(a) What do you mean by activated sludge in an STP ? (b) Explain the biological treatment of the major part of

Question

Q. (a) What do you mean by activated sludge in an STP ?

(b) Explain the biological treatment of the major part of the sludge transferred from the large aeration tank into the anaerobic sludge digesters before its final release into the natural water bodies.

CBSECBSE Class XII Board 2025Subjective· 3mImportance★★★★★

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Concept understanding — Biochemical Oxygen Demand

Imagine you have a glass of clean drinking water and a glass of water from a muddy, smelly pond. If you left both out in the sun for a few days, the pond water would turn foul and cloudy much faster. Why? Because the pond water is full of organic waste — dead leaves, sewage, or leftover food — that tiny bacteria in the water start to eat. As these bacteria feed, they breathe oxygen, just like we do. The more waste there is, the more bacteria grow, and the more oxygen they consume from the water.

That consumption of oxygen by bacteria is the core idea behind Biochemical Oxygen Demand, or BOD.

What BOD actually measures

BOD is a measure of the amount of dissolved oxygen that microorganisms (mainly bacteria) need to break down organic matter present in a water sample over a specific time, usually 5 days, at a fixed temperature (20°C). Think of it as the "appetite" of the water for oxygen. If the water has a high BOD, it means there is a lot of organic waste for bacteria to decompose, so they will use up a large amount of oxygen.

The NCERT textbook for Class 12 Biology (Chapter 16: Environmental Issues) states it clearly: "Biochemical Oxygen Demand (BOD) refers to the amount of the oxygen that would be consumed if all the organic matter in one liter of water were oxidised by bacteria." The key point is that BOD is not a measure of the waste itself, but of the oxygen required to clean that waste biologically.

Why BOD matters in the real world

BOD is the single most important indicator of organic pollution in rivers, lakes, and wastewater. Here is why it matters:

It tells you how polluted the water is. Clean river water typically has a low BOD (say, under 5 mg/L). Sewage water, on the other hand, has a very high BOD (often 200–600 mg/L). The higher the BOD, the dirtier the water.
It predicts the health of aquatic life. Fish and other aquatic animals need dissolved oxygen to survive. When BOD is very high, bacteria use up so much oxygen that there is little left for fish. This can lead to "dead zones" where nothing can live.
It guides wastewater treatment. Before sewage or industrial effluent is released into a river, treatment plants must reduce its BOD to a safe level. The government sets legal limits for BOD in treated water. If a factory releases water with high BOD, it is breaking the law. …

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Activated sludge is a mass of aerobic microbes that actively decompose organic waste in sewage; after primary treatment, it undergoes anaerobic digestion to break down remaining organics, producing biogas and stabilized sludge safe for disposal.

Sewage treatment plants handle the enormous task of cleaning wastewater before it re-enters rivers or lakes. The biological heart of this process lies in harnessing microbial communities to break down organic pollutants—first aerobically, then anaerobically.

Activated Sludge: The Aerobic Workforce

When sewage enters the secondary treatment stage of an STP, it flows into large aeration tanks. Here, air is pumped vigorously through the wastewater, creating an oxygen-rich environment. This deliberate aeration encourages the rapid growth of aerobic bacteria and other microorganisms—fungi, protozoa—that feed on the dissolved and suspended organic matter in the sewage.

Over time, these microbes multiply and clump together, forming flocs: soft, brownish masses teeming with microbial life. This entire mass of microorganisms, actively consuming organic waste and suspended in the wastewater, is called activated sludge. The term "activated" captures the idea that these microbes are metabolically active, vigorously oxidizing organic pollutants and reducing the Biochemical Oxygen Demand (BOD) of the sewage. BOD measures the amount of oxygen microbes would consume while decomposing organic matter; high BOD means heavy pollution, so lowering it is the goal.

As the wastewater passes through the aeration tank, the activated sludge settles out in a settling tank. A portion of this sludge is recycled back into the aeration tank to maintain a healthy population of decomposers—keeping the system "activated." But a significant part of the sludge, now laden with microbial biomass and undigested material, must be processed further.

Note

The effluent leaving the secondary treatment has much lower BOD—typically reduced by 90–95%—and is clearer, though not yet completely free of all contaminants.

Anaerobic Digestion: The Final Breakdown

The major portion of activated sludge that is not recycled is transferred to anaerobic sludge digesters. These are large, sealed tanks where oxygen is absent. In this oxygen-free environment, a different consortium of microbes takes over: anaerobic bacteria.

These anaerobic bacteria decompose the remaining organic matter in the sludge through a multi-step process:

Hydrolysis and fermentation: Complex organic molecules—proteins, carbohydrates, lipids—are broken down into simpler compounds like fatty acids and alcohols.
Acetogenesis and methanogenesis: Specialized bacteria convert these intermediates into acetic acid, carbon dioxide, and hydrogen. Finally, methanogenic archaea (methanogens) convert these products into methane and more carbon dioxide. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2026Set 57/1/11 markMCQ
$Q. Assertion (A) : Flocs are masses of bacteria associated with fungal filaments in secondary treatment of sewage. Reason (R) : Flocs help in digestion of solid waste by anaerobic respiration. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation for Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.$
›Reveal solutionSolution

Flocs are indeed bacterial-fungal aggregates in secondary sewage treatment, but they work through aerobic digestion, not anaerobic — so the assertion is true while the reason is false.

Sewage treatment is a multi-stage process designed to remove organic and inorganic pollutants before water is released back into the environment. The secondary treatment stage is where biological agents — primarily microorganisms — take center stage in breaking down dissolved organic matter that escaped the physical settling of primary treatment.

During secondary treatment, sewage is vigorously aerated in large tanks. This constant supply of oxygen creates an environment where aerobic bacteria thrive. These bacteria don't work in isolation; they cluster together with fungal filaments to form what we call flocs — visible, mesh-like masses that look almost like floating clumps or sludge. The fungal filaments act as a scaffold, holding the bacterial colonies together, while the bacteria do the heavy lifting of decomposing organic waste.

The assertion correctly identifies flocs as masses of bacteria associated with fungal filaments in secondary treatment. This is textbook-accurate. The structure of flocs is crucial because it increases the surface area available for microbial action and allows these aggregates to settle out later, carrying the digested waste with them.

Important

The key metabolic pathway in secondary treatment is aerobic respiration, not anaerobic. The vigorous aeration ensures oxygen is continuously available, and aerobic bacteria use this oxygen to oxidize organic matter into carbon dioxide, water, and energy — a far more efficient breakdown than anaerobic processes. …

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CBSE 2026Set EG1 mark
$Q. Write the complete form of BOD abbreviation.$
›Reveal solutionSolution

BOD = Biochemical Oxygen Demand — a measure of organic pollution in water.

BOD (Biochemical Oxygen Demand) is the amount of dissolved oxygen that would be consumed by micro-organisms while they oxidise the organic matter present in a sample of water. It is measured over a fixed period (usually 5 days at 20 °C).

…

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CBSE 2026Set ANNUAL1 markMCQ
$Q. Based on the given BOD test results (Sample A-20 mg/L, Sample B-8 mg/L, and Sample C-400 mg/L), which water sample is most likely to be least polluted?$
(a) Sample A (20 mg/L)
(b) Sample C (400 mg/L)
(c) Sample B (8 mg/L)
(d) Sample A (20 mg/L) and Sample C (400 mg/L) both.
›Reveal solutionSolution

BOD (Biochemical Oxygen Demand) measures the oxygen consumed by microbes decomposing organic waste in water; higher BOD = more organic pollution = more polluted water. So the sample with the lowest BOD is the least polluted.

When a water body receives organic pollutants (sewage, industrial effluents), aerobic decomposer microorganisms multiply and consume dissolved oxygen to break down this organic matter. The amount of oxygen they need over a fixed period (BOD, in mg/L) is therefore directly proportional to the amount of biodegradable organic pollution present — a highly polluted water body will have decomposers using up a lot of oxygen (high BOD), starving aquatic organisms of the dissolved oxygen they need.

…

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CBSE 2025Set ANNUAL1 markMCQ
$Q. What happens to the BOD of a water body with removal of effluents from it ?$
(a) Increases
(b) Decreases
(c) Remains the same
(d) Increases and then stagnates
›Reveal solutionSolution

Fewer organic pollutants means less oxygen-demanding microbial decomposition, so BOD falls.

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen consumed by microorganisms while decomposing the organic matter present in a water sample over a fixed period (usually 5 days at 20°C). Effluents discharged from sewage or industries carry large amounts of biodegradable organic matter, which decomposer microbes break down using dissolved oxygen — the more such organic effluent present, the higher the BOD (and the more polluted and oxygen-depleted the water becomes for aquati …

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CBSE 2023Set 57/1/11 markMCQ
$Q. The graph plotted below is based on the data collected by biology students with respect to the levels of oxygen at the specific points in the river flowing outside their city. Which point in the graph indicates the entry of untreated sewage in the river ?$
(a) Point
(i)
(b) Point
(ii)
(c) Point
(iii)
(d) Point (iv)
›Reveal solutionSolution

The point where the dissolved oxygen level drops sharply after a period of stability marks the entry of untreated sewage into the river — that is Point (ii).

To understand which point on the graph corresponds to the entry of untreated sewage, you first need to grasp what Biochemical Oxygen Demand (BOD) tells us. BOD is a measure of the amount of oxygen that microorganisms require to break down organic matter in water. When untreated sewage — rich in organic waste — enters a river, the natural microbial population goes into overdrive, consuming dissolved oxygen at a furious rate. This causes a steep drop in the oxygen level in the water.

Now, look at the graph. The horizontal axis shows different sampling points along the river, and the vertical axis shows the dissolved oxygen level. A healthy, unpolluted stretch of river will have a relatively high and stable oxygen level. That is what you see at Point (i) — the oxygen level is high, indicating clean water upstream.

At Point (ii), the oxygen level plunges dramatically. This sudden crash is the signature of a large influx of organic pollutants. The microbes are feasting on the sewage, using up oxygen faster than it can be replenished from the air or from photosynthesis. This is the classic response to untreated sewage being discharged into the river.

Note

The graph does not show BOD directly, but the sharp fall in dissolved oxygen is the indirect evidence. A high BOD would be measured at the same point, confirming the sewage entry. …

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CBSE 2023Set ANNUAL1 markMCQ
$Q. The BOD of wastewater is estimated by measuring the amount of which?$
(a) oxygen consumption
(b) oxygen output
(c) biodegradable organic matter
(d) total biological (organic) matter
›Reveal solutionSolution

BOD = amount of oxygen consumed by microbes.

Biochemical Oxygen Demand (BOD) is the amount of dissolved oxygen consumed by aerobic microorganisms while decomposing the organic matter in a sample of water over a fixed period (usually 5 days at 20 degrees C). The greater the organic pollution, the more oxygen the microbes …

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CBSE 2021Set ANNUAL1 markMCQ
$Q. Organic pollutants in sewage water is measured as ______.$
(a) GMO
(b) MTP
(c) BOD
(d) HGP
›Reveal solutionSolution
The amount of organic matter (pollutants) present in sewage water is measured as BOD.

BOD (Biochemical Oxygen Demand) refers to the amount of oxygen that would be consumed by micro-organisms (mainly bacteria) if all the organic matter in one litre of water were oxidised by them.

The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water, over a period of five days, and thereby estimates the amount of organic matter (pollutant load) present. …
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CBSE 2019Set ANNUAL1 markMCQ
$Q. Value of BOD in clean water will be:$
(a) More than 5 ppm
(b) Less than 5 ppm
(c) More than 17 ppm
(d) None of these
›Reveal solutionSolution
Clean water contains little organic pollution, so microorganisms need very little dissolved oxygen to decompose it — clean water has a BOD below 5 ppm.

BOD is defined as the amount of dissolved oxygen (in mg/L or ppm) needed by microorganisms to biologically break down organic matter present in a water sample, measured over 5 days at 20°C. It is used as an indicator of water pollution:

Clean water: BOD < 5 ppm

Moderately polluted water: BOD 5–17 ppm (roughly) …
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CBSE 2019Set ANNUAL1 mark
$Q. What is the significance of BOD?$
›Reveal solutionSolution
BOD measures the organic (pollution) load in water — higher BOD means more organic matter and more polluting potential.

Concept: BOD (Biochemical Oxygen Demand) is the amount of oxygen that would be consumed by microbes if all the organic matter in one litre of water were oxidised. It is measured by the amount of oxygen microbes use up.

Significance:

[!NOTE]

It indicates the amount of biodegradable organic matter present in the water sample.

Higher BOD = more organic matter = greater polluting potential of the water. …
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Q.(a) What do you mean by activated sludge in an STP ?

Activated Sludge: The Aerobic Workforce

Anaerobic Digestion: The Final Breakdown

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