Insights into membrane fouling of submerged membrane bioreactors by characterizing different fouling layers formed on membrane surfaces

Two distinct fouling layers, i.e., a gel layer and a sludge cake layer were observed in two MBRs operated under same constant fluxes but different aeration rates. The formation mechanisms and properties of the two fouling layers were systematically analyzed. It was found that the gel layer had lower porosity and higher specific resistance (αc) compared to the sludge cake layer. The mean particle size of the gel layer was smaller than that of the sludge cake layer. Soluble microbial products (SMP) were found to be the major soluble organic materials in both fouling layers. Mg, Ca and Al were the major inorganic elements in solid state of the foulants, and the monovalent elements such as Na and K were mainly detectable in the soluble component of the foulants. SMP and tightly bound extracellular polymeric substances (EPS) of both fouling layers featured macromolecular characters. The molecular weight (MW) of gel foulants was larger than that of cake foulants. The loosely bound EPS and the fouling layers had very similar MW distribution profiles. Only the peaks associated with protein-like fluorophores were detectable in gel foulants, while besides the protein-like materials the humic acid-like substances were present in sludge cake foulants.

► Gel layer had poorer porosity and higher specific fouling layer resistance compared to cake layer.
► SMP and TB-EPS of both fouling layers featured macromolecular characteristics which are different from bulk sludge.
► Protein-like substances were detected in gel layer, while protein and humic acid-like substances were present in cake layer.