The following is an excerpt from my book Simple Solar Homesteading (www.simplesolarhomesteading.com)
Composting toilets have been used for many years and commercial composting toilets are available and used by many off grid homes.
Human waste can be safely composted into valuable fertilizer that can be used around fruit trees and shrubs but not recommended for gardens.
The problem with commercial composting toilets is they require a tank inside or under a house, must have urine and feces separated, and do not keep microbes warm enough to be efficient.
Human waste is already partially composted by the bacteria present in human bodies and when placed in a composting toilet the waste is quickly consumed by microbes and turned into compost that is full of nitrogen and minerals that is valuable fertilizer.
I live off grid in a solar and wind powered cabin and I wanted a composting toilet that would be more efficient than commercial models and could be adapted to any home for use to replace conventional sewer lines or septic tank drain fields.
I designed and built a small test model that uses solar to heat the composter and remove excess moisture through evaporation. I have used my solar assisted composting toilet for 5 years and it has worked wonderfully!
Microbes thrive in warm temperatures between 50 degrees and 98 degrees. Below 50 degrees microbes hibernate or die and the compost process slows down.
By heating the composter with solar the microbes are kept warmer and work faster and the excess moisture form urine is quickly evaporated away leaving only a small amount of solids that will be composted into a fine fluffy clean smelling fertilizer.
The Solar Assisted Composter is comprised of four parts:
1) The toilet which can be a porta-potty or 5 gallon sawdust toilet
2) The tank which can be made from cement block or a poly tank
3) The solar cover which can be made from plywood and single pane safety glass or used windows
4) The exhaust vent for moisture evaporation which is a length of black pvc vent pipe 3" dia.
HOW IT WORKS:
There is no need to separate urine and feces in this composting toilet and you can use either a commercial porta-potty toilet or 5 gallon bucket sawdust toilet (see http://www.appropedia.org/Waterless_Toilet)
The human waste is collected in the toilet and emptied into the top of the solar assisted composting tank that sits outside facing the sun.
The sun will penetrate through the glass and heat the chamber keeping the microbes at a warmer temperature where they will quickly go to work composting the waste.
The heat will also evaporate off excess moisture from the waste that will escape out the exhaust vent pipe.
You can keep adding human waste as it is created until the tank is approximately half full at which time you should empty the composted material from the bottom of the tank using the compost door and this can be applied to your fruit trees and shrubs.
This solar assisted composting toilet is not designed to handle the water waste from other sources like sinks, showers, tubs, or washing machines. It is only for human waste composting.
Water from other sources is gray water and can be filtered and used on gardens, lawns, trees etc.
I have supplied basic drawings and dimensions and this can be adapted to your own needs
NOTE: If you would like to learn more about off grid homesteading, solar and wind power please visit my website www.simplesolarhomesteading.com