Biogas is a mixture of (mostly) methane and carbon dioxide produced by the process of anaerobic digestion.  Green garden waste, farmyard manure, green energy crops such as maize or sweetgrass, amenity grass clippings, and commercial food waste, are all suitable “feed stocks” to add to a digester.  The gas is then either cleaned for use as a vehicle fuel or for injection into the gas grid, or is burned to generate electricity to export to the grid.