Optimization of coagulation pre-treatment for alleviating ultrafiltration membrane fouling: The role of floc properties on Al species

Abstract

This study investigated membrane fouling in a coagulation/ultrafiltration (C-UF) process by comparing the floc properties and humic acid (HA) removal efficiency of three hydrous Al(III) species (Al_a, Al_b, and Al_c). The results indicated that the coagulation and membrane mechanisms were different for all three Al species because of the differences in floc properties. The HA removal efficiency increased with increasing Al dosage until an equilibrium was reached at the optimal dosage of 6 mg L^-1. In addition, membrane fouling gradually decreased as the Al dosages increased. Regardless of coagulant type, the OH and COOH functional groups of HA reacted with the Al species. Both external and internal membrane fouling were strongly dependent on the porosity of the cake layer and on the size distribution of the floc particulates, respectively. The pore area of the cake layer formed by the Al_a-coagulated effluent was large because of the strong charge neutralization. Moreover, Al_a generated large and loose flocs with a porous cake layer that mitigated external fouling. However, the internal fouling with the Al_c coagulant was significant because the concentration of residual aggregates in the membrane pores was high.

Keywords: Al species; Coagulation/ultrafiltration; External membrane fouling; Internal membrane fouling.