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GATE 2018 CE (CE1) – Question 43

Water and Waste Water Quality and Treatment · Water requirement, distribution system and drinking water treatment · 2 marks · Multiple choice

A rapid sand filter must produce 99 MLD potable water. Backwashing loss is 5%, filtration rate 6 m/h, and each bed has length/width 1.35 and width 5.2 m. Include one extra bed for breakdown, repair and maintenance. Total beds required are ______.

  1. 19
  2. 20
  3. 21
  4. 22

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Show answer and explanation

Correct answer: (C) 21; (D) 22

Explanation

Each bed has area $1.35(5.2^2)=36.504$ m² and gross daily throughput $36.504(6)(24)=5256.576$ m³. Required gross flow is $99000/0.95=104210.5$ m³/day, requiring 19.825 operating beds. Round up to 20 and add one standby: 21 (C). The final key also accepts 22 (D); both remain valid for grading.