Filtration behaviors and biocake formation mechanism of mesh filters used in membrane bioreactors

Membrane bioreactors (MBRs) equipped with mesh material as the filter have recently attracted increasing interest worldwide for wastewater treatment. In such systems, the biocake formed on filter surface plays a unique and critical role during filtration, but the specific mechanism of biocake formation remains unclear so far. Here, we systematically explored the filtration behavior and biocake formation mechanism of mesh filters using both a lab-scale MBR and a newly-designed dead-end filtration unit. For a quantitative description of the biocake evolution process, a two-stage mechanism model was proposed according to the filtration characteristics. The fitness of this model was well demonstrated by dead-end filtration test, and its general applicability was validated by the filtration performances under varied filtration conditions and using different filter materials. Furthermore, the long-term filtration experiments in MBR demonstrated that the gradual accumulation of extracellular polymeric substances in the biocake finally led to irreversible fouling of mesh filters. This process of biocake evolution was also well supported by microscopic observations and filtration resistance analysis. This work offers an insight into the filtration characteristics and biocake formation mechanism of mesh filters, and may lay a theoretical foundation for future application of mesh filters and other coarse-pore materials in MBR wastewater treatment processes.

► Dead-end filtration showed a two-stage filtration behavior in short-term run.
► Two-stage model offered good biocake formation process simulation.
► Irreversible fouling developed during long-term operation as EPS accumulated.
► Optical detection performed for visualized characterization and mechanism confirmation.