Composting Toilets ‐ Design 1, The Weekender, Heating the Compost


If the compost can be heated, the microbes will be more active and composting more efficient. Certain types of microbe will die out above certain temperatures, but others will take over. As discussed in previous pages, pathogens will be killed too. Having said that, the need to supply heat to my composting heap is not essential, however, when building my system I chose to install a heater in case it would be needed. In reality, I have never used it. Perhaps in periods of constant use or if I stayed on the property in the middle of winter when outside temperatures drop to about 25 ° F (-4 ° C) and midday temperatures hover around 50 ° F (10 ° C) it would be needed. It is obviously much easier to install a heater when you are making the system in case you need it, rather than add one later! Tropical aquarium heaters with built-in adjustable thermostat are ideal. They're built to be submerged, and a relatively low wattage model will suffice. Depending on the model, temperature can be set using the thermostat from around 68 ° F (20 ° C) to 95 ° F (35 ° C). The only drawback is the need for mains power. This can be overcome if you investigate the use of solar panels to charge batteries and use an inverter to convert 12v DC battery power to AC. I'll create a page about solar power at some time in the future and discuss this in detail. tropical fish tank heatertropical fish tank heater The other problem with the use of a fish tank heater in our compost bin, is the fact that you can't just leave it sitting on the plastic tray as obviously this will melt the plastic, drop through to the bottom liner and m ake its merry way melting through the bottom of the composting bin! To overcome this I created a special platform for the heater to diffuse the heat. I used a ceramic tile, the type used in bathroom decorating or floor tiling. I made the platform as "safe" as possible so the heater would not roll or slip off the platform. This can be achieved by using RTV silicone sealer (fish tank sealer) to glue short strips of ceramic tile to the main platform, creating "sides" and "corners" and some strips across the top. A couple of things to consider - make sure the heater won't touch any of the sealer and think about drainage to ensure liquids won't gather or pool. If you need to cut the ceramic tiles you can get a ceramic tile cutter cheaply from your building supply depot. Scribe a line across the surface of the tile so it's a little larger than the heater. Place the tile on a flat surface with a matchstick under the scribed line at each end. Hold one side of the tile down firmly against the floor. Using a block of wood, apply downward pressure to the end that's raised in the air. The tile should snap along the scribed line.
score the ceramic tile Use the ceramic tile cutter to "score" the surface.
Place matchsticks slightly to the left of the line you "scored" in the step above. Apply light pressure to the left (assuming you're right-handed) so the side on the right is raised in the air. Use a piece of light timber on the right and use downward pressure to "snap" along the line you "scored". preparing to make the ceramic tile cut
two pieces of tile The tile in two pieces.
I chose a 25w fish tank heater. I think this is probably the smallest and therefore cheapest you can get, and is often used for small "hospital" fish tanks. I set the thermostat to the lowest setting. My theory is that the heater will be covered in compost so the heat will be retained. We don't want to "cook" the compost either, just add a little heat to help the microbes. An alternative to using a tropical tank heater is the underground heating element used for plant propagation. You can get under soil heating mats and heated cables. These are also controlled by thermostat. You'll need to check the wattage and heat produced. As always, be careful with electrics. If you don't know what you're doing, get an expert in! Safeguards! Although the heater has a built-in thermostat, I have it plugged into an electric timer switch, similar to that shown below - my one is programmable so it's set to allow power to be on or off in 1hour sessions. This is an extra safety step I think will be useful when / if I need to use the heater. I certainly wouldn't leave the heater going if away from the property for a length of time. Another useful safety tip is to ensure the power supply to the shed is protected with a RCD device (Residual Current Device) also known as a GFCI (Ground Fault Circuit Interrupter). They act as a trip-switch in the event of a current leak and prevent electric shock and are not the same as a circuit breaker. If the cost of wiring the entire shed is an issue for you, then at least purchase one as a portable outlet plug for a few dollars from an electronics supply store. Well worth the investment don't you think? an electric appliance timer Some ideas and instructions-
  • make the ceramic tile platform as discussed above. Make sure the heater will slide into it and be sure it can't roll out
  • place the platform on the plastic tray as shown in the diagram below. If you are worried about excessive heat, consider making some "legs" for the platform to raise it off the tray. This could be done by using the RTV silicone sealant to glue extra strips of ceramic tile to the bottom.
  • put the heater into position inside the platform and run the cable to the outside of the compost bin. Plug it into a timer
plastic trays and fish tank heater inside the bin