Question 2 of 12: Design Criteria and Considerations

Question 2: Design Criteria and Considerations (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.

2.1 Composting operation design criteria (3). (i) Carbon-to-nitrogen (C:N) ratio — an initial C:N of roughly 25–30:1 balances microbial energy (carbon) and growth (nitrogen) needs; a ratio too high slows decomposition for want of nitrogen, while one too low causes excess nitrogen to volatilize as ammonia (odour, nitrogen loss). (ii) Moisture content — maintained near 50–60% by weight; too dry limits microbial activity, too wet displaces pore air and produces anaerobic, malodorous zones. (iii) Aeration/oxygen supply — turning or forced aeration keeps interstitial O₂ above roughly 5%, sustaining aerobic thermophilic decomposition and preventing the odour and slower kinetics of anaerobic pockets.

2.2 Landfill design considerations (4). (i) Site hydrogeology and subgrade suitability — low-permeability native soils and adequate depth to groundwater provide a secondary barrier beneath the engineered liner. (ii) Liner and leachate-collection system — a composite (clay + geomembrane) liner and a sloped leachate-collection layer/pipe network keep leachate head on the liner within the regulatory limit (commonly ≤300 mm). (iii) Landfill gas management — a collection, venting or flaring/energy-recovery system controls methane migration, odour and greenhouse-gas emissions. (iv) Final cover and post-closure care — cap design (barrier + drainage + vegetated topsoil), settlement allowance and a post-closure monitoring plan (commonly 25–30 years) for leachate, gas and groundwater.

2.3 Leachate control factors (3). (i) Leachate quantity generated — governed by precipitation infiltration through the working face/cover and waste moisture; controlled by minimizing the active/uncovered area and diverting clean stormwater away from refuse. (ii) Liner and collection-system integrity — a double composite liner with a leak-detection layer and correctly spaced/sloped collection piping limits both the driving head and the consequence of any breach. (iii) Leachate composition versus treatment/discharge capability — young leachate (high BOD/COD, low pH) and old, stabilized leachate (high ammonia, low biodegradability, higher metals) demand different control/treatment strategies, so the age profile of the cell governs what control measures are adequate.

2.4 Leachate treatment processes (3). (i) Biological treatment (aerobic activated sludge/SBR, or anaerobic treatment) — effective for BOD/COD and ammonia removal, particularly on high-strength "young" leachate. (ii) Physical/chemical treatment (coagulation–flocculation, chemical precipitation) — removes suspended solids and heavy metals, often as a pretreatment step ahead of biological or membrane stages. (iii) Membrane treatment (reverse osmosis/nanofiltration) — achieves high removal of dissolved organics, ammonia and salts on low-biodegradability "old" leachate, at the cost of a concentrated reject stream that itself needs disposal.

2.5 Landfill gas production variables (3). (i) Waste composition — the fraction of readily biodegradable organic material (food waste, paper) drives methanogenic potential far more than an inert construction-and-demolition fraction. (ii) Moisture content — water is required for microbial hydrolysis and methanogenesis, so gas generation rate rises markedly with moisture (the basis of "bioreactor" landfills that recirculate leachate). (iii) Waste age/time since placement — gas generation follows a lag–rise–peak–decay curve (commonly modelled as first-order decay, e.g. EPA LandGEM), with peak methanogenic conditions typically developing 1–2 years after placement and persisting for decades.

2.6 Problems associated with landfilling municipal waste (4). (i) Groundwater/surface-water contamination if the liner and leachate-collection system are breached or bypassed. (ii) Greenhouse-gas emissions and odour/air-quality nuisance from uncontrolled methane migration (CH₄ has roughly 25–28× the 100-year global-warming potential of CO₂). (iii) Loss of available siting capacity near urban centres, driven by buffer-distance requirements and public opposition (NIMBY). (iv) Long-term liability and post-closure cost — decades of required cap, gas and leachate monitoring, settlement management and financial-assurance obligations after the site stops accepting waste.