Composting Toilet

We decided early on that we were going to go with a composting toilet. I tried to draw what I thought were the best ideas from various sources and different designs and come up with the ultimate tiny house toilet. If you haven’t bought into the philosophy yet, I highly recommend Joseph Jenkins’ The Humanure Handbook. You will wonder why you ever thought that in order to be civilized one must poop into a bowl of water and then flush it away. We are breaking the natural cycle that God created! Plants produce food for humans and animals and then we produce fertilizer for the plants. Furthermore, these days all sewer water gets injected into the ground where eventually it ends up (you guessed it!) back in our own drinking water! (How civilized, right?) So basically there are two kinds of people in the world: those who poop in their own drinking water and those who do not. :o)

Also, we are sitting when we should be squatting! But I’ll leave that sermon for someone else (5 Problems With Sitting On Your Toilet).

Anyway, enough preaching. This is where the toilet box will go. I made it the full width of the wall (about 36.5 in) and made the height and depth as short as I could get away with (about 18 inches each way inside).


Framing out the sides of the box.


I made it wide enough that I can fit up to three buckets, side by side. That way the composting process can take its time. I made it tall enough that lids can be used if desired.


At first I was going to put a toilet seat on top of the box, but decided to make the seat out of the top itself. I used 3/4 inch finish plywood and traced the included Separett toilet seat as a guide to make the hole. I decided to coat this with shellac instead of polyurethane, since shellac is a natural product and I would be sitting on it.


I had to sand the nubs off of the Separett diverter since I would now be installing this under the plywood instead of on top. I should have taken my time and done a better job, but I was in a hurry.


This is how it fits in the box. In order to keep it flush, I later traced where the diverter sits on top of the wood frame and then countersunk it about an eighth of an inch with my router.


I cut a hole for the urine drain line which I fastened to the bottom of the trailer. I made a small loop for a P-trap and then plumbed it into my gray water tank. The gray water + urine can later be used for gardening. (see: Liquid Gold: The Lore and Logic of Using Urine to Grow Plants)


To keep it relatively air tight, I put some weather strip around the edges.


I wanted to ventilate my box to provide plenty of oxygen for the compost, but also to draw any smell out of the house instead of in. I looked for the most efficient computer fan I could find and decided to go with a San Ace Silent Fan (model 9S1212L402). This fan only draws 0.08 amps at 12 volts (about 1 watt) while moving plenty of air. That’s so low I can leaving it running all the time, even when I’m on battery power!


I needed a way for it to get air even when the lid is closed, so I added a piece of 1 ½ inch PVC pipe and put some screen on it to keep the bugs out. I left it kind of tall so that air would come in near the tops of the buckets and not just flow across the floor.



Finally, this is what the completed project looks like. It is quite comfortable to sit on and a pleasure to use. There is a slight downward draft with the fan running so it is impossible for any smell to flow out of the box. And between the fan and the urine diverter, no cover material is needed. This is a big plus for a composting toilet! For two adults, emptying the bucket twice a month is more than sufficient.


I made a temporary cover out of some left over paneling. This effectively makes the toilet into a nice bench when not in use (multi-purpose is part of the tiny house philosophy). Later, I plan to add a thin cushion to the top. I am also a big fan of the Squatty Potty, so I added a small squat platform as well. I may build a nicer one later. I also added a switch for the fan, but I usually never turn it off.

This is my waterless, urine diverting, off-gird, solar-powered (future upgrade), eco-friendly, odorless, multi-function, high-capacity, colon friendly, composting toilet!




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