Question 4 of 7: Air Toxics, Mobile Sources, Odour Control and Emission Trading

Question 4: Air Toxics, Mobile Sources, Odour Control and Emission Trading (20 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.

(i) Mobile Sources of Air Toxics

Given the site's water, rail and air access alongside heavy industry and dense population, three distinct mobile-source categories are all present and significant:

• Marine vessels at the port. Ocean-going ships burning heavy bunker fuel emit large quantities of SO_x, NO_x and diesel particulate, especially while manoeuvring and at berth (hotelling) close to the populated shoreline.
• Rail locomotives. Diesel-electric switching and line-haul locomotives serving the rail yard emit diesel particulate matter (a recognized air toxic) and NO_x, often idling for extended periods in the yard near residential areas.
• Aircraft at the local airport. Jet engines emit ultrafine particulate, VOCs and NO_x during the landing-and-takeoff (LTO) cycle, concentrated near the runway approach/departure corridors that frequently overlap populated neighbourhoods.

(Heavy-duty diesel trucks moving containers between the port, rail yard and industrial plants add a fourth, spatially pervasive mobile-source contribution across the whole area.)

(ii) Biofiltration for Food-Industry Odour Control

A process releasing a "pet-food-like" odour is characteristic of a rendering/protein-processing operation, whose exhaust carries odorous reduced-sulphur compounds, amines and volatile fatty acids. The appropriate control biotechnology is biofiltration: the humidified, odorous exhaust air is passed through a bed of biologically active organic media (compost, bark, peat, or an engineered synthetic media) that supports an attached microbial biofilm, which metabolizes/oxidizes the odorous compounds to CO₂, water and biomass.

Two fundamental design principles: (1) adequate empty-bed residence time (EBRT), typically 30–60 seconds, sized from the airflow and bed volume so the microbial population has enough contact time to degrade the odorous compounds before the air exits the bed; (2) maintaining proper media moisture content (typically 40–60%) and near-neutral pH, since a bed that dries out or acidifies (a known issue with sulphur-compound-heavy odours) rapidly loses microbial activity and removal efficiency, or can itself become an odour source.

(iii) Emission Trading (Cap-and-Trade)

Under a cap-and-trade system, a government first sets a declining aggregate emissions cap for a pollutant (e.g. CO₂, SO_x) across all regulated sources, then issues or auctions a matching total of tradable emission allowances (credits), one per unit of allowed emission. Sources that can reduce emissions cheaply do so and sell their surplus credits; sources facing higher abatement costs buy credits rather than reduce on-site. This creates a market price for emissions that automatically routes reductions to wherever they are cheapest across the whole economy, while the fixed, shrinking cap guarantees the environmental target is met regardless of how the reductions are distributed — combining environmental certainty (from the cap) with economic efficiency (from the trading market), in contrast to a uniform prescriptive emission-rate mandate applied to every source regardless of its abatement cost. (Real-world examples include the EU Emissions Trading System and various North American cap-and-trade programs for CO₂ and SO_x.)