The substrate consumption and microbial growth processes in a PUF(polyurethane foam particles) fluidized bed bioreactor, were formulated by introducing microbial transport between support particle and water phase and the biological predator-prey symbiosis. In formulation, assumptions used were 1) two types of bacteria- motile and flock-forming bacteria- should inhabit simultaneously, 2) two species of protozoa-free-swimming and stalked protozoa- should coexist, 3) bacteria should grow subject to the Monod equation, but flocculation and deflocculation are influenced by protozoa mass concentrations, 4) free-swimming and stalked protozoa should grow preying on motile bacteria subject to the Monod equation, but the rates are subordinatively influenced by substrate concentration. In consequence, the processes were shown to be expressed by a substrate mass balance and eight microbial mass balance equations. The numerical results to these equations were shown to explain well the actual valuations with time passage in TOC and retained and suspended microbial concentrations.