23-Chem-B2 Environmental Engineering · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B2, Environmental Engineering — December 2018. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.
Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Petroleum hydrocarbons are biodegradable, so the engineering method of choice is in-situ bioremediation enhanced by air sparging (AS) below the water table and soil vapour extraction (SVE) in the vadose zone, which supplies the oxygen that indigenous hydrocarbon-degrading microbes need while simultaneously stripping volatile fractions.
| Technology | Engineering principle / quantitative strategy 1 | Engineering principle / quantitative strategy 2 |
|---|---|---|
| Soil washing | Particle-size fractionation: contaminants (especially metals and hydrophobic organics) preferentially sorb onto the fine (silt/clay, <63 µm) fraction due to its high specific surface area, so hydrocyclones/screens concentrate contamination into a much smaller mass of fines for disposal while the clean coarse sand/gravel fraction (often >70% of the soil mass) is returned to site. | Extraction-fluid chemistry is dosed (surfactant concentration above its critical micelle concentration for organics; chelating agent stoichiometry, e.g. EDTA, matched to the metal-exchange capacity for metals) to maximize desorption from soil particles into the aqueous wash stream within an economic liquid:solid ratio. |
| Bio-remediation | First-order biodegradation kinetics (C = C₀e−kt) size the required treatment duration from the site-specific rate constant k, which itself depends on electron-acceptor availability (oxygen/nitrate), moisture and temperature. | Nutrient (N, P) and electron-acceptor amendment is dosed to a target C:N:P ratio (commonly cited near 100:10:1 for hydrocarbon-degrading consortia) so biodegradation is not nutrient-limited once the easily available oxygen is consumed. |
| Thermal desorption | Volatilization is governed by each contaminant's vapour pressure/boiling point at the desorber operating temperature (typically 90–320°C for low-temperature units, up to 560°C for high-temperature), sized so the target contaminant is driven off without vitrifying/altering the soil matrix unnecessarily. | Residence time in the desorber (rotary-kiln or thermal-screw) is sized from a heat- and mass-transfer balance so every soil particle reaches the target temperature long enough for the contaminant to diffuse out of the particle and volatilize, not just the bulk gas temperature. |
(1) Gaseous pollutants in ambient air — continuous gas analyzer (e.g. chemiluminescence for NO/NO₂/NOₓ, non-dispersive infrared (NDIR) for CO/CO₂). The key engineering principle is a species-specific physical/chemical reaction that produces a signal proportional to concentration — chemiluminescence measures the light emitted when NO reacts with ozone in a reaction chamber (intensity ∝ [NO]), while NDIR measures the attenuation of a specific infrared wavelength absorbed by the target gas (Beer–Lambert law). Calibration: a zero gas (nitrogen or clean dry air) sets the instrument's baseline, and a certified span gas of known concentration (traceable to a national standard) is introduced to set the high-end response; a multi-point calibration curve (span checks at several concentrations) improves precision across the working range, and this is repeated on a defined schedule (daily zero/span checks, periodic multi-point calibration) because detector drift is the dominant source of long-term measurement error.
(2) Pathogens in treated water — membrane filtration / defined-substrate technique for indicator organisms (e.g. E. coli/total coliform via Colilert). The key engineering principle is selective growth: a measured water volume is passed through (or incubated with) a nutrient-indicator medium that only target organisms can metabolize, producing a countable colony (membrane filtration) or a colour/fluorescence change (defined-substrate) proportional to the most-probable-number of viable organisms present. Calibration: precision is maintained by running reference/positive and negative control cultures (known organism concentrations) alongside every batch, verifying incubator temperature against a certified thermometer, and periodically confirming counting/MPN-table results against an accredited reference laboratory split-sample — because the "instrument" here is a biological assay, calibration is really quality-control of incubation conditions and reagent lot performance rather than a physical sensor adjustment.
Assumption stated: both techniques are assumed to be applied by a trained operator following the standard method's specified sample volume/dilution and holding-time requirements; deviations from those (e.g. delayed sample analysis, wrong incubation temperature) are treated as a separate quality-assurance failure mode, not a calibration issue.
