In-depth insights into the temporal-based fouling mechanism and its exploration in anaerobic membrane bioreactors: A review

The anaerobic membrane bioreactor (AnMBR) has been deemed a sustainable technology in simultaneous wastewater resource recovery and reclaimed water production. Membrane fouling is one of the most challenging issues in AnMBRs remaining to be addressed. Rather than emphasizing operational design and performance, a summarization of the existing understandings of temporal-based fouling mechanism in AnMBRs is proposed to render guidance in principle. The fouling development can be chronologically divided into three main stages: pore narrowing, pore-clogging, and gel/cake layer formation. Medium-sized soluble microbial products (SMP), soluble extracellular polymeric substances (EPS), and/or small-sized colloids dominate the pore narrowing stage through strong surface interaction of the inner pore walls. Macro-molecular SMP, cell debris, and colloidal EPS mainly lead to complete pore-clogging by membrane-foulant hydrophobic interaction in terms of sieving effect on the membrane and intermediate pore-clogging by intermolecular adhesion forces and hydrodynamic bridging on the first single foulant layer. The gel layer forming stage is mainly attributed to sticky proteins and polysaccharides in EPS, macro-molecular SMP, metal ions, and microbial colloids through the foulant-foulant interaction. Moreover, the cake layer formation stage is mainly ascribed to flocs and solids deposition, microbial biomass, humic substances, inorganic precipitation, and intracellular polymeric foulants through strong solid-liquid adsorptive interaction and/or solid precipitation. Specifically, the electrostatic, hydrophobic, spatial, complexing, adsorptive, and compressive interactions between the membrane and/or foulants are qualitatively discussed in detail for different fouling stages. Finally, the chronological transformation process of the fouling layer and the role of biofouling in the late stage of fouling development in AnMBRs is analyzed.