Does a septic tank at my factory count as a Scope 1 methane source, and how big is it?
Yes. A septic tank you own on your site releases methane, and that is a Scope 1 emission. It is small: our worked example for 200 workers on one shift comes to about 5.5 tonnes CO2e a year. Most factories can disclose it as an immaterial, excluded source with the estimate, unless a buyer wants every source.
Why a septic tank makes methane
Toilet and canteen wastewater sits in a closed tank without oxygen, and bacteria break down the organic matter into methane, which escapes through the vent. The IPCC 2006 Guidelines, Volume 5 Chapter 6, treat septic systems as a domestic wastewater source with their own methane correction factor. A tank on your premises is under your control, so its methane is Scope 1 under the GHG Protocol.
The formula
- Organic load (kg BOD a year) = workers x BOD per person per day x share of the day's load generated on site x days on site / 1,000.
- BOD per person: take the India value from IPCC 2006 Volume 5, Table 6.4, or a measured figure for your site. The example below uses 34 g per person per day as an assumed input.
- Methane (kg CH4) = organic load x Bo x MCF, where Bo = 0.6 kg CH4 per kg BOD and MCF = 0.5 for septic systems (IPCC 2006 defaults).
- CO2e = kg CH4 x GWP. Use one GWP set for the whole inventory: AR6 gives 27.0 for non-fossil methane, AR5 gives 28.
- If you know the organic load removed when a tanker empties the tank, IPCC lets you subtract it. Most sites do not have that figure, which keeps the estimate on the safe side.
When it is immaterial, and when it is not
Compare the result with the rest of Scope 1 and 2. A factory using diesel, fuel or grid power usually finds septic methane well under 1 percent of its total. List it as an excluded source with the estimate and your exclusion threshold, rather than inflating it with a precise-looking figure.
It matters more for an office or warehouse with low energy use and many staff, for a site with a large residential labour colony using its septic tanks, or where the tank also takes process effluent. Process effluent belongs in an industrial wastewater calculation, not this one.
Where it does not apply
- Toilets connected to a municipal sewer: the methane, if any, arises at the city's plant and is not your Scope 1.
- An aerobic sewage treatment plant that is well run: its methane correction factor is near zero, so treat it with the STP method instead.
- Septage hauled away by a contractor is no longer in your tank; its later treatment is outside your Scope 1.
Worked example: 200 workers, one shift
- Assumed share of a day's load generated on site for an 8 hour shift: one third
- Organic load at an assumed 34 g BOD per person per day: 200 x 34 g x 1/3 x 300 days = 680,000 g, so 680 kg BOD a year
- Methane: 680 x 0.6 x 0.5 = 204 kg CH4
- CO2e at AR6 non-fossil methane GWP 27.0: 204 x 27 = 5,508 kg, about 5.5 t CO2e
- Against Scope 1 and 2 of 2,000 t CO2e: 5.5 / 2,000 = 0.28 percent
The 34 g full-day BOD load and the one third share for an 8 hour shift are assumed inputs; replace the 34 g with the India value in IPCC Table 6.4 and state both next to the number. Field studies often find septic MCF below the 0.5 default, so this errs high.
What Senseible does here
Senseible builds your fuel and electricity records from invoices and bills. Add this septic estimate beside them as its own Scope 1 line, or as a disclosed exclusion with the number, so the whole Scope 1 picture sits in one place.
Sources: 2006 IPCC Guidelines, Volume 5, Chapter 6 Wastewater Treatment and Discharge, Tables 6.3 and 6.4, IPCC AR6 WG1 (2021), Chapter 7, Table 7.15, GHG Protocol Corporate Standard
Last reviewed 2026-10-19. Bo 0.6 kg CH4/kg BOD and septic MCF 0.5 confirmed as IPCC 2006 defaults; the per-person BOD for India was not confirmed against Table 6.4 and is shown as an assumed input (checked 19 October 2026).