Acidogenesis and acetogenesis: the stages of methane fermentation in which acids and hydrogen are formed

Acidogenesis and acetogenesis are the second and third stages of methane fermentation: in the first, microorganisms convert the simple compounds formed in hydrolysis into short-chain fatty acids (volatile fatty acids, VFA), CO₂ and hydrogen, and in the second they convert the mixture of VFA into acetic acid, releasing further amounts of CO₂ and hydrogen. Only these products serve as food for methanogens. Understanding both stages is important for the operator, because this is where the most common process disturbances begin.

Place in the chain of transformations

According to the EPA AgSTAR guide, anaerobic decomposition proceeds in four steps: hydrolysis (complex organic compounds such as proteins, carbohydrates and fats are broken down into simpler ones), acidogenesis (acid-forming microorganisms ferment simple compounds into VFA, CO₂ and hydrogen), acetogenesis (acetogenic microorganisms convert the mixture of VFA into acetic acid, releasing further amounts of CO₂ and hydrogen) and methanogenesis (methanogens convert acetic acid and hydrogen into biogas). A general picture is given in the article The four stages of methane fermentation: from hydrolysis to methane. For this whole pathway, the key is the balance between the groups of microorganisms, not the rate of any one stage.

Acidogenesis: where acids come from

This stage is carried out by heterotrophic microorganisms, which use organic compounds and grow faster than methanogens. In his 1997 review, Schink notes that fatty acids and alcohols are the main intermediate products of methanogenic decomposition, so acetate, propionate, ethanol and their higher homologues must be further converted into one-carbon compounds. The composition of the products depends on the substrate and conditions; readily degradable carbohydrates quickly yield large amounts of acids, which matters with large doses of production waste.

In practice the operator does not observe acidogenesis directly, only its effects: VFA concentration, alkalinity and pH. If the rate of acid production exceeds the rate of their consumption, accumulation occurs. Initially the carbonate buffer can maintain the pH, which is why the pH value alone is a lagging indicator — the EPA guide calls alkalinity the most reliable indicator of stability, because the accumulation of VFA and CO₂ shows up as a fall in alkalinity before a fall in pH.

Acetogenesis and syntrophy

Converting VFA to acetate does not proceed like an ordinary reaction with a positive energy balance. Schink describes these processes as secondary fermentations, that is, syntrophic ones: they are endergonic under standard conditions and depend on tight coupling with methanogenesis; the available energy is so small that it forces very efficient cooperation between organisms. In simplified terms: one group produces hydrogen (and acetate), and hydrogenotrophic methanogens consume it, so the reaction can continue. We also describe this relationship in the article Acetoclastic and hydrogenotrophic methanogens in a biogas plant. Disruption of hydrogen removal favours the accumulation of intermediate acids, and the symptoms must be interpreted together with the other process indicators: gas composition alone does not unambiguously point to a specific group of microorganisms.

Why the balance is fragile

The EPA guide stresses that methanogens multiply more slowly than the other groups, so their population must be sufficiently large: when there are too few of them, VFA accumulate, and at higher concentrations are toxic to them. To maintain methanogens, their loss with the effluent must not be greater than their growth, which is regulated by retention time. The main cause of disturbances is an increase in loading (OLR) or a sudden change in feed composition, after which acid-forming bacteria outpace methanogens and lower the pH; a second cause is the introduction of toxic substances, which usually leads to an even faster collapse. We describe the dependence of stability on dosing in the article Biogas plant feeding stability: why the daily total of feed is not enough.

Does separating the stages help? Two-stage digesters carry out the first three stages (hydrolysis, acidogenesis and acetogenesis) in one tank, and methanogenesis in another. The EPA guide notes, however, that phase separation has not shown a significant advantage over single-stage fermentation and that the choice of technology depends on the specific substrate.

What to watch in operation

1. A drop in CH₄ content in the biogas is the first indicator of disturbance according to the EPA.
2. Alkalinity and its trend: a fall signals disturbance earlier than pH.
3. VFA and the VFA-to-alkalinity ratio: analyses are performed by the distillation or chromatographic method in a specialised laboratory, usually not daily; titration is a supplement — see FOS/TAC: how to measure and interpret and VFA at a biogas plant.
4. pH in the effluent: in a properly working digester usually 7.0–8.0, but it is a lagging indicator.
5. Changes in feed: match every anomaly against the record of deliveries and doses.

In the event of acidification, the plan described in the article Digester acidification: diagnosis and response is applied. Understanding the two middle stages helps avoid the mistaken conclusion that the problem lies in a "lack of methane bacteria", when it often results simply from the speed at which readily degradable substrates reach the digester.