(579g) Field Demonstration of Brine Demineralization for High Recovery RO Desalination

Reverse osmosis desalination of brackish water in the San Joaquin Valley may be feasible only at high recoveries (>85%) because of the high cost of brine disposal. However, because brackish waters in this region tend to be at or near saturation with respect to gypsum, high recoveries cannot generally be achieved with a single stage RO process. Recent studies at UCLA have suggested that high recoveries may be achievable with a two-stage RO process with induced interstage precipitation to demineralize the primary RO concentrate, enabling a secondary RO stage to achieve the desired overall recovery level. In the present study a single-stage pilot-plant operating near its gypsum scaling imposed recovery limit (62% recovery) produced a supersaturated concentrate stream that was then demineralized by seeded gypsum precipitation to produce a desupersaturated stream (gypsum saturation index ≈ 1). On-line monitoring of pH and calcium concentrations during the induced gypsum precipitation batch process allowed the degree of gypsum precipitation and the secondary RO recovery limit to be quantified. The ability to desupersaturate the primary RO concentrate operating at 62% recovery was successfully demonstrated and would enable the two-stage RO process to operate at the desired high recoveries (>85%).