GATE 2026 BT – Question 61

Fundamentals of Biological Engineering · Bioreaction Engineering: Batch, fed-batch and continuous processes, microbial and enzyme reactors, optimization and scale up · 2 marks · Numerical answer

A microbe that follows Monod growth kinetics on a limiting substrate (maximum specific growth rate of 0.5 h$^{-1}$ and Monod constant of 0.1 mg L$^{-1}$) is cultivated in a continuous reactor for microbial growth (chemostat) with sterile feed. Given that the chemostat volume is 5 L and inlet concentration of limiting substrate is 10 g L$^{-1}$, the minimum inlet feed flow rate for chemostat washout is ______ L h$^{-1}$. (rounded off to one decimal place)

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Correct answer: 2.49 to 2.51

Explanation

Washout starts when the dilution rate exceeds $D_{crit} = \frac{\mu_{max}S_0}{K_s + S_0} = \frac{0.5 \times 10000}{0.1 + 10000} \approx 0.5$ h$^{-1}$. The feed rate is $F = D_{crit}V = 0.5 \times 5 = 2.5$ L/h.