The mechanism of fouling and synthetic membrane development for treating coating plant effluent from a pulp and paper mill.

Abstract

The use of the membrane is ever increasing with the development of membranes which can tolerate higher temperatures and a wider range of pH. In the pulp and paper industry, to comply with the strict effluent regulations and to reduce the water consumption, more and more water has to be recycled within the process after some internal treatments. Paper coating plant effluent and paper machine white-water are the streams that would allow for the application of membranes in pulp and paper. Therefore, in this research an attempt was made to develop synthetic membranes and also to study the mechanism of fouling for treating these streams. Asymmetric membranes having different pore sizes were prepared from polyethersulfone by the phase inversion technique using casting solutions of different polymer concentrations. These membranes were further modified by coating a thin layer of SPPO (sulfonated polyphenylene oxide) material. Each type of membrane used in this study was characterized in terms of mean pore size, pore size distribution, pore density, surface porosity and surface roughness by solute transport and also by atomic force microscope (AFM). Fouled membranes after ultrafiltration experiments were also characterized. Membranes were tested for laboratory made feed solutions which included clay and/or styrene butadiene rubber (SBR) as solutes to simulate coating plant effluent and white-water from a pulp and paper mill. The mean pore size of the membrane was substantially reduced on coating of a SPPO layer on the membrane surface. The mean pore sizes measured by AFM were about 3.5 times larger than those calculated from solute transport data. Pore sizes, obtained from both AFM and solute transport studies fitted remarkably well to the log-normal distribution curve. A very sharp flux decline was observed immediately after the start of the experiment when membranes were tested for a feed solution containing clay and/or SBR. Flux reduction was attributed mainly to pore plugging and cake layer formation on the membrane surface. A procedure was developed to determine the resistances of cake layer and membrane to the permeate flow. Cake layer resistance to the permeate flow was substantially higher than the resistance of the membrane and hence was the controlling factor. The resistance of the cake layer was, however, lower for the SPPO coated membranes compared to unmodified membranes. Both mass and thickness of cake layer attained their maximum in the first hour of operation and remained unchanged thereafter. Clay particles in the cake layer were substantially smaller in size compared to those in the feed solution. Specific resistance of the cake layer increased while its void space decreased with the operating time. More pore plugging was observed with the membranes having bigger pores. Reduction in the pore densities of the membranes after treating clay/SBR solution indicated that some of the pores were blocked either completely or partially. Characterization of the changes in the membrane morphology after coating a thin layer of SPPO will help in selecting design parameters for the development of a new thin film composite membrane. The usefulness of AFM was demonstrated in studying fouling and also in characterizing the membrane morphology. A detailed study of fouling was also conducted.