Coagulation

Sedimentation Aided with coagulation

• To remove very fine suspended particles and colloidal particles.
• Certain chemicals called “Coagulants" are mixed with water which form a gelatinous precipitate called “floc’ which attracts the fine suspended particles.

Commonly used coagulants

1. Alum: A12 (S04)3. 18 H2O (Aluminum Sulphate). Also called filter alum.
2. Copperas (Ferrous Sulphate + lime)
3. Chlorinated Coppers (Ferric Sulphate)
4. Sodium Sulphate.

•  Iron salts arc used more frequently for treating sewage and alum is used for treating water.
• Detention time: 2 to 4 hours
• Overflow rate : 1000 to 1250 lit/hr /m of plan area.


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Floc Chamber:

• After mixing alum to the water in the mixing basin, it is gently stirred by paddles in the floe chamber to build up the floc.
• Detention time: 15 to 40 min.
• Depth of floc  chamber is half the depth of settling chamber,

Flocculation : Slow mixing technique which promotes agglomeration of particles

Coagulation : Mixing of coagulants  +Flocculation

Dosage: It depends on the amount of turbidity present in water.

• Average dosage for normal water is 14 mg/I.
• Optimum alum dosage my be determined by Jar Test.
• Alum functions effectively only when pH value lies between 6.5 & 8.5
• Iron salts can be used over a wide range of pH values.

• Addition of alum to water imparts permanent hardness and CO2 evolved causes corrosion
• Presence of alkalinity is necessary for the functioning of coagulants. If raw water is not alkaline, lime or soda ash is added to make water alkaline.

Advantages of alum as coagulant:

1. Reduces taste and odour apart from turbidity
2. The floc is effective
3.  It is cheap

Disadvantages of alum as coagulant:

1. Difficult to dewater the sludge formed 
2. It imparts permanant hardness 
3. CO2 causes corrosion