The effect of powdered activated carbon (PAC) addition on the architecture and cohesion strength of bio-cake in a membrane bioreactor (MBR) was investigated. Two reactors, a conventional MBR and a membrane-coupled biological activated carbon reactor (MBRac), were run in parallel. The addition of PAC led to a substantial increase in membrane permeability. Based on the conventional filtration theory, microbial floc size (d), bio-cake porosity (epsilon), and total attached biomass (TAB) were determined to find the key mechanism for the enhanced permeability. Unexpectedly the addition of PAC did not significantly change either the microbial floc size (d) or the porosity (epsilon) of the bio-cake. It decreased, however, not only the concentration of extra-cellular polymeric substances (EPS) in the bulk phase but also the TAB on the membrane. Using a separate batch cohesion test, it was revealed that the cohesion strength between microorganisms in the bio-cake in the MBRac was weaker than that in the MBR. The addition of PAC led to the reduction of EPS, which act as glue-like materials, which in turn weakened the cohesion strength of the microorganisms. This resulted in a lower amount of TAB, i.e., a smaller amount of the bio-cake on the membrane, which ultimately gave rise to the enhanced permeability in the MBRac.