A/O™, A2/O, An/O
Kruger's A/O process utilizes an anaerobic selector for release of soluble phosphorus and uptake of BOD by phosphorus accumulating organisms (PAOs). Upon entering the oxic stage, the PAOs uptake soluble phosphorus, the remaining BOD is consumed, and nitrifying bacteria oxidize the ammonia to nitrate. Most of the settled sludge from the final clarifier is returned to the anaerobic selector and a fraction of the phosphorus rich sludge is removed with the wasted sludge. Automatic dissolved oxygen (D.O.) control in the oxic stage maximizes denitrification, which in addition to saving energy, reduces the amount of nitrate returned to the anaerobic selector.
Kruger's A²/O process incorporates an anoxic zone between the anaerobic selector and oxic zone for nitrate reduction. An internal recycle pump returns nitrate rich mixed liquor from the oxic zone to the anoxic zone. For more stringent TN limits, secondary anoxic zones can be added after the oxic stage for additional denitrification producing effluent with TN of less than 3 mg/L.
Kruger's An/O process is designed for WWTPs that must meet stringent TN limits without a TP limit. Nitrate-rich mixed liquor and RAS are returned to the anoxic zone where they are combined with the influent wastewater. In the primary anoxic zone, bacteria consume BOD using oxygen from the nitrate molecule. Secondary anoxic zones can be added after the oxic stage for additional denitrification producing effluent with TN of less than 3 mg/L.