Coagulant vs flocculant: what each one does
Why most plants dose both, in which order, and how to tell which one is failing.
Coagulant first
A coagulant such as alum, PAC or ferric chloride neutralises the charge that keeps fine particles apart. Particles start to clump into pin-sized flocs.
Flocculant second
A flocculant such as Sunfloc is a long-chain polymer. It bridges those pin flocs into large, heavy flocs that settle in minutes instead of hours.
Why the order matters
Polymer added before the coagulant has nothing to bridge. Added into the rapid mixer, its chains are sheared apart. It belongs in the gentle mixing zone after coagulation.
Which one is failing?
Cloudy water with no flocs points at the coagulant or the pH. Small flocs that will not grow point at the polymer: its charge, its dose or how it was made up.
When polymer works alone
Cationic polymer can act as both coagulant and flocculant on organic and biological solids, which is why it works on its own for sludge dewatering.
Related
Polyelectrolyte dosing guide
Starting doses, solution strength and how to work out kilograms per day.
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