One of the points I make in my biochar presentations is that you can make biochar from any sort of biomass. When I say that, what I generally have in mind is brush, wood, bamboo, and untreated lumber—the stuff you might otherwise burn as fuel. As a joke, I like to add that you could throw in roadkill too, and it would char up just as well.

What I generally don’t have in mind is any sort of manure. But the fact is that in many parts of the world, animal manure is a common fuel for cooking and heating. It is, after all, composed of biomass. Its energy content is partly reduced by digestion, which converts some of its embodied energy into a form that the animal can use, but animal waste is unutilized energy and unsequestered carbon. Thus, manure could be pyrolyzed and made into biochar. The pathogenic anaerobes that it contains would not survive the process.

Taking this one step further, our own waste has roughly the same unutilized energy and carbon; theoretically, it could be dried and burned. As with animal manure, the end product would be sterile, just like biochar made from plant material.

But why would anybody actually do it?

Well, it turns out that human manure (aka humanure) contains not just pathogens but microplastics, heavy metals, PFAS forever chemicals, and pharmaceuticals, all of which are taken out of circulation by pyrolysis. Where sewage sludge is being diverted to agriculture, these substances need to be effectively removed, and pyrolysis does the job. Heavy metals are immobilized within the biochar, due to the same chemical property of chelation that makes them toxic, and the other substances are broken down into their constituent elements. About half the carbon that they contain becomes sequestered, just like carbon from the biomass itself. Minerals in the sludge are conserved, fulfilling the original purpose of utilizing it for agriculture, with a 90% savings in transport cost from the reduced weight and volume.

It gets better. With the right equipment, there are valuable co-products, such as bio-oil, syngas, and wood vinegar. The economy of scale for this equipment works out to be positive for municipal operations above a certain size.

So if you hear of a sewage treatment plant approaching a sludge disposal problem with pyrolysis, take heart. As with any good permaculture practice, it makes a problem into a solution.