Energy optimization of a multistage reverse osmosis process for seawater desalination

The water recovery, net specific energy consumption (SEC_net) and osmotic pressure differential (OPD) are determined for a multistage reverse osmosis (MSRO) process relative to conventional and the recently advanced energy-efficient RO (EERO) processes. The MSRO process combines RO and nanofiltration (NF) stages in series, and blends permeate from the NF stages with the saltwater feed. This increases the water recovery and lowers the rejections required in the RO stages. The SEC_net of the MSRO process to produce water containing ≤ 350 ppm salt is evaluated at the thermodynamic limit for pump and energy-recovery-device efficiencies of 85% and 90%, respectively, which provides a basis of comparison relative to alternative processes. The MSRO process employing one RO and two NF stages in series achieves a 65% water recovery for a 35,000 ppm seawater feed producing a product water with ≤ 350 ppm salt at an OPD of 51.7 bar and SEC_net of 2.688 kWh/m³, reductions of 35% and 8%, respectively, relative to conventional SSRO. For the same conditions, the MSRO process employing two RO and two NF stages in series requires an OPD of 68.0 bar and SEC_net of 2.22 kWh/m³, reductions of 14% and 1.6%, respectively, relative to two-stage SSRO.