During coagulation, the coagulant chemicals neutralize the electrical charges of the fine particles in the water, allowing the particles to come closer together and form large clumps. The final step is flocculation. During flocculation, a process of gentle mixing brings the fine particles formed by coagulation into contact with each other.
Lime: Calcium Oxide (CaO) is commercially known as lime. It is used in water treatment to reduce acidity, to harden as a flocculant and to remove other impurities such as phosphates. Lime is added in an amount of 2 to 40% by weight according to the weight of sludge. Mixing and reacting steps have duration of 30 seconds.
Alum: Alums are useful for a range of industrial processes. They are soluble in water; have an astringent, acid, and sweetish taste; react acid to litmus; and crystallize in regular octahedral geometry. When heated they liquefy; and if the heating is continued, the water of crystallization is driven off, the salt froths and swells, and at last an amorphous powder remains. Potassium alum is the common alum of commerce, although soda alum, ferric alum, and ammonium alum are manufactured.
The basic water causes Al(OH)3 to precipitate bringing small particles with them and then making water clear. Fe2O3 is removed mainly by coagulation.
Polyelctrolytes: These are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions (water), making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes (salts) and polymers (high molecular weight compounds), and are sometimes called polysalts. Like salts, their solutions are electrically conductive. Like polymers, their solutions are often viscous.