Comparison of semi-batch reverse osmosis configurations

Semi-batch reverse osmosis (RO) is a non-conventional approach to RO intended for high-recovery desalination, whereby the RO membrane module is placed in a pressurised recirculation loop that is continually supplied with saline feed. In some variants, the loop includes an additional vessel to lengthen the cycle time or to facilitate continuous operation. This vessel can also enable a batch phase of the cycle (i.e., hybrid semi-batch/batch RO operation, HSBRO). Nonetheless, few studies have investigated the effect of the additional vessel and how best to configure it. Here, we compare four configurations: a well-mixed vessel downstream of the mixing point of the feed (Dx); a well-mixed vessel upstream of the mixing point (Ux); a plug-flow vessel downstream of the mixing point (Dg); and a plug-flow vessel upstream of the mixing point (Ug). Compared to Dx and Dg, the upstream configurations Ux and Ug consume less energy because they favour supply of low concentration feed to the membrane. This advantage grows at high concentration factor. The differences between well-mixed and plug flow are marginal except at low concentration factors. Considering practical losses, modelling shows that configuration Ux can save 9 % energy over Dx in a HSBRO system comprising two RO membranes treating brackish water with a concentration factor of 35.