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"FOS/TAC" is the ratio of FOS (volatile fatty acids - VFAs, such as acetic acid, propionic acid, and butyric acid) to TAC (total alkalinity or total bicarbonate alkalinity). Monitoring the FOS/TAC ratio is a common and effective method to assess the state of the fermentation process in a biogas plant. 

The FOS/TAC ratio is an important indicator of the stability of the biogas fermentation process. A balanced FOS/TAC ratio indicates that the production and consumption of VFAs are in equilibrium, which is a sign of a stable and healthy fermentation process. In contrast, an imbalanced FOS/TAC ratio may indicate potential issues, such as acidification or alkalisation of the system, which can negatively impact the biogas production and overall efficiency of the plant.

By regularly monitoring the FOS/TAC ratio, operators of biogas plants can make timely adjustments to the process conditions, such as the feedstock mix, temperature, and pH, to ensure optimal biogas production and maintain the stability of the fermentation process.