Composting operations: which emission factors should be used?

By Denis Dionne, eng., M.A.Sc., GHG-V, EP(GHG)

Composted organic waste

Whether you are working on your carbon footprint, setting up a GHG emission reduction project or making a funding request that requires a greenhouse gas emissions quantification you may have noticed that the emission factors are critical and often dictate whether the project actually leads to a reduction or not (see “Positive GHG results obtained from a new organic waste collection system”). Some GHG programs impose the IPCC factors while others leave the selection of the factors (with justification) up to the proponent.

Default emission factors for the treatment of organic waste are presented in Table 4.1 of the IPCC report. But the values of 4gCH₄/kg and 0.3 gN₂O/kg represent averages (the margin of error is large, the minimum value for the emission factor of CH₄ being 0.03 gCH₄/kg and the maximum value of 8gCH₄/kg… a difference of 7.97 gCH₄/kg).

As GHG emissions from composting activities are influenced by the type of composted material, temperature, moisture and oxygen fed to the process and that these parameters differ for all composting sites, the use of an average value is questionable.  A well-managed aerobic composting operation will have better oxygen level control and minimal anaerobic conditions. Similarly, a composting vessel with treatment of the exhaust air will produce lower emissions (including GHG emissions).

An example: A California project using extended aerated static piles (eASP)[1] recently demonstrated GHG emission reductions ranging from 64 to 70 % (achieved through, among other things,  the use of controlled forced aeration and the application of a biofiltration layer on the windrows).

The operating knowledge and expertise behind composting has greatly improved in the past few years and composting sites are now obligated to follow guidelines or regulations that ensure better management and monitoring.  This leads to operational practices that do not promote the formation of CH4… the use of IPCC values may therefore lead to a biased GHG quantification, especially for new sites.

Other sources of emission factors to consider:

• Australian government study on GHG emission reduction through different modes of composting.
• Amlinger, F., Peyr, S., Cuhls, C., 2008. Greenhouse gas emissions from composting and mechanical biological treatment. “An average value of the different studies has been used, that is to say 60 kg CO₂e/ton ww” (to compare with 177 kgCO₂e for the IPCC factors)
• Dutch government study for the update of N₂O and CH₄ emission factors (many references cited, proposes 0.75 kgCH₄/t and 0.096 kgN₂O/t = 45.51 kgCO₂e/t):

Although you must follow the program guidelines to which you are submitting your carbon footprint or GHG quantification, be sure to use factors that are appropriate and up-to-date for your project or site as specified in the ISO 14064-1 and 2 standards.

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