Managing an anaerobic digester by looking only at pH and biogas production is operating while looking in the rear-view mirror. When pH drops you may already have severe acidosis: the operational damage runs 5 to 14 days ahead 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 with enough lead time to act before the damage is irreversible. In anaerobic digesters, pH is the most closely watched indicator, but it is also the most delayed: when 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 when the methanogenic community is already affected; individual VFAs and the FOS/TAC ratio respond when acidogenesis begins to exceed the buffering capacity. That time difference — between 5 and 14 days depending on the substrate and the load — is the operational window that determines 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 the English-language literature) compares the concentration of total volatile fatty acids (FOS, Flüchtige Organische Säuren) with the buffering capacity of the medium (TAC, Totale Anorganische Carbonate). It is the most widespread method in industrial digester monitoring, validated in the DIN 38414-19 standard and the Nordmann procedure. The reference operational thresholds are:
- FOS/TAC < 0.3: stable process. Sufficient buffering capacity and controlled VFA production.
- FOS/TAC 0.3-0.4: monitoring zone. Increase sampling frequency. Review organic load and substrate composition.
- FOS/TAC > 0.4: alert. Reduce organic load by 10-20%. Review the full dashboard.
- FOS/TAC > 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-managed digesters, FOS/TAC detects the decoupling between acidogenesis and methanogenesis between 5 and 10 days before pH reflects it. This 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 decreases.
The propionic/acetic ratio (C3/C2) is the reference clinical indicator: values above 1.0 indicate active syntrophic decoupling. Butyrate and isovalerate have complementary diagnostic values: an increase in isovalerate indicates degradation of branched-chain amino acids, frequent in digesters with a high protein load.
How to build an operational technical dashboard
A technical dashboard for digesters is not a SCADA supervision dashboard: it is a weekly review protocol of integrated variables that makes it possible to detect trends before they become incidents. The Smallops model organises these variables into three levels of urgency.
Level 1 · daily control variables
Biogas production (m³/day and % CH4), effluent pH, digester temperature, amount of substrate fed. These variables are recorded daily and define “yesterday’s state”: they are necessary for monitoring but insufficient for prediction.
Level 2 · sentinel variables (minimum weekly frequency)
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. These variables define the “present state” of the process and its trend.
Level 3 · diagnostic variables (monthly or on incident)
Specific methanogenic activity (SMA) via a BMP test of the sludge, full chromatographic VFA profile including LCFA (Long-Chain Fatty Acids), 16S rRNA microbiological analysis, BOD5 and COD of the digestate, trace metal content of the substrate. These variables are not measured in routine operation but in response to persistent alert signals or to validate diet changes.
ORP (redox potential) deserves special mention as a variable-frequency parameter: in continuously fed digesters it can be measured continuously and anticipates both oxidative stress and the accumulation of oxidised matter or failure of the anaerobic zone. ORP can be measured with platinum electrodes and is especially useful in digesters fed with variable waste.
Sulphides (H2S) must be monitored both in the liquid phase and in the biogas. Hydrogen sulphide in biogas > 2,000 ppm indicates a high sulphate load in the substrate; below 100 ppm it is operationally irrelevant. In slurry digesters the concentration can exceed 5,000 ppm, with a corrosive impact on the engine and the instrumentation.
Key quantified data
- Nordmann FOS/TAC thresholds: <0.3 stable · 0.3-0.4 monitoring · >0.4 alert · >0.8 crisis.
- FOS/TAC vs pH lead margin: 5-10 days.
- Critical C3/C2 ratio: >1 indicates syntrophic decoupling.
- VFA vs pH lead time: 5-14 days depending on substrate type and temperature.
- Typical ORP for active methanogenesis: −300 to −550 mV.
- Free NH3 inhibition: 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 buffering 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 monitoring, and above 0.4 is an alert signal that demands a load reduction. The “normal” value for efficient operation is between 0.15 and 0.30.
Why does pH arrive late as an indicator in a digester?
pH in a digester is strongly buffered by the bicarbonate-CO2 system. The buffering capacity (TAC) neutralises the VFAs as long as the system is not overwhelmed. This means pH does not fall until the VFA concentration exceeds the buffering capacity of the medium, by which point the acidosis is already severe. VFAs accumulate between 5 and 14 days before this threshold is crossed.
Which are the key variables to anticipate operational problems?
The five variables 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 + temperature, (4) ORP in the liquid phase, and (5) the CH4 content in biogas with its trend. None of these variables alone 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 recommended?
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. ORP can be measured continuously if instrumentation is available. Sporadic measurement of these variables is worse than not measuring them, because it generates a false sense of control without sufficient temporal resolution.
How Smallops installs predictive control in your plant
Most biogas plants operate without an integrated technical dashboard. They record pH and production, and react when something has already failed. Smallops implements the sentinel variable protocol in three phases: (1) diagnosis of the current situation and definition of the baseline, (2) design of the monitoring protocol adapted to the substrate and the available resources, and (3) monthly follow-up with trend analysis and adjustment of operational thresholds.
The clinical result in plants that have implemented the full protocol is a reduction in incident response time of 60-80% and a decrease in severe acidosis episodes of more than 90% in the first year. If your plant operates without a technical dashboard, the Smallops Operational Excellence Diagnosis is the starting point.
References and regulations
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.