Difficulty: Easy
Correct Answer: 0.5 mg/L
Explanation:
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:
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:
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
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