Q.1 What is the primary purpose of a bioreactor in environmental biotechnology?
To generate electricity directly from waste
To provide a controlled environment for microbial growth and biochemical reactions
To store hazardous chemicals safely
To convert solar energy into chemical fuels
Explanation - Bioreactors are designed to maintain optimal conditions (temperature, pH, oxygen, nutrients) for microorganisms to degrade pollutants or produce valuable products.
Correct answer is: To provide a controlled environment for microbial growth and biochemical reactions
Q.2 Which parameter is most critical for aerobic bioreactors treating wastewater?
Dissolved oxygen concentration
pH
Temperature
Mixing speed
Explanation - Aerobic microorganisms require oxygen; insufficient dissolved oxygen limits the rate of pollutant degradation.
Correct answer is: Dissolved oxygen concentration
Q.3 In a continuously stirred tank reactor (CSTR), the steady‑state substrate concentration is half of the inlet concentration. Which of the following statements is true?
The dilution rate equals the specific growth rate of the microorganisms.
The reactor is operating under substrate inhibition.
The residence time is twice the hydraulic retention time.
The reactor volume must be doubled.
Explanation - At steady state in a CSTR, when the substrate concentration drops to half, the dilution rate (D) equals the microbial specific growth rate (μ) under Monod kinetics.
Correct answer is: The dilution rate equals the specific growth rate of the microorganisms.
Q.4 Which of the following is NOT a typical design consideration for a packed‑bed bioreactor used in gas‑phase bioprocesses?
Void fraction of the packing material
Mass transfer coefficient of gas to liquid
Heat removal through jacket cooling
Magnetic stirring speed
Explanation - Packed‑bed reactors rely on natural convection or forced gas flow; magnetic stirring is not applicable.
Correct answer is: Magnetic stirring speed
Q.5 What does the term 'hydraulic retention time' (HRT) represent in wastewater bioreactors?
The time required for microorganisms to double their population.
The average time the liquid spends in the reactor.
The period needed to reach steady‑state temperature.
The duration of the cleaning cycle.
Explanation - HRT = reactor volume / influent flow rate; it indicates how long wastewater remains in the bioreactor for treatment.
Correct answer is: The average time the liquid spends in the reactor.
Q.6 Which kinetic model best describes microbial growth when substrate inhibition is significant?
Monod model
Haldane model
First‑order decay model
Zero‑order model
Explanation - The Haldane model incorporates substrate inhibition, unlike the Monod model which assumes no inhibition at high substrate levels.
Correct answer is: Haldane model
Q.7 In an air‑lift bioreactor, the primary mechanism for mixing is:
Mechanical impellers
Pneumatic lift of liquid due to gas bubbles
Magnetic stirring
Ultrasonic agitation
Explanation - Air‑lift reactors use rising bubbles to create a circulation loop, providing gentle mixing and high oxygen transfer.
Correct answer is: Pneumatic lift of liquid due to gas bubbles
Q.8 Which of the following is a major advantage of using a membrane bioreactor (MBR) over a conventional activated sludge system?
Higher energy consumption
Lower sludge production and better effluent quality
Requirement of larger land area
Inability to treat high‑strength wastewaters
Explanation - MBRs combine biological degradation with membrane filtration, resulting in high-quality effluent and reduced sludge yield.
Correct answer is: Lower sludge production and better effluent quality
Q.9 What is the purpose of adding antifoam agents in aerobic bioreactors?
To increase microbial growth rate
To reduce surface tension and prevent foam formation
To sterilize the reactor
To adjust pH
Explanation - Foam can cause operational problems; antifoam agents lower surface tension, breaking down foam bubbles.
Correct answer is: To reduce surface tension and prevent foam formation
Q.10 In a fed‑batch bioprocess, why is substrate typically added gradually rather than all at once?
To minimize the risk of substrate inhibition and control growth rate
To increase the reactor temperature rapidly
To reduce the need for mixing
To avoid the formation of biofilms on reactor walls
Explanation - Gradual feeding maintains optimal substrate concentration, preventing inhibition and allowing better control over product formation.
Correct answer is: To minimize the risk of substrate inhibition and control growth rate
Q.11 Which of the following gases is most commonly used as an electron acceptor in anaerobic bioreactors treating organic waste?
Oxygen
Nitrogen
Carbon dioxide
Methane
Explanation - In methanogenic anaerobic digestion, CO₂ serves as an electron acceptor, producing methane as the reduced product.
Correct answer is: Carbon dioxide
Q.12 The term 'biomass yield coefficient' (Y_X/S) represents:
The amount of substrate consumed per unit of biomass produced
The amount of biomass produced per unit of substrate consumed
The ratio of dissolved oxygen to substrate concentration
The rate of heat generation per unit biomass
Explanation - Y_X/S = ΔX/ΔS, indicating how efficiently microorganisms convert substrate into cell mass.
Correct answer is: The amount of biomass produced per unit of substrate consumed
Q.13 Which configuration is best suited for treating high‑strength industrial effluents with short hydraulic retention times?
Batch reactor
Plug‑flow reactor (PFR)
Sequencing batch reactor (SBR)
Trickling filter
Explanation - PFRs provide high biomass concentrations and short HRT, making them ideal for high‑strength waste streams.
Correct answer is: Plug‑flow reactor (PFR)
Q.14 In bioprocess engineering, what does the term 'scale‑up' refer to?
Increasing the temperature of a bioreactor
Transferring a process from laboratory to pilot or commercial scale while maintaining performance
Adding more microorganisms to the same reactor
Changing the substrate type
Explanation - Scale‑up involves adjusting parameters (mixing, mass transfer, etc.) so that the larger system behaves similarly to the smaller one.
Correct answer is: Transferring a process from laboratory to pilot or commercial scale while maintaining performance
Q.15 Which of the following is a key limitation of using pure cultures in industrial bioreactors for wastewater treatment?
Higher resistance to toxic compounds
Broader substrate utilization
Lower overall treatment efficiency compared to mixed cultures
Simpler process control
Explanation - Mixed microbial consortia can degrade a wider range of compounds and are more robust under variable conditions.
Correct answer is: Lower overall treatment efficiency compared to mixed cultures
Q.16 What is the main function of a sparger in an aerobic bioreactor?
To inject nutrients
To provide mechanical agitation
To disperse fine bubbles for oxygen transfer
To remove excess heat
Explanation - A sparger creates a bubble curtain, increasing the gas–liquid interfacial area and enhancing oxygen dissolution.
Correct answer is: To disperse fine bubbles for oxygen transfer
Q.17 The 'Monod constant' (K_s) is analogous to which parameter in enzyme kinetics?
V_max
K_m
k_cat
k_i
Explanation - K_s describes the substrate concentration at half‑maximum specific growth rate, similar to K_m in Michaelis‑Menten kinetics.
Correct answer is: K_m
Q.18 Which type of bioreactor is most suitable for producing high‑value recombinant proteins that require strict temperature control?
Open pond system
Stirred‑tank reactor with jacketed cooling
Trickle‑bed reactor
Anaerobic digester
Explanation - Jacketed stirred tanks allow precise temperature regulation, essential for protein folding and stability.
Correct answer is: Stirred‑tank reactor with jacketed cooling
Q.19 In a bioreactor, what does the term 'shear stress' refer to?
The stress experienced by microorganisms due to fluid movement
The electrical stress applied to the reactor walls
The chemical stress caused by pH fluctuations
The mechanical stress on the reactor's external casing
Explanation - High shear can damage delicate cells (e.g., mammalian), influencing reactor design and impeller selection.
Correct answer is: The stress experienced by microorganisms due to fluid movement
Q.20 Which of the following statements about anaerobic digestion is FALSE?
It produces biogas rich in methane.
It requires oxygen for microbial metabolism.
It proceeds through hydrolysis, acidogenesis, acetogenesis, and methanogenesis.
It reduces the volume of organic waste.
Explanation - Anaerobic digestion occurs in the absence of oxygen; it is an anaerobic process.
Correct answer is: It requires oxygen for microbial metabolism.
Q.21 When designing a bioreactor for the degradation of phenolic compounds, which parameter is most critical to monitor?
pH near neutral
High dissolved oxygen
Low temperature
High salt concentration
Explanation - Phenol degradation enzymes typically have optimal activity around pH 7; extreme pH can inhibit microbial activity.
Correct answer is: pH near neutral
Q.22 What is the primary advantage of using a sequencing batch reactor (SBR) over a continuous flow system for wastewater treatment?
Constant biomass concentration
Simplified operation with no need for separate clarifiers
Higher energy consumption
Inability to handle variable influent loads
Explanation - SBR integrates fill, react, settle, and decant steps in a single tank, eliminating the need for separate secondary clarifiers.
Correct answer is: Simplified operation with no need for separate clarifiers
Q.23 In a bioprocess, the term 'product inhibition' refers to:
The substrate preventing product formation
The accumulation of product slowing down microbial growth or enzyme activity
The increase in temperature due to product formation
The reduction of pH caused by the product
Explanation - When product concentrations become high, they can inhibit enzymes or cause feedback inhibition, reducing process rates.
Correct answer is: The accumulation of product slowing down microbial growth or enzyme activity
Q.24 Which sensor is typically used to monitor dissolved oxygen (DO) levels in aerobic bioreactors?
pH electrode
Clark-type electrochemical sensor
Thermocouple
Turbidity meter
Explanation - Clark-type sensors measure oxygen concentration based on diffusion and reduction at a cathode, providing real‑time DO data.
Correct answer is: Clark-type electrochemical sensor
Q.25 The term 'mass transfer coefficient' (k_La) in bioreactors quantifies:
The rate of substrate consumption by microbes
The rate of heat removal from the reactor
The efficiency of oxygen transfer from gas to liquid phase
The speed of mixing due to impellers
Explanation - k_La combines the liquid‑film mass transfer coefficient (k_L) and interfacial area (a) to describe gas‑liquid oxygen transfer.
Correct answer is: The efficiency of oxygen transfer from gas to liquid phase
Q.26 Which bioreactor type is most commonly used for large‑scale production of bioethanol from corn mash?
Batch fermenter
Fed‑batch fermenter
Continuous stirred‑tank reactor
Anaerobic packed‑bed reactor
Explanation - Fed‑batch allows controlled addition of sugars, maintaining optimal yeast activity and preventing substrate inhibition.
Correct answer is: Fed‑batch fermenter
Q.27 In a bioreactor, what is the purpose of a pH control system?
To regulate temperature
To add nutrients automatically
To maintain optimal enzyme activity and microbial growth
To increase dissolved oxygen
Explanation - pH affects enzyme kinetics and cell membrane stability; precise control ensures consistent bioprocess performance.
Correct answer is: To maintain optimal enzyme activity and microbial growth
Q.28 Which of the following is a major source of greenhouse gas emissions from anaerobic digesters?
Carbon dioxide released during aeration
Methane leakage
Nitrous oxide from nitrification
Sulfur dioxide from sulfide oxidation
Explanation - If biogas is not captured properly, methane—a potent greenhouse gas—can escape into the atmosphere.
Correct answer is: Methane leakage
Q.29 What does the term 'specific growth rate' (μ) represent in bioprocess kinetics?
The amount of substrate consumed per hour
The rate of biomass increase per unit of existing biomass
The total mass of product formed per day
The temperature change per unit time
Explanation - μ = (1/X)(dX/dt), indicating how fast the cell population grows relative to its current size.
Correct answer is: The rate of biomass increase per unit of existing biomass
Q.30 When scaling up a bioreactor, which dimensionless number is most useful for maintaining similar mixing conditions?
Reynolds number (Re)
Péclet number (Pe)
Damköhler number (Da)
Schmidt number (Sc)
Explanation - Re = (ρND^2)/μ; keeping Re constant helps preserve the ratio of inertial to viscous forces, ensuring comparable mixing.
Correct answer is: Reynolds number (Re)
Q.31 Which microorganism is commonly employed in the biodegradation of oil spills in marine environments?
Escherichia coli
Pseudomonas aeruginosa
Saccharomyces cerevisiae
Clostridium botulinum
Explanation - Pseudomonas species can metabolize hydrocarbons, making them effective for bioremediation of petroleum contaminants.
Correct answer is: Pseudomonas aeruginosa
Q.32 In a bioprocess, the term 'downstream processing' refers to:
The steps involved in cultivating microorganisms
The removal of waste streams from the plant
The purification and recovery of the desired product after fermentation
The monitoring of pH and temperature during fermentation
Explanation - Downstream processing includes cell separation, product extraction, and purification to obtain a final product.
Correct answer is: The purification and recovery of the desired product after fermentation
Q.33 Which factor most directly influences the rate of substrate diffusion into microbial cells in a bioreactor?
Temperature
Stirring speed
Cell membrane permeability
pH
Explanation - Diffusion across the cell membrane depends on its permeability; temperature and stirring affect bulk transport but not membrane diffusion.
Correct answer is: Cell membrane permeability
Q.34 The use of immobilized enzymes in a packed‑bed reactor primarily provides which benefit?
Higher temperature operation
Ease of enzyme recovery and reuse
Reduced need for pH control
Elimination of mass transfer limitations
Explanation - Immobilization allows enzymes to be retained in the reactor, facilitating repeated use and simplifying product separation.
Correct answer is: Ease of enzyme recovery and reuse
Q.35 What is the typical range of the oxygen transfer coefficient (k_La) for well‑mixed aerobic bioreactors?
0.01–0.1 h⁻¹
0.5–2.0 h⁻¹
10–30 h⁻¹
50–100 h⁻¹
Explanation - Well‑mixed aerobic reactors generally achieve k_La values between 0.5 and 2 h⁻¹, sufficient for most microbial processes.
Correct answer is: 0.5–2.0 h⁻¹
Q.36 Which bioreactor design inherently provides a high surface‑to‑volume ratio, beneficial for biofilm growth?
Stirred‑tank reactor
Air‑lift reactor
Trickle‑bed reactor
Bubble column reactor
Explanation - Trickle‑bed reactors have packed media offering extensive surface area for biofilm attachment.
Correct answer is: Trickle‑bed reactor
Q.37 During aerobic wastewater treatment, which process is primarily responsible for the removal of nitrogen as nitrate?
Ammonification
Nitrification
Denitrification
Nitrogen fixation
Explanation - Nitrification oxidizes ammonia to nitrite and then nitrate, which can later be removed by denitrification.
Correct answer is: Nitrification
Q.38 In bioreactor design, the term 'dead zone' refers to:
A region where temperature is too high
An area with little to no mixing and low mass transfer
A zone where microorganisms are most active
A region with excessive foaming
Explanation - Dead zones can cause uneven substrate distribution and affect overall reactor performance.
Correct answer is: An area with little to no mixing and low mass transfer
Q.39 Which of the following is a common method to sterilize a bioreactor before inoculation?
Chemical disinfectants only
Autoclaving with steam at 121 °C for 15 min
UV irradiation of the reactor walls
Cooling to –20 °C
Explanation - Steam sterilization (autoclaving) is the standard method to eliminate contaminating microbes before starting a culture.
Correct answer is: Autoclaving with steam at 121 °C for 15 min
Q.40 Which parameter is most directly affected by the impeller diameter in a stirred‑tank bioreactor?
pH stability
Oxygen solubility
Shear rate and mixing intensity
Temperature gradient
Explanation - Larger impeller diameters increase tip speed, raising shear forces and mixing efficiency.
Correct answer is: Shear rate and mixing intensity
Q.41 A bioprocess aims to produce lactic acid using Lactobacillus species. Which pH range should be maintained for optimal production?
3.0–4.0
5.5–6.5
7.5–8.5
9.0–10.0
Explanation - Lactobacilli grow best in mildly acidic conditions; too low pH inhibits cell growth while too high reduces lactic acid yield.
Correct answer is: 5.5–6.5
Q.42 In bioprocess engineering, what is the main purpose of using a 'recirculation loop' in a bioreactor system?
To increase reactor volume
To enhance mass transfer and homogenize substrate concentration
To remove excess heat from the reactor
To introduce sterile air
Explanation - Recirculation improves mixing and can increase oxygen transfer by passing liquid through a high‑transfer zone.
Correct answer is: To enhance mass transfer and homogenize substrate concentration
Q.43 Which of the following statements about biofilm reactors is TRUE?
Biofilms completely prevent mass transfer limitations.
They require constant addition of fresh microorganisms.
They can achieve higher biomass densities than suspended‑cell reactors.
They are unsuitable for treating low‑strength wastewaters.
Explanation - Biofilms immobilize cells on surfaces, allowing much higher cell concentrations per reactor volume.
Correct answer is: They can achieve higher biomass densities than suspended‑cell reactors.
Q.44 Which of the following best describes the purpose of a 'heat exchanger' in a temperature‑controlled bioreactor?
To increase oxygen solubility
To regulate the temperature of the culture medium
To remove microbial cells from the broth
To adjust pH automatically
Explanation - Heat exchangers add or remove heat, maintaining the optimal temperature for microbial activity.
Correct answer is: To regulate the temperature of the culture medium
Q.45 Which gas composition is typically used for sparging in an aerobic bioreactor?
100% nitrogen
100% oxygen
Air (≈21% O₂, 78% N₂)
Carbon dioxide-rich gas
Explanation - Air provides sufficient oxygen while being inexpensive; pure oxygen is used only when higher transfer rates are needed.
Correct answer is: Air (≈21% O₂, 78% N₂)
Q.46 In a batch bioreactor, the time at which the product concentration reaches its maximum is called:
Lag phase
Stationary phase
Peak production time
Turn‑over point
Explanation - The product peaks during the stationary phase when growth slows but product synthesis continues until depletion of nutrients.
Correct answer is: Peak production time
Q.47 Which of the following is a typical by‑product of the anaerobic digestion of organic waste?
Methane-rich biogas
Oxygen gas
Sulfuric acid
Ammonia gas
Explanation - Anaerobic digestion yields biogas composed mainly of methane (CH₄) and carbon dioxide (CO₂).
Correct answer is: Methane-rich biogas
Q.48 When operating a bioreactor at high cell densities, which problem is most likely to arise?
Excessive foaming
Reduced oxygen transfer efficiency
Lower temperature control difficulty
Increased pH stability
Explanation - High cell concentrations increase oxygen demand; without adequate transfer, dissolved oxygen drops, limiting growth.
Correct answer is: Reduced oxygen transfer efficiency
Q.49 A 'chemostat' maintains a constant:
pH level
Temperature
Dilution rate
Substrate concentration
Explanation - In a chemostat, fresh medium is continuously added at a fixed rate, keeping the dilution rate constant and controlling growth.
Correct answer is: Dilution rate
Q.50 Which material is most commonly used for constructing the inner vessel of laboratory‑scale bioreactors due to its inertness?
Stainless steel
Glass
Aluminum
Plastic (polypropylene)
Explanation - Stainless steel resists corrosion, can be sterilized, and is chemically inert, making it ideal for bioreactors.
Correct answer is: Stainless steel
Q.51 In a bioprocess, the term 'upstream processing' includes:
Fermentation and cell cultivation
Product packaging
Waste disposal
Quality control testing
Explanation - Upstream processing covers all steps before product recovery, primarily microbial growth and metabolite formation.
Correct answer is: Fermentation and cell cultivation
Q.52 For a bioreactor treating nitrogen‑rich wastewater, which microbial process converts nitrate to nitrogen gas?
Nitrification
Ammonification
Denitrification
Nitrogen fixation
Explanation - Denitrifying bacteria reduce nitrate (NO₃⁻) to N₂ gas under anoxic conditions.
Correct answer is: Denitrification
Q.53 Which factor most directly influences the rate of heat generation in a high‑density aerobic fermentation?
Stirring speed
Oxygen uptake rate
pH of the medium
Substrate concentration
Explanation - Metabolism of substrates with oxygen releases heat; higher oxygen consumption leads to greater heat generation.
Correct answer is: Oxygen uptake rate
Q.54 The main advantage of a 'twin‑screw' impeller over a standard Rushton turbine in a bioreactor is:
Higher shear forces
Better oxygen transfer at low speeds
Reduced power consumption
Simpler design
Explanation - Twin‑screw impellers generate axial flow, improving gas dispersion without excessive shear.
Correct answer is: Better oxygen transfer at low speeds
Q.55 In a continuous bioreactor, the term 'steady‑state' means:
All variables change linearly with time
System variables (e.g., substrate, biomass) remain constant over time
The reactor temperature is constant
The pH fluctuates periodically
Explanation - At steady state, the rates of input and output balance, leading to constant concentrations inside the reactor.
Correct answer is: System variables (e.g., substrate, biomass) remain constant over time
Q.56 Which of the following is a primary reason to use an anaerobic membrane bioreactor (AnMBR) for wastewater treatment?
Higher oxygen transfer rates
Simultaneous removal of solids and production of biogas
Elimination of temperature control needs
Complete removal of all nutrients
Explanation - AnMBRs combine anaerobic digestion with membrane filtration, yielding solid‑free effluent and biogas.
Correct answer is: Simultaneous removal of solids and production of biogas
Q.57 The 'Péclet number' (Pe) in the context of bioreactors is used to compare:
Convective transport to diffusive transport
Thermal conductivity to viscosity
Reaction rate to mass transfer rate
Electrical conductivity to magnetic susceptibility
Explanation - Pe = (velocity × characteristic length)/diffusion coefficient; high Pe indicates convection dominates over diffusion.
Correct answer is: Convective transport to diffusive transport
Q.58 When operating a bioreactor at high pressures to increase gas solubility, which safety concern becomes most significant?
Foaming
Corrosion of the vessel
Risk of over‑pressurization and rupture
Excessive heating
Explanation - Pressurized systems must be designed with relief valves and safety factors to prevent catastrophic failure.
Correct answer is: Risk of over‑pressurization and rupture
Q.59 In the context of bioreactors, the term 'bioaugmentation' refers to:
Increasing the temperature to speed up reactions
Adding specialized microorganisms to enhance degradation of specific pollutants
Removing excess biomass from the system
Increasing the pH to alkaline levels
Explanation - Bioaugmentation introduces strains with desired metabolic capabilities to improve treatment performance.
Correct answer is: Adding specialized microorganisms to enhance degradation of specific pollutants
Q.60 Which analytical method is most commonly used to measure cell density in a liquid culture?
Gas chromatography
Spectrophotometric measurement of optical density (OD)
pH meter
Conductivity probe
Explanation - OD at 600 nm correlates with cell concentration for many microorganisms.
Correct answer is: Spectrophotometric measurement of optical density (OD)
Q.61 In a bioreactor treating phenol, the inhibitory concentration (IC₅₀) is 200 mg L⁻¹. If the inlet phenol concentration is 500 mg L⁻¹, which strategy could be employed to avoid inhibition?
Increase the temperature to 45 °C
Dilute the influent with water to reduce phenol concentration
Add more oxygen to the system
Decrease the pH to 3.0
Explanation - Dilution brings phenol concentration below the inhibitory threshold, allowing microbial degradation.
Correct answer is: Dilute the influent with water to reduce phenol concentration
Q.62 Which type of bioreactor is most appropriate for the production of antibiotics that require long residence times and low shear conditions?
Stirred‑tank reactor
Air‑lift reactor
Packed‑bed reactor
Bubble column reactor
Explanation - Air‑lift reactors provide gentle mixing with low shear, suitable for delicate filamentous microorganisms producing antibiotics.
Correct answer is: Air‑lift reactor
Q.63 The term 'substrate loading rate' (SLR) in a continuous bioreactor is defined as:
Influent flow rate divided by reactor volume
Mass of substrate entering per unit reactor volume per time
Concentration of substrate in the effluent
Rate of oxygen transfer per unit volume
Explanation - SLR = (Q_in × S_in) / V, where Q_in is flow rate, S_in is substrate concentration, V is reactor volume.
Correct answer is: Mass of substrate entering per unit reactor volume per time
Q.64 In a membrane bioreactor, fouling of the membrane primarily results from:
Excessive temperature
Accumulation of biomass, colloids, and organic matter on the membrane surface
High dissolved oxygen levels
Low pH
Explanation - Fouling reduces permeability; cleaning protocols are required to maintain performance.
Correct answer is: Accumulation of biomass, colloids, and organic matter on the membrane surface
Q.65 Which kinetic model is appropriate for describing substrate consumption when microbial growth is limited by the availability of a limiting nutrient other than the carbon source?
Monod model
Luedeking‑Piret model
Contois model
Haldane model
Explanation - Contois kinetics incorporate biomass concentration in the denominator, suitable for systems where substrate uptake depends on both substrate and cell density.
Correct answer is: Contois model
Q.66 During the downstream processing of a recombinant protein, which step typically follows cell harvest?
Chromatography purification
Fermentation
Cell lysis or disruption
Formulation into final dosage form
Explanation - After harvesting, cells must be broken to release intracellular product before purification.
Correct answer is: Cell lysis or disruption
Q.67 Which of the following is a typical indicator of excessive foaming in an aerobic bioreactor?
Drop in pH
Increase in dissolved oxygen
Rapid rise of liquid level and overflow
Decrease in temperature
Explanation - Foam expands the liquid volume, potentially causing overflow and operational issues.
Correct answer is: Rapid rise of liquid level and overflow
Q.68 In an anaerobic digester, the 'acidogenic' phase is characterized by:
Production of methane and carbon dioxide
Conversion of complex organics to volatile fatty acids
Oxidation of ammonia to nitrate
Complete mineralization of all organics
Explanation - Acidogenesis converts polymers into acids, which are later utilized by methanogens.
Correct answer is: Conversion of complex organics to volatile fatty acids
Q.69 Which parameter is most directly responsible for controlling the residence time of microorganisms in a continuous bioreactor?
Temperature
Dilution rate
pH
Agitator speed
Explanation - Residence time = 1/Dilution rate; adjusting D changes how long cells stay in the reactor.
Correct answer is: Dilution rate
Q.70 A bioprocess engineer wants to minimize the formation of inhibitory by‑products during high‑density yeast fermentation. Which feeding strategy is most appropriate?
Batch feeding all substrate at the start
Fed‑batch with exponential feeding rate matching growth
Continuous feeding at constant rate regardless of growth
No feeding after inoculation
Explanation - Matching feed to growth prevents substrate accumulation, reducing by‑product inhibition.
Correct answer is: Fed‑batch with exponential feeding rate matching growth
Q.71 The term 'bioprocess intensification' refers to:
Increasing the size of the bioreactor
Improving productivity per unit volume through advanced technologies
Reducing the number of process steps
Eliminating the need for downstream processing
Explanation - Intensification aims to achieve higher yields, rates, or efficiencies without scaling up the equipment size.
Correct answer is: Improving productivity per unit volume through advanced technologies
Q.72 Which of the following is a common method to control pH in a bioreactor during fermentation?
Adding sodium hydroxide or hydrochloric acid as needed
Increasing stirring speed
Increasing temperature
Adjusting the inlet flow rate
Explanation - Acid or base solutions are automatically added based on pH sensor feedback to maintain target pH.
Correct answer is: Adding sodium hydroxide or hydrochloric acid as needed
Q.73 In a bioreactor, the term 'gas‑liquid mass transfer coefficient' (k_La) is most affected by:
Impeller blade material
Bubble size and interfacial area
pH of the medium
Nutrient concentration
Explanation - Smaller bubbles increase surface area, enhancing k_La and oxygen transfer efficiency.
Correct answer is: Bubble size and interfacial area
Q.74 Which of the following microorganisms is typically used for the production of polyhydroxyalkanoates (PHAs) in bioreactors?
Saccharomyces cerevisiae
Cupriavidus necator
Lactobacillus acidophilus
Escherichia coli
Explanation - C. necator is a model organism for PHA accumulation under nutrient‑limiting conditions.
Correct answer is: Cupriavidus necator
Q.75 What is the main environmental benefit of using a bioreactor for treatment of industrial effluents compared to chemical precipitation?
Higher energy consumption
Generation of reusable biomass and lower sludge production
Need for expensive catalysts
Increased greenhouse gas emissions
Explanation - Biological treatment converts pollutants into biomass, reducing waste and often producing useful by‑products.
Correct answer is: Generation of reusable biomass and lower sludge production
Q.76 In a high‑cell‑density fermentation, why might a temperature shift be employed after the growth phase?
To increase oxygen solubility
To trigger product formation while slowing cell growth
To sterilize the culture
To reduce pH fluctuations
Explanation - Lowering temperature can reduce metabolic rates, diverting resources toward product synthesis rather than biomass.
Correct answer is: To trigger product formation while slowing cell growth
Q.77 Which of the following best describes the function of a 'baffles' in a stirred‑tank bioreactor?
To increase gas solubility
To reduce vortex formation and improve mixing
To provide attachment sites for biofilm
To heat the culture medium
Explanation - Baffles disrupt the formation of a central vortex, enhancing axial mixing and oxygen transfer.
Correct answer is: To reduce vortex formation and improve mixing
Q.78 During the anaerobic digestion of wastewater, the accumulation of which intermediate indicates a possible process upset?
Methane
Acetate
Carbon dioxide
Hydrogen sulfide
Explanation - Acetate buildup suggests that methanogens are inhibited or overloaded, leading to process instability.
Correct answer is: Acetate
Q.79 Which type of control strategy is most suitable for maintaining a constant dissolved oxygen level in an aerobic bioreactor?
On‑off control
Proportional‑Integral‑Derivative (PID) control
Feedforward control only
Manual adjustment
Explanation - PID controllers continuously adjust aeration and agitation to keep DO at the setpoint with minimal overshoot.
Correct answer is: Proportional‑Integral‑Derivative (PID) control
Q.80 The 'specific substrate uptake rate' (q_s) is defined as:
Rate of substrate consumption per unit biomass
Total substrate consumption in the reactor
Substrate concentration in the feed
Biomass concentration per unit time
Explanation - q_s = (1/X)(dS/dt) and is a key parameter in metabolic modeling.
Correct answer is: Rate of substrate consumption per unit biomass
Q.81 In a bioprocess, what is the purpose of a 'seed train'?
To increase the temperature gradually
To progressively scale up the inoculum volume from small to large reactors
To sterilize the bioreactor
To add nutrients continuously
Explanation - A seed train ensures a healthy, high‑density culture is transferred stepwise to the production bioreactor.
Correct answer is: To progressively scale up the inoculum volume from small to large reactors
Q.82 Which of the following is a typical method for removing dissolved gases (e.g., CO₂) from a bioreactor broth?
Increasing agitation speed
Using a degassing membrane
Adding antifoam agents
Lowering the pH
Explanation - Degassing membranes allow selective removal of gases without losing liquid, improving gas balance.
Correct answer is: Using a degassing membrane
Q.83 A bioreactor operates at a constant temperature of 37 °C, pH 7.0, and dissolved oxygen 30 % saturation. Which of the following changes would most likely increase the specific growth rate of E. coli?
Reducing the agitation speed
Increasing the inlet glucose concentration while keeping other conditions constant
Lowering the temperature to 30 °C
Adding 0.5 % (w/v) sodium chloride
Explanation - More substrate (glucose) supplies more carbon and energy, potentially raising the specific growth rate if not limited by other factors.
Correct answer is: Increasing the inlet glucose concentration while keeping other conditions constant
Q.84 Which of the following best explains why membrane fouling is more severe in high‑cell‑density cultures?
Higher temperatures increase fouling rates
Increased biomass leads to more particles and extracellular polymeric substances depositing on the membrane
Higher pH neutralizes fouling agents
Greater oxygen transfer reduces fouling
Explanation - Cellular debris and EPS readily adhere to membrane surfaces, decreasing permeability.
Correct answer is: Increased biomass leads to more particles and extracellular polymeric substances depositing on the membrane
Q.85 When modeling a bioreactor using the Monod equation, the half‑saturation constant (K_s) is 0.2 g L⁻¹. What substrate concentration corresponds to approximately 66 % of the maximum specific growth rate?
0.2 g L⁻¹
0.4 g L⁻¹
0.6 g L⁻¹
1.0 g L⁻¹
Explanation - At S = 2·K_s, μ = (2/3) μ_max ≈ 66 % of μ_max.
Correct answer is: 0.4 g L⁻¹
Q.86 In an industrial bioprocess, why is it often necessary to conduct a 'sterile filtration' step after fermentation but before downstream processing?
To remove all dissolved gases
To eliminate residual microorganisms and spores that could contaminate later steps
To increase the temperature of the broth
To add nutrients for the next stage
Explanation - Sterile filtration ensures that downstream equipment remains free of live cells, preventing product degradation or safety issues.
Correct answer is: To eliminate residual microorganisms and spores that could contaminate later steps
Q.87 Which of the following is the most appropriate method to increase the residence time in a continuous stirred‑tank reactor without changing its volume?
Decrease the influent flow rate
Increase the agitation speed
Raise the temperature
Add more antifoam
Explanation - Residence time = V/Q; reducing Q (flow rate) lengthens residence time while volume stays constant.
Correct answer is: Decrease the influent flow rate
Q.88 A bioreactor is experiencing a sudden drop in pH during fermentation. Which of the following is the most likely cause?
Excessive oxygen supply
Accumulation of acidic metabolic by‑products (e.g., lactic acid)
Increase in temperature
Loss of agitation
Explanation - Many fermentations produce acids that lower pH unless neutralized.
Correct answer is: Accumulation of acidic metabolic by‑products (e.g., lactic acid)
Q.89 What is the primary function of a 'peristaltic pump' in a fed‑batch bioprocess?
To sterilize the medium
To provide precise, contamination‑free feeding of substrate solutions
To control reactor temperature
To measure dissolved oxygen
Explanation - Peristaltic pumps move fluids through a closed tubing system, minimizing contamination risk.
Correct answer is: To provide precise, contamination‑free feeding of substrate solutions
Q.90 Which of the following best describes the 'critical micelle concentration' (CMC) in the context of using surfactants in bioreactors?
The concentration at which surfactants become toxic to microbes
The concentration at which surfactants start forming micelles, affecting mass transfer
The maximum allowable surfactant concentration for foaming control
The concentration required to sterilize the broth
Explanation - Above the CMC, surfactants aggregate, which can alter substrate availability and oxygen transfer.
Correct answer is: The concentration at which surfactants start forming micelles, affecting mass transfer
Q.91 In a bioreactor treating phenol, the degradation pathway proceeds through catechol as an intermediate. Which enzyme is responsible for the ring‑cleavage of catechol in the ortho‑cleavage pathway?
Catechol 1,2‑dioxygenase
Catechol 2,3‑dioxygenase
Phenol hydroxylase
Cytochrome P450
Explanation - Catechol 1,2‑dioxygenase catalyzes ortho‑cleavage of catechol, leading to cis,cis‑muconic acid.
Correct answer is: Catechol 1,2‑dioxygenase
Q.92 Which factor most directly determines the maximum achievable cell density in a suspended‑cell bioreactor?
Impeller blade material
Oxygen transfer rate (OTR)
Color of the reactor vessel
Length of the inlet pipe
Explanation - High cell density requires sufficient OTR to meet the metabolic oxygen demand of the culture.
Correct answer is: Oxygen transfer rate (OTR)
Q.93 During scale‑up, which of the following dimensionless numbers is most commonly kept constant to preserve the ratio of reaction rate to mass transfer rate?
Damköhler number (Da)
Reynolds number (Re)
Prandtl number (Pr)
Froude number (Fr)
Explanation - Da = (reaction rate)/(mass transfer rate); keeping Da constant helps maintain similar conversion efficiencies during scale‑up.
Correct answer is: Damköhler number (Da)
Q.94 In a bioreactor, the term 'lag phase' refers to:
The period of rapid cell division
The initial period where cells adapt to the new environment before exponential growth
The phase when nutrients are completely depleted
The stage when product accumulation is maximal
Explanation - During lag phase, cells synthesize necessary enzymes and adjust metabolism before rapid growth begins.
Correct answer is: The initial period where cells adapt to the new environment before exponential growth
Q.95 Which of the following is a key advantage of using a two‑stage bioreactor system for the production of a toxic metabolite?
Higher temperature operation in both stages
Separation of growth and production phases, protecting cells from product toxicity
Reduced need for pH control
Elimination of oxygen supply
Explanation - The first stage focuses on biomass accumulation; the second stage provides conditions favoring product synthesis while limiting growth, reducing toxic effects.
Correct answer is: Separation of growth and production phases, protecting cells from product toxicity
Q.96 Which type of bioreactor is most suitable for the production of recombinant proteins that require post‑translational modifications such as glycosylation?
Bacterial stirred‑tank reactor
Yeast fed‑batch reactor
Mammalian cell perfusion bioreactor
Algal photobioreactor
Explanation - Mammalian cells provide the necessary cellular machinery for complex post‑translational modifications.
Correct answer is: Mammalian cell perfusion bioreactor
Q.97 In bioprocess control, a 'cascade control' strategy typically involves:
Two controllers where the output of the primary controller serves as the set‑point for the secondary controller
A single controller handling multiple variables simultaneously
Manual adjustments based on operator observations
Using temperature control to regulate pH
Explanation - Cascade control improves response time and stability, e.g., DO controller adjusts agitator speed, which then controls oxygen transfer.
Correct answer is: Two controllers where the output of the primary controller serves as the set‑point for the secondary controller
Q.98 Which of the following is the most appropriate method to reduce the risk of microbial contamination during the inoculation step of a sterile bioreactor?
Opening the reactor for a prolonged period
Performing inoculation inside a laminar flow hood
Adding antifoam before inoculation
Increasing the temperature to 60 °C
Explanation - A laminar flow hood provides a clean environment, minimizing airborne contaminants during inoculation.
Correct answer is: Performing inoculation inside a laminar flow hood
Q.99 When operating a high‑temperature bioreactor (e.g., 55 °C), which type of material is most suitable for the reactor vessel to avoid corrosion?
Stainless steel 304
Glass
Aluminum alloy
Copper
Explanation - Stainless steel 304 offers good corrosion resistance at elevated temperatures, suitable for many industrial bioprocesses.
Correct answer is: Stainless steel 304
Q.100 In a bioreactor treating oil‑containing wastewater, which additive can enhance the bioavailability of hydrocarbons to microbes?
Sodium chloride
Surfactants (e.g., Tween 80)
Hydrochloric acid
Calcium carbonate
Explanation - Surfactants lower interfacial tension, increasing the solubility of hydrophobic compounds for microbial uptake.
Correct answer is: Surfactants (e.g., Tween 80)
Q.101 Which analytical technique is commonly used to monitor the concentration of volatile fatty acids (VFAs) in an anaerobic digester?
Gas chromatography (GC)
High‑performance liquid chromatography (HPLC)
Spectrophotometric pH measurement
Flow cytometry
Explanation - GC separates and quantifies volatile compounds like acetate, propionate, and butyrate in digester effluent.
Correct answer is: Gas chromatography (GC)
Q.102 During scale‑up of a bioreactor, which phenomenon often becomes more pronounced, potentially leading to reduced product yield?
Enhanced mixing
Increased oxygen transfer rates
Higher temperature gradients
Lower shear stress
Explanation - Larger reactors can develop temperature hotspots due to limited heat removal, affecting microbial activity and yield.
Correct answer is: Higher temperature gradients