Numerical model-based analysis of energy-efficient reverse osmosis (EERO) process: Performance simulation and optimization

We conducted a feasibility study of the energy-efficient reverse osmosis (EERO) process, which is a multi-stage membrane system that integrates single-stage reverse osmosis (SSRO) and a countercurrent membrane cascade with recycle (CMCR). To this end, we developed a numerical model for the 1-2 EERO process (one SSRO stage with two stages in CMCR: one nanofiltration (NF) stage followed by one terminal RO stage), then validated the model using performance data obtained from commercial RO projection software. Retentate recycle ratio was one of the key parameters to determine energy efficiency of EERO. In addition, the implementation of NF membranes in the first stage of CMCR yielded additional improvement in EERO performance and played an important role in determining optimum salt rejection. An optimal design of the NF stage was successfully achieved by hybridization of different NF membranes in a vessel (internally staged design, ISD). Under the conditions optimized, EERO exhibited not only greater energy efficiency (3-25%), but lower concentration polarization (CP) and potentials of membrane fouling than conventional SSRO for >= 55% overall recoveries because of reduced water flux in the lead elements (averagely 34%). These findings can thus provide insight into optimal design and operation of the EERO process.