Composting Toilets ‐ Design 1, The Weekender, The Composting Toilet Bin.

Choosing the bin
A tank or barrel can be acquired for small cost from a recycling center or new from a plumbing supply store. It is needed to "house" the compost heap. Plastic is probably best as you do not want to use something that will rust or rot. An old concrete header tank may also prove suitable or possibly an animal drinking trough, however, it will need a lid to keep flies out and you need to make some holes for various things so a plastic lid would be easier to use. Whatever is chosen, an air intake vent is made and covered with fine gauze to prevent flies entering the system.

The following plastic header tank, as used in older houses with gravity-feed plumbing, was recycled for use in my composting toilet system.  
plastic header tank suitable for use a DIY composting toilet binplastic header tank suitable for use a DIY composting toilet bin
The above tank measures about 26" wide (650mm) x 18" high (450mm) and if full with water would hold around 32 US Gallons (120 liters). You wouldn’t store that volume of compost in it at any one time though. This is because you need to have a large air volume inside the tank and around the compost to help with airflow i.e. the composting process. In my design, some of the volume inside the tank is also taken up with a heavy duty plastic lining (more on this later) and plastic grills and trays to encourage air flow.

The low profile of the above tank is ideal for a composting bin. If you opt for something taller like a plastic barrel, you need to consider the height people will be sitting at. With a barrel you may need to half bury it so it is not too high for the top of the toilet seat or you may have to make a step arrangement so people can get up there!

Of course, to up-scale my design to cater for more people or permanent use, you would need a larger tank. Better still, is the idea of making a dual-chamber system – one chamber is used until nearly full, when it is closed-off and the other chamber opened for use.

Now, time for some DIY plumbing. You may need a trip to your local building supply shop or plumbing supply store. Using the tank pictured above-

  • remove the float valve fitting at the top. You may need two adjustable wrenches or plumbing stilson wrench to remove the nuts locking the float valve feed pipe fitting
  • Make the top hole larger as this will be our air inlet. I made mine about 1.25" (30mm) diameter. We will cover this with plastic coated fine gauze mesh. Gauze mesh is usually sold on rolls at your hardware store. Hopefully they’ll cut off what you need. Get them to cut you a piece about 2′ (600mm) square – you’ll need some later for the exhaust. From this, cut 2 pieces of gauze about 2.5" (65mm) diameter so it’s a bit larger than the hole you cut. Glue these over the hole using a silicon sealer, maybe RTV, but make sure you read the label to check it will stick to the type of plastic your bin is made from. Get a good bead of glue around the entire hole – no gaps allowed! Glue one over the hole on the outside and one on the inside of the bin, just to make sure. We don’t want any flies getting in, only air!
  • plug the two bottom holes. On the tank pictured above, these are 3/4" (20mm) brass fittings. You may have to check the sizes used on your tank/bin. I used recycled 3/4" brass caps as they were in my plumbing box, but you can get plastic ones for a few dollars from a plumbing supply merchant, or, you could remove the pipe fittings completely and use a nylon washer or disc glued into position with silicon sealer or similar. Our plugs don’t need to make a watertight seal as they are only there to prevent creepy-crawlies entering the system.
  • cut a hole in the bottom of the tank/bin which will be plumbed to act as drainage for urine. I chose to make a urine drain at the bottom of the bin rather than utilize one of the existing 3/4" outlets as I felt these were positioned too high up the side wall of the bin and would mean there would be a lot of liquid collecting at the bottom of the compost bin. Check the pipe sizes available at your plumbing or building supply store. I used 3/4″ (20mm) diameter pipe, which is possibly a bit small, so consider using 1"-1.25" (25mm-30mm) to be safe. With my design, this pipe shouldn’t get blocked with debris, but probably a good idea to go for the slightly larger size.
Here is a diagram showing the cross-section of the compost bin- diagram of composting toilet

Positioning the bin
I have two diagrams below, showing the composting toilet bin sitting on a concrete pad, but you could equally use other materials such as timber deck planks. You'll need to decide which suits you best, but here are a few points-
  • you need to consider the height that people will be sitting at on your throne! Watch the height at the top of the bin and be aware that the toilet seat will have a "chute" attached to it, so you will need some "drop" between the underneath of the seat and the top of the bin. If you measure the height of a traditional toilet from the floor to the top of the seat, it will give you a good idea of where the top of your composting toilet seat needs to be so that people can sit at a comfortable height. Spend some time to calculate these heights correctly. It would be a good idea to read-ahead a few pages to find out about the seat and chute.
  • if you think the seat will end up being too high, the foundation can be lowered before you pour the concrete i.e. dug out of the inside of the shed.
  • if the concrete pad is already there, you can either (a) break through the concrete foundation, which in a shed won't be too thick anyway. You could hire a rotary hammer drill from your local hire equipment company (if you don't already own one) and drill a series of small holes the diameter of the urine pipe. Then break through this with a cold chisel and excavate the soil from under the foundation pad so you can get to the pipe, or, (b) take the urine drain pipe through the back wall and build a raised timber "deck" or platform inside the shed. When people go into your shed, they'll open the door and step up onto your raised timber deck. This will allow you to achieve the correct height from the floor to top of the seat
The following diagram shows the pipe from the urine drain going down through the foundation concrete, so this diagram would apply if you are building the shed at the same time as the toilet i.e. pour the concrete around the pipe-

positioning the bin on a concrete slab - urine drainage pipe below

An alternative to the above, the following shows the pipe from the urine drain going through the back wall of the shed. The pipe is then plumbed downwards into the trench. This design would typically be used if the shed exists or perhaps the concrete base has been poured.

positioning the bin on a concrete slab - urine drainage through back wall of shed

Lining the bin The composting drum/bin can be lined with heavy duty plastic. I don’t believe it is essential to follow this step, rather, it seems like a good idea to help future-proof the design. My own system gets infrequent use and as yet, has not needed to be emptied. However if it does, I will simply tie-off the plastic liner at the top, lift the heap out, and leave it to “pasteurize” before burying the contents. I used a damp proof course (DPC) polyethylene sheet, as used in the building industry to line the inside of composting bin. This is heavy plastic sheeting, typically 500 microns thick – .02″ (.5mm). This material is often used to prevent rising damp in older houses, by creating a barrier between the soil underneath houses and the underneath of floorboards/wooden bearers etc. It is also used in modern constructions when concrete foundations are poured. I was lucky to have some of this left over from a previous project. The downside with using this material, is that it is quite difficult to work with and you lose a bit of internal space in the bin because it’s hard to shape it without making large bulky folds. Something sourced from your local garden supply store or hardware supply store may be more suitable. Greenhouse plastic film might be a good option. It needs to be heavy duty but not so thick that you can’t shape it. I’m guessing polyethylene material upwards of 200 microns and less than 500 microns. You don’t want it to rot or leak, although as you’ll discover shortly, we’re going to cut some holes in it! Essentially, we want excess liquid (urine) to pass through the compost heap, hit the polyethylene sheeting and make its way towards the urine drain hole end of the bin. To assist this flow, it’s possible to make a slight “slope” by shaping a tapered sandy-pebble mix (I used concrete premix – just a sand/shingle/gravel mix without cement, available at your hardware store) between the bin and underneath of the plastic. Remove the lid from your bin and line it as follows-
  • add your sand-pebble mix (or similar) and create a gentle slope. It doesn’t need to be a thick layer and the slope doesn’t need to be steep. A maximum of 1″ (25mm) at the opposite end to the urine drain, tapered down to the urine drain end. The following diagram should make this clearer. Once you've made this "slope" you need to be careful how you move the bin - you don't want to upset the position of the slope.
  • place the plastic sheeting inside the bin. If you use the same material I did, you’ll have a fight on your hands, but you’ll win in the end! Try to cover as much of the bottom of the bin as possible and work the folds in the polyethylene sheeting in the direction of the urine drain hole we cut earlier. Try to make sure the folds in the plastic won’t trap or prevent liquid moving in this direction.
  • fold the plastic sheeting over the top of the bin and tape down with duct tape. Check your building supply store – a roll of 3″ duct tape will prove useful for other parts of this project too.
  • cut a hole in the plastic liner opposite the air-intake vent we made earlier in step 1. Make the hole a similar size to the air intake vent
  • cut a “+” shaped hole in the plastic sheet with a sharp craft knife, directly above the urine drain we made earlier. The lengths of the two cuts should be about the same as the diameter of the urine drain pipe. I made a couple extra “+” shaped holes in places I felt liquids might accumulate. This way any excess liquid will drain from the inside of the lining, into the compost bin and down the urine drain.
Here is the cross-section diagram, showing the lining (in green) composting toilet - plastic lining