(29c) Evaluation of RO Concentrate Treatment for High Recovery Desalting of High Salinity Inland Brackish Water

Development of new water sources is crucial in order to address the crisis of dwindling water supplies around the globe. Reverse osmosis (RO) desalination is a technology that is now in common practice for developing new water sources via desalting of seawater, brackish surface water and groundwater, as well as municipal wastewater. While RO is considered a mature technology, its application for desalination of inland brackish water remains challenging due to the high costs associated with residual RO concentrate disposal and membrane mineral scaling. The above challenge is particularly daunting in California's San Joaquin Valley with respect to the critical need for high-salinity agricultural drainage (AD) water management. Accordingly, in the present study, the technical feasibility of primary RO (PRO) desalting and its integration with PRO concentrate treatment was evaluated, via both experimental field tests and process analysis, to assess its potential use in enabling recovery enhancement via secondary RO (SRO) desalting. Two concentrate treatment options were considered: precipitation softening (PS) and a new chemical enhanced seeded precipitation (CESP) process. In the CESP process, initial partial lime treatment provides adsorptive removal of residual antiscalant from the PRO concentrate. This enables subsequent concentrate desupersaturation (with respect to the major scaling salt calcium sulfate) to occur via seeded gypsum precipitation, unimpeded by residual antiscalant. The desupersaturated PRO concentrate (~ 34% reduction in SIg from ~ 1.7 to ~ 1.1) can then be further desalted in an SRO step up to the limit that is feasible with antiscalant dosing for scale control. In order to assess the cost of various AD water desalination management options, on the basis of treated feed volume, a comparison was undertaken of the total cost (including brine disposal for all options) for: (a) PRO-CESP-SRO and PRO-PS-SRO at the maximum recovery limit of 93%, and (b) RO desalting up to the maximum attainable recovery of 62% without concentrate demineralization. Process and economic analysis of water salinity management options for the AD water source considered suggested that, when considering the value of the desalted product water and accounting for the disposal cost of residual brine, PRO-CESP-SRO desalting is the most economically attractive option. The break-even AD water desalination management cost (i.e., zero net cost) for PRO-CESP-SRO, for the present cost estimates, was at a product water value of 0.67–0.74 $/m3 which is lower by 22–29% compared to that of the PRO-PS-SRO process (0.95 $/m3). Admittedly, RO desalting (along with disposal of residual brine) can be more economical when brine disposal cost is below about ~ 0.44 $/m3 brine, but this is unlikely to be a feasible or acceptable water management option given the environmental challenge of disposal and loss of valuable water resource.