Quick answer: Dissolved air flotation wastewater treatment removes suspended solids, fats, oils and greases by attaching microbubbles to particles so they float to the surface for skimming. Air is dissolved into a recycle stream under pressure, then released into the tank. DAF suits low-density solids and FOG-laden effluents that settle poorly.
Key facts – DAF microbubbles are typically 20 to 100 micrometres in diameter, formed as pressure is released (technical literature). – Recycle (pressurised) flow is commonly 10 to 50 percent of the forward flow, depending on saturation and load. – Air-to-solids (A/S) ratio is the central DAF design parameter, often in the range 0.005 to 0.06 kg air per kg solids (design guidance). – With coagulation and flocculation, DAF can remove a large fraction of TSS, FOG and some associated metals. – DAF is favoured over sedimentation where solids are light, oily or otherwise float more readily than they settle.
What is dissolved air flotation?
Dissolved air flotation (DAF) is a physical separation process that removes suspended particles by floating them rather than settling them. It attaches fine air bubbles to solids and oil droplets so they rise to the surface, where they are skimmed off as a float layer.
DAF is used where particles are too light, too oily or too slow-settling for conventional sedimentation. It is common in food and beverage, oil and gas, chemicals and municipal pre-treatment.
The clarified water leaves from below the float layer, and the skimmed float becomes a sludge for further handling.
How does dissolved air flotation work?
DAF works by dissolving air into water under pressure, then releasing that pressure so the air comes out of solution as microbubbles. Those bubbles attach to particles and carry them upward.
A portion of clarified effluent is recycled, pressurised and saturated with air in a saturation vessel. When this recycle stream is injected into the flotation tank at atmospheric pressure, dissolved air forms clouds of microbubbles.
The microbubbles collide with and attach to flocculated solids and oil droplets, lowering their effective density so they float. A surface skimmer removes the float, and clarified water is drawn off below.
What are the key DAF design parameters?
The key DAF design parameters are the air-to-solids ratio, recycle ratio, saturation pressure, hydraulic and solids loading rates, and coagulant and flocculant dosing. These set removal performance and unit size.
| Parameter | Role | Typical direction |
|---|---|---|
| Air-to-solids (A/S) ratio | Bubbles available per unit solids | Higher A/S for higher or lighter solids loads |
| Recycle ratio | Fraction of flow pressurised and saturated | Often 10 to 50 percent of forward flow |
| Saturation pressure | Sets dissolved air quantity | Higher pressure dissolves more air |
| Hydraulic loading rate | Flow per unit surface area | Limited to keep the float stable |
| Coagulant / flocculant dose | Builds floatable flocs | Tuned by jar or pilot testing |
Values above are indicative planning ranges. Final design should rest on jar testing or pilot work with the actual effluent.
What does DAF remove?
DAF removes suspended solids (TSS), fats, oils and greases (FOG), and, with appropriate coagulation, a share of associated metals, phosphorus and colloidal material. It does not remove dissolved salts or truly soluble organics.
With coagulation and flocculation ahead of the unit, DAF can achieve substantial TSS and FOG reduction. Metals that can be precipitated or adsorbed onto flocs are partly removed with the float.
DAF is a solid-liquid separation step, not a polishing or desalination process. Dissolved contaminants pass through and need other treatment.
| Contaminant | DAF effectiveness |
|---|---|
| Suspended solids (TSS) | High, especially with coagulation |
| Fats, oils, greases (FOG) | High for free and emulsified-then-broken oil |
| Metals (precipitable) | Partial, via coagulation onto flocs |
| Dissolved salts / TDS | Not removed |
| Soluble organics / COD | Only the particulate or colloidal fraction |
How do you operate a DAF unit? (HowTo)
The steps below outline the DAF process train from conditioning to sludge handling. Chemistry must be tuned to the specific effluent.
- Condition the influent. Adjust pH and add coagulant to destabilise fine and colloidal particles and break oil emulsions.
- Flocculate. Add flocculant and provide gentle mixing so destabilised particles grow into floatable flocs.
- Saturate the recycle stream. Pressurise a portion of clarified effluent and dissolve air into it in the saturation vessel.
- Release and mix. Inject the saturated recycle into the flotation tank, releasing pressure so microbubbles form and attach to the flocs.
- Float and separate. Allow the aerated flocs to rise and form a stable float layer at the surface.
- Skim the float. Remove the surface float with a mechanical skimmer as flotation sludge.
- Draw off clarified water. Collect clarified effluent from below the float layer for discharge or further treatment.
- Handle the sludge. Thicken and dewater the skimmed float for disposal or recovery.
When should you use DAF instead of alternatives?
Use DAF when solids are light, oily or slow to settle, or where a compact footprint and fast start-up matter. Use sedimentation or other processes when solids are dense and settle readily.
DAF competes with clarifiers or settling tanks, and with filtration. Sedimentation is often cheaper for dense, fast-settling solids. Filtration polishes finer particles but blinds quickly on FOG-heavy streams.
| Situation | Preferred process |
|---|---|
| Light, oily, slow-settling solids | DAF |
| Dense, fast-settling solids | Sedimentation / clarifier |
| Fine polishing of low-solids water | Filtration |
| FOG-dominated effluent | DAF |
What regulatory context applies in the NL and EU?
DAF is commonly installed to meet discharge limits for suspended solids, FOG and sometimes metals set in Dutch discharge permits, issued via the Waterschappen or Rijkswaterstaat. Limits derive from the EU framework.
The EU Water Framework Directive and, for larger installations, the Industrial Emissions Directive with BAT-BREF documents shape acceptable practice. Where regulated substances (Dutch ZZS list) are present in the float, sludge handling and disposal must account for them.
What are the cost drivers?
DAF cost drivers include unit size (set by flow and loading), coagulant and flocculant consumption, energy for recycle pressurisation, and sludge handling and disposal. Chemical dose often dominates operating cost.
Sludge volume and its disposal route can be a larger lifetime cost than the unit itself. All figures are site-specific and should be treated as caveated planning ranges only.
Common mistakes
- Sizing on nominal flow while ignoring peak hydraulic and solids loads.
- Setting coagulant and flocculant doses from theory rather than jar or pilot testing.
- Expecting DAF to remove dissolved salts or soluble COD, which it cannot.
- Undersizing saturation and recycle so the air-to-solids ratio falls short under load.
- Overlooking sludge dewatering and disposal cost in the business case.
When should you bring in an independent advisor?
Bring in an independent advisor when effluent quality is variable, when regulated substances are present, or when vendor DAF proposals differ on chemistry and sizing. Independent review keeps the design honest.
Brine Consulting is independent and vendor-neutral. We specify and review DAF systems on their process merits, from jar-test-based chemistry to A/S ratio and sludge handling, with no unit to sell.
Frequently asked questions
What is the air-to-solids ratio in DAF?
The air-to-solids (A/S) ratio is the mass of dissolved air supplied per mass of solids to be floated. It is the central DAF design parameter. Too low, and there are not enough bubbles to float the solids; too high wastes energy. It is often in the range of roughly 0.005 to 0.06.
Does DAF remove dissolved solids?
No. DAF is a solid-liquid separation process that floats suspended and colloidal particles and oil droplets. It does not remove dissolved salts, dissolved organics or other truly soluble matter. Those require processes such as reverse osmosis, ion exchange or biological treatment, applied separately.
What is the recycle ratio in a DAF system?
The recycle ratio is the fraction of clarified effluent that is pressurised, saturated with air and returned to the flotation tank to form microbubbles. It is commonly 10 to 50 percent of the forward flow. A higher ratio supplies more dissolved air but increases pumping energy and unit hydraulic load.
Can DAF remove metals from wastewater?
DAF can partly remove metals when they are precipitated or adsorbed onto flocs by coagulation and then floated off with the sludge. It does not remove metals that stay dissolved. Effective metal removal therefore depends on the coagulation chemistry ahead of the flotation step.
When is DAF better than sedimentation?
DAF is better than sedimentation when solids are light, oily or slow to settle, or where FOG dominates the effluent. Sedimentation is usually more economic for dense, fast-settling solids. DAF also offers a compact footprint and quick start-up, which can matter where space is limited.
Sources and further reading
- Water and wastewater engineering reference texts (for DAF design parameters and A/S ratio)
- Technology provider technical documentation (for saturation and recycle configuration)
- European Commission, Water Framework Directive and Industrial Emissions Directive (BAT-BREF)
- Rijkswaterstaat and regional Waterschappen, industrial discharge permitting guidance
Need DAF chemistry and sizing checked independently before you buy? [Contact Brine Consulting.]
Written by the Brine Consulting advisory team. The Brine Consulting advisory practice includes Osis G. Kalache, P.Eng., a senior wastewater process engineer with over 17 years in industrial treatment and process optimization, and Cesar J.M. Chu Ortega, a water-tech strategist and electrocoagulation specialist working across textile, chemical and municipal applications. Meet our experts.