Combined Organic and Colloidal Fouling of Ultrafiltration Membranes During Potable Reuse: Role of Particle Size Distribution and Alleviation by Iron Coagulation Pretreatment

Abstract

One strategy to address the global freshwater shortage is to reclaim municipal wastewater. Ultrafiltration (UF) is an important technology for potable reuse because it can remove protozoa, bacteria, and viruses while simultaneously providing highly effective pretreatment for reverse osmosis. However, its own fouling from organic matter and colloids present in the secondary wastewater effluent continues to be an unresolved issue. In this work, fouling of a commercially available hollow-fiber UF membrane over nine filtration-backwashing cycles caused by two different secondary effluents having similar bulk composition including total organic carbon concentrations but substantially different suspended solids concentrations and particle size distributions were compared. Significant flux loss was caused by the wastewater containing colloids that were predominantly smaller than the nominal membrane pore size where standard blocking was the dominant fouling mechanism. In contrast, the other wastewater having colloids that were predominantly larger than the nominal membrane pore size did not foul the membrane to a substantial extent even though its suspended solids concentration and turbidity were much higher. Intermediate blocking was the dominant fouling mechanism in this case. Hence, location of colloidal deposition appears to be a crucial determinant of UF fouling (rather than the total amount deposited) with internal pore fouling within the polymer matrix being more difficult to alleviate by hydraulic backwashing compared with surface deposition. Additionally, pretreatment by conventional coagulation and electrocoagulation mitigated (ir)reversible UF fouling. Both types of coagulation created flocs larger than membrane pores leading to surface deposition that (i) allowed facile backwashing (i.e., reduced irreversible fouling) and (ii) formed a more permeable cake layer (i.e., reduced reversible fouling). These results demonstrated that UF productivity can be largely determined by colloids in some instances in consort with effluent organic matter.