PURPOSE: To enhance the nitration efficiency in an aerobic tank and the denitrification effect in an anaerobic tank in the nitration accelerating method of an activated sludge circulation modified method.

CONSTITUTION: In an activated sludge circulation modified method containing a process denitrifying raw water 3 in an anaerobic tank l by denitrifying bacteria, a process performing nitration in an aerobic tank 2 by nitrifying bacteria and a process performing solid-liquid separation in a sedimentation tank 7 to discharge a supernatant soln. as treated water 11, a first pH meter 15 and a measuring device 16 measuring a value wherein the oxygen consumption speed accompanied by nitration reaction is subtracted from the total oxygen consumption speed are arranged to the first stage aerobic tank 2a among a plurality of aerobic tanks. A second pH meter 21 is arranged to the final stage aerobic tank 2d among the aerobic tanks and the nitration speed in the aerobic tank is estimated from the measured pH values of the respective aeration tanks and the oxygen consumption amt. based on nitration reaction and, corresponding to the estimated value, pH control by injecting alkali 25 into the initial and final stage aerobic tanks is executed.