Anaerobic digestion produces biogas from organic waste. This biogas can then be burnt to generate heat. Biogas can also be refined into biomethane. This can be injected into the natural gas grid to generate income.

Requires: Large quantities of organic waste, Insulation.

Biogas is a low carbon, renewable fuel source which can be burnt for space, heating and hot water using a biogas boiler.

Biogas digesters are available in many sizes. The process is similar to composting but with the added benefit of capturing biogas, which can then be used in the same ways as natural gas.

Biogas digesters break down the waste by inducing anaerobic digestion. This is when organic waste such as animal or food waste decomposes in an environment without oxygen. A useful by-product of anaerobic digestion is a nutrient-rich liquid which is a highly effective fertiliser.

After one year, this biogas is typically composed of 60% methane (CH₄) and 40% carbon dioxide (CO₂). Since methane is combustible, biogas can be used as an energy source similar to natural gas.

Burning methane produces carbon dioxide and water, so biogas is not a zero-carbon fuel. However, greenhouse gas emissions from burning biogas are offset by averting:

• Emissions caused by natural decomposition, such as in landfill.
• The greenhouse gas and toxic chemical emissions caused in the sewage treatment process.
• The greenhouse gas and toxic chemical emissions caused in the production and use of fertilisers.

The efficiency of biogas digesters can be improved by using Solar Water Heating to heat up the waste.

Biogas can be purified to produce biomethane. This could be burnt for fuel in the same way as biogas, but the benefit of purification is that biomethane is closer in composition to natural gas. In the same way electricity exports to the grid are paid for, a producer can be paid for injecting biomethane into the gas network. In addition to the financial incentives, this also lowers the carbon intensity of grid gas.