18-Env-A6 Solid Waste Engineering and Management · May 2013
Question 1 of 17: Energy Content of Refuse
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.
Given. A refuse mixture: 50% paper (16,300 kJ/kg), 20% metal + glass + ash combined, and the balance (30%) food and other organic wastes (5,800 kJ/kg).
Find. The overall (weighted-average) energy content of the mixture, kJ/kg.
Approach. Mass-weighted average of each fraction's typical energy content, treating metal/glass/ash as essentially non-combustible.
Check: the printed text is ambiguous ("50% and 20% metal, glass, and ash") — read as "50% paper and 20% metal/glass/ash combined," which is the only reading that uses the table's paper value and leaves a sensible 30% balance. Metal, glass and ash are taken as ≈0 kJ/kg (standard simplification — these materials are essentially inert/non-combustible; Tchobanoglous typical values are near-zero for glass/metal/ash relative to paper or food waste).
Weight each fraction by its typical energy content.
$$E = 0.50(16{,}300) + 0.20(0) + 0.30(5{,}800)\ \text{kJ/kg}$$
$$E = 8{,}150 + 0 + 1{,}740 = \boxed{9{,}890\ \text{kJ/kg}}$$
This lands close to the table's own "refuse" (typical MSW as-collected) figure of 10,500 kJ/kg, which is a useful sanity check on the composition assumed.
