Describe the processes used in the treatment of sewage to produce water that is safe to release into rivers.

Cambridge International IGCSE Chemistry (0620) — 10.1 Water · Describe · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Screening removes large solids and debris.
2. Sedimentation allows suspended solids to settle and form sludge.
3. Biological (aerobic) treatment uses bacteria to digest organic matter.
4. Chlorination adds chlorine to kill harmful microorganisms before release.

Examiner tips

  • Use the exact terms from the mark scheme (screening, sedimentation, biological treatment, chlorination).
  • Show each step as a separate point to match the 4 marks.
  • Keep the answer concise and avoid extra explanation.

Common mistakes

  • Using 'filtration' instead of 'screening'; confusing sedimentation with filtration.
  • Omitting the step of chlorination or not stating its purpose.

Mark scheme (4 marks)

  1. Screening / removal of large solid objects or debris
  2. Sedimentation / settlement allows suspended solids to sink / form sludge
  3. Biological / aerobic treatment where bacteria break down / digest organic matter
  4. Chlorination / addition of chlorine to kill harmful microorganisms / bacteria before release

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