Contact time requirement for disinfection: What minimum contact time with free chlorine (at typical plant conditions) should be ensured before water reaches the first consumer connection?

Introduction / Context:
Disinfection efficacy depends on the product of disinfectant concentration and time (CT). Water systems must provide a minimum contact time with free chlorine to achieve pathogen inactivation targets before delivery to consumers.

Given Data / Assumptions:

• Normal temperature and pH conditions for municipal supplies.
• Free chlorine as the disinfectant.
• Typical conservative design guidance for minimum CT.

Concept / Approach:

While detailed design uses specific CT tables for target organisms, a commonly cited rule-of-thumb minimum contact time is on the order of 20–30 minutes for routine systems, ensuring adequate disinfection under average conditions with a measurable residual at the outlet.

Step-by-Step Solution:

Verification / Alternative check:

CT-credit calculations often use baffling factors and reservoir volumes to demonstrate compliance; many practice problems adopt 20–30 minutes as a baseline where detailed CT charts are not provided.

Why Other Options Are Wrong:

• 10–20 minutes may be insufficient depending on temperature/pH/organism.
• Greater than 30–60 minutes can be used but is beyond the minimum widely quoted for general cases.

Common Pitfalls:

• Ignoring baffling efficiency—short-circuiting reduces effective contact time.
• Confusing contact time with storage time; not all zones contribute to CT.

Final Answer:

20 to 30 minutes.