Fouling by inorganic-particle-containing cake layers can be reduced by microorganisms at low fluxes

A microbe-rich cake layer (i.e., a membrane biofilm) may maintain a stable hydraulic resistance at low fluxes because the microorganisms may decompose organic foulants and create cavities in the cake layer. However, these microorganisms cannot decompose the inorganic particles, which can increase cake resistance because of the synergistic fouling effect with organic foulants. To gain a better understanding of the effect of microorganisms on the microbe-rich cake layer containing inorganic particles, we investigated the change in hydraulic resistance, structure, and composition during the transition of cake layers from a microbe-poor state to a microbe-rich state, at the flux of 5, 10, and 30 L·m−2·h−1. And we used SiO2 particles (2.5 μm in diameter) and sodium alginate to simulate inorganic particles and organic foulants. The results showed that when the cake layer was transformed from a microbe-poor state to a microbe-rich state, the hydraulic resistance was reduced, the structure became looser, and inorganic particles were partially detached. These changes were more pronounced for the cake layer under an ultra-low flux of 5 L·m−2·h−1, with 62% and 47% reductions in the hydraulic resistance and inorganic particles, respectively. These results indicated that the fouling by inorganic particles could be reduced by the microorganisms at ultra-low fluxes. Cake composition analysis, forces balance analysis, and computational fluid dynamics simulation suggested that the reduced fouling could be derived from the reasons 1) the biodegradation of organic foulants could promote the detachment of inorganic particles, and 2) the microbial-induced cavities at ultra-low fluxes could reduce the synergistic fouling effect by inorganic particles and organic foulants.