Drinking-Water Quality (BIS) — What is the maximum permissible turbidity limit for potable water (in the absence of an alternate source), expressed in NTU?

Introduction:Turbidity reflects light-scattering by suspended particles and colloids. It is a key aesthetic and operational parameter in drinking-water treatment because high turbidity can protect microorganisms from disinfection and degrade consumer acceptability. Standards specify both desirable and permissible limits to guide utilities.

Given Data / Assumptions:

• BIS (IS 10500) specifies a desirable turbidity of about 1 NTU and a higher permissible value when no alternative source is available.
• Units are Nephelometric Turbidity Units (NTU).
• Question asks for the maximum permissible limit in typical practice.

Concept / Approach:

Where the desirable target (1 NTU) cannot be met, the standard allows a maximum permissible limit of 5 NTU. Water meeting this limit is generally clear to consumers and amenable to effective disinfection, though utilities are encouraged to achieve the lower desirable level whenever feasible.

Step-by-Step Solution:

Verification / Alternative check:

Utility monitoring and regulatory documents commonly reference 1 NTU (desirable) and 5 NTU (permissible) for treated water entering the distribution system.

Why Other Options Are Wrong:

B–D exceed the permissible level and would be considered substandard. E reflects the desirable target, not the maximum permissible limit referenced in the question.

Common Pitfalls:

Confusing raw-water turbidity (often much higher) with treated-water limits; always apply standards to the correct sampling point.

Final Answer:

5 NTU