23-Chem-B2 Environmental Engineering · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 16-Chem-B2, Environmental Engineering — December 2019. 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.
| Technology | Main principle | Advantages | Limitations | Application |
|---|---|---|---|---|
| (i) Catalytic reactor (gases) | The contaminated gas stream is heated (typically 200–450°C) and passed over a catalyst bed (platinum/palladium or base-metal-oxide) that lowers the activation energy of the VOC oxidation reaction, so complete combustion to CO₂ and H₂O occurs at a much lower temperature than thermal (flame) oxidation. | 1) Much lower fuel/energy cost than thermal oxidation because the catalyst substitutes for high flame temperature. 2) High destruction efficiency (often >95%) for a wide range of oxidizable VOC/hazardous gas streams. | 1) Catalyst is poisoned/deactivated by particulates, halogens, sulfur or heavy-metal compounds in the stream, requiring pre-filtration and periodic catalyst replacement. 2) Not suitable for streams with highly variable or very high VOC loading, where excessive exotherm can damage the catalyst bed. | Solvent-recovery or degreasing operations emitting hazardous volatile organics (e.g. chlorinated or aromatic solvent vapours) at a fairly steady, moderate concentration. |
| (ii) Biofilter (vapours) | The vapour stream is humidified and passed through a bed of organic media (compost, wood bark, peat) that supports an acclimated microbial population; the vapour-phase contaminant partitions into the moist biofilm and is biodegraded aerobically to CO₂, H₂O and biomass. | 1) Low capital and operating cost (no fuel, no catalyst) for a dilute, continuous, biodegradable vapour stream. 2) Low secondary environmental impact (no combustion by-products, low energy use, low carbon footprint). | 1) Removal efficiency and capacity are limited — poor for high-concentration or non-biodegradable (e.g. many chlorinated) VOCs. 2) Performance is sensitive to media moisture, pH and temperature; the bed can dry out, channel or become acidic and lose removal capacity if not maintained. | VOC emissions from a paint booth, food-processing or rendering-plant off-gas where the compounds are biodegradable and the stream is dilute and continuous. |
| (iii) Cyclone (particulates) | The dust-laden gas enters tangentially and is spun in a vortex inside a conical body; centrifugal force throws the denser particulates outward to the wall, where they lose momentum, fall, and are collected in a hopper while the cleaned gas exits through a central vortex finder. | 1) Very low capital cost, no moving parts and minimal maintenance. 2) Handles high dust loadings, high temperatures and abrasive/coarse particulates well without media fouling. | 1) Poor collection efficiency for fine particulates (below roughly 10–15 µm) because centrifugal separation force scales with particle mass. 2) Pressure drop (and hence fan energy cost) rises quickly if a higher-efficiency (smaller-diameter, higher-velocity) cyclone design is used to chase finer particle capture. | Pre-cleaner ahead of a baghouse or ESP on a quarry, sawmill or grain-handling operation emitting coarse particulates up to about PM₅₀, reducing the dust load (and wear) reaching the finer downstream control device. |
All three technologies trade off capture/removal mechanism against particle or molecule size and concentration: catalytic reactors and biofilters both convert (destroy) the gas-phase contaminant rather than merely capturing it, differing mainly in whether the oxidation is thermally/catalytically or biologically driven, while the cyclone is a purely physical separator whose efficiency is intrinsically limited to coarser particulates and is therefore almost always paired with a finer polishing device downstream.
