Anaerobic digestion

Anaerobic digestion

What is anaerobic digestion?

It is an environmental technology that, in the absence of oxygen, degrades organic waste to generate biogas, a renewable energy containing methane (CH4), carbon dioxide (CO2) and other, lesser components. Anaerobic digestion is a relatively complex process that is typically explained through 4 mechanisms:

  • Hydrolysis (solubilization), carried out by enzymes, of the higher molecular weight components (e.g., carbohydrates, proteins, fat) to form their oligomers and monomers (e.g. sugars, aminoacids, fatty acids).
  • Acidogenesis, where those oligomers and monomers are transformed into volatile fatty acids, those with less than six carbon atoms (e.g., acetic, propionic, butyric acids).
  • Acetogenesis, where those volatile fatty acids (VFA) become acetic acid and acetate.
  • Methanogenesis, where methanogenic bacteria transform the above substances into methane (CH4).

Anaerobic digestion

Conventional anaerobic digestion

In which all the above mechanisms are carried out in the same vessel (the anaerobic digester), that operates with relatively long residence times (e.g., 20 days) and generates both a gas (biogas) and a liquid (biosolid) stream.

Conventional anaerobic digestion

Advanced anaerobic digestion

Out of the four mechanisms, the kinetics of the anaerobic digestion process are limited by the hydrolysis or solubilization stage. To overcome this limitation and accelerate the process, the hydrolysis step can be carried out as a pre-treatment to the digestion, feeding the digester with an easier to biodegrade, hydrolyzed waste that increases biogas yields with shorter residence times (e.g., 12 days) and thus smaller digesters. With thermal hydrolysis as a pre-treatment, the advanced anaerobic digestion not only maximizes the energy output but also hygienizes the biosolid, that becomes a pathogen-free, organic fertilized in a paradigmatic example of circular economy.

Advanced anaerobic digestion

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