18-Env-A5 Air Quality and Pollution Control Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2015 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8½×11 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.
Benzene, present in gasoline and emitted both as unburned fuel vapour (evaporative losses) and as a partial-combustion product in vehicle exhaust, is a recognized human carcinogen (leukemia) under prolonged or intense exposure, and is the classic mobile-source air toxic tracked in Canadian and U.S. urban air toxics programs because of its ubiquity near roadways and fuel-dispensing facilities. Two effective engineering strategies: (1) three-way catalytic converters on the vehicle's exhaust system oxidize unburned hydrocarbons (including benzene) to CO2 and H2O over a Pt/Pd/Rh catalyst bed operating near stoichiometric air-fuel ratio, cutting tailpipe benzene emissions by well over 90% relative to an uncontrolled engine; (2) vapour recovery systems (onboard refuelling vapour recovery, ORVR, and Stage I/II vapour balance at fuel-dispensing stations) capture the benzene-laden gasoline vapour displaced during refuelling and return it to the storage tank or an activated-carbon canister rather than allowing it to vent to atmosphere, addressing the evaporative (non-tailpipe) exposure pathway that catalytic converters cannot touch.
A biofilter treats odorous/VOC-laden air by passing it through a moist, porous, biologically active media bed (compost, wood chips, or an inert support with an attached biofilm), and its design rests on two fundamental principles. First, mass transfer of the pollutant from the gas phase into the water film surrounding the microbial biofilm must be fast relative to the gas residence time in the bed — this sets the required empty-bed residence time (EBRT, typically 15–60 s) and depends on the pollutant's water solubility/Henry's-law constant, since a poorly soluble compound cannot transfer into the biofilm fast enough to be degraded no matter how active the microbial population is. Second, the biofilm must sustain an actively growing, acclimated microbial population capable of biodegrading the specific odorous/VOC compounds to CO2, water and biomass; this requires the bed to be maintained at the right moisture content (typically 40–60% by weight — too dry and the biofilm desiccates and mass transfer stops, too wet and the bed goes anaerobic and channels/compacts), adequate nutrients, and near-neutral pH (some odorous compounds, e.g. H2S, oxidize to acidic products that can self-inhibit the biology if not buffered).
Emission (cap-and-trade) trading works by having government set a declining aggregate cap on total permitted GHG emissions across a sector or economy, issue or auction tradeable allowances (each representing the right to emit one tonne of CO2-equivalent) equal to that cap, and require every regulated emitter to surrender allowances matching its actual annual emissions. Because allowances can be bought and sold, emitters for whom abatement is cheap have an incentive to reduce emissions below their allocation and sell the surplus allowances, while emitters facing high abatement cost can instead purchase allowances rather than reduce on-site — the market finds the lowest aggregate cost of achieving the fixed emissions cap, in contrast to a uniform command-and-control standard that forces the same percentage reduction on every source regardless of its marginal abatement cost. Canada has applied this mechanism through the federal Output-Based Pricing System for large industrial emitters and provincial cap-and-trade systems (e.g. Quebec's linkage to California's Western Climate Initiative market); the price signal created by trading also drives longer-term investment in lower-carbon technology, since holding fewer allowances than needed becomes progressively more expensive as the cap tightens over time.
