manure to gas
Ethanol production normally uses natural gas or coal as the energy source to distil ethanol from plants, and requires the input of one British thermal unit (btu) to make 2 to 2.5 btus of ethanol. Here's another method that only needs 1 btu of energy input to produce 46 btus of ethanol:
Stand cattle on slatted floors, gather the dung, extract methane and use the methane to to power an ethanol plant. By-products include fertiliser for the surrounding maize fields, and wet distiller's grain. Some of the maize is used to feed the cattle (with the distiller's grain) and the rest is distilled into ethanol.
This closed-loop system is being piloted in the Genesis project in Nebraska, where water is also being recycled.