Q.For Questions number 13 to 16, two statements are given – one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (A), (B), (C) and (D) as given below. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : Activated sludge contains large population of bacteria. Reason (R) : These bacteria help in anaerobic digestion of human waste in biogas plant.

Let’s break this down.

The Assertion says that activated sludge contains a large population of bacteria. That is true — activated sludge is rich in aerobic bacteria that break down organic matter in sewage treatment.

The Reason says that these bacteria help in anaerobic digestion of human waste in a biogas plant. That is false. The bacteria in activated sludge are primarily aerobic, not anaerobic. Anaerobic digestion in a biogas plant is carried out by a different set of bacteria, t …

Let’s place this in context. In a modern sewage treatment plant, the water that enters after primary treatment — where solid debris and grit are removed — still contains a heavy load of dissolved organic matter. To break this down, the plant relies on a biological step called the activated sludge process. Here, air is bubbled vigorously through the effluent in a large aeration tank. This oxygen-rich environment encourages the rapid growth of aerobic bacteria and other microbes. These microorganisms form flocs — sticky, clumpy masses — and they consume the organic pollutants as food. The result is a thick, dark liquid teeming with bacteria: that is the activated sludge.

So the Assertion (A) — that activated sludge contains a large population of bacteria — is perfectly correct. The NCERT textbook explicitly states that “the effluent is passed into an aeration tank where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into flocs.” Those flocs are the activated sludge, and they are indeed packed with bacteria. …