The effects of immobilizing materials on the activity of nitrifying bacteria and removal of ammonium nitrogen (NH4-N) from waste-water by immobilized nitrifying bacteria were investigated using six urethane prepolymers. With a urethane prepolymer containing 2.27 free isocyanate, a high activity yield of nitrifying bacteria was obtained. There was a drastic improvement over the conventional method of immobilization by acrylamide in the activity yield. Inorganic synthetic waste-water was treated at a high loading rate of 0.24 kg N·m−3·day−1. The NH4-N concentration of the effluent could be reduced to 2 mg·1−1or less and the removal was 90. The life of the pellets in terms of activity was at least
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