A full-scale study of nanofiltration: Separation and recovery of NaCl and Na2SO4 from coal chemical industry wastewater

Abstract This study focuses on the application of nanofiltration membrane in separating NaCl from Na2SO4 of a “zero discharge” process designed for a particular coal chemical wastewater (60 m3/h) in full scale. The front-end inlet water's residual chlorine content seriously affects the subsequent nanofiltration membrane's long-term efficiency. The nanofiltration membrane was oxidized when the front-end inlet residual chlorine exceeds 0.1 mg/L. In 1-year operation, the rejection rate of the nanofiltration membrane for chemical oxygen demand (COD) dropped from ~70% to ~50%, and the rejection rate for silica dropped from ~30% to ~20%; in the same period, the rejection rate for sodium sulfate dropped from 99.5% to ~88%. It was found that coconut shell activated carbon, a pre-treatment unit integrated in front of the nanofiltration membrane unit, could remove up to 98% of residual chlorine in the system. The chlorine concentration entering the nanofiltration unit was kept below 0.05 mg/L, thus maintaining the nanofiltration membrane's efficiency and extending its lifespan. This work provides a foundation for the nanofiltration in separation and resource of NaCl and Na2SO4 from a large-scale perspective and implication for the development of Nanofiltration membrane for coal chemical wastewaters.