How to Size a DAF System: A Step-by-Step Engineering Guide

What information do you need before sizing?

Before any calculation, characterise the wastewater and the duty:

• Design flow — average and peak flow (m³/h), including any recycle from later stages.
• Influent TSS and FOG (mg/L) — sets the solids load to be floated.
• Target effluent quality — the consent or downstream requirement.
• Coagulant/flocculant regime — affects floc strength and float behaviour.
• Temperature — governs air solubility in the saturator.

How do you calculate DAF tank size?

The flotation tank area is set by the surface loading rate (also called hydraulic loading or rise rate), defined as the total flow divided by the tank surface area:

Rearranging, the required area A = (Q + Qr) / SLR. For example, a feed of 50 m³/h with a 50% recycle (25 m³/h) at a design SLR of 8 m³/m²·h needs about (75 / 8) = 9.4 m² of surface area. The solids loading rate (kg TSS/m²·h, typically 2–8) is then checked to confirm the same area is not solids-limited; the larger of the two areas governs.

What is the air-to-solids (A/S) ratio?

The air-to-solids ratio is the mass of air supplied per mass of solids to be floated. It determines whether there is enough bubble surface to lift the flocs. Typical A/S ratios fall between 0.01 and 0.06 kg air / kg solids. The air available comes from the recycle flow, the saturator pressure and the air solubility at the operating temperature; raising the recycle ratio or saturator pressure increases available air. Designers pick an A/S target from jar/float testing, then size the recycle rate and saturator to deliver it.

Worked example: sizing a DAF for a food factory

Take a food-processing effluent of 1,000 mg/L TSS at a peak flow of 60 m³/h, with a 60% recycle and a design surface loading rate of 8 m³/m²·h. The first-pass size falls out of four short calculations.

• Recycle flow: Qr = 0.6 × 60 = 36 m³/h, so total flow Q + Qr = 96 m³/h.
• Hydraulic area: A = (Q + Qr) / SLR = 96 / 8 = 12 m².
• Solids check: 60 m³/h × 1,000 g/m³ = 60 kg/h TSS. At a solids loading of 6 kg/m²·h that needs 60 / 6 = 10 m². The hydraulic area (12 m²) is larger, so it governs.
• Air required: at an A/S ratio of 0.03, air = 0.03 × 60 = 1.8 kg air/h, to be delivered by the saturated recycle.

So a roughly 12 m² flotation unit with 36 m³/h recycle and a saturator supplying about 1.8 kg/h of dissolved air is the first-pass design — to be confirmed by float testing before procurement.

How do you size the air saturation system?

The saturator must dissolve enough air into the recycle stream to meet the air-to-solids target. The air a recycle stream can carry depends on air solubility at the operating temperature, the saturator absolute pressure and the saturator efficiency (typically 0.8–0.9 for a packed unit).

In practice the recycle ratio, saturator pressure (commonly 4–6 bar) and temperature are traded off until the delivered air mass meets or exceeds the A/S requirement. Because warm water dissolves less air, always size the saturator for the warmest expected operating temperature — otherwise the float will weaken in summer when solubility is lowest.

Worked sizing sequence

The steps below give a defensible first-pass design that you then confirm with bench float tests and a supplier's performance guarantee.

Common sizing mistakes

• Ignoring peak flow. Sizing on average flow alone leads to carry-over of solids during peaks; design to the peak hydraulic loading.
• Under-providing recycle/air. Too low an A/S ratio starves the flocs of bubbles and the float blanket collapses.
• Skipping float tests. Bench-scale flotation tests on the real effluent are the most reliable basis for SLR and A/S targets.
• Forgetting temperature. Warm water holds less air, reducing saturator output — size for the warmest expected condition.

For a difficult or high-value duty, CFD modelling of the flotation tank can confirm the hydraulics and bubble distribution before fabrication, reducing the risk of short-circuiting.

From first-pass design to final specification

A hand calculation gets you a defensible size, but a procured DAF should be backed by more than a spreadsheet. Before issuing a specification, confirm:

• Float/jar testing on representative effluent to fix the coagulant, polymer, dose, and the achievable A/S and surface loading rate.
• Peak and diurnal flow data, not just a daily average, so the unit holds up under real load.
• A supplier performance guarantee tied to the tested conditions, not generic figures.
• Materials selection for the effluent — for example corrosion-resistant grades for saline or aggressive streams.
• Sludge handling — the float volume and solids set the downstream dewatering duty, so size both together.

Treat the hand calculation as the sanity check that keeps a supplier proposal honest, not as the final design on its own.

Worked sizing sequence

1. Set the design flow. Establish average and peak feed flow, including internal recycles. Size hydraulics to the peak.
2. Choose a surface loading rate. Select an SLR (5–15 m³/m²·h) from float-test data or comparable duty; lower for weak flocs.
3. Calculate tank area from hydraulics. A = (Q + Qr) / SLR. Compute with the chosen recycle ratio.
4. Check the solids loading rate. Confirm kg TSS/m²·h is within 2–8; if exceeded, increase area. The larger area governs.
5. Set the A/S ratio and recycle. Pick an air-to-solids target (0.01–0.06), then size recycle flow and saturator pressure to deliver the required air mass.
6. Confirm with float testing. Validate SLR, A/S and chemical dose with bench flotation tests on the actual effluent before finalising.