Activated sludge should have the ability to settle quickly so that it can: (a) be rapidly pumped back from sed

NCERT Exemplar · Q18

Q. Activated sludge should have the ability to settle quickly so that it can:

(a) be rapidly pumped back from sedimentation tank to aeration tank

(b) absorb pathogenic bacteria present in waste water while sinking to the bottom of the settling tank

(d) absorb colloidal organic matter.

Rajasthan RbseMCQ· 1mImportance★★★★★

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Concept understanding — Sewage Composition Hazards

Let’s start with something you already know. Imagine the water that goes down your kitchen sink after washing dishes, or the flush from a toilet. That water doesn’t just vanish — it carries everything we rinse, flush, or pour away. That mixture is called sewage.

Now, sewage is not just “dirty water.” It is a complex, living soup. And because it comes from homes, hospitals, factories, and streets, its composition is both predictable and dangerous. Understanding its hazards means understanding what is in that soup and why it cannot be ignored.

What is sewage made of?

Sewage contains three broad categories of material:

Organic matter — human waste (faeces and urine), food scraps, paper, oils, and detergents. These are biodegradable but require oxygen to break down.
Inorganic matter — salts, metals, sand, and grit. These do not rot easily.
Pathogens — disease-causing microorganisms like bacteria, viruses, and parasites. This is the most dangerous part.

Important

The key hazard of sewage is not just that it smells bad — it is that untreated sewage is a direct carrier of waterborne diseases such as cholera, typhoid, dysentery, and hepatitis. These diseases spread when sewage contaminates drinking water or food.

Why does composition matter for hazards?

The hazard depends on what is in the sewage and how much of it there is. For example:

High organic content means bacteria will multiply rapidly, using up oxygen in water bodies. This kills fish and creates foul-smelling, dead zones in rivers.
High pathogen load means even a small leak into a well can cause an epidemic.
Industrial sewage adds toxic chemicals (acids, heavy metals, dyes) that poison soil and groundwater for decades.

Note

In India, untreated sewage is the single biggest source of pollution in rivers like the Ganga and Yamuna. The NCERT textbook highlights that over 80% of the water supplied to Indian cities returns as sewage — and most of it is discharged without treatment.

The three main hazards at a glance

Health hazard — Direct contact or ingestion causes disease. Sewage workers, children playing near drains, and communities using contaminated wells are most at risk.
Environmental hazard — Organic waste depletes oxygen in water (eutrophication). Chemicals kill aquatic life. Groundwater gets polluted.
Aesthetic and social hazard — Stagnant sewage breeds mosquitoes (malaria, dengue), produces unbearable odour, and lowers property values. It also spreads stigma and inequality — poor neighbourhoods often lack proper sewerage.

A real-world connection …

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Activated sludge must settle quickly to allow for its efficient separation from treated water and subsequent recirculation back to the aeration tank, where it acts as an inoculum of active microbes.

Wastewater treatment is a critical process to reduce pollution and make water safe for discharge into natural bodies. The secondary treatment stage, also known as biological treatment, is where the bulk of organic matter is broken down by microbes. This stage primarily occurs in large aeration tanks.

In these aeration tanks, wastewater is continuously agitated mechanically and air is pumped into it. This creates an environment conducive for the growth of aerobic microbes. These microbes grow into large masses called 'flocs', which are essentially masses of bacteria associated with fungal filaments, forming a mesh-like structure. As these flocs grow, they consume the major part of the organic matter present in the wastewater, significantly reducing its Biochemical Oxygen Demand (BOD).

Once the BOD of the wastewater is sufficiently reduced, the effluent is then pumped into a large settling tank, also known as a sedimentation tank. Here, the bacterial flocs are allowed to settle down due to gravity. This settled material is referred to as 'activated sludge'.

The ability of this activated sludge to settle quickly is paramount for the efficiency of the entire secondary treatment process. If the flocs do not settle rapidly, they would remain suspended in the water, making the separation of the treated water from the microbial biomass difficult and inefficient.

Important

The primary reason for activated sludge needing to settle quickly is to facilitate its separation from the treated effluent and, crucially, to enable its recirculation.

A significant portion of this settled activated sludge is then pumped back into the aeration tank. This returned sludge serves as an 'inoculum' – essentially, it's a starter culture of active, aerobic microbes. By continuously reintroducing these microbes, the aeration tank maintains a high concentration of the necessary microorganisms, ensuring that the breakdown of organic matter in the incoming wastewater is rapid and efficient. Without this recirculation, the process would be much slower, or new microbial cultures would constantly need to be introduced.

Let's consider the given options in light of this understanding:

(A) be rapidly pumped back from sedimentation tank to aeration tank: This aligns perfectly with the explanation. The quick settling allows for efficient collection and recirculation of the active microbial flocs, which are essential for the continuous biological treatment process. …

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Q.Activated sludge should have the ability to settle quickly so that it can:

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