DAF Design Parameters: Loading Rates, Air-to-Solids Ratio and Sizing Data

A dissolved air flotation unit is defined by six numbers: hydraulic loading rate (5–15 m/h conventional, 15–30 m/h high-rate), solids loading rate (up to about 10 kg/m²·h), saturator pressure (400–600 kPa gauge), recycle ratio (15–30% of forward flow), air-to-solids ratio (0.005–0.06 kg air/kg solids), and bubble diameter (20–100 µm). Everything else in the design follows from these.

This page gives each parameter, its governing equation where one exists, and the practical range. It also covers the temperature correction that most imported designs get wrong, and a unit conversion that produces two contradictory-looking figures in the literature.

Master parameter table

Parameter	Symbol	Conventional	High-rate	Unit
Hydraulic loading rate	vh	5 – 15	15 – 30	m³/m²·h
Solids loading rate	SLR	2 – 10	5 – 15	kg/m²·h
Saturator pressure (gauge)	P	400 – 600	400 – 600	kPa
Saturator efficiency	f	0.6 – 0.8	0.6 – 0.8	–
Recycle ratio	R/Q	15 – 30	15 – 30	%
Air-to-solids ratio	A/S	0.005 – 0.06	0.005 – 0.06	kg/kg
Bubble diameter	db	20 – 100	20 – 100	µm
Flotation cell depth	h	2.0 – 2.5	2.0 – 2.5	m
Hydraulic retention time	HRT	20 – 40	5 – 15	min
Float solids concentration	–	2 – 5	2 – 5	% DS
Length : width ratio	–	2:1 – 4:1	2:1 – 4:1	–

Hydraulic loading rate

Hydraulic loading rate is the total flow through the cell divided by its plan area, expressed in m³/m²·h — which simplifies to m/h, a rise velocity. It is the parameter that sets footprint, and it is the honest dividing line between a conventional and a high-rate machine.

Critically, the flow used is forward flow plus recycle, not forward flow alone. A 100 m³/h duty at 20% recycle puts 120 m³/h through the cell. Sizing on 100 m³/h under-designs the unit by 20% and is one of the more common specification errors.

The design rule is that hydraulic loading must stay below the rise velocity of the slowest bubble–floc aggregate you need to capture. Exceed it and those aggregates are dragged out with the underflow before they reach the surface — solids breakthrough. Well-flocculated, low-density material tolerates high rates. Poorly conditioned or dense material does not, regardless of what the nameplate says.

Air solubility and the unit trap

DAF literature quotes air solubility at 20 °C as both 18.7 and 24.3. Both are correct. They are the same quantity in different units:

18.7 mL/L × 1.3 mg/mL (density of air) = 24.3 mg/L

The standard A/S equation takes solubility in mL/L and carries the 1.3 factor separately. Substituting the mg/L figure into that equation inflates the air term by 30% and produces a badly undersized recycle system. If you take one thing from this page, take this.

Water temperature	Air solubility sa	Equivalent	Relative to 20 °C
0 °C	29.2 mL/L	38.0 mg/L	+56%
10 °C	22.8 mL/L	29.6 mg/L	+22%
20 °C	18.7 mL/L	24.3 mg/L	baseline
30 °C	15.7 mL/L	20.4 mg/L	−16%
40 °C	14.2 mL/L	18.5 mg/L	−24%

Why this matters more in Africa than in Europe

A DAF designed on 20 °C data and installed on effluent running at 30–35 °C dissolves roughly 16–20% less air per cubic metre of recycle. The delivered air-to-solids ratio falls by the same proportion, and the unit quietly underperforms — usually diagnosed in the field as a chemical problem when it is in fact a design temperature problem.

Tropical and equatorial installations, and any warm process effluent — brewery wort cooling, dairy CIP, abattoir wash-down — need either a higher recycle ratio or a higher saturator pressure to compensate. Specify the design water temperature explicitly in your enquiry. If a supplier does not ask for it, they are sizing on a default that may not apply to your site.

Air-to-solids ratio: the governing equation

For a recycle-pressurised DAF — which is effectively all modern industrial units — the air-to-solids ratio is:

A/S = 1.3 · s_a · (f · P − 1) · R ⁄ (S_a · Q)

• 1.3 — density of air, mg/mL
• s_a — air solubility at the design temperature, mL/L (see table above)
• f — saturator efficiency, 0.6–0.8 depending on vessel type and packing
• P — absolute saturator pressure in atm (gauge pressure in kPa ÷ 101.3, then + 1)
• R — recycle flow, m³/h
• S_a — influent suspended solids, mg/L
• Q — forward flow, m³/h

The (f · P − 1) term is the useful part: it is the air that actually comes out of solution when the recycle is depressurised to atmospheric. The recycle stream arrives already carrying its atmospheric-pressure load of dissolved air, and that fraction is not available for bubble formation. Designers who forget to subtract it overestimate available air by 20–30%.

Selecting a target A/S

Duty	Target A/S (kg/kg)	Comment
Low-solids clarification (<100 mg/L)	0.03 – 0.06	Little solids mass; air demand set by bubble coverage
General industrial pretreatment	0.015 – 0.03	Most food, beverage and abattoir duties
High-solids streams (>1,000 mg/L)	0.005 – 0.015	Abundant solids; air per kg falls
Sludge thickening	0.02 – 0.06	Higher A/S for a firm, thick float
Algae removal, potable water	0.04 – 0.06	Very low density cells need generous bubble coverage

Note the inverse relationship: A/S is a ratio, so dilute streams need a higher ratio and concentrated streams a lower one. This trips people up — the instinct is that dirtier water needs more air, and in absolute mass terms it does, but not per kilogram of solids.

Saturator pressure and efficiency

Saturator gauge pressure sits at 400–600 kPa (4–6 bar) in virtually all designs. Below roughly 350 kPa too little air dissolves and bubble density is inadequate. Above about 700 kPa the additional air dissolved no longer justifies the pumping energy, and the pressure drop across the release valve becomes violent enough to shear flocs already formed.

Saturator efficiency f depends on vessel design: unpacked vessels achieve roughly 0.6, packed vessels 0.7–0.8, and eductor or venturi systems vary widely with maintenance condition. Assume 0.7 for a packed vessel unless the manufacturer provides test data. A worn or fouled saturator drifts downward over time, which is why a unit that met specification at commissioning may not three years later.

Recycle ratio

Recycle ratio is the pressurised return flow as a percentage of forward flow, normally 15–30%. It is the primary operating handle: it is far easier to trim recycle than to change saturator pressure, and it is the adjustment an operator should reach for first when float quality deteriorates.

Higher recycle delivers more air but also raises the hydraulic load on the cell, so the two constraints must be checked together. Pushing recycle from 20% to 35% to fix a float problem can move the unit past its hydraulic loading limit and cause exactly the solids carry-over you were trying to eliminate. Below 10% recycle, bubble distribution across the cell typically becomes uneven regardless of total air mass.

Bubble size

The target is 20–100 µm, with most well-designed systems producing a distribution centred on 30–50 µm. Bubble size is set by the pressure differential at the release device and by the device’s geometry — not by how much air is dissolved.

Small bubbles matter for two reasons. For a fixed mass of air, halving bubble diameter multiplies the available surface area by eight, giving far more opportunity for floc attachment. And rise velocity scales with the square of diameter under Stokes’ law, so a 50 µm bubble ascends slowly enough to sweep through the floc blanket, while a 500 µm bubble shoots to the surface contacting almost nothing. Coarse bubbles — the classic symptom of worn or fouled release nozzles — deliver the same air mass with a fraction of the separation.

Tank geometry

• Depth: 2.0–2.5 m for rectangular cells, 2.5–3.0 m side-water depth for circular. Deeper does not improve separation — flotation happens in the upper zone — but too shallow risks the float blanket being disturbed by the underflow draw.
• Length:width: 2:1 to 4:1 for rectangular units. Wider cells suffer short-circuiting; narrower ones waste skimmer travel.
• Contact zone: 30–90 seconds, where bubbles and flocs first meet before entering the separation zone. Undersizing this is a frequent cause of poor performance in cheap units.
• Freeboard: allow 300–500 mm above water level for float accumulation between skimmer passes.

Design checklist

1. Characterise the effluent by composite sampling over a full production cycle — not a grab sample
2. Establish coagulant and polymer type and dose by jar test on the actual effluent
3. Fix the design water temperature and read s_a from the table above
4. Select target A/S from the duty table
5. Calculate required recycle from the A/S equation
6. Check hydraulic loading on Q + R, not Q
7. Check solids loading rate independently — both constraints must be satisfied
8. Size the contact zone for 30–90 s
9. Select materials against effluent chemistry, particularly chlorides and pH
10. Verify peak flow and shock-load cases, including CIP discharges

Frequently asked questions

What is the hydraulic loading rate for DAF?

5–15 m³/m²·h for conventional DAF and 15–30 m³/m²·h for high-rate designs. The rate must be calculated on forward flow plus recycle flow, not forward flow alone.

Is air solubility 18.7 or 24.3 mg/L at 20 °C?

Neither figure is wrong — they are different units. Air solubility at 20 °C is 18.7 mL/L, which equals 24.3 mg/L when multiplied by the density of air, 1.3 mg/mL. The standard air-to-solids equation requires the mL/L value because it carries the 1.3 factor separately.

What saturator pressure should a DAF run at?

400–600 kPa gauge. Below about 350 kPa insufficient air dissolves; above about 700 kPa the extra air does not justify the energy and the release pressure drop begins shearing formed flocs.

How does temperature affect DAF performance?

Air solubility falls as water warms. At 30 °C it is about 16% lower than at 20 °C, and at 40 °C about 24% lower. A unit designed on 20 °C data and run on 30–35 °C effluent delivers proportionally less air and a correspondingly reduced air-to-solids ratio, requiring higher recycle or higher saturator pressure to compensate.

Why does a dilute stream need a higher A/S than a concentrated one?

Because A/S is a ratio of air mass to solids mass. A dilute stream contains little solids mass, so the air needed for adequate bubble coverage represents a larger proportion of it. In absolute terms a concentrated stream still needs more total air.

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Related: DAF sizing calculator — applies every equation on this page automatically · Complete DAF technical guide for Africa

Sources

• Metcalf & Eddy — Wastewater Engineering: Treatment and Resource Recovery (air-to-solids equation, air solubility data)
• Haarhoff, J. & van Vuuren, L. — A South African Design Guide for Dissolved Air Flotation, Water Research Commission, Report TT 60/93
• Edzwald, J.K. — Dissolved air flotation and me, Water Research
• Rodrigues & Rubio — DAF: a review from the perspective of system parameters

Last reviewed July 2026. All ranges are indicative and must be confirmed by jar test or pilot trial against the actual effluent.