Energy-Efficient Reverse Osmosis Desalination Process

William B. Krantza,c,, Tzyy Haur Chongb,c, Siew-Leng Loob,c

a Dept. of Chemical & Biological Engineering, University of Colorado, Boulder CO 80309-0424

b School of Civil & Environmental Engineering, Nanyang Technological University, Singapore 639798

c Singapore Membrane Technology Center, Nanyang Technological University, Singapore 637141

A novel energy-efficient reverse osmosis (EERO) process recently patented by the Singapore Membrane Technology Center is discussed for which the retentate from single-stage reverse osmosis (SSRO) serves as the feed to a countercurrent membrane cascade with recycle (CMCR). The EERO process can reduce the osmotic pressure differential (OPD) in desalination by capitalizing on four features: coupling SSRO with a CMCR; countercurrent retentate and permeate flow; permeate recycling; and retentate self-recycling owing to the use of one or more NF stages. The EERO process can achieve the same recovery as SSRO at a significantly reduced OPD. Moreover, there is a critical recovery above which the EERO process can reduce both the OPD and specific energy consumption (SEC) relative to conventional SSRO for the same recovery. For typical seawater feed of 35000 ppm the EERO process can achieve a recovery of
75% at an OPD of 55.5 bar and SEC of 3.086 kWh/m3. This represents a 50% reduction in the OPD at the same SEC as conventional SSRO. The high recoveries achieved by the EERO process result in a highly concentrated retentate. This can be used in a hybrid process that couples EERO with pressure-retarded osmosis (PRO) for harvesting the osmotic pressure energy in the retentate. The EERO process can double the osmotic draw potential relative to using the retentate from conventional SSRO as the draw solution in PRO. Hence, the EERO process not only can significantly increase the water recovery and reduce the OPD for water desalination, but also can increase the energy flux density of PRO.