Sludge is often the most expensive part of running an ETP or STP: it has to be dewatered, stored and sent for disposal. The right polyelectrolyte makes sludge release its water quickly and form a firm cake, which cuts disposal volume and keeps the press or centrifuge running smoothly.
Cationic or anionic?
- Cationic polymers are the usual choice for organic and biological sludge, such as sludge from STPs, food plants and the biological stage of an ETP.
- Anionic polymers generally suit inorganic and mineral sludge, such as metal hydroxides, lime and alum sludge, and many primary clarifier sludges.
Within each family, products differ in charge density and molecular weight. Higher charge density suits sludge with more organic matter; higher molecular weight makes bigger, stronger flocs. Our guide Polyelectrolyte Types Explained covers the families in more detail.
Match the polymer to the machine
- Filter press: needs strong floc that resists pressure, and is often used with lime or ferric conditioning.
- Screw press and belt press: need a large, shear-resistant floc that drains quickly in the gravity zone.
- Centrifuge: needs very strong floc that survives high shear; high molecular weight cationic grades are common.
Screening polymers
Collect fresh sludge and test several grades in beakers: add polymer solution, mix gently, then pour through a filter cloth or sieve and compare how fast the water drains, how clear it is, and how firm the floc looks. Pick the two best and trial them on the machine. Dose is usually expressed as kilograms of polymer per tonne of dry solids.
Preparation makes or breaks performance
- Make a dilute solution (often 0.1 to 0.5% for powder grades) using clean water.
- Add powder slowly into the vortex so it does not form lumps ('fish eyes').
- Allow ageing time, often 30 to 60 minutes, before use, so the polymer fully uncoils.
- Do not keep made-up solution for days. Most lose strength within a day or two.
Poor preparation is the most common reason a good polymer seems not to work.
Grades we stock
We are an authorised SNF channel partner and stock cationic, anionic and nonionic polyelectrolytes in powder and liquid form. See Polyelectrolyte Supplier and Sludge Dewatering Chemicals.