finally, viral challenge tests were considered to verify the inherent redundancy of the system and the impact that recycling streams would have on virus accumulation.