18-Env-A6 Solid Waste Engineering and Management · December 2016
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.); Freeze & Cherry, Groundwater; 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.
18.1 Carbon-to-nitrogen (C:N) ratio outside the optimum (25–30:1) range. If C:N < 10, nitrogen is in excess relative to available carbon; microorganisms cannot metabolize all the nitrogen and the surplus is lost as ammonia gas (volatilization), producing a strong ammonia odour and reducing the nitrogen value retained in the finished compost. If C:N > 40, carbon is in excess relative to available nitrogen, which is the microbial population's limiting nutrient; decomposition proceeds slowly because microbial growth is nitrogen-limited, extending the time required to reach stabilization.
18.2 Temperature > 68 °C. Above roughly 65–70 °C, thermophilic microbial activity itself becomes inhibited — even the heat-tolerant thermophiles that drive composting begin to die off, decomposition rate falls, and in extreme cases spontaneous combustion or self-heating fire risk increases in a static, poorly aerated pile. The pile must be actively turned or aerated to dissipate excess heat and bring the temperature back into the effective thermophilic range (roughly 50–65 °C, which is also the range required for pathogen destruction).
18.3 pH > 8.5. A strongly alkaline pile favours volatilization of ammonia (NH₃ is favoured over NH₄⁺ at high pH), causing nitrogen loss as odorous ammonia gas, and can inhibit certain microbial populations whose activity is optimal nearer neutral pH, slowing overall decomposition; high pH can also arise from excess addition of alkaline bulking/amendment materials (e.g. lime, wood ash) and signals the mix should be rebalanced promptly with additional carbon-rich bulking agent to correct the imbalance. Operators typically log temperature, moisture and periodic pH/C:N readings on a routine schedule during active composting, since feedstock composition varies batch to batch and this kind of monitoring catches a drifting parameter before it produces a visible odour complaint or a stalled, non-decomposing pile.
