GATE 2016 CE (CE2) – Question 28

Water and Waste Water Quality and Treatment · Unit processes and operations · 1 mark · Numerical answer

A water supply board is responsible for treating 1500 m3/day of water. A settling column analysis indicates that an overflow rate of 20 m/day will produce satisfactory removal for a depth of 3.1 m. It is decided to have two circular settling tanks in parallel. The required diameter (expressed in m) of the settling tanks is __________

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Correct answer: 6.8 to 7.0

Explanation

The two identical tanks share1500m³/day equally, so each treats750m³/day. Required surface area per tank is750/20=37.5m². For a circle D=sqrt(4A/π)=sqrt(150/π)=**6.91m**. The3.1m depth does not change the surface-area requirement from the supplied overflow rate.