Running an anaerobic digester by looking only at pH and biogas production is operating with the rear-view mirror. By the time pH falls you may already have severe acidosis: the operational damage shows up 5 to 14 days earlier in the variables you should be measuring.
Robust control is built on sentinel variables with defined operational thresholds: FOS/TAC, individual volatile fatty acids (VFAs), the propionic/acetic ratio, free NH₃ and ORP. This article describes what to measure, how to interpret each variable and how to build a technical dashboard that detects decoupling before the digester shows it in the pH.
What sentinel variables are in anaerobic digestion
A sentinel variable is any parameter whose changes announce a process deviation early enough to act before the damage becomes irreversible.
In anaerobic digesters, pH is the most closely watched indicator, but it is also the latest one: by the time it falls below 6.8 there is already an advanced acidosis that has been developing for days.
The operational difference between a classic control indicator and a sentinel variable lies in the lead time. pH responds once the methanogenic community is already affected; individual VFAs and the FOS/TAC ratio respond when acidogenesis starts to outpace the buffer capacity.
That time difference (between 5 and 14 days depending on substrate and load) is the operational window that decides whether you can intervene or can only manage the damage.
FOS/TAC by the Nordmann method: thresholds and operational reading
The FOS/TAC ratio (also called VFA/TIC in English-language literature) compares the concentration of total volatile fatty acids (FOS, Flüchtige Organische Säuren) with the buffer capacity of the medium (TAC, Totales Anorganisches Carbonat).
It is the most widespread method for monitoring industrial digesters, validated in the DIN 38414-19 standard and in the Nordmann procedure.
The reference operational thresholds are:
| FOS/TAC | Status | What to do |
|---|---|---|
| < 0.3 | Stable process | Sufficient buffer capacity and controlled VFA production |
| 0.3-0.4 | Watch zone | Increase sampling frequency. Review organic load and substrate composition |
| > 0.4 | Alert | Reduce the organic load by 10-20%. Review the full dashboard |
| > 0.8 | Crisis | Stop or drastically reduce feeding. Urgent corrective intervention |
The operational advantage of FOS/TAC over pH is the lead time: in well-run digesters it detects the decoupling between acidogenesis and methanogenesis 5 to 10 days before pH reflects it.
That margin is enough to act without stopping the plant.
Individual VFAs: the propionic/acetic ratio as a predictor
The analysis of individual VFAs by chromatography (GC-FID) or HPLC provides diagnostic information that total FOS/TAC cannot offer.
Under normal conditions, acetic acid is the dominant VFA and the direct precursor of methane in the acetoclastic route. Propionic acid is the most sensitive indicator of methanogenic stress: it accumulates when acetotrophic activity declines.
The propionic/acetic ratio (C3/C2) is the reference clinical indicator: values above 1.0 indicate active syntrophic decoupling.
Butyrate and isovalerate provide complementary diagnostic value: a rise in isovalerate indicates degradation of branched-chain amino acids, common in digesters with a high protein load.
How to build a working technical dashboard
A technical dashboard for digesters is not a SCADA supervision dashboard: it is a weekly protocol for reviewing integrated variables that makes it possible to spot trends before they become incidents.
The Smallops model organises those variables into three levels of urgency.
Level 1 · Daily control variables
Recorded daily, they define the state of yesterday:
- Biogas production (m³/d and % CH₄).
- pH of the effluent.
- Digester temperature.
- Amount of substrate fed.
They are necessary for monitoring, but insufficient for prediction.
Level 2 · Sentinel variables (minimum weekly frequency)
They define the present state of the process and its trend:
- FOS/TAC (Nordmann method).
- Individual VFAs (acetic, propionic, butyric, isovaleric).
- Total ammonia and calculation of free NH₃.
- Viscosity of the incoming substrate.
- Volatile solids (VS) of the digestate.
Level 3 · Diagnostic variables (monthly or on incident)
They are not measured in routine operation, but in the face of persistent alert signals or to validate diet changes:
- Specific methanogenic activity (SMA) by BMP test of the sludge.
- Full chromatographic VFA profile, including LCFAs (long-chain fatty acids).
- 16S rRNA microbiological analysis.
- BOD₅ and COD of the digestate.
- Trace metal content of the substrate.
When what is being validated is precisely a diet change, the BMP falls short: it is a batch test that measures the maximum potential under ideal conditions. The semi-continuous test, by contrast, feeds a laboratory reactor continuously over several weeks and reveals the kinetics, the tolerance to load, the inhibitions and the acclimation time: it is the most reliable validation before scaling the change up to the plant.
ORP as a variable-frequency parameter
ORP (redox potential) deserves a special mention: in digesters with continuous feeding it can be measured continuously and anticipates both oxidative stress and the accumulation of oxidised matter or the failure of the anaerobic zone.
It is measured with platinum electrodes and is especially useful in digesters fed with variable waste streams.
Sulphides (H₂S) in the liquid phase and in the biogas
Sulphides must be monitored both in the liquid phase and in the biogas. Above 2,000 ppm in biogas they indicate a high sulphate load in the substrate; below 100 ppm they are operationally irrelevant.
In slurry digesters the concentration can exceed 5,000 ppm, with a corrosive impact on the engine and the instrumentation.
Key quantitative data
| Item | Value |
|---|---|
| FOS/TAC thresholds (Nordmann) | < 0.3 stable · 0.3-0.4 watch · > 0.4 alert · > 0.8 crisis |
| Lead time of FOS/TAC over pH | 5-10 days |
| Critical C3/C2 ratio | > 1 indicates syntrophic decoupling |
| Lead time of VFAs over pH | 5-14 days depending on substrate and temperature |
| Typical ORP with active methanogenesis | -300 to -550 mV |
| Inhibition by free NH₃ | 150-700 mg NH₃/L depending on the consortium |
Frequently asked questions
What is the FOS/TAC ratio and what value is normal?
The FOS/TAC ratio compares total volatile fatty acids with the buffer capacity of the medium in an anaerobic digester.
A value below 0.3 indicates a stable process; between 0.3 and 0.4 requires increased vigilance; and above 0.4 is an alert signal that calls for reducing the load. The normal value for efficient operation sits between 0.15 and 0.30.
Why does pH arrive late as an indicator in a digester?
pH is strongly buffered by the bicarbonate-CO₂ system. The buffer capacity (TAC) neutralises the VFAs as long as the system is not overwhelmed.
That means pH does not fall until the VFA concentration exceeds that buffer capacity, by which point acidosis is already severe. VFAs accumulate for 5 to 14 days before that threshold is crossed.
Which are the key variables for anticipating operational problems?
The five with the greatest predictive power are: (1) FOS/TAC by the Nordmann method, (2) the propionic/acetic ratio in individual VFAs, (3) free NH₃ calculated from total ammonia, pH and temperature, (4) ORP in the liquid phase and (5) CH₄ content in the biogas with its trend.
None of them on its own is sufficient: it is the integrated trend of all of them that defines the state of the process.
What measurement frequency for sentinel variables is advisable?
The minimum recommended frequency for FOS/TAC and VFAs is weekly in stable plants, escalating to twice weekly or daily if there is an upward trend. Total ammonia should be measured at least every two weeks in plants with protein-rich substrates, and ORP can be measured continuously where instrumentation is available.
Sporadic measurement of these variables is worse than not measuring them, because it creates a false sense of control without sufficient time resolution.
How do you validate a diet change before taking it to the digester?
With two complementary tests. The BMP is a batch test that measures the maximum potential of the new substrate under ideal conditions: it tells you how much it can contribute.
The semi-continuous test feeds a laboratory reactor continuously over several weeks, with the real load and retention time, and reveals the kinetics, the tolerance to load, the inhibitions and the acclimation time. It is the most reliable validation before scaling the change up to the plant, and the one that prevents the new diet from turning into a FOS/TAC rise two weeks later.
How Smallops installs predictive control in your plant
Most biogas plants operate without an integrated technical dashboard. They record pH and production, and react once something has already failed.
Smallops implements the sentinel variable protocol in three phases:
- Diagnosis of the current situation and definition of the baseline.
- Design of the monitoring protocol, adapted to the substrate and the available resources.
- Monthly follow-up with trend analysis and adjustment of operational thresholds.
In plants that have implemented the full protocol, the response time to incidents drops by 60-80% and severe acidosis episodes fall by more than 90% in the first year.
If your plant operates without a technical dashboard, the Operational Excellence Diagnosis from Smallops is the starting point.
Does your digester warn you in time, or do you find out when the pH falls?
Request a Smallops Operational Excellence Diagnosis: we review what you measure, how often and against which thresholds, and leave you a sentinel variable dashboard adapted to your substrate.
Normative and bibliographic references
Nordmann, W. (1977). Die Überwachung der Schlammfaulung. Korrespondenz Abwasser, 24 (1), 8-16.
Boe, K. et al. (2010). State indicators for monitoring the anaerobic digestion process. Water Research, 44 (20), 5973-5980.
Lindorfer, H. et al. (2008). New data on temperature optimum and temperature changes in energy crop digesters. Bioresource Technology, 99 (15), 7011-7019.
DIN 38414-19 (2010). Deutsche Einheitsverfahren zur Wasser-, Abwasser- und Schlammuntersuchung.