GATE 2026 CE (CE2) – Question 64

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

Anaerobic bacteria are being utilized for the destruction of biodegradable organic content of wastewater measured in terms of Chemical Oxygen Demand (COD). The yield coefficient ($Y$) is 0.06 gram Volatile Suspended Solids (VSS) per gram of COD. Bacterial biomass forms the sludge during the process. The proximate chemical formula of anaerobic bacteria is $C_6H_7NO_2$.

The COD (in %) converted into bacterial biomass is ______ (*rounded off to the nearest integer*).

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Correct answer: 9

Explanation

The COD of the biomass is the oxygen needed to oxidise it: $C_6H_7NO_2 + 6O_2 \rightarrow 6CO_2 + 2H_2O + NH_3$. One mole of cells (125 g) needs $6 \times 32 = 192$ g of oxygen, so 1 g of VSS has a COD of $\frac{192}{125} = 1.536$ g. Each gram of COD removed makes 0.06 g of VSS, which has a COD of $0.06 \times 1.536 = 0.092$ g. So about 9.2 %, which is 9 %, of the COD is converted into biomass.