Process for treating BOD-containing wastewater

A process for treating BOD-containing wastewater using a system employing a plurality of concentric, annular zones, the process comprising:

• introducing wastewater influent into an anaerobic zone having activated sludge;
• mixing the wastewater influent with the activated sludge in the anaerobic zone to form a mixed liquor;
• introducing the mixed liquor into a first aeration zone maintained under conditions which produce a complete mix reaction and provide insufficient oxygen to meet the biological oxygen demand of the resulting mixed liquor;
• recycling denitrified-mixed liquor from the first aeration zone to the anaerobic zone for mixing therein with wastewater;
• transferring the mixed liquor from the first aeration zone to a subsequent aeration zone maintained under conditions which produce a complete mix reaction and provide sufficient oxygen to produce an overall dissolved content of at least 0.5 mg/L;
• transferring the mixed liquor from the subsequent aeration zone to a settling zone wherein a supernatant is separated from settled sludge;
• and
• recycling at least a portion of the settled sludge to the first aeration zone as recycled activated sludge.

A process for treating BOD, nitrogen and phosphorus containing wastewater, the process comprising:

• introducing wastewater influent into an anaerobic zone having activated sludge;
• mixing the wastewater influent with the activated sludge to form a mixed liquor;
• introducing the mixed liquor into an aerated anoxic zone promoting simultaneous nitrification and denitrification;
• recycling denitrified mixed liquor from the aerated anoxic zone to the anaerobic zone for mixing therein with wastewater;
• transferring the mixed liquor to a subsequent aeration zone under conditions which produce a complete mix reaction and provide sufficient oxygen to product an overall dissolved oxygen content of at least 0.5 mg/L;
• transferring the mixed liquor from the subsequent aeration zone to a settling zone wherein supernatant is separated from settled sludge;
• and
• recycling at least a portion of the settled sludge to the aerated anoxic zone.