In treating water containing ammonium-nitrogen by a combination of a nitrification process and autotrophic denitrification bacteria, high efficiency denitrification treatment is achieved by providing adequate ratio between nitrite and ammonia in effluent of the nitrification process. Ammonium-nitrogen concentration of the raw water introduced to a nitrification reactor and the ammonium-nitrogen concentration of the nitrified liquid from the nitrification reactor is measured, and on the basis of the measurement, the flow volume for aeration by a blower would be controlled. From the difference in NH.sub.4--N concentration between the influent and the effluent of the nitrification reactor, nitrite concentration A in the nitrified liquid is calculated, and ratio A/B of the nitrite concentration A with the ammonia concentration B in the nitrified liquid is calculated. The flow volume for aeration by the blower to the nitrification reactor is controlled such that the A/B becomes 1.1 or more, preferably from 1.1 to 2.0.
A method of treating wastewater wherein the biomass associated with the wastewater is subjected to simultaneous nitrification and denitrification in a single reactor. The method entails directing a wastewater influent stream into the reactor and measuring the concentration of N--NH.sub.4 in the influent wastewater stream at a point upstream from the reactor. Air is supplied to the reactor for aerating the biomass in the reactor. The method includes controlling the quantity of air introduced into the reactor based in part at least on the measured concentration of N--NH.sub.4.
