The influence of morphology, hydrophobicity and charge upon the long-term performance of ultrafiltration membranes fouled with spent sulphite liquor

A membranes hydrophobicity, surface roughness and charge can all affect its tendency to foul and its subsequent cleanability. In this study polysulphone (PSf), polyethersulphone (PES) and regenerated cellulose (RC) ultrafiltration (UF) membranes were fouled with spent sulphite liquor (SSL) and cleaned with sodium hydroxide and Ultrasil 11 (Henkel Ecolab) over several operational cycles until a steady-state was reached. Fouling and cleaning mechanisms were evaluated using Fourier Transform Infra Red (FTIR) spectral peak heights, zeta potential and contact angle measurements. Polysulphone (PSf) membranes displayed a much large flux decline than polyethersulphone (PES) membranes that are smoother and of a similar hydrophobicity. The greater the available surface area (due to the roughness) the stronger the interaction between membrane material, foulants and cleaning agents is likely to be. Regenerated cellulose (RC), has a similar roughness to PSf but is much more hydrophilic. RC membranes displayed the smaller flux decline of the two membranes examined, demonstrating the importance of hydrophilicity in reducing fouling by the SSL feed.

The importance of examining membrane performance over multiple operational cycles has been demonstrated. Although fluxes measured are often high following cleaning procedures, FTIR and Zeta potential studies showed the inability of cleaning agents to return membrane surfaces to a pristine state. Permeate flux is therefore a poor indicator of surface condition. For membranes of a similar hydrophobicity but different surface roughnesses, (PSf and PES), flux decline over multiple fouling and cleaning cycles was found to be more significant for the rougher membrane (PSf). For the system studied, the rougher, hydrophilic RC membrane had a greater tendency to resist adhesion than the smoother but more hydrophobic PES membrane. Therefore both hydrophobicity and roughness were found to be linked to fouling tendency over the long term. In addition to the degree of fouling, the type of fouling was also determined by the hydrophobicity. The extent of fouling during the first cycle was controlled by the membrane’s surface morphology (roughness, pore size, pore size distribution), hydrophobicity and charge. Surfaces fouled during their first operational cycle so that subsequently deposit charge, hydrophobicity and morphology rather than the membrane material characteristics controlled the performance. Over many cycles, the choice of cleaning agent will still determine whether there is a long-term decline in the flux magnitude, and whether a steady state flux value is achieved. This can be seen when cleaning is carried out with Ultrasil 11. For the hydrophobic PES membrane the surfactants maintain stable fluxes over the long term. For the hydrophilic RC membrane, the surfactants only attach after the membrane is fouled; only then can the flux be recovered and maintained at a relatively high level.