Difficulty: Easy
Correct Answer: 5 NTU
Explanation:
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:
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
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