The GATE Grind

GATE 2026 CE (CE2) – Question 42

Water and Waste Water Quality and Treatment · Water quality standards, parameters and water quality index · 2 marks · Multiple choice

The 2.4 ml of raw sewage is diluted to 240 ml. The Dissolved Oxygen (DO) of the diluted sample at the beginning of Biochemical Oxygen Demand (BOD) test was 8 mg/l and it was 6 mg/l after 5-day incubation at 20 °C.

The BOD$_5$ (in mg/l) of the raw sewage is

  1. 200
  2. 100
  3. 250
  4. 150

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (A) 200

Explanation

The dilution factor is $\frac{240}{2.4} = 100$. The oxygen used in five days is $8 - 6 = 2$ mg/l. So $BOD_5 = 2 \times 100 = 200$ mg/l.