The GATE Grind

GATE 2019 CE (CE2) – Question 42

Water and Waste Water Quality and Treatment · Unit processes and operations · 2 marks · Multiple choice

Chlorination of municipal water kills 50% of E. coli in 3 minutes. Find the minimum time for 99% disinfection efficiency.

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

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Correct answer: (C) 19.93 min

Explanation

First-order survival is $e^{-kt}$. Half survival at 3 min gives $k=\ln2/3$. For 1% survival, $t=\ln100/k=3\ln100/\ln2=$ **19.93 min, C**.