To easily and efficiently perform control and management of the activated sludge concentration in the liquid mixture in an aerator without carrying out vexatious work in a process step of cleaning livestock waste water, etc.

The waste water treatment system A has a screen 2 for removing solids from the waste water, a mixing vessel 4 for mixing the waste water treated by the screen 2, an initial settling tank 5 for removing the residual solids of the waste water sent from the fixing vessel 4, the aerator 6 for subjecting the waste water sent from the initial settling tank 5 to aeration treatment and solid-liquid separation treatment by a separating membrane, a sludge storage tank 10 for subjecting the excess sludge sent from the aerator 6 and the precipitate sent from the initial settling tank 5 to aeration treatment and a filter press 11 for dewatering the sludge sent from the sludge storage tank 10. The aerator 6 is so controlled as to stop the aeration by each prescribed time, to settle the activated sludge by standing the sludge still for the prescribed time after the stoppage of the aeration, to withdraw the precipitated sludge allowing the prescribed amount thereof to remain and to repeat the process steps from the aeration to the withdrawal of the activated sludge.