Energy-efficient reverse osmosis desalination process

• New desalination process combines SSRO with countercurrent membrane cascade.
• Multistage process requires no interstage pumping on retentate side.
• Reduced osmotic pressure difference relative to conventional SSRO at all recoveries.
• Critical recovery above which SEC is less than that for conventional SSRO.
• 75% recovery with 33% reduction in pressure and 11% in SEC relative to SSRO.

A novel energy-efficient reverse osmosis (EERO) process is proposed for which the retentate from single-stage reverse osmosis (SSRO) serves as the feed to a countercurrent membrane cascade with recycle (CMCR). The 3-stage EERO process employs two, whereas the 4-stage EERO process employs three stages in the CMCR. The EERO process is advantageous because of four features: (i) coupling SSRO with a CMCR; (ii) countercurrent retentate and permeate flow; (iii) permeate recycling; and (iv) retentate self-recycling owing to the use of one or more nanofiltration stages. The EERO process was compared to conventional SSRO for both processes operating at the thermodynamic limit and employing an energy-recovery device. For the same overall recovery the osmotic pressure differential is reduced by 33% and 50% relative to SSRO for the 3- and 4-stage ERRO processes, respectively. There is a critical recovery above which the EERO process also can reduce the specific energy consumption (SEC) relative to SSRO for the same recovery. For a typical seawater feed of 35 g/L the 3-stage EERO process can achieve a 75% recovery at a net SEC of 2.746 kWh/m3, an 11.0% reduction in the SEC relative to SSRO for the same recovery. The 4-stage EERO process can achieve a 75% recovery at the same net SEC as SSRO (3.086 kWh/m3). Accounting for the additional membrane area required for the EERO process increases its cost relative to that for SSRO by at most 8%. An additional benefit of the EERO process relative to SSRO is the highly concentrated retentate that reduces the brine disposal volume or can be used to greatly increase the draw potential to harvest its osmotic potential energy via the pressure-retarded osmosis process.

(a) Osmotic pressure differential, (b) gross specific energy consumption (SEC), and (c) net specific energy consumption versus overall recovery for the 35 g/L seawater feed for single-stage reverse osmosis (SSRO) and the 3-stage and 4-stage energy-efficient reverse osmosis (EERO) processes; the arrows indicate the recovery range for which the EERO processes have a lower SEC than SSRO; both EERO processes have a lower OPD than SSRO for all recoveries.Figure optionsDownload high-quality image (101 K)Download as PowerPoint slide