Regulating oxygen input in treatment of effluent

In the introduction of a finely dispersed gas containing at least about 20% of oxygen by volume with intensive mixing into a vessel having a gas space and effluent containing biochemically oxidizable constituents, in the presence of activated sludge particles, the improvement which comprises

• a. introducing oxygen into the gas space of said vessel in such amount that the oxygen content is from about 40 to 80% by volume,
• b. a component volume of the gases in the gas space being ejected as a fine dispersion into the effluent,
• c. directly introducing oxygen into the effluent in such amount that the oxygen content in the effluent is from about 1 to 4 mg/l,
• d. forwarding effluent from said vessel to a second vessel having a gas space and establishing an oxygen content of about 30 to 60% by volume in said second vessel gas space,
• e. ejecting a component volume of the gases in the second vessel gas space as a fine dispersion into the effluent in said second vessel,
• f. directly introducing oxygen into the effluent of said second vessel in such amount that the oxygen content in the effluent therein is from about 4 to 8 mg/l, and
• g. recycling partially treated effluent in an amount ranging from about 10 to 300% by volume of fresh effluent and ejecting them together,
• h. the conditions of ejection in (b) and (e) being such that the activated sludge particles are sub-divided to from about 1/3 to 1/20 their normal size due to the action of shear forces, whereby there is obtained an improved utilization of oxygen even in the event of fluctuations in load of the effluent.