This is a Fermentation process. An anaerobic digestion of organic material. It uses kitchen waster i.e.Food and water.The Compound of two tanks one floating in the cows storage while one at the bottom in the digester. Methane gas is therefore produced from the process . This pushes the floating tank up and the weight of the fumes the gas to the user point.
Cheap and any household unit can access the technology because they at least have both human and kitchen waste.