Threshold odour number (TON) – limit for public water supplies The dilution ratio at which odour is just detectable is the threshold odour number (TON). For public water supplies, what maximum TON is generally acceptable?

Introduction / Context:
Odour is a key aesthetic parameter in drinking water. The threshold odour number (TON) represents the dilution factor at which a panel can just detect an odour. Maintaining a low TON is vital to consumer acceptability and confidence in potable water.

Given Data / Assumptions:

• TON determined by standard panel dilution methods.
• Question asks for a typical maximum limit recommended for public supplies.

Concept / Approach:
Many design guides adopt a conservative TON limit to ensure that even sensitive consumers do not detect unpleasant odours. A commonly cited target is TON ≤ 3 for distributed water, reflecting high acceptability. Larger numbers indicate stronger odours and deteriorating aesthetic quality.

Step-by-Step Solution:
Recognize that lower TON values correspond to better quality.Recall typical target values for distribution systems → not exceeding 3.Select the smallest threshold consistent with standard recommendations: 3.

Verification / Alternative check:
Utilities often adopt internal objectives stricter than regulatory maxima to avoid consumer complaints; a TON of 3 is widely used as an operational criterion.

Why Other Options Are Wrong:

• 5, 7, 9, 10 indicate increasingly stronger odours and are typically unacceptable for finished water.

Common Pitfalls:
Confusing TON with taste thresholds; both are sensory but evaluated separately. Also, odour can be episodic due to algal metabolites; control requires upstream treatment, not just chlorination.

Final Answer:
3