Let's say I have the option to send my food scraps to either a composting site, or a biogas digester. Both are industrial scale, following best practices.

The output from each is high quality and replaces conventional product in the market, to the extent possible (i.e., humus from the compost operation is sold in stores, and biogas from the digester is used for heating and/or electricity production). Let's assume in both cases the quantity is insufficient to significantly distort the market (i.e., the biogas digester will offset some level of natural gas combustion, but won't put the power plant out of business).

Which of these two options is better for the environment? I'm most interested in CO2 emissions (or global warming potential), but would also be interested in data on toxicity, water use, air pollution, etc.

A few related questions which prompted this one:

  • I'd say, biogas first, then use the leftovers from that process for compost. But I'm not an expert in this. – Jan Doggen Oct 8 '18 at 12:55

Carbon dioxide release occurs when compounds containing carbon are oxidised. In conventional aerobic decomposition, this process is relatively direct and carried out by a range of organisms. In a biogas digester, whilst there is no initial release of carbon dioxide, it is released when the resulting methane is burnt. At the end of both processes, you are left with carbon compounds that are only very slowly decomposed. So the in effect they are equivalent.

The difference is anaerobic digesters make it easier to recover energy from waste (although it is possible to extract heat from compost piles). They also have an advantage over composting in that most compost piles release some methane and other greenhouse gases. These gases are captured during the recovery of gas in an anaerobic digester and then combusted to carbon dioxide which has a lower warming potential than methane.

So probably best to use an anaerobic digester first.

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