Polyelectrolytes are flocculant aids that work by charge — which is why picking the wrong ionic type for your suspended solids can mean poor floc formation no matter how much you dose.
Anionic Polyelectrolytes
Anionic (negatively charged) polyelectrolytes are used after a primary coagulant like alum, PAC or ferric chloride has already neutralized most of the negative charge on suspended particles — the anionic polymer then bridges the destabilized particles into larger, faster-settling flocs. This is the standard combination in most municipal and industrial clarification and sludge-thickening applications.
See Anionic Polyelectrolyte (Polyacrylamide) and the Ultra High Molecular Weight grade for higher-shear applications like belt-press dewatering.
Cationic Polyelectrolytes
Cationic (positively charged) polyelectrolytes can act as a standalone coagulant-flocculant on streams carrying organic or biological suspended solids (which tend to carry a negative surface charge naturally), and are also the standard choice for sludge dewatering ahead of a belt press or centrifuge, since they bind well to the negatively-charged biosolids typical of that application.
See Cationic Polyelectrolyte — High Molecular Weight, the Very High Molecular Weight grade, and Cationic Polymer Liquid Coagulant for a ready-to-dose liquid option.
Nonionic Polyelectrolytes
Nonionic polyelectrolytes carry no charge and work purely through physical bridging between particles, which makes them useful where the water chemistry is sensitive to added ionic load, or as a complement when neither anionic nor cationic charge gives clean flocculation on a particular stream.
See Polyelectrolyte Nonionic (Powder & Liquid).
Choosing the Right Type
Start from what you're already dosing upstream: if a positively-charged coagulant (alum, PAC, ferric) is already neutralizing your raw water, an anionic polyelectrolyte usually completes the job best. For sludge dewatering or organically-loaded streams, start with cationic. A simple jar test comparing 2-3 charge types and molecular weights at bench scale is still the fastest way to confirm the right choice for your specific water — every stream behaves a little differently.