Effect of initial feed and draw flowrates on performance of an 8040 spiral-wound forward osmosis membrane element

This study investigated the effects of initial feed (20–50 L/min) and draw flowrates (2–5 L/min) on 8040 spiral-wound FO element performances in serial configuration for a forward osmosis and reverse osmosis (FO-RO) hybrid system employing single element-based tests. Average J_w,ave values for varying feed and draw flowrates were found to be 20.93, 19.38 and 18.71 LMH at E1, E2 and E3 (first, second and third elements in a serial configuration), respectively, with averaged diluted draw concentrations of 12.55, 7.88 and 5.77 g/L (initial conc. = 35 g/L). The draw stream dilution was not governed by J_w,ave but by the initial draw flowrates at the inlet that governs the retention time of the draw water body in the element. To sum up the performance results, it was concluded that initial draw flowrate is found to govern the performances of FO elements in series in terms of both production of diluted draw stream, determined by the averaged water flux of the FO element, J_w,ave, and the degree of draw stream dilution. Specific energy consumptions (SECs) of RO were estimated with varying RO feed concentrations (i.e. diluted draw concentration); it was observed the efficiency of SEC reduction by the dilution significantly decays after a critical RO recovery rate. This study successfully provides a valuable insight for feasible application of the FO-RO hybrid system.