A method and a device are provided with which ammonia nitrogen-containing water can be stably treated even when the water temperature is 15ºC or lower.The method includes a nitrification step in which ammonia nitrogen-containing water having an ammonia nitrogen concentration of 1-300 mg/L is brought into contact with a carrier on which nitrifying bacteria have been preferentially propagated to such a degree that the bacteria are detected after 8-week cultivation in an ammonium sulfate solution having a concentration as high as 5,000 mg/L or above, in an aerobic environment at a temperature of 0-15ºC to thereby nitrify the ammonia contained in the ammonia nitrogen-containing water.
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