Dissolved air flotation (DAF) removes fats, oils, grease and suspended solids by dissolving air in water under pressure, then releasing it to form fine bubbles that attach to chemically conditioned flocs and float them to the surface for skimming. DAF is the most versatile flotation option; cavitation air flotation (CAF) is simpler and lower in energy for easier wastewaters, and shallow air flotation saves footprint on high-flow, low-density solids such as paper fiber.
How does dissolved air flotation work?
Dissolved air flotation (DAF) separates light solids, fats, oils and grease (FOG) from water by attaching very fine air bubbles to them so they rise rather than settle. Part of the clarified effluent is pressurized and saturated with air, then released into the incoming wastewater; the pressure drop forms a cloud of microbubbles that lift chemically conditioned flocs to the surface, where a skimmer removes them.
DAF is widely used as primary or pretreatment for industrial wastewater because it handles material that settles poorly. Below we explain the recycle-pressurization process, compare DAF with cavitation air flotation (CAF) and shallow air flotation, and outline the chemistry and sizing factors. See our dissolved air flotation equipment for available models.
What happens inside a recycle-pressurization DAF system?
Most modern DAF units use recycle pressurization: only a portion of treated water is pressurized, rather than the whole flow. The process runs in this order:
- Chemical conditioning. Raw wastewater receives coagulant and flocculant in a pipe flocculator or reaction tank so that fine particles and emulsified oil form flocs.
- Recycle pressurization. A pump draws a portion of clarified effluent, typically around 10–30% of the influent flow, and pressurizes it, commonly to about 4–6 bar.
- Air saturation. Compressed air is injected and the water passes through a saturation vessel (or an equivalent dissolving device) so that air dissolves close to saturation at that pressure.
- Pressure release. The saturated recycle flows through release valves or nozzles into the contact zone. The sudden drop to atmospheric pressure forms microbubbles, typically in the range of roughly 30–100 microns.
- Contact and attachment. Bubbles and flocs mix in the contact zone; bubbles attach to or become trapped within the flocs, lowering their apparent density.
- Flotation and skimming. The floc-bubble aggregates rise to form a float blanket, which a chain-and-flight or rotating skimmer pushes into a sludge hopper.
- Clarified water discharge. Clear water leaves from the bottom of the separation zone, often over an adjustable weir that controls the water level and float thickness. Heavy solids that settle are removed by a bottom scraper or drain.
Why are fine bubbles important?
Smaller bubbles have more surface area per unit of air and rise slowly, giving them more chance to collide with and attach to particles. Large bubbles rise fast, create turbulence and can break up fragile flocs. This is why pressure-dissolved air generally outperforms coarse aeration for difficult separations.
How do DAF, CAF and shallow air flotation differ?
Cavitation air flotation (CAF)
CAF uses a cavitation aerator, a rotating impeller that draws air down a hollow shaft and shears it into bubbles at the tip, instead of a pressure saturation system. It has fewer components (no saturation tank, recycle pump or compressor), lower energy use and simple operation. The bubbles are generally larger than DAF microbubbles, so separation efficiency on fine or emulsified material is usually lower.
Shallow air flotation
Shallow air flotation (also called shallow pool or rotating flotation) uses a circular tank with a shallow water depth and a rotating distribution system that moves with the flow, so the water in the tank is nearly stationary relative to the inlet. Combined with dissolved air, this gives short retention times and very compact footprint per unit of flow. It is common in pulp and paper for fiber recovery and in other high-flow, low-density applications.
| Factor | DAF (recycle pressurization) | CAF (cavitation) | Shallow air flotation |
|---|---|---|---|
| Bubble generation | Pressurized saturation, release valves | Cavitation impeller | Dissolved air, rotating distribution |
| Typical bubble size | Fine microbubbles | Larger than DAF | Fine microbubbles |
| Removal of emulsified oil and fine solids | High | Moderate | High |
| Energy use | Moderate | Low | Moderate |
| Footprint | Medium | Medium | Smallest per unit flow |
| Mechanical complexity | Moderate | Low | Higher (rotating structure) |
| Typical uses | Food, dairy, meat, oily, refinery, textile, municipal thickening | Pretreatment of moderate-load wastewater, budget projects | Paper mills, fiber recovery, large flows, water treatment |
What chemistry does a DAF system need?
Flotation separates particles, but chemistry creates particles that float well. Most wastewaters need two steps.
Coagulation
Coagulants neutralize the negative surface charge on colloids and help break oil-in-water emulsions. Common choices are polyaluminum chloride (PAC), ferric chloride, ferric sulfate and aluminum sulfate. Each works best in a particular pH range, so pH correction with caustic soda, lime or acid is often part of the system. For proteins in dairy and meat processing wastewater, pH adjustment is especially important.
Flocculation
A polymer flocculant, usually anionic or cationic polyacrylamide (PAM), bridges the small coagulated particles into larger, stronger flocs that trap bubbles. Mixing intensity must be gentle enough not to break flocs before they reach the contact zone.
Setting doses
Chemical type and dose should be chosen by jar testing on representative samples, then fine-tuned on site. Overdosing increases sludge volume and cost; underdosing gives a cloudy effluent and a weak float blanket.
Where is dissolved air flotation used?
- Food processing: Removing FOG, starch and suspended solids ahead of biological treatment.
- Dairy: Recovering fats and proteins from cheese, milk and whey processing streams.
- Slaughterhouses and meat processing: Removing blood, fat and fine solids that load biological plants heavily.
- Pulp and paper: Recovering fiber and fillers from white water, and clarifying effluent.
- Oily wastewater: Metalworking, refineries, car washes and vehicle workshops, usually after gravity oil separation for free oil.
- Textile and printing: Removing color and suspended solids after coagulation.
- Municipal and drinking water: Thickening waste activated sludge and removing algae from surface water.
In many plants, DAF is preceded by fine screening such as a rotary drum screen and followed by biological treatment in a package sewage treatment plant or larger system.
What factors determine DAF sizing?
DAF selection is driven by both hydraulic and solids loading. Key factors include:
- Design flow and peak flow. The separation area is based on hydraulic loading rate, typically expressed in m³/m²·h. Equalization upstream often allows a smaller unit.
- Influent TSS and FOG. Higher loads need more air and a larger skimming capacity.
- Air-to-solids (A/S) ratio. The mass of air released per mass of solids to be floated; textbook values are typically in the range of about 0.005–0.06, with the right value found by testing.
- Recycle rate and saturation pressure. These set how much air is available.
- Water temperature. Air solubility falls as temperature rises, so warm wastewater needs more recycle or pressure for the same air delivery.
- Particle characteristics. Density, size and whether oil is free or emulsified.
- Effluent targets. Stricter limits may require lower loading rates or a second stage.
- Float handling. Plan how float sludge will be stored and dewatered; a screw press or other sludge dewatering machine is commonly used.
How does DAF fit into a complete treatment line?
DAF is rarely a standalone process. A typical industrial line runs in this order: coarse and fine screening to remove rags and large solids, an equalization tank to even out flow and concentration, pH adjustment, chemical conditioning, DAF, then biological treatment for dissolved organics, and finally sludge dewatering for the combined float and biological sludge.
Equalization is particularly valuable. Food, dairy and meat plants often discharge in batches during washdown, so flow and load can swing sharply over a shift. A buffer tank ahead of DAF keeps chemical dosing stable, prevents hydraulic overload of the separation zone and allows a smaller unit. Where free oil is present in large amounts, a simple gravity oil separator ahead of DAF reduces the chemical and air demand on the flotation stage.
Common DAF operating problems
| Symptom | Likely cause | Typical fix |
|---|---|---|
| Cloudy effluent | Wrong chemical dose or pH | Repeat jar test, correct pH and dose |
| Large bubbles boiling at surface | Low saturation or faulty release valves | Check pressure, air supply, clean valves |
| Float sinking or breaking up | Weak flocs or skimmer too slow | Adjust flocculant, skimmer speed |
| Solids in bottom of tank | Heavy grit or settled solids | Add upstream screening or grit removal |
Talk to us about your flotation project
Send your flow rate, wastewater source, TSS, FOG, COD and pH data, and discharge target to [email protected], or use our contact page. We will recommend DAF, CAF or shallow air flotation with the right chemical dosing and float handling for your application.
FAQ
What is the difference between DAF and CAF?
DAF dissolves air into a recycle stream under pressure and releases it as very fine bubbles, typically tens of microns across. CAF uses a cavitation aerator to shear air into the water without a pressure saturation tank, producing larger bubbles at lower energy, which suits less demanding applications.
What chemicals does a DAF system need?
Most DAF systems use a coagulant such as polyaluminum chloride, ferric chloride or aluminum sulfate to destabilize particles and break emulsions, followed by a polymer flocculant to build larger flocs. pH adjustment is often needed, and doses should be set by jar testing.
Can DAF remove dissolved COD?
DAF mainly removes suspended and colloidal material, including emulsified oil. It removes dissolved organic matter only to the extent that chemistry precipitates or adsorbs it, so biological treatment usually follows for dissolved BOD and COD.
How is DAF float sludge handled?
Float sludge is skimmed into a hopper and usually dewatered with a screw press, decanter centrifuge or filter press. Its high fat and oil content makes screw presses a common choice.
