The hydrodynamic characteristics, such as minimum fluidization velocity (Ulmf for liquid-solid (LS) system and Ug,if for gas-liquid-solid (GLS) system) and bed expansion ratio (BER), of liquid-solid and gasliquid-solid inverse fluidized beds (LSIFB and GLSIFB) with bare particles and particles with biofilm were investigated. In the LSIFB system, Ulmf and BER of the bare particles were independent of the solids loading. For bioparticles, the increase of the biofilm thickness reduced Ulmf and increased BER, suggesting that the fluidizability increases with the presence of the biofilm. In the GLSIFB system, the initial fluidization gas velocity (Ug,if) and the complete fluidization gas velocity (Ug,cf) both increased with increasing particle diameter and decreasing particle density under fixed superficial liquid velocities. Biofilm attachment led to a decrease of both Ug,if and Ug,cf, and an increase of bed expansion, again suggesting increased fluidizability with the presence of biofilm. (c) 2021 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.