Treatment of nanofiltration and reverse osmosis concentrates: Systematic study of coagulation, softening and anion exchange

Membrane processes are becoming more prevalent. However, disposal and treatment of membrane 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 system recovery rates and decrease the volume of concentrate produced. However, there is a need to better understand physical-chemical treatment of membrane concentrate. This work systematically evaluated lime softening, ferric sulfate coagulation, and ion exchange as individual, intermediate treatment processes for membrane concentrate. Six membrane concentrates with varying concentrations of calcium, dissolved organic matter (DOM) and sulfate were chosen for this study. Maximum removal of calcium, DOM and sulfate was achieved by ion exchange. The results of this work show that physical-chemical treatment of NF/RO concentrate is capable of producing treated concentrate with calcium and DOM concentrations approximately equal to the initial source water.