My earlier blog about greenhouse gas emissions from composting generated a lot of good discussion so I am writing to respond.
- Yes, composting is certainly better than some outcomes like food scraps going into a garbage dump which does not do anything to capture the methane
- Yes, an anaerobic digester would be a very good thing to use for most waste streams. A recent example is what was done at Gill’s Onions
- Many wastes can also be put through a fast-pyrolysis process to form syngas and biochar. This is another way to get at the carbon-neutral energy that is in the manure or other waste
- Greenhouse gas emissions are not the only metric that matters as was pointed out, but manures in particular are undesirable fertilizers based on multiple other metrics as well: tendency to leach more nitrates because of extended release, more nitrous oxide emissions for the same reason, and excess levels of phosphorus relative to nitrogen leading to water pollution
- Compost is indeed a very good way to build soil carbon and that is a super important thing to do for true sustainable farming, but there are other ways to accomplish that that don’t have the greenhouse gas issues. One is the use of biochar. The other is to practice no-till farming and grow cover crops which I describe in another post.
- There may be ways of composting that don’t emit as much methane, but I’ve seen far more theoretical arguments that way with no actual measurements taken. As a microbiologist I have a hard time imagining how you could avoid having some anaerobic conditions in a big pile of manure. Starting from 14 times as much carbon equivalents as synthetic nitrogen, the process would have to be vastly improved to be acceptable
- Un-composted manure has similar drawbacks as a fertilizer. When it is stored for later use on a farm, at least 1-2 percent of its total methane potential gets released even with very good manure management practices
- Chicken manure is more attractive to farmers as a nitrogen source because the levels are higher, but there is every reason to believe it would generate methane in storage and during composting if someone bothered to measure it