Q.1 What is the primary component of biogas produced from anaerobic digestion of organic waste?
Methane (CH₄)
Carbon dioxide (CO₂)
Hydrogen (H₂)
Nitrogen (N₂)
Explanation - Anaerobic digestion converts organic matter into biogas, which typically contains 50‑70% methane, the main energy carrier.
Correct answer is: Methane (CH₄)
Q.2 Which of the following processes converts lignocellulosic waste into ethanol?
Fermentation after enzymatic hydrolysis
Direct combustion
Pyrolysis
Electrolysis
Explanation - Lignocellulosic biomass must first be broken down into sugars by enzymes; these sugars are then fermented by microorganisms to produce ethanol.
Correct answer is: Fermentation after enzymatic hydrolysis
Q.3 In a biodiesel production plant, what is the role of a catalyst such as NaOH?
To accelerate transesterification of triglycerides
To increase the boiling point of the product
To remove water from the feedstock
To act as a fuel additive
Explanation - NaOH provides hydroxide ions that speed up the reaction between triglycerides and alcohol, forming fatty acid methyl esters (biodiesel).
Correct answer is: To accelerate transesterification of triglycerides
Q.4 Which waste-derived biofuel has the highest energy density per kilogram?
Biodiesel
Biogas
Bioethanol
Biobutanol
Explanation - Biobutanol has an energy density (~33 MJ kg⁻¹) close to gasoline and higher than ethanol, biodiesel, and biogas (by mass).
Correct answer is: Biobutanol
Q.5 What is the main advantage of using micro‑algae over agricultural crops for biofuel production?
Higher photosynthetic efficiency and non‑competition with food land
Requires no water
Produces only methane
Can grow in total darkness
Explanation - Micro‑algae grow faster, have higher lipid content, and can be cultivated on non‑arable land, avoiding food‑fuel conflict.
Correct answer is: Higher photosynthetic efficiency and non‑competition with food land
Q.6 Which of the following is a common pretreatment method for municipal solid waste before anaerobic digestion?
Mechanical shredding
Flame‑spraying
Cryogenic grinding
Electroplating
Explanation - Shredding reduces particle size, increasing surface area and enhancing microbial access during digestion.
Correct answer is: Mechanical shredding
Q.7 In a solid oxide fuel cell (SOFC) using biogas, which component is primarily responsible for removing sulfur compounds?
Sulfur‑tolerant anode material
Cathode catalyst
Electrolyte membrane
Fuel reformer
Explanation - A reformer or desulfurization unit processes biogas to remove H₂S before it reaches the SOFC anode, protecting the cell.
Correct answer is: Fuel reformer
Q.8 Which type of reactor is most commonly used for continuous production of biodiesel?
Plug‑flow reactor
Batch reactor
CSTR (Continuous Stirred‑Tank Reactor)
Fluidized‑bed reactor
Explanation - CSTRs provide uniform mixing and temperature control, enabling steady‑state biodiesel production.
Correct answer is: CSTR (Continuous Stirred‑Tank Reactor)
Q.9 What is the main greenhouse gas avoided when waste‑derived biogas replaces natural gas in power generation?
Methane
Carbon monoxide
Nitrous oxide
Sulfur dioxide
Explanation - Biogas captures methane that would otherwise be emitted from waste; using it for energy avoids releasing this potent greenhouse gas.
Correct answer is: Methane
Q.10 Which of the following waste streams is richest in lignin, making it less suitable for ethanol production?
Food waste
Paper mill sludge
Sugarcane bagasse
Algae biomass
Explanation - Bagasse contains high lignin content, which is resistant to enzymatic hydrolysis, limiting its conversion to fermentable sugars.
Correct answer is: Sugarcane bagasse
Q.11 What is the purpose of adding a co‑solvent like methanol in the transesterification reaction for biodiesel?
To increase reactant solubility and drive the reaction forward
To act as a catalyst
To lower the reaction temperature
To neutralize acids formed
Explanation - Methanol dissolves triglycerides and reacts with them, shifting equilibrium toward biodiesel formation.
Correct answer is: To increase reactant solubility and drive the reaction forward
Q.12 Which parameter most directly affects the hydraulic retention time (HRT) in an anaerobic digester?
Volumetric flow rate of feedstock
Temperature of the digester
pH of the reactor
Type of inoculum
Explanation - HRT = reactor volume / influent flow rate; changing the flow rate directly changes retention time.
Correct answer is: Volumetric flow rate of feedstock
Q.13 Which biofuel can be directly blended with gasoline up to 85% without engine modification?
E85 (85% ethanol)
B20 (20% biodiesel)
Biogas
Bio‑hydrogen
Explanation - Flex‑fuel vehicles are designed to run on gasoline‑ethanol blends up to 85% ethanol.
Correct answer is: E85 (85% ethanol)
Q.14 During the pyrolysis of waste plastics, which product is primarily targeted for bio‑fuel use?
Liquid oil (pyrolysis oil)
Char
Syngas
Ash
Explanation - Pyrolysis of plastics yields a hydrocarbon‑rich liquid that can be refined into gasoline‑like fuels.
Correct answer is: Liquid oil (pyrolysis oil)
Q.15 What is the main function of a gas‑tight membrane in a bioreactor producing hydrogen from waste water?
To separate hydrogen from oxygen and prevent recombination
To increase temperature
To filter out solid particles
To catalyze the reaction
Explanation - A membrane keeps the produced H₂ away from O₂, avoiding explosive mixtures and improving yield.
Correct answer is: To separate hydrogen from oxygen and prevent recombination
Q.16 Which of the following is NOT a typical by‑product of biodiesel production?
Glycerol
Methane
Soap
Free fatty acids
Explanation - Methane is not generated in the transesterification process; glycerol, soaps, and free fatty acids are common side‑products.
Correct answer is: Methane
Q.17 In a bio‑refinery concept, what is the term for converting waste lignocellulose into platform chemicals like furfural?
Bioconversion
Biogasification
Bioliquefaction
Bioprocessing
Explanation - Bioconversion encompasses chemical transformations of biomass into value‑added intermediates such as furfural.
Correct answer is: Bioconversion
Q.18 Which type of microbial community is most efficient for degrading complex polymers in solid waste?
Mixed anaerobic consortia
Pure yeast cultures
Algal monocultures
Aerobic nitrifiers
Explanation - A diverse anaerobic community provides synergistic enzymes capable of hydrolyzing cellulose, hemicellulose, and lignin.
Correct answer is: Mixed anaerobic consortia
Q.19 What is the primary reason that bio‑hydrogen production from waste water is often coupled with photo‑bioreactors?
Light provides energy for photosynthetic bacteria to split water
It reduces temperature
It eliminates the need for nutrients
It increases carbon dioxide capture
Explanation - Photosynthetic microbes use light energy to drive water splitting, generating hydrogen as a by‑product.
Correct answer is: Light provides energy for photosynthetic bacteria to split water
Q.20 Which of the following waste‑to‑energy pathways has the highest overall conversion efficiency (energy out / energy in)?
Direct combustion of waste
Anaerobic digestion to biogas + CHP
Pyrolysis to bio‑oil
Gasification to syngas
Explanation - Combined heat and power (CHP) from biogas typically achieves 80‑90% of the chemical energy, higher than most thermal conversion routes.
Correct answer is: Anaerobic digestion to biogas + CHP
Q.21 When producing bio‑ethanol from municipal solid waste, why is it important to control the C/N ratio of the feedstock?
To balance microbial growth and prevent ammonia inhibition
To increase the boiling point of ethanol
To reduce the need for catalysts
To enhance flame stability
Explanation - An optimal carbon‑to‑nitrogen ratio ensures sufficient nutrients for fermentative microbes without excess ammonia that can inhibit fermentation.
Correct answer is: To balance microbial growth and prevent ammonia inhibition
Q.22 Which of the following gases is NOT typically found in biogas produced from waste?
Hydrogen sulfide (H₂S)
Methane (CH₄)
Carbon dioxide (CO₂)
Oxygen (O₂)
Explanation - Biogas is generated in anaerobic conditions, so oxygen is absent; H₂S, CH₄, and CO₂ are common constituents.
Correct answer is: Oxygen (O₂)
Q.23 Which catalytic material is most commonly used for upgrading bio‑oil obtained from fast pyrolysis?
Hydrotalcite
Zinc oxide
Nickel‑based hydrodeoxygenation catalyst
Silica gel
Explanation - Hydrodeoxygenation removes oxygen from bio‑oil, improving stability and heating value; nickel catalysts are widely employed.
Correct answer is: Nickel‑based hydrodeoxygenation catalyst
Q.24 What is the key advantage of using a membrane‑assisted reactor for simultaneous biogas production and CO₂ capture?
Continuous removal of CO₂ shifts equilibrium toward more CH₄ production
It increases temperature drastically
It eliminates the need for inoculum
It reduces reactor size by 90%
Explanation - By extracting CO₂, the partial pressure of methane increases, enhancing methane yield according to Le Chatelier's principle.
Correct answer is: Continuous removal of CO₂ shifts equilibrium toward more CH₄ production
Q.25 Which of the following is a major limitation of using waste‑derived bio‑hydrogen in fuel cells?
Presence of impurities like H₂S that poison the catalyst
Low energy density compared to gasoline
Inability to be stored as a liquid
Excessive CO₂ emissions
Explanation - Trace sulfur compounds can deactivate platinum catalysts in PEM fuel cells, requiring thorough gas cleaning.
Correct answer is: Presence of impurities like H₂S that poison the catalyst
Q.26 In the context of waste‑to‑biofuel, what does the term 'hydrothermal liquefaction' refer to?
Conversion of wet biomass into a crude oil‑like liquid under high temperature and pressure
Combustion of waste in a water‑filled furnace
Electrolysis of water using waste heat
Fermentation of sugars in aqueous solution
Explanation - Hydrothermal liquefaction processes wet feedstocks at 250‑350 °C and 10‑25 MPa, producing a bio‑crude without prior drying.
Correct answer is: Conversion of wet biomass into a crude oil‑like liquid under high temperature and pressure
Q.27 Which waste‑derived feedstock typically requires a pretreatment step of acid hydrolysis before fermentation?
Food waste
Lignocellulosic agricultural residues
Sewage sludge
Algal oil
Explanation - Acid hydrolysis breaks down the rigid cellulose/hemicellulose matrix, releasing fermentable sugars.
Correct answer is: Lignocellulosic agricultural residues
Q.28 What is the main environmental benefit of using bio‑diesel made from waste cooking oil compared with petroleum diesel?
Reduced net CO₂ emissions due to carbon recycling
Higher cetane number
Zero NOₓ emissions
Higher energy density
Explanation - The carbon in waste oil originally came from atmospheric CO₂ fixed by plants, so burning it closes a short carbon loop.
Correct answer is: Reduced net CO₂ emissions due to carbon recycling
Q.29 Which of the following analytical techniques is most suitable for quantifying the methane content in biogas?
Gas chromatography (GC)
Mass spectrometry (MS) of liquids
Fourier‑transform infrared spectroscopy (FTIR) of solids
UV‑Vis spectroscopy of liquids
Explanation - GC separates gaseous components and provides accurate quantitative analysis of CH₄, CO₂, and trace gases.
Correct answer is: Gas chromatography (GC)
Q.30 When producing bio‑ethanol from waste starch, why is it essential to maintain a temperature around 30 °C during fermentation?
Optimal growth temperature for Saccharomyces cerevisiae
To prevent ethanol evaporation
To keep the reaction exothermic
To enhance enzyme activity of cellulases
Explanation - Yeast ferments efficiently at 30 °C; higher temperatures can kill the cells, while lower temperatures slow metabolism.
Correct answer is: Optimal growth temperature for Saccharomyces cerevisiae
Q.31 Which waste‑derived substrate is most suitable for producing biobutanol via the ABE (acetone‑butanol‑ethanol) fermentation process?
Molasses
Lignin
Petroleum coke
Silicon chips
Explanation - Molasses provides fermentable sugars that Clostridium acetobutylicum can convert into butanol, acetone, and ethanol.
Correct answer is: Molasses
Q.32 What is the effect of increasing the hydraulic retention time (HRT) in a digester on methane production?
Typically increases methane yield up to an optimum point
Decreases methane concentration
Has no effect on methane production
Causes immediate reactor failure
Explanation - Longer HRT allows more complete degradation of organics, but excessively long HRT reduces productivity per unit time.
Correct answer is: Typically increases methane yield up to an optimum point
Q.33 Which of the following best describes the term 'circular economy' in the context of bio‑energy from waste?
Recovering energy and materials from waste and reinserting them into production cycles
Burning all waste to generate electricity
Exporting waste to other countries
Storing waste indefinitely
Explanation - Circular economy aims to minimize waste by turning it into resources, such as bio‑fuels, fertilizers, or chemicals.
Correct answer is: Recovering energy and materials from waste and reinserting them into production cycles
Q.34 In the production of bio‑hydrogen via dark fermentation, which metabolite is often a by‑product that can inhibit the process if accumulated?
Acetic acid
Methane
Ethanol
Oxygen
Explanation - Acidic metabolites lower pH, inhibiting hydrogen‑producing bacteria; pH control is essential.
Correct answer is: Acetic acid
Q.35 Which of the following is a major challenge when integrating waste‑derived bio‑fuel production with existing power grids?
Variability in fuel quality and supply
Excessively high energy density
Zero emissions
Instantaneous combustion
Explanation - Fluctuating feedstock composition leads to inconsistent fuel properties, complicating grid integration and engine tuning.
Correct answer is: Variability in fuel quality and supply
Q.36 What role does a 'buffer tank' play in a continuous anaerobic digestion system?
Smooths out fluctuations in influent flow and composition
Catalyzes methane formation
Removes dissolved gases
Increases temperature
Explanation - Buffer tanks store incoming waste temporarily, providing a stable feed to the digester.
Correct answer is: Smooths out fluctuations in influent flow and composition
Q.37 When converting waste glycerol (a biodiesel by‑product) into 1,3‑propanediol, which microorganism is commonly employed?
Clostridium butyricum
Escherichia coli (engineered)
Saccharomyces cerevisiae
Algae Chlorella vulgaris
Explanation - Genetically engineered E. coli strains can efficiently ferment glycerol to 1,3‑propanediol, a valuable chemical.
Correct answer is: Escherichia coli (engineered)
Q.38 Which factor most strongly influences the rate of hydrolysis in the anaerobic digestion of solid waste?
Particle size of the substrate
Color of the waste
Magnetic field strength
Ambient light intensity
Explanation - Smaller particles increase surface area for microbial enzymes, accelerating hydrolysis.
Correct answer is: Particle size of the substrate
Q.39 In a two‑stage bio‑fuel process, why is it advantageous to separate the fermentation and product recovery steps?
To avoid product inhibition and improve yields
To increase the temperature dramatically
To eliminate the need for microorganisms
To produce solid waste
Explanation - Separating stages prevents accumulation of toxic products (e.g., ethanol) that can inhibit microbial activity.
Correct answer is: To avoid product inhibition and improve yields
Q.40 Which of the following waste-derived gases is most suitable for direct injection into a natural‑gas turbine?
Biogas (after desulfurization and CO₂ removal)
Pure hydrogen
Syngas rich in CO
Methane from landfill without treatment
Explanation - Cleaned biogas (mostly CH₄) can be upgraded to pipeline quality and used in turbines.
Correct answer is: Biogas (after desulfurization and CO₂ removal)
Q.41 What is the typical energy conversion efficiency of a modern combined heat and power (CHP) unit running on biogas?
80‑90 %
30‑40 %
10‑20 %
95‑100 %
Explanation - CHP systems capture both electricity and useful heat, achieving high overall efficiencies.
Correct answer is: 80‑90 %
Q.42 Which waste stream is most commonly used as a feedstock for producing renewable diesel via hydro‑processing?
Used cooking oil
Paper waste
Construction debris
Electronic waste
Explanation - Used cooking oil contains triglycerides that can be hydrotreated to renewable diesel (hydrotreated vegetable oil).
Correct answer is: Used cooking oil
Q.43 In bio‑fuel production, the term 'valorization' most closely means:
Adding value to waste by converting it into useful products
Burning waste for heat
Storing waste underground
Disposing waste in landfills
Explanation - Valorization transforms low‑value waste into higher‑value fuels or chemicals.
Correct answer is: Adding value to waste by converting it into useful products
Q.44 Which of the following is a key parameter for monitoring the performance of a biogas upgrading unit?
Methane purity (CH₄ concentration)
Color of the gas
Magnetic susceptibility
Viscosity of the gas
Explanation - Higher methane purity indicates effective removal of CO₂ and impurities, making the gas suitable for grid injection.
Correct answer is: Methane purity (CH₄ concentration)
Q.45 Why is it necessary to control the pH of an anaerobic digester within the range 6.8‑7.2?
Methanogenic archaea are most active in this neutral range
It prevents corrosion of the reactor steel
It enhances the solubility of gases
It increases the temperature of the process
Explanation - Methanogens have optimal activity near neutral pH; deviations inhibit methane production.
Correct answer is: Methanogenic archaea are most active in this neutral range
Q.46 Which technology enables the conversion of municipal solid waste directly into a synthetic natural gas (SNG) without intermediate liquid fuels?
Gasification followed by methanation
Incineration
Anaerobic digestion
Fermentation
Explanation - Gasification produces syngas (CO + H₂), which can be methanated to form SNG.
Correct answer is: Gasification followed by methanation
Q.47 During the transesterification reaction for biodiesel, what is the stoichiometric ratio of methanol to triglyceride typically used?
3:1 (molar)
1:1 (molar)
6:1 (molar)
0.5:1 (molar)
Explanation - Three moles of methanol react with one mole of triglyceride to produce three moles of fatty acid methyl ester.
Correct answer is: 3:1 (molar)
Q.48 Which waste‑derived substrate is best suited for the production of biogas via thermophilic digestion (55 °C)?
Food waste with high moisture content
Dry sawdust
Plastic bags
Metal scraps
Explanation - Thermophilic microbes thrive on readily degradable, moist organic matter like food waste.
Correct answer is: Food waste with high moisture content
Q.49 What is the main function of a 'scrubber' in a biogas upgrading plant?
To remove acidic gases (e.g., H₂S, CO₂) from the biogas stream
To increase the pressure of the gas
To add oxygen for combustion
To cool the gas to cryogenic temperatures
Explanation - Scrubbers use liquids or solids to absorb contaminants, improving methane purity.
Correct answer is: To remove acidic gases (e.g., H₂S, CO₂) from the biogas stream
Q.50 Which of the following best explains why lignin-rich waste is less favorable for bio‑ethanol production?
Lignin is resistant to enzymatic hydrolysis, limiting sugar release
Lignin combusts spontaneously
Lignin reacts violently with ethanol
Lignin is toxic to yeast
Explanation - Lignin forms a complex matrix that protects cellulose, making it difficult to access fermentable sugars.
Correct answer is: Lignin is resistant to enzymatic hydrolysis, limiting sugar release
Q.51 In the context of bio‑fuel standards, what does the term 'GHG intensity' refer to?
Amount of greenhouse‑gas emissions per unit of energy produced
The brightness of the fuel flame
The heat capacity of the fuel
The viscosity of the fuel
Explanation - GHG intensity measures CO₂‑equivalent emissions per megajoule, used for lifecycle assessment.
Correct answer is: Amount of greenhouse‑gas emissions per unit of energy produced
Q.52 Which of the following reactors operates without a catalyst and relies solely on thermal cracking to convert waste plastic into fuel?
Pyrolysis reactor
Fluidized‑bed reactor
Catalytic cracking unit
Photobioreactor
Explanation - Thermal pyrolysis thermally decomposes polymers into hydrocarbons without a catalyst.
Correct answer is: Pyrolysis reactor
Q.53 What is the primary benefit of using a two‑step anaerobic digestion system (hydrolysis/acidogenesis followed by methanogenesis) for waste treatment?
Improved control over intermediate volatile fatty acids, leading to higher methane yields
Elimination of the need for temperature control
Production of solid fuel directly
Complete removal of all pathogens
Explanation - Separating stages allows optimization of each microbial group, minimizing acid inhibition and maximizing methane production.
Correct answer is: Improved control over intermediate volatile fatty acids, leading to higher methane yields
Q.54 When upgrading biogas to biomethane, which process uses a selective membrane that preferentially allows CO₂ to pass through?
Membrane separation (permeation)
Water washing
Amine scrubbing
Cryogenic separation
Explanation - Selective polymer membranes let CO₂ permeate faster than CH₄, concentrating methane on the retentate side.
Correct answer is: Membrane separation (permeation)
Q.55 Which waste‑derived material can serve as a catalyst support for biodiesel transesterification?
Calcined rice husk ash
Plastic beads
Glass shards
Rubber granules
Explanation - Rice husk ash contains silica and can be calcined to form a porous support for basic catalysts.
Correct answer is: Calcined rice husk ash
Q.56 What is the main reason for adding trace amounts of nutrients (e.g., nitrogen, phosphorus) to an anaerobic digester fed with food waste?
To supply essential elements for microbial growth and enhance biogas production
To increase the temperature of the reactor
To neutralize the biogas
To change the color of the effluent
Explanation - Nutrients prevent microbial limitation, leading to higher digestion rates and methane yields.
Correct answer is: To supply essential elements for microbial growth and enhance biogas production
Q.57 Which of the following best describes the term 'syngas' in the context of waste conversion?
A mixture of carbon monoxide (CO) and hydrogen (H₂) produced by gasification
Pure methane from anaerobic digestion
Compressed biogas after CO₂ removal
Liquid bio‑oil from pyrolysis
Explanation - Syngas (synthetic gas) results from high‑temperature gasification of carbonaceous feedstocks.
Correct answer is: A mixture of carbon monoxide (CO) and hydrogen (H₂) produced by gasification
Q.58 In a life‑cycle assessment (LCA) of waste‑derived bio‑fuel, which stage typically contributes the most to total GHG emissions?
Feedstock collection and transportation
Combustion of the final fuel
Catalyst manufacturing
End‑of‑life disposal
Explanation - Moving large volumes of low‑density waste can dominate emissions unless logistics are optimized.
Correct answer is: Feedstock collection and transportation
Q.59 Which microbial pathway is primarily responsible for converting acetate into methane in an anaerobic digester?
Acetoclastic methanogenesis
Hydrogenotrophic methanogenesis
Fermentative pathway
Nitrification
Explanation - Acetoclastic methanogens split acetate directly into CH₄ and CO₂.
Correct answer is: Acetoclastic methanogenesis
Q.60 What is a common method for removing residual water from biogas before compression?
Molecular sieve drying
Centrifugation
Magnetic separation
UV irradiation
Explanation - Molecular sieves adsorb water molecules, delivering dry gas suitable for compressors and pipelines.
Correct answer is: Molecular sieve drying
Q.61 Why is the use of waste‑derived bio‑fuel considered carbon‑neutral over its lifecycle?
The carbon released during combustion was originally captured from the atmosphere by plants
It does not emit any CO₂ when burned
The process uses no energy
The waste is not biodegradable
Explanation - Biomass fixes atmospheric CO₂; when the bio‑fuel is burned, this carbon returns, creating a closed loop.
Correct answer is: The carbon released during combustion was originally captured from the atmosphere by plants
Q.62 Which of the following waste streams can be directly used as a feedstock for producing bio‑hydrogen via steam reforming?
Sewage sludge
Plastic bottles
Glass bottles
Metal cans
Explanation - Sewage sludge contains moisture and organic matter suitable for steam reforming to generate H₂.
Correct answer is: Sewage sludge
Q.63 In a biorefinery, what is the purpose of 'hydrodeoxygenation' (HDO) of bio‑oil?
To remove oxygen atoms, improving stability and heating value
To add more oxygen for better combustion
To increase the acidity of the oil
To polymerize the oil into plastics
Explanation - HDO reduces oxygen content, decreasing viscosity and increasing energy density of the fuel.
Correct answer is: To remove oxygen atoms, improving stability and heating value
Q.64 Which waste‑derived substrate is most commonly used for producing renewable diesel through the Fischer‑Tropsch (FT) process?
Syngas generated from gasification of municipal solid waste
Pure ethanol
Vegetable oil
Methane from biogas
Explanation - FT synthesis converts CO and H₂ (syngas) into long‑chain hydrocarbons suitable for diesel.
Correct answer is: Syngas generated from gasification of municipal solid waste
Q.65 What is the main environmental concern associated with the combustion of bio‑diesel derived from waste cooking oil?
Potential for higher NOₓ emissions compared with petroleum diesel
Generation of radioactive waste
Release of chlorine gases
Excessive ash formation
Explanation - Biodiesel can raise combustion temperatures, leading to increased NOₓ formation unless mitigated.
Correct answer is: Potential for higher NOₓ emissions compared with petroleum diesel
Q.66 Which analytical method is best suited for measuring the fatty acid methyl ester (FAME) content in biodiesel?
Gas chromatography with flame ionization detection (GC‑FID)
Infrared spectroscopy of solids
Atomic absorption spectroscopy
pH titration
Explanation - GC‑FID separates and quantifies individual FAME components with high sensitivity.
Correct answer is: Gas chromatography with flame ionization detection (GC‑FID)
Q.67 Why is it important to maintain an anaerobic environment in a digester?
To favor methanogenic archaea that produce methane
To speed up oxidation of organic matter
To increase the temperature automatically
To prevent formation of carbon monoxide
Explanation - Methanogens are strict anaerobes; oxygen inhibits their activity, reducing methane output.
Correct answer is: To favor methanogenic archaea that produce methane
Q.68 Which waste‑derived fuel can be used directly in existing gasoline engines without modification when blended up to 10%?
Ethanol derived from waste sugars
Hydrogen from electrolysis
Biogas
Methanol
Explanation - Gasoline engines can handle low blends (E10) of ethanol without hardware changes.
Correct answer is: Ethanol derived from waste sugars
Q.69 In the context of bio‑energy, what does the term 'feedstock pre‑treatment' refer to?
Physical, chemical, or biological processes applied to waste before conversion
Storing the waste in a freezer
Mixing the waste with oil
Burning the waste directly
Explanation - Pre‑treatment enhances accessibility of the feedstock for downstream biochemical or thermochemical processes.
Correct answer is: Physical, chemical, or biological processes applied to waste before conversion
Q.70 Which of the following is a major advantage of using a 'two‑phase' (acidogenic‑methanogenic) reactor over a single‑phase digester?
Separate optimization of acid production and methane production
Elimination of the need for temperature control
Direct production of liquid fuels
Removal of all pathogens
Explanation - Each phase can be operated at its optimal pH and temperature, enhancing overall performance.
Correct answer is: Separate optimization of acid production and methane production
Q.71 Which parameter is most directly linked to the specific methane yield (m³ CH₄ per kg VS) in an anaerobic digester?
Volatile solids (VS) content of the feedstock
Color of the waste
Magnetic properties of the reactor
Altitude of the plant location
Explanation - VS represents the organic fraction available for conversion to methane.
Correct answer is: Volatile solids (VS) content of the feedstock
Q.72 What is the main reason to add a 'buffer' (e.g., sodium bicarbonate) to a high‑acidic waste stream before anaerobic digestion?
To maintain pH within the optimal range for methanogens
To increase the temperature
To make the waste more flammable
To reduce the water content
Explanation - Buffers counteract acid spikes, protecting methanogenic activity.
Correct answer is: To maintain pH within the optimal range for methanogens
Q.73 Which of the following waste‑derived fuels can be directly used in a standard diesel engine without any modification?
Hydrotreated vegetable oil (HVO) renewable diesel
Biogas
Bio‑ethanol
Hydrogen
Explanation - Renewable diesel is chemically similar to petroleum diesel and can be blended up to 100%.
Correct answer is: Hydrotreated vegetable oil (HVO) renewable diesel
Q.74 In a waste‑to‑biofuel process, what is the purpose of 'solid–liquid separation' after fermentation?
To recover the liquid bio‑fuel and remove biomass residues
To increase the temperature of the mixture
To add more microbes
To dissolve the solid waste completely
Explanation - Separating solids simplifies downstream purification of the liquid fuel.
Correct answer is: To recover the liquid bio‑fuel and remove biomass residues
Q.75 Which of the following technologies enables the simultaneous production of electricity and heat from biogas?
Combined heat and power (CHP) unit
Electrolyzer
Cryogenic distillation column
Photovoltaic panel
Explanation - CHP converts the chemical energy of biogas into electrical and thermal energy efficiently.
Correct answer is: Combined heat and power (CHP) unit
Q.76 When producing biodiesel, why is excess methanol often recycled back into the reactor?
To drive the equilibrium toward complete conversion of triglycerides
To increase the temperature of the reaction
To act as a catalyst
To reduce the viscosity of the product
Explanation - Le Chatelier’s principle: excess reactant pushes the reaction toward product formation.
Correct answer is: To drive the equilibrium toward complete conversion of triglycerides
Q.77 Which waste‑derived material can be used as a carbon source for producing polyhydroxyalkanoates (PHAs) in microbial fermentation?
Food waste hydrolysate
Glass shards
Metal filings
Concrete powder
Explanation - Hydrolyzed sugars from food waste serve as carbon for PHA‑producing bacteria.
Correct answer is: Food waste hydrolysate
Q.78 In a biogas plant, the presence of which gas is most detrimental to the performance of a methanogenic culture?
Hydrogen sulfide (H₂S)
Carbon dioxide (CO₂)
Methane (CH₄)
Nitrogen (N₂)
Explanation - H₂S is toxic to anaerobic microbes and can corrode equipment.
Correct answer is: Hydrogen sulfide (H₂S)
Q.79 Why is it advantageous to operate a biogas reformer at temperatures above 700 °C?
To ensure complete steam reforming of methane into syngas (CO + H₂)
To melt the reactor walls
To produce liquid fuel directly
To generate electricity without catalysts
Explanation - High temperatures favor endothermic steam reforming reactions, yielding a high-quality syngas.
Correct answer is: To ensure complete steam reforming of methane into syngas (CO + H₂)
Q.80 Which of the following statements about bio‑hydrogen production by photo‑fermentation is correct?
Light energy is used to split water, generating hydrogen without oxygen release
The process requires high temperatures (>200 °C)
It produces large amounts of methane as a by‑product
It can only use inorganic substrates
Explanation - Photo‑fermentative bacteria use light to drive hydrogen evolution from organic substrates.
Correct answer is: Light energy is used to split water, generating hydrogen without oxygen release
Q.81 Which waste‑derived fuel is classified as a 'drop‑in' fuel for existing natural‑gas infrastructure after upgrading?
Biomethane (purified biogas)
Bio‑ethanol
Biodiesel
Biobutanol
Explanation - Biomethane meets the same specifications as pipeline natural gas, allowing direct injection.
Correct answer is: Biomethane (purified biogas)
Q.82 When using lignocellulosic waste for bio‑fuel production, which step typically follows acid hydrolysis?
Fermentation of released sugars
Combustion of the residue
Electrolysis
Direct compression
Explanation - Acid hydrolysis breaks polymers into monomeric sugars that are then fermented to ethanol or other bio‑fuels.
Correct answer is: Fermentation of released sugars
Q.83 What is the primary environmental benefit of using anaerobic digestion to treat animal manure?
Reduces methane emissions from uncontrolled decomposition and produces renewable energy
Eliminates the need for water in farms
Produces plastic waste
Increases the odor of the farm
Explanation - Digesters capture methane that would otherwise be emitted, converting it into usable biogas.
Correct answer is: Reduces methane emissions from uncontrolled decomposition and produces renewable energy
Q.84 Which of the following processes is most suitable for converting waste plastics into a gasoline‑like fuel?
Catalytic cracking (fluid catalytic cracking, FCC)
Fermentation
Anaerobic digestion
Electrolysis
Explanation - FCC breaks down long‑chain polymers into shorter hydrocarbons resembling gasoline.
Correct answer is: Catalytic cracking (fluid catalytic cracking, FCC)
Q.85 In bio‑fuel production, what does the term 'energy return on investment (EROI)' represent?
The ratio of energy obtained from the fuel to the energy invested in its production
The monetary profit from selling the fuel
The amount of CO₂ captured per liter of fuel
The speed at which the fuel can be delivered
Explanation - EROI evaluates the net energy benefit of a fuel source.
Correct answer is: The ratio of energy obtained from the fuel to the energy invested in its production
Q.86 Which waste‑derived substrate typically requires an 'acid pretreatment' to improve its fermentability?
Corn stover
Sugar beet juice
Liquid ethanol
Methane
Explanation - Acid pretreatment hydrolyzes hemicellulose and disrupts lignin, making cellulose more accessible for enzymes.
Correct answer is: Corn stover
Q.87 What is the main reason for adding a 'desulfurization unit' before feeding biogas to a solid oxide fuel cell (SOFC)?
Sulfur compounds poison the SOFC anode and degrade performance
To increase the pressure of the gas
To add oxygen for combustion
To cool the gas to ambient temperature
Explanation - H₂S and other sulfur species can corrode and deactivate the high‑temperature anode material.
Correct answer is: Sulfur compounds poison the SOFC anode and degrade performance
Q.88 Which analytical technique is most appropriate for quantifying the lipid content of algae used for biodiesel production?
Gravimetric extraction followed by gas chromatography
X‑ray diffraction
UV‑Vis spectroscopy of water
Thermogravimetric analysis (TGA) of solids
Explanation - Lipid extraction gives total mass, and GC quantifies fatty acid methyl esters after transesterification.
Correct answer is: Gravimetric extraction followed by gas chromatography
Q.89 Which waste‑derived product can be directly used as a feedstock for the production of synthetic gasoline via the FT process?
Syngas from municipal solid waste gasification
Pure ethanol
Biodiesel
Biogas
Explanation - FT synthesis converts CO + H₂ (syngas) into hydrocarbons in the gasoline range.
Correct answer is: Syngas from municipal solid waste gasification
Q.90 Why is it essential to maintain a low oxygen concentration (<0.1 %) in the headspace of an anaerobic digester?
Oxygen inhibits strict anaerobes, especially methanogens
Oxygen increases methane production
Oxygen is needed for acidogenesis
Oxygen improves mixing
Explanation - Methanogens are obligate anaerobes; even trace O₂ can suppress their activity.
Correct answer is: Oxygen inhibits strict anaerobes, especially methanogens
Q.91 In a waste‑to‑biofuel process, what is the function of a 'heat exchanger' placed after the reactor?
To recover heat from the hot effluent and improve overall energy efficiency
To increase the acidity of the stream
To add more microorganisms
To evaporate the water content completely
Explanation - Heat exchangers capture waste heat, which can be reused for pre‑heating feedstock or other processes.
Correct answer is: To recover heat from the hot effluent and improve overall energy efficiency
Q.92 Which waste‑derived feedstock is most suitable for the production of renewable diesel via hydrodeoxygenation (HDO)?
Waste cooking oil
Sewage sludge
Paper pulp
Coal ash
Explanation - Waste cooking oil is rich in triglycerides that can be hydrotreated into hydrocarbon diesel.
Correct answer is: Waste cooking oil
Q.93 What is the main purpose of adding a 'seed inoculum' to a newly started anaerobic digester?
To introduce a healthy microbial community and reduce start‑up time
To raise the temperature
To increase the pH dramatically
To add extra solid waste
Explanation - Inoculum contains active methanogens and bacteria that accelerate digestion of the fresh substrate.
Correct answer is: To introduce a healthy microbial community and reduce start‑up time
Q.94 Which of the following is a typical by‑product of the Fischer‑Tropsch synthesis of bio‑diesel from syngas?
Heavy waxes that require further hydrocracking
Pure ethanol
Biogas
Methane
Explanation - FT produces a broad range of hydrocarbons, including long‑chain waxes that must be cracked to diesel range.
Correct answer is: Heavy waxes that require further hydrocracking
Q.95 In the context of waste‑derived bio‑fuels, what does the term 'drop‑in replacement' imply?
The bio‑fuel can be used directly in existing engines/infrastructure without modification
The fuel drops out of the system quickly
The fuel is only usable in laboratory settings
The fuel requires special storage tanks
Explanation - Drop‑in fuels meet the same specifications as conventional fuels, allowing immediate substitution.
Correct answer is: The bio‑fuel can be used directly in existing engines/infrastructure without modification
Q.96 Which of the following factors most strongly influences the rate of enzymatic hydrolysis of cellulose in waste biomass?
Degree of polymerization and crystallinity of cellulose
Color of the biomass
Magnetic susceptibility of the reactor
Ambient sound level
Explanation - Highly crystalline, long‑chain cellulose resists enzyme attack, slowing hydrolysis.
Correct answer is: Degree of polymerization and crystallinity of cellulose
Q.97 Why is it advantageous to integrate a waste‑to‑bio‑fuel plant with a district heating network?
Recovered heat from processes can be supplied to the network, improving overall energy efficiency
It eliminates the need for any electricity generation
It allows the plant to operate at room temperature
It removes the need for waste pretreatment
Explanation - Heat from reactors, condensers, and CHP can be utilized for heating, raising total system efficiency.
Correct answer is: Recovered heat from processes can be supplied to the network, improving overall energy efficiency
Q.98 Which waste‑derived feedstock typically contains the highest proportion of lipids suitable for biodiesel production?
Used cooking oil
Paper waste
Sewage sludge
Wood chips
Explanation - Cooking oil is composed mainly of triglycerides, the raw material for biodiesel transesterification.
Correct answer is: Used cooking oil
Q.99 What is the most common method for measuring the methane content in biogas on‑site?
Portable infrared gas analyzer
pH meter
Thermocouple
Conductivity probe
Explanation - IR analyzers can rapidly quantify CH₄ concentration in the field with good accuracy.
Correct answer is: Portable infrared gas analyzer
Q.100 Which of the following waste streams can be directly used as a substrate for hydrogen production by dark fermentation?
Organic kitchen waste
Glass bottles
Metal scrap
Concrete slurry
Explanation - Organic waste provides fermentable sugars that dark fermentative bacteria convert to hydrogen.
Correct answer is: Organic kitchen waste
Q.101 In a biorefinery, the term 'valorization of glycerol' most often refers to:
Conversion of glycerol into higher‑value chemicals such as 1,3‑propanediol
Burning glycerol for heat
Disposing glycerol in landfills
Mixing glycerol with diesel
Explanation - Glycerol is a low‑value by‑product; upgrading it creates additional revenue streams.
Correct answer is: Conversion of glycerol into higher‑value chemicals such as 1,3‑propanediol
Q.102 Which waste‑derived bio‑fuel can be used directly in a gasoline engine after simple blending, without any engine modifications?
E85 (85% ethanol)
Pure hydrogen
Biogas
Methanol
Explanation - Flex‑fuel vehicles are designed to run on high‑ethanol blends without hardware changes.
Correct answer is: E85 (85% ethanol)
Q.103 Why is a 'pH control system' crucial during the acidogenesis stage of anaerobic digestion?
Acidogenic bacteria produce volatile fatty acids that lower pH, which can inhibit further digestion if not controlled
It heats the reactor
It adds extra nutrients
It removes methane
Explanation - Maintaining pH around 5.5‑6.0 ensures optimal activity of acid‑producing microbes.
Correct answer is: Acidogenic bacteria produce volatile fatty acids that lower pH, which can inhibit further digestion if not controlled
Q.104 Which of the following best describes the concept of 'energy integration' in a waste‑to‑bio‑fuel plant?
Utilizing waste heat from one unit to power another process, reducing external energy demand
Exporting all waste heat to the environment
Operating each unit independently
Increasing the amount of waste fed to the plant
Explanation - Energy integration improves overall plant efficiency by recycling internal heat streams.
Correct answer is: Utilizing waste heat from one unit to power another process, reducing external energy demand
Q.105 What is the main limitation of using municipal solid waste (MSW) gasification for bio‑fuel production?
Heterogeneity of the feedstock leads to variable syngas composition
It produces too much liquid fuel
It requires no pretreatment
It eliminates all emissions
Explanation - Variations in plastic, paper, and organic fractions cause fluctuations in gas quality, complicating downstream processing.
Correct answer is: Heterogeneity of the feedstock leads to variable syngas composition
Q.106 In a biogas upgrading plant, which technique uses an amine solution to absorb CO₂ and H₂S?
Amine scrubbing
Membrane separation
Cryogenic distillation
Water washing
Explanation - Amines chemically bind acidic gases, allowing selective removal from the methane stream.
Correct answer is: Amine scrubbing
Q.107 Which waste‑derived feedstock is most suitable for producing bio‑hydrogen through thermophilic dark fermentation?
Food waste slurry
Plastic pellets
Glass fragments
Metal shavings
Explanation - Thermophilic bacteria efficiently ferment the organic acids present in food waste to hydrogen.
Correct answer is: Food waste slurry
Q.108 What is the typical carbon‑to‑hydrogen ratio (C/H) of biogas after upgrading to biomethane?
Approximately 1:4 (CH₄)
10:1
0.5:1
5:1
Explanation - Methane (CH₄) has a C/H ratio of 1:4, reflecting the stoichiometry of the upgraded gas.
Correct answer is: Approximately 1:4 (CH₄)
Q.109 Which of the following is a key advantage of using algae as a feedstock for bio‑fuel production from waste CO₂ streams?
High growth rates and ability to fix CO₂ directly from flue gases
Zero water requirement
Production of pure hydrogen without processing
No need for nutrients
Explanation - Algae can utilize CO₂ from waste emissions, turning a pollutant into biomass for fuel.
Correct answer is: High growth rates and ability to fix CO₂ directly from flue gases
Q.110 Why is it necessary to dewater the digestate after anaerobic digestion before using it as a fertilizer?
To reduce the moisture content for easier handling and transport
To increase its methane content
To convert it into liquid fuel
To make it more acidic
Explanation - Dewatering concentrates nutrients, making the digestate more manageable as a soil amendment.
Correct answer is: To reduce the moisture content for easier handling and transport
Q.111 Which of the following processes can convert waste glycerol directly into propylene glycol, a valuable chemical?
Catalytic hydrogenolysis
Fermentation with yeast
Anaerobic digestion
Electrolysis
Explanation - Hydrogenolysis over suitable catalysts transforms glycerol into propylene glycol efficiently.
Correct answer is: Catalytic hydrogenolysis
Q.112 In a waste‑to‑biofuel system, why is 'process integration' important for sustainability?
It reduces overall energy consumption and waste generation by sharing utilities among units
It makes the plant larger
It increases the need for external electricity
It eliminates the need for any pretreatment
Explanation - Integrating heat, mass, and energy streams lowers the environmental footprint and improves economics.
Correct answer is: It reduces overall energy consumption and waste generation by sharing utilities among units