Semi-batch and batch low-salt-rejection reverse osmosis for brine concentration

Abstract

Brine management strategies require efficient methods of desalinating high-salinity streams (> 7 wt%). Low-salt-rejection reverse osmosis (LSRRO) provides salt leakage to reduce the trans-membrane osmotic pressure, and has higher water recovery than reverse osmosis (RO). However, LSRRO has high energy consumption (e.g., >5kWh/m3 for seawater feed). In this work, we develop novel transient multistage processes (i.e., semi-batch and batch) that employ LSRRO membranes to desalinate high-saline streams. Herein, we use a simplified lumped model to capture the system performance under ideal and non-ideal conditions and optimize salt rejection per pass for the multistage system. The results show that the batch LSRRO system performs dewatering with fewer stages and more efficiently than the steady-state LSRRO. In particular, the multistage batch system significantly outperforms the steady-state LSRRO by reducing energy consumption between 38 % to 73 %. The ZLD application for 4-stage batch systems requires energy consumption between 1.07 and 6.96kWh/m3 for the feed salinity range between 0.1 and 1 mol/L. Conversely, semi-batch LSRRO is less efficient than steady-state and batch LSRRO. It is constrained by the loss due to the mixing of streams which results in more stages, tighter range for membrane selectivity, and higher energy consumption than batch LSRRO.