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 don’t 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-
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 and when it is nearly full 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, 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-