6.1 Coagulation flocculation sedimentation treatments.
Coagulation flocculation sedimentation treatments are generally used to eliminate organic substances, but the chemicals normally used in this process have no effect on the elimination of soluble dyestuffs. Although this process effectively eliminates insoluble dyes, its value is doubtful because of the cost of treating the sludge and the increasing number of restrictions concerning the disposal of sludge. The electrical charge of colloidal type of very small size suspended mater in the effluent cause repulsion and prevent their aggregation. Therefore, electrolytic products such as aluminium sulphate, ferric sulphate, ferric chloride are added to eliminate the surface electrical charges of the colloids. This effect is named as coagulation. Normally the colloids bring negative charges, so the coagulants are usually inorganic or organic cationic coagulants (with positive charge in the water). The metallic hydroxides and the organic polymers can help the particle aggregation into flocks in addition to coagulation, thereby increasing the sedimentation. The combined action of coagulation, flocculation and setting is named as 'clarifloccultion'.