A new pretreatment approach for applying high-recovery reverse osmosis desalination to highly scaling brackish groundwater

Abstract

A new approach is presented for pretreating very hard, divalent-ion-laden brackish-water, to allow applying reverse-osmosis desalination at reasonable recovery-ratio and competitive cost. The approach consists of a cation-exchange step applied on the raw-water to reduce the divalent-cation concentrations thereby enabling operation at high recovery (>80 %) without chemical scaling. For cost-effectiveness, the exhausted ion-exchange regeneration solution undergoes nanofiltration separations to recover the NaCl, enabling multi-cycle usage. The process was exemplified on brackish groundwater from New-Mexico's National-Desalination-Research-Facility. Two treatment alternatives were considered for the exhausted regeneration brine, consisting of several NF in series (alternative 1) and additional NF-retentate treatment (alternative 2). Following simulation and selection of alternative 1, empirical-evaluation and cost-assessment ensued. The results showed that when the regeneration-solution contained total-hardness < 2 % of the total-cation concentration (eq/eq), the cation-exchange breakthrough curves did not deteriorate. To maintain this criterion the exhausted regeneration-solution should undergo 3–4 sequential NF passes after each regeneration cycle. This approach enables operating the BWRO plant with RR = 81 % at additional cost of ∼US$0.4/m3 (CAPEX+OPEX), which ∼doubles the overall brackish desalination cost, but also produces more desalted water and much less retentate. The approach is competitive when the brackish water is laden with divalent ions and the retentate discharge-cost is high.