(IMCS Second Place Best Poster Award) In-Situ Analysis of Volatile Organic Compounds in Biogas Fermentation Processes Using a Metal Oxide Gas Sensor Array
Authors: Ojha Binayak , Aleksandrova Margarita , Kohler Heinz , Franzreb Matthias , Schwotzer Matthias , Abegg Sebastian , Güntner Andreas 
Published: July 2021

1. Introduction and Motivation

Considering the global climate change, the efficient use of clean renewable energy source is one of the key research areas. In this context, the biological production of methane and other biogas produced by anaerobic fermentation of organic wastes is gaining importance. These fermentation processes attract more interest mainly due to the possibility to use different organic wastes as feeding substrates. However, efficient biomass conversion to biogas is only possible if the process parameters taking direct influence on the fermentation process are continuously and reliably monitored. This allows efficient process control. Some of key process parameters are the volatile fatty acids (VFAs) like acetic, propionic, butanoic acids formed during the biomass conversion process. The reliable monitoring of VFAs and other volatile organic compounds (VOCs) gives valuable information and their analysis at low concentrations (<2000 ppm) allows to model the actual microbial state and to adapt the feeding to keep their concentration and their inhibiting influence on the fermentation process low.
Conventionally, the analysis of the VFAs in anaerobic fermentation processes is done by sophisticated methods like gas chromatography [1], infrared spectroscopy [2], and high pressure liquid chromatography (HPLC) [3]. However, the major disadvantages of these methods are their complicated and time consuming sample pre-treatment routines and high costs.
In this paper, an automated measuring system developed by combining a silicon rubber membrane-based carrier gas probe (Fig. 1) with a thermo-cyclically operated metal oxide gas sensor array [4] is introduced. This automated system might enable in-situ monitoring of different VOCs developed during the bio-fermentation processes in time periods of about one hour.
2. Method of VOC monitoring

Metal oxide gas sensors (MOGs) are well established as gas sensing devices for monitoring of VOCs and oxidizable gases like CO, H2 and CH4. Thus, before measurement of dissolved VOCs, CH4, H2 and other cross-sensitivity contributing gas components present in the biogas fermentation sample must be driven out. In a first step, a small amount of the fermentation liquid (about one liter) is extracted from the main reactor and its pH is shifted to an alkaline value by dosage of KOH. This allows transformation of the dissolved VFAs to the dissociated state and enables purging out CH4, H2 and all other non-acidic, physically dissolved gas components, by a high flow of N2. Then, pH is shifted to a value near or even lower than the pKa value of the VFAs by dosage of H3PO4. Now the dissolved VFAs are in the undissociated molecular state and equilibrate with the gas state (Henry´s law). This enables the uptake of molecular dissolved VFAs from the liquid state into the constant flow of synthetic air (carrier gas: 5ml/min) by permeation through the gas permeable silicon rubber membrane of the gas carrier probe. By the help of the carrier gas the VFAs are transported to the metal oxide gas sensor array (Fig 1a) for analysis.
3. Results and Discussion

In a screening process several additive/SnO2 gas sensing composites have been prepared and the most interesting candidates with respect to their remarkably high sensitivity and most characteristic conductance vs. time profile (CTP) shapes when operated in thermocyclic mode [4,5] will be reported. When the carrier gas probe is immersed in acetic acid/deionized water admixtures some metal oxides show characteristic CTP-shapes (Fig. 1b) representing the individual surface reaction processes with acetic acid.
4. Conclusions and outlook

By some extended in-situ monitoring experiments of acetic acid in deionized water as a model substance using a gas carrier probe combined with a 4-fold metal oxide sensor array, several additive/SnO2 materials could be identified as good candidates for monitoring of organic acids in biogas fermentation processes.
Monitoring experiments in real fermentation samples and analysis of the CTPs yielded gas formation changing with pH and time. Some CTPs sampled at real fermentation liquid (pH 3) and referenced by the CTPs at pH 8 allowed the conclusion that the VFAs formed by the fermentation process can be really detected by the metal oxide sensor array. But to rule out any doubts, these gas formation processes have to be referenced by simultaneous Gas Chromatograph-Mass Spectrometry (GC-MS) analysis and these results will be reported as well in context with the MOG sensitivity data.
5. Acknowledgement

This work is part of the EBIPREP collaboration project (www.ebiprep.eu/). It is financed by the EU International Programme INTERREG V Oberrhein 2017-2020

References

[1] V. Diamantis, P. Melidis, A. Aivasidis; Continuous determination of volatile products in anaerobic fermenters by on-line capillary gas chromatography, Analytica Chimica Acta 573-574(2006)189-194
[2] H.M. Falk, P. Reichling, C. Andersen, R. Benz; Online monitoring of concentration and dynamics of volatile fatty acids in anaerobic digestion processes with mid-infrared spectroscopy, Bioprocess Biosyst Eng (2015) 38:237-249
[3] P.v. Zumbusch, T. Meyer-Jens, G. Brunner, H. Märkl; On-line monitoring of organic substances with high-pressure liquid chromatography (HPLC) during the anaerobic fermentation of waste-water, Appl Microbiol Biotechnol (1994)42:140-146
[4] K. Frank, V. Magapu, V. Schindler, H. Kohler, H.B. Keller, R. Seifert; Chemical analysis with tin oxide gas sensors: choice of additives, method of operation and analysis of numeric signal, 7th East Asian Conference on Chemical Sensors, Dec. 3-5, 2007, Singapore, SENSOR LETTERS 6 (2008) 908-911.
[5] Navas Illyaskutty, Jens Knoblauch, Matthias Schwotzer, Heinz Kohler; Thermally modulated multi sensor arrays of SnO2/additive/electrode combinations for enhanced gas identification, Sensors and Actuators B 217 (2015) 2-12

Figure 1