Learn
How anaerobic digestion works
It is the same process your body uses to break down food, run deliberately, at scale, in a sealed vessel with no oxygen.
The basics
Anaerobic digestion is decomposition without oxygen. Seal organic material in a vessel, keep out the air, hold it at a stable temperature, and a mixed community of microorganisms will break it down in a predictable sequence. Two things come out: a combustible gas and a nutrient-rich residue.
None of this is new. What is new in much of the United States is doing it at commercial scale, on purpose, and treating the outputs as products rather than as a disposal problem that has been made slightly smaller.
The four stages
- 01
Hydrolysis
Bacteria break the long-chain polymers in the feedstock - carbohydrates, proteins and fats - into simple sugars, amino acids and fatty acids. Nothing produces gas yet; this stage simply makes the material available to everything that follows.
- 02
Acidogenesis
Fermentative bacteria convert those simple compounds into volatile fatty acids, alcohols, hydrogen and carbon dioxide. This is the fastest stage, and the one most likely to run away if the vessel is overfed.
- 03
Acetogenesis
Acetogenic bacteria convert the volatile fatty acids into acetate, hydrogen and carbon dioxide - the only substrates the final stage can actually use.
- 04
Methanogenesis
Methanogenic archaea produce methane, primarily from acetate and from hydrogen with carbon dioxide. These organisms are the slowest growing and the most sensitive in the vessel, which is why stable feeding and temperature matter more than throughput.
The four stages run simultaneously and continuously in a working digester. They are separated here for explanation, not because the vessel processes them in sequence.
What goes in
- Distillery stillage. The spent mash left after distillation - abundant along the bourbon trail and expensive to dry or land-apply.
- Food processing residuals. Off-spec product, trimmings, wash water solids and fats, oils and greases.
- Agricultural material. Manure, bedding and crop residues.
- Source-separated food waste. Pre- and post-consumer organics diverted from landfill.
What comes out
- Biogas. Typically 50-70% methane, with most of the balance carbon dioxide plus trace hydrogen sulphide that must be removed.
- Digestate. The remaining liquid and fibre, with its nutrients largely intact and much of its odour and pathogen load reduced.
Biogas can be burned on site in a combined heat and power unit to make electricity and usable heat, cleaned and upgraded to renewable natural gas for pipeline or vehicle fuel, or used directly as boiler fuel to displace purchased gas.
Why Kentucky
Kentucky concentrates two things a digester wants: a dense cluster of distilleries producing high-strength organic waste year round, and farmland close enough to use the digestate. The feedstock and the offtake are already in the same place.