Coagulants, polymers and jar testing for DAF
A dissolved air flotation unit is a separator. The chemistry decides what there is to separate. Get the coagulant and the pH right and a mediocre machine performs well; get them wrong and the best machine on the market will not save you.
Dissolved air flotation needs a coagulant to neutralise particle charge and a flocculant polymer to bind the neutralised particles into flocs that bubbles can lift. Polyaluminium chloride at 50–600 mg/L is the most widely applicable coagulant because it works across pH 6.0–8.5 and produces less sludge than alum. Ferric chloride at 100–800 mg/L is preferred where phosphorus or sulphide must also be removed, and on tannery and high-ionic-strength streams. Anionic polyacrylamide at 1–5 mg/L suits most food, oil and municipal duties; cationic polymer at 0.5–3 mg/L is correct for paper mill whitewater. Doses must be confirmed by jar test on a real composite sample — published ranges are starting points, not design values.
Coagulant selection
| Coagulant | Working pH | Dose range | Strengths | Weaknesses |
|---|---|---|---|---|
| Polyaluminium chloride (PAC) | 6.0 – 8.5 | 50 – 600 mg/L | Wide pH window, low alkalinity consumption, 30–50 % less sludge than alum, fast floc | Costs more per kg; aluminium residual matters for potable duty |
| Aluminium sulphate (alum) | 5.8 – 7.5 | 100 – 500 mg/L | Cheap and widely available across Africa | Narrow pH window, consumes alkalinity, more sludge, slow at low temperature |
| Ferric chloride | 4.5 – 6.5 and >8.5 | 100 – 800 mg/L | Dense strong floc, removes phosphorus and sulphide, works at high ionic strength | Corrosive, stains, colours the float, high alkalinity demand |
| Ferric sulphate | 4.0 – 6.0 and >8.5 | 100 – 700 mg/L | Less corrosive than ferric chloride, no chloride added | Adds sulphate, which matters where anaerobic treatment follows |
| Cationic organic coagulant (polyDADMAC) | 4 – 10 | 10 – 150 mg/L | Almost no added sludge, no metal residual, wide pH tolerance | Costs considerably more; less effective alone on heavy solids |
| Lime | >9.5 | Variable | Cheap; precipitates phosphorus, metals and some hardness | Very high sludge volume; handling and scaling problems |
Polymer selection
| Charge | Use on | Dose | Molecular weight |
|---|---|---|---|
| Anionic | Food, dairy, abattoir, edible oil, municipal, after metal-salt coagulation | 1 – 5 mg/L | High to very high |
| Cationic | Paper mill whitewater, sludge thickening, streams with heavy anionic trash | 0.5 – 3 mg/L | Medium to high |
| Non-ionic | Rare; very low or very high pH where ionic polymers hydrolyse | 1 – 4 mg/L | High |
Starting doses by industry
| Industry | Coagulant and dose | Target pH | Polymer |
|---|---|---|---|
| Abattoir / meat | FeCl₃ 100 – 400 mg/L or PAC 150 – 400 | 6.0 – 7.0 | Anionic 1 – 4 mg/L |
| Edible oil | PAC 150 – 600 mg/L, after acid split | 6.5 – 8.0 | Anionic 2 – 5 mg/L |
| Dairy | PAC 80 – 300 mg/L | 6.5 – 7.5 | Anionic 1 – 3 mg/L |
| Brewery | PAC 50 – 200 mg/L | 6.5 – 7.5 | Anionic 1 – 3 mg/L |
| Poultry | PAC 120 – 350 mg/L | 6.0 – 7.0 | Anionic 1 – 4 mg/L |
| Tannery | FeCl₃ 200 – 800 mg/L | 7.5 – 8.5 | Anionic 2 – 6 mg/L |
| Pulp & paper | PAC 50 – 250 mg/L or none | 6.5 – 8.0 | Cationic 0.5 – 3 mg/L |
| Textile dyeing | PAC 150 – 500 mg/L + decolourant | 6.0 – 8.0 | Cationic 1 – 4 mg/L |
| Municipal | FeCl₃ 30 – 120 mg/L or alum 40 – 150 | 6.0 – 7.5 | Anionic 0.5 – 2 mg/L |
| Surface water / algae | PAC 10 – 60 mg/L | 6.0 – 7.5 | Often none |
A jar test that predicts full-scale performance
Most jar tests fail to predict plant behaviour because they replicate settling, not flotation, and because the mixing regime bears no relation to the machine. This protocol addresses both.
- Sample properly. Composite over a full production cycle, including CIP and shift changeover. Test within 6 hours or refrigerate; a sample left overnight at African ambient has already changed.
- Measure the baseline. pH, temperature, TSS, total COD and filtered COD (0.45 µm). The filtered COD sets the ceiling on what flotation can achieve, before any chemistry is chosen.
- Set pH first. Adjust each jar to the target for the coagulant under test. Testing coagulant dose at the wrong pH tells you nothing.
- Rapid mix. Add coagulant at 200 rpm for 30 seconds. This approximates G ≈ 500 s⁻¹.
- Slow mix with polymer. Reduce to 40 rpm, dose polymer, continue 10 minutes. Approximates G ≈ 40 s⁻¹. Watch when the floc first becomes visible — that timing informs flocculation chamber sizing.
- Simulate flotation, not settling. This is the step usually omitted. Use a bench flotation cell, or inject 10–20 % by volume of water previously saturated at 5 bar in a small pressure vessel. Stop stirring and observe for 5 minutes.
- Assess. Record float thickness and firmness, clarity of the subnatant, whether any floc sinks, and how the float behaves when disturbed — a float that collapses when nudged will collapse under a skimmer.
- Analyse the subnatant. TSS, FOG, total COD. These figures, not the visual appearance, are the result.
- Bracket the optimum. Run at least five doses spanning half to twice the expected value. The optimum is a range, not a point, and the width of that range tells you how much control the plant will need.
Reading the result
| Observation | Likely cause | Action |
|---|---|---|
| No visible floc | Underdosed coagulant, or pH outside window | Check pH first, then increase coagulant |
| Fine floc that will not grow | Insufficient polymer, or polymer sheared | Increase polymer; reduce slow-mix speed |
| Good floc that sinks | Overdosed metal coagulant; floc too dense | Reduce coagulant, increase polymer |
| Slimy float that will not firm up | Overdosed polymer, re-stabilisation | Reduce polymer by half and re-test |
| Clear subnatant, thin float | Insufficient air, not a chemistry problem | Increase simulated recycle; check plant A/S ratio |
| Cloudy subnatant, good float | Carry-over of unflocculated fines | Extend flocculation time; check for short-circuiting |
Frequently asked questions
Which coagulant is best for DAF?
Polyaluminium chloride for most industrial streams, because it works across pH 6.0–8.5, forms floc quickly, consumes little alkalinity and produces 30–50 % less sludge than alum. Ferric chloride is better where phosphorus or sulphide must also be removed, where ionic strength is high (tannery, produced water), or where a denser, more shear-resistant floc is needed. Alum remains a reasonable choice where price and local availability dominate and pH is stable.
How do I calculate coagulant dose?
You do not calculate it — you measure it. Coagulant demand depends on particle charge, natural organic matter, alkalinity and ionic strength in combinations that no formula predicts reliably. Use the published range for your industry as a starting point, bracket it with a jar test across at least five doses, then confirm at plant scale during commissioning. Recheck whenever the process, the season or the coagulant supplier changes.
Can a DAF run without chemicals?
Only where the target material is already buoyant and unstabilised: free oil, coarse fibre, some algae. Anything emulsified, colloidal or charge-stabilised will pass straight through an unchemically-dosed flotation cell, because the particles are too small for a bubble to attach to usefully. If reliable chemical supply is genuinely doubtful at your site, that constraint should shape the process selection at the start, not surface as a complaint later.
Why does my jar test look good but the plant performs badly?
Four usual reasons. The jar test simulated settling rather than flotation, so it never tested whether the floc floats. The mixing energies did not match the plant, so floc that survived a paddle is shearing in a pump or nozzle. The sample was a grab rather than a composite, so it did not represent what the plant actually receives. Or the plant’s dosing is not delivering what the setpoint claims — measure the pump output with a calibration column before blaming the chemistry.