Treated drinking water: What is the maximum permissible free residual chlorine at the consumer end (after typical contact time), expressed in mg/L?

Introduction:
Residual chlorine ensures continuing disinfection in the distribution system. Too little leaves water vulnerable; too much affects taste and safety.

Given Data / Assumptions:

• Treated water, post contact (about 30 minutes).
• Consumer end residual target in mg/L.
• Normal municipal distribution conditions.

Concept / Approach:
Disinfection uses chlorine dose, demand, and contact time. A small but effective residual must remain when water reaches consumers to control regrowth.

Step-by-Step Solution:
Step 1: Typical guideline residual at consumer tap is roughly 0.2 to 0.5 mg/L.Step 2: Many standards operationalize a narrower band near 0.2 to 0.3 mg/L to balance safety and taste.Step 3: Hence, the maximum permissible value in this context aligns with 0.3 mg/L upper bound.

Verification / Alternative check:
Higher values (2 to 3 mg/L, 5 to 10 mg/L) cause strong odor/taste and are generally beyond routine potable practice.

Why Other Options Are Wrong:
0.001–0.01: Ineffective. 2–3 and 5–10: Excessively high. 0.05–0.1: Often insufficient to guarantee residual at the far end.

Common Pitfalls:
Confusing applied chlorine dose with residual; they are not the same.

Final Answer:
0.2 to 0.3 mg/L