Anaerobic digestion (AD) produces biogas and digestate fertiliser. Biogas is a mixture of gases which can be upgraded to produce biomethane, the biological equivalent of natural gas fossil fuel. Although biomethane is chemically identical to natural gas, there are several reasons why it is better for the planet.
1. Burning of fossil fuels releases carbon, which was previously locked up underground, contributing to increasing global greenhouse gas (GHG) emissions. Burning of biomethane also releases carbon dioxide (CO2), but carbon from organic materials has recently been captured from the atmosphere via photosynthesis. In other words, AD is recycling carbon already circulating in the biosphere, whereas the carbon from fossil fuels is adding to it.
2. By converting waste to biomethane, then using the biomethane as fuel, AD avoids the release of methane directly from decomposing waste to the atmosphere. Methane is an especially potent GHG because it has 28 times the global heating potential compared to CO2.
3. Using AD to convert waste to power can divert organic waste materials from landfill sites. AD plants support waste management by providing means to process curb side food waste collections, organic waste from businesses and the food manufacturing sector, agricultural waste and sewage.
4. As well as biogas, the AD process produces nutrient-rich digestate which farmers can use as an alternative to artificial fertilisers. The production of manmade fertilisers is energy intensive, so replacing demand for this with digestate biofertiliser can help reduce greenhouse gas emissions. The digestate biofertiliser also adds organic matter, improving soil structure as well as providing nutrients.
5. AD plants which produce biomethane can incorporate carbon capture technology. Biogas is a mixture of biomethane, CO2 and small quantities of other gases. When the biogas is upgraded to biomethane, the CO2 can be captured. This can be used in other industries, such as for the manufacture of carbonated drinks, packaging of foods or in abattoirs, and can displace demand for manufactured CO2.
AD is a true circular economy technology because nothing is wasted. Organic materials can be processed in digesters to produce energy, biofertiliser and CO2, all of which can be used by households or businesses. It provides an alternative to fossil fuels which can contribute to tackling the climate crisis.