Reduction of cake layer by re-aggregation in coagulation-crossflow microfiltration process

Abstract

Cake layer in crossflow microfiltration(CFMF) can be reduced by coagulation, enhancing membrane flux. This is because enlarging particle size by coagulation increases shear-induced diffusivity and the back-transport of rejected particles. However, it is known that the enlarged particles are disaggregated by the shear force of the pump while passing through it. This study looks at the disaggregation in relation to cake layer reduction. Kaolin and polysulfon hollow fiber microfilters are used for experiments. The reduction of cake resistance by coagulation is observed in a range of 17% to 53% at the various coagulation conditions. Particle size analysis results of the experiments show that aggregated particles in feed are completely disaggregated by the pump but re-aggregation of particles occurs in the membrane. This suggests that the re-aggregation of particles is critical to cake reduction and flux enhancement, since the aggregated particles are completely broken. The mechanisms for re-aggregation in the membrane are the same as those for coagulation in the feed tank. Charge neutralization is better for CCFMF than sweep flocculation although it has two drawbacks in operation.