Water Quality Parameters MCQ with answer and solution

Concept

Residual Chlorine in Drinking Water

Formula

N/A

Step-by-step solution

1. Review the drinking water quality standards for residual chlorine. 2. Recall that free residual chlorine must be at least 0.2 ppm for proper disinfection against bacteria. 3. Identify Option (b) as incorrect, making it the correct choice.

Explanation

Option B is not correct because to safely protect drinking water against pathogenic bacteria, the residual chlorine level should be maintained at a minimum of 0.2 ppm (typically between 0.2 ppm and 0.5 ppm or up to 1.0 ppm). A range starting as low as 0.05 ppm is insufficient to ensure proper disinfection.

Shortcut

To remember safe residual chlorine levels in drinking water, use the rule "0.2 is the Minimum Safe Limit": - Minimum Level: Free residual chlorine must be at least 0.2 ppm (typically 0.2 to 0.5 ppm) to kill pathogenic bacteria. - Danger Zone: Levels below 0.2 ppm (such as 0.05 ppm) are insufficient to ensure proper disinfection.

Common mistake

A common mistake is selecting option A by incorrectly assuming that chlorides in water must strictly be under 250 ppm and treating option A as incorrect. In reality, chlorides are often permitted up to 500 ppm or higher depending on local standards/cause for rejection. Another frequent mistake is overlooking option B's dangerously low range of 0.05 ppm, assuming any presence of chlorine is sufficient, whereas safe residual chlorine must be at least 0.2 ppm to protect against pathogenic bacteria.