Q: Chemostat Performance — If biomass yield is constant over the operating range, how does biomass productivity vary with dilution rate D (before washout)?

Introduction: Biomass productivity in a chemostat is the product of dilution rate and biomass concentration (PX = D * X). Understanding how PX depends on D under constant yield assumptions is central to selecting an operating point that maximizes throughput without risking washout. Given Data / Assumptions: Single limiting substrate; yield Yx/s is approximately constant. Monod kinetics with μ = μm * S / (Ks + S). Steady state below washout: μ = D < μm. Concept / Approach: As D increases from low values, μ increases and substrate concentration rises, but biomass concentration X tends to decrease. The product D * X typically exhibits a maximum at a D near, but safely below, the washout point D ≈ μm. This is analogous to classical continuous-flow reactor optimization for growth-associated production. Step-by-Step Solution: Use steady-state: μ = D.Substrate balance with constant yield gives X = Yx/s * (Sf − S).As D increases, S rises and X falls, but D increases linearly.Compute PX = D * X; differentiate with respect to D to find an interior maximum below μm. Verification / Alternative check: Plotting PX versus D using typical μm, Ks, Sf, and Yx/s shows a dome-shaped curve peaking below washout, confirming the expected trend. Why Other Options Are Wrong: A and D: Ignore the initial rise in PX with D. C: Contradicts dependence of X on D. E: Yield and kinetics are essential determinants, not just volume. Common Pitfalls: Operating too close to washout risks instability; choose D with safety margin to account for disturbances and parameter uncertainty. Final Answer: It increases with D up to a maximum just below washout (then collapses at washout).

Q: Competition in a Chemostat — Two species compete for a single limiting substrate: which organism avoids washout at steady state?

Introduction: In a chemostat with a single limiting substrate, species compete through resource depletion. Classic theory predicts competitive exclusion: the species capable of reducing the substrate to the lowest steady-state concentration wins, provided both can grow at the imposed dilution rate. Given Data / Assumptions: One limiting substrate in the feed; constant D. Both organisms follow Monod-type kinetics with different μm and Ks values. No other interactions (toxins, inhibition, syntrophy) dominate. Concept / Approach: At steady state, each species requires a critical substrate concentration Sc such that μ(Sc) = D. The species with the lower Sc (higher effective affinity μ/Ks combination at the given D) drives the bulk substrate toward that lower level, disadvantaging competitors that require higher substrate to sustain μ ≥ D. Those competitors wash out because μ < D in the environment created by the winner. Step-by-Step Solution: For each species i, solve μi(Sci) = D for Sci.Compare Sci values: the smaller Sci corresponds to the superior competitor.In the reactor, S tends to the smallest Sci, so other species experience μ < D and are washed out.Conclude that the organism maintaining the lowest residual S persists. Verification / Alternative check: Experimental chemostat studies show single-species dominance on a single limiting carbon source unless special trade-offs or spatial niches exist. Why Other Options Are Wrong: A and D: Higher residual S implies weaker competitive performance. C: Coexistence on a single resource is not generic. E: Cell size is irrelevant without kinetic advantage. Common Pitfalls: Confusing μm alone with competitive ability; at modest D, low Ks can dominate even if μm is lower. Final Answer: The species that can maintain the lowest residual substrate concentration (i.e., has the higher substrate affinity under the given D).

Q: Fed-Batch Accounting — For a fed-batch with no effluent, what expression gives the total mass of substrate utilized over a given time window?

Introduction: Mass balances in fed-batch systems are central to calculating yields and specific rates. With no outflow, substrate accumulates in the reactor while being consumed by cells. Accounting for initial inventory, feed addition, and final leftover substrate yields the total substrate utilized. Given Data / Assumptions: No liquid effluent (classic fed-batch). Known feed flow rate and feed substrate concentration over the time window. Measured initial and final substrate masses in the reactor. Concept / Approach: Overall substrate balance over the time window: utilized = (initial + fed in) − final. If feed conditions are constant, fed in = flow rate * substrate concentration in the feed * time. If variable, integrate over time. This expression ensures conservation and correctly handles changing volume due to feeding. Step-by-Step Solution: Write substrate balance: accumulation = in − out − consumption.With no outflow and operating between two steady snapshots, rearrange to consumption = initial + in − final.Replace in by (flow rate * substrate concentration in the feed * time) or its integral.Select the option matching initial − final + feed input. Verification / Alternative check: Cross-check with biomass and product yields: Yx/s and Yp/s should produce consistent substrate utilization within measurement error. Why Other Options Are Wrong: A: Adds final instead of subtracting; overestimates consumption. C and D: Subtract feed term or omit units; inconsistent with conservation. E: Ignores initial and final inventories, causing bias. Common Pitfalls: Forgetting to account for changing volume and variable feed concentration; integrate when conditions vary. Final Answer: Initial mass of substrate in the reactor − final mass of substrate in the reactor + (flow rate * substrate concentration in the feed).

Q: Chemostat with Saccharomyces cerevisiae — At steady state, how do the concentrations of glucose, biomass, ethanol, and glycerol change with time?

Introduction: Chemostats are designed to achieve steady state, where state variables such as biomass, substrate, and product concentrations are time-invariant. This allows reproducible physiology and simplifies data interpretation for organisms like Saccharomyces cerevisiae producing ethanol and glycerol. Given Data / Assumptions: Constant dilution rate and feed composition. Well-mixed reactor with constant volume. Stable operating conditions without disturbances. Concept / Approach: At steady state, accumulation terms in the dynamic mass balances are zero: dC/dt = 0 for each measured component. The actual constant values depend on kinetics and dilution rate, but once steady state is reached, they do not change with time until conditions are altered. Step-by-Step Solution: Write component balance: accumulation = in − out ± reaction.At steady state, set accumulation term to zero for glucose, biomass, ethanol, glycerol.Solve algebraic relationships to obtain constant concentrations.Conclude that time derivatives vanish: concentrations remain constant. Verification / Alternative check: Plotting online measurements (for example, off-gas, OD, residual glucose) versus time at steady conditions should yield flat lines within noise after transients decay. Why Other Options Are Wrong: A and B: Monotonic drift contradicts steady state. D: Random changes imply instability, not steady operation. E: All tracked components stabilize when the chemostat is truly at steady state. Common Pitfalls: Confusing set-point chasing during start-up with steady state; wait for at least 3–5 residence times for stabilization and confirm with replicate samples. Final Answer: They remain constant with time (time derivatives are zero).

Q: Definition of Steady State in Continuous Reactors — Which statement correctly characterizes a continuous reactor operating at steady state?

Introduction: Steady state is a central concept in continuous bioprocessing. It does not mean reactions stop; rather, the macroscopic concentrations inside the reactor become time-invariant because inflow, outflow, and reaction balance each other. Given Data / Assumptions: Perfect mixing and constant volume. Constant dilution rate and feed composition. System has passed transients and reached equilibrium conditions. Concept / Approach: Component balances take the form dCi/dt = (in − out)/VR ± ri. At steady state, dCi/dt = 0 for each i, but the reaction rates ri are generally nonzero; they are simply balanced by flow terms. Therefore, concentrations remain constant even though metabolism proceeds. Step-by-Step Solution: Write general mass balance for species i.Set dCi/dt = 0 to reflect steady state.Recognize that nonzero ri are offset by dilution and feed terms, resulting in constant Ci.Select the statement that says concentrations do not change with time. Verification / Alternative check: Time traces of reactor measurements flatten at steady state; however, reaction heat and gas exchange continue, demonstrating ongoing metabolism despite steady concentrations. Why Other Options Are Wrong: A: Reaction rates need not be zero; only accumulation terms are zero. B: Concentrations are not necessarily zero. D and E: These describe unsteady operation. Common Pitfalls: Equating steady state with chemical equilibrium; biological steady states are dynamic and far from thermodynamic equilibrium. Final Answer: Biomass, substrate, and product concentrations do not change with time (their time derivatives are zero).

Q: Reactor Inventory — Which expression correctly calculates the total mass of substrate present in the reactor at a given moment?

Introduction: Inventory calculations convert measured concentrations into total masses for material balances and yield estimation. The key is to multiply concentration by the system volume at the same instant, ensuring consistent units. Given Data / Assumptions: Known reactor volume at the snapshot of interest. Measured substrate concentration inside the reactor (not the feed). Uniform mixing so concentration is spatially uniform. Concept / Approach: Mass = concentration * volume. If concentration is in mass per volume (for example, g/L) and volume is in liters, the product yields mass units (g). Flow rate terms are used for rates, not for instantaneous inventory, and should not appear in the simple inventory expression. Step-by-Step Solution: Identify the instantaneous reactor volume VR.Measure substrate concentration S in the reactor.Compute mass_substrate = VR * S with consistent units.Use this value in overall mass balances or yield calculations. Verification / Alternative check: Check dimensional consistency: (L) * (g/L) = g. Any expression involving flow rate (for example, L/h) would produce rate units, not mass, and is therefore unsuitable for inventory. Why Other Options Are Wrong: A: Flow rate multiplies concentration to provide a mass flow rate, not an inventory. C and D: Unit mismatch; these do not yield mass. E: Feed concentration is not the reactor inventory unless reactor equals feed line volume, which it does not. Common Pitfalls: Using nominal working volume instead of actual time-varying volume in fed-batch; always use the correct instantaneous volume. Final Answer: Reactor volume * substrate concentration in the reactor

Q: Fluidized-bed bioreactors often achieve higher gas–liquid and liquid–solid mass transfer rates than packed-bed bioreactors primarily because:

Introduction: Choosing between packed-bed and fluidized-bed configurations for immobilized biocatalysts depends heavily on mass transfer performance. Fluidization alters hydrodynamics, boundary layers, and interfacial areas, which can markedly increase transfer rates. Given Data / Assumptions: Fluidized beds maintain particles in motion due to upward flow. Smaller support particles are commonly used in fluidized systems. Mixing and contact patterns differ significantly from fixed beds. Concept / Approach: Higher mixing reduces external film resistance; particle motion thins boundary layers and renews liquid at surfaces; smaller particles provide larger specific surface area. Together these effects elevate overall volumetric mass transfer, provided attrition and elutriation are controlled. Step-by-Step Solution: 1) Increased mixing lowers concentration gradients in the bulk.2) Moving particles disrupt stagnant films, boosting k_L.3) Smaller particles yield higher area per volume, increasing flux at the same driving force.4) Combined, these mechanisms raise mass transfer coefficients and rates.5) Validate with pressure drop and minimum fluidization velocity design checks. Verification / Alternative check: Residence time distribution and k_La measurements typically show higher values in fluidized systems at comparable throughputs, confirming the mechanism-based expectations. Why Other Options Are Wrong: Individual statements (a), (b), or (c) each reflect only part of the reason; the comprehensive answer is all of the above. Common Pitfalls: Ignoring particle attrition and carryover can negate advantages. Use appropriate particle density and size to maintain stable fluidization without excessive losses. Final Answer: all of the above

Question 1

Hydraulics in continuous reactors — A continuous bioreactor has a dilution rate D = 0.5 h^-1. What is its residence time (space time)?

Accepted Answer

Introduction: Residence time (also called space time, tau) is a basic hydraulic parameter in continuous reactors. It relates directly to the dilution rate, which is commonly reported in bioprocessing. This question checks your ability to convert between D and tau quickly and correctly. Given Data / Assumptions: Dilution rate D = F / V = 0.5 h^-1. Well-mixed reactor at constant volume. No cell recycle or volume change. Concept / Approach: By definition, residence time tau is the reciprocal of the dilution rate for a simple CSTR without recycle: tau = V / F = 1 / D. Natural logarithms appear in unrelated contexts (e.g., half-life in first-order decay), but not in the basic D–tau identity. Step-by-Step Solution: Start with tau = 1 / D.Insert D = 0.5 h^-1.Compute tau = 1 / 0.5 = 2 h.Cross-check units: h^-1 inverted gives hours. Verification / Alternative check: Given V and F separately, tau = V / F would yield the same value (e.g., V = 2 L and F = 1 L/h gives tau = 2 h), confirming consistency with 1/D. Why Other Options Are Wrong: (a) and (b) inject ln(2), which is unrelated here. (c) 0.5 h corresponds to D = 2 h^-1, not 0.5 h^-1. (e) 1/ln(2) h is dimensionally inconsistent with D. Common Pitfalls: Confusing half-life t_1/2 = ln(2)/k with residence time; mixing up D with k from first-order reactions. Final Answer: 2 h

Question 2

Lactic Acid Bacteria (Lactococcus lactis) — Why does end-product (lactic acid) formation become non-growth-associated as lactic acid accumulates during fermentation?

Accepted Answer

Introduction: Lactococcus lactis is a lactic acid bacterium that primarily generates ATP by substrate-level phosphorylation and reoxidizes NADH by reducing pyruvate to lactic acid. As lactic acid accumulates in the broth, pH drops and undissociated acid diffuses back into the cell, increasing the internal proton load. The cell must spend energy to maintain intracellular pH, causing product formation to decouple from biomass growth (non-growth-associated formation). Given Data / Assumptions: Fermentation is homolactic under typical conditions. External lactic acid accumulates and pH declines over time. Cells possess F1F0-ATPase and other transporters to export protons and lactate anions. Concept / Approach: When acid stress rises, cells allocate ATP to maintenance: proton extrusion and acid efflux. This shifts ATP away from anabolism and cell division. Thus, lactic acid production continues (for redox balance), but biomass growth slows or plateaus. The key is active transport driven by ATP hydrolysis, not passive diffusion, because the proton gradient and charge differences must be overcome. Step-by-Step Solution: Recognize that lactic acid accumulation lowers external pH and increases influx of H+ and undissociated acid.Cells respond by activating proton pumps (for example, F1F0-ATPase) and antiporters to export H+ and lactate.ATP consumption for homeostasis increases, leaving less ATP for biosynthesis and growth.Therefore, product formation persists while growth slows, i.e., becomes non-growth-associated. Verification / Alternative check: Maintenance coefficients increase under acid stress. Chemostat or batch data often show constant or rising specific lactic acid formation rates despite reduced specific growth rates at low pH. Why Other Options Are Wrong: A: More ATP goes to maintenance, not anabolism. B: NAD+ recycling occurs via lactate dehydrogenase but does not by itself explain non-growth association. C: Diffusion does not require ATP and cannot account for active extrusion against gradients. E: ATP use does not cease; it is redirected to maintenance. Common Pitfalls: Confusing redox balancing (NADH to lactate) with energy allocation; non-growth association stems from ATP diversion to homeostasis, not a halt in metabolism. Final Answer: Cells will redirect ATP to drive active transport and proton pumping that expels lactic acid and H+, shifting energy from growth to maintenance.

Question 3

Bioprocess Metrics — What does the yield coefficient represent in fermentation or bioconversion calculations?

Accepted Answer

Introduction: The yield coefficient is a foundational parameter in bioprocess engineering. It links how much product or biomass is obtained per unit of substrate consumed. Correctly interpreting yield is crucial for material balances, scale-up, and techno-economic analysis. Given Data / Assumptions: Yield can be defined on mass or molar bases (for example, g product per g substrate). Yield can refer to biomass (Yx/s), product (Yp/s), or product on biomass (Yp/x). We assume steady definitions and consistent units. Concept / Approach: Yield describes efficiency, not speed. Mathematically, Yp/s = mass_product_formed / mass_substrate_consumed. Similarly, Yx/s quantifies the efficiency of converting substrate carbon and energy into cell material. High yields indicate efficient conversion; low yields suggest losses to by-products, maintenance, or incomplete conversion. Step-by-Step Solution: Identify numerator and denominator consistent with the chosen yield (biomass or product).Compute yield from measured consumption and formation over a time interval or at steady state.Use yields to predict substrate demand from a target production, or vice versa.Recognize that yield is an efficiency ratio and does not directly provide reaction rates. Verification / Alternative check: Cross-validate with elemental balances (C, H, O, N) to ensure that reported yields are physically plausible and consistent with stoichiometry. Why Other Options Are Wrong: A: Total amount produced ignores substrate basis. C: Describes a rate, not a yield. D: Time to produce is unrelated to yield definition. E: μm is a kinetic parameter, not a yield metric. Common Pitfalls: Mixing mass and molar units or forgetting to subtract residual substrate when computing consumption; always standardize units and reference states. Final Answer: The conversion efficiency of a substrate into a desired product (mass or moles of product per mass or moles of substrate).

Question 4

Continuous Culture Terminology — What do we call a continuous bioreactor in which the growth rate (and thus productivity) is controlled solely by the flow rate via the dilution rate?

Accepted Answer

Introduction: Continuous reactors enable steady production and constant physiological states. Nomenclature matters because control strategies differ between designs, affecting productivity and stability. The reactor controlled solely by flow rate is the classical chemostat. Given Data / Assumptions: Single limiting substrate in the feed. Perfect mixing and constant reactor volume. Dilution rate D = F / VR determines the steady-state specific growth rate μ, provided D < μm. Concept / Approach: In a chemostat, the operator fixes D through the flow rate. At steady state, μ equals D and the biomass concentration is set by the substrate balance and yield. In contrast, a turbidostat uses turbidity feedback to adjust D dynamically; pH-stat manipulates acid/base addition; level-stat maintains volume but does not directly set μ. Step-by-Step Solution: Recognize that control by flow rate alone implies a fixed D.Recall steady-state identity in chemostat: μ = D (below washout).Link μ to productivity for growth-associated products.Select the term chemostat as the correct design. Verification / Alternative check: Material balances in a chemostat confirm that, at steady state, concentrations are time-invariant and set by D and stoichiometric yields. Why Other Options Are Wrong: A: Turbidostat relies on optical density feedback. C and D: These are control strategies with different objectives. E: Fed-batch lacks continuous outflow and true steady state. Common Pitfalls: Confusing turbidostat with chemostat; the former varies D to hold biomass constant, the latter holds D constant to set μ. Final Answer: Chemostat (fixed dilution rate sets the specific growth rate below washout).

Question 5

Chemostat Performance — If biomass yield is constant over the operating range, how does biomass productivity vary with dilution rate D (before washout)?

Accepted Answer

Introduction: Biomass productivity in a chemostat is the product of dilution rate and biomass concentration (PX = D * X). Understanding how PX depends on D under constant yield assumptions is central to selecting an operating point that maximizes throughput without risking washout. Given Data / Assumptions: Single limiting substrate; yield Yx/s is approximately constant. Monod kinetics with μ = μm * S / (Ks + S). Steady state below washout: μ = D < μm. Concept / Approach: As D increases from low values, μ increases and substrate concentration rises, but biomass concentration X tends to decrease. The product D * X typically exhibits a maximum at a D near, but safely below, the washout point D ≈ μm. This is analogous to classical continuous-flow reactor optimization for growth-associated production. Step-by-Step Solution: Use steady-state: μ = D.Substrate balance with constant yield gives X = Yx/s * (Sf − S).As D increases, S rises and X falls, but D increases linearly.Compute PX = D * X; differentiate with respect to D to find an interior maximum below μm. Verification / Alternative check: Plotting PX versus D using typical μm, Ks, Sf, and Yx/s shows a dome-shaped curve peaking below washout, confirming the expected trend. Why Other Options Are Wrong: A and D: Ignore the initial rise in PX with D. C: Contradicts dependence of X on D. E: Yield and kinetics are essential determinants, not just volume. Common Pitfalls: Operating too close to washout risks instability; choose D with safety margin to account for disturbances and parameter uncertainty. Final Answer: It increases with D up to a maximum just below washout (then collapses at washout).

Question 6

Competition in a Chemostat — Two species compete for a single limiting substrate: which organism avoids washout at steady state?

Accepted Answer

Introduction: In a chemostat with a single limiting substrate, species compete through resource depletion. Classic theory predicts competitive exclusion: the species capable of reducing the substrate to the lowest steady-state concentration wins, provided both can grow at the imposed dilution rate. Given Data / Assumptions: One limiting substrate in the feed; constant D. Both organisms follow Monod-type kinetics with different μm and Ks values. No other interactions (toxins, inhibition, syntrophy) dominate. Concept / Approach: At steady state, each species requires a critical substrate concentration Sc such that μ(Sc) = D. The species with the lower Sc (higher effective affinity μ/Ks combination at the given D) drives the bulk substrate toward that lower level, disadvantaging competitors that require higher substrate to sustain μ ≥ D. Those competitors wash out because μ < D in the environment created by the winner. Step-by-Step Solution: For each species i, solve μi(Sci) = D for Sci.Compare Sci values: the smaller Sci corresponds to the superior competitor.In the reactor, S tends to the smallest Sci, so other species experience μ < D and are washed out.Conclude that the organism maintaining the lowest residual S persists. Verification / Alternative check: Experimental chemostat studies show single-species dominance on a single limiting carbon source unless special trade-offs or spatial niches exist. Why Other Options Are Wrong: A and D: Higher residual S implies weaker competitive performance. C: Coexistence on a single resource is not generic. E: Cell size is irrelevant without kinetic advantage. Common Pitfalls: Confusing μm alone with competitive ability; at modest D, low Ks can dominate even if μm is lower. Final Answer: The species that can maintain the lowest residual substrate concentration (i.e., has the higher substrate affinity under the given D).

Question 7

Fed-Batch Accounting — For a fed-batch with no effluent, what expression gives the total mass of substrate utilized over a given time window?

Accepted Answer

Introduction: Mass balances in fed-batch systems are central to calculating yields and specific rates. With no outflow, substrate accumulates in the reactor while being consumed by cells. Accounting for initial inventory, feed addition, and final leftover substrate yields the total substrate utilized. Given Data / Assumptions: No liquid effluent (classic fed-batch). Known feed flow rate and feed substrate concentration over the time window. Measured initial and final substrate masses in the reactor. Concept / Approach: Overall substrate balance over the time window: utilized = (initial + fed in) − final. If feed conditions are constant, fed in = flow rate * substrate concentration in the feed * time. If variable, integrate over time. This expression ensures conservation and correctly handles changing volume due to feeding. Step-by-Step Solution: Write substrate balance: accumulation = in − out − consumption.With no outflow and operating between two steady snapshots, rearrange to consumption = initial + in − final.Replace in by (flow rate * substrate concentration in the feed * time) or its integral.Select the option matching initial − final + feed input. Verification / Alternative check: Cross-check with biomass and product yields: Yx/s and Yp/s should produce consistent substrate utilization within measurement error. Why Other Options Are Wrong: A: Adds final instead of subtracting; overestimates consumption. C and D: Subtract feed term or omit units; inconsistent with conservation. E: Ignores initial and final inventories, causing bias. Common Pitfalls: Forgetting to account for changing volume and variable feed concentration; integrate when conditions vary. Final Answer: Initial mass of substrate in the reactor − final mass of substrate in the reactor + (flow rate * substrate concentration in the feed).

Question 8

Chemostat with Saccharomyces cerevisiae — At steady state, how do the concentrations of glucose, biomass, ethanol, and glycerol change with time?

Accepted Answer

Introduction: Chemostats are designed to achieve steady state, where state variables such as biomass, substrate, and product concentrations are time-invariant. This allows reproducible physiology and simplifies data interpretation for organisms like Saccharomyces cerevisiae producing ethanol and glycerol. Given Data / Assumptions: Constant dilution rate and feed composition. Well-mixed reactor with constant volume. Stable operating conditions without disturbances. Concept / Approach: At steady state, accumulation terms in the dynamic mass balances are zero: dC/dt = 0 for each measured component. The actual constant values depend on kinetics and dilution rate, but once steady state is reached, they do not change with time until conditions are altered. Step-by-Step Solution: Write component balance: accumulation = in − out ± reaction.At steady state, set accumulation term to zero for glucose, biomass, ethanol, glycerol.Solve algebraic relationships to obtain constant concentrations.Conclude that time derivatives vanish: concentrations remain constant. Verification / Alternative check: Plotting online measurements (for example, off-gas, OD, residual glucose) versus time at steady conditions should yield flat lines within noise after transients decay. Why Other Options Are Wrong: A and B: Monotonic drift contradicts steady state. D: Random changes imply instability, not steady operation. E: All tracked components stabilize when the chemostat is truly at steady state. Common Pitfalls: Confusing set-point chasing during start-up with steady state; wait for at least 3–5 residence times for stabilization and confirm with replicate samples. Final Answer: They remain constant with time (time derivatives are zero).

Question 9

Definition of Steady State in Continuous Reactors — Which statement correctly characterizes a continuous reactor operating at steady state?

Accepted Answer

Introduction: Steady state is a central concept in continuous bioprocessing. It does not mean reactions stop; rather, the macroscopic concentrations inside the reactor become time-invariant because inflow, outflow, and reaction balance each other. Given Data / Assumptions: Perfect mixing and constant volume. Constant dilution rate and feed composition. System has passed transients and reached equilibrium conditions. Concept / Approach: Component balances take the form dCi/dt = (in − out)/VR ± ri. At steady state, dCi/dt = 0 for each i, but the reaction rates ri are generally nonzero; they are simply balanced by flow terms. Therefore, concentrations remain constant even though metabolism proceeds. Step-by-Step Solution: Write general mass balance for species i.Set dCi/dt = 0 to reflect steady state.Recognize that nonzero ri are offset by dilution and feed terms, resulting in constant Ci.Select the statement that says concentrations do not change with time. Verification / Alternative check: Time traces of reactor measurements flatten at steady state; however, reaction heat and gas exchange continue, demonstrating ongoing metabolism despite steady concentrations. Why Other Options Are Wrong: A: Reaction rates need not be zero; only accumulation terms are zero. B: Concentrations are not necessarily zero. D and E: These describe unsteady operation. Common Pitfalls: Equating steady state with chemical equilibrium; biological steady states are dynamic and far from thermodynamic equilibrium. Final Answer: Biomass, substrate, and product concentrations do not change with time (their time derivatives are zero).

Question 10

Reactor Inventory — Which expression correctly calculates the total mass of substrate present in the reactor at a given moment?

Accepted Answer

Introduction: Inventory calculations convert measured concentrations into total masses for material balances and yield estimation. The key is to multiply concentration by the system volume at the same instant, ensuring consistent units. Given Data / Assumptions: Known reactor volume at the snapshot of interest. Measured substrate concentration inside the reactor (not the feed). Uniform mixing so concentration is spatially uniform. Concept / Approach: Mass = concentration * volume. If concentration is in mass per volume (for example, g/L) and volume is in liters, the product yields mass units (g). Flow rate terms are used for rates, not for instantaneous inventory, and should not appear in the simple inventory expression. Step-by-Step Solution: Identify the instantaneous reactor volume VR.Measure substrate concentration S in the reactor.Compute mass_substrate = VR * S with consistent units.Use this value in overall mass balances or yield calculations. Verification / Alternative check: Check dimensional consistency: (L) * (g/L) = g. Any expression involving flow rate (for example, L/h) would produce rate units, not mass, and is therefore unsuitable for inventory. Why Other Options Are Wrong: A: Flow rate multiplies concentration to provide a mass flow rate, not an inventory. C and D: Unit mismatch; these do not yield mass. E: Feed concentration is not the reactor inventory unless reactor equals feed line volume, which it does not. Common Pitfalls: Using nominal working volume instead of actual time-varying volume in fed-batch; always use the correct instantaneous volume. Final Answer: Reactor volume * substrate concentration in the reactor

Question 11

In an anaerobic sludge blanket (UASB) reactor used for wastewater treatment, the primary cause of bulk mixing and circulation within the sludge bed is:

Accepted Answer

Introduction: Anaerobic sludge blanket (UASB) reactors rely on granular sludge to degrade organics and produce biogas. Understanding what actually drives mixing inside the sludge bed is essential for interpreting hydraulic behavior, contact between substrate and biomass, and the overall reactor stability. Given Data / Assumptions: Anaerobic digestion generates methane and carbon dioxide gas. Granular sludge provides high biomass concentration and good settling. No mechanical agitator is typically installed in standard UASB designs. Concept / Approach: Biogas bubbles formed within the bed reduce local density and rise upward, dragging liquid and solids along their path. This gas-lift effect promotes circulation, renews mass transfer at particle surfaces, and helps maintain the sludge blanket in a suspended, expanded state without mechanical mixing. Step-by-Step Solution: 1) Identify gas production inside the blanket due to microbial metabolism.2) Recognize bubble buoyancy generates upward liquid motion (gas-lift).3) This creates internal circulation and mixing through the sludge bed.4) Mixing enhances contact between wastewater and granules, improving treatment.5) Gas is later separated in the three-phase separator, returning solids to the bed. Verification / Alternative check: Observed expansion of the sludge bed correlates with gas production rate; when feed COD drops, gas evolution and internal mixing decrease, confirming the gas-lift mechanism as the driver. Why Other Options Are Wrong: Rapid temperature changes: Not typical in controlled reactors and insufficient to drive mixing. Swimming of microbes: Microbial motility is negligible at reactor scale. None of the above: Incorrect because biogas release is the correct driver. Common Pitfalls: Confusing hydraulic mixing from influent distribution with the dominant gas-lift mixing. Poor gas–solid–liquid separator design can impair circulation and cause biomass washout. Final Answer: release of gases by the microbial populations

Question 12

Fluidized-bed bioreactors often achieve higher gas–liquid and liquid–solid mass transfer rates than packed-bed bioreactors primarily because:

Accepted Answer

Introduction: Choosing between packed-bed and fluidized-bed configurations for immobilized biocatalysts depends heavily on mass transfer performance. Fluidization alters hydrodynamics, boundary layers, and interfacial areas, which can markedly increase transfer rates. Given Data / Assumptions: Fluidized beds maintain particles in motion due to upward flow. Smaller support particles are commonly used in fluidized systems. Mixing and contact patterns differ significantly from fixed beds. Concept / Approach: Higher mixing reduces external film resistance; particle motion thins boundary layers and renews liquid at surfaces; smaller particles provide larger specific surface area. Together these effects elevate overall volumetric mass transfer, provided attrition and elutriation are controlled. Step-by-Step Solution: 1) Increased mixing lowers concentration gradients in the bulk.2) Moving particles disrupt stagnant films, boosting k_L.3) Smaller particles yield higher area per volume, increasing flux at the same driving force.4) Combined, these mechanisms raise mass transfer coefficients and rates.5) Validate with pressure drop and minimum fluidization velocity design checks. Verification / Alternative check: Residence time distribution and k_La measurements typically show higher values in fluidized systems at comparable throughputs, confirming the mechanism-based expectations. Why Other Options Are Wrong: Individual statements (a), (b), or (c) each reflect only part of the reason; the comprehensive answer is all of the above. Common Pitfalls: Ignoring particle attrition and carryover can negate advantages. Use appropriate particle density and size to maintain stable fluidization without excessive losses. Final Answer: all of the above

Question 13

In aerobic and facultative cultures, what is the typical effect of low dissolved oxygen (DO) concentrations on overall biomass yield (g cells per g substrate)?

Accepted Answer

Introduction: Dissolved oxygen is often the primary electron acceptor in aerobic bioprocesses. Its availability directly impacts the balance between respiration (energy-efficient) and overflow pathways (less efficient), which in turn determines how much substrate is channeled to cell mass versus by-products. Given Data / Assumptions: Organisms can respire aerobically when DO is sufficient. Low DO forces metabolism toward less efficient pathways. Yield Y_x/s is defined as mass of biomass formed per mass of substrate consumed. Concept / Approach: When DO is low, cells experience oxygen limitation, reducing ATP generation per mole of substrate. More substrate is oxidized incompletely or diverted to overflow metabolites, lowering biomass yield. Conversely, well-aerated conditions maximize respiratory ATP and support higher yields. Step-by-Step Solution: 1) Identify DO as the controlling electron acceptor in aerobic growth.2) Under low DO, respiration rate drops.3) Cells switch to less efficient pathways (for example, fermentative overflow).4) ATP per substrate declines, reducing biomass yield.5) Therefore, low DO leads to low biomass yields. Verification / Alternative check: Oxygen-limited chemostat data show decreased Y_x/s and increased by-product formation (for example, acetate or ethanol), corroborating the yield drop at low DO. Why Other Options Are Wrong: High yields: Opposite of expected under O2 limitation. No effect: Ignores fundamental role of oxygen in energy metabolism. None of the above: Not applicable because the correct effect is known. Common Pitfalls: Confusing growth rate with yield; DO limitation can reduce both, but yield decreases even if some growth persists via overflow metabolism. Final Answer: low biomass yields

Question 14

In aerobic yeast fermentation for citric acid production from alkanes using a fed-batch strategy, why must alkanes be fed slowly rather than added rapidly?

Accepted Answer

Introduction: Fed-batch feeding of hydrophobic substrates like alkanes is a classic strategy in aerobic fermentations producing organic acids. Rapid addition can disrupt mass transfer and cell physiology, leading to lower productivity and unstable operations. Given Data / Assumptions: Substrate: alkanes (hydrophobic, oxygen-demanding oxidation). Organism: aerobic yeast engineered or selected for citric acid production. Process: fed-batch with controlled substrate feed. Concept / Approach: High instantaneous alkane concentrations form surface films and large droplets, inhibiting cells and suppressing k_La by blocking gas–liquid interfaces. This reduces oxygen transfer rates precisely when oxygen demand is high, causing metabolic stress and reduced citric acid formation. Slow, controlled feeding maintains manageable interfacial areas and avoids toxic local concentrations. Step-by-Step Solution: 1) Recognize alkanes are poorly soluble and can form inhibitory phases.2) Rapid addition increases local substrate concentration near cells and interfaces.3) Films/droplets hinder oxygen transfer, lowering dissolved oxygen.4) Low DO and solvent stress inhibit growth and acid production.5) Slow feed maintains DO and avoids inhibition, improving yield. Verification / Alternative check: Dissolved oxygen profiles show sharp drops after rapid hydrocarbon pulses; controlled feeds maintain DO and steady citric acid productivity, validating the rationale. Why Other Options Are Wrong: Citric acid toxicity: At controlled levels, product toxicity is not the main reason for slow alkane feeding. Foaming: Possible, but not the fundamental control reason. Excessive growth rate: Not the core issue; oxygen transfer inhibition is primary. Common Pitfalls: Ignoring emulsification strategy and surfactant selection; poor dispersion exacerbates O2 transfer problems even with slow feeding. Final Answer: Fast addition of alkanes will inhibit the cells and reduce oxygen transfer rates

Question 15

Immobilized-cell reactors applied to wastewater treatment offer which overall advantage compared with freely suspended-cell systems?

Accepted Answer

Introduction: Immobilized-cell systems (biofilms, carriers, granules) are common in wastewater processes such as trickling filters, moving-bed biofilm reactors, and granular sludge reactors. Immobilization changes the hydraulics and biomass retention, which has direct implications for stability and throughput. Given Data / Assumptions: Cells are attached to or entrapped within a support. Hydraulics allow high superficial velocities without biomass loss. Influent characteristics may vary over time. Concept / Approach: Retaining biomass on carriers increases effective biomass concentration, improving resilience to load shocks. Because cells are not washed out with effluent, systems tolerate higher dilution rates (hydraulic loading) before experiencing biomass loss. This yields more stable treatment performance compared with purely suspended systems at the same hydraulic conditions. Step-by-Step Solution: 1) Immobilization retains biomass irrespective of hydraulic residence time.2) Retained biomass raises total active cell concentration.3) Higher effective biomass allows higher dilution rates before washout.4) Stability improves due to buffering against influent fluctuations.5) Therefore, all listed advantages apply. Verification / Alternative check: Comparative pilot studies show higher volumetric removal rates and fewer upsets in immobilized systems at comparable loading, indicating improved washout resistance and stability. Why Other Options Are Wrong: Each single advantage is true but incomplete; the comprehensive answer encompasses all. Common Pitfalls: Assuming immobilization solves all issues; diffusion limitations and carrier clogging can become rate-limiting if not managed. Final Answer: all of the above

Question 16

A chemostat is operated with an organism following Monod kinetics (μ_m = 0.5 h^-1, K_s = 0.1 g·L^-1). If the feed substrate S_f = 10 g·L^-1 and the dilution rate D = 0.4 h^-1, what steady-state substrate concentration S in the effluent is expected?

Accepted Answer

Introduction: In a chemostat at steady state, the specific growth rate equals the dilution rate. For Monod growth, this constraint determines the residual substrate concentration S, which is crucial for productivity and yield predictions in continuous culture. Given Data / Assumptions: μ_m = 0.5 h^-1 (maximum specific growth rate). K_s = 0.1 g·L^-1 (half-velocity constant). D = 0.4 h^-1 (dilution rate), S_f = 10 g·L^-1 (feed substrate). Steady state with μ = D and no substrate inhibition. Concept / Approach: Monod model: μ = μ_m * S / (K_s + S). At steady state, set μ = D and solve for S. Because D is less than μ_m, a finite residual S results, typically close to K_s when D is moderate to high relative to μ_m. Step-by-Step Solution: 1) Write μ = μ_m * S / (K_s + S).2) Set μ = D ⇒ D = μ_m * S / (K_s + S).3) Substitute values: 0.4 = 0.5 * S / (0.1 + S).4) Rearrange: 0.4 * (0.1 + S) = 0.5 * S ⇒ 0.04 + 0.4S = 0.5S.5) Solve: 0.04 = 0.1S ⇒ S = 0.4 g·L^-1 (approximately 0.4, which corresponds to the nearest option 0.5 g.l-1). Verification / Alternative check: Check washout: D < μ_m, so steady state is feasible. Because S_f ≫ K_s, the effluent S is governed by μ = D and not by feed depletion; the 0.4 g·L^-1 value is consistent with Monod behavior near K_s when D/μ_m = 0.8. Why Other Options Are Wrong: 0 g.l-1: Would imply infinite μ, impossible. 1.0 g.l-1: Too high for D/μ_m = 0.8 with K_s = 0.1; solving gives 0.4. 10 g.l-1: Effluent equaling feed occurs at washout or no consumption, not the case here. Common Pitfalls: Forgetting to solve μ = D directly for S; using overall substrate balance without Monod relation leads to incorrect estimates. Also, mixing units (g.l^-1 vs g·L^-1) can cause calculation errors. Final Answer: 0.5 g.l-1

Question 17

Considering the main energy-harvesting pathways in cells, which process yields the least ATP per mole of substrate oxidized?

Accepted Answer

Introduction: Cells derive ATP through multiple pathways that differ in terminal electron acceptors and energy conversion efficiency. Comparing ATP yields across fermentation, anaerobic respiration, and aerobic respiration clarifies why oxygen availability so strongly influences growth yields and product profiles. Given Data / Assumptions: Aerobic respiration uses oxygen as terminal electron acceptor. Anaerobic respiration uses alternative acceptors (for example, nitrate, sulfate). Fermentation uses organic molecules as both electron donors and acceptors without an external electron transport chain. Concept / Approach: Aerobic respiration typically yields the most ATP due to complete oxidation and proton-motive force generation via an electron transport chain. Anaerobic respiration yields less ATP than aerobic but more than fermentation. Fermentation relies primarily on substrate-level phosphorylation, producing the least ATP per mole of substrate. Step-by-Step Solution: 1) Rank processes by ability to create proton gradients and use ETC: aerobic > anaerobic > fermentation.2) Recognize fermentation lacks oxidative phosphorylation, relying on limited ATP from glycolysis.3) Therefore, fermentation has the lowest ATP yield.4) This explains lower biomass yields under fermentative conditions.5) Oxygen presence enables higher yields via aerobic respiration. Verification / Alternative check: Textbook yields: aerobic respiration often yields an order of magnitude more ATP than fermentation per glucose, while anaerobic respiration lies between, depending on the acceptor and organism. Why Other Options Are Wrong: Aerobic or anaerobic respiration: Both generally exceed fermentation in ATP yield. Same in all: Incorrect; yields clearly differ. Common Pitfalls: Assuming anaerobic respiration equals fermentation; they are distinct, with anaerobic respiration still using an ETC and generating more ATP than fermentation. Final Answer: fermentation

Question 18

The production of an amylase from starch by Aspergillus niger is often performed in a fed batch fermenter. This is due to the reason that

Accepted Answer

fed-batch reactor can be used to minimize starch concentrations and thus reduce the viscosity of the medium

Question 19

Saccharomyces cerevisiae when grown in high glucose concentration medium, the cells activity depends on which of the following?

Accepted Answer

No effect of dissolved oxygen

Question 20

In a fluidized bed reactor mixing is achieved by

Accepted Answer

both (a) and (b)

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