Biogas, also known as biomethane or renewable natural gas (RNG), is a carbon-neutral gas fuel derived from microbiological process. This fuel can be used to replace fossil natural gas or also compressed and liquefied to power vehicles [1].

Biogas Production

Biogas is produced through anaerobic digestion, which actually occurs in the natural environment; to harness this energy, we have to perform the process in a sealed and controlled manner. During anaerobic digestion, organic matters such as animal manure, food waste, and wastewater solids are broken down by microbial communities in the absence of oxygen [2]. In detail, there are 4 stages [3]:

• Hydrolysis: Hydrolytic bacteria convert carbohydrates, lipids, and proteins into simpler molecules - sugars, fatty acids, and amino acids. Enzymes can be added to enhance the breakdown of substances which are more difficult to break such as lignin and cellulose.
• Acidogenesis: The products of hydrolysis are absorbed by acidogenic bacteria and processed into volatile fatty acids (VFAs) including acetates and larger organic acids.
• Acetogenesis: Acetogenic bacteria convert larger organic acids into acetates, rendering the VFAs suitable to produce methane.
• Methanogenesis: Methanogenic bacteria process acetates into methane. This process takes about 40 days and the end is marked by the halt of biogas production.

How Is Biogas Carbon Neutral?

When we burn fossil fuels, we release a huge amount of carbon dioxide that has been stored in the ground for thousands or millions of years as part of the slow carbon cycle. In a short-term view, it is a net increase of emission for the period we currently live in. Unlike fossil fuels, the burning of biogas results in net zero emission. This is because the carbon we emit is the carbon which has been stored in living organisms through photosynthesis and food chains not long ago, part of the fast carbon cycle [1]. Decaying organic materials are going to release greenhouse gases after all; with biogas, we are sequestering these emissions for energy. The burning of biogas also converts methane into carbon dioxide, a greenhouse gas that is 27 times less harmful [4].

Other Benefits of Biogas

After anaerobic digestion, liquid and solid residues remain - this is called digestate. Digestate can be used as nutrient-rich fertilizer, animal bedding, and raw material for bio-based products such as bioplastic [1]. Biogas from micro-scale digesters are most often used for cooking and heating especially in rural households, substituting firewood and charcoal. This results in better air quality and health, and in the long term may lead to reduced deforestation. It is estimated that anaerobic digestion has the potential to reduce global emissions by 10-13% through avoiding emissions from fossil fuels, crop burning, deforestation, landfills, and fertilizer manufacture [5].

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References:

[1] Payne, C. (2023). How is biomethane carbon neutral? Cycle0. https://www.cycle0.com/how-is-biomethane-carbon-neutral/

[2] United States Environmental Protection Agency. (2019). How does anaerobic digestion work? EPA. https://www.epa.gov/agstar/how-does-anaerobic-digestion-work

[3] Meegoda, J., Li, B., Patel, K., & Wang, L. (2018). A Review of the Processes, Parameters, and Optimization of Anaerobic Digestion. International Journal of Environmental Research and Public Health, 15(10), 2224. https://doi.org/10.3390/ijerph15102224

[4] Greenhouse Gas Protocol. (2024). IPCC Global Warming Potential Values. In GHG Protocol. https://ghgprotocol.org/sites/default/files/2024-08/Global-Warming-Potential-Values (August 2024).pdf

[5] European Biogas Association. (n.d.). Avoided emissions from biogas and biomethane can lead to a negative carbon footprint. EBA. https://www.europeanbiogas.eu/avoided-emissions-from-biogas-and-biomethane-can-lead-to-a-negative-carbon-footprint/#_ftn1