23-CS-3 Sustainability, Engineering and the Environment · 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.
In environmental and occupational risk assessment the two factors are the hazard (the intrinsic capacity of an agent or situation to cause harm, i.e. its toxicity or energy) and the exposure (how much of it actually reaches a person, and for how long): risk = hazard × exposure. There is no risk unless both are present. High hazard, low exposure: a sealed, locked store of concentrated arsenic trioxide or a shielded industrial radiography source. The agent is extremely toxic, but with containment nobody contacts it, so the risk is low. Low hazard, high exposure: the table salt or caffeine people ingest every day, or chlorinated tap water drunk for a lifetime. Exposure is continuous, but the agent is of low toxicity at those doses, so the risk is again low. This is the same frame as part (b): controls act either on the hazard at its source or on the exposure along the path and at the person. (Safety engineering expresses the same idea as likelihood × consequence, where likelihood corresponds to exposure and consequence to hazard 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 both ends of the range Health Canada treats as "essentially negligible" (10⁻⁵ to 10⁻⁶ excess lifetime cancer risk), so this is not a safe exposure on the cancer criterion. Note that 7 ppb is below Health Canada's maximum acceptable concentration for arsenic (0.010 mg/L = 10 µg/L); that MAC is set as low as is reasonably achievable by treatment rather than at the 10⁻⁶ risk level, which is why a water that meets it can still fail a strict cancer-risk test.
(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.
