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Chlorine is used as the disinfectant in a municipal water treatment plant. It achieves 50 percent of disinfection efficiency measured in terms of killing the indicator microorganisms (E-Coli) in 3 minutes. The minimum time required to achieve 99 percent disinfection efficiency would be

1. 9.93 minutes
2. 11.93 minutes
3. 19.93 minutes
4. 21.93 minutes

Correct answer: 19.93 minutes

Solution

The relationship between disinfection efficiency and time is often logarithmic, meaning that each additional log reduction in microorganisms requires a significantly longer time. Since achieving 50% disinfection takes 3 minutes, reaching 99% disinfection (which is a 2-log reduction) typically requires about 6 times that initial time, leading to approximately 19.93 minutes.

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