Question 4 of 5: Landfill Gas Composition Trend and MSW Composition Analysis

Question 4: Landfill Gas Composition Trend and MSW Composition Analysis (25 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) Key landfill gas constituents and their trend with landfill age. The key constituents are methane (CH₄), carbon dioxide (CO₂), nitrogen (N₂), oxygen (O₂) and trace hydrogen sulphide, ammonia and non-methane organic compounds. Their relative abundance changes systematically as the refuse mass ages through the five biological phases (Question 5): freshly placed refuse is aerobic (Phase I), so the gas in the void space starts close to atmospheric — roughly 80% N₂ and 20% O₂, with a small amount of CO₂ from aerobic respiration. As the trapped oxygen is consumed and anaerobic conditions establish (Phase II, transition), N₂ and O₂ both fall steeply while CO₂ rises, since anaerobic and facultative organisms are now respiring without an external oxygen supply. Through Phase III (acid formation), CO₂ continues to climb toward a peak while CH₄ is still negligible, because the acidogenic/acetogenic bacteria of Question 2(a) are producing CO₂, H₂ and organic acids, not methane. Once the methanogenic population is established in Phase IV, CH₄ rises rapidly at CO₂'s expense until the two settle into an approximately steady 50–60% CH₄ / 40–50% CO₂ mixture, with N₂ and O₂ both essentially at zero; this composition then persists with only minor drift through Phase V (maturation) as gas generation gradually declines.

Given. A 100 kg sample of municipal solid waste is broken down by component, each with its own moisture content and (as-discarded) density, as tabulated below.

Find. The overall percent moisture, percent dry solids, and bulk density of this MSW sample.

Approach. Sum each component's moisture mass to get the total moisture mass and hence percent moisture directly from the 100 kg basis; convert each component's mass to a volume via its own density, sum those volumes, and take total mass over total volume for the correctly volume-weighted bulk density — never mass-weighting the individual densities directly, which systematically overstates density by over-crediting the small-volume, high-density components (metal, glass).

1. Total moisture mass. Moisture mass per component $= m_i \times w_i$: Paper $25 \times 0.07 = 1.75$ kg; Organics $55 \times 0.70 = 38.50$ kg; Metal $10 \times 0.03 = 0.30$ kg; Glass $7 \times 0.02 = 0.14$ kg; Ash $3 \times 0.08 = 0.24$ kg. $$M_{moist} = 1.75+38.50+0.30+0.14+0.24 = 40.93\ \text{kg}$$
2. Percent moisture and dry solids. On the 100 kg total-mass basis, $$\%\text{Moisture} = \dfrac{M_{moist}}{M_{tot}}\times 100\% = \dfrac{40.93}{100}\times100\% = \boxed{40.9\%}$$ Substituting, $$\%\text{Dry solids} = 100\% - 40.9\% = \boxed{59.1\%}$$
3. Total volume, by component. Volume per component $= m_i / \rho_i$: Paper $25/100 = 0.2500$ m³; Organics $55/300 = 0.1833$ m³; Metal $10/480 = 0.0208$ m³; Glass $7/160 = 0.0438$ m³; Ash $3/480 = 0.0063$ m³. $$V_{tot} = 0.2500+0.1833+0.0208+0.0438+0.0063 = 0.5042\ \text{m}^3$$
4. Bulk density. Dividing total mass by total volume, $$\rho_{bulk} = \dfrac{M_{tot}}{V_{tot}} = \dfrac{100\ \text{kg}}{0.5042\ \text{m}^3} = \boxed{198.3\ \text{kg/m}^3}$$