Question 1 of 5: Key MSW Management Terms

Question 1: Key MSW Management Terms (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.

i. Key concerns with a landfill. The principal concerns are: uncontrolled leachate migration to groundwater or surface water if the liner and collection system underperform; landfill gas generation, which is both a methane explosion/migration hazard for nearby structures and a major greenhouse-gas source if not collected and flared or used; the rate at which permitted airspace/capacity is being consumed against the community's growing waste stream; siting and land-use conflict, since traffic, odour, noise and perceived property-value impact routinely trigger public opposition during permitting; long-term post-closure liability, since monitoring and maintenance cost continues for decades after tipping-fee revenue ends; and differential settlement of the refuse mass and cover as decomposition continues. Together these are why a landfill is one of the most heavily engineered and regulated facility types in the solid waste system.

ii. Landfill leachate. Leachate is the liquid generated once infiltrating precipitation, the refuse's own initial moisture and the water released by decomposition together exceed the waste mass's field capacity and begin to drain; as it percolates it dissolves and entrains contaminants, so it typically carries very high BOD₅/COD, elevated ammonia-nitrogen, dissolved solids and, depending on the waste stream, heavy metals. Its composition is not static — it evolves through the five biological phases described in Question 5 — and it is the single greatest uncontrolled environmental liability of a landfill, which is why the composite liner and collection system sketched in Question 2(b) exist specifically to intercept and route it to treatment.

iii. Landfill closure and post closure care. Closure is the set of engineered activities performed at the end of a cell's or site's active operating life: placing a final low-permeability cap and drainage layer, grading for positive surface drainage, establishing vegetated topsoil, and capping or tying off remaining gas and leachate infrastructure. Post-closure care is the extended monitoring and maintenance period that follows, typically 25–30 years under most provincial regulation, during which leachate is still collected and treated, landfill gas is monitored and collected or flared, groundwater monitoring wells are sampled on a fixed schedule, and the cap is inspected and repaired as it settles. Because waste keeps generating leachate and gas for decades after the last load is placed, regulators require an approved closure/post-closure plan and financial assurance before an operating permit is even issued.

iv. Principal landfill gas constituents. Landfill gas (LFG) generated by anaerobic decomposition of the biodegradable fraction is dominated by methane (CH₄, typically 45–60% by volume at maturity) and carbon dioxide (CO₂, typically 40–60%), produced in roughly that ratio because the underlying methane fermentation reaction converts organic acids into CH₄ and CO₂ in near-equal molar proportion. Minor/trace constituents include nitrogen and oxygen (residual atmospheric air trapped in the refuse or drawn in through the cover, both of which decline sharply as the waste mass turns anaerobic — see Question 4(a)), water vapour (saturated, since the gas forms in a moisture-rich environment), and trace levels of hydrogen sulphide, ammonia and non-methane organic compounds (NMOCs), which are responsible for LFG's characteristic odour and for most of its air-quality/toxicity concern even though they make up well under 1% by volume.

v. Windrow and aerated static pile composting techniques. Both are mechanical means of supplying the aeration and moisture control that aerobic composting needs. In the windrow method, feedstock is formed into long, triangular or trapezoidal piles roughly 1–2 m high that are periodically turned by a mechanical windrow turner, which re-introduces oxygen, releases excess heat and re-homogenizes moisture and temperature across the pile. In the aerated static pile (ASP) method, the pile is built once over a network of perforated pipes connected to a blower; forced or induced air supplies oxygen without physically turning the material, and the pile is often finished with a layer of screened compost or a biofilter cover to control odour. Windrow composting needs more land and mobile turning equipment but has lower capital cost; ASP has a smaller footprint and tighter process control at higher capital cost — the choice is a land/budget/odour-sensitivity tradeoff a design engineer makes case by case.