Evaluation of fouling mechanisms in the nanofiltration of solutions with high anionic and nonionic surfactant contents using a resistance-in-series model

The effects of feed concentration and transmembrane pressure (TMP) on membrane fouling in the treatment of cleaning-in-place (CIP) wastewater originated from the production of liquid dishwashing detergent with a nanofiltration (NF) membrane were investigated. The resistance-in-series model was used to evaluate the flux decline caused by a gel layer, a concentration polarisation layer, and internal pore blocking in the NF membrane for CIP solutions of varying concentration termed CIP 5, CIP 10 and CIP 20. With an increase in feed concentration and TMP, it was observed that resistance of the gel layer (Rg) played a more important role in the flux decline than that of the concentration polarisation layer or internal pore fouling (Rcp + in). Considering the membrane resistance (Rm) values, the CIP solutions containing high concentrations of anionic and nonionic surfactants and low dye and salt concentrations did not cause serious fouling of the NF membrane. Characterisation of the membrane surface by atomic force microscopy (AFM) and contact angle measurements also showed that the deposition of surfactant aggregates on the NF membrane surface likely played an important role in the gel layer fouling. The NF membrane showed a rejection efficiency of over 98% for anionic surfactants, nonionic surfactant and dye in all CIP solutions. Salt rejection was not achieved in the CIP 5 solution because of the Donnan effect, whereas salt rejections were around 11–34% and 28–54% in the CIP 10 and CIP 20 solutions, respectively. The resistance-in-series model was successfully tested to evaluate the flux decline for the CIP solutions containing anionic and nonionic surfactants, dye and salt at various TMPs.