The hydrodynamic behaviors of a Multi-Section Bioreactor (MSB) and a Trickle Fixed-bed Column (TFC) packed with porous concrete block medium were investigated in terms of liquid holdup and RTD modeling in regimes with and without biofilm. It was found liquid static holdup was always higher than the dynamic holdup due to the porous structure of medium, rather than the configuration of bioreactor. Dynamic holdup increased when the hydraulic load increased. The presence of biofilm retarded the liquid release through the bioreactor; however, it enhanced the mass diffusion between static and dynamic fractions. The RTD measurements and modeling showed that the presence of biofilm lengthened the liquid residence time, and it promoted the mass diffusion between the liquid and biofilm. Liquid film thickness estimation implied that the biofilm reduced the liquid film thickness resulting both from the increase of surface area and reduction of dynamic holdup volume. (C) 2013 Elsevier B.V. All rights reserved.