(460b) Salt Harvesting of Reverse Osmosis Concentrate By Continuous Chemically-Enhanced Seeded Precipitation

Desalination of inland brackish water via reverse osmosis membrane (RO) technology is often confronted by the need to attain high recovery permeate product recovery in order to reduce the challenge of concentrate (brine) management. Driving toward high recovery is particularly problematic in source water that are of high scaling propensity as is typically the case with agricultural water source and especially agricultural drainage. In order to reach high recovery RO operation while avoiding membrane mineral scaling, a process of chemically-enhanced seeded precipitation (CESP) was proposed as an intermediate process to desupersaturate (with respect to sparingly water soluble salts such as gypsum and calcite) the RO concentrate from a primary RO stage so as to enable subsequent additional desalting in a secondary RO state and hence increased the overall recovery. In the CESP process CaCO₃ precipitation (as calcite) is first induced via partial lime-dosed treatment for antiscalants (originating from the primary RO) scavenging, followed by subsequent CaSO₄ (as gypsum) seeded concentrate desupersaturation. The CESP process, which was previously demonstrated as a batch process, was explored in the present study to evaluate the feasibility of utilizing the approach as a practical means for achieving high recovery RO under continuous desalting process operation. Accordingly, a novel fluidized bed CESP reactor system, for continuous RO system operation, was developed to systematically investigate the impact of various process conditions (e.g., feed composition and pH, temperature, CESP crystallizer residence time). Continuous RO concentrate demineralization tests were conducted using antiscalant-containing, gypsum-supersaturated RO concentrate solutions. These studies demonstrated that precipitated calcite particles in the initial partial lime treatment step (with minimum dose compared to conventional softening process) enabled significant antiscalants scavenging; this facilitated subsequent mineral salts precipitation via non-chemical (i.e., gypsum seeded) heterogeneous precipitation. Continuous mode operation of the partial-lime and seeded-precipitation fluidized reactor, along with model simulations, demonstrated that steady-state operation with respect to the required level of calcium desupersaturation and a particle size distribution was achievable by optimizing the size and initial mass loading of gypsum seed, as well as the fluidized CESP reactor recycle ratio (i.e., fluidized bed recirculation flow rate relative to the CESP feed flow rate). Results of the current study indicate that enhancement of overall RO water recovery RO from 63% via primary RO up to 90% (or higher) with CESP followed by secondary RO are feasible for raw source water sources that are supersaturated with respect to both gypsum and calcite.