Multi-cycle operation of semi-batch reverse osmosis (SBRO) desalination

Semi-batch reverse osmosis (SBRO) desalination was investigated via process modeling and experimentally using a spiral-wound SBRO pilot system. The salinity increase during SBRO filtration necessitates periodic flushing of the excess salinity buildup in the system. Accordingly, concentrate flushing efficacy and its impact on SBRO performance were investigated for multi-cycles of filtration and concentrate flushing. Progressive cycle-to-cycle concentrate salinity rise, due to incomplete (concentrate) flushing, was apparent leading to corresponding SEC increase, but a stable cycle-to-cycle operation was ultimately achieved. Flushing duration longer than the convective residence time improved concentrate flushing. However, for a given recovery, longer flushing required longer filtration. The analysis confirmed that the SEC for an ideal SBRO operation (i.e., complete flushing of excess concentrate salinity) is lower relative to SPRO. However, ideal operation is likely to be a challenge with practical SBRO systems as it would necessitate complete concentrate flushing over the course of one convective residence time. Indeed, experimental evaluation of SBRO demonstrated higher SEC relative to single-pass RO (SPRO). Clearly, experimental quantification of concentrate flushing efficacy is critical for establishing SBRO effectiveness. Irrespective of SEC considerations, both SBRO and SPRO with partial recycle offer the flexibility of desalting operation over a wide recovery range.