We investigated the hydrodynamic characteristics of two types of inverse fluidized bed reactors having different driving force for fluidization: aeration and centrifugal force. In the first reactor, only an upward gas flow allows floating low-density polyethylene beads to sink down into liquid phase and to be uniformly distributed over the entire column. The gas velocity at which the solid concentration is uniform throughout the bed expansion decreases with increasing particle loads. In the second reactor, the particle loads do not greatly affect the critical rotating velocity for the homogeneous distribution of solid particles while the geometry of reactor spacing and the type of impeller are more important for the distribution of particles. For the application of wastewater treatment, the inverse fludized bed with aeration was found to be more efficient than the second type of reactor.