So, I don't know if this will go anywhere but I thought I'd put the idea out there. Should I end up actually building it I'll document the process and learnings here.
So, I'm feeling frustrated by the composting toilet options available and thinking about a DIY build. The nature's head unit seems well liked, but doesn't really compost and seems likely to be on the small side for a family of 5 planning to full time off grid as much as possible. And it seems expensive for what it is. The "real" composting toilets are ridiculously expensive for what they are, generally don't rate very well, and are not really configured in a convenient layout for Skoolie use. And the all in one units still may not be big enough for our family. The idea may end up being completely impractical but we'll see where it goes.
So if we go this route, the design spec may be a little different than most. I love many of the low cost designs folks have come up with, but if I can stay under the cost of the Natures Head ($1000) and have something that work and fits our needs better, that would be a win. It would also be somewhat driven by the specific layout of our soon to be bus, which has a side emergency door directly over a belly storage bin.
I still need to read up more on the specifics (get the humanure book), but here's my basic understanding from using a Sun-Mar at my parents camp and having a compost tumbler at home. As I understand, effective composting takes heat, time, aeration and mixing, as well as the basic balancing of nitrogen, carbon and moisture to promote digestion. Time in this case means several stages of "finishing" and sufficient storage volume in each stage to let time pass before material is moved to the next stage. Adding an external heat source, air circulation, and mechanical mixing can all help accelerate the process and reduce the time / storage volume required. Since space is a premium on a bus, and we won't have an off site storage option, maximizing these acceleration factors seems key.
One of the main things most commercial composting (or desiccating) toilets seem to have that most DIY's don't is the mixing / aeration component unless it's done manually. Air circulation and heat are relatively straight forward, but mixing less so. Which brings me to the cement mixer idea ;) There are a number of upright style designs that might work, but I'm leaning toward these externally driven side dump type designs at the moment. Harbor freight has two sizes, a 1.25 cuft. (9.4 gallon) for $160, and a 3.5 cuft. (26.2 gallon) for $210.
Turns out I'm not quite so clever as I thought, as their are numerous examples on the internet of folks using old cement mixers as composters.
My initial though is to build a relatively air/bug tight box over the upright mixer, which would be shortened by modifying the legs below the crossbar. Ideally the inside of the box would be water tight as well, maybe shower polywall or something, to contain any inevitable spillage. The box would then have a top hole and toilet seat, and heat source / ventilation like most more traditional DIY units to accelerate composting and help control odor. The mixer could either just be manually run every so often, or perhaps put on a timer to run at regular intervals.
From there I have two thoughts. One is the bottom of the box becomes the first finishing tray stage. When the mixer drum gets to a certain fullness, it would just be dumped into the tray below. Then the drum is ready for the next load, and the first load has time to mature. Ideally this tray would be designed in a way that it could be pulled out through the open emergency door for disposal. Or, designed to slide out and dump into a second slide out tray / bin located directly below in the belly bin for storage and more finishing. Alternatively the whole mixer drum assembly could be designed to slide out the emergency door and dump directly into the belly storage bin.
Thought / comments welcome!