Many systems are in operation at a variety of scales.
At the largest scale, some municipal sewage treatment works use composting of untreated or partially treated sludges with other materials (often wood waste or other green and plant materials) to compost under controlled conditions. These systems often use large machinery to aerate the large compost heaps (also known as windrows) to aid growth of beneficial aerobic microbes. An example of this is at the Arcata wastewater treatment works, where a system is in use to force air into the windrows using a pipe laid into the concrete beneath it.
In all scales, the concept is the same: human feces is a rich substrate of potentially useful plant nutrients. However it also represents a threat to human health when handled without proper treatment or allowed to contaminate water supplies. Composting aims to encourage the conditions for the growth of beneficial aerobic micro-organisms as these will create the conditions to aid destruction of harmful pathogenic microbes in the feces.