Anaerobic digester instability: symptoms, causes and prevention

Instability is the problem that costs the most production in a biogas plant, which translates into lower profit. It rarely arrives all at once: it almost always warns days or weeks in advance, if you know how to read the signals.

A stable digester is an ecosystem in balance. When that balance breaks, the process enters a spiral that, if not cut in time, ends in a costly biological shutdown.

This is the reference guide on digester instability: how it manifests, why it happens and how to avoid it. It is the starting point for stabilising the anaerobic digester.

What digester instability is

A digester is stable when the rate at which the volatile fatty acids (VFA) are produced matches the rate at which the methanogenic archaea transform them into methane. As long as that balance holds, the pH is stable and production is optimal.

Instability is the breaking of that balance: the VFA are produced faster than they are consumed and begin to accumulate. As the methanogens are slow and sensitive, it is almost always this population that cannot keep up and the whole system becomes unbalanced.

Symptoms: how instability manifests

The symptoms appear in a fairly predictable order. Watching them allows you to act before the problem is serious:

• Propionic acid rises: it is the earliest signal. A propionic/acetic ratio above 1 warns before almost anything else.
• FOS/TAC rises: stable below 0.4; between 0.4 and 0.6 you have to watch; above 0.6-0.8, the process is at risk.
• Biogas production falls and the methane percentage drops.
• The pH drops: it is a late symptom; when the pH falls, acidification is already advanced.
• Physical signs: foaming, crusts or changes in the appearance of the digestate.

Most common causes of instability

Behind almost every episode there is one of these causes:

• Organic overload: raising the organic loading rate (OLR) too fast or adding a substrate all at once.
• Inhibition: ammonia (protein-rich substrates), H₂S, LCFA (fats) or salts above their thresholds.
• Temperature changes: methanogens tolerate sudden variations badly.
• Toxics: antibiotics, disinfectants or metals that arrive with the substrate.
• Lack of micronutrients: a consortium short of trace elements (Ni, Co, Fe) loses its capacity to recover.

The sentinel variables: detecting before it collapses

The key to not reaching collapse is monitoring. The sentinel variables (FOS/TAC, individual VFA, propionic acid, pH and specific biogas production) allow you to see instability while it is still reversible.

Propionic acid and FOS/TAC anticipate the gas drop and the pH decline, which are late symptoms. That is why a good monitoring plan is worth more than any corrective measure: it is the basis of biogas plant diagnosis.

How to prevent instability

Instability is prevented with disciplined operation, not with patches:

• Raise the load gradually (0.2-0.5 kg VS/m³·day per week) and respect the acclimatisation times.
• Characterise the substrates before adding them and control the inhibitors.
• Keep the temperature stable, without sudden swings.
• Monitor the sentinel variables at a frequency appropriate to the risk.

What to do when the digester is already unstable

If the symptoms have already appeared, the priority is to stop the spiral:

• Reduce or stop feeding to give the methanogens time to consume the accumulated VFA.
• Check and correct the inhibitor if there is one (dilute ammonia, reduce fat, review toxics).
• Buffer carefully if the pH has dropped, to gain margin while the process recovers.
• Resume the load very slowly once the FOS/TAC and propionic acid come back down.

An acidified digester can be recovered, but it takes weeks. Preventing is always much cheaper than correcting.

Frequently asked questions about digester instability

How do I know if my digester is unstable?

The first signals are analytical: the propionic acid and the FOS/TAC ratio rise (above 0.4 you have to watch; above 0.6-0.8, the process is at risk). Then biogas production and the methane percentage fall, and finally the pH drops, which is already a late symptom. Foaming or crusts can also appear. Watching the propionic acid and the FOS/TAC allows you to detect it in time.

What are the most common causes of instability?

Organic overload (raising the load too fast or adding a substrate all at once) and inhibition by ammonia, H₂S or fats (LCFA). Also sudden temperature changes, the entry of toxics (antibiotics, disinfectants) and the lack of trace micronutrients. In most cases, the trigger is a rushed feeding decision.

What is FOS/TAC and what value indicates problems?

FOS/TAC is the ratio between the volatile fatty acids (FOS) and the buffering capacity or alkalinity (TAC) of the digester. It is the most-used stability indicator. Below 0.4 the process is stable; between 0.4 and 0.6 it is worth watching and slowing the load; above 0.6-0.8 the digester is at risk of acidification. The exact value varies with the plant type, so the trend matters more than the isolated number.

Can an acidified digester be recovered?

Yes, but it takes time. The first thing is to reduce or stop feeding so the methanogens consume the accumulated acids; then the cause is corrected (the inhibitor or the overload) and, if the pH has dropped a lot, it can be buffered carefully. Full recovery usually takes several weeks, so preventing instability always comes out cheaper than correcting it.

Smallops and digester stability

Detecting instability in time is a matter of reading the sentinel variables well. At Smallops we monitor and diagnose your digester to anticipate problems with an Operational Excellence Diagnosis.

References and standards

Drosg, B. (2013). Process monitoring in biogas plants. IEA Bioenergy Task 37.

Switzenbaum, M.S. et al. (1990). Monitoring of the anaerobic methane fermentation process. Enzyme and Microbial Technology, 12 (10), 722-730.

Boe, K. et al. (2010). State indicators for monitoring the anaerobic digestion process. Water Research, 44 (20), 5973-5980. → doi.org/10.1016/j.watres.2010.07.043

Chen, Y. et al. (2008). Inhibition of anaerobic digestion process: a review. Bioresource Technology, 99 (10), 4044-4064. → doi.org/10.1016/j.biortech.2007.01.057