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Microbes in Sewage Treatment

Microbes in sewage treatment are mainly aerobic and anaerobic microorganisms used to reduce organic waste, lower biochemical oxygen demand and digest sludge before treated water is released.

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Student-friendly explanation

Sewage contains organic matter and many microbes, so it must be treated before discharge. Primary treatment physically removes floating and suspended solids by filtration and sedimentation. Secondary treatment is biological: aeration encourages aerobic microbes to form flocs, which consume organic matter and reduce BOD. A part of activated sludge is returned as inoculum, while the remaining sludge is sent to anaerobic digesters. Anaerobic bacteria digest sludge and produce gases such as methane, carbon dioxide and hydrogen sulphide. CBSE often asks why BOD falls and why untreated sewage is harmful to water bodies.

How to write this in exams

  1. 1

    Start with the exact idea

    Microbes in sewage treatment are mainly aerobic and anaerobic microorganisms used to reduce organic waste, lower biochemical oxygen demand and digest sludge before treated water is released.

  2. 2

    Then show how to use it

    First write primary treatment as physical removal of solids. Then describe aeration and floc formation. Next explain oxidation of organic matter and BOD reduction. Finally mention settling of flocs, return of activated sludge and anaerobic digestion of sludge.

  3. 3

    Add one concrete example

    In an aeration tank, microbial flocs consume organic matter from sewage. As the organic load decreases, the biochemical oxygen demand of the effluent becomes lower, making it safer for release after further checks.

  4. 4

    Avoid this incomplete answer

    BOD increases after successful secondary treatment because more microbes are present; this is wrong because the treatment reduces organic matter, so the oxygen demand of the effluent falls.

Definition

Microbes in sewage treatment are mainly aerobic and anaerobic microorganisms used to reduce organic waste, lower biochemical oxygen demand and digest sludge before treated water is released.

Example

In an aeration tank, microbial flocs consume organic matter from sewage. As the organic load decreases, the biochemical oxygen demand of the effluent becomes lower, making it safer for release after further checks.

Rule to remember

Key rule: high organic matter causes high BOD; microbial oxidation during secondary treatment lowers organic load and therefore lowers BOD.

Memory hook

Primary removes particles; secondary uses microbes; sludge digestion makes biogas-like gases.

Examples and method

Worked example

Question: Explain the role of flocs in sewage treatment. Answer: Flocs are masses of aerobic bacteria associated with fungal filaments. They consume organic matter in aerated sewage and help reduce BOD during secondary treatment.

Method to apply

First write primary treatment as physical removal of solids. Then describe aeration and floc formation. Next explain oxidation of organic matter and BOD reduction. Finally mention settling of flocs, return of activated sludge and anaerobic digestion of sludge.

Diagram support

Important labels: primary settling tank, aeration tank, microbial flocs, settling tank, activated sludge return, anaerobic sludge digester and treated effluent outlet. Notice the return of activated sludge and separate digestion of sludge.

How CBSE asks it

Asked as a flow-chart completion, diagram labelling, reason for BOD reduction, difference between primary and secondary treatment, or role of activated sludge.

Avoid common mistakes

Common confusion

Students often write that primary treatment kills microbes. Primary treatment is physical removal of solids, while major organic matter reduction occurs during biological secondary treatment.

Common wrong answer

BOD increases after successful secondary treatment because more microbes are present; this is wrong because the treatment reduces organic matter, so the oxygen demand of the effluent falls.

Exam tip

Use the terms flocs, activated sludge, aeration tank, BOD reduction and anaerobic digester in a sequence-based answer.

Quick check

Why does BOD decrease during secondary sewage treatment?

BOD decreases because aerobic microbes in flocs oxidise and consume organic matter present in sewage. With less organic matter left, less oxygen is required for microbial decomposition.

Answer writing and exam use

1-mark answer

Define sewage treatment briefly, list primary physical treatment, explain secondary treatment with aeration, flocs and BOD reduction, then add activated sludge settling and anaerobic digestion of sludge.

2-mark answer

Define sewage treatment briefly, list primary physical treatment, explain secondary treatment with aeration, flocs and BOD reduction, then add activated sludge settling and anaerobic digestion of sludge.

3-mark answer

Define sewage treatment briefly, list primary physical treatment, explain secondary treatment with aeration, flocs and BOD reduction, then add activated sludge settling and anaerobic digestion of sludge.
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