Our Compost Toilet deadline

We have been using our Nature’s Head Compost toilets for 4.5 years and been very happy with them.

However, we are going to have to change things within the next few months. At the moment we empty the urine bottles in the boatyard toilets (that’s fine and can continue). The situation with the solids is different.

Currently, we take the solids home (at the moment maybe 6 times a year), double bagged in black sacks, and add them to our compost barrels. The consistent advice is that after 12 months humanure is perfectly safe to use as compost on all plants. What we don’t want to do is leave a compost barrel that isn’t fully composted for the next people. So we need to stop adding solids to the compost barrels by July 2024. Then when we leave we can empty both compost bins onto the garden and leave no issues for the next people to live there.

We face a similar challenge once we are sailing. What do we do with the solids? I’m becoming increasingly uncomfortable with the options we have seen already. That’s basically either to use multiple bags and throw it away with the other trash or throw it overboard. At present it seems very unlikely that we would find places where we could add our solids to a compost heap.

We think that it is unethical and unjust to visit communities and leave human waste in their trash system for them to deal with. Dealing with their own trash is a difficult challenge for many remote/poor/island communities already without us making it worse.

There is also the issue of plastic bags, it is always good to find ways to reduce plastic waste.

So I’ve been reading “The Humanure Handbook” by Joseph Jenkins. A simplified version of his recommendations is:

• Use a simple, single-bucket toilet (no urine separation). No stirring. Simply add enough cover material (eg damp sawdust) for it not to smell.
• Use a two-bin composting setup. His basic setup uses old pallets for the base and 4 sides of each bin. Line the base and sides with a thick layer of straw cut from a bale. Cover the top of the pile with about 25cm of straw.
• When emptying the buckets of poop into the pile, remove the cover straw, create a dip in the middle. Pour in all the buckets. Wash the buckets and add all the water. Put the cover straw back on.
• He uses a long compost thermometer to check the centre of the pile is hot enough.

This isn’t going to work on a boat as:

• Using a single bucket uses a lot more cover material. We don’t have the space for a wheelie bin of sawdust. Nor do we have the space for lots of 5 gallon buckets.
• We don’t have space for two large compost bins.
• It would be horrific if a single 5 gallon bucket, with urine and poop mixed up, tipped over, or emptied itself all over the boat in a knockdown.

Fortunately, our Nature’s Head toilets and other urine separating toilets produce a lot less volume of waste and use a lot less cover material (because you don’t need to cover the urine to prevent smells). So we need to figure out how to turn a smaller quantity of pretty dry solids into a safe state (pathogen free).

On p196 he says that, after 3 months, Septage (the solids from a urine diverting compost toilet) should be free from all pathogens (some intestinal worm eggs might still survive). However, if the solids are composted at higher temperatures then it can be completely safe more quickly. According to the EPA, 3 days at 55˚C will result in “Class A” material with no detectable pathogens that can be used on food crops etc (p197-198).

So we are looking at creating a composting box to empty a single Nature’s Head solids container into. It takes us several weeks to fill a Nature’s Head when using that as our only toilet.

Our plan is to insulate a box, provide an airflow and monitor the temperature inside with a RaspberryPi Pico with environmental sensor. All we have to do is record the temperature to be sure that it has been at 55˚C for 3 days, then we know it is safe. To reduce the volume and compost it as much as possible we would leave it in the insulated box until the next time we need to empty a toilet. Then we can move it into a simpler box for it to gently continue to compost until we are somewhere we can put it into a compost bin (or if fully composted around a tree or bush in a deserted place). During the time in the insulated composting box we may need to adjust the airflow or add liquid (could be from our urine containers) so that the composting process generates enough heat (if we track moisture with the Raspberry Pi as well as temperature we should gradually learn how to adjust the levels of air flow and moisture to keep the temperature high enough.

This should mean that from our toilets we never need to take anything to the trash disposal that can cause problems or costs for the communities we visit – that seems a good Sustainable outcome.