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GATE 2020 CE (CE1) – Question 47

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

A water scheme transports 10 MLD through a 450 mm pipe for 2.5 km. Chlorine dose at entry is 3.50 mg/L for the required downstream disinfection. Flow rises to 13 MLD; Chick–Watson concentration exponent is 0.86. Find entry chlorine dose for equal disinfection (mg/L).

  1. 3.95
  2. 4.40
  3. 4.75
  4. 5.55

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Correct answer: (C) 4.75

Explanation

Equal disinfection requires constant $C^{0.86}t$. Residence time is inversely proportional to flow. Thus $C_2=3.50(13/10)^{1/0.86}=4.7485$ mg/L. **4.75, C**.