Plain chlorination – typical chlorine dosage For plain chlorination (without ammonia), an indicative quantity of chlorine used during treatment is closest to:

Introduction / Context:In disinfection, “plain chlorination” refers to dosing chlorine to achieve a target residual after a specified contact time. While exact dose depends on chlorine demand of the water, exam problems use representative values.

Given Data / Assumptions:

• Potable water after clarification/filtration.
• Target free residual often about 0.2 mg/L after ≈ 30 minutes contact.
• Moderate chlorine demand in typical raw waters.

Concept / Approach:The applied dose must satisfy initial demand plus desired residual. A common indicative dose for plain chlorination is around 0.5 mg/L (may range from roughly 0.2–0.5 mg/L). For a single best-choice question, 0.5 mg/L is the most representative among the discrete options provided.

Step-by-Step Solution:Recognize that residual of ~0.2 mg/L typically requires a higher applied dose.Pick the commonly cited exam value closest to practice → 0.5 mg/L.

Verification / Alternative check:Operator logs often show 0.3–0.8 mg/L dose adjustments depending on demand; 0.5 mg/L is a typical starting point.

Why Other Options Are Wrong:

• 0.1–0.4 mg/L may be insufficient to overcome demand and still leave a protective residual.

Common Pitfalls:Assuming a fixed dose always works; real plants measure residual and adjust dosage continuously with changing raw-water quality.

Final Answer:0.5 mg/L