Question

Which is the best and most effective method for the removal of organic contaminant present in the polluted water in very small quantity (< 200 mg/litre)?
1. Biological oxidation pond
2. Lagooning
3. Activated carbon adsorption
4. Chemical coagulation

Answer 1

Correct Answer - Option 3 : Activated carbon adsorption

Concept:

Biological oxidation pond:

• Biological oxidation pond is basically an extended aeration system of a modified activated sludge process. It is designed and operated on the same principle as of the activated sludge process. It is an aerobic suspended system.

Activated carbon adsorption:

• Activated carbon is a specially treated carbon that possesses the property of absorbing and attracting impurities such as gases liquids and finely divided solids. Because of its excellent property of absorbing impurities, it is widely used for removing tastes and odors from public supplies.
• The use of activated carbon many thus serve the following advantages:
• When used in the powdered form before coagulation it aids in coagulation.
• It reduces the chlorine demand for treated water.
• It removes the organic matter present in water.
• It removes the taste odors and colors caused by the presence of iron, manganese, phenols, excess chlorine hydrogen sulfide, etc.
• Its overdose is not harmful.
• This method is the most effective method for the removal of organic contaminant present in the polluted water in very small quantity.

Chemical coagulation:

• The surface charge on colloidal particles gives them long-term stability and hence the particles which might otherwise settle or coalesce are mutually repelled by their like charges. 
• Coagulation is a chemical technique that is directed towards the destabilization of the charged colloidal particles.
• Flocculation, on the other hand, is the slow mixing technique that promotes the agglomeration of the destabilized particles.
• For all practical purposes, however, the entire process of addition of chemicals (coagulants) and mixing (flocculation) is usually referred to as coagulation.
• The coagulated water is finally made to pass through the sedimentation tank where the flocculated particles settle down and are thus removed.

Lagooning:

• This method is sometimes used at smaller places for disposing of raw sludge without digestion. In the method, the raw sludge is kept at rest in a large shallow open pond, called a lagoon.
• The detention period is 1 to 2 months and may extend up to 6 months.
• During its detention in the lagoon the sludge undergoes anaerobic digestion thereby getting stabilised.