Anionic polyacrylamide (APAM)
The workhorse flocculant behind almost every dissolved air flotation cell. It does not neutralise charge — that is the coagulant’s job. It bridges the neutralised particles into a floc large and strong enough for a bubble to lift.
Anionic polyacrylamide is a high-molecular-weight copolymer of acrylamide and sodium acrylate, supplied as an 88–92% active white granular powder with a molecular weight of 8–20 million and an anionic charge density of 10–40 mol%. It is dosed after the coagulant, into a gently mixed flocculation stage, at 0.5–5 mg/L for most flotation duties.
Its role is bridging, not charge neutralisation. Dosing anionic polymer into an uncoagulated stream achieves very little, and dosing it before the coagulant achieves less than nothing.
Selecting the grade
Two variables define an anionic polymer: molecular weight, which sets bridging reach and floc strength, and charge density, which sets how strongly it attaches to the coagulated particle. Getting them wrong is the most common reason a DAF produces a small, fragile floc that shears apart in the recycle nozzle.
| Grade | Molecular weight | Charge density | Best on | Typical dose |
|---|---|---|---|---|
| Low charge, very high MW | 15–20 million | 8–15% | Mineral fines, quarry water, coal washery, clay | 0.5–2 mg/L |
| Medium charge, high MW | 12–18 million | 20–30% | General DAF duty behind PAC or alum: food, beverage, municipal | 1–3 mg/L |
| High charge, high MW | 10–15 million | 30–40% | Streams coagulated with a high coagulant dose; edible oil, tannery | 2–5 mg/L |
| Medium charge, medium MW | 6–10 million | 20–30% | Belt press and centrifuge dewatering of mineral sludge | 2–6 kg/t DS |
| Emulsion, medium charge | 10–15 million | 20–30% | Sites with no reliable make-up operator | 2–8 mg/L as supplied |
Specification
| Parameter | Powder | Emulsion |
|---|---|---|
| Active polymer | 88–92% | 30–50% |
| Appearance | White free-flowing granule | White to off-white viscous liquid |
| Particle size | 90% through 1.0 mm, ≤ 10% through 0.15 mm | n/a |
| Residual acrylamide monomer | ≤ 0.05%; ≤ 0.025% for potable grade | ≤ 0.10% |
| Bulk density | 0.7–0.8 kg/L | 1.0–1.05 kg/L |
| Dissolution time | 30–60 min at 0.3–0.5% w/v | 10–20 min inversion |
| pH, 0.5% solution | 6.0–8.0 | 5.0–7.0 |
| Shelf life sealed | 12–24 months | 6–12 months |
Make-up: where most polymer problems start
Dry polyacrylamide is the most badly handled chemical in industrial water treatment. Dumped into a tank as a lump, it forms gel balls with a dry core that never dissolve, and the plant then runs on a fraction of the polymer it is paying for.
- Wet the powder as it enters the water, not before and not after. A dry-polymer eductor, a vibrating tray feeder over a wetting cone, or in the smallest plants a person sifting powder slowly into the vortex of a stirred tank.
- Make up at 0.3–0.5% w/v. Above 0.5% the solution is too viscous to mix or pump reliably. Below 0.2% the make-up tank becomes impractically large.
- Mix at 200–400 rpm during addition, then reduce. High shear during ageing breaks the polymer chain, which is exactly the property you are paying for. A polymer that has been over-sheared performs like a much lower molecular weight product.
- Age for 30–60 minutes. The chain has to hydrate and uncoil. Polymer dosed straight after make-up typically delivers half its potential.
- Dilute to 0.05–0.1% immediately before injection. In-line dilution water at the injection point disperses the polymer into the stream instead of dropping a viscous rope into it. This single step commonly cuts polymer consumption by 20–30%.
- Use made-up solution within 24 hours. Anionic polymer solution degrades by hydrolysis and by any residual chlorine in the make-up water. Beyond 24–48 hours it is measurably weaker.
Make-up water matters: use clean, non-chlorinated water below 30 °C. Chlorinated water attacks the polymer chain. Hard water at over about 500 mg/L as CaCO₃ will reduce the activity of a high-charge anionic grade noticeably.
Dose ranges by application
| Application | Dose as active polymer | Recommended MW | Recommended charge |
|---|---|---|---|
| Municipal primary DAF | 0.5–2 mg/L | 12–18 million | 20–30% |
| Dairy processing DAF | 1–3 mg/L | 12–18 million | 20–30% |
| Brewery and beverage DAF | 1–3 mg/L | 12–18 million | 20–30% |
| Abattoir and poultry DAF | 2–5 mg/L | 10–15 million | 30–40% |
| Edible oil refinery DAF | 2–5 mg/L | 10–15 million | 30–40% |
| Tannery DAF | 2–5 mg/L | 10–15 million | 30–40% |
| Paper mill whitewater | 0.5–2 mg/L | 12–18 million | 10–25% |
| Mining and quarry thickener | 5–25 g/t solids | 15–20 million | 8–15% |
| Mineral sludge dewatering | 2–6 kg/t dry solids | 6–10 million | 20–30% |
Handling and storage
- Slip hazard. Wetted polyacrylamide on a floor is dangerously slippery and water makes it worse. Clean spills dry — sweep or vacuum the powder first, then absorb the residue with sand or vermiculite. Hosing a polymer spill creates a skating rink.
- Storage: sealed bags on pallets, dry, out of direct sun, 5–35 °C. Polyacrylamide is hygroscopic; a bag opened and left in African humidity will cake within days.
- Dust: nuisance dust rather than acute toxicity, but use a P2 mask and local extraction at the make-up point.
- Shelf life: 12–24 months sealed. Made-up solution: 24 hours.
- Materials: no corrosion concern. Stainless steel, HDPE and polypropylene are all fine. The engineering constraints are shear and viscosity, not chemical attack — use progressive-cavity or low-shear diaphragm pumps and generously sized lines.
Frequently asked questions
Anionic or cationic polymer for my DAF?
Anionic in most cases, dosed behind an aluminium or iron coagulant that has already neutralised the negative surface charge. Cationic is used where the stream is heavily organic and protein-rich — poultry, some abattoir and rendering duty — either as a primary coagulant replacing the inorganic entirely, or as a first stage ahead of anionic. If in doubt, jar test anionic behind PAC first; it is right more often than not. See cationic polyacrylamide for the cases where it is not.
How long does made-up polymer solution last?
24 hours as a working rule, 48 at the outside. Anionic polyacrylamide hydrolyses slowly in solution and is attacked by any residual chlorine in the make-up water. A plant that makes up a large batch on Friday and doses it through Monday is running on degraded polymer for most of that period and will see the dose creep up to compensate.
Why is my floc small and fragile?
In order of likelihood: the polymer has been over-sheared during make-up or by the transfer pump; the molecular weight is too low for the duty; the polymer is being injected neat instead of at 0.05–0.1% dilution; the ageing time is too short; or the coagulant is not reaching the charge-neutralisation point so there is nothing for the polymer to bridge. Work through them in that order before changing product.
Powder or emulsion?
Powder is 88–92% active, emulsion 30–50%. On a long sea freight leg you are paying to ship oil and water with emulsion, and emulsion has a shorter shelf life in heat — both arguments point to powder for African supply. Emulsion wins only where the site cannot run a reliable powder make-up unit, in which case the simpler handling is worth the freight penalty.
Is polyacrylamide safe in drinking water treatment?
The polymer itself is not toxic and is used worldwide in potable treatment. The concern is residual acrylamide monomer, which is a neurotoxin. Potable grades are certified to NSF/ANSI 60 or EN 1407 with residual monomer at or below 0.025%, and dose limits are set so that the monomer contribution to finished water stays within regulatory limits. Industrial grade at 0.05% monomer should not be used on potable duty.
Can I dose polymer and coagulant at the same point?
No. The coagulant needs rapid, high-shear mixing for a few seconds to neutralise charge; the polymer needs gentle mixing over 10–20 minutes to bridge. Dosed together, the polymer is destroyed by the rapid-mix shear and the coagulation is incomplete. Separate the injection points with a coagulation stage in between — this is the single most common design fault we find on underperforming flotation plants.