Operating Strategy for the Energy-Efficient Reverse Osmosis (EERO) Desalination Process

Operating Strategy for the Energy-Efficient Reverse Osmosis (EERO) Desalination Process

William B. Krantz^a,c,, Tzyy Haur Chong^b,c, Siew-Leng Loo^b,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

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).

Optimal operation of the EERO process can require using NF membranes with very low salt rejections in one or more of the CMCR stages. If a membrane with a higher than optimal rejection is used, near optimum EERO performance can be achieved at the cost of a small increase in the SEC_net by boosting the pressure in the NF stage.

EERO process performance for different operating strategies is compared to that for SSRO and two SSROs in series based on the OPD and SEC_net. The annualized total cost of water production for EERO operation at 75% recovery is shown to be less than that for SSRO operation at 50% recovery.

At 75% recovery EERO results in a two-fold increase in the retentate concentration that not only reduces the brine disposal, but also translates to a four-fold increase in the energy density possible via a hybrid EERO-PRO (pressure-retarded osmosis) process for recovering the osmotic potential energy in the brine. As such, this hybrid process would cross the 10 W/m² threshold for economic PRO operation.

Acknowledgment

This research was supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Program and administered by the Environment & Water Industry Program office (EWI) of the PUB.  The Singapore Membrane Technology Center, Nanyang Environment and Water Research Institute. Nanyang Technological University is supported by the Economic Development Board of Singapore.