A Bioelectrochemical Sensing System for Measuring Acidic Bioprocess Fermentation End Products
Monitoring is an important tool in the management of bioprocesses such as anaerobic digesters, wastewater treatment plants and bioelectrochemical systems. There are several important parameters which are considered for regular monitoring, including pH, temperature, conductivity, gas composition and volatile fatty acids (VFAs) concentration. To date the most reliable methods for offline VFA measurement are gas chromatography (GC) and high performance liquid chromatography (HPLC). Attempts have been made to develop in situ enzyme based sensor for online VFA measurements but the long-term stability and speciation is not yet satisfactory. In this study microbial fuel cell (MFC) technology has been tested for its capabilities to measure VFAs and the method explored for its stability, repeatability, recovery and implementation on real wastewater samples. The amperometric signals derived from MFCs were correlated to corresponding, cross VFA and sucrose concentration that ranged up to 40 mg/l using electrochemical methods such as coulombic efficiency (CE) and cyclic voltammetry (CV); with response time of >20 hours (for 20 mg/l) and <3-4 minutes respectively. The stability of the sensor responses were studied further enhanced by fabricating anode electrodes with various natural and functionalised conductive polymers. Poly (alkyl) ammonium showed improved response, shortened start up and recovery times for sensor arrays. The range of the MFC sensors was also improved to reach up to 200 mg/l, by poising the enriched anodes at specific potentials. The individual VFA enriched MFC sensor array was calibrated offline to measure VFA concentrations in the samples collected from a sucrose fed scaled up MFC and hydrogen fermentation reactor with electrodialysis cell. Acetate and propionate enriched MFCs showed a satisfactory response when compared with GC analysis and could be further improved. However, the poor response from the butyrate enriched MFCs required more study with regards to calibration, degradation pathways and electron transfer mechanisms. This study also revealed various operating strategies to inhibit methanogenesis in MFCs enriched with individual VFA and enhanced degradation of recalcitrant fermentation end products, which could contribute to further increasing the overall performance of the proposed sensor array. The study presented here is the first attempt to use MFC as a sensing system to detect VFAs and is an important contribution in the field of bioprocess monitoring and will provide a platform for further improvements in MFC based VFA sensing.
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