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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

IR efficiency

—%

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 →