18-Env-A6 Solid Waste Engineering and Management · December 2018
Question 4 of 5: Landfill Gas Composition and MSW Composition Analysis
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Examination, December 2018 — 18-Env-A6, Solid Waste Engineering and Management. 3 hours duration, closed book (one aid sheet permitted, written on both sides). Question 1 is compulsory; candidates were instructed to attempt any three of the remaining four questions — all five are answered in full below as a complete study resource.
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; Canadian Environmental Protection Act, 1999.
Question 4: Landfill Gas Composition and MSW Composition Analysis (25 marks)
(a) Key constituents of landfill gas and how composition trends with age. The major constituents are methane (CH₄) and carbon dioxide (CO₂), which together make up the bulk of landfill gas once anaerobic decomposition is established; nitrogen (N₂) and oxygen (O₂), both present early from air trapped in the refuse at placement and consumed/displaced as anaerobic conditions take hold; and a range of trace constituents — hydrogen (H₂, a transient intermediate), hydrogen sulphide (H₂S) and ammonia (NH₃, both odorous), and numerous trace non-methane organic compounds (NMOCs, e.g. vinyl chloride, benzene) that are environmentally significant despite their very low concentration. Gas composition follows the same five biological phases described in Question 5: it starts dominated by trapped air (high N₂/O₂), then as oxygen is consumed and anaerobic acid-forming bacteria take over, CO₂ rises steeply while N₂/O₂ are displaced; CO₂ peaks during the acid phase before methanogenic bacteria establish and CH₄ rises to become co-dominant with CO₂ through a long, roughly steady methane-fermentation phase; and finally, as biodegradable substrate is depleted in the maturation phase, both CH₄ and CO₂ decline while trace N₂/O₂ slowly reappear as air infiltrates the now less-active refuse mass. This generalized trend is plotted below.
Generalized trend of landfill gas composition with landfill age: trapped-air N₂/O₂ dominate at placement (Phase I), CO₂ rises sharply through the transition and peaks in the acid phase (Phases II–III), and CH₄ rises to co-dominate with CO₂ through the long methane-fermentation phase (Phase IV) before both decline and trace air re-enters during maturation (Phase V). Curves are illustrative/qualitative, matching the question's own "generalized trend chart" wording.
(b) Percent moisture, dry solids and bulk density of the analyzed MSW.
Check
This paper's printed table gives only the five rows above, summing to 85 kg per "100 kg-solid waste". The table is solved literally on the 85 kg of material it actually characterizes, i.e. moisture/dry-solids/density are all computed relative to the 85 kg total these five components actually sum to, rather than forcing an assumed sixth component to reach 100 kg.
Given. An 85 kg composite sample split across five components (table above), each with its own moisture split and as-discarded density.
Component
Sample mass (kg)
Moisture (%)
Moisture (kg)
Dry solids (%)
Dry solids (kg)
Density (kg/m³)
Paper
45
7
3.2
93
41.9
80
Organics
20
70
14.0
30
6.0
300
Metal (Fe)
7
3
0.2
97
6.8
480
Glass
10
2
0.2
98
9.8
160
Ashes
3
8
0.2
92
2.8
480
Total
85
Find. The overall percent moisture content, percent dry solids content, and bulk (as-discarded) density of this MSW.
Approach. Moisture and dry-solids fractions are the ratio of each mass total to the 85 kg sample total; bulk density requires converting each component's mass to its own volume (mass ÷ its own density), summing those volumes, and dividing the total sample mass by the total volume — densities themselves must never be averaged directly.
Total moisture and percent moisture. Summing each row's moisture mass: $3.2+14.0+0.2+0.2+0.2 = 17.8$ kg.
$$\%\text{Moisture} = \dfrac{17.8}{85}\times100\% = \boxed{20.9\%}$$
Total dry solids and percent dry solids. By mass balance, dry solids $= 85 - 17.8 = 67.2$ kg (the individually-rounded dry-solids column above sums to 67.3 kg due to rounding each row separately; the mass-balance figure is used to avoid compounding that rounding).
$$\%\text{Dry solids} = \dfrac{67.2}{85}\times100\% = \boxed{79.1\%}$$
Volume of each component (mass ÷ density): paper $45/80=0.5625$ m³; organics $20/300=0.0667$ m³; metal $7/480=0.0146$ m³; glass $10/160=0.0625$ m³; ashes $3/480=0.0063$ m³.
Total volume and overall bulk density.
$$V_{tot} = 0.5625+0.0667+0.0146+0.0625+0.0063 = 0.7125\ \text{m}^3$$
$$\rho_{overall} = \dfrac{85\ \text{kg}}{0.7125\ \text{m}^3} = \boxed{119.3\ \text{kg/m}^3}$$
