A review on mitigation of pressure-driven membrane fouling by coagulation/flocculation as a pretreatment in membrane separation processes

Abstract

Membrane fouling is an inevitable consequence of the membrane operation process. Coagulation/flocculation is simple, effective, and low-cost membrane pretreatment method. The performance of coagulation/flocculation, including its coupling processes with other pretreatment technologies, in controlling pressure-driven membrane fouling has been reviewed in detail according to fouling type (including inorganic, organic, and microbial fouling) in this paper. The development prospects of this technology are also explored to further improve its application performance. Coagulation/flocculation generally shows high efficiency in removal of insoluble inorganic, organic and microbial colloids in effluents by charge neutralization, bridging and sweeping effects to thus mitigate membrane fouling. For soluble matter, especially for low molecular-weight ones, the mitigation effect highly depends on the selected coagulant/flocculant, in addition to the coupling pretreatment technologies. A desired coagulant/flocculant can effectively bind to and transform the solute from hydrophilic to hydrophobic by some valid interactions, and cause an easy destabilization, aggregation and final formation of large and loose flocs. The resultant flocs can be trapped on the membrane and further form loose and porous cake layer, thus increasing membrane flux, decreasing membrane resistance, and alleviating membrane fouling. Based on an improved understanding of the membrane fouling and also coagulation/flocculation mechanisms at the molecular levels, designing and selecting an advanced coagulant/flocculant, with the advantages of high performance, environmental friendliness and low cost, is crucial to achieve the desired pretreatment effect.