How do I size an aeration tank?

Size from F:M ratio (V = QS_0 / FM·X) and check against minimum hydraulic detention time (V_HRT = Q × t_min). Use the larger of the two. For typical municipal POTW design, F:M-based volume governs.

What's a typical MLSS?

Conventional: 1500–3000 mg/L. Extended aeration: 3000–5000. MBR: 8000–15,000. Higher MLSS allows smaller tank but loads the secondary clarifier harder. Practical limit: 3000 mg/L for conventional clarifiers.

What side-water depth do I use?

Common: 12–20 ft (3.7–6.1 m). Length-to-width: 4:1 to 8:1 for plug flow. Aspect ratios outside these can produce hydraulic dead zones — verify with baffles or CFD if geometry is unusual.

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Aeration Tank Sizing

Required aeration basin volume for an activated-sludge plant. Solves the F:M relationship for V given target F:M, design MLSS, and BOD load. Cross-checks against minimum hydraulic detention time.

Units

Design flow, QMGD

Influent BOD, S₀mg/L

Target F:M ratio/day (0.2–0.5 conventional)

Design MLSS, Xmg/L

Min. hydraulic detentionhours

Required volume (F:M)—MG

Required volume (HRT min)—MG

Design volume (governing)—MG

Resulting HRT—hours

Defaults: 2 MGD plant, conventional activated sludge. Pick the larger of F:M-based and HRT-min volumes. Real designs add 10–20% for mixing dead zones and side-water depth limits.

F:M-based volume:

$$ V = \frac{Q \, S_0}{\text{F\!:\!M} \cdot X} $$

HRT-based minimum:

$$ V_{HRT} = Q \cdot t_{min} $$

V aeration basin volume · Q design flow · S₀ influent BOD · F:M target food-to-microorganism ratio · X design MLSS · t_min minimum hydraulic detention time.

F:M ranges and process selection

Conventional / step-feed: F:M 0.2–0.5/d, HRT 4–8 hr, MLSS 1500–3000 mg/L. Standard secondary treatment.
Extended aeration / oxidation ditch: F:M 0.05–0.15/d, HRT 18–36 hr, MLSS 3000–5000 mg/L. Stable, low effluent BOD, large footprint.
High-rate: F:M 0.5–1.5/d, HRT 1.5–3 hr, MLSS 1000–2000 mg/L. Compact, sensitive to upsets, often roughing-stage only.
Membrane bioreactor (MBR): F:M 0.05–0.20/d, HRT 4–8 hr, MLSS 8000–15000 mg/L. Smaller basin, replaces clarifier with membrane.

Why two sizing checks

The F:M equation gives the volume needed for the biomass to consume the food load. But there's a separate hydraulic limit: even at very high MLSS, the basin must be large enough that water spends at least the minimum detention time mixing with biomass. For dilute waste (low S₀), the F:M equation might give a tiny volume, but you still need HRT > ~3 hr for adequate mixing and oxygen transfer.

For typical municipal POTW design, F:M-based volume governs. For dilute industrial waste or very-low-strength domestic, HRT minimum often governs.

Design MLSS — picking X

Higher MLSS lets you size a smaller basin but loads the secondary clarifier harder. The clarifier's ability to thicken sludge limits the practical X. Conventional secondary clarifiers handle MLSS up to about 3000 mg/L without loss of effluent quality. Above that, sludge volume index (SVI) and clarifier surface loading become critical. MBRs sidestep this by replacing the clarifier with membranes.

Side-water depth, length-to-width

Once you've sized V, pick a depth. Common: 12–20 ft (3.7–6.1 m) side-water depth. Length:width ratio: 4:1 to 8:1 for plug-flow basins. Aspect ratios outside these tend to produce hydraulic dead zones — use baffles or CFD to verify if the geometry is unusual.

Reference: Metcalf & Eddy / AECOM (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th ed., McGraw-Hill, ch. 8.

Common mistakes

Using average-day flow for every check. Volume is usually set at design average flow, but oxygen demand and clarifier loading must be checked at peak conditions, and the minimum HRT should hold at peak hour flow.
Picking MLSS without checking the clarifier. A higher MLSS shrinks the basin but increases solids loading on the clarifier. Basin and clarifier have to be sized together.
Treating F:M as the only design variable. Solids retention time (SRT) controls nitrification and sludge production. Modern design generally sets SRT first and checks F:M afterwards.
Forgetting cold-weather nitrification. Nitrifiers grow slowly in cold water, so the SRT needed in winter can be much longer than in summer.

When this calculator doesn’t apply

This tool sizes the basin volume from F:M and HRT for conventional suspended-growth processes. Aeration requirements, nutrient removal (biological phosphorus and denitrification), membrane systems and sequencing batch reactors need process-specific design. Municipal plants must meet state design standards, such as Ten States Standards where adopted, and permit limits, which this tool does not check.