Thermal NOx formation in combustion increases with:
A.Higher flame temperature and excess air
B.Reduced oxygen presence in the combustion zone
C.Shorter residence time in combustion chamber
D.Low flame temperature below 800°C threshold
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Answer · A
Thermal NOx formation requires high temperature (>1300°C) and oxygen to oxidize atmospheric N₂. Excess air provides additional O₂ and higher flame temperatures exponentially increase the Zeldovich mechanism rate constant, driving NOx formation.
Question 2Biology
A biological reactor is operated at steady-state with influent substrate S = 2K_s. According to the Monod equation (μ = μ_max × S / (K_s + S)), what is the specific growth rate as a fraction of μ_max?
A.μ = 2/3 μ_max
B.μ = 1/2 μ_max
C.μ = 3/4 μ_max
D.μ = 1/3 μ_max
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Answer · A
Plug S = 2K_s into the Monod equation: μ = μ_max × 2K_s / (K_s + 2K_s) = μ_max × 2K_s / 3K_s = (2/3) μ_max. Understanding Monod kinetics is crucial for biological reactor design and operation.
Question 3Biology and Ecology
During anaerobic digestion, acetogenesis converts:
A.Volatile fatty acids to acetate, CO2, and H2
B.Acetate to methane in the methanogenesis stage
C.Cellulose directly to methane without hydrolysis
D.Methane to carbon dioxide through oxidation
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Answer · A
Anaerobic digestion proceeds in four sequential stages: hydrolysis → acidogenesis → acetogenesis → methanogenesis. During acetogenesis, acidogens (syntrophic acetogens) oxidize volatile fatty acids (VFAs) such as propionate and butyrate into acetate, CO₂, and H₂: e.g., CH₃CH₂COO⁻ + 2H₂O → CH₃COO⁻ + CO₂ + 3H₂ (propionate oxidation). This step is thermodynamically unfavorable unless H₂ partial pressure is kept low by hydrogenotrophic methanogens (interspecies hydrogen transfer). Choices B and C describe methanogenesis (conversion of acetate or H₂/CO₂ to methane), not acetogenesis.
Question 4Chemistry
Total inorganic carbon (CT) in water equals:
A.[CO2] + [HCO3-] + [CO3²-]
B.[HCO3-], the dominant carbonate species
C.[CO2] + [CO3²-], excluding bicarbonate
D.Alkalinity (measured via titration)
Show solution
Answer · A
Total inorganic carbon (T_IC or C_T) = [CO₂(aq)] + [HCO₃⁻] + [CO₃²⁻]. In natural waters at typical pH (6–9), HCO₃⁻ dominates. CO₂ dominates below pH ~6.3 and CO₃²⁻ becomes significant above pH ~10.3. The carbonate system is the primary pH-buffering system in natural and engineered water. Note that dissolved CO₂ in the equation represents both CO₂(aq) and H₂CO₃ (carbonic acid), which are often grouped as H₂CO₃* in textbooks. Choice B omits bicarbonate, the dominant species; Choice C omits CO₂; Choice D includes Ca²⁺, which is a separate parameter.
Question 5Environmental Engineering
In a sanitary landfill, the primary purpose of a composite liner system (compacted clay overlain by a geomembrane) is to:
A.Increase the structural stability of the waste mass
B.Prevent leachate migration into groundwater
C.Promote aerobic decomposition of waste
D.Collect landfill gas for energy recovery
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Answer · B
The composite liner system is designed to minimize leachate percolation into the subsurface and protect groundwater. The geomembrane provides a near-impermeable barrier while the compacted clay provides a secondary barrier. Reference: FE Reference Handbook – Solid Waste (Environmental Engineering section).
Question 6Environmental Health and Safety
The SI unit of absorbed radiation dose is:
A.Gray (Gy) — equal to 1 J/kg
B.Curie (Ci) — measure of radioactivity
C.Becquerel (Bq) for activity rate
D.Roentgen (R) for exposure dose
Show solution
Answer · A
Units: Becquerel (Bq) = 1 disintegration/second (activity). Gray (Gy) = 1 J/kg absorbed dose. Sievert (Sv) = equivalent dose (Gy × radiation weighting factor). Curie = 3.7×10¹⁰ Bq (old unit). Roentgen = exposure in air (old unit). EPA cancer risk slope factors available for common radionuclides.
Question 7Ethics and Professional Practice
Engineers issuing public statements must:
A.Include all relevant facts and identify if statements are speculative
B.Prioritize the employer's business interests over public safety and welfare
C.Only communicate statements through legal counsel to minimize liability exposure
D.Avoid all written communication and only use verbal statements to minimize documentation
Show solution
Answer · A
NSPE Code of Ethics, Section III.3: Engineers shall issue public statements only in an objective and truthful manner, including pertinent information and, where relevant, noting that the statement is personal opinion. Engineers must not misrepresent facts or omit information that would make a statement misleading. Choice B is fundamentally wrong — engineers must hold public safety paramount above employer interests. Choice C is not required by any code of ethics. Choice D would violate NSPE requirements for open and honest communication.
Question 8Fluid Mechanics
The Froude number Fr = V/√(gL) characterizes:
A.The ratio of inertial to gravitational forces
B.The ratio of inertial to viscous forces
C.The ratio of pressure to inertial forces
D.Turbulence intensity
Show solution
Answer · A
Fr < 1: subcritical (gravity dominates); Fr > 1: supercritical (inertia dominates). Critical for open channel flow.
Question 9Groundwater
At a petroleum spill site, dissolved oxygen in groundwater is depleted, and nitrate levels are low. To enhance in situ bioremediation, which amendment should be injected, and what is its primary role?
A.Inject phosphate to provide a carbon source for microbes.
B.Inject sulfate as an electron acceptor for anaerobic degradation.
C.Inject sand to increase aquifer porosity.
D.Inject hydrogen peroxide to increase nitrate concentration.
Show solution
Answer · B
With oxygen and nitrate depleted, sulfate can serve as an alternative electron acceptor to stimulate anaerobic biodegradation of petroleum hydrocarbons. Phosphate is a nutrient, not a carbon source. Hydrogen peroxide increases dissolved oxygen, not nitrate. Increasing porosity with sand is not a standard enhancement method.
Question 10Mass and Energy Balances
A CSTR receives two waste streams: Stream 1 is 1000 L/hr at 150 mg/L BOD, and Stream 2 is 2000 L/hr at 180 mg/L BOD. The effluent BOD is 60 mg/L. What is the total BOD mass removal rate in g/hr?
A.230 g/hr
B.330 g/hr
C.270 g/hr
D.180 g/hr
Show solution
Answer · B
Compute the influent BOD mass rate from both streams: (1000 L/hr)(150 mg/L) + (2000 L/hr)(180 mg/L) = 150,000 + 360,000 = 510,000 mg/hr.
The total effluent flow is 3000 L/hr, so the effluent BOD mass rate is (3000 L/hr)(60 mg/L) = 180,000 mg/hr.
The removal rate is 510,000 - 180,000 = 330,000 mg/hr = 330 g/hr. The other choices reflect errors such as using an incorrect influent concentration, using the wrong effluent concentration, or reporting the effluent mass rather than the removed mass.
Question 11Material and Energy Balances
A secondary clarifier receives 10 MGD with MLSS = 3500 mg/L. The solids loading rate (SLR) with surface area 10,000 ft² is:
