Drinking water quality – maximum permissible colour (Cobalt–Platinum scale) For domestic water supplies, what is the maximum permissible colour value on the Cobalt–Platinum (Pt–Co) colour scale, assuming no alternate source is available?

Introduction / Context:
Colour in drinking water is an important aesthetic parameter that affects consumer acceptance and can indicate the presence of natural organic matter, corrosion products, or industrial contaminants. Standards often specify a desirable limit and a maximum permissible limit for colour measured on the Cobalt–Platinum (Pt–Co) scale.

Given Data / Assumptions:

• Measurement uses the standard Pt–Co (Cobalt–Platinum) colour scale.
• Question asks for maximum permissible value used in many public water supply standards when no better source is available.
• Domestic (potable) supplies are intended.

Concept / Approach:
Typical guidance distinguishes between a desirable limit (often about 5 Pt–Co units) and a permissible limit in the absence of an alternate source (commonly about 15 Pt–Co units). Above the permissible limit, water can appear visibly coloured, affecting acceptability and potentially interacting with disinfection by-product formation if associated with organics.

Step-by-Step Solution:
Identify the standard metric: Pt–Co units (sometimes called Hazen units).Recall typical limits: desirable ≈ 5, permissible (in absence of alternate) ≈ 15.Select the option matching the maximum permissible value: 15 Pt–Co units.

Verification / Alternative check:
Many national standards and design manuals list colour desirable 5 and permissible 15 Pt–Co units. Utilities strive to be at or below the desirable value to minimize consumer complaints.

Why Other Options Are Wrong:

• 5 and 10: These reflect desirable/stricter targets but not the upper permissible limit.
• 20 and 25: These exceed typical permissible limits and are likely to be objectionable.

Common Pitfalls:
Confusing “desirable” with “permissible in absence of alternate source.” Also, mixing up Pt–Co colour with turbidity; they are different parameters measured by different methods.

Final Answer:
15 PPM (Pt–Co units)