Advances in hybrid semi-batch/batch reverse osmosis (HSBRO) using a novel system configuration

Hybrid semi-batch/batch reverse osmosis (HSBRO) is an emerging desalination technology suited to minimal and zero liquid discharge applications, thanks to its high recovery and low energy consumption. To further lower the energy consumption of HSBRO, we explore an improvement to the initial semi-batch phase of operation using a novel system configuration in which the work exchanger is positioned upstream, rather than downstream, of the feed mixing point. Feed pressure is measured and compared between the upstream and downstream configurations. The pressure increases linearly with time, at a gradient represented by simple analytic expressions. Measurements confirm that the upstream configuration yields a less steep gradient and thus lower Specific Energy Consumption (SEC). The verified analytic expressions are applied to model scaled-up HSBRO systems with up to seven RO elements in series, at water recovery ≤98.5%. Whereas increasing the number of elements incurs an energy penalty in the downstream configuration, the upstream configuration mitigates this penalty, maintaining SEC almost constant for 1–4 elements in series. The upstream configuration favours lower recirculation pump flow and provides overall energy saving of 8% using available membranes (operating ≤120 bar), potentially increasing to 12% with future membranes operating at up to 200 bar.