Chlorination guideline in drinking water Approximately what dose of chlorine (as Cl2) is typically applied for routine disinfection of potable water under normal conditions?

Introduction / Context:
Chlorination remains one of the most widely used and cost-effective disinfection methods in drinking water treatment. Engineers must select doses that inactivate pathogens while limiting disinfection by-product formation and maintaining a residual in the distribution network.

Given Data / Assumptions:

• Treated water with low turbidity and typical pH (around neutral).
• Standard contact times and demand conditions.
• Question asks for a representative dose, not a regulatory residual requirement.

Concept / Approach:
Typical chlorination practice applies doses on the order of a few tenths of a milligram per litre to achieve pathogen control and a small free-chlorine residual at consumers’ taps. While actual doses vary with chlorine demand (ammonia, organics) and CT (concentration*time) targets, a commonly cited nominal figure for clear water is about 0.2–0.5 mg/L, making 0.3 mg/L a representative textbook value.

Step-by-Step Solution:

Verification / Alternative check:
Many design manuals present starting-point values near 0.3 mg/L for low-demand waters, with field adjustment as needed.

Why Other Options Are Wrong:

• 0.01 or 0.1 mg/L: Often insufficient to establish a durable residual in typical systems.
• 1 or 5 mg/L: Excessive for routine finished water under normal demand; may be used in shock chlorination or high-demand scenarios.

Common Pitfalls:
Confusing applied dose with required residual; failing to account for chlorine demand and contact time when setting dose.

Final Answer:
0.3 mg/L