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Difficulty: MediumPurification and Treatment of Water for Supply

During the municipal treatment of river water containing dissolved iron(II) salts and unpleasant odors caused by dissolved hydrogen sulfide gas, raw water is initially sprayed into the air during an aeration process. What is the primary chemical purpose of this aeration step prior to coagulation?

  1. To oxidize soluble iron(II) compounds into insoluble iron(III) hydroxide while expelling volatile dissolved gasesAnswer
  2. B
    To destroy pathogenic bacteria and harmful microorganisms present in the raw water supply
  3. C
    To precipitate calcium and magnesium hydrogen trioxocarbonates(IV) causing temporary water hardness
  4. D
    To cause fine colloidal clay suspensions to aggregate into larger settleable flocs

Answer

Aeration oxidizes soluble iron(II) compounds into insoluble iron(III) hydroxide and expels volatile dissolved gases from the water.
Aeration introduces atmospheric oxygen into raw water, which oxidizes soluble iron(II) compounds into an insoluble iron(III) hydroxide precipitate (Fe(OH)3Fe(OH)_3). Additionally, the splashing action strips out volatile gases responsible for bad tastes and odors, such as hydrogen sulfide (H2SH_2S).

Step-by-Step Solution

1
Identify the chemical changes occurring when air contacts raw water during aeration.
Dissolved atmospheric oxygen reacts with dissolved Fe2+Fe^{2+} ions, while volatile gases like H2SH_2S escape into the atmosphere.
Aeration increases dissolved oxygen concentration and promotes liquid-gas equilibrium exchange.
2
Determine the resulting chemical products formed.
Soluble iron(II) ions form insoluble iron(III) hydroxide, Fe(OH)3Fe(OH)_3, precipitate.
Insoluble precipitates formed during aeration can subsequently be settled and removed during sedimentation and filtration.

Key Concept

Aeration in Municipal Water Purification
Estimated Time:1m 0s
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Topics

Purification and Treatment of Water for Supply

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