The A²O process—Anaerobic-Anoxic-Oxic—is the most widely deployed biological nutrient removal (BNR) configuration in the world. It removes organic carbon, nitrogen, and phosphorus in a single continuous-flow system with three sequential zones and a return sludge line. The concept is elegant. The commissioning is not.
Unlike a simple activated sludge plant where you’re mainly managing dissolved oxygen and MLSS, an A²O has three interdependent biological zones where changing one parameter shifts the performance of the other two. Crank up the internal recycle to improve nitrate removal, and you might dilute the anaerobic zone enough to kill phosphorus removal. Lower the sludge age to waste more phosphorus, and the nitrifiers wash out.
This article covers the practical commissioning sequence for an A²O plant—what order to tune things, what targets to aim for, and how to diagnose the most common failure modes.
The Biology in 60 Seconds
| Zone | What Happens | Key Condition |
|——|————-|—————|
| Anaerobic | PAOs (phosphorus-accumulating organisms) release phosphorus and store VFAs as PHA. No free oxygen, no nitrate. | ORP < -200 mV; NO₃-N < 0.5 mg/L |
| Anoxic | Denitrifiers convert NO₃-N to N₂ gas using influent carbon. PAOs may also denitrify (DPAOs). | DO < 0.3 mg/L; NO₃-N present |
| Aerobic | Nitrifiers convert NH₃-N to NO₃-N. PAOs take up excess phosphorus (luxury uptake). Organic carbon oxidized. | DO 1.5-3.0 mg/L |
The process has three recycle streams:
1. Return Activated Sludge (RAS): From clarifier bottom back to anaerobic zone inlet. Typically 50-100% of influent flow.
2. Internal Recycle (IR): From aerobic zone end back to anoxic zone inlet. Typically 200-400% of influent flow.
3. Waste Activated Sludge (WAS): Removes excess biomass and the phosphorus stored in the biomass. The only phosphorus removal pathway—if you don’t waste sludge, you don’t remove phosphorus.
Commissioning Sequence: What to Tune First
There’s a specific order. Don’t skip steps.
Step 1: Establish MLSS and SRT (Week 1-3)
Seed the plant if possible. If starting from scratch, you’re looking at 4-6 weeks to build a biomass from raw wastewater—faster in warm weather, painfully slow below 15°C.
Target MLSS: 3,000-4,000 mg/L for municipal wastewater at design load.
Target SRT: 10-20 days (15 is a safe starting point).
How SRT controls everything: SRT determines which organisms survive. Nitrifiers need SRT >8 days at 20°C, >12 days at 12°C. PAOs need SRT >10 days. If you set SRT too short, both nutrient removal functions collapse before carbon removal is affected—BOD removal survives at SRTs as low as 3-4 days.
Set SRT by controlling WAS flow rate, not by guessing.
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WAS flow (m³/day) = (V_aerator × MLSS) / (SRT × RAS_conc)
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Where V_aerator is the aerobic zone volume, MLSS is in mg/L, and RAS_conc is the return sludge concentration (typically 8,000-12,000 mg/L).
Step 2: Achieve Full Nitrification (Week 2-4)
Before you can remove nitrogen, you need to make nitrate. Before you can make nitrate, you need a healthy nitrifier population. This takes time—nitrifiers have a doubling time of 1-2 days at 20°C and 4-6 days at 12°C.
How you know nitrification is working:
– NH₃-N in aerobic zone effluent < 1 mg/L
– NO₃-N in aerobic zone effluent > 5 mg/L (and rising as nitrifiers establish)
– DO in aerobic zone: 1.5-3.0 mg/L
The DO trap: New operators often run DO at 4-5 mg/L “to be safe.” This wastes energy and can inhibit denitrification by carrying too much DO into the anoxic zone through the internal recycle. Set DO to 2.0 mg/L and adjust from there.
Step 3: Tune Internal Recycle for Denitrification (Week 3-5)
Once nitrification is established (ammonia low, nitrate high in the aerator), adjust the internal recycle to send nitrate back to the anoxic zone.
The IR rate formula:
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IR rate = (Target TN removal efficiency) / (1 – Target TN removal efficiency)
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For 75% TN removal: IR = 0.75/(1-0.75) = 300% of influent flow.
But: The formula assumes perfect mixing and 100% denitrification efficiency, neither of which is true. Start at 200-250% IR and increase incrementally while monitoring.
The IR trap: More IR sends more nitrate back = more denitrification. But IR also carries DO from the aerator (typically 1.5-2.0 mg/L) into the anoxic zone. Each mg/L of DO in the recycle flow consumes about 0.35 mg/L of the BOD that denitrifiers need. If you’re carbon-limited (common in low-strength municipal wastewater), too much IR starves denitrification and your effluent nitrate doesn’t improve despite higher IR.
How to check if you’re IR-limited or carbon-limited:
– Increase IR from 200% to 300%. If effluent nitrate drops, you were IR-limited. Keep increasing until you see diminishing returns.
– If increasing IR doesn’t reduce effluent nitrate, you’re carbon-limited. Reducing IR might actually improve TN removal by concentrating available carbon in the anoxic zone.
Step 4: Establish Anaerobic Conditions for Bio-P (Week 4-6)
Phosphorus removal is the last function to develop because PAOs grow slower than ordinary heterotrophs. The anaerobic zone must be truly anaerobic—no dissolved oxygen AND no nitrate.
The RAS nitrate problem: RAS from the clarifier carries 5-15 mg/L NO₃-N back to the anaerobic zone. Nitrate in the anaerobic zone allows ordinary denitrifiers to outcompete PAOs for VFAs. PAOs starve. Phosphorus removal fails.
Solutions, in order of preference:
1. Pre-anoxic zone: A small anoxic zone before the anaerobic zone where RAS nitrate is consumed. This is the standard A²O upgrade—technically making it an “A-A²O” or “Modified A²O” process.
2. Reduce RAS rate: Lower RAS = less nitrate returned to anaerobic zone. But lower RAS also risks sludge blanket in the clarifier. Not below 50% of influent flow.
3. Improve denitrification in the main anoxic zone: If effluent nitrate is low, RAS nitrate is low. Better denitrification fixes the PAO problem indirectly.
Step 5: Fine-Tune the Carbon Split (Week 5+)
Influent carbon (BOD) is the limiting resource. It’s consumed by:
1. PAOs in the anaerobic zone (for PHA storage)
2. Denitrifiers in the anoxic zone (for nitrate reduction)
3. Ordinary heterotrophs in the aerobic zone (for BOD removal)
If denitrifiers starve, nitrate passes through. If PAOs starve, phosphorus passes through. The art of A²O operation is splitting the limited carbon correctly.
The rule of thumb: 60-70% of influent BOD should be consumed in the anaerobic + anoxic zones combined. If more than 30% reaches the aerobic zone, you’re wasting carbon on ordinary heterotrophs that don’t contribute to nutrient removal.
Signs of carbon shortage:
– Effluent nitrate >8 mg/L despite high IR
– TP removal unstable (below 50% some days, above 80% others)
– Anaerobic zone ORP not dropping below -100 mV
If carbon is short: Consider supplemental carbon (methanol, acetate, or a high-BOD industrial side stream). Or reduce the SRT slightly to increase the F/M ratio, which directs more carbon to PAOs and denitrifiers relative to maintenance energy demands.
The Dashboard: What to Monitor Daily
| Parameter | Location | Target | Action if Off |
|———–|———-|——–|—————|
| DO | Aerobic zone end | 1.5-2.5 mg/L | Adjust blower output or valve position |
| DO | Anoxic zone | <0.3 mg/L | Check for surface aeration; reduce IR if high |
| ORP | Anaerobic zone | <-200 mV | Check RAS nitrate; check for surface aeration |
| NH₃-N | Effluent | <1 mg/L | If high: check aerobic DO, SRT, temperature |
| NO₃-N | Effluent | <8 mg/L | If high: increase IR or check carbon availability |
| TP | Effluent | <1 mg/L (typical) | If high: check anaerobic ORP, RAS nitrate, WAS rate |
| SVI | Aerobic zone end | 80-150 mL/g | >150: check for filamentous organisms |
| pH | Aerobic zone | 6.8-7.5 | Nitrification consumes alkalinity; pH <6.8 inhibits nitrifiers |
| Alkalinity | Effluent | >50 mg/L as CaCO₃ | Add alkalinity if low (soda ash, lime, or sodium bicarbonate) |
The Three Most Common Failure Modes
1. “Good nitrification, no phosphorus removal”
Symptoms: NH₃-N < 1, NO₃-N 10-15, TP removal <30%.
Diagnosis: RAS nitrate killing the anaerobic zone. ORP in anaerobic zone > -50 mV.
Fix: Add a pre-anoxic zone (re-route RAS through a small anoxic contact zone before the anaerobic zone). This is a piping modification—a day of work, a week of benefit. Or reduce RAS rate and accept a higher sludge blanket.
2. “Good phosphorus removal in summer, gone in winter”
Symptoms: TP removal 80%+ at 22°C, <30% at 12°C. Nitrification also marginal in winter.
Diagnosis: SRT too short for cold-weather nitrification. Nitrifiers wash out, less nitrate available for denitrification, PAOs lose VFA competition to ordinary heterotrophs in the anaerobic zone. A cascade failure triggered by temperature.
Fix: Increase MLSS in autumn to maintain SRT through winter. Target SRT 15+ days at 12°C (vs 10 days at 20°C). This means either reducing WAS in October or accepting higher MLSS year-round.
3. “Rising sludge blanket, rising SVI, everything falling apart”
Symptoms: SVI >200 mL/g, sludge blanket in clarifier, effluent TSS rising, nutrient removal collapsing.
Diagnosis: Filamentous bulking. The most common cause in A²O is low F/M in the selector zone (anaerobic zone), which favors filaments over floc-formers. The anaerobic zone IS the selector—if it’s too large relative to influent BOD, you’ve inadvertently created a low-F/M selector that breeds filaments.
Fix: Reduce anaerobic zone HRT by taking a portion offline (if multiple trains or compartments). Increase WAS to reduce MLSS and increase F/M. Add a small dose of chlorine or peroxide to the RAS to selectively kill filaments extending from flocs (temporary measure while fixing the root cause).
Summary
Commission an A²O in this order:
1. MLSS and SRT first. Everything else depends on the right organisms being present at the right concentration.
2. Nitrification second. You can’t remove what you can’t convert.
3. Internal recycle third. Match the nitrate load to the carbon available.
4. Anaerobic conditions fourth. PAOs are the most sensitive organisms in the system. Protect them.
5. Carbon split last. This is the ongoing tuning—the art of A²O operation.
The plant will tell you what it needs if you measure the right parameters daily. The operators who track ORP in the anaerobic zone, DO in the anoxic, and NO₃ in the RAS have plants that run. The ones who only check effluent compliance numbers are always reacting to yesterday’s problem.
📖 Related Reading
- MBR Membrane Bioreactor Operation and Maintenance Guide
- Industrial Wastewater Treatment Process Selection Guide
EHS compliance checklists, waste management logs, incident investigation forms — ready to download and use.