How do I size the anaerobic, anoxic and oxic zones of an A2/O reactor?

Each zone is sized from its own loading - the anaerobic zone for phosphorus release, the anoxic zone for denitrification rate, the oxic zone for nitrification - and the calculator returns the three volumes.

How is sludge age (SRT) determined for biological nutrient removal?

SRT is checked against what the biomass needs for nitrification, and the calculator reports the SRT implied by the design so you can see whether the zones are sized consistently.

Is biological phosphorus removal enough, or is chemical dosing needed?

The calculator compares bio-P capacity against the phosphorus load and, when capacity falls short, tells you that chemical polishing is required and how much it would take.

Does it account for tropical conditions and QCVN limits?

Yes - the calculator is set up for tropical conditions and is aligned to QCVN 14:2025.

Design Manual › Biological treatment › A²/O BNR

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A²/O Biological Nutrient Removal Calculator

Anaerobic–anoxic–oxic zone sizing · bio-P capacity vs chemical polishing · internal recycle · tropical conditions · QCVN 14:2025

iWhat this tool does

Sizes the three A²/O zones and decides how much phosphorus biology can remove before chemicals must step in:

Feed / targets — the same influent case as our aeration and chemical-P tools, so the three pages chain end-to-end.
Process — SRT, MLSS, anaerobic HRT, denitrification rate, WAS phosphorus content, recycles.
Results — zone volumes + bio-P capacity; the shortfall is handed to the chemical-P tool.

Bio-P is not guaranteed. Above ~28 °C glycogen-accumulating organisms (GAO) can out-compete phosphorus-accumulating organisms (PAO) for the volatile fatty acids — exactly the temperature band of Southeast-Asian sewage. This tool sizes the biology, then honestly shows what a chemical hedge must carry.

1Wastewater & effluent targets

Design flow Q m³/d

Temperature^[5] (26–34)°C

Influent BOD₅^[1][5] (150–250)mg/L

Effluent BOD₅^[1]mg/L

Influent COD^[5] (300–500)mg/L

Influent TKN^[5] (30–50)mg/L

Effluent NH₃-N^[9]mg/L

Effluent TN^[9] (≤ 15)mg/L

Effluent organic N^[4] (1–3)mg/L

Influent total P^[5] (4–8)mg/L

Effluent TP target^[9] (0.1–2)mg/L

Influent TSS^[1] (150–300)mg/L

Inert (nb)VSS in TSS^[1] (10–20 % TSS)mg/L

Peak factor^[1] (1.3–2.0)×

2Process & Bio-P assumptions

MLSS (aerobic)^[1][8] (2.5–5.0)g/L

MLVSS/MLSS^[1] (0.75–0.85)—

SRT mode

Anoxic-zone design basis^[2][4]

Sizes the anoxic zone by specific denitrification rate (SDNR 0.03–0.11 g N/g VSS·d) — temperature already inside the value.

2 h risks secondary P release">Anaerobic HRT^[5][8] (0.5–1.5)h

SDNR (denitrification)^[2][3] (0.03–0.11)g N/g VSS·d

WAS phosphorus content^[5][6] (3–6)% of TSS

anoxic). Efficiency = IR/(1+IR)">Internal recycle IR^[8] (100–400)% of Q

Return sludge RAS^[8] (50–100)% of Q

Aerobic DO^[1] (1.5–2.5)mg/L

Kinetic coefficients (advanced — same engine as our aeration tool)

True yield Y^[1] (0.40–0.80)g VSS/g BOD₅

Decay kd @20 °C^[1] (0.06–0.15)d⁻¹

θ for kd^[1] (1.02–1.06)—

Nitrifier yield Yn^[1] (0.10–0.20)g VSS/g N

AOB μmax @20 °C^[6] (0.60–1.00)d⁻¹

θ for μmax^[6] (1.06–1.12)—

AOB decay b^[6] (0.05–0.15)d⁻¹

Ks, ammonia^[1] (0.50–1.00)mg/L

Ko, oxygen^[1] (0.40–0.60)mg/L

SRT safety factor^[1] (1.5–3.0)—

Identical defaults to the aeration tool, so sludge production and AOR agree across pages.

Results — zone volumes

Anaerobic

—m³

Anoxic

—m³

Oxic (aerobic)

—m³

Item	Value	Unit

Results — biological P vs chemical polishing

P to remove

—mg/L

Bio-P capacity

—mg/L

Chemical polish

—mg/L

WAS P content	Scenario	P removed (mg/L)	Verdict

Results — nitrogen & recycles

N denitrified

—kg N/d

Denit. share

—%

AOR (avg)

—kg O₂/d

Item	Value	Unit

Feed & effluent checks

Ratio	Value	Bio-P / denitrification requirement	Status

Dosing-room-adjacent instrumentationMAPPED TO OUR RANGE

Process instruments we carry and quote at factory-direct price. Submersible mixers (anaerobic / anoxic), internal-recycle and RAS pumps are high-flow low-head duties outside our standard range — normally supplied by the EPC or sourced on request.

Duty	Instrument	Model / spec	Qty

Quote this A²/O's instruments at factory price

DO, MLSS, ORP/pH and flow instruments at the manufacturer's ex-works price — our margin comes solely from the export tax rebate, so you see exactly what they cost. The buttons below carry this run's duty across: aeration for the full SOR/air/blower sizing, the clarifier for solids separation, the chemical-P tool when the bio-P shortfall needs a metal salt, and the sludge train with this run's WAS (kg DS/d) pre-filled.

Request a quote for this duty Clarifier sizing → Sludge train → ← Size aeration / blowers Chemical-P polish →

Response within 24 hours · Your inputs stay in your browser — nothing is uploaded.

Sources & parameter basis

[1]	Metcalf & Eddy / AECOM, Wastewater Engineering, 5th ed. 2014 — SRT→P_X→V, kinetics, MLSS/DO ranges.
[2]	US EPA, Nutrient Control Design Manual (2009) & Design Manual Phosphorus Removal — SDNR = F_d·SOUR/2.86 (F_d 0.41–0.55), SDNR₁ = 0.03(F/M)+0.029.
[3]	Tchobanoglous et al. (2002/2003) — SDNR by carbon source: influent 0.03–0.11, methanol 0.21–0.32, endogenous 0.005–0.048.
[4]	DWA-A 131 (2016) — effluent organic-N assumption, V_DN/V_T reference table.
[5]	WEF, Water Environment Technology (2021-08) — PAO ecology (Accumulibacter/Tetrasphaera), COD:P 45, VFA:P 4–16, GAO ≥28 °C.
[6]	WaterWorld (2026-05) — anaerobic mass fraction 15–25 %, avoid comfort SRT 15–25 d, PAO min aerobic SRT 2–3 d @20–25 °C.
[7]	China GB 50014 — empirical zone split reference band (qualitative; verify current clause numbers).
[8]	CH2M Hill BNR parameter compilation — A²/O SRT 5–25 d, anaerobic 0.5–1.5 h, anoxic 0.5–1 h, oxic 4–8 h, IR 100–400 %, RAS 50–100 %.
[9]	QCVN 14:2025 / QCVN 40:2025 (Vietnam, Circulars 05 & 06/2025); PERMENLHK 68/2016 (Indonesia).

On the nitrogen-assimilation convention. This tool follows the kinetic form used by the aeration sheet (N in sludge = 0.124 × P_X,VSS), which makes the oxygen balance self-consistent across both pages. A simpler design convention (0.05 × BOD removed) puts more nitrogen in sludge, less in the anoxic zone — expect a ~20–25 % difference in the nitrogen to denitrify depending on which convention you apply. We show the kinetic value and flag the difference rather than hide it.

Questions about a duty, a specification, or transparent sourcing? Contact our application engineers →

Cite this tool / 引用此工具

If you use this result in a report, specification or client document, please cite it as below. It takes one line and lets engineers find us. 如果您把计算结果用于报告、规格书或客户文件，请按下列方式引用。

Jiuyuhub. "A²/O Biological Nutrient Removal Calculator". Jiuyuhub, https://jiuyuhub.com/a2o-bnr-design.html

Licensed for internal and commercial technical use (CC BY 4.0). 内容可用于内部及商业技术用途。 Questions? Ask our engineers

iWhat this tool does

1Wastewater & effluent targets

2Process & Bio-P assumptions

Results — zone volumes

Results — biological P vs chemical polishing

Results — nitrogen & recycles

Feed & effluent checks

Dosing-room-adjacent instrumentationMAPPED TO OUR RANGE

Quote this A²/O's instruments at factory price

Sources & parameter basis

Cite this tool / 引用此工具

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