Batch reverse osmosis (RO) is a promising approach to high-recovery desalination. It has low energy consumption, but system size increases sharply with recovery because of the need for a large work exchange vessel. In this study, we propose a compact hybrid batch/semi-batch reverse osmosis (HBSRO) system incorporating aspects of each approach. HBSRO works in three phases, i.e. semi-batch pressurisation phase, batch pressurisation phase, and finally purge-and-refill phase. We analyse ideal and practical cases of HBSRO to gain understanding about the specific energy consumption (SEC) and size of the system. In the ideal analysis, HBSRO can halve the size of work exchange vessel while incurring just a 5% energy penalty compared to batch RO at all recoveries. In the practical case, accounting for non-idealities, HBSRO has lower SEC than batch RO at recovery over 0.9, because a smaller volume of work exchange vessel minimises the energy penalty of the purge-and-refill phase in HBSRO. The reduced volume not only makes HBSRO more practical, but also improves energy-efficiency through reduced losses. Thus, our study highlights that HBSRO is highly flexible, achieving high recovery, compact size, and low SEC – advantages that are especially important in minimal or zero liquid discharge applications.


  • Desalination harvests large fresh water resources from seawater and brackish water [1,2,3]

  • The results reveal that hybrid batch/semi-batch reverse osmosis (HBSRO) can improve the practicality of the design, by reducing the size of work exchange vessel significantly while maintaining the high energy efficiency of the batch reverse osmosis (RO) for high-recovery desalination and brine concentration

  • HBSRO is a good candidate for desalination systems with high recovery and high energy efficiency, and the potential will grow as RO membrane technology advances

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Desalination harvests large fresh water resources from seawater and brackish water [1,2,3]. Recent research has sought to extend the application of batch RO towards largescale treatment of inland brackish water desalination [47] and full-scale system for seawater desalination [45,48] These studies found that batch RO suffers the drawback of a large work exchange vessel (in the freepiston design) or large flexible bladder, when recovery over about 0.8 is required [45,47,48].

Rationale and working principles of HBSRO
Practical design model of the HBSRO process
Non-ideal analysis of HBSRO
Results and discussion
Energy and feasibility analysis of HBSRO in the ideal case
Energy and feasibility analysis of HBSRO in the practical case
Recommended operating strategy for effective utilisation of the HBSRO

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