How do I estimate methane from my effluent treatment plant?
Use the IPCC 2006 method: take the organic load in kg COD a year, multiply by 0.25 kg CH4 per kg COD and by a methane correction factor for how the plant treats it, then subtract methane recovered. A well run aerobic ETP emits almost none; anaerobic tanks or lagoons without gas capture emit a lot.
The formula
From the 2006 IPCC Guidelines, Volume 5, Chapter 6, for industrial wastewater:
CH4 = (TOW minus S) x EF minus R, where TOW is total organic load in kg COD a year, S is organic load removed as sludge, R is methane recovered, and EF = Bo x MCF.
Bo, the maximum methane producing capacity, is 0.25 kg CH4 per kg COD by default.
Choosing the methane correction factor
These are IPCC defaults for industrial wastewater. If your ETP has several stages, apply the factor to the COD each stage removes.
- Aerobic treatment, well managed: 0.
- Aerobic treatment, overloaded: 0.3.
- Anaerobic reactor or digester without methane recovery: 0.8.
- Shallow anaerobic lagoon, under 2 m deep: 0.2.
- Deep anaerobic lagoon, over 2 m: 0.8.
Data you need
- Daily inflow in m3, from a flow meter or pump hours.
- Inlet and outlet COD in kg per m3 or mg per litre, from your lab or pollution control board returns.
- Sludge removed, and any biogas captured and burnt or used.
Which scope it falls in
Methane from an ETP you own and run is Scope 1. Effluent sent to a common effluent treatment plant (CETP) run by others is Scope 3 Category 5. Wastewater also releases nitrous oxide, which IPCC estimates separately from nitrogen load.
Worked example: anaerobic tank without gas capture
- Organic load: 50 m3 a day x 3 kg COD per m3 = 150 kg COD a day
- Per year: 150 x 365 = 54,750 kg COD
- Methane: 54,750 x 0.25 x 0.8 = 10,950 kg CH4, about 11 t
- CO2e at GWP 28 (IPCC AR5): 10,950 x 28 = 306,600 kg, about 307 t CO2e
- At GWP 27 (IPCC AR6, non-fossil methane): 10,950 x 27 = 295,650 kg, about 296 t CO2e
No sludge removal or recovery is assumed here. Subtract both if they apply, and use one GWP set for the whole inventory.
What Senseible does here
Senseible does not calculate wastewater methane; it has no wastewater factor. It reads ETP chemical and power invoices, so the ETP's electricity shows up in Scope 2.
Sources: 2006 IPCC Guidelines, Volume 5, Chapter 6 (wastewater treatment and discharge), IPCC AR6 WG1, global warming potentials
Last reviewed 2026-10-15.