A New Mathematical Model for the Accurate Determination of Volatile Fatty Acids Concentration in Anaerobic Digesters

Abstract

One of the key success factors for optimal management of anaerobic digestion reactors is to control the volatile fatty acids (VFA)
production. This is justified to the extent that at a concentration greater than 3000 mg/l, VFA can inhibit the methanation process. In other words, their concentration is a reliable indicator of reactor stability or stress. Several analysis methods for these VFA
have been developed such as high performance liquid chromatography (HPLC), gas chromatography (GC), colorimetry and
various titration techniques. The titration technique offers several advantages: it is the simplest, least expensive, fastest, and
accessible to all countries, including developing countries where laboratory equipment’s are generally lacking. The titration
method can, therefore, be a useful tool if its reproducibility, precision and interpretation can be improved. The present study
focused on the development and validation of a new simplified, the improved and accurate titrimetric method for the determination of VFA concentration, sufficiently close to chromatographic methods (GC or HPLC), considered as the gold
reference method today. A mathematical model taking into account the main factors affecting the accuracy of VFA concentration
in anaerobic reactors (e.g chemical species, pH, temperature, ion activity coefficients) was developed. Its validation was carried
out using measurements on thirty-three different samples with this new titration method on the one hand and the gas chromatography (GC) method on the other hand. At 99% confidence interval, the VFA concentration determined using this new
titration method/mathematical model were close to those obtained by the GC method with a determination coefficient R2 of 0.97.