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 have already been constructed at 11 sewage treatment plants in Japan.