Cationic polyacrylamide (CPAM)
Two jobs, both of which anionic polymer does badly: flocculating heavily organic effluent where the particles are protein and fat rather than mineral, and dewatering biological sludge on a belt press or centrifuge.
Cationic polyacrylamide is a copolymer of acrylamide with a quaternary cationic monomer, supplied as an 88–92% active powder or a 40–50% emulsion, with molecular weight of 5–12 million and cationic charge density from 10 to 80 mol%. Unlike anionic polymer it carries positive charge, so it neutralises and bridges in a single step — which means it can act as the primary coagulant on a stream whose particles are organic.
On dissolved air flotation duty, doses run 1–10 mg/L. On sludge dewatering, doses are expressed against dry solids and run 3–12 kg of active polymer per tonne of dry solids.
Charge density decides everything
With anionic polymer, molecular weight is the main lever. With cationic, charge density is. The rule of thumb is that charge demand tracks the organic and colloidal load of the stream: the dirtier and more biological it is, the higher the charge needed.
| Charge density | Molecular weight | Best on | Typical duty and dose |
|---|---|---|---|
| Low, 10–20% | 10–12 million | Lightly loaded streams; paper mill whitewater; primary sludge | Flotation 1–3 mg/L; dewatering 3–5 kg/t DS |
| Medium, 30–40% | 8–12 million | Mixed primary and secondary sludge; food effluent; general DAF duty | Flotation 2–5 mg/L; dewatering 5–8 kg/t DS |
| High, 50–60% | 6–10 million | Waste activated sludge; poultry and abattoir effluent | Flotation 3–8 mg/L; dewatering 7–10 kg/t DS |
| Very high, 65–80% | 5–8 million | Digested sludge; rendering effluent; heavily surfactant-loaded streams | Flotation 5–10 mg/L; dewatering 9–12 kg/t DS |
Where cationic replaces the inorganic coagulant
On some streams a high-charge cationic polymer used alone outperforms the conventional coagulant-plus-anionic regime, and the reason is sludge volume. An aluminium or iron coagulant contributes its own hydroxide precipitate to the float — often more mass than the contaminant being removed. A cationic polymer contributes almost nothing.
| PAC plus anionic | Cationic alone | |
|---|---|---|
| Chemical cost per m³ | Lower | Higher, typically 1.5–3× |
| Float sludge volume | Higher | 30–60% lower |
| Float sludge dry solids | 3–5% | 4–7% |
| Sludge disposal cost | Higher | Lower |
| Recoverability of the float | Poor — contaminated with metal hydroxide | Good — can go to rendering or digestion |
| Alkalinity consumed | Significant | None |
| pH adjustment needed | Usually | Rarely |
This is the standard regime on poultry processing across much of the world, and it is worth testing on any abattoir, rendering or fish processing stream where the float has a downstream value.
Specification
| Parameter | Powder | Emulsion |
|---|---|---|
| Active polymer | 88–92% | 40–50% |
| Appearance | White granular powder | White viscous liquid |
| Charge density | 10–80 mol%, specified per grade | 10–80 mol% |
| Molecular weight | 5–12 million | 5–10 million |
| Residual acrylamide monomer | ≤ 0.05% | ≤ 0.10% |
| Dissolution time | 40–60 min at 0.3–0.5% w/v | 10–20 min inversion |
| pH, 0.5% solution | 3.0–5.0 | 3.0–5.0 |
| Shelf life sealed | 12–24 months | 6–12 months |
| Solution stability | 8–24 hours | 8–24 hours |
Make-up and dosing
The procedure is the same as for anionic polymer — wet the powder as it enters the water, 0.3–0.5% w/v, low shear, age 30–60 minutes, dilute to 0.05–0.1% at injection — with three differences that matter:
- Cationic dissolves more slowly. Allow 40–60 minutes rather than 30, and expect a more viscous solution at the same concentration.
- Solution life is shorter. Eight to twenty-four hours rather than twenty-four to forty-eight. In a hot plant room, make up daily and size the tank accordingly.
- Overdosing reverses the effect. Past the charge-neutralisation point the particle surface flips to net positive and re-stabilises. With anionic polymer, overdosing wastes money; with cationic, it actively destroys performance. If the float has thinned as the dose went up, come back down.
Sludge dewatering doses
| Sludge type | Dose, kg active/t DS | Charge density | Cake dry solids achievable |
|---|---|---|---|
| Primary sludge, belt press | 3–5 | 20–35% | 22–28% |
| DAF float sludge, food industry | 4–7 | 30–45% | 18–25% |
| Mixed primary and WAS, belt press | 5–8 | 40–55% | 18–24% |
| Waste activated sludge, belt press | 7–10 | 50–65% | 14–20% |
| Waste activated sludge, centrifuge | 8–12 | 55–75% | 16–22% |
| Anaerobically digested sludge, centrifuge | 9–12 | 65–80% | 20–26% |
| Mineral and inorganic sludge | 1–3 | Anionic preferred | 35–55% |
Frequently asked questions
Can cationic polymer be used without a coagulant?
On organic streams, yes, and it is standard practice in poultry processing. A high-charge cationic polymer neutralises and bridges in one step. The trade is a higher chemical bill against a substantially smaller and cleaner float sludge that may have resale value to a renderer. On mineral or inorganic streams it does not work — those need an inorganic coagulant.
How do I choose the charge density?
Charge demand tracks organic load. Start at 40% for a general food effluent, 55–65% for poultry or waste activated sludge, 70–80% for digested sludge or rendering effluent, and 20–35% for primary sludge and lightly loaded streams. Then bracket it on the bench with three grades either side. Charge density is the variable to test first; molecular weight second.
Why did performance get worse when I increased the dose?
You have gone past charge neutralisation and re-stabilised the particles with net positive surface charge. This is specific to cationic polymer and it is abrupt — a 20% overdose can visibly thin the float. Reduce the dose in steps until the float recovers, and treat that point as the ceiling rather than the target.
How long does cationic polymer solution keep?
Eight to twenty-four hours, shorter than anionic. The quaternary group hydrolyses faster than the acrylate group, and heat accelerates it. In a plant room above 30 °C, make up daily and size the ageing tank for one day’s consumption rather than three.
What dose should I expect on a belt press?
Between 3 and 12 kg of active polymer per tonne of dry solids, depending on the sludge. Primary sludge sits at the bottom of that range, waste activated sludge and digested sludge at the top. A centrifuge on the same sludge will need roughly 20–30% more polymer than a belt press because of the higher shear.
Can I use cationic and anionic polymer together?
Yes, in sequence — high-charge cationic first to neutralise, high-molecular-weight anionic second to bridge. It is a common regime on poultry, rendering and paper mill duty. What you cannot do is mix them in the same tank or dose them at the same point: they will react with each other and precipitate, achieving nothing except a blocked injection line.