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
Step 1: The Toilet
The toilet used with the solar assisted composter can be either a commercial porta-potty or a 5 gallon bucket style toilet.
Sawdust bucket toilets are used by many off grid homes and as waste is added a scoop of sawdust or peat moss is also added to absorb moisture, reduce smell and assist microbes.
Toilet paper can be added to composting toilets but you should avoid heavily perfumed toilet papers.
Never add chemicals or cleaners to a composting toilet as these will kill the natural microbes that are necessary for decomposition.
Many people design very nice sawdust toilets for their homes and to see examples go here:
Step 2: The Composter Front View
The composter sits outside and when the indoor toilet is full it is emptied into the composter through the trap door on top.
The top of the composter is a plywood box with 2x4 framing and either clear plexiglass or safety glass front panel angled at 45 degrees. Old storm windows or shower doors work well for this purpose
I have not specified the dimensions as this will depend on the size tank you use and your needs.
An average family of four would best be served by a 4' X 8' tank and compost top.
The tank can be made from cement block stacked and grouted or a poly tank. As long as it is sealed to prevent leakage and will not deteriorate in moisture it will work.
There is a vent stack which is just a piece of black 3 inch pvc attached to the top of the composter.
there is a trap door in the top of the composter on the opposite side of the compost removal door for emptying in the waste.
The composter MUST sit facing the sun so sunlight will penetrate the glass and warm the composter.
Step 3: The Composter Side View
The side view shows the compost removal door which should be hinged and large enough that you can use a hoe to scoop out the compost into a wheel barrow or bucket.
Wear gloves when handling fresh compost and human waste compost is only recommended for fruit trees and shrubs and not on gardens.
You will want to remove compost after the composter is approximately half full and it takes about two weeks for human waste to be completely composted into usable fertilizer.
The human waste at the bottom of the tank will compost faster and be ready as you add fresh waste to the top of the composter. Do not rush the compost and allow the microbes to do the work to break it down into its basic components.
Excess moisture will quickly evaporate and leave just a small amount of clean smelling fluffy compost at the bottom of the tank when finished.