Energy efficiency of staged reverse osmosis (RO) and closed-circuit reverse osmosis (CCRO) desalination: a model-based comparison Simeng Li, Karla Duran, Saied Delagah, Joe Mouawad, Xudong Jia and Mohamadali Sharbatmaleki ABSTRACT Reverse osmosis (RO) technologies have been widely implemented around the world to address the rising severity of freshwater scarcity. As desalination capacity increases, reducing the energy consumption of the RO process per permeate volume (i.e., specific energy consumption) is of particular importance. In this study, numerical models are used to characterize and compare the energy efficiency of one-stage continuous RO, multi-stage continuous RO, and closed-circuit RO (CCRO) processes. The simulated results across a broad range of feed salinity (5,000–50,000 ppm, i.e., 5–50 g kg 1 ) and recovery (40%–95%) demonstrate that, compared with the most common one- stage continuous RO, two-stage and three-stage continuous RO can reduce the specific energy consumption by up to 40.9% and 53.6%, respectively, while one-stage and two-stage CCRO can lead to 45.0% and 67.5% reduction, respectively. The differences in energy efficiencies of various RO configurations are more salient when desalinating high-salinity feed at a high recovery ratio. From the standpoints of energy saving and capital cost, the simulated results indicate that multi-stage CCRO is an optimal desalination process with great potential for practical implementation. Key words | closed-circuit reverse osmosis, desalination, energy efficiency, pressure recovery, specific energy consumption, staging HIGHLIGHTS • Numerical models are used to compare energy efficiencies of different RO processes. • Multi-stage continuous RO may not be energy-efficient at a high recovery rate. • Multi-stage closed-circuit RO configurations can greatly improve energy efficiency. • Long-term stability in a larger-scale CCRO system needs to be explored. Simeng Li (corresponding author) Karla Duran Mohamadali Sharbatmaleki Department of Civil Engineering, California State Polytechnic University Pomona, 3801 West Temple Avenue, Pomona, CA 91769, USA E-mail: sli@cpp.edu Saied Delagah Denver Federal Center, US Bureau of Reclamation, PO Box 25007, Denver, CO 80225, USA Joe Mouawad Eastern Municipal Water District, 2270 Trumble Road, Perris, CA 91570, USA Xudong Jia College of Engineering and Computer Science, California State University Northridge (CSUN), Northridge, CA 91330, USA INTRODUCTION Challenged by rapidly growing human population, increas- ing living standards, and deteriorating climate change around the world, freshwater scarcity is becoming an increasingly severe issue (Gosling & Arnell ). With over two-thirds of the global population (∼4 billion people) living under severe water scarcity for at least one month each year (Mekonnen & Hoekstra ), there is an increased demand for producing freshwater from different saline water sources (e.g., seawater and brackish ground- water) through desalination (Gude ). For most water sources, membrane-based desalination, particularly reverse osmosis (RO), is the most prevalent technology for desalination on a large scale because of its relatively high energy-efficiency and low operational costs 3096 © IWA Publishing 2020 Water Supply | 20.8 | 2020 doi: 10.2166/ws.2020.208 Downloaded from http://iwaponline.com/ws/article-pdf/20/8/3096/814692/ws020083096.pdf by guest on 11 August 2021