18-Env-A6 Solid Waste Engineering and Management · May 2015
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.
Particle size governs composting rate through two opposing effects, and the design objective is to balance them. Reducing particle size (by shredding or grinding) increases the surface-area-to-volume ratio of the feedstock exposed to microbial attack, which accelerates the rate of decomposition and heat generation — a finer feedstock reaches thermophilic temperature faster and decomposes more completely for a given retention time. However, particle size that is too fine collapses the free air space (porosity) between particles, restricting the oxygen diffusion the aerobic process depends on and promoting compaction into anaerobic, odour-generating pockets, particularly in a wet feedstock. The practical design response is to shred to a moderate, reasonably uniform size (typically a few centimetres) and, where the feedstock is wet or fine (e.g. food waste, biosolids), to blend in a coarser bulking agent (wood chips, shredded yard waste) specifically to preserve structural porosity independent of the fine material's own particle size.
