23-CS-3 Sustainability, Engineering and the Environment · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 11-CS-3 Sustainability, Engineering and the Environment. Open book; non-communicating calculator permitted. Any four questions constitute a complete paper; all questions are of equal value (25 marks each).
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.
Risk = likelihood × consequence. Living downwind of a coal plant: continuous emissions make the likelihood of exposure high and the consequence per exposure low–medium (chronic health effects). Living just downriver of a hydro dam: the likelihood of a dam failure is low but the consequence (catastrophic flooding) would be high. Coal is a high-likelihood/low-consequence chronic risk causing more routine harm; hydro is a low-likelihood/high-consequence catastrophic risk.
(ppb converted to µg/m³ at 25 °C and 1 atm, molar volume 24.45 L/mol.) A lifetime risk of about 3.7 × 10⁻³ (roughly 4 extra cancers per 1,000 people) exceeds the 10⁻⁶ acceptable level by more than three orders of magnitude, and is well above both Health Canada's “essentially negligible” range of 10⁻⁵ to 10⁻⁶ and the 10⁻⁴ upper bound sometimes tolerated for existing exposures, so this is not safe. The unit risk factor is already defined for a 70 kg adult breathing 20 m³/day, which is exactly this man. The dose route gives the same answer: intake = 0.913 mg/m³ × 20/70 = 0.261 mg/kg-d; inhalation slope factor = 4.1×10⁻⁶ × 70/20 × 1000 = 0.0144 (mg/kg-d)⁻¹; risk = 0.261 × 0.0144 ≈ 3.7×10⁻³.
Concentration 6 ppb = 6×10⁻³ mg/L. Intake dose when exposed = (6×10⁻³ × 2)/50 = 2.4×10⁻⁴ mg/(kg·day). Cancer risk (lifetime-averaged over 70 yr):
Hazard quotient (non-carcinogenic; dose averaged over the exposure period, ≈ 2.4×10⁻⁴ × 350/365 = 2.30×10⁻⁴):
The cancer risk of ~1.5×10⁻⁴ is unsafe (it exceeds the 10⁻⁶ benchmark by more than a hundredfold), while the hazard quotient of ~0.77 is below 1, so the non-carcinogenic exposure is acceptable (if the exposure-day dose is used without the 350/365 averaging, HQ = 2.4/3.0 = 0.80, still below 1). As is typical for arsenic, the carcinogenic endpoint governs, so the exposure is unsafe overall and warrants reduction. In the Canadian frame the verdict is the same: 1.5×10⁻⁴ is well above Health Canada's “essentially negligible” range of 10⁻⁵ to 10⁻⁶, even though 6 µg/L is below the arsenic maximum acceptable concentration of 10 µg/L. That MAC is set at the lowest level treatment can reliably achieve, not at a negligible cancer risk, so Health Canada asks that arsenic be kept as low as reasonably achievable; a water that meets the MAC is not thereby shown to be safe on the cancer endpoint.
At the source: do not introduce fine, dry dust outside the fuel's design spec into the conveyor, and control dust accumulation—keeping the fuel within the particle-size/moisture range assessed as non-explosive removes the explosive mixture at its origin. Along the path: install explosion-protection engineering controls—dust collection/ventilation to keep airborne dust below the explosive range, explosion venting/suppression, and removal of ignition sources on the conveyor (as Section 63 requires). At the worker: keep workers clear of the conveyor discharge during operation, train them on the dust-explosion hazard, and use PPE (which offers little protection against a fireball). Best action: the source control—not introducing out-of-spec fine dust and controlling accumulation—is best, because it removes the fuel for the explosion entirely; the incident occurred precisely because fine dry dust outside the assessed fuel spec created an explosive mixture the system was never designed to handle.
