Evolution And Mechanisms Of Membrane Fouling During Microfiltration In Membrane Bioreactors

Membrane bioreactors (MBRs), a novel combination of biological process andmicrofiltration, are attracting increasing attention and being popularized in the area ofwastewater treatment and reclamation. However, the fouling of microfiltrationmembranes in MBRs remains a major obstacle to the wider application of thistechnology. The entire fouling process can be divided into two stages—the initialfouling stage followed by the gel-type fouling stage. Although studies on membranefouling been undertaken for more than a decade, systematic and quantitative knowledgeis yet to be further explored regarding the entire fouling process and foulingmechanisms. In view of this, the objective of this study is to provide systematic insightinto the process of fouling evolution and reveal the underlying mechanisms withquantitative descriptions.The adsorptive fouling pertaining to the initial fouling stage was characterized byThomas dynamic adsorption model. As carefully verified in this study, Thomas modelwas adequate for the parameterization of the equilibrium constants and rate constants ofadsorption between membranes and foulants. A semi-empirical equation relating theadsorption equilibrium constants with contact angles and zeta potentials of bothmembranes and foulants was established, which aims at elucidating the combined effectof hydrophobicity and charge on fouling. The knowledge about interfacial energy wasexpanded to non-ideal systems, quantitatively evaluating the different contribution ofhydrophobic interaction and electrostatic interaction to adsorptive fouling. At the pointof adsorption equilibrium, electrostatic interaction was found to play a negligible role(less than5%) as compared to hydrophobic interaction. This indicates that hydrophobicinteraction was the predominant mechanism affecting adsorptive fouling.The second stage of fouling process, namely the gel-type fouling stage, wasinvestigated by the gel-layer-evolution model established based on the theories of cakelayer filtration and concentration polarization (CP). Three characteristic points wasdefined using this model, including the critical point, the pseudo-steady point and thetransition point. This model also provides parameters regarding gel layer properties(specific resistance and gel point), CP boundary layer properties (thickness) and foulant properties (diffusion coefficient and hydrodynamic diameter). The hydrophobicity andpore size of membranes, the size and concentration of foulants, and the shear rate at themembrane surface was found to exert combined effects on fouling evolution. This studyquantified the synergistic effect of membrane hydrophobicity and foulant concentration,as well as that of membrane hydrophobicity and shear rate.Based on the aforementioned mechanisms of adsorptive fouling and gel-typefouling, membrane fouling was further investigated using the supernatant from a largefull-scale MBR. The major fractions in the MBR supernatant were hydrophilicsubstances and hydrophobic acids among all the organics. Hydrophobic acids showedhigher potential to induce fouling than hydrophilic substances during the developmentof gel layers. In addition, the gel layers formed by hydrophobic acids possessed higherspecific resistance. A comprehensive fouling evolution model that incorporates theproperties of membranes and foulants was semi-empirically established, which may beprospectively applied to practical MBR systems.