DAF solutions by treatment objective
Most people arrive at flotation with a number they have to hit — a FOG limit, a TSS consent, a phosphorus target, a water-reuse volume. This page maps each objective to what the process can actually deliver, and says plainly where it cannot.
Dissolved air flotation removes particulate and floatable material: fats, oils and grease (90–99 %), suspended solids (85–98 %), particulate COD (40–70 % of total COD), phosphorus once precipitated (60–90 %), algae (90–99 %), and coagulated colour (50–90 %). It also thickens sludge from 0.5–1 % to 4–6 % dry solids. It does not remove dissolved organics, ammonia, nitrate, dissolved salts, dissolved metals, or hardness. Any objective expressed in those terms needs biological treatment, precipitation chemistry, ion exchange or membranes — with flotation as a supporting stage, not the answer.
What DAF achieves, by parameter
| Objective | Achievable removal | Conditions | Verdict |
|---|---|---|---|
| Fats, oils and grease | 90 – 99 % | Free oil intercepted upstream; emulsion broken by coagulant or acid split | Best-in-class process |
| Total suspended solids | 85 – 98 % | Correct coagulant and pH; adequate flocculation time | Best-in-class process |
| Turbidity | 85 – 99 % | To <1 NTU on surface water with filtration downstream | Excellent |
| Algae and cyanobacteria | 90 – 99 % | Cells removed intact, avoiding toxin release | Process of choice |
| Total COD | 40 – 70 % | Only the particulate fraction; soluble COD passes through | Partial — needs biology |
| Soluble COD / BOD | 10 – 30 % | Incidental only | Wrong process |
| Total phosphorus | 60 – 90 % | Requires Fe or Al coagulant to precipitate first | Good, as a co-benefit |
| Total nitrogen | 15 – 35 % | Only organic N bound to removed solids | Wrong process |
| Ammonia | <10 % | Fully soluble | Wrong process |
| Colour (dye) | 50 – 90 % | Requires coagulant, often with a decolourant | Good, stream dependent |
| Heavy metals | 90 – 99 % | Only after precipitation as hydroxide or sulphide | Good, as separation stage |
| Dissolved salts / TDS | 0 % | — | Wrong process — needs RO |
| Sludge thickening | 0.5–1 % → 4–6 % DS | 2 – 5 kg DS/m²·h, 1 – 3 mg/L polymer | Excellent, handles bulking sludge |
The test that predicts your outcome
Before anyone specifies anything, run a filtered COD. Take a composite sample, measure total COD, then filter through 0.45 µm and measure again. The difference is particulate COD — the only fraction flotation can address.
- Particulate fraction above 60 % (abattoir, edible oil, paper) — flotation will do most of the work and may reach consent alone.
- Particulate fraction 30–60 % (dairy, poultry, municipal) — flotation is a strong pretreatment; biology follows.
- Particulate fraction below 30 % (brewery, distillery, soft drinks) — flotation protects the biology and thickens the solids, but the consent will be met biologically. Buy the DAF for that reason or not at all.
Water reuse and recycling
Water scarcity, not compliance, is increasingly what drives these projects across Southern and East Africa. Flotation is a strong first stage in a reuse train because it removes the particulate load and most of the FOG that would otherwise foul whatever follows.
| Reuse application | Required quality | Train |
|---|---|---|
| Irrigation of non-food crops | TSS <50 mg/L, FOG <10 mg/L | DAF alone, often sufficient |
| Cooling tower make-up | TSS <10 mg/L, low hardness and TDS | DAF → filtration → softening |
| Yard and vehicle washing | TSS <20 mg/L, no odour | DAF → filtration → disinfection |
| Boiler feed | Near-demineralised | DAF → filtration → RO → polishing |
| Process water, food contact | Potable standard | DAF → filtration → RO → disinfection, with regulatory approval |
Pretreatment before biological treatment
Where a site already has an activated sludge plant, anaerobic reactor or lagoon system that cannot cope, flotation upstream is usually the cheapest fix available. It removes 70–90 % of the solids load and 40–60 % of the organic load before the biology sees it, which restores capacity without building anything new. In anaerobic systems the case is stronger still: FOG in an UASB or lagoon causes sludge flotation, scum blankets and washout, and removing it upstream is the difference between a reactor that works and one that does not.
Frequently asked questions
Can DAF remove COD?
It removes the particulate fraction of COD, typically 40–70 % of total COD on a solids-rich stream and 10–30 % where COD is mostly dissolved. Flotation is a physical separation process: if the organic material is in solution, there is nothing to float. Run a filtered COD before deciding — the ratio of filtered to total COD tells you the ceiling on what a DAF can achieve, before anyone quotes you anything.
Can DAF remove heavy metals?
Only after precipitation. Dissolved metals pass straight through. Raise pH to the minimum-solubility point for the metal in question — roughly pH 9–10 for zinc and nickel, 8.5–9 for chromium(III), 9–10.5 for lead — or precipitate as sulphide, and the resulting hydroxide or sulphide floc is then removed by flotation at 90–99 %. Precipitation and separation are two steps; a DAF performs the second one only.
Can DAF remove phosphorus?
Yes, 60–90 % of total phosphorus, essentially free of charge where an iron or aluminium coagulant is already being dosed for solids removal. Ferric chloride at 1.5–2.5 mol Fe per mol P precipitates phosphate as ferric phosphate, which the flotation stage then removes with the rest of the floc. This makes DAF attractive wherever a phosphorus consent exists alongside a solids consent.
Is DAF suitable for water reuse?
As the first stage, yes and often essentially mandatory. It removes the particulate and FOG load that would foul filtration and membranes downstream. On its own it produces water fit for non-food irrigation and dust suppression. For cooling water, boiler feed or process reuse it must be followed by filtration and, for the last two, reverse osmosis.