An “Inconvenient Truth” about 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.
Inconvenience
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.
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.
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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.
Environmental benefits or harms cannot be reduced to a single metric - even if climate change is the biggest threat we face.
Composting converts what might otherwise be tossed into a landfill (or burned) into useful soil organic matter. Carbon will be released no matter what is done - the proper question is which choice produces the most appropriate benefit along with those costs.
The greenhouse gases are part of the natural cycle though, as opposed to being locked up in a toxic landfill. Maybe if we were wiser we would not only compost at the city level, but we would harness the heat and gases that it puts off for better use as well as generating electricity and fertilizer.
-Tyler
Tyler,
Exactly. Manures and food scraps have more value to society as a fuel than as a problematic fertilizer. This could be done with an anaerobic digester or in a fast pyrolysis system
There’s no need for the two to be mutually exclusive - anaerobic digestion can produce compost-like materials as well as methane.
Landfills also produce massive amounts of methane. Why not harvest it like some landfills are doing.
Just as what Ken said, if we were to toss these same items into the trash, then it would produce the same amount of greenhouse gases, but nobody is getting use out of them. With the state of our landfills being overcrowded, anyway we can prevent things from ending up in the landfill help, whether it’s for composting or fuel as Steve suggests.
My point isn’t that a city like SF shouldn’t to something with food scraps. Its that they should consider other options like methane digesters or pyrolysis. Some landfills also harvest the methane they generate which is a not as bad option
Regarding the nitrogen source, no produce farmers I ever knew use cow manure to boost nitrogen. They use it to build their soil. For nitrogen, chicken manure compost is popular.
Regarding the implied technique, power-intensive hot composting, it’s important to note that it’s not the only way to compost. Layered windrows that are never turned produce the best compost in my experience. Layered windrows may emit less greenhouse gases.
Even if there is a short-term greenhouse gas to creating compost, it may pay off well in the end in terms of that same metric. Getting more carbon into the soil, the basic premise of composting, can help poor soils such as desert soils develop positive feedback loops where they progressively capture more carbon instead of remaining barren.
Finally, I think the basic point to keep in mind is that we need to get as much carbon back into our soil as possible. Composting is currently the best-known way to do that. Perhaps this article can spur some improvements in composting techniques.
Steve:
GREAT post! May I have your permission to “quote” it in my Examiner.com article (I’m the Seattle Green Living Examiner)? I’m putting together an article on green living called “Now What? What to do when your efforts to be green… aren’t”. This is a great example.
Another one is the State of Washington’s law that collecting rainwater is (technically) illegal. (Colorado and Utah have similar laws).
Good work!
Dave Kuhns, web content writer, marketing consultant,
http://www.cyranowriter.com
http://www.examiner.com/x-17874-Seattle-Green-Living-Examiner
Would it possible for you to do some more research and get back to us with a follow=up article, preferably with a less dramatic title and a little more science than a back-of-the-napkin calculation? Perhaps you could google “Stop Trashing the Planet” and read that first. Then look for Dr. Sally Brown’s PowerPoint “A Green House Gas Balance for Compost.” These two pieces alone might get you thinking about the carbon sequestration properties of composting and carbon-amended soil, the avoidance of landfill-generated methane (72 times more potent as a greenhouse gas, not 21 times), and life cycle analysis considerations that might help you look at composting in a more rational perspective. My apologies, I do not mean to be snide, but your argument is quite unfair, specious, and misleading to readers who would be much better served to be presented the full picture of composting and organic resource management in cyclical, not linear form.