Question 4 of 6: Odour Control, Noxious Pollutants, Emission Trading and Air Toxics

Problem 4: Odour Control, Noxious Pollutants, Emission Trading and Air Toxics (25 marks)

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.

Part (i) — two odour control methods. Odour is typically a mixture of reduced-sulphur and volatile organic compounds (e.g. hydrogen sulphide, mercaptans, amines) emitted from biological/organic sources such as wastewater treatment plants, composting facilities, rendering plants and pulp mills. Method 1 — biofiltration. Governing principle: odorous process air is humidified, then passed slowly through a bed of living microbial media (compost, bark, peat); resident microorganisms biologically oxidise the odorous VOCs and reduced-sulphur compounds to CO₂, H₂O and biomass as the air percolates through the bed. It is effective and low-cost for dilute, biodegradable odour streams (a wastewater headworks, a rendering plant) but needs a large footprint and consistent media moisture. Method 2 — chemical scrubbing (wet absorption). Governing principle: the odorous gas stream is contacted counter-currently with a chemical solution (e.g. sodium hypochlorite for H₂S, or an acid scrubber for amines) in a packed tower, which oxidises or neutralises the odour-causing compound into a non-volatile, water-soluble form. It is compact and fast-responding, well-suited to a concentrated, variable odour stream (a lift station or digester off-gas), at higher chemical operating cost than biofiltration.

Part (ii) — noxious pollutants. A noxious pollutant is a substance released to the atmosphere that is harmful or injurious to human health, property, or the environment even at relatively low concentrations — the term is used broadly for any substance regulated because of its acute or chronic toxicity, irritancy, or ecological harm, as distinct from pollutants regulated primarily for aesthetic/nuisance reasons. Three examples: (1) hydrogen sulphide (H₂S), from wastewater treatment, oil/gas processing and pulp mills — acutely toxic (respiratory paralysis) at high concentration and a strong odour nuisance at low concentration; (2) ammonia (NH₃), from livestock operations, fertiliser manufacturing and refrigeration leaks — corrosive to the respiratory tract and eyes; (3) chlorine (Cl₂), from pulp bleaching and water/wastewater disinfection — a strong respiratory irritant and historically a chemical-warfare agent, illustrating the acute-toxicity end of the noxious-pollutant spectrum.

Part (iii) — emission trading. Emission trading (cap-and-trade) is a market-based regulatory mechanism in which a regulator sets an overall emissions cap for a pollutant (e.g. CO₂e) across a sector, issues (or auctions) a matching quantity of tradable emission allowances, and lets firms buy and sell allowances among themselves; a firm that can reduce emissions cheaply sells its surplus allowances to a firm facing a higher marginal abatement cost, so the sector-wide reduction target is met at the lowest total economic cost. Purpose: achieve a fixed environmental outcome (the cap) while letting the market find the cheapest combination of abatement across many emitters, rather than mandating the same technology or reduction percentage on every source. Canadian application: British Columbia's CleanBC industrial carbon-pricing system (and the federal Output-Based Pricing System, OBPS) apply an output-based cap-and-trade-style mechanism to large industrial emitters (>10,000 t CO₂e/yr), where a facility emitting below its sector benchmark earns tradable credits and one emitting above must buy credits or pay the carbon price on the excess — a direct real-world example of emission trading operating in a Canadian jurisdiction today.

Part (iv) — air toxics. An air toxic (hazardous air pollutant, HAP) is a substance known or suspected to cause serious health effects — cancer, birth defects, reproductive or neurological harm — at exposures far below the concentrations of a criteria pollutant, and is therefore regulated individually (source-specific technology standards) rather than through an ambient air-quality standard alone. Two examples: (1) benzene, a known human carcinogen from vehicle exhaust, gasoline vapour and petrochemical processing; (2) mercury (Hg), a persistent, bioaccumulative neurotoxin emitted from coal combustion and metal smelting, which accumulates in fish and poses a particular risk to fetal neurological development.