PhD example by Dr. Jörg Kretzschmar
Characterisation of a microbial electrochemical sensor platform for anaerobic digestion process control
Energy generation from biogas accounts for approx. 8 % of gross energy production in Germany. While electricity generation from solar and wind power plants depends on fluctuating weather conditions and the time of day, electricity generation from biogas is able to provide electricity and heat on demand. Flexible energy generation from biogas can be made possible, for example, by installing additional gas storage facilities. Another approach is to produce biogas according to the demand by supplying substrates such as maize silage, cattle manure and straw in variable amounts at variable times. However, since high substrate input rates can interfere with the process, there are special process monitoring requirements.
In order to establish successful process control in the future, real-time monitoring of central process indicators, such as acetate concentration, is necessary. Currently, the acetate concentration and the concentration of other important volatile fatty acids or their salts (e. g. propionate and butyrate) are determined using time-consuming and costly methods such as gas or liquid chromatography or automated titration. The aim of this work was therefore to develop and characterise a microbial electrochemical sensor for real-time measurement of acetate in the biogas process. The most important part of the biosensor, the receptor, consists of a Geobacter sp.-dominated biofilm on a graphite electrode. The bacteria oxidise acetate, among other things, as part of their energy metabolism. The amount of oxidised acetate correlates with the number of electrons transferred and thus with the sensor current. Within the scope of the work, basic sensor parameters, such as measuring range, measurement resolution, cross-sensitivity and functional stability of the biosensor were examined.
In addition to determining these parameters in artificial wastewater, experiments were carried out to verify the sensor’s function in its future process environment, the biogas process. Furthermore, possible impurities from the biogas process were investigated for the microbial receptor. The focus here was on a high salt and ammonium concentration and the effect of fumarate as an alternative electron acceptor. Finally, the suitability of electrochemical impedance spectroscopy as a tool for the in situ monitoring of biosensor functionality was investigated. For this purpose, electrochemical impedance measurements were carried out on metabolically active receptors.
The characterisation of the biosensor showed a measuring range of 0.5–5 mmol L-1 acetate and a measuring resolution of 0.25–1 mmol L-1. When the biosensor is used in the biogas process, the upper measuring limit of 5 mmol L-1 acetate must be raised to at least 20 mmol L-1. This can be achieved by using a membrane as an additional diffusion barrier. The proof of function in the biogas process proved there is a clear correlation between sensor current and acetate concentration. However, the investigations identified an inhibition of the sensor and the microbial receptor over a period of 1–8 days. The investigation for possible impurities revealed the biofilms have a tolerance to a high salt concentration of (13.5 g L-1) and an ammonium concentration of up to 3 g L-1 NH4+, a typical value for biogas reactors.
Based on the current state of knowledge, using the biosensor to monitor the biogas process or other processes in which acetate plays a role seems possible in principle. In order to further develop the sensor concept into a market-ready product, further measures must be tested and implemented in order to increase the measuring range and to stabilise and monitor the biological receptor.