An “Inconvenient Truth” about Composting

commercial scale composting

Composting is a really green thing to do, right? I’ve always thought so since my Grandfather taught me to do it in the early sixties. Large-scale composting is getting to be quite the rage. The City of San Francisco attracted a great deal of attention with it’s mandatory food scrap recycling program and lots of local wineries are bragging about their use of that compost to fertilize their vineyards.

I just read today about how the Langley Parish Council in England is setting up a village compost and “set an example to small villages as the UK strives to battle climate change.”  Unfortunately, I recently learned that they and San Francisco and the Napa wineries might actually be doing is contributing to climate change.

Climate change science often ends up challenging things we think we know.


The idea of composting is to provide plenty of moisture and oxygen so that microbes will digest the easily available organic matter and generate a great deal of metabolic heat in the process.  What is left at the end is a sterilized source of more resistant organic matter that can enrich a soil.

Composting of wastes is done with very good intentions, but there is the inconvenient truth that even a very well run large-scale compost operation emits some methane.

But if you stop to think about it, as much as you intend to have oxygen available to the whole pile (aerobic conditions), there are definitely going to be micro-sites that are going to lack oxygen (anaerobic conditions) particularly when there is huge oxygen demand during the peak of the process. That is where methane gets made.

The Science

There are actually very few published scientific studies about greenhouse gas emissions from composts, but the two that I have been able to find show that around 2-3% of the original carbon in the manure or green waste is emitted as methane (21X carbon dioxide in GHG potential) and there is also a little nitrous oxide as well (310X carbon dioxide in GHG potential).  That doesn’t sound at all bad until you do some math with the values in these publications.

The Math

If you think of it in terms of delivering a hundred pounds of nitrogen/acre (as you would for something like an organic vegetable crop) you would need to start with 8600 pounds (on a dry weight basis) of cow manure (because there is a loss of mass and because the compost is only 1.7% nitrogen).  The greenhouse gas emissions are the equivalent of 0.74 lbs CO2 per dry lb of manure.  That means that the “carbon footprint” of the 100 lbs of N in compost fertilizer is 6,403 lbs CO2.  That is 14.6 times as much as for synthetic urea fertilizer! It is the equivalent of burning 331 gallons of gasoline! (if you are interested you can see a more detailed explanation).

Now What?

When I first did these calculations I was shocked. I did them over and over to make sure I wasn’t in error. I’ve run this by a number of appropriate USDA scientists. They too were surprised, but confirmed my math.  All of a sudden, compost isn’t looking like such a “green” fertilizer.  I’ve tried to find out whether anyone has measured the emissions from San Francisco’s operation but haven’t had any luck.

I’ll discuss this in a later post, but there are some better options for these waste streams as carbon-neutral energy sources.  I wouldn’t worry about your back yard compost, this is probably just an issue at a commercial scale.

Image credit: Julep67 via photopin cc

Written by sdsavage

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