Microbes in Sewage Treatment — Revision Notes

⚡ 30-Second Revision

Sewage: — Wastewater with organic matter, pathogens.
Primary Treatment: — Physical removal (screening, grit, sedimentation). No microbes.
Secondary Treatment: — Biological removal. Aeration tanks $\rightarrow$ aerobic microbes (flocs) $\rightarrow$ consume organic matter $\rightarrow$ reduce BOD.
BOD: — Biological Oxygen Demand. High BOD = high pollution. Goal: reduce BOD.
Flocs: — Masses of aerobic bacteria + fungal filaments.
Activated Sludge: — Settled flocs. Part recycled to aeration tank (inoculum).
Anaerobic Sludge Digester: — Anaerobic microbes (methanogens) break down sludge.
Biogas: — Product of anaerobic digestion. Major components: $CH_4$ (methane), $CO_2$. Used as fuel.

2-Minute Revision

Sewage treatment is vital to clean wastewater using microbes. It starts with Primary Treatment, a physical process involving screening, grit removal, and sedimentation to remove large solids and grit.

This stage yields primary effluent and primary sludge, with minimal microbial involvement. The primary effluent then moves to Secondary Treatment, the biological core. Here, in large aeration tanks, aerobic microbes (bacteria and fungi) grow rapidly, forming flocs.

These flocs actively consume the dissolved organic matter, significantly reducing the Biological Oxygen Demand (BOD) of the wastewater. A high BOD indicates high organic pollution. After aeration, the flocs settle, forming activated sludge.

A crucial step is recycling a portion of this activated sludge back to the aeration tanks to provide a fresh inoculum of active microbes. The remaining primary and excess activated sludge are then transferred to anaerobic sludge digesters.

In these oxygen-free tanks, anaerobic bacteria (methanogens) break down the organic matter, producing biogas, a valuable fuel primarily composed of methane ($CH_4$) and carbon dioxide ($CO_2$). This entire microbial-driven process transforms hazardous waste into environmentally safe water and useful energy.

5-Minute Revision

Mastering microbes in sewage treatment for NEET requires understanding the sequential stages and the specific microbial roles. Raw sewage, a complex mix of organic waste and pathogens, first undergoes Primary Treatment. This is a physical process: large debris is removed by screening, grit settles in grit chambers, and suspended solids settle in primary settling tanks, forming primary sludge. The liquid, primary effluent, still has high organic content and BOD.

Next is Secondary Treatment, the biological heart of the process. Primary effluent enters aeration tanks where air is continuously pumped in. This aerobic environment fosters the growth of aerobic heterotrophic microorganisms, mainly bacteria and fungi.

These microbes aggregate into flocs, which are masses of bacteria held together by fungal filaments. These flocs are the workhorses, consuming the dissolved and colloidal organic matter, converting it into $CO_2$, water, and new microbial biomass.

This activity drastically reduces the Biological Oxygen Demand (BOD) – the amount of oxygen microbes need to decompose organic matter. A high BOD signifies high pollution; secondary treatment aims for a 85-95% BOD reduction.

For example, if raw sewage has a BOD of $300,\text{mg/L}$, secondary treatment reduces it to $30,\text{mg/L}$ or less.

After aeration, the mixture flows into a secondary settling tank, where the flocs settle as activated sludge. The clear water, secondary effluent, is then discharged. A vital step is recycling a small portion of the activated sludge back to the aeration tank.

This 'inoculum' ensures a continuous supply of active, efficient microbes for the incoming primary effluent. The remaining primary sludge and excess activated sludge are then pumped into anaerobic sludge digesters.

These are large, closed tanks where anaerobic bacteria, particularly methanogens (e.g., *Methanobacterium*), break down the organic matter in the absence of oxygen. This process yields biogas, a mixture predominantly of methane ($CH_4$, 50-75%) and carbon dioxide ($CO_2$, 25-45%), along with trace $H_2S$.

Methane is a valuable fuel, often used to power the STP itself, making the process sustainable. This comprehensive microbial action is essential for environmental protection and resource recovery.

Prelims Revision Notes

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Sewage Composition: — Domestic wastewater containing organic matter, pathogens, nutrients. High BOD.
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Purpose of Treatment: — Reduce pollution, pathogens, BOD before discharge.
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Primary Treatment (Physical):

* Screens: Remove large floating debris (rags, sticks). * Grit Chambers: Remove grit (sand, pebbles) by reducing flow velocity. * Primary Settling Tanks: Sedimentation of suspended organic solids to form primary sludge. Scum (oil, grease) floats. * Output: Primary effluent (still high BOD), primary sludge. * Microbial Role: Minimal.

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Secondary Treatment (Biological):

* Aeration Tanks: Primary effluent + air pumped in $\rightarrow$ aerobic environment. * Microbes: Aerobic heterotrophic bacteria and fungi grow rapidly. * Flocs: Masses of bacteria associated with fungal filaments.

Consume organic matter. * BOD Reduction: Significant (85-95%). BOD is oxygen required by microbes to decompose organic matter. High BOD = high pollution. * Secondary Settling Tanks: Flocs settle $\rightarrow$ activated sludge.

Clear water $\rightarrow$ secondary effluent (low BOD). * Activated Sludge Recycling: Part of activated sludge pumped back to aeration tank as inoculum for fresh primary effluent.

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Sludge Treatment:

* Anaerobic Sludge Digesters: Primary sludge + excess activated sludge. * Conditions: Anaerobic (no oxygen). * Microbes: Anaerobic bacteria, especially methanogens (e.g., *Methanobacterium*, *Methanococcus*). * Process: Break down organic matter. * Product: Biogas ($CH_4$ 50-75%, $CO_2$ 25-45%, trace $H_2S$). Methane is combustible fuel.

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Tertiary Treatment (Optional): — Advanced purification (filtration, disinfection) for specific reuse/discharge.
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Key Microbes: — Aerobic bacteria/fungi (flocs), Anaerobic methanogens (biogas).
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Environmental Impact: — Untreated sewage causes oxygen depletion (due to high BOD), kills aquatic life, spreads diseases. Treated sewage is safe.

Vyyuha Quick Recall

To remember the key stages and microbial roles: Primary Secondary Anaerobic Biogas.

Primary: Physical removal (screens, grit, settling).
Secondary: Super microbes (aerobic flocs) eat Sewage, reducing Significant BOD.
Anaerobic: Absence of oxygen, Anaerobic bacteria (methanogens) digest sludge.
Biogas: Byproduct of anaerobic digestion, Burnable fuel ($CH_4$).

Think of it as: People Should Always Be clean!