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

Question 13 of 16: Variables Governing Landfill Gas Production

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 13: Variables Governing Landfill Gas Production (3 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) Waste composition (biodegradable organic fraction) — the proportion of readily-degradable organics (food, yard waste) versus slowly- or non-degradable material (plastics, inerts) sets the total gas-generation potential of the refuse mass. (2) Moisture content — anaerobic microbial activity requires water; refuse moisture below field capacity sharply retards decomposition and gas generation, which is why leachate recirculation (bioreactor landfill operation) is used to accelerate gas production. (3) Age of the refuse and time since placement — gas generation follows a characteristic lag–rise–peak–decay curve over years to decades as the biodegradable fraction is progressively consumed, so the fraction of refuse by age within a cell governs the site's current total generation rate.

These three variables also interact with the moisture-recirculation life-extension measure discussed elsewhere in this paper: deliberately raising moisture toward field capacity (a bioreactor-landfill operating strategy) accelerates gas generation specifically because it acts on the moisture variable, converting a site's normally decades-long lag-rise-peak-decay curve into a compressed, higher-peak curve over just a few years. This is a genuine engineering trade-off, not a pure benefit — a bioreactor site's gas-collection system must be sized for the resulting higher peak flow rate, and the compressed timeline also means the operator has a narrower window in which to capture the resource productively (via a power-purchase agreement or pipeline-injection arrangement) before generation declines, compared with a conventionally operated (dry) cell's much longer, lower-peak curve. This trade-off should be quantified explicitly (peak flow rate vs. total energy captured) before an operator commits to a bioreactor conversion.