Diffused Aeration Blower Sizing for STPs

To size a diffused aeration blower you need two numbers: the air flow the process demands and the discharge pressure the tank depth demands. Air flow comes from oxygen demand and diffuser efficiency; pressure comes from submergence plus losses. This post gives you both calculations with worked examples, so you can arrive at a specific Yash model instead of guessing.

The two numbers that decide everything

A blower's duty point is a pair — capacity at a pressure. A YEBL-1-345 delivers 345 m³/hr but only up to ~125 mbar; a YEBL-DS-320V delivers 320 m³/hr up to 500 mbar. Same air flow, very different pressure class. Size for one without the other and the blower will not perform in the tank.

Part A — Air flow from oxygen demand

Step 1: Oxygen demand

Oxygen demand is driven by the organic load removed:

• BOD load (kg/day) = Q (m³/day) × BOD removed (mg/L) ÷ 1000
• O₂ demand (kg/day) = BOD load × f

The oxygen factor f is standard design guidance: 1.0–1.5 for carbonaceous BOD only, and up to ~2.0 when nitrification is included. This cluster uses f = 2.0 as a conservative default so figures stay comparable across posts. The framework itself is explained in the complete STP blower guide.

Step 2: Convert oxygen to air

One cubic metre of air carries roughly 0.28 kg O₂, but only the SOTE fraction transfers into the water:

Air flow (m³/hr) = O₂ demand (kg/hr) ÷ (0.28 × SOTE_field)

Typical field SOTE, labelled as design guidance:

Worked example: 300 KLD, fine bubble

• Q = 300 m³/day, BOD removed ≈ 265 mg/L
• BOD load = 300 × 265 ÷ 1000 = 79.5 kg/day
• O₂ demand = 79.5 × 2.0 ≈ 159 kg/day = 6.6 kg/hr
• Air = 6.6 ÷ (0.28 × 0.18) ≈ 131 m³/hr from oxygen

Add ~30% for peak load, diffuser ageing and mixing, and the duty air flow lands near 250–300 m³/hr, which points at a YEBL-1-345 if the pressure class allows.

Part B — Discharge pressure from submergence

Now the pressure check. The blower must overcome the water column above the diffuser plus system losses.

• 1 metre of water ≈ 98 mbar
• Fine-bubble diffuser loss ≈ 30–50 mbar
• Pipe and fitting friction ≈ 20–50 mbar

Worked pressure examples

Notice how a 300 KLD plant sized on air flow alone might suggest a single-stage YEBL-1-345 — but if that plant's diffusers sit at 3.5 m, the ~418 mbar demand rules single-stage out and forces a double-stage turbine blower. Always run both parts.

Part C — Select the model

Pick the smallest model that meets both air flow and pressure at its rated duty point. For the 300 KLD example:

• Shallow tank (~2.5 m): air ~300 m³/hr, pressure ~315 mbar → single-stage high-pressure candidate.
• Deeper tank (~3.5 m): air ~300 m³/hr, pressure ~418 mbar → YEBL-DS-320V (320 m³/hr, 500 mbar) with margin on both.

Then add an identical standby (N+1), an inlet filter and a pressure relief valve. The redundancy pattern is covered in blower redundancy: duty/standby for STPs.

Correcting the SOTE you actually get

The SOTE band in the table above is field guidance that already accounts for the real tank. If you start instead from a diffuser maker's clean-water SOTE, correct it before sizing:

• Alpha (α) — dirty-water to clean-water transfer ratio; ~0.4–0.7 typical for fine bubble in activated sludge.
• Beta (β) — saturation correction for salts and solids; ~0.9–0.98 for sewage.
• Temperature — warm water holds less oxygen, so check the summer peak where transfer is worst.

Field SOTE = clean-water SOTE × α × (temperature and saturation corrections). Skip this and you will specify a blower that meets the catalogue but starves the tank in July. Designing to the actual (AOTR) figure at worst-case temperature is the safe path.

A worked full example, start to finish

Bring both parts together for a 400 KLD plant, 4 m submergence, fine-bubble diffusers.

Air ~230 m³/hr at ~472 mbar rules out any single-stage unit and points squarely at a YEBL-DS-230P (230 m³/hr, 490 mbar) as the duty, with an identical standby. Had the tank been only 2 m deep, the ~266 mbar demand would have allowed a single-stage YEBL-1-345 — same air flow, different machine. Depth decides the class.

A note on turndown and margin

Real STPs see load swing between night and morning peak. Two smaller blowers in parallel give better turndown than one big unit — you drop to a single duty machine at night and save energy. Alternatively a VFD on one blower trims output to match dissolved oxygen. Both approaches feed into the running-cost discussion in energy cost of STP aeration.

Quick sizing checklist

• Flow Q in m³/day and BOD removed in mg/L → BOD load
• Multiply by f (≈2.0) → oxygen demand
• Divide by (0.28 × SOTE_field) → air flow from oxygen
• Add ~30% margin and confirm mixing → duty air flow
• Submergence × 98 mbar + diffuser + pipe loss → discharge pressure
• Smallest model meeting both → duty; add identical standby

Common sizing errors

• Using clean-water SOTE. The catalogue number flatters you; correct it for α, saturation and temperature.
• Sizing on flow, ignoring load. Two 300 KLD plants with different BOD need different blowers.
• Forgetting the pressure check. A blower that meets air flow but not head cannot push air through submerged diffusers.
• No margin for fouling. Field SOTE falls as diffusers age; design with headroom.
• Oversizing "to be safe." An oversized blower is throttled or short-cycled and wastes energy every hour — see energy cost of STP aeration.

Serving sewage treatment plants and effluent treatment plants across India, with the complete STP blower guide as the full reference.

Get your duty point sized

Send flow, BOD in/out and submergence depth and we will return a duty-and-standby selection with the pressure check applied.

• WhatsApp: +91 9311693322
• Email: sales@yashblowers.org