Secondary Sedimentation Tank or Clarifier
It is a rectangular tank constructed with brick masonry. Baffle walls are provided in the tank. to divide it into two chambers. Inlet and outlet pipes with valves are provided on opposite corners.
A sludge removal pipe is provided at the bottom of the tank on both chambers. Very fine suspended particles, present in wastewater, which cannot be removed in plain sedimentation tank, can be removed in the secondary sedimentation tank.
Coagulation of sewage
Sedimentation of sewage can be assisted by adding certain chemicals, known as coagulants.
Coagulants react with colloidal matter in sewage and form flock. The phenomenon of the formation of flock is termed as flocculation.
Commonly used coagulants are alum, chlorinated copperas, lime, ferric sulphate, ferric chloride, sodium silicate etc. Ferric chloride is widely used for sewage treatment on a large scale.
Coagulants are mixed with sewage properly with some devices. The commonly used devices are Flash mixer, Deflector plate mixer, floculator.
The choice of chemical as coagulant depends upon its cost, dosage requirement, degree of treatment required, characteristics of waste, pH value of the waste etc.
Alum is preferred in sewage treatment when iron salts are not available and the wastewater is alkaline in nature. If natural alkalinity is not present, sufficient lime is added for the effective working of alum.
Ferric sulphate is effective in a wider pH range of sewage (pH value 4 – 10). Ferrous sulphate works well when pH value is above 9.
If wastewater is highly alkaline due to the presence of industrial waste, it is cheaper to use large doses of ferrous sulphate than ferric sulphate. Ferric salt reacts quickly but they are corrosive and impart colour.