A DAF (dissolved air flotation) system is a water treatment process that removes suspended solids, oils and greases by dissolving air into water under pressure and then releasing it as microscopic bubbles. The bubbles attach to flocculated particles and float them to the surface, where a skimmer removes the resulting sludge for a clarified effluent.
How does a DAF system work?
A DAF system works by making contaminants float rather than settle. A portion of the clarified effluent — the recycle stream — is saturated with air in a pressure vessel at roughly 4–6 bar. When this air-saturated water is injected back into the flotation tank at atmospheric pressure, the dissolved air comes out of solution as a cloud of bubbles 20–50 microns across. These microbubbles attach to and become enmeshed in the chemically formed flocs, lowering their effective density below that of water so they rise to the surface.
The floated solids form a sludge blanket that a surface skimmer scrapes into a collection channel. Clarified water leaves from near the bottom of the tank. Because flotation is far faster than gravity settling for light, oily or low-density solids, DAF units are compact relative to the load they handle.
What does a DAF system remove?
DAF is most effective on contaminants that are light, buoyant or readily flocculated:
- Total suspended solids (TSS) — typically 90–98% removal with correct chemical conditioning.
- Fats, oils and greases (FOG) — DAF is the workhorse for emulsified and free oils in food, dairy and oleochemical effluent.
- Free and emulsified hydrocarbons in refinery and metalworking wastewater.
- Phosphorus, when precipitated with a metal coagulant ahead of flotation.
DAF does not significantly reduce dissolved organic load (soluble BOD/COD) or ammonia — those require biological treatment downstream.
DAF system applications by industry
Dissolved air flotation appears across many sectors:
| Industry | Typical DAF duty |
|---|---|
| Food & beverage | FOG and solids removal before discharge to sewer or biological stage |
| Dairy | Fat and protein recovery; load levelling ahead of an MBR/MBBR |
| Oil & gas / refining | De-oiling produced and process water |
| Municipal | Thickening waste activated sludge; algae removal in potable pre-treatment |
| Pulp & paper | Fibre and filler recovery, whitewater clarification |
In practice the configuration is matched to the duty — for example containerised and fixed DAF units sized to the specific solids and FOG load.
Key components of a DAF unit
- Coagulation/flocculation stage — rapid-mix and flocculation chambers or an inline pipe flocculator.
- Saturation (air dissolution) system — recycle pump, air compressor and a saturation vessel or unpacked saturator.
- Flotation tank — rectangular or circular, sized on surface loading rate.
- Surface skimmer — chain-and-flight or rotating scoop to remove float sludge.
- Bottom scraper and sludge hopper — for the small settled fraction.
How efficient is a DAF system?
A correctly designed and dosed DAF typically removes 90–98% of total suspended solids and a similar proportion of fats, oils and greases, with useful phosphorus reduction when a metal coagulant is dosed. Performance depends far more on the coagulation chemistry and tank hydraulics than on the size of the vessel.
The figures most engineers design around are:
| Parameter | Typical range |
|---|---|
| Surface (hydraulic) loading rate | 5–15 m³/m²·h |
| Recycle ratio | 30–120% of feed flow |
| Saturator pressure | 4–6 bar |
| Air-to-solids ratio | 0.01–0.06 kg air / kg solids |
| Microbubble size | 20–50 µm |
| Float sludge solids | 2–5% dry solids |
These are starting points. Bench-scale flotation tests on the actual effluent are the reliable basis for a guaranteed design, because the right coagulant, polymer and dose vary widely between waste streams.
Advantages and limitations of DAF
DAF earns its place where solids are light, oily or hard to settle, but it is not a universal clarifier. Weigh it against the duty:
Advantages
- Excellent on low-density solids, FOG and emulsified oils that settle slowly or not at all.
- Compact footprint and short residence time versus gravity settling for the same duty.
- Produces a thicker float sludge (2–5% DS) than many clarifiers, reducing downstream dewatering load.
- Rapid start-up and easy to automate; tolerates flow variation when sized for peaks.
Limitations
- Does little for dissolved (soluble) organic load or ammonia — biological treatment is still needed.
- Relies on chemical dosing; running cost and sludge volume rise with coagulant demand.
- Energy is consumed by the recycle pump and air compressor maintaining saturation.
- Performance is sensitive to chemistry and temperature, so it needs competent operation.
Operating a DAF: common problems and maintenance
Most DAF problems trace back to chemistry or air supply rather than the mechanical plant. The recurring issues are poor float formation, solids carry-over and loss of saturation.
- Poor or patchy float — usually a coagulant/polymer dose or pH issue; confirm with a jar/float test and adjust dosing.
- Solids carry-over in the effluent — often hydraulic (flow above design) or too little air; check the loading rate and the air-to-solids ratio.
- Loss of saturation — saturator fouling, low recycle flow or a failing compressor reduces dissolved air and collapses the float blanket.
- Recycle pump cavitation — caused by air ingress or suction problems; degrades both flow and bubble formation.
Routine maintenance covers the skimmer flights and drive, recycle pump and seals, saturator and nozzle inspection, compressor service, and periodic calibration of dosing pumps and instrumentation. Keeping a simple log of dose rates, recycle flow and effluent quality makes most faults quick to diagnose.
The DAF process step by step
- Coagulation. A coagulant (ferric or aluminium salt) is dosed and rapidly mixed to neutralise particle charge and destabilise emulsified oils and fine solids.
- Flocculation. Gentle mixing with a polymer grows the destabilised particles into larger, robust flocs that bubbles can attach to.
- Air saturation and bubble release. A recycle stream saturated with air at 4–6 bar is injected into the tank; the pressure drop releases 20–50 micron microbubbles that attach to the flocs.
- Float separation and skimming. Bubble-laden flocs rise to form a surface sludge blanket, which a skimmer removes; clarified water is drawn off near the base of the tank.
Frequently asked questions
What does DAF stand for in water treatment?
DAF stands for dissolved air flotation. It is a clarification process that uses air dissolved under pressure and released as microbubbles to float contaminants to the surface for removal, rather than settling them by gravity.
What is the difference between DAF and sedimentation?
Sedimentation removes solids by letting them settle to the bottom under gravity, which works well for dense particles. DAF floats solids to the top using microbubbles, which is far more effective for light, oily or low-density solids and gives a smaller footprint for the same duty. Many plants use DAF specifically where settling is too slow or incomplete.
How much TSS can a DAF system remove?
With correct coagulant and flocculant dosing, a well-run DAF typically removes 90–98% of total suspended solids and a similar proportion of fats, oils and greases. Removal of dissolved (soluble) organic load is minimal — that needs biological treatment.
Does a DAF system need chemicals?
Usually yes. A coagulant (such as a ferric or aluminium salt) destabilises fine and emulsified particles, and a polymer flocculant builds them into larger, bubble-friendly flocs. Some lightly loaded streams can be floated with minimal chemistry, but most industrial DAF duties rely on coagulation and flocculation for reliable performance.
How much energy does a DAF system use?
The main energy users are the recycle pump and the air compressor that maintain saturation, so consumption scales with the recycle ratio and saturator pressure rather than the full flow. Keeping the recycle no higher than needed to hit the target air-to-solids ratio is the simplest way to control DAF running cost.
Can a DAF system run continuously?
Yes. Most industrial DAF units run continuously, with the skimmer, dosing and recycle operating automatically. Float sludge is removed on a timer or level control. Continuous operation is generally more stable than intermittent running because the chemistry and float blanket stay in equilibrium.
Where does a DAF sit in a treatment train?
DAF is normally a primary or pre-treatment stage. It comes after coarse screening and any equalisation, removing solids, fats, oils and greases before biological treatment so the downstream stage is not overloaded. It can also be used after a biological stage to thicken waste activated sludge, or as a final solids-polishing step.
What size of operation suits a DAF?
DAF scales from small packaged or containerised units treating a few cubic metres per hour up to large municipal and industrial installations handling hundreds of cubic metres per hour. Because sizing is driven by surface loading rate and solids load rather than a fixed capacity, a unit can be matched closely to almost any duty.