Indirect Evidence that Colloidal Deposition Inside a Hollow Fiber Ultrafiltration Membrane Exacerbated Fouling during Municipal Wastewater Reclamation

Abstract

Ultrafiltration (UF) or microfiltration is often employed during potable reuse for reverse osmosis pretreatment, and their own fouling from organics and colloids present in the secondary wastewater effluent remains an unresolved issue. Herein, we compared the fouling of a hollow fiber UF membrane over multiple filtration–backwashing cycles caused by two different secondary-treated effluents having similar concentrations of effluent organic matter (EfOM) and dissolved constituents but substantially different turbidity, total suspended solids (TSS), and particle size distributions. Contrary to expectations, significantly worse fouling was caused by the less turbid (low TSS) wastewater, which was explained by the presence of particles that were smaller than the membrane pores. In this case, pore plugging was the dominant fouling mechanism, which was mechanistically quantified as “standard blocking,” i.e., colloid deposition inside the pores. On the other hand, fouling was negligible for the more turbid (high TSS) wastewater that contained particles larger than the pores. In this case, surface accumulation of colloids blocked pores, which was mechanistically quantified as “intermediate blocking.” Hence, the location of particle deposition was a crucial determinant of ultrafilter productivity when colloids combined with EfOM to (ir)reversibly foul membranes operating on real-world secondary effluents. Additionally, pretreatment by electrocoagulation and conventional coagulation mitigated (ir)reversible fouling by creating large flocs that deposited on the membrane surface, (i) allowing facile backwashing (i.e., reduced irreversible fouling) and (ii) forming a more permeable cake layer (i.e., reduced reversible fouling). Hence, UF productivity was primarily determined by colloid size relative to pore size in conjunction with secondary contributions from EfOM. Importantly, internal pore fouling within the membrane’s polymer matrix was more difficult to alleviate by hydraulic backwashing than by surface deposition.