18-Env-A6 Solid Waste Engineering and Management · May 2013

Question 10 of 17: Life-Cycle Analysis of a Composting Solution

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

Notes on this paper

National Examination, May 2013 — 04-Env-A6 / 18-Env-A6, Solid Waste Engineering and Management. 3 hours duration, closed book, one letter-sized aid sheet permitted. All 17 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.

Check: several questions on this paper (Q1, Q13, Q14, Q17) supply an incomplete data set and explicitly invite the candidate to "make and state" assumptions. Every assumed value below is called out where it is introduced and is chosen from standard solid-waste-engineering practice; the governing METHOD, not the specific assumed number, is what the exam is testing.

Question 10: Life-Cycle Analysis of a Composting Solution (7 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.

A life-cycle analysis (LCA) of a composting solution follows the standard ISO 14040 four-stage framework, applied specifically to the composting system:

1. Goal and scope definition. State the purpose (compare composting against the city's other options — landfilling, incineration) and define the system boundary: collection of organics, transport to the composting facility, processing (windrow/in-vessel), curing, and end use/distribution of the finished compost. Define the functional unit (e.g., "management of 1 tonne of source-separated organics").

2. Life-cycle inventory (LCI). Quantify every input and output crossing the system boundary: fuel and electricity for collection vehicles and turning/aeration equipment, water added during processing, land area occupied, and outputs including finished compost mass, fugitive emissions (CH₄, N₂O, VOCs, odour), leachate/runoff from the pad, and residuals (contraries) sent to landfill.

3. Life-cycle impact assessment (LCIA). Translate the inventory into impact categories — global warming potential (CH₄/N₂O emissions vs. the avoided landfill methane and avoided synthetic fertilizer manufacture), energy consumption, water use, and local air/odour and water-quality effects — using standard characterization factors.

4. Interpretation and comparison. Compare the composting option's impact profile against the alternatives (landfilling with gas capture, incineration with energy recovery) on a consistent functional-unit basis, identify the dominant contributors (e.g., transport distance, energy for turning, avoided-fertilizer credit), test sensitivity to key assumptions, and use the results to recommend whether — and at what scale — composting should be part of the city's integrated plan, along with any design changes (e.g., closer siting, in-vessel vs. windrow) that would most improve its performance.

Topic: Life-cycle assessment (LCA) methodology applied to composting.

Key relations: ISO 14040 four stages: goal/scope → inventory → impact assessment → interpretation.

Why this works: LCA captures impacts across the whole system (collection through end-use), preventing a narrow comparison that only looks at, say, facility-gate emissions and misses upstream transport or downstream avoided-product credits.

Common pitfall: comparing composting to landfilling using only greenhouse-gas emissions at the processing facility, without crediting composting for displacing synthetic fertilizer or debiting landfilling for its long-term uncaptured methane fraction.

Source: ISO 14040/14044, Environmental Management — Life Cycle Assessment; Tchobanoglous, Theisen & Vigil, Integrated Solid Waste Management, Ch. 15 (composting) & Ch. 1 (LCA in SWM planning).

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