DAF Sizing Calculator
Enter your flow, suspended solids and design water temperature. The calculator returns the required air mass, recycle flow and ratio, flotation area and indicative tank dimensions, using the standard air-to-solids equation with proper temperature correction for air solubility.
How the calculation works
The calculator applies the standard air-to-solids relationship for a recycle-pressurised DAF, rearranged to solve for recycle flow:
R = (A/S · Sa · Q) ⁄ (1.3 · sa · (f · P − 1))
- Solids load = Q × TSS ÷ 1,000, in kg/h
- Air required = solids load × target A/S, in kg/h
- Air solubility sa is interpolated from published data at your design temperature — 18.7 mL/L at 20 °C, falling to 15.7 mL/L at 30 °C
- Air released per m³ recycle = 1.3 × sa × (f · Pabs − 1). The −1 removes the air already dissolved at atmospheric pressure, which never forms bubbles
- Recycle flow = air required ÷ air released per m³
- Flotation area = (Q + R) ÷ hydraulic loading rate — note the recycle is included
Tank dimensions assume a 2.5:1 length-to-width ratio, mid-range for rectangular cells.
Choosing your inputs
| Input | If you don’t know it |
|---|---|
| Influent TSS | Use the industry ranges in the DAF guide, but sample before specifying |
| Design temperature | Use the warmest sustained effluent temperature, not ambient air |
| Target A/S | 0.03 for general industrial; 0.015 for streams above 1,000 mg/L; 0.05 for dilute or algal water |
| Saturator pressure | 500 kPa is the usual default |
| Saturator efficiency | 0.7 for a packed vessel; 0.6 unpacked |
| Hydraulic loading | 8 m/h conventional; 20 m/h for a high-rate design |
What this calculator does not do
It sizes the flotation cell and recycle system. It does not tell you whether your solids will float, which coagulant to use, or at what dose — those come from a jar test on your actual effluent, and no calculator substitutes for one. It also assumes steady flow. Real plants see diurnal peaks, CIP slugs and seasonal swings, and a unit sized on average flow will fail at peak.
Treat the output as the starting point of a specification, not the end of one.
Frequently asked questions
How do I calculate DAF recycle flow?
Divide the required air mass by the air released per cubic metre of recycle. Air required is the solids load multiplied by the target air-to-solids ratio. Air released is 1.3 × sa × (f · Pabs − 1), where sa is air solubility in mL/L at the design temperature.
Should recycle flow be included in the flotation area calculation?
Yes. Hydraulic loading is calculated on forward flow plus recycle. Omitting the recycle undersizes the cell by 15–30%, which is one of the most common DAF specification errors.
What flotation area does a 100 m³/h DAF need?
At 8 m/h conventional loading with 20% recycle, about 15 m². At 20 m/h high-rate loading, about 6 m². The difference in footprint is why high-rate designs are specified on space-constrained sites.
Related: DAF design parameters reference · Complete DAF technical guide for Africa