Energy-efficient reverse osmosis desalination process

Abstract

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 35g/L the 3-stage EERO process can achieve a 75% recovery at a net SEC of 2.746kWh/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 (3.086kWh/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.