Accumulation and dispersion of mixed liquor particles on membrane surfaces in submerged wastewater membrane bioreactors

Membrane fouling was studied using three pilot-scale submerged membrane bioreactors (MBR) and one bench-scale filtration apparatus operated at a series of permeation and aeration conditions to treat municipal wastewater. The mixed liquor quality was quantified by measuring the critical flux, colloidal particle concentration, mixed liquor suspended solids concentration, time-to-filter, and diluted sludge volume index. Simultaneously a set of online filterability tests were used to study particle accumulation and dispersion. Transmembrane pressure increases were monitored to calculate the fouling ratios in order to compare the relative fouling rates.; The results show that the resistance per unit particle mass carried onto the membrane by permeation strongly correlates to the critical and sustainable fluxes as well as the colloidal TOC. Although the fouling mechanisms differ, there appears to be a relation between the cake resistances at air sparged and non-air sparged operating conditions perhaps due to the influence of colloidal particles at both conditions. Under non-aeration conditions there also appears to be a relation between particle deposition on the pilot-scale MBRs and on the bench-scale filtration apparatus. The trends of air sparged fouling resistances differ greatly, depending on the permeate flux and mixed liquor characteristics. A stable fouling resistance can result when the filtration is operated at sustainable permeate flux conditions while at the unsustainable permeate flux conditions, the fouling resistance increases exponentially as the filtration progresses. The effects of aeration intensity on the fouling ratios are independent of the permeate flux and vice versa. Furthermore, the variation of fouling ratios at different operating conditions strongly depends on the sludge characteristics of mixed liquor. It appears that more than one parameter of mixed liquor is needed to define their relationship. Finally, the fraction of reversible resistance removed is dependent on the operating conditions applied during particle deposition and dispersion, in addition to the characteristics of the mixed liquor.