11-CS-3 Engineering Management · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2016 — 11-CS-3 Sustainability, Engineering and the Environment. Closed book; approved 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 is assessed from two factors: the likelihood (probability) that a harmful event occurs, and the consequence (severity) if it does; risk = likelihood × consequence. High likelihood, low consequence: a minor cut from routine handling of paper—very common but trivial in effect. Low likelihood, high consequence: a catastrophic dam failure or a major nuclear accident—extremely rare but devastating if it happens. Recognizing both dimensions is essential, because the two examples can carry similar computed "risk" by opposite routes, and controlling each calls for different strategies (reducing frequency versus limiting severity).
At the source: prevent the accumulation of fine, dry combustible dust—do not dump swept-up fine dust (outside the design particle-size/moisture spec) into the conveyor, and keep the fuel within the moisture/particle-size range for which the system was assessed as non-explosive; better still, control dust generation and housekeeping so no explosive dust cloud can form. Along the path: install explosion-protection engineering controls—dust collection/ventilation to keep airborne concentrations below the explosive range, plus explosion venting/suppression and removal of ignition sources on the conveyor (as Section 63 of the Regulation for Industrial Establishments requires). At the worker: keep workers away from the conveyor discharge during operation, provide training on the dust-explosion hazard and safe procedures, and use PPE—though PPE offers little protection against a fireball. Best action: the source control—not introducing out-of-spec fine dry dust and controlling dust accumulation—is best, because it removes the fuel for the explosion entirely; the incident arose precisely because fine dry dust (outside the assessed fuel spec) created an explosive mixture the system was never designed to handle.
7 ppb = 7 µg/L = 7×10⁻³ mg/L:
Cancer risk uses the lifetime average daily dose (LADD): the 10 years × 350 d/yr of exposure are averaged over an assumed 70-year lifetime, the standard averaging time for carcinogens.
The cancer risk of about 4 × 10⁻⁵ exceeds the 10⁻⁶ acceptable threshold, so this is not a safe exposure on the cancer criterion (7 ppb is below the 10 ppb standard, but the standard itself corresponds to a risk above the strict 10⁻⁶ benchmark—a known feature of the arsenic MCL).
(Strictly, the non-cancer dose is averaged over the exposure period, i.e. 350/365 of the exposure-day dose, giving $HQ = 1.92\times10^{-4}/3.0\times10^{-4} = 0.64$; either way the value is below 1.) Since $HQ < 1$, the exposure is below the reference dose and is safe for non-carcinogenic effects. As usual, the carcinogenic risk governs the overall judgment for arsenic.
