The effect of hetero-aggregated feeds on critical flux

Membrane fouling by flocculated particulate mixtures formed by silica (SiO2) and alumina (Al2O3) was studied. The properties of the particulate mixtures formed depended on the properties of the solutions. The effect of floc properties and temperature on the apparent critical flux was studied by flux stepping. Critical flux increased with operating temperature and decreased with pH and ionic strength of the mixture.

Critical flux is a phenomenon which is known to depend on the way in which foulants deposit on membrane surfaces. The critical flux is believed to be a measure of where a fouling or cake layer first forms on a membrane surface. Direct observation methods [10–12] have however shown that even before the critical flux is achieved in a membrane system, foulants are continuously being deposited on to and removed from the membrane surface. This observation is common across a range of foulant materials and not restricted to particulate systems. The critical flux is, therefore, most likely the point where the rate of deposition first significantly exceeds the rate of removal and/ or a significant increase in resistance is obtained due to deposited material. In particulate systems, deposition rates are likely to be significantly affected by the particle or aggregate size. In the experiments reported here, the major factors known to dominate particle size were investigated using a full factorial design to determine the relative contribution of these factors to critical flux. The results for this system show that operating temperature and mixture properties (pH and ionic strength) all had statistically significant effects on the critical flux of heteroaggregated silica and alumina mixtures. Of the factors investigated, the temperature has the largest effects on critical flux both independ-ently and due to interactions, followed by ionic strength.