Field Evaluation of Brine Demineralization for High Recovery RO Desalination

RO desalination of brackish water in the San Joaquin Valley may be feasible only at high recoveries that may be achieved through the use of a two-stage RO process with inter-stage demineralization of scale precursors. In the present work, the concentrated brine from a pilot RO system operating in the San Joaquin Valley was demineralized by seeded gypsum precipitation to reduce the gypsum scaling propensity sufficiently to enable secondary RO recovery corresponding to an overall recovery greater than 85%. Brine was collected from the RO pilot system during scale-free operation at 52.3% recovery and then treated to remove scale precursors (e.g. calcium ion, silica). At this recovery the antiscalant concentration in the retentate was ~6 ppm. Demineralization was achieved via seeded gypsum precipitation (SGP) of the supersaturated retentate to induce gypsum precipitation. SGP was performed in a 20-liter stirred-tank batch crystallizer with continuous monitoring of calcium ion activity, pH, and temperature. The ability to desupersaturate the pilot system’s primary RO concentrate was successfully demonstrated for brackish water in the San Joaquin Valley. Calcium concentration was reduced by 32% to 34% and the gypsum saturation index was reduced to near saturation (to ~1.1). This successful demineralization was achieved by seeded gypsum precipitation and would enable a two-stage RO process to operate at the desired high recoveries (>85%) necessary for a cost effective desalination process.

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