Which of the following statements explains the reason why activated sludge should have the ability to settle quickly?

So that it can be rapidly pumped back from the sedimentation tank to the aeration tank

The correct answer is (a) be rapidly pumped back from the sedimentation tank to the aeration tank.

Activated sludge refers to the flocculent mass of microorganisms that is suspended in wastewater during the secondary treatment process. It is formed by mixing sewage with a microbial culture in the aeration tank, where the microorganisms consume organic matter and reduce the biochemical oxygen demand.

After the aerobic treatment in the aeration tank, the wastewater containing the suspended activated sludge is transferred to a settling tank, also known as a sedimentation tank. In the settling tank, the primary purpose is to separate the solid sludge from the clarified effluent.

To achieve efficient separation, the activated sludge should have the ability to settle quickly. This means that the flocculated mass of microorganisms should rapidly sink to the bottom of the settling tank, allowing the clarified effluent to be collected from the top. The settled sludge, called primary sludge, can then be pumped back from the settling tank to the aeration tank for further treatment.

Option (a) is the correct answer because the ability of activated sludge to settle quickly enables its efficient recycling and re-entry into the aeration tank, ensuring continuous treatment of wastewater. Options (b), (c), and (d) are incorrect explanations as they do not accurately describe the purpose and function of settling the activated sludge.