Question 1 of 5: Municipal Solid Waste Management Fundamentals

Question 1: Municipal Solid Waste Management Fundamentals (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. A sanitary landfill's principal long-term risks are all consequences of the biodegradation of the buried organic fraction: leachate generation, a contaminated liquid formed as infiltrating precipitation percolates through refuse and dissolves organic and inorganic decomposition products, which can migrate to groundwater or surface water if not intercepted by an engineered liner and collection system; landfill gas migration, since methane generated by anaerobic decomposition is both an explosion/asphyxiation hazard if it migrates laterally into buildings or confined spaces and a potent greenhouse gas if vented uncontrolled; differential settlement of the refuse mass as organic content decomposes and voids compress over years to decades, which can damage the final cover, surface drainage and any post-closure structures; vector and nuisance control (odour, litter, birds, rodents and insects) during active operation; and siting/land-use constraints — a landfill requires a large land footprint, is difficult to site near communities, and permanently constrains the site's future use even decades after closure. Each of these concerns drives a specific engineered control covered in the parts below (liners and leachate collection for the first; gas venting/flaring for the second; a robust final cover and long-term monitoring for the third and fifth).

(ii) Waste transformation via composting. Composting is a controlled, accelerated aerobic biological decomposition process in which microorganisms (bacteria, fungi, actinomycetes) convert the biodegradable organic fraction of MSW (yard waste, food waste, some paper) into a stable, humus-like product with continuous oxygen supply, adequate moisture (typically 50–60% by weight) and a favourable carbon-to-nitrogen ratio (roughly 25–30:1) maintained by turning or forced aeration. The process passes through a mesophilic phase, a self-heated thermophilic phase (55–65 °C, which pathogen-kills and weed-seed-kills the material) and a cooling/curing phase where the product stabilizes. Composting transforms a waste stream that would otherwise occupy landfill airspace and generate methane into a beneficial soil amendment, diverting the single largest biodegradable fraction of MSW from disposal.

(iii) Landfill Liners. A liner is the engineered hydraulic barrier placed beneath (and up the side slopes of) a landfill cell to prevent leachate from migrating into the underlying soil and groundwater. A modern composite liner combines a low-permeability compacted clay layer (typically ≥0.6 m, hydraulic conductivity ≤1×10^-9 m/s) with an overlying synthetic geomembrane (commonly HDPE), which together perform far better than either material alone — the geomembrane provides an essentially impermeable barrier while any small defect (pinhole, seam flaw) is backed up by the clay's much lower permeability, and the clay in turn provides puncture protection and attenuation should the geomembrane ever be breached. A drainage/leachate collection layer is placed directly above the liner to keep the head of leachate ponding on the liner as low as regulations require (commonly ≤0.3 m), since leakage through any liner defect increases with the hydraulic head driving it.

(iv) Principal landfill gas constituents. Landfill gas is generated by anaerobic microbial decomposition of the buried organic fraction and, once anaerobic conditions are established, is dominated by roughly equal parts methane (CH₄, typically ~45–60%) and carbon dioxide (CO₂, ~40–55%). Smaller amounts of nitrogen (N₂) and oxygen (O₂) are present, mostly trapped air from initial placement that is progressively consumed/displaced as anaerobic conditions develop. Trace constituents include hydrogen sulphide (H₂S) and other reduced-sulphur/organic compounds responsible for the characteristic landfill odour, ammonia (NH₃), and numerous trace non-methane organic compounds (NMOCs) that, despite their very low concentration, are significant for air-toxics regulation. The CH₄/CO₂ ratio and total gas generation rate both evolve over the landfill's biological phases (see Question 3(a)).

(v) Landfill closure and post-closure care. Closure is the engineered capping of a filled cell once its final design elevation is reached: a low-permeability barrier layer (compacted clay and/or a geomembrane) minimizes long-term infiltration, an overlying drainage layer sheds the water that does infiltrate the cap itself, and a vegetated topsoil layer controls erosion and supports the approved post-closure land use, all graded to shed surface runoff without ponding. Post-closure care is the regulatory-mandated monitoring and maintenance period that follows (commonly 25–30 years or longer under Canadian provincial regulation) during which the owner must continue leachate collection and treatment, landfill gas monitoring and control, groundwater monitoring at the perimeter wells, and cap maintenance (repairing settlement cracks, erosion and vegetation), because biological decomposition, gas generation and leachate production continue for decades after the last refuse is placed, long after the site stops accepting waste.