Abstract: The method of the invention is characterized by adding comprises adding an inorganic flocculant to waste water treated with activated sludge process, adjusting the waste water between pH 4.0 and 5.0, waste water in this state for at least 1 minute, adjusting the waste water between pH 5.5 and 9.0 and adding an anionic polymer flocculant to generate flocs and separating the flocs to obtain supernatant. According to the invention, flocs become large, especially since flocs grow by adsorbing fine particles during maturation. The clarity of the treated water is sharply improved, and the qualities of the treated water, such as COD and chromaticity are also improved. Moreover, the scale of sedimentation tank can be made compact due to the improvement in precipitatability of flocs by growing into large size.

Abstract: Waste water is treated by an activated sludge process, the equipment for which is provided with an aeration tank unit comprised of three tanks of a first tank, a second tank and a third tank, said waste water and microorganisms being fed into the first tank while the ratio of aeration in the first tank ranges from 0.5 to 0.8 by volume/minute relative to the amount of liquid in the first tank, the ratio of aeration in the second tank ranges from 0. 1 to 0.4 by volume/minute relative to the amount of liquid in the said second tank and the ratio of aeration in the third tank ranges from 0.1 to 0.3 by volume/minute relative to the amount of liquid in the third tank, the improvement which comprises carrying out said treatment by passing air containing ozone in an amount of from 0.01 to 0.16 wt. % relative to the amount of oxygen during the aeration into the first tank.

Abstract: A method for treatment of waste water by an activated sludge process wherein the aeration tank system is composed of three tanks: a first tank, a second tank and a third tank, in which sludge microorganisms adsorb the organic substances in the waste water to be treated and a part of them begin oxidation and decomposition of the substances in the first tank, the organic substances which are adsorbed by the sludge microorganisms are further decomposed by the microorganisms of the second tank, and the sludge microorganisms are activated in the third tank, is disclosed. The aeration amount to the respective tanks is controlled to fall within specified ratios. Further, the pH value in the aeration tank system is controlled to be slightly acidic, and the pH of the tanks increases in order from the first tank to the third tank. The process may also be carried out in a single tank using a three-step batchwise process, where the batches are subject to the aeration and pH requirements.