Residual Chlorine after chlorination of water should be?

Appeared in: NIMHANS Nursing Officer - 2019

Explanation

The correct standard for water disinfection is to have a free residual chlorine level of at least 0.5 mg/L.
This concentration must be measured after a minimum contact time of 30 minutes (1/2 hour) to ensure effective disinfection.
This standard is sufficient to inactivate most harmful microorganisms, including bacteria and viruses, ensuring the water is safe for consumption.
Maintaining this residual level also provides a margin of safety, protecting the water from potential re-contamination as it travels through the distribution system.

Option A: This represents an unnecessarily high concentration and an overly long contact time for routine water disinfection. It is not cost-effective and can lead to complaints about taste and odor.
Option B: A concentration of 1.0 mg/L is higher than the routine requirement. This level is typically used for super-chlorination, which is a response to heavy contamination or during an epidemic of waterborne disease.
Option C: While the concentration of 0.5 mg/L is correct, the standard contact time for testing is 30 minutes, not one hour. Waiting an hour is inefficient, as effective disinfection is achieved earlier.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Water disinfection standards as background academic context rather than a clinical decision trigger.
Nurses, particularly in community and public health settings, must be knowledgeable about safe water standards to prevent outbreaks of waterborne diseases like cholera, typhoid, dysentery, and hepatitis A.
During disaster relief or in areas with questionable water quality, nurses may be responsible for teaching community members how to properly chlorinate and test their water.
What if? If a community reports a cholera outbreak, the nursing action would be to recommend increasing the residual chlorine target, possibly to 1.0 mg/L after 30 minutes (super-chlorination), to ensure rapid and thorough disinfection of the water supply.

How to Approach the Question

Identify the core of the question: it's asking for a specific, standard value related to public health.
Recognize that both concentration (mg/L) and contact time (hours/minutes) are critical components of the answer.
Recall the standard guideline for water chlorination. The key numbers are '0.5' and '30' (or '1/2').
Evaluate each option against this standard. Option A has the wrong concentration and time. Option B has the wrong concentration. Option C has the wrong time.
Select the option that matches both the correct concentration (0.5 mg/L) and the correct minimum contact time (1/2 hour).

Concept Tested & Keywords

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E6 Parks TextBook of Preventive & Social Medicine part 2 — Subpart B (pp 233-449 of 464) pp. 38-40, 32-34

Practise the full NIMHANS Nursing Officer - 2019

Attempt every question from this paper in a timed mock, then review the full solution for each one.