Sustainable operation of hybrid semi-batch/batch reverse osmosis by additional purge-and-refill phase: Optimization and life cycle assessment

Abstract

Hybrid semi-batch/batch reverse osmosis (HSBRO) has been recently proposed as a high-recovery, low-energy, and compact desalination system. In this study, we introduce an additional purge-and-refill phase to minimize internal salt accumulation while enhancing energy efficiency and permeate quality. Experiments were conducted using NaCl- and Na<inf>2</inf>SO<inf>4</inf>-based feedwaters under varying semi-batch operation times, purge-and-refill times, and permeate flux conditions. The results show that extending the purge-and-refill first phase effectively reduces residual brine, thereby mitigating the initial total dissolved solids (TDS) spike, defined as the increase in internal salinity at the start of the semi-batch phase. However, an excessively long purge-and-refill first phase reduces recovery. This trade-off was addressed by compensating with a longer semi-batch operation time, which enabled high recovery with minimal increase in specific energy consumption (SEC). At a concentration of 4000 ppm NaCl, recovery dropped to 84 % with a 60 s purge-and-refill first phase, but was restored to 91 % by adjusting the semi-batch operation time, confirming that careful balancing of operational phases is critical for maintaining system performance. Experimental results identified optimal purge-and-refill first phase times of approximately 20 s for both NaCl and Na<inf>2</inf>SO<inf>4</inf>, effectively minimizing internal salinity and operating pressure. Life cycle assessment (LCA) using the ReCiPe 2016 (H) method revealed that Na<inf>2</inf>SO<inf>4</inf>-based wastewater treatment yielded up to 10 % higher environmental impacts compared to NaCl, highlighting the need for salt-specific operational strategies. These findings establish a practical operational framework for HSBRO, enabling high recovery and low energy demand while incorporating salt-specific environmental considerations.