Human excreta -- urine and feces -- can be treated one of two ways,  either aerobically (with oxygen) or anaerobically (without oxygen).  The objectives of treatment are to contain this material, eliminate disease-causing organisms (pathogens), and -- in ecological systems like composting toilets -- return the nutrients to the soil. 

 

There are different styles of composting toilets -- some are built on  site and others are commercially manufactured -- but all have these  objectives in common. For instance, a single chamber composting  toilet contains excreta, toilet paper, and even kitchen food waste,  in a tank below the bathroom floor. There, biological activity, much  like that found in a backyard compost pile, aerobically digests this material. During the treatment process, up to 90% of the volume is  driven out of the ventilation stack as carbon dioxide and water. What  remains is called humus, a fertilizer that looks and smells like the  kind of soil you would find in a greenhouse or on a Lancaster farm. 

 

Time, not heat, kills pathogens in the tank. Pathogens generally need  their human host to survive. But it's the highly competitive environment in the tank, where they cannot compete with the  composting organisms, that does them in. This takes time, which the composting toilet has because of proper sizing and volume reduction. 

 

The maintenance involved is simple but necessary for proper functioning. It includes maintaining airflow in the tank, periodically adding a bulking agent like coarse sawdust, and removing and using the compost and liquid fertilizer.