GATE 2022 CE (CE2) – Question 60

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

A sewage treatment plant receives sewage at a flow rate of 5000 m3/day. The total suspended solids (TSS) concentration in the sewage at the inlet of primary clarifier is 200 mg/L. After the primary treatment, the TSS concentration in sewage is reduced by 60 %. The sludge from the primary clarifier contains 2 % solids concentration. Subsequently, the sludge is subjected to gravity thickening process to achieve a solids concentration of 6 %. Assume that the density of sludge, before and after thickening, is 1000 kg/m3. The daily volume of the thickened sludge (in m3/day) will be_________. (round off to the nearest integer)

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Correct answer: 10

Explanation

TSS entering the clarifier is $5000(0.2)=1000$ kg/day because 200 mg/L=0.2 kg/m³. Removal at 60% captures 600 kg/day of solids.

At 6% solids and sludge density 1000 kg/m³, each m³ contains 60 kg solids. Thickened volume =600/60=**10 m³/day**. Thickening conserves solids mass.