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.
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 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.