Evaluation of nanofiltration membranes for filtration of paper mill total effluent

Abstract

In the pulp and paper industry, membrane technology can be applied both in recycling of the valuable materials within the process, and in pollution control. A major challenge for membrane technology is purification of process water, so that it can be reused, for example, as shower water. The study is part of a project to develop a single-stage nanofiltration process for recirculation of paper mill water, and to reuse this water as shower water. Different membranes were used to treat effluent, which consisted of water from a thermomechanical pulp plant and paper machine. In the ultrafiltration experiments it was seen that the resulting permeates were practically free from highly conjugated lignin residuals, anionic trash and turbidity. The reductions in total carbon, chemical oxygen demand and inorganic matter were between 50% and 60%. Even better reductions were achieved from experiments with nanofiltration membranes. It was seen that the resulting permeates were totally free from turbidity, colour and anionic trash. Reductions of more than 80% were measured for chemical oxygen demand, total carbon and inorganic matter. Chloride ions were not well retained by NF membranes. The permeabilities of NF membranes were between 3 and 13 I/(M²h bar), depending on the piece of membrane and the pretreatment of the membrane. The use of parallel spacers greatly decreased the reduction in the permeate flux during the filtration. Reductions in chloride ions were better using RO membranes, as expected. The reductions in chemical oxygen demand, total carbon and sugar were above 95% for filtration using RO membranes. The permeabilities of these membranes were below 2.5 l/(M²h bar). Thus they are not suitable for industrial-scale filtration. The best membranes for treatment of the paper mill effluent used here were the Desal-5 and NF45 membranes. These had a low flux reduction of the pure water flux, stable permeate flux and good permeabilities. The reductions in the measured parameters were also acceptable.