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Consider the following statements regarding contact stabilization process: 1. Primary settling tank is not required in some cases. 2. BOD removal occurs in two stages. 3. Aeration volume requirements are approximately 50% of those of a conventional – or tapered – aeration plant. 4. Returned sludge is aerated for 30 min to 90 min in sludge aeration tank. Which of the above statements are correct?

Consider the following statements regarding contact stabilization process: 1. Primary settling tank is not required in some cases. 2. BOD removal occurs in two stages. 3. Aeration volume requirements are approximately 50% of those of a conventional – or tapered – aeration plant. 4. Returned sludge is aerated for 30 min to 90 min in sludge aeration tank. Which of the above statements are correct? Correct Answer 1, 2 and 3 only

Concept:

In contact stabilization, primary treatment is not required. The activated sludge is mixed with influent in the contact tank where the organics are absorbed by microorganisms.

The MLSS is settled in the clarifier. The returned sludge is aerated in the reaeration basin to stabilize the organics. The process requires approximately 50% less tank volume.

In this process returned sludge aerated for 3 to 6 hours before taking back to aeration tank. This reduces aeration period thus reduce the volume of aeration tank and there is possibility of BOD reduction in two stages at stabilization unit and at aeration tank.

It is a type of activated sludge process. Activated sludge is a process in which air or oxygen is forced into sewage liquor to develop a biological floc which reduces the organic content of the sewage.

The sludge is returned to the aeration system to re-seed the new sewage entering the tank.

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