Sludge Biogas (Captured for Combustion)

Every large wastewater treatment plant sits on an energy resource: the anaerobic digesters that stabilise sewage sludge produce a steady stream of methane-rich digester gas. Captured and burned in cogeneration engines or boilers, that gas powers the plant itself — and because its carbon is biogenic, the combustion carries almost no charge in your greenhouse gas inventory.

This entry covers the NGA Factors 2025 sludge biogas factor, the biogenic CO₂ logic behind it, and three worked examples.

Quick Verdict

Sludge biogas captured for combustion has a combined Scope 1 emission factor of 6.43 kg CO₂-e per gigajoule under the NGA Factors 2025 (Table 5). The CO₂ component is zero because the carbon is biogenic — it is reported separately, not in your CO₂-e total — leaving only residual methane (6.4) and nitrous oxide (0.03). The factor applies to water utilities and treatment plant operators combusting captured digester gas, reported under Scope 1, at an energy content of 0.0377 GJ per cubic metre of methane. Per gigajoule, combusted digester gas emits about 87% less than natural gas, and combustion versus venting is the difference between 6.43 kg CO₂-e and methane at a GWP of 28.

How to Calculate Sludge Biogas Emissions

Emissions (kg CO₂-e) = Energy consumed (GJ) × 6.43 kg CO₂-e/GJ

Convert cubic metres of methane at 0.0377 GJ/m³ and megajoules at 1,000 MJ per GJ.

Worked Example 1: Cogeneration engines

A metropolitan treatment plant’s cogeneration engines combust 15,000 GJ of digester gas to generate electricity and heat.

15,000 GJ × 6.43 = 96,450 kg CO₂-e

96.45 tonnes CO₂-e (Scope 1)

Worked Example 2: Digester heating boiler

A regional plant burns 4,000 GJ of digester gas in a boiler that keeps the digesters at operating temperature.

4,000 GJ × 6.43 = 25,720 kg CO₂-e

25.72 tonnes CO₂-e (Scope 1)

Worked Example 3: Digester output metered in cubic metres

A digester complex delivers 500,000 m³ of methane to combustion over the year.

500,000 m³ × 0.0377 GJ/m³ = 18,850 GJ

18,850 GJ × 6.43 = 121,205.5 kg CO₂-e

121.21 tonnes CO₂-e (Scope 1)

How sludge biogas compares with other gaseous fuels

NGER and AASB S2 Reporting

Combustion of captured digester gas is reported as Scope 1 under the NGER scheme, while process methane from wastewater treatment itself is calculated separately under waste-sector methods. Under AASB S2, the combustion figure sits in your Scope 1 inventory, with biogenic CO₂ disclosed separately where material — a distinction assurance providers increasingly check.