Off Grid Toilet at Century Wood

The toilet shed was the first building I put up at the wood, way back in 2009. This post talks through the original design with photos I took at the time. All before it was ever used in case you’re worried 🙂 In short, it’s an off grid composting toilet set up with separate chambers and seats for liquids vs solids+liquids.

I based the building on a shed kit from Tiger Sheds. It’s an 8ft by 6ft apex shed which cost £248 in 2009. Since 2009 the average price rise across the economy has only been about 30%, but timber has sky rocketed. The equivalent shed now costs over £700!

Clean toilets that people are happy to use are important to many visitors and in the past we’ve sometimes had one or two dozen people in the wood so you do need to be organised. I decided to go for a composting toilet design, with a chamber under the seats that is dug about 30-40cm into the ground.

If you prefer to watch and listen, I’ve made this video which uses the photos to talk you through how the toilet was designed. It covers most of the same ground as this post.

I began by clearing space at the side of the roadway and felling poplars that might conceivably drop branches on the roof. I brought the components of the shed kit to the wood in a van along with the big brown box in the photos. This was made of marine ply treated with creosote. At home I had cut a rectangular hole for the box in the kit’s floor panel and I climbed inside with the help of the step ladder and dug down to let the box gradually sink down into the hole until it was at the right height compared to the floor.

Next I assembled the four wall panels, two roof panels, and nailed the roll of felt onto the roof. You can see the two normal toilet seats that go on top of the box, one for each of the two chambers inside which are separated for most of their height by a plywood partition. One is dedicated to liquids only and the other for liquids and solids.

I based the design on the book “Sanitation without water” by Uno Winblad and Wen Kilama. Decomposition of waste can either happen with air (aerobic decomposition) or without air (anaerobic) and it’s the air-less form that causes bad smells, particularly hydrogen sulphide (H2S) and ammonia (NH3). Several measures promote smell-free aerobic decomposition: a good air flow to carry moisture and any smells away; reducing moisture in the first place by having a separate chamber for liquid only visits; putting material into the mix which let’s air in and soaks up moisture rather than letting water pool; ensuring there is a lot more carbon in the mix than nitrogen, or a big C/N ratio in other words. Aerobic bacteria “eat” carbon rich compounds like cellulose by combining them with oxygen but they also need to reuse nitrogen compounds from their food to make the proteins of their own cells. If there is too little carbon, you don’t have enough growing bacteria around to use the nitrogen up and it escapes as ammonia instead.

To get a good air flow I put a black plastic chimney up the back wall which runs from the back wall of the box. This tends to suck air out of the box by the chimney effect, which keeps a good air flow in to the box from the shed and then out of the box via the chimney. This tends to stop any short-lived bad smells getting out into the shed.

We normally use shredded paper from an airtight plastic container as the material to drop into the box after each visit. The cellulose in the paper is full of carbon, soaks up liquids very well too, and helps create gaps the air can enter more easily.

People do still want water to wash their hands with, and the first attempt was this improvised sink and drain. Water came from 2 litre bottles filled up at home.

The sink drains through a trap under the sink and then out through the shed wall and down to a trap dug behind the shed, filled with gravel and then covered over. This keeps the sink water well away from the composting toilet box inside the shed.

I did originally try this white solar light, with an A5 solar panel on the roof, but the wiring was attacked by mice and we were using torches to give better lighting at night anyway. The coat hook on the back of the door was a success though. The shed kit provided sheets of clear plastic for the windows but I converted them to translucent sheets for privacy by just sand papering them. During daylight there is plenty of illumination through these windows still.

In 2012 I replaced the improvised sink with a cheap but proper plastic bathroom sink bowl with a cold water tap. This is a very similar set up to the tap on the Log Cabin metal sink I installed at the same time. In both cases, I have camping water carriers on shelves which a flexible plumbing connector running from the carrier to the tap. The sink and water carrier are shown in the Spring at the Log Cabin video.

Over the years the Toilet Shed has made such a difference in its understated way. The kind of thing you forget about but would really notice if it wasn’t there.

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