Coagulant and factors affecting coagulation

Coagulant

A coagulant is a chemical added to water to destabilise the fine suspended and colloidal particles and make them join into large, settleable flocs. Common coagulants are alum (aluminium sulphate), ferrous sulphate (copperas) with lime, ferric chloride, ferric sulphate, sodium aluminate and polyelectrolytes (as coagulant aids).

Alum reaction:

$$Al_2(SO_4)_3\cdot 18H_2O + 3Ca(HCO_3)_2 \rightarrow 2Al(OH)_3\downarrow + 3CaSO_4 + 18H_2O + 6CO_2$$

Factors affecting coagulation

1. Type and dose of coagulant: an under- or over-dose gives poor floc; the optimum is found by the jar test.
2. pH of water: each coagulant works best in a range (alum about pH 6.5 to 8.5; ferrous sulphate with lime pH above 8.5).
3. Alkalinity: alum needs about 0.5 mg/l alkalinity (as CaCO$_3$) for each mg/l of alum.
4. Turbidity and nature of particles: low turbidity needs more coagulant or a coagulant aid, since there are fewer particles to collide.
5. Temperature: low temperature slows the reaction and makes weak floc, increasing the dose and time.
6. Mixing: rapid mixing for 30 s to 2 min ensures instant dispersal; slow stirring then builds floc. Too fast mixing breaks the floc.
7. Detention time in the flocculator (20 to 40 min).
8. Presence of ions and organic matter (e.g. sulphates, colour compounds) alters the dose.
9. Coagulant aids (activated silica, polyelectrolytes, bentonite) help form stronger and heavier floc.