DAF flotation: when you need it and how to size it

Effluent from a meat plant, a dairy or a vehicle wash settles badly: fat and emulsified oil are lighter than water, and floc formed after coagulation hangs in the bulk instead of sinking. Dissolved air flotation solves exactly this problem — it does not wait for contaminants to settle, it lifts them to the surface. Here is how a DAF unit works, which figures you need for sizing, and what to do with what it removes.

Dissolved air and microbubbles

DAF stands for dissolved air flotation. The principle is simple: air is dissolved in water under pressure, then the pressure is released. Solubility drops, and the excess air comes out not as large bubbles but as a cloud of very fine ones — throughout the whole volume at once.

From there it is the physics of adhesion. A microbubble attaches to a contaminant particle, and together they become lighter than water. The "particle plus bubbles" aggregate floats up and collects on the surface as a dense layer. A stainless steel chain scraper removes it, and the clarified water is drawn off from below.

Bubble size matters here. A large bubble shoots up through the water vertically and carries almost nothing with it, breaking up loose floc on the way. A fine bubble rises slowly, has time to meet a particle and hold it. That is why the air is dissolved rather than blown in by a compressor through a diffuser.

Saturation is maintained by recirculation: part of the already treated water is returned to the pressure line, saturated with air under pressure and fed back into the flotation chamber. The point is that clean water, not raw effluent, passes through the pump and the saturator — otherwise the unit fouls quickly.

What a flotation unit removes, and why not a settling tank

Flotation takes on what gravity settling handles poorly: low-density suspended solids, fats, petroleum products, fibre, and floc formed after coagulation. Hence the typical duties — preparing effluent for discharge to the sewer, taking load off the biological stage, polishing biologically treated water and recovering valuable raw material from process water. Flotation units are installed in meat and fish processing, dairy production, petrochemicals, the pulp and paper industry and food plants.

A settling tank works on the density difference the other way round: the particle is heavier than water and sinks to the bottom. That is cheap and needs almost no energy, but it only works with what genuinely sinks — sand, scale, mineral solids, heavy chemical sludge. Fat and oil in a settling tank either float by themselves and need separate grease removal, or stay in emulsion and travel further down the line. If the effluent analysis shows a noticeable share of fats and petroleum products, the choice shifts towards flotation; if heavy mineral solids dominate, towards settling.

Coagulant and flocculant: the unit will not cope without them

A single dissolved or colloidal particle is too small to hold a bubble. It has to be enlarged first, and that is the job of chemical conditioning ahead of the flotation chamber.

The coagulant neutralises the charge on colloids, so they stop repelling each other and stick together into microfloc. The flocculant — a long polymer molecule — stitches microfloc into large loose floc that holds air bubbles well. Sequence and mixing time matter: coagulation runs under intensive mixing, flocculation under slow mixing, otherwise the formed floc is destroyed. In the unit the coagulation and flocculation chambers are built into the body together with the clarified water tank, so the line takes up less space.

Doses are selected on the actual water, not from a reagent catalogue. Underdosing — the floc does not form and the unit sends turbidity to the outlet. Overdosing — wasted reagent and more sludge, which then has to be dewatered. pH noticeably affects coagulant performance, so it is often corrected before the chamber. The chemical section is assembled on dosing stations with a separate pump for each reagent.

What to calculate when sizing

The range is selected by flow: from HLDAF-2.5 at 2–2.5 m³/h to HLDAF-100 at 95–100 m³/h, with HLDAF-5, HLDAF-10, HLDAF-15, HLDAF-20, HLDAF-30, HLDAF-40, HLDAF-50, HLDAF-60 and HLDAF-75 in between. Sizing is done on the peak hour of the shift, not on the daily average: a slug discharge from a wash-down or the end of a shift passes through the unit in full.

The second figure is the pressurised water flow. This is that same recirculation stream, and it is not symbolic: on the HLDAF-10 with a capacity of 8–10 m³/h the pressurised water flow is 3.5 m³/h, on the HLDAF-100 at 95–100 m³/h it is 50 m³/h. It goes into pump selection, pipework diameters and the power consumption calculation.

Third — hydraulic loading and residence time. The flotation chamber takes a certain flow per unit of water surface area: the higher that loading, the less time the floc has to rise. That is exactly why you cannot "push a bit more" through a smaller size — the water reaches the outlet before the floc has risen, and you lose clarification on equipment that is, on paper, in perfect order.

Fourth — the body material for the particular effluent. Carbon steel with an epoxy coating suits ordinary municipal and food effluent; steel lined with glass-reinforced plastic and stainless steel 304 and 316L suit aggressive media, acidic rinse waters and effluent containing chlorides.

Float sludge: where it goes next

The scraper pushes the floated layer into the sludge trough, and from that point the question is no longer water treatment but sludge handling. Skimming is set to the actual amount of sludge: skim too often and you carry water off with it and end up with a thin sludge; skim too rarely and the layer compacts and works its way back into the water.

Float sludge comes out very wet. Hauling it as is costs a lot, because what you mostly haul is water, so the next stage is dewatering on a screw press or its equivalents. After dewatering what remains is a cake, many times smaller in volume than the feed.

How it is handled after that depends on composition: fatty sludge from food production and oily sludge from a vehicle wash are different stories, and the acceptable options are set by the local regulator or the landfill operator. This question is worth closing before buying a flotation unit, not after: equipment with nowhere to unload stops the whole line.

What next

Sizing a flotation unit needs: effluent flow — average and peak by hour of the shift; an analysis report covering suspended solids, fats or petroleum products, COD and pH; effluent temperature; the site's operating mode — shift-based or continuous; the discharge requirements from your permit, since the limit is set by the water utility and differs at every site. Separately — the available floor area and ceiling height, which immediately rule out part of the range.

With those figures you can look at DAF flotation units in the catalogue and match the capacity range against your peak hour. If you do not have an effluent analysis yet, start there: sizing a flotation unit without figures for fats and solids is choosing blind, and the mistake here costs a whole size.