According to the morphology investigation of the biodiatomite dynamic membrane (BDDM), the deposits on its surface was fine, homogeneous and of high porosity. The ratio of volatile suspended solids (VSS) to suspended solids (SS) of the BDDM was less than that in the biodiatomite mixed liquor. The research results showed that the hydraulic scour of the precoating stage caused microorganisms in the biodiatomite at the side of the cake layer near the support mesh to slough off, and therefore enhanced the porosity and augment the asymmetric pore structure of the cake layer. Diatomite, the core of biodiatomite, is the key factor of the dynamic semicompressibility of the BDDM. Thus, the BDDM has characteristics of good retention efficiency, high filtration flux and easy backwash. (C) 2009 Elsevier B.V. All rights reserved.