The anaerobic digestion (AD) process is widely used to convert organic-based matter by bacteria into biogas in the absence of oxygen. Controlling the digestion process is one of the most important ways of making the biogas production process more efficient. Volatile Fatty Acids (VFAs) are proved to be the best parameters for direct indication of the status and health of the AD process. The AD process can be optimized to produce either biogas or volatile fatty acids that are even more valuable products than biogas.
In a two years EU-project OPTI-VFA, VTT developed a measurement solution for VFA monitoring. Another important goal in the project was to develop a Fabry-Perot Interferometer-based spectrometer prototype, which is cheaper and smaller than the FT-IR spectrometer.
As opposed to existing reference analytical methods, the OPTI-VFA measurement system requires no sample preparation and provides the results in real-time, allowing better process control in digestion processes. The system could also be used in the monitoring of other liquid processes, for example fermentable sugar production in for instance lignocellulosic biorefineries or the fermenting of wine, beer, distilled beverages, brandies, sweet liquors, and juices.
The OPTI-VFA measurement solution consists of a spectrometer, optical probe and chemometric models for VFAs. A commercial FT-IR and the new Fabry-Perot based prototype were used as spectrometers in the project. Chemometric models were developed in order to predict VFA concentrations based on the FT-IR spectral data. The OPTI-VFA measurement system developed was successfully validated in a laboratory digester and in a full-scale biogas plant. Acetic and propionic acids as well as total VFA concentrations could be measured reliably with a detection limit of 1 g/l.
Commercialization possibilities for the OPTI-VFA measurement system are being analysed by Rikola Ltd together with other consortium members. The goal is to further develop the system into a compact sensor type solution.