Experimental work on the biological phenol degradation by immobilized activated sludge in gel beads using a three-phase fluidized-bed reactor with a draft tube (DTFBR) was carried out. A simulation model for the DTFBR was developed. The overall biodegradation model consisted of a single gel bead biodegradation model and a reactor model for the three-phase fluidized-bed reactor with a draft tube. In the single gel bead degradation model, simultaneous diffusion and consumption of both phenol and oxygen in a gel bead were considered. The single gel bead reaction model was found to describe the phenol degradation behaviour by immobilized activated sludge in a batch-operated small bubble column. The reactor model was an integrated multiple plug flow reactor model, which consisted of three plug flow reactors. The DTFBR was divided into three parts; a draft tube region, a region above the draft tube, and an annular region. Each part of the reactor is assumed to be a plug flow reactor. The integrated multiple plug flow reactor model could estimate qualitatively the phenol degradation behaviour obtained experimentally in a DTFBR.