In this section you will find how the GHG (Greenhouse Gas) module estimates methane emissions from a landfill, what every number means, and how to use each tab. The methodology follows the IPCC 2006 Guidelines for National Greenhouse Gas Inventories, Volume 5 (Waste) — the First Order Decay (FOD) model for Solid Waste Disposal Sites.
In one sentence: the module estimates how much methane (CH₄) the organic waste in your site produces as it slowly rots, subtracts what you capture (flaring / energy), accounts for what oxidises in the cover soil, and can project the remaining "tail" of emissions decades into the future.
A landfill gives off roughly half CH₄ and half CO₂, so a natural question is "where is the CO₂?"
Under the IPCC method the CO₂ from landfilled waste is not counted as an emission. The carbon in food, paper, wood and garden waste is biogenic — the plants recently took it out of the air — so returning it as CO₂ is treated as part of the natural carbon cycle (net zero). The methane is counted, because rotting that same carbon anaerobically into CH₄ turns it into a far more powerful greenhouse gas.
So everything this module computes is CH₄. For reporting, the Results tab also shows the CH₄ converted to a CO₂-equivalent (CO₂e) figure using a Global Warming Potential (GWP) factor — see Step 5 below. The CO₂ physically leaving the site is left out on purpose.
The engine runs one calculation per waste category, per year, from the site's starting year to the present (and, optionally, into the future). The waste categories are:
food, garden, paper, wood, textile, nappies, sludge, MSW, industrial. Inert waste (rubble, glass, metal, most plastics) generates nothing and is ignored.
For each category, the mass deposited that year is converted into decomposable organic carbon (in IPCC terms, DDOCm — mass of decomposable degradable organic carbon):
DDOCm_deposited = W × DOC × DOCf × MCF
| Symbol | Meaning | Typical value |
|---|---|---|
| W | Mass of that waste type deposited in the year (converted to Gg) | from your data |
| DOC | Fraction of the waste that is degradable organic carbon | food 0.15, paper 0.40, wood 0.43, garden 0.20 … |
| DOCf | Fraction of that carbon that actually decomposes | 0.5–0.7 depending on category |
| MCF | Methane Correction Factor — how anaerobic the site is (an open dump ferments less than a managed cell) | 1.0 for a managed anaerobic site |
Waste does not rot all at once — it decays a little each year. The undecomposed carbon left in the ground is carried forward and a fixed fraction of it breaks down every year, following exponential decay:
Decomposed this year = (carbon still in the ground) × (1 − e^(−k))
Carbon carried to next year = (carbon still in the ground) × e^(−k)
k is the decay-rate constant — how fast a category rots:
| Fast (k ≈ 0.085) | Medium (k ≈ 0.045–0.065) | Slow (k ≈ 0.025) |
|---|---|---|
| food, sludge | paper, textile, garden, nappies, MSW | wood |
This is why a projection shows food waste disappearing within a few years while wood keeps off-gassing for decades.
CH₄ generated = Decomposed carbon × F × (16 / 12)
Not all generated methane reaches the atmosphere. Gas you capture and flare or use for energy is subtracted, and a fraction of what is left is oxidised to CO₂ as it passes through the cover soil:
Emitted = (CH₄ generated − CH₄ recovered) × (1 − OX)
Emitted is the headline number: the methane actually leaving the site after everything is accounted for.
Methane and CO₂ warm the climate with very different strength, so reportable figures are expressed in CO₂-equivalent (CO₂e) — the amount of CO₂ that would cause the same warming:
CO₂e = CH₄ × GWP
The Global Warming Potential (GWP) says how many times more warming one tonne of CH₄ causes than one tonne of CO₂, integrated over a chosen time horizon. The inventory standard is a 100-year horizon (GWP-100).
The GWP of methane is not one fixed number — it is re-assessed in each IPCC Assessment Report (AR), and different regulators mandate different reports. That is why the module lets you pick the basis instead of hardcoding one:
| Basis | Year | CH₄ GWP-100 | Notes |
|---|---|---|---|
| AR4 | 2007 | 25 | still required by some older reporting schemes |
| AR5 | 2014 | 28 | a common current requirement in national and corporate inventories |
| AR6 | 2021 | 27 non-fossil / 30 fossil | the latest science — the module uses the non-fossil value |
| Custom | — | any value | for schemes with their own factor |
Two subtleties worth knowing:
Where to set it: Parameters tab → Global warming potential (GWP) — pick AR4 / AR5 / AR6, or type any custom factor. The Results tab then shows CO₂e generated and CO₂e emitted columns next to the CH₄ columns, and always states the factor and its source (e.g. CO₂e = CH₄ × 27 (GWP-100, AR6)) so the basis of the figure is never ambiguous.
CO₂e is not CO₂. The CO₂e columns express the methane's warming in CO₂ terms. They are not the physical CO₂ leaving the site — that CO₂ is biogenic and deliberately not counted (see "Why the module reports methane" above).
The buttons above the Results chart control the decay projection:
| Button | Meaning |
|---|---|
| — | Off — show only real data, up to the current year |
| +10 | Extend 10 years into the future |
| +25 | Extend 25 years into the future |
| +50 | Extend 50 years into the future |
When a projection is on, the engine keeps running the decay model into future years but assumes no new waste is landfilled (W = 0 for every future year). So the projected line shows the methane that will keep coming off the waste already in the ground as it finishes decaying — a declining tail, not a forecast of continued dumping.
It answers: "if we stopped landfilling today, how much methane would this site still emit over the next 10 / 25 / 50 years?"
Projected years are drawn as a shaded band in the chart and marked in the table so you can always tell real data from projection.
Note: projected years carry zero recovery by default. If your site currently captures gas, the projected Emitted line may look like it jumps up in the first projected year — that is because capture is assumed to stop, not a modelling error. Enter future recovery values if you plan to keep flaring.
Set the IPCC coefficients for your site: the starting year, per-category DOC, the DOCf levels, decay rates k, MCF, the oxidation factor OX, the delay before decomposition begins, and the GWP basis used for the CO₂e conversion (AR4 / AR5 / AR6 / custom). Sensible IPCC 2006 default values are pre-filled — only change them if you have site-specific data. If you save no parameters at all, the engine falls back to those same defaults.
This is the waste tonnage that feeds the model. Values come from your imported waste data, and you can override any year manually — a manual row replaces the derived values for that whole year. Amounts are entered in tonnes.
The output table shows CH₄ per category, per year, plus the Total, the Recovery you enter, the resulting Emitted figure, and the CO₂e generated / CO₂e emitted columns.
| Term | Meaning |
|---|---|
| FOD | First Order Decay — the IPCC model where waste decomposes exponentially over time |
| DDOCm | Decomposable Degradable Organic Carbon (mass) — the carbon that can actually turn into gas |
| DOC | Degradable Organic Carbon — fraction of a waste type that is organic carbon |
| DOCf | Fraction of DOC that actually decomposes under landfill conditions |
| k | Decay-rate constant — how fast a waste type rots (per year) |
| MCF | Methane Correction Factor — how anaerobic (methane-producing) the site is |
| F | Fraction of landfill gas that is methane (0.5) |
| OX | Oxidation factor — fraction of methane oxidised to CO₂ in the cover soil |
| Gg | Gigagram = 1,000 tonnes |
| GWP | Global Warming Potential — how many times stronger a gas warms the climate than CO₂ over a given horizon (GWP-100 = 100 years) |
| AR4 / AR5 / AR6 | IPCC Assessment Reports (2007 / 2014 / 2021); each gives a CH₄ GWP-100 value: 25 / 28 / 27 (non-fossil) |
| CH₄ / CO₂e | Methane / its carbon-dioxide-equivalent warming after applying a GWP |
Methodology reference: IPCC (2006), 2006 IPCC Guidelines for National Greenhouse Gas Inventories, Volume 5, Chapter 3 — Solid Waste Disposal.