I have been making compost piles for many years,
as the main process for transforming garden, landscape and household waste
into a valuable material for amending the soil in the vegetable gardens.
And most of the time the material heats up quite a bit, which is a sign of active decomposition,
and hopefully that there will be fewer weed seeds surviving the composting process to be spread back
out into the gardens. But I have often wondered about using this heat for other purposes, and
have seen examples of people making large compost piles to provide hot water or to heat their house,
or to simply to provide a steady source of warmth for growing tender plants in the spring.
This summer I had the opportunity to experiment with building a compost pile specifically for
heating the water for an off-grid shower, which was part of a temporary camping area,
and I was really pleased with the results.
The composting process involves a wide range of different types and sizes of organisms, but it is
the microscopic thermophilic bacteria that do most of the work to create the higher temperatures.
These heat loving microbes are apparently most active in the temperature range between
55º and 70ºC (135-160ºF) which is hotter than most other organisms can survive.
Below this range other organisms tend to take over most of the decomposition process,
and above about 70º C (160ºF) even the thermophilic bacteria slow down,
which generally creates an upper limit on how hot a compost pile can get. And
if there is a decent balance between the amount of nitrogen and carbon in the material that is to be
composted, the material is broken down into small enough pieces, and there is a large enough volume
to allow the heat to build up, and appropriate amounts of air and moisture, then the conditions
can be created for these thermophilic bacteria to thrive. And we can adjust the conditions to enable
everything to heat up as much as possible or we can aim to keep the temperature in a lower range,
depending on what we want to achieve. In this case I wanted to create as much heat as possible,
and to be able to extract it when needed, and I wanted the heat to last for more than a week,
but I figured even slightly warm water would be adequate.
The context for this experiment was a 10 day long Permaculture Course which included
15 to 20 campers in an area right beside the RED Gardens, which was also the focus
of parts of the course. Although the campers had access to one shower in a nearby building,
we wanted to improve the facilities by creating another shower closer to the camping area,
where there was access to municipal tap water, but no electricity supply nearby. So
this was an opportunity to build a compost pile filled with grass and rough material that was
gathered from clearing the areas for the campsite, and mowing the grass from the area, and to use the
heat from this compost pile to heat the water for an off-grid outdoor shower. To start everything,
I dug a few holes and filled them with stones, set a pallet as a base on top of this soak-away,
and built a simple wood frame for a shower cubicle on top of this base, which was lined with tarps
for privacy. We had proper mixing taps and shower head to use, with the cold water connected to a
pipe bringing water down to the area. And another long length of pipe was tee'd off this supply pipe
to coil through the compost pile before being attached to the hot water tap. It was a simple
system, powered by the pressure of the municipal water supply, with both hot and cold running
water, if the compost heated up as expected, and enough heat was able to be extracted.
I built a container for a vertical compost pile out of a welded wire mesh tied together in order
to create a cylinder about 1.2m (4 ft) high and 1.2m (4 ft) in dimeter, which would enclose a
volume of about 1.35m3 (1.75 yards or 50 ft3). The wire mesh frame was placed directly on the ground
with the start of the long coil of pipe coming out of the cylinder from the bottom and fed out
the top. I filled the compost with alternating layers of rough material that had been scythed
to clear the area, and grass clippings from mowing the main lawn area around the gardens. The
rough material included older grasses, nettles and other plants which would
hopefully serve as a decent source of carbon and allow more air to penetrate into the pile,
and the grass clippings would provide a lot nitrogen and enable the pile to heat
up quickly. After each layer of the rough materials and finer clippings, I laid down
another loop of the long coil of pipe, trying to keep it in from the outside edge of the cylinder.
I slowly built up these layers of organic material with pipe in between as I cleared
more space and mowed the grass. I packed the material down lightly as the cylinder filled
in order to be able to fit more in, but not so much that all the air was pressed out.
When the cylinder was filled, I watered the material from the top, added some cross braces
to keep the cylinder stable, and capped it off with an insulating layer of straw. I
fed the end of the long pipe out of the top of the cylinder and connected it to the hot tap of
the shower. Then I wrapped the whole cylinder with a type of compost cover that would keep
most of the rain off, and hopefully keep in some of the heat but would let more air in.
The next morning some of the campers had warm showers and it was great that
it worked so quickly. The temperature near the core of the compost cylinder was reading
55º C (130ºF) but the outer edge closer to where the water pipe was located was slightly cooler,
between 40-45º which is definitely hot enough for a shower. But apparently it didn't heat
up fast enough for another person to have a shower right after one had been finished. By
late afternoon, about 24 hours after the compost pile was built, the core temperature was almost
70º and peaked at about 74º C the next day, a high temperature that lasted for 3 or 4 days. The
temperature of the material around the pipe was closer to 60ºC which is too hot for a shower and
needed to be mixed with the cold water. When the shower was started the water ran cold for a short
bit, then quickly became too hot to keep your hand under. This lasted for a few minutes until all the
water that had been sitting in the length of pipe buried in the compost pile had passed through,
then the temperature gradually cooled down rather then going cold suddenly. The
incoming cold water had a chance to absorb the residual heat from the pipe, but eventually the
amount of heat transferred wasn't enough. But if it was left for 10 to 15 minutes, the temperature
went back up to approximately where it had been before. I didn't figure out how many showers the
compost could heat in a morning, or what effect this would have on the temperature of the compost,
but that would have been an interesting thing to test. About half way through the course the
temperature of the core of the compost started to fall and it continued to fall after that. I would
have expected this given the mix of materials that I used and the usual rise and fall of temperatures
of compost piles that aren't turned or aerated, and I'm not sure what effect removing some of the
heat each day would have had on the process.
Two weeks after starting the compost the temperature at the core was still
52º C (125ºF) with the temperature around the pipes closer to 32º
(90ºF). I ran the tap and found that it produced about 5 litres (1.3 gallons) of water that was
33º C which is a bit cool for a shower, but still useable. I finally got around to dismantling it
a week later, or three weeks after starting the compost, and the material had shrunk to about 25%
of the original volume, with the coil of pipe compressing with it. The outside of
the cylinder was quite dry, and the core was a mix of the sections of grass clippings that had
compressed into a wet mass and the rougher material being much drier in sections. I
had removed the cover and it had rained quite a bit, which definitely didn’t help. The
concentrated sections of nitrogen rich grass clippings had really helped heat
everything up really quickly, which was what I wanted. But it probably would have lasted
longer if the materials had been mixed in together a lot more. But considering
the pile hadn’t been turned or aerated it was close to what I would have expected. Pulling
the pipe out from the matted long rough material was a bit difficult
and I had to move a lot of the layers to the side by hand, but it all came
apart fairly cleanly and the pipe and wire mesh are ready to use again in the future.
In the end the compost did what was intended, it provided hot showers or at least warm showers
over the 10 day period, though I didn’t get an approximate number of how many
showers it actually heated. People were delighted with the unusual experience,
and it was also a good teaching opportunity, and I was happy that this first attempt was successful.
But there are of course some improvements that I would make if I ever did it again,
the easiest of which would be to use a longer coil of pipe. If I doubled the length of pipe
buried in the compost it would double the amount of water that was available for longer showers. I
think it would also allow for more frequent showers as the incoming cold water would
have more time to absorb the residual heat of the pipe. I think I would also coil every second loop
of the pipe closer to the centre of the compost, which would extract more heat from the hot core.
Mixing the material before filling the cylinder would improve things, and shredding or chipping
all the material first would make it easier to load and to take apart, and allow for woodier
material to be added. I think more care in all of this would help the heat of the compost to last
longer, though it might take a few days for the compost to really heat up at first. And
wrapping the cylinder with a thicker insulated blanket would help retain the heat,
but it might be easier to just make wider cylinder with a greater diameter, out of the wire mesh,
and to back straw into the outside perimeter. But then it would become more important to provide
enough air into the core of the compost, either with a blower or perforated pipes, but it would
be important to do it in such a way that it didn’t cool down the compost pile too much.
I learned a lot making this off-grid shower that was heated by a compost pile,
and I also gained a better understanding about composting in general, especially about how to
control the amount of heat generated. I had thought that the higher the temperature the
faster the decomposition would take place, but now I realise that the maximum rate of decomposition
actually occurs at a slightly lower temperature range. When the heat rises too close to the limit
of about 73° C the rate of decomposition slows right down, until enough heat dissipates or is
extracted so that the bacteria can become much more active again. And this is something that
we can control to a certain extent, depending on what the purpose of the compost pile is.
So if I want to store the hot water for occasional use in a shower, I would want to insulate the pile
to try to get the temperature as high as possible, so that there is a store of water available.
But if I want to extract heat continuously, to get as much heat as possible from the process,
to help heat a house for example, it would be better to try to remove the heat at a rate that
keeps the temperature within the ideal range in which the thermophilic bacteria
are most active. If I want take advantage of the ambient warmth to grow tender seedling on top of
a compost pile in the spring, then I perhaps won't want too heat it up so much, but to use a mix of
materials that would stretch out this heat for several months. If on the other hand wanted to
use composting process to destroy weed seeds or disease organisms in plants, or to quickly process
a large amount of material, then frequent turning could be a much more important factor. But if I
want to make the best possible compost, with the maximum benefit for vegetable plants, a lower
temperature and much greater oxygen flow right through centre of material might be a lot better.
I still have a lot of explorations to do in order to better achieve that last one, but making
compost in a different context, with radically different objectives has helped me understand
everything a little bit better. And it was great to be able to provide the facility for an off-grid
shower in the process, but now I'm beginning to wonder what else I can use that heat for.
Most of the videos on this channel are about growing vegetables,
but I do find it interesting to be able to explore related areas,
especially if I'm able to share all the cool and interesting things that I learn. But that's what
this RED Gardens Project is all about, exploring a wide range of different issues and methods,
trying to gain a better understanding about the complex interrelationships between all the various
factors and issues that are involved in growing vegetables. And then to share them as wide as I
can, in hopes to be able to help more people grow more of their own food. If you value the
work that I do, and like the videos that I make, it would be great if you would consider supporting
me and this channel through Patreon or Paypal. But more importantly, than you for watching.