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CraftExam
medium difficulty9 min

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.

In one line: Primary removes particles; secondary uses microbes; sludge digestion makes biogas-like gases.

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Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Question: Explain the role of flocs in sewage treatment.

  1. 1Answer: Flocs are masses of aerobic bacteria associated with fungal filaments.
  2. 2They consume organic matter in aerated sewage and help reduce BOD during secondary treatment.

Method you can reuse

  1. 1First write primary treatment as physical removal of solids.
  2. 2Then describe aeration and floc formation.
  3. 3Next explain oxidation of organic matter and BOD reduction.
  4. 4Finally mention settling of flocs, return of activated sludge and anaerobic digestion of sludge.

Visual tip

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does BOD decrease during secondary sewage treatment?

Think first, then show answer
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.

Common wrong answer

BOD increases after successful secondary treatment because more microbes are present; this.

Why it loses marks: The treatment reduces organic matter, so the oxygen demand of the effluent falls.

Don't mix this up

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.

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

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.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    Sewage contains organic matter and many microbes, so it must be treated before discharge.

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