Oxygen Transfer Rate (SOTR ↔ AOTR)
Convert manufacturer's standard oxygen transfer rate (SOTR, clean water at 20°C, 0 DO) to actual field rate (AOTR, mixed liquor at design DO). Required for aerator sizing in activated-sludge systems.
Defaults: SOTR = 100 lb/hr, fine-bubble diffuser α=0.5, β=0.95, design DO 2 mg/L, 20°C, 15 ft submergence. The unit (lb vs kg) is consistent across SOTR and AOTR — convert input to your preferred system.
α, β, F factors explained
Alpha (α) is the largest source of uncertainty. It accounts for surfactants, dissolved organics, and salts that depress oxygen transfer in mixed liquor compared to clean water. Fine-bubble diffusers (small holes, lots of surface area) are most affected — typical α = 0.4–0.6. Coarse-bubble and surface aerators are less sensitive — α = 0.7–0.9. The factor degrades over time as biofilm forms on diffuser surfaces (the "F" fouling factor).
Beta (β) adjusts the dissolved oxygen saturation concentration. Salinity reduces DO saturation. Domestic wastewater has β = 0.95–0.98. High-strength industrial waste or seawater can drop β to 0.7–0.9.
F (fouling) is the new-vs-old factor. New diffusers transfer at the spec rate; biofilm and chemical fouling reduce transfer 10–30% over a year. F = 1.0 for new, F = 0.7–0.9 for end-of-life sizing.
Why SOTR can't be used directly
Manufacturers test aerators in clean water at 20°C with zero DO — the conditions giving the highest possible transfer rate. These are NOT field conditions. Real basins are warmer/cooler, contain mixed liquor, and operate at non-zero DO. The AOTR adjustment is required by ASCE Standard 2-91 for valid aerator sizing.
Typical AOTR/SOTR ratios
- Fine-bubble disc, fresh, design DO 2 mg/L: 0.40–0.55 of SOTR
- Coarse-bubble, fresh, design DO 2 mg/L: 0.55–0.70
- Surface mechanical, fresh: 0.55–0.65
- Pure-oxygen system: 0.70–0.85 (different reference saturation)
If your AOTR/SOTR comes out below 0.35 or above 0.75, recheck α, β, F and the basin depth before sizing the blower.
Going from AOTR to blower sizing
Once you have AOTR (lb O₂/hr that the aerator must deliver in the basin), sizing the blower follows from O₂ density (0.0827 lb/scf) and the aerator's standard aeration efficiency (SAE, lb O₂/hp-hr or lb/kWh). Typical SAE: 3–5 lb/hp-hr for fine-bubble at 15 ft submergence. Power demand (hp) = AOTR / (SAE × AOTR/SOTR). Adjust for blower efficiency (~70%) and motor efficiency (~90%) to get connected hp.
Reference: ASCE (2007). Standard Guidelines for In-Process Oxygen Transfer Testing, ASCE/EWRI 2-06. Metcalf & Eddy / AECOM (2014). Wastewater Engineering, 5th ed., ch. 5.