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18-Env-A6 Solid Waste Engineering and Management · May 2017

Question 11 of 16: Steps in Composting

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Examination, May 2017 — 04-Env-A6 / 18-Env-A6, Solid Waste Engineering and Management. 3 hours duration, closed book, non-communicating calculator permitted. All 16 questions constitute a complete paper (100 marks total).

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; ISO 14040/14044, Environmental Management — Life Cycle Assessment.

Q6 below is solved from the six printed per-component rows, which are unambiguous exam-given data — see the callout at Q6 for the arithmetic. Table 2's "5.800 kJ/kg" organics value (period instead of comma) is read as 5,800 kJ/kg.

Question 11: Steps in Composting (5 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.

(1) Feedstock preparation — receiving, contaminant removal/screening, size reduction (shredding/grinding) and blending of the carbon-rich (bulking agent, e.g. yard waste, wood chips) and nitrogen-rich (food waste) fractions to reach a target C:N ratio of roughly 25–30:1. (2) Active (thermophilic) composting — piles/windrows/in-vessel reactors are aerated (turning or forced air) and moisture maintained (~50–60% by weight) so aerobic microbial activity raises pile temperature into the thermophilic range (55–65°C), which drives rapid decomposition and pathogen destruction. (3) Curing — after the active phase, material is held in lower, unturned piles for several weeks to months to allow slower mesophilic organisms to stabilize the remaining organic matter and let the compost mature. (4) Screening/refining — finished compost is screened to remove oversize/uncomposted material (returned to the active process) and any residual contraries. (5) Storage and distribution/marketing — the finished, screened compost is stored and distributed to its end market (agriculture, landscaping, topsoil blending).

The active and curing phases (steps 2–3) together typically span 8–16 weeks depending on feedstock and technology (windrow composting runs toward the longer end, in-vessel systems can compress the active phase to 1–2 weeks by forcing aeration), and skipping directly from active composting to marketing without adequate curing is the most common operational shortcut that produces an unstable, phytotoxic product. Process monitoring throughout — pile temperature, oxygen level, and moisture — lets an operator confirm the thermophilic pathogen-kill requirement (a sustained temperature, e.g. ≥55°C for several days) has actually been met before the material moves to curing, rather than assuming it based on elapsed time alone.