Q.53 Under complete cell washout condition in a chemostat with sterile feed, which of
the following statements is/are correct?
(A) Biomass concentration in the reactor is maximum
(B) Substrate concentration in the exit stream is less than that in the inlet stream
(C) Substrate concentration in the exit stream is equal to that in the inlet stream
(D) Substrate concentration in the exit stream is zero
Chemostat Complete Cell Washout
Chemostat complete cell washout occurs when dilution rate exceeds maximum growth rate in sterile feed systems, leading to zero biomass. Only one statement holds true under these conditions.
Correct Answer
Option (C) is correct: Substrate concentration in the exit stream equals that in the inlet stream.
Option Explanations
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(A) Biomass concentration in the reactor is maximum: Incorrect. Biomass approaches zero as cells wash out faster than they grow (D > μ_max).
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(B) Substrate concentration in the exit stream is less than that in the inlet stream: Incorrect. No consumption occurs without biomass, so substrate remains unchanged.
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(C) Substrate concentration in the exit stream is equal to that in the inlet stream: Correct. Sterile feed passes through unconsumed due to absent microbial growth.
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(D) Substrate concentration in the exit stream is zero: Incorrect. Inlet substrate flows out identically without utilization.
Chemostat Basics
Chemostats maintain steady-state growth via dilution rate (D = F/V). Washout happens beyond critical D, nullifying growth (dX/dt = (μ – D)X < 0, X → 0). Sterile feed (S_0 > 0, X_0 = 0) ensures no inlet cells, amplifying washout effects.
