Biogas Magazine | Edition 20 |
22
A critical review of the
VFA/TA (FOS/TAC) method
A
mong the many techniques used for field monitoring of biogas plants, none has created more “rural legends” than the VFA/TA test (a.k.a. FOS/ TAC in the German technical literature). Some biogas plant owners trust this simple analysis, despite of the fact that in practise, the canonical VFA/TA quotient rarely corresponds to optimal plant production. In this article, we will analyse the logical flaws of the VFA/TA method as well as a case study of a biogas plant where it was proven not only useless, but also detrimental.
breviation TA stands for Total Alkalinity, expressed in mg equivalent of calcium carbonate per litre. According to the proponents of this method, the anaerobic digestion process is stable if the VFA/TA ratio falls in the range of values between 0.23 and 0.3. When the ratio is less than 0.23, the plant begins to “feel hungry” and requires more feed ration to digest, whereas a value greater than 0.3 indicates the beginning of “indigestion.
Probably this ease of operation has popularized the VFA/ TA method but, at the same time, has opened the door to the risk of biological collapse of the anaerobic system. The reasons are two fold; the method is overly reductive because it only monitors a single parameter and, worst of all, it is based on a logical fallacy, as explained below.
The VFA/TA method makes it simple to measure both the VFA and the TA using a technique known as titration. What the VFA/TA is? When using an automatic titrator, the same procedure VFA is an abbreviation for takes only a few minutes and Volatile Fatty Acids, usually does not require expert perexpressed in milligrams of sonnel. acetic acid per litre. The ab-
It is a well established scientific principle that a conclusion can be true if it is founded on two true propositions. Nevertheless, even if both propositions are true, the conclusion can be false; in other words, it is a necessary but insufficient condition. On the other hand, a conclusion drawn from false
The logical flaws in the VFA/ TA method
