Methane, a renewable biofuel: from organic waste to bioenergy

Biogas is a renewable, sustainable fuel with a lower environmental impact than fossils, with great potential to supply, at least in part, the constantly growing world energy demand. Anaerobic digestion (AD) is an alternative to producing biogas using substrates from livestock, forestry, agriculture, and other wastes, which harness feedstocks of little economic value. In AD, different microorganisms participate in 4 different stages. The last stage is performed by a particular group of methanogenic archaea, which present some peculiar characteristics: they are sensitive to oxygen and pH changes and belong to an individual branch in the tree of life. The biogas obtained by anaerobic digestion contains around 50% methane (energy component). It can be subjected to an upgrading process, which allows obtaining more than 90% methane, having as many applications as natural gas. However, a disadvantage of biogas production is that one molecule of methane equals 30 molecules of CO₂ as a greenhouse gas if it leaks to the atmosphere. By combining technical and biological aspects of the AD process, this review can be helpful for those who are beginning to address the issue or who have already worked on it from a technological point of view and have biological doubts.