Effects of finite flux and flushing efficacy on specific energy consumption in semi-batch and batch reverse osmosis processes

Abstract

This work focuses on Specific Energy Consumption (SEC) in batch and semi-batch reverse osmosis (RO) processes. It is proved from optimal control theory that the minimal SEC in semi-batch RO occurs at constant flux conditions, similar to conclusions in both batch RO and continuous, infinite-stage RO. While semi-batch RO is comparable to two- or three-stage RO at the thermodynamic limit (i.e. zero flux), its performance may be severely compromised by finite flux applied in desalination and efficacy of flushing before the next filtration step. Batch RO is even more susceptible to salt retained during flushing. On the basis of typical flux used in industrial desalination and a 95% flushing efficacy, semi-batch RO at the cyclic steady state is only similar to one-stage RO at low recoveries (e.g. 30 and 40%) and excels slightly at high recoveries (e.g. 50 and 60%). Batch RO may outperform one-stage RO but is still not as good as two-stage RO. Reducing salt retention and operating at a reduced flux are ways to make batch and semi-batch ROs energy advantageous over their continuous counterparts.