NCERT grounding
This subtopic corresponds to Section 8.3, Microbes in Sewage Treatment, of the NCERT Class XII Biology chapter Microbes in Human Welfare. NCERT defines the problem first: large quantities of waste water are generated every day in cities and towns, and a major component of this waste water is human excreta. This municipal waste water is called sewage, and it contains large amounts of organic matter and microbes, many of which are pathogenic. Such water cannot be discharged directly into natural water bodies like rivers and streams, so it is first treated in sewage treatment plants (STPs) to make it less polluting.
The single most important idea NCERT stresses is that the treatment itself is biological — it is carried out by the heterotrophic microbes naturally present in the sewage. No microbe is added from outside to start the process; the plant simply creates conditions that let the right microbes flourish. NCERT divides the treatment into two stages, primary and secondary, and the chapter Summary reinforces that microbes have been used to treat sewage by the process of activated sludge formation for more than a hundred years.
Till date, no man-made technology has been able to rival the microbial treatment of sewage.
NCERT Class XII Biology — Microbes in Human Welfare
Sewage treatment, stage by stage
Sewage treatment is best understood as an ordered sequence in which the water becomes progressively cleaner. The dirty water that arrives at the plant carries floating debris, suspended grit, dissolved and colloidal organic matter, and a teeming population of microbes. Treatment removes these in a fixed order — first the bulky physical solids, then the dissolved organic load — because each stage is designed to handle a different kind of impurity. Understanding why the order is fixed is the key to never confusing the stages in an exam.
The plant works on naturally occurring heterotrophic microbes. Heterotrophs cannot make their own food; they obtain carbon and energy by breaking down organic matter. Sewage is essentially a rich broth of organic matter, so it is an ideal feeding ground. The job of the engineer is not to supply microbes but to supply the conditions — oxygen, mixing, retention time — under which the useful microbes outcompete everything else and consume the pollutants.
Primary treatment — physical removal
Primary treatment involves only the physical removal of particles, both large and small, from the sewage. There is no biology in this stage; nothing is digested. The particles are taken out in two ordered steps. First, floating debris such as rags, paper and plastic is removed by sequential filtration through screens of decreasing mesh size. Next, the grit — soil and small pebbles that are heavier than water — is removed by sedimentation, in which the water is allowed to stand so the dense particles sink.
All the solids that settle out during primary treatment form the primary sludge. The liquid that lies above the settled sludge — the clarified water — is the supernatant, and this supernatant forms the effluent. The effluent from the primary settling tank is not yet clean; it still carries dissolved and colloidal organic matter that no screen or sedimentation step can remove. This primary effluent is therefore taken forward to secondary treatment.
Figure 1. Primary treatment removes solids physically. Floating debris is screened out by sequential filtration, grit settles by sedimentation as primary sludge, and the clarified supernatant becomes the effluent passed to secondary treatment.
Secondary treatment — biological treatment
Secondary treatment, also called biological treatment, is where the microbes do their work. The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This combination of mixing and oxygen supply allows the vigorous growth of useful aerobic microbes, which clump together into flocs — masses of bacteria associated with fungal filaments that form mesh-like structures.
While they are growing, the microbes in the flocs consume the major part of the organic matter present in the effluent. Because the organic matter is the food and pollutant, removing it cleans the water; and because oxidising organic matter consumes oxygen, removing it significantly reduces the BOD (biochemical oxygen demand) of the effluent. The sewage water is treated in the aeration tank until the BOD has fallen low enough.
Figure 2. Secondary treatment. Aerobic microbes grow into flocs in the aerated tank and consume the organic matter, reducing BOD. The flocs settle as activated sludge — a small part is recycled as inoculum, the major part goes to the anaerobic digester where biogas is produced.
From flocs to activated sludge and biogas
Once the BOD of the sewage or waste water has been reduced significantly, the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. This sediment is called activated sludge. The word "activated" reflects that the sludge is full of living, actively metabolising microbes — it is not inert mud like the primary sludge.
The activated sludge is divided into two unequal parts. A small part is pumped back into the aeration tank, where it serves as the inoculum — a starter population of microbes that immediately begins working on the next batch of incoming effluent. The remaining major part of the sludge is pumped into large tanks called anaerobic sludge digesters. Here, other kinds of bacteria, which grow anaerobically, digest the bacteria and fungi present in the sludge.
During this anaerobic digestion the bacteria produce a mixture of gases — methane, hydrogen sulphide and carbon dioxide. This mixture is biogas, and because it is inflammable it can be used as a source of energy. The effluent that finally emerges from the secondary treatment plant is generally released into natural water bodies like rivers and streams.
Sewage through the treatment plant
-
Step 1
Filtration
Sequential screening removes floating debris from raw sewage.
Primary · physical -
Step 2
Sedimentation
Grit settles as primary sludge; the supernatant forms the effluent.
Primary · physical -
Step 3
Aeration
Air and agitation grow aerobic flocs that eat organic matter; BOD falls.
Secondary · biological -
Step 4
Settling
Flocs sediment as activated sludge; clean effluent decants off.
Secondary · biological -
Step 5
Digestion
Major sludge enters anaerobic digesters; biogas is generated.
Sludge handling
The strength of this methodology is also its modesty: it relies on processes microbes have performed for billions of years, simply concentrated and accelerated in engineered tanks. The activated-sludge process has been practised for more than a century in almost every part of the world, and NCERT notes that no man-made technology has yet been able to rival it for treating millions of gallons of waste water every day.
The Ganga and Yamuna Action Plans
Increasing urbanisation means sewage is now produced in far larger quantities than before, but the number of sewage treatment plants has not grown enough to handle it. As a result, untreated sewage is often discharged directly into rivers, polluting them and increasing the spread of water-borne diseases. To address this, the Ministry of Environment and Forests initiated the Ganga Action Plan and the Yamuna Action Plan to save these major rivers from pollution. Under these plans it is proposed to build a large number of sewage treatment plants so that only treated sewage is discharged into the rivers.
Primary versus secondary treatment
The single distinction NEET tests most often is the difference between the two stages. Primary treatment is purely physical — it removes particles by filtration and sedimentation and produces primary sludge. Secondary treatment is biological — it removes dissolved organic matter using aerobic microbial flocs and produces activated sludge while lowering BOD. The comparison below lays the two side by side.
Primary treatment
Physical
filtration + sedimentation
- Removes large and small particles from sewage
- Floating debris removed by sequential filtration
- Grit (soil, small pebbles) removed by sedimentation
- Settled solids form the primary sludge
- Supernatant forms the effluent; no microbes involved
Secondary treatment
Biological
aerobic flocs in aeration tanks
- Removes dissolved and colloidal organic matter
- Aerated, agitated tanks grow flocs of microbes
- Microbes consume organic matter, reducing BOD
- Settled flocs form the activated sludge
- Major sludge digested anaerobically to biogas
Notice how the two sludges differ. Primary sludge is inert physical debris — grit and settled solids removed before any biology happens. Activated sludge is a living mass of bacterial flocs produced by secondary treatment. Mixing up which sludge goes where is a frequent error, so anchor it firmly: only the activated sludge has a fate involving recycling and anaerobic digestion.
Track the four products: primary sludge and effluent come out of primary treatment; activated sludge and clean effluent come out of secondary treatment.
Primary sludge
All solids that settle during primary treatment — grit and physical debris. Inert, not living.
Effluent
The supernatant above the primary sludge; carries organic matter on to secondary treatment.
Activated sludge
Sedimented bacterial flocs from secondary treatment — living microbes, partly recycled.
Biogas
Methane, hydrogen sulphide and carbon dioxide from anaerobic digestion of the sludge.
BOD — the pollution yardstick
BOD stands for biochemical oxygen demand. NCERT defines it precisely: BOD is the amount of oxygen that would be consumed if all the organic matter in one litre of water were oxidised by bacteria. The BOD test measures the rate of uptake of oxygen by micro-organisms in a sample of water, and so, indirectly, BOD is a measure of the organic matter present in that water.
The logic is straightforward. Microbes need oxygen to oxidise organic matter; the more organic matter a water sample contains, the more oxygen its microbes will consume, and therefore the higher its BOD. Because organic matter is the chief pollutant in sewage, BOD acts as a numerical stand-in for how dirty the water is. NCERT states the conclusion plainly: the greater the BOD of waste water, the more is its polluting potential.
The goal of secondary treatment
Aerobic microbes in the flocs consume the major part of the organic matter in the effluent. Less organic matter means less oxygen would be needed to oxidise it — so the BOD drops, and the water is treated only until that BOD has fallen significantly.
This is why BOD is the control parameter of the whole plant. Treatment continues in the aeration tank until the BOD is reduced; only then is the effluent moved to the settling tank. A high-BOD water released into a river would let river microbes strip dissolved oxygen from the water as they break down its organic load — which is exactly what kills fish and other aquatic life. Lowering the BOD before discharge is therefore the protective purpose behind the entire process.
Worked examples
Three water samples — river water, untreated sewage water and secondary effluent from a sewage treatment plant — were subjected to a BOD test. The values recorded were 20 mg/L, 8 mg/L and 400 mg/L. Which sample is most polluted, and which label fits which value, given that the river water is relatively clean?
BOD rises with polluting potential, so the highest BOD marks the most polluted water. The sample with 400 mg/L is the most polluted — it must be the untreated sewage water. The relatively clean river water carries the lowest organic load, so it is 20 mg/L. The secondary effluent, treated until its BOD is reduced, takes the intermediate value of 8 mg/L. Note that the treated effluent has an even lower BOD than the natural river water — a measure of how effective secondary treatment is.
In a sewage treatment plant, which material is pumped into the anaerobic sludge digester, and why is the digester anaerobic?
The major part of the activated sludge — the sedimented bacterial flocs from the settling tank — is pumped into the anaerobic sludge digester. (The small remaining part of activated sludge is returned to the aeration tank as inoculum.) The digester is kept anaerobic because the bacteria that digest the bacteria and fungi of the sludge there grow without oxygen, and it is precisely this anaerobic digestion that yields biogas — methane, hydrogen sulphide and carbon dioxide.
What exactly are flocs, and in which tank of the sewage treatment plant do they form?
Flocs are masses of bacteria associated with fungal filaments that together form mesh-like structures. They develop in the aeration tank during secondary treatment, where constant mechanical agitation and pumped-in air allow vigorous growth of useful aerobic microbes. As the microbes in the flocs grow, they consume the major part of the organic matter in the primary effluent, which is what reduces the BOD.
Common confusion & NEET traps
Sewage treatment carries a cluster of look-alike terms — two sludges, two effluents, two kinds of tanks. NEET writes stems that swap one for another and rewards students who can keep the pairs apart. The traps below mark the swaps that catch candidates most often.