Q.Removal of large pieces of floating debris, oily substances, etc. during sewage treatment is called ................

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.

Now look at the Reason (R). It claims that these bacteria help in the anaerobic digestion of human waste in a biogas plant. That is a confusion of two entirely different processes. The bacteria in activated sludge are aerobic — they need oxygen to function. A biogas plant, on the other hand, operates under anaerobic conditions (no oxygen), where a different set of microbes, such as methanogens, break down organic matter to produce methane. The NCERT makes this distinction very clear: the activated sludge process is an aerobic treatment step in sewage management, while biogas production from human waste (or cattle dung) is a separate, anaerobic process carried out in a biogas plant.

Therefore, the Reason (R) is false. It incorrectly attributes an anaerobic function to bacteria that are fundamentally aerobic, and it mixes up two distinct stages of waste treatment.