Size-dependent microbial diversity of sub-visible particles in a submerged anaerobic membrane bioreactor (SAnMBR): Implications for membrane fouling.

Sub-visible particles, an often-overlooked fine particle (0.45-10 μm) with a size between sludge solids and soluble microbial products (SMP), have recently been identified as a critical foulant in anaerobic membrane bioreactors (AnMBRs), and our recent new insights into the size-fractionation and composition of sub-visible particles in AnMBRs have enabled fouling to be understood in more depth. Here, we investigated the microbial diversity of the sub-visible particles in three size fractions (i.e., 5-10, 1-5, and 0.45-1 μm) from bulk and cake solutions in a lab-scale AnMBR, and their fouling potential was further explored based on their filtration behavior and biofilm formation. Results show that with decreasing particle size, a significant shift in microbial communities was observed for the sub-visible particles in both bulk and cake solutions; (a) with notable decreases in filamentous microbes in the order SJA-15, GCA004, and Anaerolineales of phylum Chloroflexi, and, (b) with substantial increases in sulfate-reducing bacteria (i.e., the family Syntrophobacteraceae, genus DCE29 of family Thermodesulfovibrionaceae, Desulfovibrio, and Geobacter). More importantly, the filamentous microbes associated with micro-particles (5-10 μm) led to higher cake fouling resistances while free living cells in the form of colloidal particles (0.45-1 μm) induced severer pore blocking. Moreover, the micro-particles had an enhanced capacity to favor biofilm formation (OD595 = 1.0-2.5, categorized as highly positive), thus potentially aggravating biofouling. This work advances our knowledge on the effect of particle size on communities and underlying fouling behavior of microbes associated with fine particles in AnMBRs.