Dissolved air flotation (DAF): how it works and when you need it

An oil skimmer removes oil that floats as a layer on the surface. A gravity separator removes oil droplets large enough to rise on their own. Much industrial effluent also carries oil that will not rise by itself: fine droplets held in an emulsion by detergents, soaps or high-shear pumping, together with fine suspended solids. Dissolved air flotation, or DAF, is the usual way to remove them.

At a glance

• What DAF removes: emulsified and dispersed oil, fats and grease, and fine suspended solids.

• How: very fine air bubbles attach to oil droplets and flocs and float them to the surface, where a skimmer removes them.

• Where it sits: after an oil skimmer or gravity separator, and before biological treatment or discharge.

• What it needs: chemical dosing chosen by jar tests, and a design based on flow, oil and solids content.

How a DAF system works

• Chemical conditioning. A coagulant breaks the emulsion and a flocculant gathers the released droplets and fine particles into flocs. A pipe flocculator gives the mixing and contact time needed.

• Air saturation. Part of the treated water is pumped into a pressure vessel where air dissolves into it.

• Release. At the inlet of the flotation tank the pressure is released and the dissolved air comes out as a cloud of bubbles only tens of micrometres across. They attach to the flocs and lift them.

• Float removal. The float layer collects on the surface and a chain and flight skimmer pushes it up a beach into a sludge trough.

• Settled solids and clear water. Heavier solids settle into bottom hoppers, and clear water leaves over an adjustable outlet weir.

Skimmer, separator or DAF?

What is in the water	Equipment that removes it
A layer of free oil on the surface	Oil skimmer (belt, disc, drum, tube)
Free oil droplets that rise given time	Gravity separator (API, TPI or CPI)
Emulsified oil, grease and fine solids	Dissolved air flotation (DAF) system
Dissolved oil and organics	Biological treatment or adsorption, after the steps above

Most plants use more than one step. A skimmer or separator takes the bulk of the free oil first, which keeps chemical use and sludge volume in the DAF down.

What decides the design

• Average and peak flow, and how much they vary through the day

• Oil and grease, and suspended solids, at the inlet

• Whether the oil is emulsified, and by what (detergents, soaps, cutting fluids, pumping)

• Temperature and pH

• The treated water quality you need, and where the water goes next

• Space available, and the materials suited to the effluent

Chemicals and jar tests

The choice and dose of coagulant and flocculant decide how well a DAF works. They are chosen by jar tests on a sample of your effluent. Send us a representative sample, or the results of tests already done, and we will size the dosing with the system.

Checklist: what to send us

• Source of the effluent and the process that produces it

• Flow rate: average, peak and hours of operation

• Oil and grease, suspended solids, COD, temperature and pH, if known

• Required outlet quality and discharge route

• Existing treatment steps, such as skimmers, separators or equalisation tanks

• Site layout, power supply and any hazardous area classification

Frequently asked questions

Does a DAF system replace an oil skimmer?

Not usually. A skimmer removes free oil cheaply and without chemicals. A DAF system handles the emulsified oil and fine solids that remain, and runs better when the free oil has already been taken out.

Can DAF work without chemicals?

Free oil and some solids float with air alone, but emulsified oil needs a coagulant to break the emulsion first. Jar tests show what your effluent needs.

What happens to the float?

It is a wet sludge of oil, grease and solids. It is usually thickened or dewatered before disposal, and oil can sometimes be recovered from it.

Is the system supplied complete?

Yes. The Vens DAF system comes as a skid with the flotation tank, pipe flocculator, dosing, saturation vessel, recycle pump, float skimmer, sludge hoppers and control panel.

Not sure whether you need a skimmer, a separator or DAF? Send us your details and our engineers will propose the right combination for your effluent. Customers in the European Union can also order through our European company, VDT Europa OÜ in Tallinn, Estonia.