COMPARISON OF MEMBRANE SEPARATION PROCESSES IN THE INTERNAL PURIFICATION OF PAPER-MILL WATER

In recent years the paper industry has been studying different technol ogies to reduce fresh water consumption in the mills. In this study mi cro-, ultra- and nanofiltration of some paper mill waters both on a la boratory and a-pilot scale were compared. The performances of differen t membranes and modules are reported and some general conclusions are given. The performance of the membranes was evaluated by determining t he reduction of certain measurable parameters in the feed solution suc h as chemical oxygen demand (GOD), total organic carbon (TOC), total s olids (TS), lignin, sugar, anionic trash (CD), ions, conductivity, tur bidity, silt density index (SDI) and brightness decreasing substances. The tested tubular 0.4 mu m microfiltration membrane was not suitable as a pretreatment step to nanofiltration. In ultrafiltration the test ed cross rotational (CR) filter using flat sheet membranes gave signif icantly higher fluxes than the tested polymeric or ceramic tubular mem branes. It was shown that the relatively low cut-off (30 kD) hydrophil ic C30G membrane made from regenerated cellulose had higher fluxes bot h at neutral and acidic pH. Also the long time behaviour (duration fou r or six days) of the C30G was very good. It was also shown that the p H of the original feed, the experimental set-up and the membrane cut-o ff affected fouling the most in ultrafiltration. In nanofiltration the membrane structure and feed pH affected the results the most. Very go od results were obtained by all the tested nanofiltration membranes. I t was shown that fluxes similar to microfiltration are obtained by tub ular ultrafiltration membranes and that the reductions were better. Mo reover, the ultrafiltration process was more reliable than microfiltra tion as a pretreatment for nanofiltration because the obtained silt de nsity index was always below the value 5 with ultrafiltration. However , it was also shown that in nanofiltration better reductions were obta ined if no pretreatment was done, if it was not required by the module configuration.