FLOCCULANTS

Flocculation and coagulation are two terms to describe the coalescence of particles suspended in water. These terms describe two separate modes of operation, and care should be taken not to confuse them.

Coagulation results from the reduction in the natural repulsive forces between particles (i.e. the barrier is lowered). It is electrostatic in nature and can be brought about by the introduction of highly charged species e.g. Al³⁺ and Fe³⁺. It can also be brought about by highly charged polyelectrolytes of low Mwt, e.g. polyaluminium chloride.

Flocculation results from the physical joining of two or more particles together, by high molecular weight polymers (i.e. the barrier is overcome). It is by this operation that the majority of polymers effect coalescence of particles.

Of course sometimes coagulation and flocculation occur simultaneously, e.g. with highly charged, high Mwt polymers. This is still referred to as flocculation.

It is relatively easy to tell the difference between flocculation and coagulation in a coalesced (or destabilised) system. 

1.   Size of flocs – coagulation exhibits very small, tight flocs which settle slowly. Flocculation on the other hand gives rise to large loose flocs (Flocculus – Latin for tuft of wool) which settle very quickly.

2.    Shear stability of flocs – coagulated flocs will reform with no detrimental effect even under the severest agitation. Flocculated flocs, however are very shear sensitive – the larger the floc the more fragile. Agitation will break down the flocs and will result in an increase in the number of fines. The flocs will never reform and subsequent flocculation with the original polymer will not achieve the original high settlement rates.