A method of biologically nitrifying wastewater utilizing an integrated fixed film activated sludge process that includes biomass supported on carriers and suspended biomass, the method comprising: mixing the wastewater with activated sludge to form mixed liquor; biologically nitrifying the mixed liquor in a reactor utilizing the biomass on carriers and the biomass suspended in the mixed liquor; wherein the amount of biomass on the carriers varies in function of temperature and the loading of organic matter and ammonia nitrogen in relation to the amount of the activated sludge in the reactor; wherein said method includes: - measuring the concentration of ammonia in the mixed liquor and comparing it to an ammonia concentration target; - measuring the concentration of dissolved oxygen in the mixed liquor; - gradually increasing aeration rate when the ammonia concentration exceeds the ammonia concentration target or reducing aeration rate in the reactor when the ammonia concentration is below the ammonia concentration target, wherein the dissolved oxygen concentration in the reactor is controlled by varying the aeration rate to keep the dissolved oxygen concentration as close as practically possible to a set point value within a setpoint range; wherein, in a first mode of operation a dissolved oxygen concentration setpoint range of 0.5-2.0 mg/L is employed, the nitrification of the wastewater being primarily performed by the suspended biomass in the mixed liquor; and wherein, in a second mode of operation a dissolved oxygen concentration setpoint range of 2.5-5.0 mg/L is employed, the nitrification of the wastewater being performed by the suspended biomass in the mixed liquor and the biomass on the carriers. mixing the wastewater with activated sludge to form mixed liquor; biologically nitrifying the mixed liquor in a reactor utilizing the biomass on carriers and the biomass suspended in the mixed liquor; wherein the amount of biomass on the carriers varies in function of temperature and the loading of organic matter and ammonia nitrogen in relation to the amount of the activated sludge in the reactor; wherein said method includes: - measuring the concentration of ammonia in the mixed liquor and comparing it to an ammonia concentration target; - measuring the concentration of dissolved oxygen in the mixed liquor; - gradually increasing aeration rate when the ammonia concentration exceeds the ammonia concentration target or reducing aeration rate in the reactor when the ammonia concentration is below the ammonia concentration target, wherein the dissolved oxygen concentration in the reactor is controlled by varying the aeration rate to keep the dissolved oxygen concentration as close as practically possible to a set point value within a setpoint range; - measuring the concentration of ammonia in the mixed liquor and comparing it to an ammonia concentration target; - measuring the concentration of dissolved oxygen in the mixed liquor; - gradually increasing aeration rate when the ammonia concentration exceeds the ammonia concentration target or reducing aeration rate in the reactor when the ammonia concentration is below the ammonia concentration target, wherein the dissolved oxygen concentration in the reactor is controlled by varying the aeration rate to keep the dissolved oxygen concentration as close as practically possible to a set point value within a setpoint range; wherein, in a first mode of operation a dissolved oxygen concentration setpoint range of 0.5-2.0 mg/L is employed, the nitrification of the wastewater being primarily performed by the suspended biomass in the mixed liquor; and wherein, in a second mode of operation a dissolved oxygen concentration setpoint range of 2.5-5.0 mg/L is employed, the nitrification of the wastewater being performed by the suspended biomass in the mixed liquor and the biomass on the carriers.