FOS/TAC ratio and what it characterizes

The FOS/TAC ratio is an important indicator used to assess the state of a fermenter in a biogas plant. FOS (from German: Flüchtigen Organischen Säuren) represents volatile organic acids, and TAC (from English: Total Anorganic Carbon) measures the total inorganic carbon, i.e., the alkaline buffering capacity of the system. In English literature, this indicator is sometimes referred to as the alkalinity ratio: Alkalinity ratio = IA/PA, where IA stands for intermediate alkalinity, and PA stands for partial alkalinity.

FOS represents organic acids that are easily broken down by microorganisms during fermentation. They are the primary source of energy for methanogenesis.

TAC is a measure of the digestate’s ability to resist acidification. It is determined by the presence of buffering substances in the digestate, such as carbonates, bicarbonates, and organic acids.

The FOS/TAC ratio is an important parameter because it characterizes factors such as:

1. Fermentation process stability: The optimal FOS/TAC ratio ensures a stable fermentation process and continuous biogas production.
2. Biogas production: An optimal FOS/TAC ratio promotes increased biogas production by providing sufficient energy for methanogenesis.
3. Digestate acidification: A FOS/TAC ratio above 1 indicates the risk of digestate acidification, which can lead to a decrease in biogas production and even the cessation of the fermentation process.

In simpler terms, this ratio characterizes the acidity and alkalinity in the system, helping to assess the risk of acidification in a biogas reactor. Regular monitoring of this ratio ensures the system responds effectively and maintains efficient reactor operation.

The optimal range for the FOS/TAC ratio is 0.3 to 0.4. An increase in the FOS/TAC ratio above 0.4 indicates overloading of the bioreactor, which reduces biogas production, while a decrease in the ratio below 0.3 indicates the need to add substrate to the reactor.

Required equipment:

• Calibrated pH-meter
• Burette or automatic titrator
• Magnetic stirrer
• Laboratory glassware
• 0.1N H2SO4 solution

Analysis Method

Sample preparation involves filtering the sample through filter paper or centrifugation. 20 ml of the filtrate is titrated using a burette with 0.1N H2SO4 solution until reaching pH 5.0 with the magnetic stirrer turned on. After fixing the volume, continue titration until pH 4.4 is reached, and record the volume of solution used for the second titration.

Calculation of the Indicator

FOS/TAC = (((V_pH=4.4 * 1.66) – 0.15) * 500) / (V_pH=5.0 * 250),

V_pH=4.4 is the volume in ml of 0.1N H2SO4 solution used for titration to pH 4.4,

V_pH=5.0 is the volume in ml of 0.1N H2SO4 solution used for titration to pH 5.0.

Regular monitoring of the FOS/TAC ratio is an essential part of biogas plant operation. Routine monitoring of this indicator allows for timely detection of deviations from the norm and taking corrective actions to address them.