Potable water quality – hardness limit: What is the maximum permissible total hardness (as CaCO3) commonly adopted for public water supplies?

Introduction / Context:
Hardness in potable water arises mainly from calcium and magnesium salts. While not a direct health hazard, it affects soap consumption, scaling in pipes and heaters, and consumer acceptability. Engineers use guideline limits to judge the need for softening.

Given Data / Assumptions:

• Total hardness is expressed as mg/litre as CaCO3.
• Question targets the upper permissible value commonly cited in exam standards.

Concept / Approach:
Typical textbook/exam practice accepts up to around 150 mg/litre as CaCO3 as a permissible level for public supplies without mandatory softening, although “desirable” limits may be lower. Above this, hardness-related issues become more evident and treatment may be considered depending on context.

Step-by-Step Solution:
Identify the parameter: total hardness as CaCO3.Recall the accepted public-supply threshold often taught: approximately 150 mg/litre.Select the matching option: 150 mg/litre.

Verification / Alternative check:
Field practice tolerates moderate hardness in many systems; customer complaints and appliance scaling increase for higher values, prompting softening or blending strategies.

Why Other Options Are Wrong:

• 95–125 mg/litre: represent lower thresholds; water at these levels is typically acceptable without softening.

Common Pitfalls:

• Confusing “desirable” versus “permissible” limits; the latter allows a higher ceiling before mandatory action.

Final Answer:
150 mg/litre