In water treatment, aeration is primarily carried out to remove which of the following from raw water?

Introduction:
Aeration is a unit operation where water is brought into intimate contact with air to strip volatile gases and oxidize certain dissolved constituents. It improves aesthetic quality and prepares water for downstream processes.

Given Data / Assumptions:

• Options list odour, colour, bacteria, hardness, turbidity.
• We seek the principal target of aeration in conventional treatment.

Concept / Approach:
Aeration removes or reduces volatile compounds (like H2S) and dissolved gases (CO2), thereby eliminating odours and correcting corrosivity (via CO2 stripping). It can also oxidize ferrous iron and manganous ions to forms more amenable to subsequent removal. It is not a disinfectant, softener, or clarifier by itself.

Step-by-Step Solution:
List typical aeration objectives: remove odorous gases (H2S), strip CO2, raise DO, oxidize Fe/Mn. Map each option: odour is directly addressed by stripping; colour from humic substances needs coagulation/adsorption; bacteria require disinfection; hardness requires softening; turbidity needs coagulation–flocculation–sedimentation/filtration. Select odour as the best single answer.

Verification / Alternative check:
Plant practice shows marked odour reduction after cascade or diffused aeration, even before coagulation.

Why Other Options Are Wrong:

• Colour: Only marginally affected; primary removal is via coagulation/activated carbon.
• Bacteria: Aeration is not a disinfecting step.
• Hardness: Requires precipitation/ion exchange, not aeration.
• Turbidity: Needs clarification and filtration.

Common Pitfalls:

• Confusing increased dissolved oxygen with microbial kill; DO does not equal disinfection.
• Expecting aeration to remove all tastes/colours regardless of cause.

Final Answer:
Odour.