Microorganisms have been silently shaping our world, playing crucial roles in crucial processes ranging from decomposition and nutrient cycling to the production of our food, such as cheese, bread, and even beer. However, the capability of microbes extend far beyond these. Specifically, in the renewable energy industry, microorganisms have been harnessing the process of anaerobic digestion to produce biogas, a flexible source of renewable energy, commonly used for heating, electricity generation, and vehicle fuel.
Understanding Anaerobic Digestion
Anaerobic digestion is a biological process where microorganisms break down organic materials in the absence of oxygen. Western University explained this mechanism as a series of reactions occurring within four phases: Hydrolysis, Acidogenesis, Acetogenesis, and Methanogenesis[^1^]. It primarily occurs in anaerobic environments, like swamps, the digestive system of ruminants, and in engineered environments such as an anaerobic digester.
A Glimpse into Anaerobic Digestion Microbiology
Anaerobic digesters are essentially the home for a myriad of microorganisms that interact symbiotically, called syntrophic relationships, to break down organic matter and generate energy in the form of biogas. Over 100 species of bacteria are involved in the process which is why understanding the microbiology of anaerobic digestion is essential to maintaining an optimal and efficient digestion process.
Hydrolysis: The First Step
In the primary process of hydrolysis, complex organic compounds comprising long-chain carbohydrates, proteins, and fats are broken down into simpler compounds, such as sugars, amino acids, and fatty acids. Bacteria within the systems, referred to as hydrolytic bacteria, orchestrate this step.
Acidogenesis: Creation of Organic Acids
Following hydrolysis, the second stage of acidogenesis begins. This is where fermentative bacteria convert the products generated in hydrolysis into organic acids, alcohol, hydrogen, and carbon dioxide.
Acetogenesis: From Simple Organic Acids to Acetic Acid
Acetogenesis follows acidogenesis, and results in the conversion of simple organic acids into acetic acid, carbon dioxide, and hydrogen. This process is facilitated by acetogenic bacteria.
Methanogenesis: Final Offensives
Finally, methanogenic Archaea take charge in Methanogenesis, converting the products of acetogenesis into methane, carbon dioxide and trace amounts of other compounds.
The collective synergy between different microbial communities throughout these stages results in the decomposition of organic matter without oxygen, and generation of biogas as a by-product[^2^].
Parameters Affecting Anaerobic Digestion Microbiology
To sustain a viable population of microorganisms conducive to efficient biogas production, several operational parameters within the anaerobic digester must be carefully regulated. These include the temperature (mesophilic digestion usually at 30 to 40°C, thermophilic digestion at 50 to 60°C), pH (slightly alkaline at around 7.1 to 7.5), hydraulic retention time (HRT), solid retention time (SRT), and organic loading rate (OLR). Ammonia toxicity and sulfide toxicity are common inhibitors, but are necessary in limited concentrations for optimal microbial growth[^3^].
Conclusion: The Future of Renewable Energy
Understanding the microbiology behind anaerobic digestion allows us to fine-tune this process and to explore inventive ways to maximize efficiency. Furthermore, with organic waste management becoming increasingly imperative, synergizing waste treatment with renewable energy production offers a promising future for sustainable development. The power of microbiology in renewable energy, while hidden, certainly cannot be ignored.
[^1^]: Western University. (2012). Fundamentals of Anaerobic Digestion. [online] Available at: https://www.uwo.ca/enviro/wheonline/anaerobicdigestion/A1/1.htm
[^2^]: Hidalgo, D., Cano, R. (2017). The anaerobic digestion microbiome. A collection of species or interactive communities. Water Science and Technology, 75(5), 964-975.
[^3^]: Demirel, B., Scherer, P. (2011). The roles of acetotrophic and hydrogenotrophic methanogens during anaerobic conversion of biomass to methane. Reviews in Environmental Science and Biotechnology, 10(2), 173-190.
