WATER QUALITY SYSTEM INTEGRATION FOR ACCEPTING DESALINATED SEAWATER – A BENCH-SCALE INVESTIGATION

摘要

The West Basin Municipal Water District (WBMWD) evaluated the feasibility of utilizing oceanrnwater desalination as a means to diversify their drinking water supply portfolio. As part of thernevaluation, the Metropolitan Water District of Southern California (MWD) conducted the OceanrnWater Desalination Water Quality Integration Bench-Scale Study to investigate the potentialrnwater quality impacts of integrating corrosion-stabilized desalinated ocean water (Desal) intornexisting drinking water distribution systems. The focus of this bench-scale study was chloraminernresidual stability and disinfectant byproduct (DBP) formation in the distribution system afterrnintegration of desalinated ocean water. One recommendation resulting from this study is torndesign the full-scale desalination facility a chloramine contact time of at least 5 hours withrnsubsequent chlorine and ammonia feed systems to stabilize chloramine residual prior torndistribution. Chloramine stability tests conducted on various source waters and blends generallyrnyielded the following ranking from most stable to least stable: (1) MWD water, (2) blends withrnnon-breakpoint chlorinated groundwater (GW), (3) Desal, and (4) blends with breakpointrnchlorinated groundwater (GWbp). The chloramine residuals in the MWD test water had alreadyrndecayed in the distribution system when it was collected, affording it greater stability in theserntests. High ammonia levels in the non-breakpoint chlorinated GW stabilized the chloraminernresidual in the GW blends. In breakpoint chlorinated GW blends there was no free ammonia tornstabilize the residual, and the relatively high organics and high bromide levels increasedrnchloramine decay. Chloramine decay rates decreased at lower temperatures, and increasing pHrnfrom 8.2 to 8.6 after the addition of ammonia had very little impact on chloramine decay. Withrnthe exception of NDMA, the integration of desalinated ocean water with MWD water, nonbreakpointrnchlorinated groundwater, and breakpoint chlorinated groundwater had a beneficialrndilution effect on regulated and unregulated DBP formation in simulated distribution systemrn(SDS) tests. The highest NDMA formation was measured in a blend of 50% Desal/50% MWDrnwater. However, the NDMA concentration was below 4.5 ng/L in all of the tested source watersrnand blends. This is well below the California notification level of 10 ng/L. Overall, the levels ofrnall DBPs were low, and all regulated DBPs were well below drinking water maximumrncontaminant levels. Examination of trihalomethanes (THMs) and haloacetic acids (HAAs)rndemonstrated little DBP increase from day 3 to day 7 in the SDS tests, as DBP formation wasrnrapid. Blends with GWbp formed the highest THMs, HAAs, and iodo-THMs due to thernpresence of bromide, iodide, and organic precursors. Blends with non-breakpoint chlorinated GW formed the highest iodo-HAAs. Finally, increasing pH from 8.2 to 8.6 had little impact onrnTHMs or HAA9 formation.