What is a flocculant?

Coagulation turns fine particles into tiny micro-flocs — but those are often still too small to settle fast or dewater well. A flocculant grows them into large, strong flocs. Here is what a flocculant is, where it is used, the problems it solves, and how to choose and use one.

1. What is a flocculant?

Flocculants used in water treatment are polymers, also called polyelectrolytes: long-chain organic molecules with chemical groups along the chain that become charged when the polymer is dissolved in water.^[1] Depending on those groups, a polymer is:

• Cationic — positively charged
• Anionic — negatively charged
• Non-ionic — no charge
• Amphoteric — both positive and negative charges^[1]

A flocculant brings small particles together into larger agglomerates. It can be used as a flocculant aid alongside aluminium or iron coagulants to improve their effectiveness, and in special cases as the main (primary) coagulant.^[1] Its long chains form bridges between particles, gathering them into large flocs that settle.^[2]

2. Where are flocculants used?

• Clarification — after the coagulant, to grow flocs that settle in a clarifier or float in a dissolved air flotation (DAF) unit.^[1]
• Heavy metal removal — once the pH has been set to precipitate the metals, a polymer gathers the fine metal particles into a coarse floc that settles rapidly.^[2] See What is a pH adjuster?
• Oily wastewater — a polymer that neutralises the charges keeping oil droplets apart helps break the emulsion so the oil can separate.^[2]
• Sludge thickening and dewatering — every belt filter press has a polymer conditioning zone,^[3] and sludge fed to a centrifuge must be conditioned with polymer for optimum performance.^[4] Dewatering reduces sludge volume, and with it storage and transport costs.^[3]

3. What problems do flocculants solve?

4. Choosing and using a flocculant — and what ours do

The right charge and grade depend on the particles and the process, so the choice is made by testing on your actual water or sludge. Accurate doses of treatment chemicals should be determined by jar tests and confirmed in the field.^[2]

Getting the best from a flocculant

• Dose after the coagulant — allow from one to several minutes for micro-flocs to form before the polymer is added.^[1]
• Dilute before dosing — polymer solutions are extremely viscous, and dilute solutions disperse into the flow more easily.^[1] Make-up systems need enough mixing and aging capacity.^[4]
• Mix well at the dosing point, gently afterwards — rapid, thorough mixing is needed where the polymer goes in,^[1] but turbulence after that shears the floc and wastes polymer.^[4]
• Store properly — powders normally keep for 12 months if stored dry; liquid products and emulsions usually keep for 4 to 12 months if protected from frost.^[1] Spilled polymer is very slippery — avoid spillage.

Accot flocculants

Used together with

• Coagulants — ACCOT K101, ACCOT K40FC and ACCOT PAC, dosed before the flocculant.
• pH adjusters — to set the pH for coagulation and metal precipitation. See What is a pH adjuster?