Composting toilets work very well, Maddie and I both use ours everyday and I dump it every 6 months! The reason I dump it at 6 months is not that it "needs" to be emptied, but because I think that it has been long enough and it is "time to do it".
I'm going to spare you the pictures of emptying it, but "most" of the container is filled with a rich black dirt. Then there are some "fresh" additions to the pile that haven't broken down yet. Dumping it is easy. I place the container in a dock cart with a bag over it and carry it over to the dumpster. Then I dump the container into the trash bag and place the trash bag in the dumpster. Easy and mess free!
Then I let the container sit in the sun for a few days to "dry and air out". Luckily, it doesn't smell, so Maddie lets me dry it out in the cockpit of the boat. After everything is good and dried out, I begin the filling process again.
The instructions that came with my toilet (by Natures Head) state careful measurements of how much dirt you need to add, and how often you need to add more dirt. Just like with the Dickinson Heater, the supplied instructions are much more complicated than they need to be.
I simply fill the bottom up to the crank with peat moss, then I add diatomaceous earth to the peat.
The diatomaceous earth comes in a 4 pound bag from a hardware store. Diatomaceous earth is very safe to use. In the United States it is marketed as a pesticide, but outside of the United States it is used to treat stomach issues by eating it. Diatomaceous earth works as an absorbent which helps settle upset stomachs and runaway intestines by absorbing whatever is irritating the area and passed in a more controlled manner.
When mixed in with the peat, it acts as a mechanical pesticide by drying out the fats and oils in the insects exoskeleton. Poop and peat makes a wonderful breeding ground for gnats and flies, add some diatomaceous earth and no insects will thrive!
I just dump half the bag (approximately 2 pounds) into the peat and turn the crank until the lot of it looks grey in color.
The rest of the diatomaceous earth fits nicely in an old container of pretzles, with a secure screw on lid. This will prevent acidental spills in the locker while heeling over or being tossed around by the seas.
For the next six months, no further soil or peat is added to the solid container. I simply turn the crank after every use and empty the liquid container when it is near full! While explaining how to use a composting toilet is "interesting" to some guests, never needed to repair a blocked joker valve because they flushed something they shouldn't have is awesome! I've been using a composting toilet for over 3 years now, and this is all I have to do every 6 months.