Q.Describe the role of microbes in secondary treatment of sewage. (3)
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🔒 Start your 14-day free trial to unlock the full solution →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.
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.
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 …
In secondary (biological) sewage treatment, aerobic microbes ('activated sludge') consume the dissolved/suspended organic matter in the primary effluent, significantly reducing its BOD, while the leftover sludge is further broken down anaerobically to produce biogas. …
Secondary sewage treatment relies on aerobic microbes to break down dissolved organic pollutants, drastically cutting the sewage's BOD before it can be safely released.
After primary treatment (which physically removes large debris and settles solid particles as sludge), the remaining liquid effluent still contains substantial dissolved and suspended organic matter, and this is where secondary (biological) treatment, driven by microbes, takes over.
The primary effluent is passed into large aeration tanks and continuously agitated and supplied with air (aerated). This encourages the rapid growth of naturally occurring aerobic microorganisms, which form floating flocs called activated sludge. These aerobic microbes consume and metabolise the dissolved/suspended organic matter present in the effluent for their own growth, significantly reducing the effluent's Biochemical Oxygen Demand (BOD), an important measure of the degree of organic pollution.
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- CBSE 2025Set ANNUAL1 markQ.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 …
- CBSE 2025Set ANNUAL1 markQ.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). …
- CBSE 2024Set GG1 markQ.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 markQ.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 markQ.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 markQ.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 …
- CBSE 2018Set ANNUAL1 markMCQQ.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 …
- CBSE 2016Set ANNUAL1 markQ.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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