The effect of transmembrane pressure and pH on treatment of paper machine process waters by using a two-step nanofiltration process: Flux decline analysis

In this work, the treatment of paper machine whitewater obtained from paper production by using loose and tight Nanofiltration (NF) membranes in a lab-scale plant with a plate and frame module was investigated. The NF membranes were evaluated for fouling, flux decline due to concentration polarization and their suitability for removing of components present in wastewater. The loose NF membrane was used for the first step under different transmembrane pressure (8 to 32 bars) and pH (3, 5.6 and 10) values. The best filtration performances were provided under a pressure of 32 bar and a pH of 5.6. The fouling mechanism at different transmembrane pressures and pH values was investigated by “Hermia's model”. Also, fouling on pores and on the surface of NF membranes was characterized by Scanning electron microscopy (SEM) and contact angle measurements, respectively. For the advanced treatment of permeate obtained from the loose NF membrane that provided the lowest fouling and flux decline, a tight NF membrane was used in the second step. Even better retentions were achieved from experiments with this NF membrane. Results obtained in this study showed that the combination loose and tight NF membranes achieved reuse of paper machine white water for use as shower water of paper machine.