We recently submitted a short report on our hot-fast-composting system, which gave some detail on the theory and practice of producing compost in as short a time as 21 days. But Permaculture principals tell us that we should always be looking to yield as many useful products and functions from any process or element as possible and it is obvious that one bi-product of hot-composting is heat. If you get it really right, a heap should reach 80°C, which is literally too hot to touch. Feeling is believing. Once we were getting to that stage with our compost competency I began pondering how we could effectively catch and store some of that heat so that it can be used for a hot shower.
I finally realised that lots of mucking about with coils and heat exchange loops, lagging and insulation etc. could be avoided if we simply have the hot water tank inside the compost heap. Jean Pain, the old French Roi de Compost did something like this in his place, but took it a lot further, even to the point of having a biogas digester inside a cooling jacket inside a giant compost heap. He was able to heat his house and get compost and biogas all out of the one system. Ours is simpler, but the good thing about that is that you don’t have to be a practical genius to do it.
When we analyse the potential connections between a composting system and a shower, there more useful connections we can realise to gain a yield. Firstly, water is an essential input for the composing process itself. Since any shower has a water point that makes the vicinity of a shower a good place for preparing compost – there is water there already. Since we are in a dry-land all water points are highly valuable nuclei where plant growth can be established to create ground cover and shading canopy. On our site we have such points around all the showers, grey water areas, where we have plated sweet potato, sugar cane, beans, banana and papaya. However in our eroded, cracking clay soils these nuclei will not be able to spread very far as the water loss from the soil continues at a high rate even in the shade, and it will take ages for those plants to create enough organic material and humus around themselves to effectively improve the water’s soil retention, which will allow the moist area and the canopy to expand. But with a hot composting system in the immediate vicinity we can obviously accelerate the development of soil humus around the grey-water points by untold fold.
The hot composting process produces humus much faster than the normal rate of breakdown of material that would occur underneath a clump of bananas or sugar cane. As well as that there will be a concentration of organic material (which is brought in to produce the compost) from else-where on the site or from off the site into the composting/grey water area, so by making compost around the shower we are not only making the humus faster, but we are making more of it than the plants would do by themselves. That makes the soil much better at holding onto and storing the water as moisture inside its bulk. We can also put the water outflow on the northern side (or south if you are in the southern hemisphere) of the shower building so that it is shaded by the building to at least some extent for most of the day. So not only do we have a hot shower, but we have a much better grey water area than we did without compost water heating.
Well, why do you need a hot shower when you are in a hot place, you may think? The thing about dry-lands is that they get quite chilly at night and in the early morning, especially if you are at the altitude we are. So you may not want a hot shower in the middle of the day, but with a solar heater, that is when you get one. Of course a good solar system will store the energy in the water and keep it well insulated, so it’s still hot at night. The problem with that in our case is that such technology is either far too expensive to buy in or too complex to improvise. In Africa there is a lack of machinery and technical skills. The compost heap on the other hand is just a pile of shit mixed with grass, soaked with water and then covered with a plastic sheet. Anybody can make one. As for the hot water tank, it’s just a 200 litre oil drum, which is stuck in the middle of the heap. It’s hot 24/7 and it insulates the tank by itself. It doesn’t even necessarily need any plumbing, you can just take the water out with a bucket and mix it with cold water to get the right temperature if you want. An opaque or black plastic sheet wrapped around the top and sides of the heap also allows it to heat up even more when the sun is shining, but at the same time keeps it moist and shaded inside.
Now as far as the one we built goes, we needed an overhead shower, so there had to be pressure. We used the standard 200 litre oil drum which comes with two threaded apertures in the top, one is ¾” and one is 2”. Pipe fittings in Ethiopia follow the US specifications, as do the oil drums, so are also in inches. This conveniently means you can easily plumb an oil drum into a water system to act as a tank. My prototype for the hot tank used a ball valve, the idea being that as the level falls in the hot tank, it will automatically refill from the main tank.
I later realised this is an over-complication and disadvantageous for several reasons. Firstly there is a loss of pressure so that the hot tank has to be above the level of the shower house, which will be difficult on a flat site. Secondly the plumbing is more difficult and there are more parts which can go wrong and need changing.
In fact, if the hot tank is sealed (as a barrel will be once you have plumbed it into the system) your main (cold) tank just needs to have a few meters of fall on it. If there is the odd drip coming out of the barrel that does not matter either as it will just make sure the compost stays moist inside. There will be no loss of pressure across the hot tank, so you can have the barrel at or below the level of the shower house floor. You take a branch off the main line near to the shower building and which leads into the barrel and it then feeds directly into the shower house to rejoin the main line just before the shower. The barrel can be on its side, raised up a bit with planks so you can get the compost mix in underneath it. The cold water comes into the upper aperture, and the hot water leads out of the lower aperture. That means there is mixing of hot and cold as new water comes into the barrel so there is no thermo cline effect and the shower temperature is as stable as possible as people use it.
What remains is to build the heap. It’s done in layers with grass or crushed up crop trash, kitchen wastes or green plant materials and animal manures put in layers. You can get more info on this in the previous composting article we did. The difference with this system though is that you want this heap to be big. At least 2m across and 2m high. It needs to be well aerated with air spaces at the base, so that air can get in from underneath. We also used a 50mm PVC pipe for an air vent to create a heat chimney effect with the hot air rising out of the heap to pull in fresh air from the bottom. Its then all wrapped in an opaque plastic sheet which keeps it moist and keeps the UV of the sun out, as that kills the bacteria. Make sure you douse it down thoroughly with water as each layer goes on to the heap. It should stay hot for at least 10 days and when it’s really going you won’t believe how hot it can get, till it burns you! Once it starts cooling down you can remove the material and build a new heap round the tank. It can either be made into another heap or put into the grey water area as mulch and covered over with more dry grass or crop trash to keep the sun off. It will continue to break down slowly with the aid of fungi and termites. The surrounding plants will love you for it!
Editor’s Note: Alex and team are doing some amazing work at their community-oriented eco-lodge in Ethiopia. In case you missed it check this out for example! If you want to support this work, or are just itching to go there to see what they’re up to and learn a lot at the same time, then there’s a great opportunity to take a PDC with David Spicer and Alex, planned for March 2011. Details here.