There is provided a reaction system capable of, in the feeding of liquid stored after gas-liquid mixing into waste water to be treated of a reaction tank, conditioning the interior of the reaction tank to a pressurized state so as to simultaneously accomplish enhancement of utilization efficiency of gas phase and further reduction of the amount of excess sludge generated. There is further provided a biological purification process characterized in that microorganisms fulfill purification activity in a pressurized reaction tank having a pressure other than deep water pressure applied thereto. Preferably, together with water to be treated, reactant gas such as pure oxygen, ozone-containing oxygen (ozonized oxygen) or air is fed into the pressurized reaction tank. With respect to gas and liquid fed into the reaction tank, preferably, reactant gas is fed in the form of dissolved gas or microbubbles by means of a gas-liquid mixing unit (line atomizer). Preferably, the pressurized reaction tank is fitted with a support for microorganisms as means for increasing the habitant density of microorganisms engaging in reaction and retaining such microorganisms.

Disclosed is a waste water disposal process in the biosolid method according to a line atomizing treatment in which a reactive gas containing oxygen or oxygen and ozone as hardly soluble gases is dissolved/stored in water as being converted into ultrafine bubbles. The characteristic feature is that, by forming a gas-dispersion liquid in which the reactive gas containing oxygen or oxygen and ozone is dispersed in the form of ultrafine bubbles in returned biosolid water or in clean water at outside of the vessels (pools) of the waste water treatment system and by introducing the aforementioned gas-dispersion liquid into a reaction vessel (aerobic or anaerobic), oxygen is supplied to the microorganisms. Alternatively, the aforementioned gas-dispersion liquid is introduced into a vessel in the step preceding the reaction vessel or succeeding the reaction vessel. Further, the kind, concentration and volume of the reactive gas, the vessel (pool) for returning and the duration of introduction are set in accordance with the proceeding conditions of the waste water treatment and unitarily controlled.