18-Env-A6 Solid Waste Engineering and Management · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management: Engineering Principles and Management Issues; Vesilind, Worrell & Reinhart, Solid Waste Engineering; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); CCME, Guidance Document on Landfill Gas Management; Canadian Environmental Protection Act, 1999.
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.
Carbon-to-nitrogen (C:N) ratio — optimum roughly 25–30:1; too high (excess carbon) starves the microbial population of nitrogen and slows decomposition, while too low (excess nitrogen) results in nitrogen loss as ammonia gas, which both wastes a valuable nutrient and causes odour. Moisture content — optimum roughly 40–60%; too low limits microbial activity (cells need water to function), while too high excludes air from the pore spaces, driving the pile anaerobic. Oxygen/aeration — aerobic decomposition requires free oxygen (typically kept above ~5% within the pile); inadequate turning or aeration creates anaerobic pockets that slow stabilization and cause odour. Particle size — smaller particles increase the surface area available for microbial attack (faster decomposition), but particles that are too fine reduce porosity and restrict airflow, working against aeration. Temperature — the pile progresses from an initial mesophilic phase through a thermophilic phase (50–65 °C), which is needed to destroy pathogens and weed seeds; temperatures above ~65–70 °C begin to inhibit even thermophilic microorganisms. pH — near-neutral pH is optimal for most composting microorganisms; strongly acidic or alkaline conditions inhibit microbial activity and slow the process.
