Comparison of nanofiltration and tight ultrafiltration membranes in the filtration of paper mill process water

Several nanofiltration and tight ultrafiltration membranes were tested by mill-site filtrations using a DSS-Labstak M20-filter. Retention, flux and fouling were analysed. Membranes were also characterised by measuring contact angles and pure water permeability. These data were used to explain the experimental results with effluents and to choose the possible membranes for the paper mill process waters. Comparison of different membranes in the constant pressure filtrations showed that usually the membranes with high pure water permeability or contact angle had worse fouling (fouling that had to be removed by alkaline cleaning). In addition, the fouling was stronger in filtrations made at acidic pH than at neutral conditions. The organic substances were retained significantly better at neutral pH than at acidic pH. Generally, when the hydrophilicity of the membrane increased the effect of pH on the retention properties increased as well. However, an individual membrane may deviate remarkably from the general trends. The results showed that some commercially available membranes have both high permeability and good retention properties at the same time.