How to Choose the Right Polyelectrolyte for Your ETP or STP

Polyelectrolytes (also called polymers or flocculants) are among the most important chemicals in an ETP or STP. They turn fine, slow-settling particles into large flocs that settle in the clarifier or release water in a dewatering press. The wrong grade, or the right grade prepared badly, gives cloudy overflow and wet sludge.

• Cationic (positive charge) – usually best for organic sludges: sewage, food, dairy, paper and textile biological sludge, and for dewatering.
• Anionic (negative charge) – usually best for inorganic and mineral suspensions, and after metal-salt coagulants such as PAC, alum or ferrous sulphate.
• Non-ionic – for specific cases where charge must be neutral, often in acidic conditions.

Within each type, molecular weight (chain length) and charge density vary. Higher molecular weight builds bigger flocs; charge density is matched to the particles you are treating.

Powders are fully active and economical to transport but need careful dissolving. Emulsions are liquid and quick to dilute, which suits small plants and automatic dosing, but they contain less active polymer per kilogram.

• Collect a fresh, representative sample of effluent or sludge.
• Fill four to six beakers with the same volume.
• If you use a coagulant, add it first and stir fast for a minute.
• Add different polymer grades or doses from a freshly prepared 0.1% solution.
• Stir slowly for a few minutes and watch floc size and how fast it settles.
• Compare the clarity of the water above the sludge. The best result at the lowest dose is your starting point.

Most polymer problems start here. Sprinkle powder slowly into the vortex of a stirred tank – never dump it in – to make a dilute stock, commonly 0.1–0.25%. Mix gently for 30–60 minutes so the chains uncoil, and use the solution within about a day. Avoid violent pumping, which breaks the long chains.

Add the polymer where there is enough mixing to spread it, but not so much turbulence that flocs break up. In dewatering, dose just before the press or centrifuge, at the point recommended by its maker.

• Fine, pin-point floc and cloudy overflow – dose too low or the wrong charge.
• Sticky, slimy sludge or blinded press cloths – often overdosing.
• Good floc in the jar but poor in the plant – check solution preparation and mixing.

GreenFission supplies cationic, anionic and non-ionic polyelectrolytes and can test your sample. Send us your details to get started.