Non-woven fabric filter- (NWFF) and microfilter-MBR modules were made using 100 μm polypropylene and 0.25 μm polyethylene materials, respectively. The performances and mechanisms of the two processes were investigated, including additional batch filtration tests to find the function of the dynamic gel layer on the membrane surface. The HRT of both MBRs was 9 h and the operating permeate flux was 13 L/m(2)/h. The two MBRs consisted of an anoxic and aerobic reactor. The NWFF or microfilter (MF) was submerged in each of the aerobic reactors. The two MBRs showed similar performances for the removal of organic matters, suspended solids and nitrogen. Cake formation on the NWFF contributed to major resistance, while the gel layer on the microfilter or internal fouling of the pores played a key role in the fouling of the membrane surface. The amount of soluble extracellular polymer substances (EPS) (13 mg/L) of the attached sludge on the NWFF surface was larger than that (11 mg/L) of that suspended sludge. Consequently, the functional gel layer for the coarse and microfilter is established based on the relationship among the EPS, transmembrane pressure and MLSS.