Treatment of nanofiltration and reverse osmosis concentrates: comparison of precipitative softening, coagulation, and anion exchange.

Disposal and treatment of concentrate from nanofiltration (NF) and reverse osmosis (RO) are major challenges to implementing membrane treatment processes. Intermediate treatment of membrane concentrate, between primary and secondary membrane stages, has the potential to increase membrane recovery rates and decrease the volume of concentrate produced. To achieve this, however, there is a need to better understand treatment of membrane concentrate. As a result, this work systematically evaluated lime softening, ferric sulfate coagulation, and magnetic ion exchange (MIEX) as individual, intermediate treatment processes for membrane concentrate. Six membrane concentrates, from NF and RO, with varying concentrations of calcium, dissolved organic matter (DOM), and sulfate were chosen for this study. Maximum removal of calcium was achieved by lime softening, whereas maximum removals of DOM and sulfate were achieved by MIEX. The results of this work show that intermediate treatment of NF/RO concentrate is capable of producing treated concentrate with water quality approximately equal to the initial source water.