The minimum recommended level of residual chlorine in the drinking water is for one hour
## **Core Concept**
The question pertains to the disinfection of drinking water, specifically focusing on the residual chlorine level required to ensure water safety. Chlorine is widely used for water treatment due to its effectiveness against a broad spectrum of pathogens. The residual chlorine level is a critical parameter as it indicates the amount of chlorine left in the water after the initial disinfection process, ensuring continued protection against microbial contamination.
## **Why the Correct Answer is Right**
The correct answer, **0.5 mg/L**, is recommended by various health organizations, including the World Health Organization (WHO), as the minimum level of free residual chlorine in drinking water to ensure it remains safe from microbial contamination. This level is considered sufficient to protect against a wide range of pathogens for a specified contact time, typically at least one hour. The mechanism behind chlorine's effectiveness involves its ability to denature proteins and disrupt cell membranes of microorganisms, thereby killing them.
## **Why Each Wrong Option is Incorrect**
- **Option A: 0.1 mg/L** - This level is too low to provide adequate protection against all pathogens. Levels below 0.5 mg/L may not be sufficient to inactivate certain viruses and protozoa.
- **Option B: 1.0 mg/L** - While 1.0 mg/L is above the minimum recommended level and provides a higher margin of safety, it is not the minimum required level.
- **Option D: 2.0 mg/L** - This is well above the minimum required level and, although it offers an increased safety margin, it is not necessary for all drinking water applications.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the presence of residual chlorine in drinking water not only ensures microbial safety but also provides a residual effect that protects the water distribution system from re-contamination. The WHO and other health organizations emphasize monitoring residual chlorine levels as part of water quality assessment.
## **Correct Answer: C. 0.5 mg/L.**