Microbes in Sewage Treatment — Revision Notes

⚡ 30-Second Revision

Sewage — Wastewater from domestic/industrial sources.
Primary Treatment — Physical removal (screening, sedimentation). Removes large solids, some suspended solids. Modest BOD reduction.
Secondary Treatment — Biological treatment. Microbes degrade organic matter.

- Aeration Tanks: Aerobic microbes (bacteria, fungi) form flocs. - Flocs: Consume organic matter, significantly reduce BOD (85-95%). - Activated Sludge: Settled flocs. A portion recycled to aeration tanks.

Anaerobic Sludge Digesters — Excess activated sludge treated by anaerobic microbes.

- Produce biogas ($CH_4$, $CO_2$, $H_2S$).

BOD — Biological Oxygen Demand. Measure of organic pollution. High BOD = high pollution.
Effluent — Treated water discharged.
Sludge — Solid waste from treatment.

2-Minute Revision

Sewage treatment is vital to prevent water pollution and disease. It begins with Primary Treatment, a physical process where large debris is screened out, and suspended solids settle in sedimentation tanks, forming primary sludge.

The liquid, primary effluent, then moves to Secondary Treatment, the biological stage. Here, in large aeration tanks, air is pumped in to foster the growth of aerobic microbes. These microbes, primarily bacteria and fungi, aggregate into flocs, which consume the dissolved organic matter in the sewage.

This microbial action drastically reduces the Biological Oxygen Demand (BOD) of the wastewater, typically by 85-95%. After aeration, the flocs settle in secondary settling tanks, forming activated sludge.

A small, crucial portion of this activated sludge is recycled back to the aeration tanks to inoculate incoming sewage, ensuring continuous and efficient microbial activity. The remaining excess activated sludge is transferred to anaerobic sludge digesters, where anaerobic microbes break down the organic matter, producing biogas (mainly methane and carbon dioxide), a valuable energy source.

The treated water, or effluent, is then disinfected and safely discharged.

5-Minute Revision

Sewage, or wastewater, is a complex mixture of organic and inorganic pollutants, along with pathogens, that requires treatment before discharge. This process is broadly divided into primary, secondary, and tertiary stages, with microbes playing a central role in the latter two.

1. Primary Treatment (Physical): - Screening: Removes large floating debris (rags, sticks). - Grit Removal: Settles heavier inorganic solids (sand, grit). - Primary Settling Tanks: Suspended organic solids settle as primary sludge. The supernatant is primary effluent. This stage reduces BOD by about 25-35%.

2. Secondary Treatment (Biological): This is where microbial action is paramount. - Aeration Tanks: Primary effluent is pumped into these tanks. Air is continuously supplied and the mixture is agitated.

This creates an aerobic environment, ideal for the growth of aerobic microorganisms (bacteria and fungi). - Flocs Formation: These aerobic microbes grow in masses called flocs, which are aggregates of bacteria associated with fungal filaments.

The flocs actively consume the dissolved organic matter in the sewage. - BOD Reduction: As organic matter is consumed, the Biological Oxygen Demand (BOD) of the wastewater is significantly reduced (typically 85-95%).

A lower BOD indicates less organic pollution. - Secondary Settling Tanks: After aeration, the wastewater with flocs is moved here. The flocs settle by gravity, forming activated sludge. - Activated Sludge Recycling: A small, vital portion of the activated sludge, rich in active microbes, is pumped back into the aeration tanks to inoculate the incoming primary effluent.

This ensures a continuous supply of efficient microbes and maintains the treatment process's speed and effectiveness.

3. Sludge Treatment (Anaerobic Digestion): - The large volume of excess activated sludge is transferred to anaerobic sludge digesters. These are closed tanks where anaerobic microbes (which thrive without oxygen) break down the complex organic polymers in the sludge.

- Biogas Production: This anaerobic digestion process produces biogas, a mixture primarily of methane ($CH_4$), carbon dioxide ($CO_2$), and trace amounts of hydrogen sulfide ($H_2S$). Methane is a valuable combustible gas used as a renewable energy source.

4. Tertiary Treatment (Optional): - Further purification (e.g., nutrient removal, disinfection) if higher water quality is required. - Disinfection: The treated water (effluent) is disinfected (e.g., with chlorine, UV) to kill remaining pathogens before discharge into natural water bodies.

Key Takeaways: Microbes are the workhorses, converting harmful organic pollutants into simpler, less toxic substances and even generating energy (biogas). Understanding BOD is crucial for assessing treatment efficiency and environmental impact.

Prelims Revision Notes

Microbes in Sewage Treatment: NEET Revision Notes

1. What is Sewage?

Domestic wastewater containing organic matter, inorganic substances, and microorganisms (pathogenic & non-pathogenic).
High BOD (Biological Oxygen Demand) indicates high organic pollution.

2. Stages of Treatment:

Primary Treatment (Physical Process):

* Goal: Remove large floating debris and settleable solids. * Steps: 1. Screening: Removes large objects (rags, sticks) using bar screens. 2. Grit Removal: Settles sand, grit, small pebbles in grit chambers. 3. Primary Settling Tanks (Sedimentation): Suspended solids settle by gravity, forming primary sludge. The supernatant is primary effluent. * BOD Reduction: Modest, typically 25-35%. * Microbial Role: Minimal direct active role in removal.

Secondary Treatment (Biological Process):

* Goal: Significantly reduce BOD by microbial degradation of dissolved organic matter. * Steps: 1. Aeration Tanks: Primary effluent is pumped in. Air is continuously supplied, and the mixture is agitated.

2. Microbial Action: Aerobic microbes (bacteria, fungi) grow rapidly in this oxygen-rich environment. 3. Flocs Formation: Microbes form masses called flocs (bacteria associated with fungal filaments).

Flocs consume organic matter. 4. BOD Reduction: Highly effective, typically 85-95% reduction. 5. Secondary Settling Tanks: Wastewater with flocs moves here. Flocs settle, forming activated sludge.

6. Activated Sludge Recycling: A small portion of activated sludge is pumped back to aeration tanks as inoculum for incoming effluent. 7. Excess Activated Sludge: Remaining sludge sent for anaerobic digestion.

Sludge Treatment (Anaerobic Digestion):

* Location: Large, closed anaerobic sludge digesters. * Process: Anaerobic microbes break down organic matter in the absence of oxygen. * Product: Biogas – a mixture of gases (primarily $CH_4$ (50-75%), $CO_2$ (25-45%), trace $H_2S$). Biogas is a renewable energy source. * Digested Sludge: Reduced in volume, dewatered, can be used as fertilizer.

3. Key Terms & Concepts:

BOD (Biological Oxygen Demand): — Amount of dissolved oxygen consumed by aerobic microbes to decompose organic matter. High BOD = more pollution.
Flocs: — Aggregates of aerobic bacteria and fungal filaments in aeration tanks.
Activated Sludge: — Settled microbial flocs, rich in active aerobic microbes, recycled to aeration tanks.
Biogas: — Gaseous product of anaerobic digestion, primarily methane.

4. Environmental Significance:

Prevents water pollution (eutrophication, oxygen depletion).
Protects aquatic life.
Prevents spread of waterborne diseases.
Generates renewable energy (biogas).

Vyyuha Quick Recall

To remember the stages of sewage treatment: Please Stop All Sludge Disposal.

Primary Treatment
Secondary Treatment
Aeration (in Secondary Treatment)
Settling (Activated Sludge)
Digestion (Anaerobic Sludge Digester)