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Difficulty: MediumPurification and Treatment of Water for Supply

Arrange the following sequential processing stages involved in the treatment of raw river water for municipal supply in the correct order from initial treatment to final distribution.

  1. 1Aeration of raw water to expel volatile gases and oxidize soluble iron and manganese compounds.
  2. 2Addition of potash alum to aggregate fine colloidal impurities into gelatinous flocs.
  3. 3Gravitational settling of coagulated flocs at the bottom of a large basin.
  4. 4Passage of water through coarse and fine sand beds to remove microscopic suspended solids.
  5. 5Disinfection using chlorine gas or calcium hypochlorite to eliminate pathogenic microorganisms.

Answer

The correct sequence for municipal water purification is: Aeration → Coagulation (addition of alum) → Sedimentation → Sand Filtration → Chlorination (Disinfection).
Municipal water treatment follows a logical progression designed to clear large and dissolved impurities first before fine physical filtering and final chemical disinfection. Aeration removes volatile gases and oxidizes metals; coagulation aggregates colloidal particles using alum; sedimentation settles the heavy flocs; sand filtration removes micro-particles; and chlorination destroys disease-causing microorganisms as the final safeguard before piping to households.

Step-by-Step Solution

1
Identify the initial physical treatment step for raw water.
Aeration is performed first to remove volatile odors/gases and oxidize dissolved iron salts into insoluble forms.
Air exposure improves taste and prepares dissolved metals for precipitation.
2
Determine the step that precipitates fine colloidal particles.
Coagulation with potash alum follows aeration.
Alum causes fine, non-settling colloidal particles to aggregate into larger flocs.
3
Identify how the aggregated flocs are removed bulk-wise.
Sedimentation allows these heavy flocs to settle to the basin floor.
Gravity settling removes the bulk of suspended solids before filtration.
4
Identify the step that removes residual microscopic suspended solids.
Sand filtration passes clear water through sand and gravel beds.
Filtration traps any residual suspended particles that escaped sedimentation.
5
Identify the final biological safety step prior to distribution.
Chlorination is carried out as the final step.
Sterilization must occur after physical clarity is achieved so chlorine acts effectively on pathogenic bacteria.

Key Concept

Sequential Stages of Municipal Water Treatment
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Topics

Purification and Treatment of Water for Supply

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