Civil Engineering > GATE 2026 SET-1 > Sewage Treatment
The average sewage from a city is 90 million litres per day and the average 5-day Biochemical Oxygen Demand (BOD5) is 300 mg/l. Average standard BOD5 of the domestic sewage is 0.08 kg/day/person. The population equivalent of the city is
Correct : a
Population equivalent (PE) is the number of people whose domestic sewage would produce the same BOD load as the actual sewage being treated. It is used to size treatment plants for mixed municipal and industrial waste.
Step 1 — Total BOD₅ load:
Total BOD = Flow × Concentration
= 90 × 10⁶ L/day × 300 mg/L
= 90 × 10⁶ × 300 × 10⁻⁶ kg/day
= 27,000 kg/day
Step 2 — Population Equivalent:
PE = Total BOD load / Per capita BOD
= 27,000 / 0.08
= 337,500
Correct answer: A — 337,500 ✓
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