FOS/TAC analytics provide plant operators with a quick and cost-efficient test of the health of their biogas plant. What is behind this analysis technique and what are the benefits and limits of the technique?

The FOS value (volatile organic acids) describes the sum total of all acids in the sample and is expressed as acetic acid without any further distinguishing between other different acids. The value does not tell you which acids exist in which concentrations. But in particular the time-dependent behaviour of the acids indicates quickly, in which direction the biology is developing.

The TAC value (total inorganic carbonate buffer), also called buffer, is – like the FOS value – a so-called totalling parameter. It contains all buffering substances (carbonates, phosphates and ammonium compounds) and is indicated as if they all were carbonates. The buffer ensures a certain stability of the pH value, which is essential for the bacteria’s health. These single-cell organisms thrive under constant conditions, which also include the pH.

The buffering substances bind and then release the acids without any major impact on the pH. The pH measures the content of free acids. As mentioned above, it is an important parameter for the work that is done by the bacteria. However, most often it only shows changes to the biological process when it is already too late.
The acid range (acetic acid, propionic acid and butyric acid)

The determination of the acid range is more time-consuming than the FOS/TAC analysis. Its big advantage is in its higher significance, because it determines the concentration and ratio of the individual acids. For a healthy plant, which is fed with renewable raw materials, the ratio between the acetic acid and propionic acid should be 3 to 1 or higher.

An increased propionic acid value can be an indication of a process fault. This might be due to a lack of trace elements or due to certain drugs or chemicals (for example in the liquid manure).