Indirect evidence for deposit rearrangement during dead-end microfiltration of iron coagulated suspensions

Constant pressure microfiltration of colloidal silica suspensions following coagulation using ferric chloride was investigated under conditions where surface deposition was the predominant cause of fouling. Experimental fluxes were found to be higher than that predicted by conventional cake filtration and blocking laws alone or in combination especially during the initial stages of filtration. Experimental inverse instantaneous fluxes depicted a concave downward behavior when analyzed as a function of volume of water filtered per unit membrane area, which cannot be explained by blocking laws. Further, flux decline under a wide range of pH, Fe3+ dosage, and pressure was accurately simulated by introducing a particle rearrangement term to conventional dead-end cake filtration theory. Equilibrium fluxes predicted by the rearrangement model increased in a straight-line fashion with increasing mean floc size whereas the cake specific resistance decreased as flocs became larger. These results are shown to be consistent with the rearrangement of coagulated aggregates and changes in cake morphology when the pressure increases with cake depth. In contrast, rearrangement effects may not be significant during microfiltration of highly colored waters because natural organic matter can bind particles to each other and to the membrane.