Aesthetics in potable water: What is the maximum permissible Threshold Odour Number (TON) typically specified for public water supplies?

Introduction / Context:
Beyond safety, drinking water must be aesthetically acceptable. Threshold Odour Number (TON) quantifies odour detectability by dilution and is used to indicate consumer acceptance and the absence of nuisance compounds from algae, industrial discharges, or disinfection by-products.

Given Data / Assumptions:

• Standard TON test method (serial dilution until odour is just perceptible).
• Public supply guidelines for finished water.

Concept / Approach:

Most guidelines accept a TON not exceeding about 3 for potable supplies. Higher odour numbers can prompt complaints and signal treatment deficiencies (e.g., ineffective activated carbon, manganese issues, or reservoir stratification impacts).

Step-by-Step Solution:

Verification / Alternative check:

Utilities track taste-and-odour events; maintaining TON around or below 3 aligns with good consumer acceptability and many national standards/handbooks.

Why Other Options Are Wrong:

• TON 1–2: While better aesthetically, these are not common upper limits.
• TON 4–5: Usually considered objectionable; may breach guideline targets.

Common Pitfalls:

• Treating odour control only at the plant and ignoring distribution issues (biofilm, dead-ends).
• Confusing TON with threshold flavour number; both are related but distinct measures.

Final Answer:

3.