Biogas systems use bacteria to break down wet organic matter like animal dung, human sewage or food waste. This produces biogas, which is a mixture of methane and carbon dioxide, and also a semi-solid residue. The biogas is used as a fuel for cooking, lighting or generating electricity. Using biogas can save the labour of gathering and using wood for cooking, minimise harmful smoke in homes, and cut deforestation and greenhouse gas emissions. Biogas plants can also improve sanitation, and the residue is useful as a fertiliser.