A submerged anaerobic dynamic membrane bioreactor (AnDMBR) was operated for treatment of concentrated wastewater. The dynamic membrane (DM) or cake layer was characterized on its physicochemical and biological composition and the role of the DM layer in treatment and filtration performances was assessed. The results showed that the DM layer had an important role in organic matter removal. Both organic and inorganic materials, such as sludge particles, soluble microbial products (SMP), extracellular polymeric substances (EPS), and Ca, N, P, Mg precipitations contributed to the DM layer formation. Thus, effective retention of very small particles by the DM layer was achieved. The DM layer had higher microbial diversity and different microbial population composition in comparison to the bulk sludge. Overall, this study provided a better understanding about the DM layer structure in AnDMBRs, which might lead to increased applicability of this promising technology for the treatment of concentrated wastewaters. (c) 2015 Wiley Periodicals, Inc.