Irreversible fouling in hollow-fiber PVDF MF/UF membranes filtering surface water: Effects of precoagulation and identification of the foulant

Abstract

In this study, microfiltration (MF) and ultrafiltration (UF) of surface water were compared in terms of irreversible fouling and effective pretreatment. Hollow-fiber MF/UF membranes made from the same polymer (polyvinylidene fluoride (PVDF)) were examined with multiple samples of surface water used for drinking water production. Experiments were carried out in a realistic style: with a constant flow rate and periodic backwashing. Regardless of the source of the feed water, evolution of fouling was less significant in the case of UF. In the case of MF, anion exchange followed by coagulation using poly-aluminium chloride (PACl) efficiently mitigated membrane fouling, whereas PACl coagulation alone was sufficient for UF. Artificial alteration of the composition of natural organic matter (NOM) in the feed water was carried out by MF/UF with different pore sizes, and it was revealed that different fractions of NOM were responsible for evolution of irreversible fouling in MF and UF: large biopolymers and small biopolymers were responsible for irreversible fouling in MF and UF, respectively. The mechanism of fouling mitigation by coagulation was also investigated. Cake (coagulated flocs) deposited on the membrane surface mitigated irreversible fouling in UF, whereas it did not affect irreversible fouling in MF. It was postulated that advection flow caused by membrane filtration made more adsorption sites available inside the flocs for removal of small biopolymers (i.e., UF foulants).