The process of removal of suspended, colloidal impurities by allowing water to stand undisturbed in big tanks about 5m deep. Most of the suspended particles settle down at the bottom, due to the force of gravity. The retention period in a sedimentation tank ranges from 2-6 hours.
Sedimentation with coagulation:
Fine suspended particles like mud etc. cannot settle down by plain sedimentation. They are removed by this process in which we add coagulant to water. The coagulant forms an insoluble gelatinous, flocculent precipitate, which traps very fine suspended impurities forming bigger flocks, which further settle down.
Some of the examples of coagulants are alum, ferrous sulphate (FeSO47H2O), sodium aluminate etc.
General reactions during coagulation are:
NaAIO2 + 2H2O AI(OH)3+NaOH (pH = 5.5-8.0)
NaOH + MgSO4 Mg(OH)2 + Na2SO4