GATE 2024 CE (CE2) – Question 60

Water and Waste Water Quality and Treatment · Primary, secondary and tertiary treatment, effluent standards · 2 marks · Numerical answer

A circular settling tank is to be designed for primary treatment of sewage at a flow rate of 10 million liters/day. Assume a detention period of 2.0 hours and surface loading rate of 40000 liters/m$^2$/day. The height (in meters) of the water column in the tank is _______________ (rounded off to 2 decimal places).

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Correct answer: 3.31 to 3.35

Explanation

The flow is $10 \times 10^6$ L/d $= 10000$ m³/d. The surface area is $\frac{10000}{40} = 250$ m² (the loading rate is 40 m³/m²/d). The volume is $Q \times t = 10000 \times \frac{2}{24} = 833.3$ m³. The depth is $\frac{833.3}{250} = 3.33$ m.