Thermo-cyclically operated metal oxide gas sensor arrays for analysis of dissolved volatile organic compounds in fermentation processes: Part II – Quasi online monitoring in biogas fermentation

This study presents a quasi-online method for monitoring of dissolved volatile fatty acids (VFAs) in biogas fermentation processes with a carrier gas probe by use of thermo-cyclically operated metal oxide gas sensor arrays. Each of the two sensor arrays comprises a pure SnO$_2$ and three different SnO$_2$/additive-composites (additives: alumina, YSZ, NASICON) but differ by SnO$_2$ synthesis routes, namely Flame Spray Pyrolysis (FSP) and Sol-Gel (SG) technique, respectively. This allowed comparative studies of the influence of layer morphology on VFA sensing characteristics. For sensitive determination of the dissolved VFAs besides high concentrations of biogas components like CO or CH$_4$, first a pre-treatment routine of the fermentation sample was introduced to remove those physically dissolved gases without losing VFAs. The Conductance-over-Time-Profiles (CTPs) of eight different sensing layers were measured simultaneously at exposure to the gases extracted from the fermentation sample at different pH conditions. Almost all the investigated SnO$_2$/additive-composites show CTP-features clearly correlating with the undissociated VFA even at concentrations below 120 ppm as referenced by GC-analysis. ... mehrThe lower detection limit is well below inhibitory concentration for fermentation processes. As expected, most pronounced CTPs representing actual VFAs situation were measured at pH 3, well below the pKa of the VFAs. The FSP-layers highlighted clearly better sensitivity and CTP specificity of higher quality compared to SG-layers. Among the SnO$_2$/additives, the CTP-features of the SnO$_2$(FSP)/NASICON and SnO$_2$(SG)/NASICON layers showed the best specificity to acetic and propionic acid. For the first time, quasi-online analysis of VFAs using metal oxide gas sensors for early warning of VFA-development in biogas fermentation processes was demonstrated.