Question 6 of 7: Air Toxics Control, Solid Waste Management and Environmental Standards/Guidelines

Question 6: Air Toxics Control, Solid Waste Management and Environmental Standards/Guidelines (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) Two Air Toxics, Matching Treatment Technologies and Key Design Principles

1. Benzene (a volatile organic air toxic) — controlled by regenerative thermal oxidation (RTO). RTO destroys VOCs by heating the contaminated air stream (with ceramic heat-exchange media recovering most of the thermal energy from the previous cycle) to a temperature high enough, and for long enough residence time, to oxidize benzene to CO₂ and water. Key design principles: sizing for the required combustion temperature (typically ~800–850°C for VOC destruction), adequate residence time at that temperature (destruction efficiency depends on both together), and turbulent mixing to ensure the entire gas stream — not just a fraction — reaches destruction conditions.
2. Mercury (a metallic air toxic, e.g., from coal combustion) — controlled by activated-carbon injection (ACI) with particulate capture. Powdered activated carbon injected into the flue gas adsorbs elemental and oxidized mercury vapour onto its surface; the mercury-laden carbon particles are then captured downstream by an existing particulate control device (baghouse or electrostatic precipitator). Key design principles: adequate carbon injection rate and gas-phase contact time for adsorption to occur before the particulate collector, carbon type/treatment (e.g., halogen-impregnated carbon) matched to whether the mercury is elemental or oxidized, and sufficient downstream particulate-collection efficiency so the mercury-laden carbon is actually captured rather than passing through.

(ii) Three Strategies to Maximize Landfill Longevity Over a 25-Year Life Cycle

1. Waste diversion through source reduction, recycling and organics diversion (composting/anaerobic digestion). Diverting recyclable and compostable material out of the waste stream before it ever reaches the landfill directly reduces the annual tonnage requiring burial, which is the single largest lever on remaining airspace life for a fixed-capacity site.
2. Waste compaction and daily-cover optimization to maximize in-place density. Higher compaction (heavier compactors, thinner and more efficient daily/intermediate cover using alternative cover materials rather than a full soil layer) increases the mass of waste stored per unit of consumed airspace, extending the site’s effective capacity without changing its footprint or accepted tonnage.
3. Vertical expansion (increased final grades) and/or engineered lateral expansion within the existing or an adjacent permitted footprint. Where geotechnically and regulatory feasible, raising the landfill’s final design elevation (with corresponding slope-stability and leachate/gas-management design) adds airspace directly; where vertical expansion is constrained, a permitted lateral expansion onto adjacent land extends the site’s remaining life without siting an entirely new facility.

(iii) Environmental Quality Standards Versus Guidelines, Used Together

An environmental quality standard is a legally enforceable numeric or narrative limit set in regulation (e.g., a maximum permitted concentration in a discharge permit or an ambient water-quality objective backed by legislation) — exceeding it carries a compliance/enforcement consequence. An environmental quality guideline is a science-based recommended value (such as the CCME water-quality guidelines) that is not itself legally binding but represents the concentration below which a given use (e.g., recreation, aquatic life, drinking-water source protection) is protected; it informs and can be adopted into standards but does not carry independent enforcement authority on its own.

Used together for a lake serving both recreational use and as drinking-water source for a large municipality, the more stringent, use-specific guideline (here, likely the drinking-water-source guideline, since potable-water protection is typically more restrictive than recreational-contact criteria) can be adopted as the enforceable ambient standard for the lake, so that meeting the single enforceable limit automatically protects the less-restrictive recreational use as well — achieving both objectives with one monitoring and compliance program rather than duplicating effort managing two separate numeric targets, which is the cost-effectiveness the question is pointing at.