What is the minimum contact time required for the chlorination of water to take place?

Appeared in: AIIMS Patna NO - 2020

Explanation

The standard minimum contact time required for chlorine to effectively disinfect water is 30 minutes.
This duration allows sufficient time for chlorine to kill or inactivate most common waterborne pathogens, such as bacteria and viruses.
Effective disinfection also requires a free residual chlorine level of at least 0.5 mg/L after this contact period to protect against recontamination.
Chlorine's effectiveness is optimal when the water is clear (low turbidity) and the pH is below 8.0.

Option A: 10 minutes is insufficient time for chlorine to reliably kill or inactivate all targeted pathogens, leaving the water potentially unsafe.
Option B: While 90 minutes would certainly be an effective contact time, it is much longer than the required minimum. The question asks for the minimum time needed.
Option C: While a 60-minute (one hour) contact time is often recommended to ensure a higher margin of safety and is used for calculating chlorine demand, it is not the absolute minimum time required for disinfection.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Water Purification and Disinfection as background academic context rather than a clinical decision trigger.
Nurses are key educators in community health settings for preventing waterborne diseases like cholera, typhoid, and dysentery. Teaching the '30-minute rule' for chlorination is a simple, life-saving intervention.
During disaster relief (e.g., floods, earthquakes), nurses often manage or advise on emergency water supplies, where proper chlorination is critical for public health.
What if the water is cloudy or turbid? The nurse should advise filtering the water through a clean cloth first before adding chlorine, as turbidity can protect germs from the disinfectant. The contact time may also need to be extended.

How to Approach the Question

First, identify the core of the question: it's asking for the 'minimum contact time' for 'chlorination of water'.
Recognize that this is a factual recall question based on standard public health guidelines.
Scan the options: 10, 90, 60, and 30 minutes. These are all plausible time frames in different contexts.
Recall or deduce the most common standard taught for basic water disinfection. The 30-minute rule is a widely disseminated public health message.
Evaluate the other options. 10 minutes is too short for reliable disinfection. 60 and 90 minutes are longer than the minimum required, although they would also be effective.
Select 30 minutes as the best answer representing the minimum standard requirement.

Concept Tested & Keywords

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

Practise the full AIIMS Patna NO - 2020

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