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

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 1year 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.

Automated summary

This study demonstrates the importance of controlling residual chlorine content in front-end inlet water to maintain the long-term efficiency of nanofiltration membrane. The use of a pre-treatment unit such as coconut shell activated carbon can effectively remove residual chlorine, ensuring the efficiency and lifespan of the nanofiltration membrane. This work provides valuable insight for the development of nanofiltration membrane for large-scale separation and resource recovery of NaCl and Na2SO4 from coal chemical wastewaters.