Drinking water aesthetics – colour limit What is the permissible colour for domestic water supply, expressed as ppm on the platinum–cobalt (Hazen) colour scale?

Introduction / Context:
Besides safety, drinking water must meet aesthetic standards so that consumers accept it. Colour is an important aesthetic parameter caused by dissolved organics (humic substances), metals, or pipe corrosion products. Regulatory and guideline documents typically express colour using the platinum–cobalt (Pt–Co) or Hazen colour scale, historically reported as “ppm” or “true colour units.”

Given Data / Assumptions:

• Domestic (potable) supply after conventional treatment.
• Colour reported on the Pt–Co (Hazen) scale in ppm units.
• Question targets a typical permissible limit used in practice and standards.

Concept / Approach:
Most drinking water standards and design manuals recommend that the true colour be low enough to be essentially unnoticeable. A widely cited threshold for acceptability is about 15–20 units on the Pt–Co scale, with many texts stating 20 ppm (Hazen units) as a practical upper limit for distribution systems. Thus, among the options offered, 20 ppm aligns with customary permissible levels for domestic supply aesthetics.

Step-by-Step Solution:

Verification / Alternative check:
Water quality handbooks commonly reference 15 units as desirable and 20 units as a permissible level; utilities often design to keep colour comfortably below this ceiling.

Why Other Options Are Wrong:

• 1 or 5: Very stringent; achievable, but the question asks the permissible limit, not the ideal.
• 100 or 1000: Excessive colour; visibly unacceptable to consumers.

Common Pitfalls:
Confusing “ppm” on the Pt–Co scale with mass concentration; Hazen units are based on a standard colour solution rather than direct mg/L of a specific compound.

Final Answer:
20