The energy-efficient reverse osmosis (EERO) process, patented in 2014 by the Singapore Membrane Technology Center, reduces the osmotic pressure differential (OPD) and the cost for potable water production from a saline water feed by combining Single-Stage Reverse Osmosis (SSRO) with a countercurrent membrane cascade with recycle (CMCR). The retentate from the SSRO stage is the feed to the CMCR and its permeate is blended with that from the CMCR to produce the potable water product. Key features of the EERO process are: (1) optimum injection of the SSRO retentate into the CMCR; (2) operating all stages at the same OPD; (3) countercurrent retentate and permeate flow; (4) permeate recycle to the retentate side in the CMCR; and (5) retentate recycle to the permeate side by employing one or more NF stages in the CMCR. These features reduce the concentration difference across the membrane in each stage, thereby reducing the OPD and net specific energy consumption (SEC_(net)). Since the permeate from an NF stage has a reduced divalent salt concentration and is the feed to the terminal CMCR stage, the latter can be operated at a higher recovery. Operating the EERO stages at the same OPD requires that the recoveries of adjacent CMCR stages add to one. Hence, fouling in the NF stage whose feed has a high divalent salt concentration can be mitigated by operating it at a lower recovery (higher safety factor).
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