Dissolved Air Flotation (DAF): Complete Technical Guide for Africa
Dissolved air flotation (DAF) is a solid–liquid separation process that removes suspended solids, fats, oils and grease from water by dissolving air under pressure and releasing it as microbubbles that attach to particles and float them to the surface, where they are skimmed off. It is the dominant pretreatment technology for industrial wastewater with high FOG or low-density solids, and it is widely used across Africa in food processing, tanning, edible oil refining and municipal treatment.
This guide covers the engineering that matters: design loadings, air-to-solids ratio, achievable removal, how to size a unit, and how DAF behaves under African operating conditions. Every figure carries a unit and a source.
DAF design parameters at a glance
These are the values a DAF unit is designed around. Conventional and high-rate designs differ mainly in hydraulic loading, which drives footprint.
| Parameter | Conventional DAF | High-rate DAF | Unit |
|---|---|---|---|
| Hydraulic loading rate | 5 – 15 | 15 – 30 | m³/m²·h (m/h) |
| Saturator pressure (gauge) | 400 – 600 | 400 – 600 | kPa |
| Recycle ratio | 15 – 30 | 15 – 30 | % of forward flow |
| Microbubble diameter | 20 – 100 | 20 – 100 | µm |
| Air-to-solids ratio (A/S) | 0.005 – 0.06 | 0.005 – 0.06 | kg air / kg solids |
| Tank depth (rectangular) | 2.0 – 2.5 | 2.0 – 2.5 | m |
| Side-water depth (circular) | 2.5 – 3.0 | 2.5 – 3.0 | m |
| Hydraulic retention time | 20 – 40 | 5 – 15 | minutes |
Each of these is covered in depth, with the governing equations and the air solubility correction for warm water, on the DAF design parameters reference.
How does dissolved air flotation work?
DAF works in four stages. Air is dissolved into a pressurised recycle stream, released as microbubbles, attached to chemically conditioned flocs, and the resulting float is skimmed from the surface.
1. Chemical conditioning
A coagulant — typically ferric chloride, alum or PAC — neutralises the surface charge on colloidal particles. A high-molecular-weight polymer flocculant then bridges the destabilised particles into flocs large enough for bubbles to attach. Without correct conditioning, no DAF will perform, regardless of how well the mechanical design is executed. This is the single most common cause of underperformance in the field.
2. Air saturation
A fraction of the clarified effluent is recycled to a saturation vessel, where it is mixed with compressed air at 400–600 kPa. At this pressure air dissolves into the water well beyond its atmospheric solubility of roughly 24 mg/L at 20 °C. Saturator efficiency is typically 60–80%, and this figure directly determines how much recycle flow is required.
3. Microbubble release and attachment
The saturated recycle passes through release valves or nozzles into the flotation chamber. The sudden pressure drop forces dissolved air out of solution as a cloud of microbubbles, 20–100 µm in diameter. Bubble size matters more than bubble volume: small bubbles provide far more surface area per unit of air and rise slowly enough to contact flocs rather than shooting straight to the surface.
4. Separation and skimming
Bubble–floc aggregates become buoyant and rise to form a float layer, which a surface skimmer removes to a sludge channel. Clarified water is drawn from the bottom of the chamber. Denser material that does not float settles and is removed by a bottom scraper. DAF float is typically 2–5% dry solids — considerably thicker than the 0.5–1% from gravity sedimentation, which reduces downstream dewatering cost.
What is the air-to-solids ratio and why does it govern DAF design?
The air-to-solids ratio (A/S) is the mass of air released divided by the mass of solids entering the unit, in kg air per kg solids. It is the governing design variable: it determines whether enough bubble surface exists to float the solids present.
- General clarification: 0.005 – 0.06 kg air/kg solids
- Sludge thickening: 0.02 – 0.06 kg air/kg dry solids
Below the required A/S, a fraction of flocs never gain enough attached air to become buoyant and are lost to the underflow — the classic symptom of solids breakthrough in the effluent. Above it, the float blanket is robust but you are paying for air and recycle pumping you do not need. The designer trades recycle ratio, saturator pressure and water temperature against the influent solids load to land in range.
Temperature is not a minor correction in African applications. Air solubility falls as water warms. A unit designed on 20 °C solubility and operated on 32 °C effluent will dissolve meaningfully less air per unit of recycle, and the delivered A/S drops accordingly. Systems specified against European ambient data routinely underperform in tropical installations for exactly this reason.
What removal efficiency can a DAF achieve?
| Parameter | Typical removal | Notes |
|---|---|---|
| Total suspended solids (TSS) | 85 – 95% | With correct coagulation and flocculation |
| Fats, oils and grease (FOG) | 90 – 99% | Free and emulsified; emulsified FOG needs chemical breaking |
| COD | 30 – 70% | Only the particulate fraction; soluble COD passes through |
| BOD | 30 – 60% | As above — DAF is physical, not biological |
| Total phosphorus | 70 – 95% | With metal-salt coagulant dosing |
| Algae (potable water) | 90 – 99% | DAF outperforms sedimentation on low-density algal cells |
The most frequently misunderstood line in that table is COD. DAF removes the particulate fraction only. If your effluent COD is largely soluble — as it is in brewery and distillery streams — DAF alone will not meet a discharge consent, and it must be followed by biological treatment. Any supplier promising 90% COD removal from a DAF alone is selling you something that will not work.
DAF compared with sedimentation and lamella clarification
| DAF | Gravity sedimentation | Lamella clarifier | |
|---|---|---|---|
| Removes particles that are | Low density, buoyant, oily | Dense, settleable | Dense, settleable |
| Surface loading | 5 – 30 m/h | 0.5 – 1.5 m/h | 5 – 15 m/h |
| Footprint | Small | Very large | Small |
| FOG removal | Excellent | Poor | Poor |
| Sludge solids | 2 – 5% | 0.5 – 1% | 1 – 3% |
| Energy demand | Moderate | Very low | Low |
| Startup time | Minutes | Hours | Hours |
| Operator skill | Moderate | Low | Low |
The decision rule is straightforward: if the solids settle, use sedimentation or a lamella clarifier and save the energy. If they float, refuse to settle, or the stream carries FOG, use DAF. Where land is expensive or unavailable, DAF wins on footprint alone — it does the work of a clarifier ten to twenty times its plan area.
Worked example: sizing a DAF for a 40 m³/h brewery effluent
Design basis: flow 40 m³/h, influent TSS 400 mg/L, target A/S 0.03 kg/kg, saturator gauge pressure 500 kPa, saturator efficiency 70%, water 20 °C, hydraulic loading 8 m/h.
- Solids load: 40 m³/h × 400 g/m³ = 16 kg TSS/h
- Air required: 16 kg/h × 0.03 = 0.48 kg air/h
- Air released per m³ recycle: at 6 bar absolute, saturation ≈ 24 mg/L × 6 = 144 mg/L; at 70% efficiency ≈ 101 mg/L; less the 24 mg/L already in solution at atmospheric pressure ≈ 77 g/m³ released
- Recycle flow: 480 g/h ÷ 77 g/m³ ≈ 6.2 m³/h → recycle ratio ≈ 16% (within the 15–30% design range)
- Total flow through the cell: 40 + 6.2 = 46.2 m³/h
- Flotation area: 46.2 ÷ 8 m/h ≈ 5.8 m², say 6 m² — roughly a 3.9 m × 1.5 m rectangular cell
This is a first-pass sizing, not a design. It assumes the solids are floatable at the chosen A/S and that coagulant and polymer dose have been established by jar test. Peak flow, diurnal load variation and CIP discharges from the brewhouse will all move the answer. Note also that the same 40 m³/h duty on a high-rate design at 20 m/h needs only about 2.3 m² — which is why footprint-constrained sites specify high-rate units despite the higher capital cost.
You can run this calculation against your own numbers, with automatic temperature correction, using the free DAF sizing calculator.
Which industries use DAF?
| Industry | Typical influent TSS | Typical influent FOG | Primary DAF duty |
|---|---|---|---|
| Abattoir / slaughterhouse | 800 – 3,000 mg/L | 200 – 1,500 mg/L | FOG and blood solids removal |
| Edible oil refining | 300 – 1,000 mg/L | 500 – 4,000 mg/L | Oil recovery and FOG removal |
| Brewery | 200 – 800 mg/L | 10 – 100 mg/L | Solids removal before biological stage |
| Dairy processing | 300 – 1,500 mg/L | 100 – 800 mg/L | Fat and protein removal |
| Poultry processing | 500 – 2,500 mg/L | 150 – 1,000 mg/L | FOG, feathers, fine solids |
| Tannery | 1,000 – 4,000 mg/L | 100 – 800 mg/L | Solids, sulphide, chromium floc |
| Fish and seafood | 500 – 3,000 mg/L | 200 – 2,000 mg/L | FOG and protein recovery |
| Textile dyeing | 100 – 800 mg/L | 10 – 100 mg/L | Colour and coagulated dye floc |
| Pulp and paper | 500 – 5,000 mg/L | Low | Fibre recovery, white-water clarification |
| Mining and minerals | Highly variable | Low | Fine tailings, process water reuse |
| Municipal pretreatment | 150 – 400 mg/L | 50 – 150 mg/L | Primary clarification, sludge thickening |
| Drinking water | Low turbidity | — | Algae, colour, low-density floc |
Why DAF suits African operating conditions
DAF is not merely transferable to African sites — several of its characteristics are specifically advantageous here.
- Recovers quickly from power interruption. A DAF is a physical process. After an outage it returns to specification within minutes. An activated sludge plant that loses aeration for a day can take weeks to recover its biomass. On grids with frequent load-shedding this difference decides whether a site stays compliant.
- Matches the industrial base. Food and beverage processing, abattoirs, tanning and edible oil refining dominate African industrial effluent — all high-FOG streams, all classic DAF duties.
- Minimal civil works. Packaged units arrive prefabricated and need a level slab, power and pipework. This matters where construction supervision is costly and concrete quality is variable.
- Small footprint. Many African industrial sites are landlocked within urban areas with no room for a conventional clarifier.
- Tolerates intermittent operation. Seasonal processing — sugar, fruit, fish — suits a process that can be stopped and restarted without biological consequence.
The counterweights are honest ones. DAF needs continuous chemical supply, and coagulant and polymer availability varies considerably by country. It requires a competent operator who understands jar testing. And it consumes more energy than gravity settling. Where a stream settles readily and land is available, DAF is the wrong answer.
DAF system capacities
| Class | Flow range | Daily equivalent | Typical application |
|---|---|---|---|
| Packaged / skid-mounted | 2 – 150 m³/h | ~50 – 3,600 m³/d | Single industrial site pretreatment |
| High-rate / roll-flow | 200 – 1,200 m³/h | ~4,800 – 28,800 m³/d | Municipal works, WWTP upgrade, surface water |
Construction materials are selected against the effluent: carbon steel with epoxy coating for general duty, FRP lining or SS304 for moderately aggressive streams, SS316L for chloride-bearing or acidic effluent, and duplex 2205 or 2507 for severe service such as tannery or certain mining applications. Getting this wrong is expensive — a carbon steel unit on a chloride-rich tannery effluent will not survive its warranty period.
Frequently asked questions
What does DAF stand for?
DAF stands for dissolved air flotation. It is also referred to as a flotation clarifier or flotation unit. Related but distinct processes are induced air flotation (IAF) and cavitation air flotation (CAF), which generate larger bubbles by mechanical means and suit heavier oil loads at lower capital cost but poorer fine-solids removal.
Can DAF be used on its own?
Sometimes. Where the discharge consent is set on TSS and FOG only, a correctly designed DAF can meet it alone. Where the consent includes COD or BOD limits and the effluent carries significant soluble organic load, DAF must be followed by biological treatment. DAF is most often installed as pretreatment ahead of an activated sludge, SBR, MBR or anaerobic stage.
How much space does a DAF need?
A packaged 40 m³/h unit occupies roughly 6–7 m × 3.5 m including the saturator and control skid. Allow 1 m clearance on all sides for access, plus space for chemical dosing and sludge handling. High-rate designs reduce the flotation cell area by 50–70% for the same duty.
What chemicals does a DAF need?
Typically a coagulant — ferric chloride, aluminium sulphate or polyaluminium chloride — and an anionic or cationic polymer flocculant. pH correction with caustic or acid is often required to bring the stream into the coagulant’s effective range. Doses must be established by jar test on the actual effluent; there is no reliable way to predict them from the industry alone.
Why is my DAF passing solids into the effluent?
In order of likelihood: coagulant or polymer dose has drifted; influent load has risen above design; saturator pressure has dropped; the recycle pump is cavitating or worn; release nozzles are fouled and producing coarse bubbles instead of microbubbles; or influent temperature has risen and reduced air solubility. Check the chemistry before touching the mechanical equipment — it is the cause four times out of five.
How long does a DAF system last?
A correctly specified unit gives 15–25 years of service. Material selection against the effluent chemistry is the dominant factor. Wear parts — skimmer chains and flights, recycle pump seals, release nozzles — are consumables on a 2–5 year cycle depending on duty.
Sources and further reading
- Haarhoff, J. & van Vuuren, L. — A South African Design Guide for Dissolved Air Flotation, Water Research Commission, Report TT 60/93
- Haarhoff, J. — Dissolved air flotation in Southern Africa, Water Science & Technology (survey of 26 operating plants)
- Metcalf & Eddy — Wastewater Engineering: Treatment and Resource Recovery
- Edzwald, J.K. — Dissolved air flotation and me, Water Research
Last reviewed July 2026. Design ranges are indicative and must be confirmed by jar test or pilot trial against the actual effluent.