New insight into the membrane fouling of anaerobic membrane bioreactors treating sewage: Physicochemical and biological characterization of cake and gel layers

As the main cause of membrane fouling, layer fouling necessitates further study to better understand its formation process and facilitate fouling control in anaerobic membrane bioreactors (AnMBRs). This study investigated the physicochemical and biological differences between the cake and gel layers in an AnMBR. The results indicated that total solids in the cake layer is forty times that in the gel layer. Nonetheless, the gel layer has a higher specific foulant resistance than that of the cake layer (6.3 × 10¹⁴/m/kg vs. 0.18 × 10¹⁴/m/kg), which results in comparable contributions of the two layers to membrane fouling. Sludge flocs with abundant extracellular proteins are the main foulants in the cake layer. Conversely, the gel layer has a homogeneous network structure formed by proteins and β-D-glucopyranose polysaccharides with pore channels rich in cells. The genera Azovibrio (28.93%), Desulfovibrio (6.54%), and Syntrophobacter (2.84%) were predominant microbes in gel foulants, suggesting that biofouling triggered by quorum sensing played a more significant role in the formation of the gel layer when compared with the cake layer. The results of this study bring out the distinction between the cake and gel layers and highlight the role of the gel layer in the layer fouling of AnMBRs. Consequently, it provides fundamental knowledge for the control of membrane fouling.