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Difficulty: Very hardPurification and Treatment of Water for Supply

A municipal water treatment facility processes raw river water containing fine colloidal clay particles, dissolved hydrogen sulfide, soluble iron(II) compounds, and harmful microorganisms. During the treatment process, the water is subjected to aeration, addition of potash alum, sedimentation, sand filtration, and chlorination. Which of the following statements correctly distinguishes the primary chemical roles of aeration and potash alum in this treatment sequence?

  1. Aeration oxidizes soluble iron(II) compounds and expels dissolved foul-smelling gases, whereas potash alum induces coagulation of fine colloidal clay particles.Answer
  2. B
    Aeration precipitates dissolved calcium tetraoxosulfate(VI) to remove permanent hardness, whereas potash alum sterilizes harmful microorganisms.
  3. C
    Aeration adjusts the pH of acidic water by dissolving atmospheric carbon(IV) oxide, whereas potash alum removes dissolved magnesium ions to soften the water.
  4. D
    Aeration kills pathogenic bacteria through dissolved oxygen toxicity, whereas potash alum neutralizes organic foul odors and tastes.

Answer

Aeration oxidizes soluble iron(II) compounds and expels dissolved foul-smelling gases, whereas potash alum induces coagulation of fine colloidal clay particles.
In municipal water purification, aeration serves to bubble air through raw water, oxidizing dissolved iron(II) salts to insoluble iron(III) precipitates and stripping out dissolved volatile gases (such as hydrogen sulfide and carbon(IV) oxide). Potash alum (hydrated potassium aluminium tetraoxosulfate(VI)) supplies aluminium ions that coagulate fine suspended colloidal particles into larger flocs that readily settle out during sedimentation.

Step-by-Step Solution

1
Analyze the primary chemical and physical effects of aeration in municipal water treatment.
Aeration increases dissolved oxygen, which oxidizes soluble Fe2+\text{Fe}^{2+} ions to insoluble Fe(OH)3\text{Fe(OH)}_3 precipitates, while simultaneously purging volatile dissolved gases such as H2S\text{H}_2\text{S} and excess CO2\text{CO}_2.
Aeration targets volatile odorous compounds and oxidizable dissolved inorganic species.
2
Analyze the role of potash alum (KAl(SO4)2⋅12H2O\text{KAl(SO}_4\text{)}_2 \cdot 12\text{H}_2\text{O}) during water purification.
Potash alum dissociates to yield Al3+\text{Al}^{3+} ions, which neutralize the negative surface charges on microscopic colloidal clay particles, allowing them to coagulate and settle during sedimentation.
Colloidal suspensions will not settle naturally under gravity without chemical coagulation.
3
Evaluate the choices to distinguish these functions from water softening and sterilization.
Sterilization is achieved by chlorination, and water softening requires removal of Ca2+\text{Ca}^{2+} or Mg2+\text{Mg}^{2+} ions via chemical precipitation or ion exchange, confirming the statement regarding oxidation/gas expulsion for aeration and coagulation for alum.
Distinguishing municipal water treatment for domestic supply (removal of turbidity and pathogens) from water softening avoids common conceptual confusion.

Key Concept

Distinct roles of aeration (gas removal and oxidation) and coagulation (potash alum flocculation) in municipal water treatment
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Topics

Purification and Treatment of Water for Supply

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