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23-Chem-B2 Environmental Engineering · December 2015

Question 3 of 7: PCB soil remediation, and reliability/reproducibility in air-quality measurement

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. EGBC 04-Chem-B2 Environmental Engineering, December 2015, 3 hours, closed-book with a candidate-prepared double-sided 8½×11-inch aid sheet. Seven problems, each worth 20 marks; candidates attempt any five, and only the first five answers in the workbook are marked. All seven problems are solved below as a complete study resource.

Reference texts: G. Tchobanoglous, F. L. Burton & H. D. Stensel (Metcalf & Eddy), Wastewater Engineering: Treatment and Reuse (4th ed., McGraw-Hill) — BOD kinetics, dissolved air flotation, activated-sludge design; M. L. Davis & D. A. Cornwell, Introduction to Environmental Engineering (5th ed., McGraw-Hill) — drinking-water treatment, air pollution control, ion exchange, reverse osmosis, soil remediation; C. D. Cooper & F. C. Alley, Air Pollution Control: A Design Approach — cyclones, scrubbers, fabric filtration, electrostatic precipitation, odour control; S. P. Turner, Workbook of Atmospheric Dispersion Estimates (2nd ed., CRC Press) — the Gaussian plume model and Pasquill–Gifford stability classes. Canadian context follows the Canadian Environmental Protection Act (CEPA 1999), the Canadian Council of Ministers of the Environment (CCME) Municipal Wastewater Effluent and Drinking Water Quality guidelines, and provincial air/water permitting practice (e.g. BC Environmental Management Act, Metro Vancouver air-quality bylaws).

Question 3: PCB soil remediation, and reliability/reproducibility in air-quality measurement (20 marks)

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.

(i) Engineering method and key steps for PCB-contaminated soil remediation

Engineering method: ex-situ thermal desorption / high-temperature incineration. PCBs (polychlorinated biphenyls) are thermally and chemically stable, persistent organic compounds that resist the biological and chemical treatment routes effective for many other organic contaminants (unlike petroleum hydrocarbons, PCBs are poorly biodegradable at the concentrations and congener profiles typical of contaminated sites), so their remediation standard of practice is thermal destruction or volatilization/capture rather than in-situ biological or chemical treatment.

Excavation &CharacterizationEx-situ ThermalDesorption / IncinerationConfirmationSamplingPCB-impactedsoilexcavated soiltreated soilBackfill(<1 ppm PCB)off-gas -> secondarycombustion + APC
Fig. 2: Ex-situ thermal treatment train for PCB-impacted soil — excavation and characterization, thermal desorption/incineration (with off-gas routed to secondary combustion and air pollution control), and confirmation sampling before backfill.
  1. Site characterization and excavation. Delineate the horizontal/vertical extent of PCB impact above the applicable regulatory soil-quality standard (in Canada, under CEPA's PCB regulations) via a statistically designed sampling grid, then excavate and stage the impacted soil separately from clean soil.
  2. Thermal treatment (desorption or incineration). Excavated soil is heated in a rotary kiln or thermal desorption unit to a temperature sufficient to volatilize (desorption, typically 300–550 °C) or fully combust (incineration, >1,000 °C) the PCBs; the resulting off-gas is routed to a secondary combustion chamber (destroying the volatilized PCBs at a temperature/residence time validated to achieve the required destruction and removal efficiency, DRE ≥ 99.9999% for PCB incineration under most regulatory regimes) followed by air pollution control (acid-gas scrubbing, particulate control) before discharge.
  3. Confirmation sampling and backfill. Treated soil is sampled against the regulatory PCB soil-quality standard (a statistically defensible sampling density) before being certified clean and returned/backfilled to the site; any soil that fails confirmation is re-processed rather than backfilled.
Check — DRE stringency for PCBs

PCB incineration is regulated to a materially higher DRE standard (commonly ≥99.9999%, "six nines") than a typical VOC/hazardous-air-pollutant thermal oxidizer (99.5–99.9%), reflecting PCBs' persistence and toxicity; this is stated here as an assumption of the treatment design and should be confirmed against the specific jurisdiction's regulation for a real site.

(ii) Reliability and reproducibility in air-quality measurement techniques

(a) Reliability is the consistency of an instrument's response to a given true concentration across repeated measurements and over the extended, often largely unattended deployment periods typical of continuous ambient air-quality monitoring. It is maintained through automated, scheduled zero/span calibration checks against a certified reference gas, redundant sensors or parallel monitoring at critical stations, and a documented data-validation/QA program that flags and excludes drifted or failed readings; a reliable network produces a continuous, gap-free, trustworthy record even though it operates largely unattended between site visits.

(b) Reproducibility is the degree to which independent measurements of the same air sample or parameter — by different instruments, operators, laboratories, or on different days — agree with one another. It is maintained through standardized measurement protocols (a common reference method, e.g. USEPA/ECCC Federal Equivalent Method designations), inter-laboratory proficiency testing/round-robin comparisons, and traceable calibration of all participating instruments to a common reference standard; reproducibility is what allows air-quality data collected at different stations, by different operators, or across different time periods to be meaningfully compared against a single regulatory standard.

Distinguishing the two. Reliability concerns a single instrument's own internal consistency over time; reproducibility concerns agreement between independent measurement systems. A network can be highly reliable (each station reads consistently over time) yet poorly reproducible (different stations, using non-standardized methods, disagree systematically) — both properties must be separately verified for the resulting air-quality dataset to be defensible for regulatory or public-health decisions.