Quick answer: Use DAF when the solids float rather than settle — oil, grease, fats, fibres and light flocculated solids. If your solids settle readily, a clarifier does the same job for less money and less maintenance. DAF almost always belongs before biological treatment, not after it.
The one question that decides it
Do your solids sink or float? That is the whole decision, and it is answerable in an afternoon with a settling test on a bucket of your own effluent.
- Solids settle readily → a clarifier or lamella settler is cheaper to buy, cheaper to run and simpler to maintain. Buying DAF here is spending money to do what gravity was doing for free.
- Solids float, stay suspended, or the problem is oil and grease → a clarifier will never fix it no matter how much coagulant you dose, and DAF is the right answer.
DAF works by dissolving air into water under pressure and releasing it as microbubbles that attach to particles and carry them to the surface, where a skimmer removes them. It makes light material rise deliberately instead of hoping it sinks.
Where DAF is normally essential
| Industry | What DAF is removing |
|---|---|
| Slaughterhouse and meat processing | Fats, oils, grease, blood solids |
| Dairy and food processing | Fats and light organic solids; high shock loads |
| Palm oil mills | Residual oil ahead of anaerobic digestion |
| Refineries and metalworking | Free and emulsified oil |
| Textile dyeing | Light flocculated solids, surfactants, some colour |
| Pulp, paper and recycling | Fibres and fillers that will not settle |
Put it before the biology
This ordering matters more than most buyers expect. Oil and grease reaching a biological stage causes three problems at once: it coats the biomass and reduces treatment efficiency, it promotes filamentous growth and bulking sludge, and it adds organic load that then has to be aerated at high energy cost.
Removing it physically first is far cheaper than treating it biologically. Every kilogram of COD taken out by DAF is a kilogram the aeration blowers do not have to work on — the same logic as taking the cheap COD removal first.
DAF performance is a chemistry problem, not a machine problem
Most underperforming DAF units are dosing problems rather than equipment problems. Three things dominate:
- Coagulant type and dose. Emulsified oil must be destabilised before bubbles can lift it. Without the right coagulant, the microbubbles pass straight by.
- pH. Every coagulant has an effective window. Influent pH drifting out of that window is a common cause of a DAF that “stopped working” with no equipment change.
- Flocculant and mixing energy. Floc must be large enough for bubbles to attach but not so fragile that mixing shears it apart.
Run a jar test against your current effluent rather than the effluent the plant was commissioned on. It takes an afternoon and settles arguments that otherwise run for months.
The float is sludge, and it has to go somewhere
DAF produces float — a sludge that is typically wet and often oily. It is easy to overlook at purchase and it appears in the disposal invoice every month afterwards.
Oily float is also the material that blinds filter cloth fastest and that belt presses handle worst. Two routes normally apply:
- A multi-disc screw press, whose self-cleaning rings cope with oily sludge that blinds other equipment, running continuously and unattended
- A chamber or membrane filter press where disposal is charged by weight and the driest possible cake is worth paying for
Deciding between them is covered in filter press versus screw press, and how DAF fits the full chain is in industrial wastewater treatment equipment and complete systems.
Frequently asked questions
When do I need DAF instead of a clarifier?
When your solids float or stay suspended rather than settling, or when the problem is oil and grease. If a settling test on your own effluent shows solids dropping out readily, a clarifier does the same job for less money and less maintenance.
Should DAF go before or after biological treatment?
Before, almost always. Oil and grease reaching the biological stage coats the biomass, promotes bulking sludge, and adds load that must then be aerated at high energy cost. Removing it physically first is considerably cheaper.
My DAF stopped working but nothing was changed. Why?
Usually chemistry drift rather than equipment. Influent pH may have moved outside the coagulant’s effective window, dosing pumps may have lost calibration, or polymer preparation may have changed. Run a jar test against current effluent before inspecting the machine.
Does DAF remove COD?
It removes the particulate and oil-associated fraction, not dissolved COD. If your COD is mainly soluble, DAF will not move the number much and biological treatment is what you need. Ask the laboratory for total and filtered COD separately.
What do I do with the float sludge?
Dewater it. DAF float is typically wet and often oily — the material that blinds filter cloth fastest. A multi-disc screw press handles oily sludge well and runs unattended; a filter press gives a much drier cake where disposal is charged by weight.
Can DAF handle shock loads?
Better than a clarifier, but it is not a substitute for equalisation. Sharp discharges change both hydraulic loading and chemistry at once, and dosing cannot track that in real time. An equalisation tank ahead of the DAF is usually the cheaper fix.
Send your oil and grease figures, suspended solids and flow and we will say honestly whether you need DAF or a clarifier: ask our engineering team.