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GATE 2026 CE (CE1) – Question 21

Water and Waste Water Quality and Treatment · Unit processes and operations · 1 mark · Multiple choice

A rapid sand filter bed of depth 0.8 m has 40 % porosity during service cycle. It is recommended that during backwash operation, the expanded filter bed should have 70 % porosity. The uniform expanded depth (in m) of the filter bed during backwash is

  1. 1.0
  2. 1.2
  3. 1.4
  4. 1.6

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Correct answer: (D) 1.6

Explanation

The volume of sand grains does not change, so $(1 - n_1)L_1 = (1 - n_2)L_2$. This gives $0.6 \times 0.8 = 0.3 \times L_2$, so $L_2 = 1.6$ m.