Part a in Figure 7.38.8 is a typical schematic representation of the A2/O process. The process consists of three zones in series: anaerobic, anoxic, and oxic. Each zone is further divided into several compartments. The mechanisms of phosphorus removal and nitrification are described in the preceding sections. This process recycles the nitrified mixed liquor containing nitrate and the nitrite at the end of the oxic zone. This internal recycled stream goes to the anoxic zone for denitrification at a rate of 100 to 200% of the influent wastewater flow rate. The process uses the organics in the influent wastewater as the carbon source for denitrification.
Influent detention times for the anaerobic and anoxic zones are generally 1 to 2 hr, while the oxic zone is 4 to 8 hr, depending on the influent wastewater characteristics. This process typically uses submersible mixers for solids mixing in the anaerobic and anoxic zones. Different types of aeration devices are used to satisfy oxygen demands in the oxic zone.
The A2/O process can achieve an effluent quality with 1 mg/l or less in total phosphorus and ammonia. However, effluent NOx-N concentration is typically limited to 6 to 10 mg/l and depends on the NH3-N concentration in the influent wastewater as well as the internal recycling rate. This process is used in more than twenty installations in
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