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Anaerobic Digestion is a biochemical process of decomposition of organic matter in the absence of oxygen which results in the degradation of complex organic substances and the production of biogas.


Biogas is a completely ecological renewable energy and consists primarily of carbon dioxide and methane.  It provides an excellent solution for the disposal of organic waste such as waste from food and drink production and is helping to decarbonize the agricultural and food and drink sectors in Ireland.  The only by-product of the AD process is a nutrient decomposed substrate called  Digestate which makes an excellent eco-fertilizer and ensures a closed nutrient cycle.  The AD insdustry provides a win-win for sustainable energy, organic fertilizer and a solution for waste management that would normally be sent to landfill.

There are several steps to biogas formation. Specific groups of micro-organisms are involved in each process and these organisms decompose the products of the process that went before it.  There are four main processes: hydrolysis, acidogenesis, acetogenesis and methanogenis.  

Hydrolysis is the first step of AD, in which the complex organic matter (polymers) is decomposed into smaller units  During this stage, polymers such as proteins, carbohydrates, lipids, and nucleic acids are converted into glucose, glycerol, purines and pyridines.  Hydrolytic microorganisms excrete hydrolytic enzymes which convert biopolymers into simpler compounds.  There are several different types microorganisms involved in hydrolysis which produce exoenzymenes which in turn decompose the undissolved material. These are further decomposed by the microorganisms and utilized in heir own metabolic processes.

During during the Acidogenesis process, the products of hydrolysis are converted by acidogenic bacteria into methanogenic substrates.  Simple sugars, amino acids and fatty acids are converted into acetate, carbon dioxide and hydrogen (70%) as well as into volatile fatty acides (VFA) and alcohols (30%).

Acetogenisis:  The resultant product of acidogenesis, which can not be directly converted to methane by methanogenic bacteria, is converted into methanogenic substrates during this phase. VTAs and alcohols are oxidized into hydrogen, carbon dioxide and acetate.  VFAs with carbon chains longer than two units and alcohols,with carbon chains linger than one unit, are oxidized into acetate and hydrogen.  During methanogenesis hydrogen is converted into methane.  Acetogenesis and methanogenesis usually run parallel as symbiosis of two groups of organisms.  

Methanogenesis:  Methane and carbon dioxide production is carried out by methanogenic bacteria.  70% of the formed methane originates from acetate, while the remaining 30% results from the conversion of hydrogen (H) and carbon dioxide (CO2). Methanogenesis is influenced by composition of feedstock feeding rate, temperature, and pH.  Digester overloading, temperature changes or large entry of oxygen can affect the production of methane.  Digestate is the decomposed substrate resulting from the production of biogas.

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