To immobilize the ammonium-oxidizing bacteria, gel entrap- ment technologies have been reported. Sumino et al. [4] developed an immobilization method by entrapping ammonium-oxidizing bacteria in a polyethylene glycol (PEG) carrier. Using this technique, they attained a high nitrification rate of 2.4 kg N/m/day at 20 °C [5]. On the other hand, Asano et al. [1] used a polyvinyl alcohol matrix to entrap ammonium-oxidizing bacteria and reported a high nitrification rate of 2.7 kg N/m'/day at 20°C. Because the gel carrier is separated from the wastewater by a screen, entrapped bacteria are not washed out the reac- tor. Thus, gel entrapment enables prolonged biomass retention times even under short hydraulic retention times (HRTS). Gel entrapment is therefore quite effective for the immobilization and cultivation of slow-growing bacteria in reactors. Moreover, the efficiency of gel entrapment techniques has been demon- strated in nitrification treatment facilities using gel carriers that ave already been constructed at 11 sewage treatment plants Japan.