Selective removal of organic matters from high-salinity chemical industrial wastewater: Ultrafiltration or nanofiltration?

The effective separation and treatment of high-salinity chemical industrial wastewater has become a critical issue for the chemical industry. Membrane separation technology, including ultrafiltration (UF) and nanofiltration (NF) membranes, are potential candidates for selectively separating inorganic salts and organic compounds. In this study, we investigated the selective separation performance of real high-salinity chemical industrial wastewater by a series of commercial UF (UF10k, UF5k, UF3k, and UF1k) and NF (NF270, NF90, BSY90, BSY60, BSY30) membranes. UF1k exhibited the best separation performance among the various UF membranes, owing to its lowest molecular weight cut-off (MWCO) of 1 kDa. Further two-pass UF and coagulation-UF cannot effectively improve the separation performance of UF as the presence of low molecule weight organics in the wastewater. Among the NF membranes, we surprisingly found that some NF membranes, despite having smaller pore sizes theoretically, exhibited even lower rejection of organic matters than UF1k in high-salinity environments. Mechanistic investigation revealed that increased salt concentrations led to pore swelling, pore-wall dehydration and charge shielding effects in NF membranes, which resulted in a substantially enlarged MWCO. BSY90, the tightest NF membrane, exhibited the best performance in the rejection of organic compounds from the high-salinity chemical industrial wastewater, owing to its smallest pore size and highest zeta potential. Our findings offer guidance for the proper selection of UF and NF in the precise selective separation of substances in high-salinity chemical industrial wastewater.