Why Blower Sizing Matters

Undersized blowers starve your biological treatment process of oxygen, causing poor BOD removal, odour, and effluent non-compliance. Oversized blowers waste electricity — often the single largest recurring cost in an STP or ETP's operating budget. Getting the sizing right the first time avoids both problems.

Step 1: Determine Your Oxygen Demand

Oxygen demand is driven by the organic load entering your aeration tank, expressed as kg BOD per day. As a general rule of thumb for extended aeration / MBBR systems:

  • Oxygen required ≈ 1.0–1.5 kg O₂ per kg of BOD removed
  • Add 10–20% safety margin for peak loading and equipment ageing

Example: A 200 KLD (kilolitres/day) STP with an inlet BOD of 300 mg/L and target outlet BOD of 10 mg/L removes approximately 58 kg BOD/day. At 1.2 kg O₂/kg BOD, that's roughly 70 kg O₂/day required, before safety margin.

Step 2: Convert Oxygen Demand to Airflow

Standard diffused aeration systems achieve roughly 15–25% oxygen transfer efficiency depending on diffuser type and submergence depth. Air itself is about 23% oxygen by weight. Combining these factors, a commonly used rule of thumb for fine-bubble diffused aeration is approximately 10–15 m³ of air per kg of oxygen required.

Continuing the example: 70 kg O₂/day × 12 m³/kg ≈ 840 m³/day of air, or roughly 35 m³/hr of continuous airflow — before accounting for peak-hour loading, which typically pushes the design flow to 50–70 m³/hr for this size of plant.

Step 3: Calculate Discharge Pressure

Blower discharge pressure must overcome three things:

  • Diffuser submergence depth — roughly 0.1 kg/cm² per metre of water depth
  • Diffuser head loss — typically 0.1–0.2 kg/cm² depending on diffuser type
  • Piping friction losses — distance and pipe diameter dependent, usually 0.05–0.15 kg/cm²

A tank with 4-metre diffuser submergence typically requires a blower rated for approximately 0.5–0.65 kg/cm² discharge pressure.

Step 4: Select the Blower Type

For the airflow and pressure ranges typical of small-to-mid STPs (up to a few thousand m³/hr), twin-lobe Roots blowers are the standard choice — reliable, tolerant of variable back pressure, and low-maintenance. For large plants running continuously above ~2,000–3,000 m³/hr, turbo blowers become worth the higher upfront cost due to their lower power consumption at steady, high-volume operation.

Try Our Free Calculator

Want to run these numbers for your own plant instantly? Use our free Air Blower Sizing Calculator — enter your plant capacity and BOD figures to get an instant airflow and pressure estimate.

Get a Free Sizing Calculation

These formulas give you a working estimate, but real installations involve variables — diffuser type, altitude, temperature, and future expansion plans — that are worth a proper engineering review. QuaSys Inc.'s technical team provides free blower sizing calculations for STP and ETP projects across India. Share your tank dimensions and BOD load or call +91 95408 15659 for a recommendation.