Review of fouling by mixed feeds in membrane filtration applied to water purification

Fouling is, and will remain, a significant problem in membrane filtration. Most previous fouling studies have determined the effects caused by a single type of foulant. However, in many water purification applications, the feed solutions are often complex, containing multiple foulant types. Existing analyses based on the contribution of individual foulant types do not suffice to explain the complex interactions which occur during mixed-feed fouling behaviour. This article presents a review of fouling studies for feeds containing a mixture of foulant types, with an emphasis on microfiltration and ultrafiltration systems for water purification. Two types of mixed-feed fouling systems are considered: mixtures of different types of macromolecules (such as NOM and polysaccharides); and mixtures of macromolecular and colloidal particles (such as kaolinite). The presence of metallic ions and organic solutes is also considered. Macromolecular adsorption often initiates the fouling, and leads to irreversible flux decline; the presence of cations may or may not exacerbate this phenomenon. The behaviour of combined macromolecule and colloidal particle systems is complex and interactive, and cannot easily be inferred from the behaviour of the individual components; this behaviour may be synergistic (greater than the sum) or it may, in some cases, be compensatory (less than the sum). Such synergy results from macromolecule-particle interactions, affecting colloid stability and macromolecular deposition onto the membrane, as well as leading to altered cake resistance and hindered back diffusion of macromolecules through the cake. Mixed feeds are found to have significant effects on the critical flux phenomenon, and this remains a poorly understood area.