The A²/O process is widely used when a wastewater treatment plant needs to remove both nitrogen and phosphorus biologically.
At first glance, the process looks simple: wastewater flows through an anaerobic zone, an anoxic zone and an aerobic zone before entering the secondary clarifier.
In practice, however, the piping and recycle flows between these zones are what make the process work.
The important flows are not all doing the same thing. Influent brings in organic matter and nutrients, internal recycle moves nitrate back to the anoxic zone, and aeration supplies oxygen to the aerobic zone.
Understanding these flows makes it much easier to understand how an A²/O plant operates.
1. Influent Flow: Where the Process Starts
In a conventional A²/O plant, the normal wastewater flow is generally:
Coarse screening → Fine screening → Grit removal → Distribution well → Influent main → Anaerobic tank
The headworks remove large solids and grit before the wastewater enters the biological treatment stage.
A distribution well may include an inlet valve, flow meter and other control equipment. These allow operators to monitor and regulate the influent flow entering the biological process.
The anaerobic tank is the first biological zone in the conventional A²/O arrangement. This is important for biological phosphorus removal because phosphorus-accumulating organisms (PAOs) need suitable anaerobic conditions to release phosphorus and take up readily biodegradable carbon.
For this reason, the normal influent flow is directed to the anaerobic zone rather than directly to the anoxic or aerobic zone. EPA's description of the conventional A²/O process also shows influent entering the anaerobic zone, followed by the anoxic and aerobic zones.
This does not mean every nutrient-removal plant has exactly the same hydraulic arrangement. Modified processes can use different configurations and recycle streams. The important point is that the piping arrangement needs to match the biological process being used.
Emergency Bypass
Some plants also have bypass piping for maintenance, equipment failure or other abnormal conditions.
A bypass is an emergency or maintenance arrangement, not part of the normal biological process. Whether wastewater can be bypassed, stored or routed to another treatment stage depends on the plant design and local discharge requirements.
2. Anaerobic Zone: Creating the Right Conditions for Phosphorus Removal
The anaerobic zone contains little or no dissolved oxygen and is intended to provide conditions favorable for phosphorus release by PAOs.
Return activated sludge (RAS) is normally returned from the secondary clarifier to the head of the anaerobic zone, where it mixes with the incoming wastewater.
The readily biodegradable carbon in the influent is particularly important here.
PAOs use energy obtained from intracellular polyphosphate to take up readily biodegradable carbon and store it as intracellular carbon compounds. Phosphate is released into the liquid during this anaerobic stage.
Later, under aerobic-and in some systems anoxic-conditions, PAOs take up phosphorus again. When phosphorus-rich biomass is eventually wasted from the system, phosphorus leaves the treatment process with the waste activated sludge.
So the anaerobic zone is not simply a tank without air. It is an important part of the biological phosphorus-removal mechanism.
3. Anoxic Zone: Where Denitrification Takes Place
After the anaerobic zone, the mixed liquor enters the anoxic zone.
The anoxic zone is designed for denitrification. Unlike the aerobic zone, it is not supplied with aeration air. Instead, mixers are normally used to keep the mixed liquor suspended.
But where does the nitrate come from?
This is where the internal recirculation line becomes important.
A portion of the nitrified mixed liquor from the aerobic zone is pumped back to the anoxic zone:
Aerobic tank → Internal recycle pump → Anoxic tank
The recycled mixed liquor contains nitrate produced during nitrification.
In the anoxic zone, microorganisms use available biodegradable carbon to reduce nitrate to nitrogen gas.
The basic nitrogen-removal sequence is:
Ammonia → Nitrification → Nitrate → Internal recycle → Denitrification → Nitrogen gas
This internal recycle is one of the defining features of the conventional A²/O process.
4. Internal Recirculation: More Is Not Always Better
The internal recycle flow needs to be large enough to return sufficient nitrate to the anoxic zone, but there is no single recycle ratio that is correct for every plant.
Depending on the design and operating conditions, internal recycle rates can be in the range of one to several times the influent flow.
For example, 100%–400% of influent flow may be encountered in practice, but the actual operating point should be determined from the plant's process requirements rather than from a fixed rule.
Important factors include:
Influent nitrogen load
Biodegradable COD availability
Nitrate concentration
Required effluent TN
Anoxic-zone volume
Dissolved oxygen carryover
Pumping energy
Biological phosphorus-removal performance
Increasing the recycle rate does not automatically mean better nitrogen removal. Once the process has sufficient nitrate transfer to the anoxic zone, additional pumping may provide little benefit while increasing energy consumption.
The recycle should therefore be adjusted based on actual plant performance.
5. Aerobic Zone: Nitrification and Phosphorus Uptake
The mixed liquor then enters the aerobic zone.
This is where oxygen is supplied to support biological reactions such as:
Nitrification
Organic matter oxidation
Aerobic phosphorus uptake
The aeration system normally consists of:
Blower → Main air header → Branch air piping → Aeration equipment
Fine-bubble diffusers are commonly used, although other aeration systems are also possible.
The airflow to individual tanks or compartments can be adjusted to maintain the required dissolved oxygen (DO) concentration.
Too little oxygen can limit nitrification. Too much aeration, however, is not necessarily beneficial. Excessive aeration can increase energy consumption and may affect biological phosphorus-removal performance under certain operating conditions. Research on A²/O systems has also reported deterioration in biological phosphorus removal under excessive aeration conditions.
The goal is therefore not simply to supply as much air as possible. The goal is to provide enough oxygen for the required biological reactions while maintaining suitable conditions throughout the process.
6. From the Aerobic Tank to the Secondary Clarifier
After biological treatment, the mixed liquor flows to the secondary clarifier.
A typical arrangement is:
Aerobic tank → Effluent channel → Secondary clarifier
Where the plant's hydraulic profile allows it, this flow can be driven by gravity rather than by a transfer pump.
The secondary clarifier separates the biological solids from the treated water.
The settled sludge is then divided into two important streams:
Return activated sludge (RAS) → returned to the biological process
Waste activated sludge (WAS) → removed from the system
The clarified water continues toward downstream treatment or discharge.
7. The A²/O Process Is Really a System of Different Flows
Looking only at the tanks can make an A²/O plant seem straightforward.
But the biological process depends on several flows working together:
Influent: brings wastewater and biodegradable carbon into the system.
RAS: returns activated sludge and PAOs to the biological process.
Internal recycle: returns nitrate-rich mixed liquor to the anoxic zone.
Aeration air: supplies oxygen to the aerobic zone.
WAS: removes excess biomass and phosphorus-rich solids.
Each flow has a different purpose.
That is why A²/O operation is not simply about maintaining the right tank volumes. The hydraulic connections, recycle rates, oxygen supply and sludge wasting all have to work together.
In wastewater treatment, piping is part of the process. It determines where carbon, nitrate, biomass and oxygen go-and that ultimately affects nitrogen and phosphorus removal.