Spatial variability of cake layer in membrane fouling of full-scale MBR: New insights and implications

The spatial variability of the cake layer formed on the hollow fiber membranes during the operation of a full-scale membrane bioreactor (MBR) has been thoroughly investigated, samples from the bulk phase of the reactor and cakes formed on the membrane surface with different locations (i.e., inner layer of the cake (Cake-I), central layer of the cake (Cake-C), and outer layer of the cake (Cake-O)) were collected. The results showed that the viscous nature of proteins, the main component of extracellular polymeric substances (EPS),as well as the hydrophobicity of the protein secondary structure, were factors that led to variations in particle size (Cake-I = 566 μm > Cake-C = 283 μm > Cake-O = 92.2 μm > Bulk sludge = 59.7 μm) and total interaction energy (Cake-I = −216.39 × 10⁴ KT > Cake-C = −68.35 × 10⁴ KT > Cake-O = −30.42 × 10⁴ KT > Bulk sludge = −30.08 × 10⁴ KT) between the spatially varying cake layers. Numerous filamentous bacteria were found in the MBR, and genes related to membrane plugging processes (such as polysaccharide synthesis, surface attachment, and amino acid degradation) of the microbial community composition were regulated within the cake layers. These new insights could be beneficial to developing fouling control and membrane cleaning strategies.