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.
| Contaminant / technology | (a) Key design parameter | (b) Operational issue | (c) Maintenance issue |
|---|---|---|---|
| (i) PM₁₀ from quarry — fabric filter (baghouse) | Air-to-cloth ratio (superficial face velocity through the bag media, typically 0.6–1.2 m/min for reverse-air/shaker designs) — set too high and the dust cake forms unevenly and blinds/penetrates the media. | Maintaining a stable, low pressure drop across the bags via timed or ΔP-triggered pulse-jet (or reverse-air) cleaning, without cleaning so aggressively that the protective residual dust cake is stripped and the fresh cloth loses its own filtration efficiency. | Periodic bag inspection and replacement (abrasion holes and thermal/chemical degradation are the main failure modes for quarry dust, which is abrasive); cleaning-system (solenoid valve, pulse header) upkeep to prevent uneven cleaning and localized blinding. |
| (ii) VOCs from spray paint booth — activated-carbon adsorption | Bed depth / empty-bed contact time and the VOC's adsorption isotherm capacity, which together set the breakthrough time for a given inlet VOC loading and superficial gas velocity. | Switching from the on-line bed to the regenerated standby bed before breakthrough occurs (VOC starting to leak through the mass-transfer zone), while also keeping the paint-booth exhaust free of overspray particulate that would blind the carbon surface. | Periodic carbon replacement or steam/thermal regeneration as adsorption capacity declines with repeated cycling (competitive adsorption of higher-boiling paint solvents progressively fouls the lighter-VOC capacity); upstream particulate-filter maintenance to protect the bed. |
| (iii) Odorous vapours from rendering plant — biofilter | Empty-bed residence time (typically 30–60 s) and media moisture content (55–65%), which together control the microbial degradation rate of the odorous compounds (H₂S, amines, organic sulfides) within the media bed. | Maintaining media moisture and pH within the range that keeps the acclimated microbial population active — a rendering-plant off-gas stream can be hot and dry, which will desiccate the bed and collapse removal efficiency if not pre-humidified. | Periodic media replacement/turning as it compacts and channels over 2–5 years, and irrigation system upkeep (nozzles, distribution header) to prevent dry zones that stop biodegrading and start short-circuiting untreated gas. |
Across all three technologies the underlying design logic is the same: performance depends on giving the contaminant enough residence time / contact area at the correct velocity to reach the pollutant-removal mechanism (cake filtration, adsorption mass transfer, or microbial degradation) before the gas exits, and each technology's maintenance burden tracks directly back to whatever physically degrades that contact mechanism over time (bag abrasion, carbon fouling, or media desiccation/compaction).
