Index >> Waste Water Microbiology >>Coagulation

Coagulation - The coagulant is a metal salt which reacts with alkalinity in the water to produce an insoluble metal hydroxide floc which incorporates the colloidal particles. This fine precipitate is then flocculated to produce settleable solids. The coagulant is usually required in doses of 30-100 mg/l.

The commonly used coagulant for water treatment is aluminium sulphate which is commercially called as alum. Complex reactions take place when alum is added to water.

For satisfactory coagulation and to leave a suitable residual in the treated water sufficient alkalinity must be available to react with the alum.

But the precipitation must be complete when using iron salts otherwise residual iron in solution will be troublesome. Coagulant has to be dispersed throughout the body of water before flocculation occurs.

Floc formation will be difficult if the concentration of colloidal matter is very low. This can be overcome by the use of coagulant aids. These are simple additives like clay particles which form nuclei for precipitation of the hydroxide or polyelectrolytes. Heavy long chain synthetic polymers can also be used as coagulant aid which when added in small amounts [<l mg/l] promote agglomeration.

There is an alternative way to the conventional coagulation process. In this a suspension of finely divided magnitite is used to adsorb color, turbidity, iron and aluminium from water. This reaction takes place in the presence of pH control reagents and a coagulant aid.