NOVELTY - Producing (M1) a nitrite type nitrification-reactive sludge, involves: subjecting an activated sludge to alkali treatment so that the pH of the activated sludge containing at least the ammonium oxidizing bacteria and nitrite oxidizing bacteria is greater than or equal to 10, to accumulate ammonium oxidizing bacteria with higher priority in the activated sludge.    USE - For producing nitrite type nitrification-reactive sludge (claimed).    ADVANTAGE - The alkali treatment for the activated sludge is conducted at a lower cost than that of heat treatment for the carrier, and accordingly can reduce the energy cost. The production method also can prevent a rate of the nitrification reaction from decreasing due to the damage of the carrier, by accumulating the ammonium oxidizing bacteria not on a carrier but in activated sludge, and accordingly can omit a complicated process for recovering the carrier. The production method can omit the neutralization treatment because even though the activated sludge which had been subjected to the alkali treatment was discharged into the nitrification reaction tank without being neutralized, the rate of the nitrification reaction does not decrease. The activated sludge can be produced in which the ammonium oxidizing bacteria have been more selectively accumulated while the nitrite oxidizing bacteria are surely deactivated. The method reduces the quantity of oxygen to be supplied in the nitrification reaction and the quantity of a hydrogen donor to be supplied in the reduction reaction.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) an apparatus for producing a nitrite type nitrification-reactive sludge, comprising: an alkali treatment apparatus which subjects the activated sludge containing at least ammonium oxidizing bacteria and nitrite oxidizing bacteria to alkali treatment in it so that the pH of the activated sludge is greater than or equal to 10, to accumulate ammonium oxidizing bacteria with higher priority in the activated sludge;    (2) a waste water treatment method (M2) for treating a waste water containing ammonium nitrogen, involving: subjecting the activated sludge containing at least ammonium oxidizing bacteria and nitrite oxidizing bacteria to alkali treatment so that the pH of the activated sludge is greater than or equal to 10; oxidizing the ammonium nitrogen contained in the waste water into nitrite with the activated sludge which has been subjected to the alkali treatment; and subjecting the nitrite to denitrification treatment; and    (3) a waste water treatment apparatus for treating a waste water containing ammonium nitrogen, comprising: an alkali treatment apparatus which subjects an activated sludge containing at least ammonium oxidizing bacteria and nitrite oxidizing bacteria to alkali treatment in it so that the pH of the activated sludge is greater than or equal to 10; a nitrite-forming tank in which the ammonium nitrogen contained in the waste water is oxidized to nitrite with the activated sludge which has been subjected to the alkali treatment; and a denitrification tank in which the nitrite is subjected to denitrification treatment.