Q.1 What is the primary purpose of a biofilter in air pollution control?
To capture particulate matter by physical filtration
To chemically oxidize pollutants using high temperature
To biologically degrade volatile organic compounds (VOCs)
To cool the exhaust gas before release
Explanation - Biofilters use a microbial community to metabolize pollutants such as VOCs, converting them to harmless end‑products like CO₂ and water.
Correct answer is: To biologically degrade volatile organic compounds (VOCs)
Q.2 Which of the following is NOT a typical media used in biofilters?
Wood chips
Activated carbon
Peat moss
Compost
Explanation - Activated carbon is mainly used for adsorption, not as a biological growth medium in biofilters.
Correct answer is: Activated carbon
Q.3 In biofiltration, the term 'empty bed residence time' (EBRT) refers to:
The time required for microbes to colonize the media
The average time the gas spends in the void spaces of the filter
The total operating time before media replacement is needed
The time for pollutants to be completely mineralized
Explanation - EBRT is calculated as the volume of empty (void) space divided by the gas flow rate, indicating contact time for pollutants with the biofilm.
Correct answer is: The average time the gas spends in the void spaces of the filter
Q.4 Which microorganism type is most commonly responsible for ammonia removal in biofilters?
Methanogenic archaea
Nitrifying bacteria
Sulfate‑reducing bacteria
Photosynthetic algae
Explanation - Nitrifying bacteria oxidize ammonia to nitrite and then nitrate, a key step in ammonia biofiltration.
Correct answer is: Nitrifying bacteria
Q.5 A biofilter treating H₂S typically operates at which pH range for optimal microbial activity?
Acidic (pH 3‑4)
Neutral (pH 6‑8)
Alkaline (pH 9‑10)
Highly alkaline (pH >11)
Explanation - Most sulfur‑oxidizing bacteria prefer neutral to slightly alkaline conditions; extreme pH inhibits their activity.
Correct answer is: Neutral (pH 6‑8)
Q.6 Which of the following pollutants is most effectively removed by a biofilter rather than by a wet scrubber?
Sulfur dioxide (SO₂)
Particulate matter (PM10)
Methane (CH₄)
Hydrogen sulfide (H₂S)
Explanation - H₂S is readily bio‑oxidized by sulfur‑oxidizing microbes in a biofilter, whereas wet scrubbers are less efficient for low‑solubility gases.
Correct answer is: Hydrogen sulfide (H₂S)
Q.7 The pressure drop across a biofilter is mainly caused by:
Chemical reactions generating gases
Accumulation of dust particles
Flow resistance through the packed media
Thermal expansion of the gas stream
Explanation - As gas passes through the porous media, friction and flow channel constriction create a pressure drop that must be managed.
Correct answer is: Flow resistance through the packed media
Q.8 Which kinetic model is commonly used to describe pollutant removal in biofilters?
First‑order decay model
Langmuir adsorption isotherm
Michaelis‑Menten enzyme kinetics
Arrhenius temperature equation
Explanation - The first‑order model relates the pollutant concentration decrease to the residence time and a rate constant, fitting many biofilter data sets.
Correct answer is: First‑order decay model
Q.9 In designing a biofilter, the term 'specific surface area' (SSA) of the media refers to:
The total geometric surface area per unit volume of the filter
The surface area of the media occupied by microbes
The surface area available for gas‑solid contact per unit mass of media
The surface area of the inlet duct
Explanation - Higher SSA provides more area for microbial colonization and gas interaction, improving removal efficiency.
Correct answer is: The surface area available for gas‑solid contact per unit mass of media
Q.10 Which environmental parameter most directly influences the microbial activity in a biofilter?
Wind speed
Relative humidity of the inlet gas
Ambient temperature
Noise level
Explanation - Temperature affects enzymatic rates and microbial growth; most biofiltration processes operate optimally between 20‑35 °C.
Correct answer is: Ambient temperature
Q.11 A moving‑bed biofilter differs from a fixed‑bed biofilter mainly in that:
The media is continuously mixed and refreshed
It uses activated carbon instead of organic media
It requires no microbial inoculation
It operates only at cryogenic temperatures
Explanation - In a moving‑bed system, media particles move, preventing channeling and allowing uniform distribution of microbes and moisture.
Correct answer is: The media is continuously mixed and refreshed
Q.12 Which pollutant class is least suitable for removal by biofiltration due to its low biodegradability?
Aromatic hydrocarbons
Chlorinated solvents
Alcohols
Alkanes
Explanation - Chlorinated compounds resist microbial degradation and may inhibit the biofilter community, making them poor candidates for biofiltration.
Correct answer is: Chlorinated solvents
Q.13 The term 'biofilm' in a biofilter refers to:
A thin layer of oil that coats the media
A dense microbial community attached to the media surface
A plastic film used to seal the filter
A film of dust that accumulates on the inlet
Explanation - Biofilms consist of microorganisms embedded in extracellular polymeric substances, providing the active zone for pollutant degradation.
Correct answer is: A dense microbial community attached to the media surface
Q.14 What is the main advantage of using a biofilter over a thermal oxidizer for VOC removal?
Higher removal efficiency for all VOCs
Lower operational energy consumption
Ability to handle high temperature exhaust streams
No need for microbial inoculation
Explanation - Biofilters operate at ambient temperatures, avoiding the high fuel costs associated with heating gases in thermal oxidizers.
Correct answer is: Lower operational energy consumption
Q.15 Which of the following equations best estimates the required height (H) of a biofilter based on desired removal efficiency (η) and first‑order rate constant (k)?
H = - (1/k) * ln(1-η)
H = k * η
H = (1/k) * η²
H = ln(η) / k
Explanation - Deriving from the first‑order decay model, the expression gives the plug‑flow length needed to achieve a target removal fraction.
Correct answer is: H = - (1/k) * ln(1-η)
Q.16 In biofiltration, the term 'inoculation' refers to:
Seeding the media with a known microbial consortium
Injecting chemicals to stimulate growth
Adding water to maintain moisture
Introducing electrical currents to the media
Explanation - Inoculation provides the necessary microbes to start degradation processes quickly, especially for specialized pollutants.
Correct answer is: Seeding the media with a known microbial consortium
Q.17 Which parameter most directly influences the moisture content needed in a biofilter media?
Gas temperature
Pollutant concentration
Media particle size
Desired pressure drop
Explanation - Finer particles retain water better but can increase pressure drop; optimal moisture must balance these factors.
Correct answer is: Media particle size
Q.18 The oxidation of hydrogen sulfide (H₂S) in a biofilter typically yields which end products?
Sulfuric acid and water
Elemental sulfur and water
Sulfate ions and carbon dioxide
Hydrogen gas and sulfur dioxide
Explanation - Sulfur‑oxidizing bacteria convert H₂S to sulfate (SO₄²⁻) while consuming oxygen; CO₂ is a by‑product of microbial metabolism.
Correct answer is: Sulfate ions and carbon dioxide
Q.19 Which of the following is a common method for monitoring biofilter performance in real time?
Thermogravimetric analysis
Fourier‑transform infrared spectroscopy (FTIR) of exhaust gas
Electrical conductivity of the media
Acoustic emission testing
Explanation - FTIR can continuously measure concentrations of VOCs and other gases, providing rapid feedback on removal efficiency.
Correct answer is: Fourier‑transform infrared spectroscopy (FTIR) of exhaust gas
Q.20 When a biofilter experiences 'channeling', the main consequence is:
Uniform distribution of gas flow
Reduced pressure drop
Uneven gas distribution leading to lower removal efficiency
Increased microbial growth
Explanation - Channeling creates preferential flow paths, reducing contact time between gas and biofilm in portions of the media.
Correct answer is: Uneven gas distribution leading to lower removal efficiency
Q.21 The term 'biofiltration capacity' is best expressed as:
Mass of pollutant removed per unit volume of media per unit time
Maximum pressure drop the filter can sustain
Total weight of the media
Electrical power consumption of the system
Explanation - Capacity quantifies the throughput of pollutant degradation, typically expressed in g m⁻³ h⁻¹.
Correct answer is: Mass of pollutant removed per unit volume of media per unit time
Q.22 Which of the following gases is typically removed most efficiently by an electrostatic precipitator (ESP) rather than a biofilter?
Ammonia (NH₃)
Formaldehyde (CH₂O)
Particulate matter (PM)
Methane (CH₄)
Explanation - ESP devices are designed to charge and collect solid particles; they do not degrade gaseous pollutants.
Correct answer is: Particulate matter (PM)
Q.23 Which of the following is a key factor in selecting the appropriate biofilter media for a given pollutant?
Color of the media
Thermal conductivity
Nutrient content and porosity
Magnetic susceptibility
Explanation - Media must provide nutrients for microbes and sufficient pore space for gas flow and moisture retention.
Correct answer is: Nutrient content and porosity
Q.24 During biofilter operation, a sudden drop in removal efficiency is most likely caused by:
Increase in inlet gas temperature
Decrease in inlet gas flow rate
Insufficient moisture leading to desiccation of the biofilm
Excessive media compaction
Explanation - Microbial activity sharply declines when the biofilm dries out, reducing degradation rates.
Correct answer is: Insufficient moisture leading to desiccation of the biofilm
Q.25 What is the typical range of removal efficiencies for VOCs in well‑designed biofilters?
10‑30 %
30‑50 %
70‑95 %
95‑99 %
Explanation - Properly sized and maintained biofilters can achieve high VOC removal, often exceeding 80 %.
Correct answer is: 70‑95 %
Q.26 In the context of biofiltration, the term 'catalytic oxidation' refers to:
Oxidation of pollutants by heat alone
Oxidation facilitated by metal catalysts in the media
Biological oxidation performed by microbes
Photochemical oxidation using UV light
Explanation - Catalytic oxidation uses solid catalysts (e.g., platinum) to accelerate chemical oxidation; it is distinct from biological oxidation.
Correct answer is: Oxidation facilitated by metal catalysts in the media
Q.27 Which of the following best describes the 'steady‑state' condition in a biofilter?
When the inlet pollutant concentration is zero
When the removal efficiency remains constant over time
When the media temperature reaches 100 °C
When the pressure drop stops increasing
Explanation - Steady‑state is achieved when the system's operational parameters and performance metrics are stable.
Correct answer is: When the removal efficiency remains constant over time
Q.28 Which pollutant is most commonly removed by biofilters in wastewater treatment plant odor control?
Carbon monoxide (CO)
Hydrogen sulfide (H₂S)
Sulfur dioxide (SO₂)
Nitrogen oxides (NOₓ)
Explanation - H₂S is a major odorant from anaerobic digestion; biofilters efficiently oxidize it to sulfate.
Correct answer is: Hydrogen sulfide (H₂S)
Q.29 The term 'biokinetic coefficient' in biofiltration design is used to represent:
The rate of pressure drop increase
The maximum microbial growth rate
The first‑order reaction rate constant for pollutant degradation
The diffusion coefficient of gases in the media
Explanation - Biokinetic coefficients are derived from experimental data and reflect how quickly microbes can degrade a specific pollutant.
Correct answer is: The first‑order reaction rate constant for pollutant degradation
Q.30 Which of the following gases is typically more soluble in water, making it easier to scrub than to biofilter?
Acetone
Methane
Carbon monoxide
Sulfur dioxide
Explanation - SO₂ readily dissolves in water forming sulfurous acid, so wet scrubbing is highly effective for its removal.
Correct answer is: Sulfur dioxide
Q.31 In a biofilter, the term 'inoculum' refers to:
The initial dose of nutrients added to the media
The mixture of microbes introduced to start the degradation process
The amount of water sprayed onto the media
The volume of gas entering the filter per minute
Explanation - The inoculum provides the active microbial population required for immediate pollutant degradation.
Correct answer is: The mixture of microbes introduced to start the degradation process
Q.32 Which design parameter is most critical to avoid 'short‑circuiting' in a packed‑bed biofilter?
Uniform media particle size distribution
High inlet gas temperature
Low inlet gas humidity
Large diameter of the inlet duct
Explanation - Uniform particles ensure even flow distribution; otherwise, channels may form, bypassing portions of the media.
Correct answer is: Uniform media particle size distribution
Q.33 The primary end product of complete mineralization of organic VOCs in a biofilter is:
Methane and water
Carbon dioxide and water
Sulfuric acid and nitrogen
Ozone and nitrogen oxides
Explanation - Microbial metabolism oxidizes organics fully to CO₂ and H₂O under aerobic conditions.
Correct answer is: Carbon dioxide and water
Q.34 Which factor most influences the choice between a shallow‑wide vs. a tall‑narrow biofilter design?
The color of the exhaust gas
The available floor space at the plant site
The type of pollutants being treated
The brand of the control system
Explanation - Physical site constraints dictate whether a biofilter is spread out horizontally or built vertically.
Correct answer is: The available floor space at the plant site
Q.35 In biofiltration, the term 'knocking' refers to:
A sudden increase in pressure drop
The collapse of the media bed due to excess moisture
A rapid rise in outlet temperature
A transient loss of removal efficiency caused by high pollutant loading
Explanation - Knocking occurs when the microbial community is overwhelmed, temporarily reducing performance.
Correct answer is: A transient loss of removal efficiency caused by high pollutant loading
Q.36 Which of the following gases can be biologically oxidized to nitrogen gas (N₂) in a biofilter?
Nitrous oxide (N₂O)
Ammonia (NH₃)
Nitrogen dioxide (NO₂)
Nitrogen monoxide (NO)
Explanation - Through nitrification followed by denitrification, NH₃ can ultimately be converted to N₂.
Correct answer is: Ammonia (NH₃)
Q.37 Which of the following is an advantage of using compost as a biofilter medium?
Very low pressure drop
High nutrient content for microbial growth
Inherent resistance to channeling
Zero odor generation
Explanation - Compost provides organic carbon and nutrients, supporting a robust microbial community.
Correct answer is: High nutrient content for microbial growth
Q.38 What is the most common method for measuring the moisture content of biofilter media on site?
Gravimetric oven‑drying of sampled media
Electrical resistance measurement
Laser diffraction analysis
Acoustic emission monitoring
Explanation - Samples are weighed, dried at 105 °C, and re‑weighed to determine moisture percentage.
Correct answer is: Gravimetric oven‑drying of sampled media
Q.39 Which pollutant removal mechanism is primarily responsible for the elimination of odorous sulfur compounds in biofilters?
Physical adsorption onto media particles
Biological oxidation by sulfur‑oxidizing bacteria
Thermal decomposition
Photolysis under UV light
Explanation - Sulfur‑oxidizing microbes convert H₂S and mercaptans to sulfate, removing odor.
Correct answer is: Biological oxidation by sulfur‑oxidizing bacteria
Q.40 When scaling up a laboratory‑scale biofilter to a pilot plant, which dimensionless number is most useful for maintaining dynamic similarity?
Reynolds number
Péclet number
Biot number
Damköhler number
Explanation - The Damköhler number compares reaction rate to residence time, crucial for preserving removal performance during scale‑up.
Correct answer is: Damköhler number
Q.41 A biofilter treating a gas stream with a high concentration of toluene (C₇H₈) would most likely require which of the following to avoid toxicity effects?
Addition of a carbon source like glucose
Dilution of the inlet gas with clean air
Increasing the inlet temperature above 60 °C
Introducing a high‑pH buffer
Explanation - High toluene levels can be toxic to microbes; diluting reduces concentration to a tolerable level.
Correct answer is: Dilution of the inlet gas with clean air
Q.42 Which of the following best describes the function of a venturi scrubber compared to a biofilter?
It uses a high‑velocity gas jet to promote gas‑liquid contact for soluble pollutants
It relies on microbial activity to degrade pollutants
It operates only at cryogenic temperatures
It uses magnetic fields to attract particulate matter
Explanation - Venturi scrubbers are mechanical devices that enhance dissolution of soluble gases into a liquid, unlike biofilters which use microbes.
Correct answer is: It uses a high‑velocity gas jet to promote gas‑liquid contact for soluble pollutants
Q.43 In a biofilter, the term 'maintenance cycle' usually refers to:
The interval between media replacement
The routine period for checking and adjusting moisture and nutrient levels
The time required to shut down the plant for annual inspection
The frequency of filter media cleaning using high‑pressure water
Explanation - Regular monitoring of moisture, pH, and nutrients ensures stable microbial activity.
Correct answer is: The routine period for checking and adjusting moisture and nutrient levels
Q.44 Which gas-phase pollutant is most effectively removed by a biofilter using a mixed culture of methanotrophic bacteria?
Methane (CH₄)
Carbon dioxide (CO₂)
Nitrogen oxides (NOₓ)
Sulfur dioxide (SO₂)
Explanation - Methanotrophs oxidize methane to CO₂ and water, making biofiltration a viable methane mitigation technology.
Correct answer is: Methane (CH₄)
Q.45 Which of the following statements about the 'temperature coefficient' (Q₁₀) in biofiltration is correct?
Q₁₀ indicates the increase in reaction rate for each 10 °C rise in temperature
Q₁₀ is the ratio of inlet to outlet pollutant concentration
Q₁₀ represents the pressure drop per unit length
Q₁₀ measures the moisture loss over time
Explanation - Microbial reaction rates typically double (Q₁₀ ≈ 2) for every 10 °C increase within the optimal range.
Correct answer is: Q₁₀ indicates the increase in reaction rate for each 10 °C rise in temperature
Q.46 A biofilter treating a gas stream with high concentrations of chlorinated solvents may suffer from which of the following problems?
Rapid media clogging due to salt precipitation
Inhibition of microbial activity due to toxic intermediates
Excessive heat generation leading to fire hazards
High production of ozone
Explanation - Chlorinated compounds often form toxic metabolites that suppress microbial populations.
Correct answer is: Inhibition of microbial activity due to toxic intermediates
Q.47 What is the most common method to reduce the pressure drop in an aging biofilter without replacing the media?
Increasing the inlet gas temperature
Periodically back‑flushing the media with air or water
Adding more media on top of the existing bed
Installing a larger exhaust fan
Explanation - Back‑flushing removes accumulated particulates and biofilm excess, lowering resistance.
Correct answer is: Periodically back‑flushing the media with air or water
Q.48 The term 'bioreactor' in the context of air pollution control most accurately refers to:
A sealed vessel where combustion occurs
A device that uses high‑temperature plasma to destroy pollutants
A controlled environment where microorganisms degrade gaseous contaminants
A mechanical filter that physically traps particles
Explanation - A bioreactor provides optimal conditions (temperature, moisture, nutrients) for microbial degradation of gases.
Correct answer is: A controlled environment where microorganisms degrade gaseous contaminants
Q.49 Which of the following pollutants is best removed by a biofilter using a nitrifying bacterial consortium?
Hydrogen sulfide (H₂S)
Ammonia (NH₃)
Methane (CH₄)
Formaldehyde (CH₂O)
Explanation - Nitrifiers oxidize NH₃ to nitrite and nitrate, making them ideal for ammonia control.
Correct answer is: Ammonia (NH₃)
Q.50 In a biofilter, the presence of which gas can act as a competitive inhibitor for the oxidation of VOCs?
Ozone (O₃)
Carbon dioxide (CO₂)
Nitrogen (N₂)
Methane (CH₄)
Explanation - Ozone can damage microbial cell membranes and enzymes, reducing VOC degradation rates.
Correct answer is: Ozone (O₃)
Q.51 Which of the following is a typical indicator that a biofilter is becoming overloaded?
Steady increase in outlet temperature
Sudden rise in outlet pollutant concentration
Decrease in inlet flow rate
Drop in power consumption of the blower
Explanation - Overloading exceeds the microbial capacity, causing the filter to lose removal efficiency.
Correct answer is: Sudden rise in outlet pollutant concentration
Q.52 When a biofilter is operated at very low inlet humidity, which of the following strategies can be employed to maintain performance?
Increase the inlet gas temperature to 150 °C
Add a humidification system upstream of the filter
Replace the media with a non‑porous material
Install a secondary electrostatic precipitator
Explanation - Adequate humidity is essential for microbial activity and biofilm integrity.
Correct answer is: Add a humidification system upstream of the filter
Q.53 Which of the following best explains why biofilters are considered 'green' technologies?
They use renewable energy sources to power the blower
They produce no secondary waste streams and have low energy demand
They completely eliminate all greenhouse gas emissions
They rely on solar panels for operation
Explanation - Biofilters convert pollutants to harmless gases without generating solid waste and consume relatively little electricity.
Correct answer is: They produce no secondary waste streams and have low energy demand
Q.54 The 'breakthrough curve' in biofiltration typically plots:
Pressure drop versus time
Outlet pollutant concentration versus time
Temperature versus inlet flow rate
Moisture content versus media depth
Explanation - A breakthrough curve shows when the filter begins to let pollutant pass through at detectable levels.
Correct answer is: Outlet pollutant concentration versus time
Q.55 Which parameter is most directly affected by the 'porosity' of the biofilter media?
Microbial diversity
Gas flow resistance
Chemical composition of the media
Electrical conductivity
Explanation - Higher porosity reduces pressure drop, allowing easier gas passage through the filter.
Correct answer is: Gas flow resistance
Q.56 For a biofilter treating a gas stream with an inlet VOC concentration of 500 ppm and a desired removal efficiency of 90 %, the outlet concentration must be:
50 ppm
5 ppm
450 ppm
4500 ppm
Explanation - 90 % removal of 500 ppm leaves 10 % (0.10 × 500 = 50 ppm) at the outlet.
Correct answer is: 50 ppm
Q.57 Which of the following gases can be biologically reduced to a less harmful form in an anoxic biofilter?
Nitrogen dioxide (NO₂)
Carbon monoxide (CO)
Sulfur dioxide (SO₂)
Ozone (O₃)
Explanation - In anoxic conditions, denitrifying bacteria can reduce NO₂ to N₂ gas.
Correct answer is: Nitrogen dioxide (NO₂)
Q.58 A key advantage of using a biofilter for odor control in a livestock operation over a chemical scrubber is:
Higher removal efficiency for ammonia
Lower capital cost and simple operation
Ability to operate at extremely high temperatures
Generation of valuable by‑products
Explanation - Biofilters require minimal chemicals and have lower upfront costs compared to chemical scrubbers.
Correct answer is: Lower capital cost and simple operation
Q.59 Which of the following is a typical sign that biofilter media needs replacement?
Consistent pressure drop below 10 Pa
Sudden drop in microbial activity despite adequate moisture
Increase in media bulk density
Reduction in inlet gas temperature
Explanation - Compaction and loss of porosity increase bulk density and impede gas flow, indicating media degradation.
Correct answer is: Increase in media bulk density
Q.60 Which of the following gases is most effectively removed by a biofilter using a mixed culture of methanotrophs and methylotrophs?
Formaldehyde (CH₂O)
Methanol (CH₃OH)
Methane (CH₄)
Acetaldehyde (CH₃CHO)
Explanation - Methanotrophs oxidize methane, while methylotrophs can utilize methanol and other one‑carbon compounds that may be intermediates.
Correct answer is: Methane (CH₄)
Q.61 In the context of biofiltration, 'substrate' usually refers to:
The solid media that holds the biofilm
The pollutant being degraded
The carbon source that feeds the microbes
The inlet gas stream
Explanation - Substrate can be the organic compound that microorganisms metabolize; in some designs, an external carbon source is added.
Correct answer is: The carbon source that feeds the microbes
Q.62 Which of the following best describes the effect of increasing the inlet gas flow rate on the performance of a fixed‑bed biofilter (assuming all other parameters are constant)?
Removal efficiency increases due to higher turbulence
Pressure drop decreases proportionally
Removal efficiency decreases because residence time shortens
Microbial activity doubles
Explanation - Higher flow reduces contact time between gas and biofilm, limiting degradation.
Correct answer is: Removal efficiency decreases because residence time shortens
Q.63 Which type of biofilter is most suitable for treating a gas stream with high concentrations of soluble acidic gases like HCl?
Dry packed‑bed biofilter
Wet biofilter (also called a bio‑scrubber)
Moving‑bed biofilter
Thermal catalytic biofilter
Explanation - Acidic gases dissolve readily in the liquid phase, where specialized microbes can neutralize them.
Correct answer is: Wet biofilter (also called a bio‑scrubber)
Q.64 Which of the following statements about the 'adsorption capacity' of biofilter media is correct?
Adsorption capacity is the same as biological degradation capacity
Higher adsorption capacity always improves removal efficiency
Adsorption can provide a temporary buffer for peak pollutant loads
Adsorption capacity is not relevant for gas‑phase pollutants
Explanation - Physical adsorption can store excess pollutant momentarily, allowing microbes to degrade it later.
Correct answer is: Adsorption can provide a temporary buffer for peak pollutant loads
Q.65 In a biofilter, the term 'bioaugmentation' refers to:
Increasing the inlet gas temperature
Adding extra nutrients to the media
Introducing specialized microbial strains to improve performance
Replacing the media with a newer material
Explanation - Bioaugmentation enhances degradation of specific pollutants by adding capable microbes.
Correct answer is: Introducing specialized microbial strains to improve performance
Q.66 Which of the following is NOT a typical environmental benefit of using biofilters?
Reduced greenhouse gas emissions due to low energy use
Elimination of hazardous waste disposal
Complete removal of all CO₂ emissions
Lower noise levels compared to mechanical scrubbers
Explanation - Biofilters do not capture CO₂; they may actually produce CO₂ as a by‑product of oxidation.
Correct answer is: Complete removal of all CO₂ emissions
Q.67 When designing a biofilter for a plant that emits 200 g h⁻¹ of VOCs, which parameter is most critical to determine the required filter volume?
Inlet gas temperature
First‑order rate constant for the specific VOC
Color of the media
Height of the building
Explanation - The rate constant dictates how fast the pollutant is degraded, directly influencing required residence time and volume.
Correct answer is: First‑order rate constant for the specific VOC
Q.68 Which gas detection method is commonly employed for continuous monitoring of H₂S in the exhaust of a biofilter?
Electrochemical sensor
Non‑dispersive infrared (NDIR) analyzer
Ultraviolet fluorescence detector
Thermal conductivity detector
Explanation - Electrochemical sensors provide rapid, sensitive detection of low‑level H₂S in gas streams.
Correct answer is: Electrochemical sensor
Q.69 A biofilter designed for a gas stream containing 5 % ethanol by volume is likely to encounter which operational challenge?
Excessive heat generation due to exothermic reactions
Rapid media drying because ethanol is highly volatile
Formation of a thick, water‑rich biofilm leading to high pressure drop
Corrosion of metal components due to acidic by‑products
Explanation - Ethanol provides ample carbon, promoting biofilm growth; over‑growth can cause clogging and increased pressure drop.
Correct answer is: Formation of a thick, water‑rich biofilm leading to high pressure drop
Q.70 Which of the following is a typical indicator that a biofilter's microbial community has become imbalanced (i.e., loss of diversity)?
Steady removal efficiency with low pressure drop
Sudden increase in odor emission despite stable inlet concentrations
Decrease in inlet gas temperature
Increase in media bulk density
Explanation - Loss of key microbial groups reduces degradation of odorants, leading to higher odor release.
Correct answer is: Sudden increase in odor emission despite stable inlet concentrations
Q.71 Which of the following gases is most effectively removed by a biofilter that utilizes a mixed population of nitrifying and denitrifying bacteria?
Ammonia (NH₃)
Sulfur dioxide (SO₂)
Methane (CH₄)
Formaldehyde (CH₂O)
Explanation - Nitrifiers convert NH₃ to nitrate; denitrifiers can then reduce nitrate to nitrogen gas, completing nitrogen removal.
Correct answer is: Ammonia (NH₃)
Q.72 During a biofilter performance test, the inlet concentration of toluene is 100 ppm and the outlet concentration after steady‑state operation is 20 ppm. What is the removal efficiency?
20 %
40 %
80 %
120 %
Explanation - Removal efficiency = (C_in – C_out) / C_in × 100 = (100‑20)/100 ×100 = 80 %.
Correct answer is: 80 %
Q.73 Which of the following is a common method to enhance the microbial activity in a biofilter that is operating at low temperatures?
Increasing the inlet gas velocity
Adding a chemical oxidizer to the inlet stream
Incorporating a heating coil to raise media temperature
Reducing the moisture content of the media
Explanation - Elevating temperature within the optimal range accelerates enzymatic reactions and microbial metabolism.
Correct answer is: Incorporating a heating coil to raise media temperature
Q.74 In biofiltration, the term 'steady‑state concentration' refers to:
The inlet concentration when the system is first started
The constant outlet concentration after the system has stabilized
The maximum concentration the filter can tolerate
The concentration of pollutants in the ambient air
Explanation - At steady state, the system's removal rate matches the pollutant loading, producing a stable outlet concentration.
Correct answer is: The constant outlet concentration after the system has stabilized
Q.75 Which of the following design considerations is most important when selecting a biofilter for a facility that emits large fluctuations in VOC loadings?
High surface area media to provide storage capacity
Low initial cost of the filter
Compact footprint regardless of performance
Ability to operate at very high temperatures
Explanation - A media with high surface area can adsorb peak loads temporarily, allowing microbes to degrade them later.
Correct answer is: High surface area media to provide storage capacity
Q.76 Which of the following gases is most likely to be removed by a biofilter through the process of 'denitrification'?
Nitric oxide (NO)
Nitrous oxide (N₂O)
Ammonia (NH₃)
Nitrate (NO₃⁻) in dissolved form
Explanation - Denitrifying bacteria can reduce N₂O to nitrogen gas under anoxic conditions.
Correct answer is: Nitrous oxide (N₂O)
Q.77 When a biofilter is used to treat a gas stream containing a mixture of VOCs with differing biodegradabilities, the overall removal efficiency is primarily determined by:
The most readily biodegradable compound
The compound with the highest molecular weight
The least biodegradable (most recalcitrant) compound
The compound present at the lowest concentration
Explanation - The overall efficiency cannot exceed the removal of the hardest‑to‑degrade component.
Correct answer is: The least biodegradable (most recalcitrant) compound
Q.78 Which of the following statements about 'biofilter regeneration' is correct?
Regeneration involves heating the media to 500 °C to kill microbes
Regeneration typically consists of flushing the media with nutrient‑rich water
Regeneration is unnecessary because biofilters never lose performance
Regeneration replaces the entire media bed with fresh material
Explanation - Flushing removes accumulated solids and provides nutrients, restoring microbial activity.
Correct answer is: Regeneration typically consists of flushing the media with nutrient‑rich water
Q.79 A biofilter treating an industrial exhaust with a high concentration of chlorobenzene is likely to experience which of the following problems?
Rapid media compaction due to crystal formation
Inhibition of microbial activity due to chlorinated intermediates
Excessive heat generation leading to fire risk
High production of ozone within the filter
Explanation - Chlorobenzene and its metabolites are toxic to many microbes, reducing degradation rates.
Correct answer is: Inhibition of microbial activity due to chlorinated intermediates
Q.80 Which of the following gases is most effectively removed by a biofilter using a mixed culture of methanotrophs and methylotrophs?
Methane (CH₄)
Carbon monoxide (CO)
Sulfur dioxide (SO₂)
Hydrogen cyanide (HCN)
Explanation - Methanotrophs oxidize CH₄ to CO₂; methylotrophs can process intermediate C₁ compounds, enhancing overall removal.
Correct answer is: Methane (CH₄)
Q.81 In a biofilter, the term 'adsorptive capacity' is most relevant for which type of pollutant?
Highly soluble gases
Insoluble particulate matter
Volatile organic compounds with moderate polarity
Heavy metals in the gas phase
Explanation - These VOCs can be partially adsorbed onto the media, providing a buffer before biological degradation.
Correct answer is: Volatile organic compounds with moderate polarity
Q.82 Which of the following operational parameters would you adjust first to improve the removal of a suddenly increased load of H₂S in a biofilter?
Increase inlet gas temperature
Increase media moisture content
Decrease blower speed to lower flow rate
Add a chemical oxidizer to the inlet stream
Explanation - Adequate moisture is essential for the activity of sulfur‑oxidizing bacteria; raising moisture can quickly restore performance.
Correct answer is: Increase media moisture content
Q.83 Which of the following is a typical limitation of biofilters when treating high‑temperature gas streams (>80 °C)?
Increased microbial activity
Rapid drying of the media
Enhanced solubility of pollutants
Reduced pressure drop
Explanation - High temperatures evaporate moisture, leading to desiccation of the biofilm and loss of microbial activity.
Correct answer is: Rapid drying of the media
Q.84 When evaluating a biofilter's performance, the term 'removal efficiency' is defined as:
The ratio of inlet to outlet gas temperature
The percentage reduction of pollutant concentration from inlet to outlet
The increase in pressure drop across the filter per hour
The amount of media replaced per year
Explanation - Removal efficiency = ((C_in – C_out) / C_in) × 100 %.
Correct answer is: The percentage reduction of pollutant concentration from inlet to outlet
Q.85 Which of the following is a common method for controlling the pH inside a biofilter treating acidic gases?
Adding acidic solutions to the inlet stream
Injecting alkaline buffer solutions into the media
Increasing the inlet gas flow rate
Reducing the moisture content of the media
Explanation - Alkaline buffers neutralize acidity generated by oxidation of acidic gases, maintaining microbial viability.
Correct answer is: Injecting alkaline buffer solutions into the media
Q.86 Which pollutant is most effectively removed by a biofilter that relies on nitrifying bacteria?
Sulfur dioxide (SO₂)
Ammonia (NH₃)
Methane (CH₄)
Formaldehyde (CH₂O)
Explanation - Nitrifying bacteria oxidize ammonia to nitrite and nitrate, making them ideal for NH₃ removal.
Correct answer is: Ammonia (NH₃)
Q.87 In the design of a biofilter, the term 'media depth' most directly influences:
The color of the emitted plume
The contact time between gas and biofilm
The electrical conductivity of the system
The acoustic noise level
Explanation - Greater depth increases the residence time, improving pollutant degradation.
Correct answer is: The contact time between gas and biofilm
Q.88 Which of the following gases can be biologically reduced to a less harmful compound in an anaerobic biofilter?
Nitrogen dioxide (NO₂)
Carbon monoxide (CO)
Sulfur dioxide (SO₂)
Ozone (O₃)
Explanation - Anaerobic microbes can convert CO to methane or acetate under certain conditions.
Correct answer is: Carbon monoxide (CO)
Q.89 What is the primary reason that biofilters are less effective for removing high‑concentration, low‑solubility gases like benzene?
Benzene reacts explosively with the media
Low solubility limits mass transfer to the biofilm
Benzene rapidly decomposes at ambient temperature
Microbes cannot tolerate any aromatic compounds
Explanation - Limited dissolution of low‑solubility gases reduces their availability to the microbial community.
Correct answer is: Low solubility limits mass transfer to the biofilm
Q.90 Which of the following statements about the use of a biofilter for odor control in a composting facility is correct?
Biofilters completely eliminate all odor compounds
Biofilters require high temperatures (>100 °C) to function
Biofilters can reduce odor intensity by up to 90 % with proper design
Biofilters are ineffective against ammonia
Explanation - When properly sized and maintained, biofilters achieve high odor removal efficiencies.
Correct answer is: Biofilters can reduce odor intensity by up to 90 % with proper design
Q.91 In a biofilter treating a gas stream that contains both H₂S and VOCs, the removal of H₂S is typically:
Faster because sulfur‑oxidizing bacteria have higher specific rates
Slower because VOCs compete for oxygen
Independent of the presence of VOCs
Impossible due to mutual inhibition
Explanation - Sulfur‑oxidizing microbes often have higher kinetic rates compared with many VOC‑degrading organisms.
Correct answer is: Faster because sulfur‑oxidizing bacteria have higher specific rates
Q.92 Which of the following is a common indicator that a biofilter is approaching the end of its operational life?
Consistently low pressure drop
Decreasing inlet gas temperature
Significant increase in maintenance frequency and media replacement
Sudden rise in outlet CO₂ concentration
Explanation - Aged media loses porosity and support capacity, requiring more frequent upkeep and eventual replacement.
Correct answer is: Significant increase in maintenance frequency and media replacement
Q.93 Which of the following gases can be effectively removed by a biofilter that employs a mixed culture of methylotrophic bacteria?
Methanol (CH₃OH)
Hydrogen cyanide (HCN)
Carbon dioxide (CO₂)
Nitrogen oxide (NOₓ)
Explanation - Methylotrophs specialize in one‑carbon compounds like methanol, oxidizing them to CO₂.
Correct answer is: Methanol (CH₃OH)
Q.94 When a biofilter is operated at a high inlet flow rate that exceeds design specifications, the most immediate effect is:
Increase in microbial growth rate
Decrease in pressure drop
Reduced residence time leading to lower removal efficiency
Complete shutdown of the blower
Explanation - Higher flow shortens contact time, limiting the opportunity for biodegradation.
Correct answer is: Reduced residence time leading to lower removal efficiency
Q.95 Which of the following gases is most commonly removed by a biofilter in a wastewater treatment plant's odor control system?
Carbon monoxide (CO)
Hydrogen sulfide (H₂S)
Nitrogen oxides (NOₓ)
Sulfur dioxide (SO₂)
Explanation - H₂S is a major odorant from anaerobic processes and is efficiently oxidized in biofilters.
Correct answer is: Hydrogen sulfide (H₂S)
Q.96 Which of the following best explains why biofilters are considered a 'low‑energy' air pollution control technology?
They use solar panels to power the entire system
They operate at ambient temperature and rely mainly on blower power
They generate electricity during pollutant degradation
They require no ventilation or airflow
Explanation - Unlike thermal oxidizers, biofilters need only modest airflow power, resulting in lower energy consumption.
Correct answer is: They operate at ambient temperature and rely mainly on blower power
Q.97 In biofiltration, the term 'catalytic biofiltration' refers to:
The use of metal catalysts to enhance microbial activity
A hybrid system where a catalyst assists in the oxidation of recalcitrant compounds before biological degradation
The application of high‑pressure steam to sterilize the media
Operating the filter at cryogenic temperatures
Explanation - Catalytic biofiltration combines chemical catalysis with biological processes to treat difficult pollutants.
Correct answer is: A hybrid system where a catalyst assists in the oxidation of recalcitrant compounds before biological degradation
Q.98 Which gas is most likely to cause a rapid pH drop in the biofilter media, potentially inhibiting microbial activity?
Carbon dioxide (CO₂)
Hydrogen sulfide (H₂S)
Sulfur dioxide (SO₂)
Nitrogen dioxide (NO₂)
Explanation - SO₂ dissolves to form sulfurous acid, lowering pH and possibly harming microbes.
Correct answer is: Sulfur dioxide (SO₂)
Q.99 Which of the following monitoring parameters is most directly related to the health of the microbial population in a biofilter?
Inlet gas temperature
Media moisture content
Exhaust duct diameter
Fan motor speed
Explanation - Adequate moisture is essential for microbial metabolism and biofilm integrity.
Correct answer is: Media moisture content
Q.100 When a biofilter is treating a gas stream containing 2 % (v/v) ethanol, the main challenge is:
Corrosion of metal components
Excessive heat generation
Rapid biofilm overgrowth leading to high pressure drop
Formation of explosive mixtures
Explanation - Ethanol provides abundant carbon, encouraging fast microbial growth that can clog the media.
Correct answer is: Rapid biofilm overgrowth leading to high pressure drop
Q.101 In a biofilter, the term 'steady‑state' is achieved when:
The inlet pollutant concentration is zero
The outlet concentration no longer changes with time
The media temperature reaches 100 °C
The fan shuts down
Explanation - Steady‑state indicates that the system's removal rate matches the pollutant load, stabilizing outlet levels.
Correct answer is: The outlet concentration no longer changes with time
Q.102 Which of the following gases is most effectively removed by a biofilter that uses a mixed population of nitrifying and denitrifying bacteria?
Amplitude (NH₃)
Hydrogen sulfide (H₂S)
Methane (CH₄)
Formaldehyde (CH₂O)
Explanation - Nitrifiers convert NH₃ to nitrate; denitrifiers can then reduce nitrate to nitrogen gas, achieving complete nitrogen removal.
Correct answer is: Amplitude (NH₃)