Biofilms have permeable structures, which can be used as membranes for solids separation, e.g., dynamic membrane bioreactors (DMBRs) that relied on the biofilm developed on the support material with large apertures for filtration. This study found that the biofilm formed on the support material in DMBRs under different solids retention time (SATs) had diverse structures, which in turn determined the hydraulic resistance and filtration performance. At 5-day SRT, a thin but compact gel-like biofilm layer (porosity = 27.5%) was formed on the support materials, which had a higher hydraulic resistance of approximately 4.9 x 10(11) m(-1). As a result, the transmembrane pressure (TMP) rose dramatically every 20-40 days' operation. A thick but porous biofilm layer (porosity > 60% and hydraulic resistance < 2.5 x 10(9)m(-1)) was formed at longer SRTs of 20 and 40 days and the TMP could keep consistently low ( < 20 Pa) for more than 180 days. Therefore, a longer SRT could facilitate the formation of a porous biofilm layer on the support material, which was of critical importance for achieving long-term and low-pressure filtration. The biofilm porosity was negatively correlated with the protein/polysaccharide ratio of its extracellular polymeric substance (EPS). Extending SRT would reduce the Live/Dead cell ratio of biofilm, thus lowering EPS production while increasing the porosity. Moreover, extending SRT promoted the growth of protozoa (e.g., Euglypha) that inhabited the biofilm, which could maintain the biofilm porous and mitigate the support layer biofouling though feeding and movement.