Biogas Methane from COD Calculator (anaerobic digester)

Understanding the biogas methane estimate — concept and principle

This calculator estimates how much methane an anaerobic process can produce from the chemical oxygen demand (COD) it removes, and how much energy that methane contains. Engineers use it for first estimates of biogas yield from anaerobic digesters and anaerobic wastewater reactors, and as a theoretical ceiling when judging landfill or digester performance.

Each kilogram of COD removed can become up to 0.35 m³ of methane.

How it is calculated

CH₄ at STP (m³/d) = COD removed (kg/d) × 0.35
CH₄ at temperature T = CH₄ at STP × (T + 273.15) ÷ 273.15, correcting the volume from 0 °C to the gas temperature at constant pressure.
Energy (kWh/d) = COD removed × 0.35 × 9.97, using about 9.97 kWh per m³ of methane at STP (lower heating value).

The principle

COD measures how much oxygen is needed to oxidise the organic matter completely. Methane itself can be oxidised: CH₄ + 2 O₂ → CO₂ + 2 H₂O, so 16 g of methane uses 64 g of oxygen, meaning 1 kg of COD corresponds to 0.25 kg of methane. Under anaerobic conditions there is no oxygen to remove, so the COD is not destroyed but transferred into methane. 0.25 kg of CH₄ is 15.6 mol, which occupies 0.35 m³ at 0 °C and 1 atm (22.4 L/mol). This is a stoichiometric upper limit. Energy is calculated on the STP volume because heating value depends on the amount of gas, not on its temperature.

Worked example

Inputs: COD removed 1,000 kg/d, gas temperature 35 °C.
CH₄ at STP = 1,000 × 0.35 = 350 m³/d.
At 35 °C: 350 × 308.15 ÷ 273.15 = 394.8 m³/d.
Energy = 350 × 9.97 = 3,489.5 kWh/d (chemical energy in the gas).

Each point is the calculator re-run with only “COD removed (kg/d)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

COD removed (kg/d)	Methane (m³/d, at T)
250	98.71178839
500	197.4235768
750	296.1353652
1,000	394.8471536
1,250	493.558942
1,500	592.2707304
2,000	789.6943072

How to read the result

Real digesters produce less than this figure because part of the COD becomes new bacterial biomass, some methane stays dissolved in the effluent, and sulfate-reducing bacteria divert COD to hydrogen sulfide. Biogas is typically about 50–70% methane, so total biogas volume is larger than the methane volume. A gas engine converts only roughly a third to two-fifths of the energy into electricity, the rest becoming heat. A measured yield close to 0.35 m³ CH₄ per kg COD removed at STP indicates very efficient conversion.

Common mistakes

Using COD fed instead of COD removed (converted).
Comparing a measured volume at 35 °C with the 0.35 STP figure without temperature correction.
Treating total biogas as pure methane.
Applying it directly to a landfill without a decay-rate model: landfill gas is released slowly over decades.

Keep in mind: The result is a theoretical maximum for screening. Plant design, landfill gas collection and energy projects require site data, kinetic models and a qualified engineer, and must follow local safety rules for flammable gas.

Frequently asked questions

Why 0.35 and not 0.40?

0.35 m³ is at 0 °C. At 35 °C the same methane occupies about 0.395 m³, which is why some sources quote higher values.

How much electricity can I expect?

Multiply the energy by your engine's electrical efficiency, for example 3,489.5 × 0.35 ≈ 1,221 kWh/d.

Sources: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery, McGraw-Hill (anaerobic processes chapter) · IPCC 2006 Guidelines for National Greenhouse Gas Inventories, Volume 5 (Waste)

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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