Reverse Osmosis (RO) membrane desalination of inland brackish water is confronted by high costs associated with residual concentrate disposal and membrane mineral scaling. Therefore, high permeate recovery is critical for both increasing productivity and reducing the RO concentrate discharge volume. However, achieving high recovery is a challenge when confronted with source water of high mineral scaling potential. The above challenge was addressed in his study by an approach to high recovery desalination of source water of high scaling propensity that integrates (a) primary reverse osmosis and nanofiltration (NF) (PRONF) desalination with (b) interstage PRONF concentrate desupersaturation and subsequent treatment, followed by (c) secondary RONF (SRONF) desalination. Ther study focused on brackish groundwater from the Salinas Valley, CA in which calcium carbonate is the main potential scalant that has been determined to limit high desalination recovery. PRONF and SRONF was carried out in a unique flexible RO-NF system (FLERONF) (fitted with 2.5-inch spiral-wound elements) having permeate production capacity of ~1 m³/h operating in the mode of partial concentrate recycle. Processing of brackish water having Langelier Saturation Index (LSI) of 0.5. The PRONF concentrate (resulting from operation at recovery of 70%) produced had LSI of 1.8 (upper limit for deployment of antiscalant dosing). PRONF concentrate desupersaturation via accelerated seeded precipitation (ASP) was accomplished at alkaline pH (~9-11) along with accelerated nucleation achieved with the addition of calcite seeds. The LSI of the PRONF concentrate was reduced from 1.8 to 0.2, enabling further SRONF desalting (post-MF/UF filtration) with antiscalant feed dosing to attain overall recovery >94%. The levelized cost of the produced permeate streams was assessed considering the cost of concentrate management via discharge and the price of water for agricultural and potable applications.
