Q.16 Wash out (as defined by D = μ_max) of a continuous stirred tank fermenter is characterized by
(X = biomass, S = substrate concentration in bioreactor, S₀ = substrate concentration in feed, P = product concentration in bioreactor)

• (A) X = 0, S = 0, P = 0
• (B) X = 0, S = S₀, P = 0
• (C) X = 0, S < 0, P = 0
• (D) X < 0, S < 0, P < 0

  Washout occurs when dilution rate (D) equals maximum specific growth rate (μmax), causing biomass washout while substrate equals feed concentration. This SEO-optimized article answers the MCQ: Wash out (as defined by D = μmax) of a continuous stirred tank fermenter is characterized by (A) X=0,S=0,P=0, (B) X=0,S=S0,P=0, (C) X=0,S<0,P=0, (D) X<0,S<0,P<0—critical for bioprocess engineering and microbial fermentation students.​

  Correct Answer: Option (B) X = 0, S = S0, P = 0

  At washout (D = μmax) in steady-state CSTR:

  X = 0 (biomass): D = μmax means washout rate equals maximum growth—no net biomass accumulation.

  S = S0 (substrate): No cells consume substrate, so bioreactor concentration equals sterile feed.

  P = 0 (product): No biomass production eliminates product formation.

  Mass balance derivation: D(X) = μX → X(D-μ)=0. When D=μmax, X=0 satisfies equation.

  Explanation of All Options

  Option	Biomass (X)	Substrate (S)	Product (P)	Physically Possible?	Correct/Incorrect
  (A)	0	0	0	Impossible—S0>0 in feed	Incorrect
  (B)	0	S0	0	Correct—classic washout	Correct ​
  (C)	0	<0	0	Impossible—negative concentration	Incorrect
  (D)	<0	<0	<0	Impossible—negative concentrations	Incorrect

  Critical Washout Curve Analysis

  text

  Steady-state: X = Y(S0-S)/(1+kd/μ) where μ = μmax S/(Ks+S)
Critical D = μmax(1-a)/(1-a+Ks/S0) where a = mS/Y
Washout boundary: D ≥ μmax → X → 0, S → S0


  This matches your balanced growth (Q.12) and lag phase (Q.15) kinetics knowledge—beyond μmax, exponential washout occurs.

  Bioreactor Design Implications

  Washout defines critical dilution rate for chemostat operation. Your fermentation expertise requires D<0.9μmax for stable X>0. Post-washout recovery demands reinoculation, costing 24-48 hours.