Composting Toilets ‐ Design 1, The Weekender, Removing Urine.

Urine is a relatively safe item to deal with compared to the human waste we are composting. You may have heard of cases where human beings have survived by drinking their own urine - as gross as that sounds!! But too much urine or liquid in the compost heap will turn compost to "stodge" - the microbes will drown so it's really much better to remove it.

There are a number of ways to deal with urine and/or liquid. It can be-
  • drained into a collection tank and disposed of later
  • drained to an evaporation system - good if you live in an all-year warm climate, or, an electric element arrangement could be used
  • drained to a urine composter - this can be achieved simply by placing the outlet pipe on a bale of straw! Microbes will deal to it. This system works ok but you may not like the idea of urine being exposed like this. Another disadvantage is that you are relying on gravity to get the urine down the pipe to the bale of straw. If you have flat site you might have to raise the garden shed to achieve this drop.
  • drained to an underground soak pit, called a French Drain.

I chose to use the French Drain option as it is quick and easy to make, cheap, and well tried and tested. Unlike the straw bale method, it is hidden. There is also no smell. French Drains have been around for hundreds of years - in the 1850's half-round clay pipes were placed in trenches with a small gap between each pipe to allow water to seep. This allowed surface water to be drained from one area and distributed over a larger underground area. They were presumably used before this time too, although it was a Mr French who gained credit for their use.

Our French Drain is a scaled-down version of this and will remove urine from the composting loo, draining it safely underground where microbes will do their thing. If water is also added, urine becomes less concentrated in salts and nitrogen etc and therefore even less of an issue. In my system, the waste water (grey water) from the small hand wash basin in the composting toilet shed, is plumbed into the same pipe and used to help dilute the urine output from the composting loo. There is a reasonably high rainfall where I am, so I guess this also helps dilute things underground.

Some more plumbing supplies will be needed. Earlier, I suggested using a 1″-1.25″ (25mm-30mm) fitting for the urine drain so you'll need to get a length of PVC pipe for the diameter you chose for the urine drain. This size is often used for plumbing waste water. You'll need relevant plumbing fittings such as "elbows".

If the shed you're placing your composting loo inside has a concrete floor, you may need to raise the bin off the floor so there is clearance for the pipe to be taken out through the back wall of the shed.

If you're lucky, the shed won't have been built and you can prepare the trench at the same time. The floor of the shed can be partially excavated to get the bin lower, if needed, so the toilet seat will end up at the correct height. The trench will go under (or through) the wall foundation at the back of the shed and continue for 3 yards, at least, to the drain.

Here are the steps-
  • from the base of the shed, dig the trench at least 3 yards (about 3 meters) long in a direction away from the shed. Further if you want or need. It only needs to be about 1' (30cm) deep at the shed but must slope down slightly towards the end of the 3 yard length as we are relying on gravity to assist the flow. It needn't be a wide trench either, as it's only going to carry our pvc pipe
  • at the end of the trench, dig a 3' (1 meter) x 3' (1 meter) square pit that tapers down. It needs to go down at least 3' (1 meter) from the bottom of the trench. Depending where you live and the type of soil you have, e.g. heavy clay, you might need to go deeper. You might want to consider hiring a mechanical or manual post hole borer from your local hire company, but that may depend on the soil type you have. The diagram at the bottom should show the shape I mean. NB - the length of the trench and depth of the hole are not drawn to scale!
  • fill the bottom of the pit with a mix of stones and small rocks with diameters ranging from 1" (25mm) stones up to approx 6" (150mm) diameter small rocks. Continue in this fashion until you get level with the bottom of the trench you dug earlier. The rocks will allow the urine to seep underground and the air cavities will assist microbes.
  • place the composting bin inside the shed in the position you think it will sit permanently, rotated so the urine drain is at the back (as you look at it) and over the trench. I can't really give you step by step instructions for this step, as it depends if you have a concrete floor, a flat aspect, the type of shed etc. Essentially, you need to lay the pipe so it goes from the urine drain on the bin, down the trench to the soak hole using "elbows" and connectors
  • cover the end of the pipe with more of the pebble and small rock mix, except for the top few inches. Backfill the trench with soil and replace the turf.
French drain, to separate urine in composting toilet