For years I’ve been taking a pee jug along when I go camping. I buy a 2-gallon jug of kitty litter and keep the nice wide-mouth jug it comes in. They’re firm plastic and have a nice handle. I keep one right outside my tent for midnight pees. Way easier than hoofing all the way to the bathrooms or whatever.
This time we actually brought the pee back and added it to the compost pile! It’s like two of my hobbies finally came together after years. Huzzah!
It’s not the browns for a compost, it’s an addition of 5-10%. Humanure usually goes out into another composting system for 12 months before use, the addition of biochar carbon during the pooping phase would be an experiment to see if it cuts smells (which is what the addition of small amounts of sawdust does). You’re not really creating a browns:greens compost ratio in a composting toilet, particularly with the small addition of sawdust each time you poop.
The char attracts ions and other charges, the structure holds microbes. It’s still doing stuff even though it’s mostly inert.
You can read a few articles here if you have account: https://medium.com/tag/biochar