After significant reduction of BOD during secondary treatment of sewage, what further processes are carried ou

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Q. After significant reduction of BOD during secondary treatment of sewage, what further processes are carried out ? Mention any one important outcome of these processes.

CBSECBSE Class XII Board 2026Subjective· 2mImportance★★★★★

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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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After secondary treatment significantly reduces the BOD of sewage, the sludge produced is passed into an anaerobic sludge digester, where bacteria break it down to produce biogas — a mixture of methane, hydrogen sulphide and carbon dioxide that can be used as a source of energy.

Secondary (biological) treatment of sewage takes place in a large aeration tank, where the effluent from primary treatment is agitated mechanically and air is pumped through it. This encourages the growth of useful aerobic microbes, which form flocs (masses of bacteria associated with fungal filaments) and consume most of the remaining organic matter. This activity significantly reduces the sewage's Biochemical Oxygen Demand (BOD) — the standard measure of the amount of organic matter present.

Once the BOD has been reduced significantly, the effluent is passed into a settling tank, where the bacterial flocs are allowed to sediment as activated sludge. A small part of this activated sludge is pumped back into the aeration tank to act as the inoculum (seed) of microbes for the next batch of incoming sewage — this is what keeps the aeration tank's bacterial population active. The remainder of the activated sludge, now in excess, is passed into large tanks called anaerobic sludge digesters.

Note

The 'activated' in activated sludge refers to the fact that it is teeming with living, actively metabolising microbes that are seeded back into the process — it is not a chemical treatment step.

In the anaerobic sludge digester, other kinds of bacteria — ones that grow anaerobically (in the absence of oxygen) — digest the bacteria and fungi present in the sludge. During this digestion, these anaerobic microbes produce a mixture of gases, mainly methane, hydrogen sulphide and carbon dioxide, together known as biogas. …

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

CBSE 2025Set ANNUAL1 mark
$Q. What is primary sludge?$
›Reveal solutionSolution

In sewage treatment, primary sludge refers to the settled solid organic matter removed by physical settling in the primary treatment stage, before the wastewater proceeds to biological (secondary) treatment.

Raw sewage first undergoes primary treatment, which is essentially physical: it passes through filters to remove large floating debris, then through a grit chamber (settling of sand/grit), and finally into a primary settling/sedimentation tank. Here, the heavier, particulate suspended solids (faecal matter and other organic debris) settle down under gravity to form primary sludge, while the floating material (scum) is skimmed off separately. The clarified supernatant liquid (called effluent) is then passed on to secondary treatmen …

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CBSE 2025Set ANNUAL1 mark
$Q. Differentiate between primary sludge and activated sludge by giving one point.$
›Reveal solutionSolution
Primary sludge is settled solid waste from physical treatment; activated sludge is a microbial floc formed during aerobic biological treatment.

Sewage (waste water) treatment happens in stages:

Primary treatment (physical): the sewage passes through filters and settling tanks. Heavier solid particles simply settle down under gravity, forming primary sludge, while the floating debris is removed by skimming. This is purely a physical, non-biological separation step.

Secondary treatment (biological): the effluent from primary treatment is passed into large aeration tanks and constantly agitated and aerated. This allows aerobic micro-organisms (bacteria, protozoa, etc., collectively called "activated sludge") to grow vigorously, forming flocs, and to consume/oxidise the remaining organic matter in the effluent, reducing its BOD (biochemical oxygen demand). …
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CBSE 2024Set GG1 mark
$Q. What is sewage?$
›Reveal solutionSolution

Sewage is the municipal waste water full of organic matter and microbes.

Concept. Large amounts of waste water are generated every day in cities and towns. This municipal waste water, called sewage, is carried through sewers and contains human excreta, domestic and industrial liquid waste, a high load of organic matter, and many microbes, several of which are pathogenic.

…

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CBSE 2022Set ANNUAL1 mark
$Q. Name the bacteria that bring about decomposition of sewage.$
›Reveal solutionSolution

Both aerobic floc-forming bacteria (secondary treatment) and anaerobic methanogens like Methanobacterium (sludge digestion) decompose sewage.

In secondary (biological) sewage treatment, the primary effluent is passed into large aeration tanks and continuously agitated and aerated. This allows vigorous growth of aerobic heterotrophic bacteria, which grow along with fungal filaments into masses called flocs (a mesh-like microbial community). These bacteria consume the dissolved and suspended organic matter in the sewage, significantly reducing its Biochemical Oxygen Demand (BOD).

…

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CBSE 2020Set ANNUAL1 mark
$Q. Which bacteria is commonly found in the anaerobic sludge during sewage treatment?$
›Reveal solutionSolution

The anaerobic sludge in sewage treatment is digested by methanogenic bacteria (methanogens), such as Methanobacterium, which produce a mixture of methane, hydrogen sulphide and CO₂ (biogas).

Concept. Sewage treatment in a Sewage Treatment Plant (STP) has two main stages — primary (physical) and secondary (biological). In secondary treatment the effluent is passed through aeration tanks where aerobic microbes form flocs and reduce the BOD. The settled solids form the activated sludge, a part of which is passed into large anaerobic sludge digesters.

…

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CBSE 2018Set ANNUAL1 mark
$Q. What is flocs?$
›Reveal solutionSolution

Flocs are microbe-rich clumps that form during activated sludge treatment and are key to purifying sewage.

During secondary (biological) treatment of sewage, the effluent from primary treatment is passed into large aeration tanks and constantly agitated and aerated. This allows aerobic microbes (bacteria and fungi) to grow vigorously, forming flocs — masses of bacteria held together in a matrix by filamentous fungi, forming a slimy, mesh-like structure. These flocs actively consume the remaining organic matter in the sewage, significantly reducing its Biochemical Oxygen Demand (BOD). The floc-laden sludge is then allowe …

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CBSE 2018Set ANNUAL1 markMCQ
$Q. Masses of bacteria associated with fungal filament to form a mesh like structure of called-$
(a) Flocs
(b) Methanogen
(c) Plasminogen
(d) None of these
›Reveal solutionSolution

Mesh-like microbial masses in sewage treatment are called flocs.

In the secondary/biological treatment of sewage, the primary effluent is passed into large aeration tanks where vigorous aerobic bacteria grow. These bacteria grow along with fungal filaments to form mesh-like clumps called flocs. The microbes in the flocs consume the organic matter, reducing the BOD (bi …

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CBSE 2016Set ANNUAL1 mark
$Q. Name the steps involve in primary sewage treatment.$
›Reveal solutionSolution

Primary treatment of sewage is a purely physical process that removes solid material before the sewage undergoes microbial (secondary) treatment.

Sewage treatment plants (STPs) purify wastewater in stages. In primary treatment, sewage is first passed through screens to filter out large floating objects (rags, plastic, paper), then through grit chambers where sand and small stones settle out by gravity. The partially clarified sewage is then held in large settling (sedimentation) tanks, where the heavier suspended solids settle at the bottom as primary sludge, while the lighter material floats up as scum and is skimmed off. The clarified liquid, now called the effluent, is passed on to second …

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Q.After significant reduction of BOD during secondary treatment of sewage, what further processes are carried out ? Mention any one important outcome of these processes.

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