Critical flux in NF of high molar mass polysaccharides and effluents from the paper industry

High molar mass polysaccharides (locust bean gum and karaya gum) and efflue nts from a mechanical pulp mill and a paper mill were nanofiltered with com mercially available nanofiltration (NF) membranes. The effect of the filtra tion conditions on the flux (critical flux), retention, and the fouling of the membranes was studied. The experiments were conducted by increasing and decreasing the pressure and measuring the flux thus obtained. The critical flux was observed to increase with increasing flow velocity an d decreasing concentration. An increase in pH increased the electrostatic r epulsion between the membrane and the dissociated (charged) components in t he paper mill effluents. As a result, a higher critical flux was obtained a nd also the retentions of the charged substances improved. Only a weak form of the critical flux was observed with the mill effluents. The permeate fl ux deviated from the pure water flux even at the lowest pressure, but incre ased linearly with pressure until the weak form of the critical flux was ex ceeded. The small decrease in flux immediately after filtration was started was probably caused by the plugging of the free spaces in the membranes or by the adsorption of foulants onto the membrane surface. In the filtrations with the high molar mass polysaccharides, a strong form of the critical flux as well as a weak form were observed, The significant irreversible fouling of the most hydrophobic membrane was due to adsorption of the model substances by hydrophobic interaction. A precleaning of the m embranes with an alkaline cleaning agent improved the pure water fluxes by up to 30%, but it had only a small effect on the critical or the Limiting f lux. The pure water fluxes of precleaned membranes after filtration were st ill higher than the pure water fluxes of the untreated membranes before fil tration. (C) 2000 Elsevier Science B.V. All rights reserved.