Monitoring and quantification of bioelectrochemical biofilms by means of OCT in novel and customized reactor-setups

In the last 40 years, bioelectrochemical systems (BESs) have been increasingly discussed within the scope of debates about sustainable energy sources and production of value added chemicals independent of fossil sources. Since the produced current in microbial fuel cells as well as the turnover rates in microbial electrosynthesis cells are dependent on the biocatalysts´ activity, control of the growing biofilm plays a major role in BESs. Moreover, the knowledge about the interplay between biofilm development and electrochemical parameters is crucial for optimizing these sytems.

In the last 3 years, various electroactive biofilms (anodic and cathodic) were cultivated and characterized in a versatile and house made lab-scale flow cell system as well as in a rotating disc biofilm contactor (RDBC). Both systems allow for control of substrate (liquid and gaseous), and nutritional conditions as well as hydrodynamics and other physical parameters. The monitoring of biofilm development was conducted non-invasively by means of optical coherence tomography (OCT). For cathodic biofilms, quantitative analysis of generated 3D OCT data sets revealed a correlation between substratum coverage and measured current density. The increase of substratum coverage led to a decrease of measured current density due to less abiotic redox processes on the cathode surface. A stable current density was achieved when a substratum coverage of 99 % was reached. Furthermore, calculated biofilm accumulation rates could also be correlated with the substratum coverage. The overall biofilm accumulation rate decreased when the substratum was fully covered. Both correlations support the hypothesis that the availability of electrons from the cathode surface is a limiting factor in microbial electrosynthesis.

A 10-liter RDBC was designed to continuously harvest biomass from the electrode to extract intracellularly stored products. In future, this approach could be applied for biotechnological processes. Additionally, the RDBC can be used to obtain reliable mass balances and turnover rates because of its larger scale.