Orbal wastewater treatment process

Claim 1

In a process for treating BOD-containing wastewater employing a plurality of concentric, annular aeration zones having surface aeration means and including the steps of contacting a wastewater influent with activated sludge in a first aeration zone to form a mixed liquor, oxidizing the mixed liquor in a subsequent aeration zone, separating a sludge from the mixed liquor and recycling the separated sludge to the first aeration zone as activated sludge, the improvement comprising introducing the wastewater and recycled activated sludge into said first aeration zone and homogeneously mixing same therein by a combination of said surface aeration means and introducing an oxygen-containing gas into the lower portion of said first aeration zone under conditions which produce a complete mix reaction in said first aeration zone and sufficient oxygen is provided to meet, but not substantially exceed, the biological oxygen demand of the resulting mixed liquor, said oxygen-containing gas being introduced in the form of fine bubbles which have a diameter of less than about 4 mm; transferring the mixed liquor from said first aeration zone to a subsequent aeration zone wherein the mixed liquor is contacted with an oxygen-containing gas under conditions which produce a complete mix reaction in said subsequent aeration zone and sufficient oxygen is provided to produce an overall dissolved content of at least 0.5 mg/l; transferring the oxidized mixed liquor from said subsequent aeration zone to a settling zone wherein a supernatant liquid is separated from settled sludge; and recycling at least a portion of the settled sludge to said first aeration zone as recycled activated sludge.