18-Env-A5 Air Quality and Pollution Control Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five (5) questions constitute a complete paper (the first five answers as they appear are marked); all seven are solved below for completeness. Each question is worth 20 marks with section marks shown in brackets.
Reference texts. Cooper & Alley, Air Pollution Control: A Design Approach (4th ed.); Wark, Warner & Davis, Air Pollution: Its Origin and Control (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Canadian Environmental Protection Act, 1999 (CEPA) and the Canadian Ambient Air Quality Standards (CAAQS) administered by Environment and Climate Change Canada.
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.
Air toxics: benzene and diesel particulate matter (diesel PM). Benzene is a known human carcinogen present in gasoline vapour and exhaust; diesel PM carries adsorbed polycyclic aromatic hydrocarbons (PAHs) and is itself classified as a probable human carcinogen.
Technology 1 — three-way catalytic converter (gasoline vehicles). A platinum/palladium/rhodium washcoat simultaneously oxidizes unburned hydrocarbons (including benzene) and CO to CO2/H2O and reduces NOx to N2, working most effectively when the engine is run at the stoichiometric air/fuel ratio (closed-loop oxygen-sensor control keeps the exhaust in the narrow window where all three reactions proceed together).
Technology 2 — diesel particulate filter (DPF) with a diesel oxidation catalyst (DOC). The DOC oxidizes gas-phase hydrocarbons and CO (and converts some NO to NO2), while the downstream DPF is a porous ceramic wall-flow filter that physically traps PM (and the PAHs adsorbed on it) as exhaust is forced through its porous walls; trapped soot is periodically oxidized ("regenerated") using the NO2 from the DOC (passive regeneration) or an active fuel-injection burn, preventing the filter from plugging.
A biofilter passes the odorous exhaust stream slowly upward through a bed of biologically active media (compost, wood chips or a synthetic packing colonized by a microbial biofilm). Odorous compounds (H2S, reduced-sulfur VOCs, organic acids) partition from the gas phase into the moist biofilm and are then metabolized by the resident bacteria — sulfur-oxidizing bacteria convert H2S to sulfate, and heterotrophic bacteria oxidize VOCs ultimately to CO2 and water — so control is biological degradation rather than physical capture, meaning there is no spent-media disposal problem in the way a carbon adsorber has. Key O&M issues: (1) moisture content of the media must be maintained (typically 40–60% by weight) since a biofilm that dries out loses microbial activity, usually requiring a humidification/pre-wetting stage ahead of the bed; (2) empty-bed residence time (EBRT, bed volume divided by gas flow, typically 30–60 seconds) must be sized for the target removal efficiency and monitored, since channelling, media compaction or bed drying over time reduce the effective residence time and degrade performance even though the bed looks intact.
A well-designed cap-and-trade programme for GHGs, linked between neighbouring jurisdictions, can effectively reduce aggregate emissions, because the environmental outcome is set by the cap itself, not by the trading. Once a jurisdiction sets a declining aggregate emissions cap and issues (or auctions) a matching number of allowances, every tonne emitted anywhere in the linked market must be matched by a surrendered allowance — total emissions across the linked system cannot exceed the sum of the caps regardless of who reduces where. Trading then adds economic efficiency on top of that guaranteed physical outcome: because marginal abatement costs differ widely between emitters (a cheap efficiency retrofit versus an expensive process redesign), a market lets abatement happen first wherever it is cheapest, achieving the same aggregate reduction at lower total cost than a uniform emissions-intensity regulation applied identically everywhere. Linking two neighbouring countries' markets (as with the Quebec–California WCI linkage) widens the pool of low-cost abatement options available to both, in principle lowering the market clearing price for the same combined cap. The scientifically necessary condition for this to work, however, is that the linked caps themselves are set consistent with the emissions trajectory required to limit warming (i.e., the cap must actually decline over time and cannot be inflated by weak monitoring, reporting and verification, MRV) — a trading mechanism cannot compensate for a cap that is too loose, but given a rigorous, declining, well-verified cap it is the cap-and-trade architecture, not a uniform regulation, that has repeatedly delivered the target reduction at the lowest observed system-wide cost (e.g., the U.S. Acid Rain sulfur-trading programme).
