A long journey from bioanode to biocathode

Doctoral thesis, 2018

Better utilization of renewable sources of energy and recovery of resources from waste streams are important challenges for researchers. Bioelectrochemical systems (BESs) are new technologies which e.g. could be used to produce green energy from waste sources or store renewable electricity as chemical fuels. They rely on microorganisms which can catalyse oxidation/reduction reactions on anodes/cathodes. BESs have a wide range of potential applications such as sensing, bioremediation, recovery of nutrients and metals, valorisation of wastewater organics, and production of energy carriers and other chemicals. However, further research is needed before these applications can be realized.

The goal of this thesis was to understand the effect of three different dynamic conditions and disturbances that bioanodes and biocathodes may encounter namely storage, starvation, and potential change. Storage and starvation are disturbances that can affect biological electrodes in all kinds of systems, and it is important to understand their consequences for performance. Changing electrode potential has been shown as a promising method for start-up of biocathodes from enriched bioanodes, but little is known about the long-term performance and changes in microbial community composition as the biocathode develops.

First, the possibility for storage of acetate-oxidizing bioanodes using refrigeration, glycerol freezing, and acetone dehydration was investigated. It was shown that storage of acetate-oxidizing bioanodes was possible. Bioanodes stored using refrigeration were the only electrodes that showed biological activity right after five weeks of storage. Then, starvation of acetate-and glucose-fed bioanodes was investigated. It was shown that the acetate- and glucose-fed bioanodes can survive 10 days starvation. However, the overall performance of the glucose-fed bioanodes deteriorated more after each starvation phase compared to the acetate-fed bioanodes. The conversion of acetate- and glucose-fed bioanodes to biocathodes was also compared. Immediately after the potential change, the glucose-fed bioanodes showed better cathodic activity but over time the performance converged. Then, we compared the conversion of bioanodes to biocathodes with direct start-up of biocathodes from a wastewater inoculum. Bare electrodes started-up faster compared to pre-enriched bioanodes. In the end, both types of enrichment procedures led to very similar biocathode communities, which were completely different from the bioanode communities. Indeed, for the microbial communities, it was a long journey from bioanode to biocathodes. Hydrogen appeared to be an important intermediate in the biocathode biofilms, therefore, start-up of biocathodes with pre-enriched hydrogenotrophic cultures was investigated. Hydrogenotrophic microorganisms could facilitate start-up of the biocathodes. All the microbial electrolysis cells inoculated by the enrichment cultures started to generate noticeable current directly after inoculation.

In summary, the bioelectrodes in our experiments were robust and could handle storage and starvation periods although the results depended on the experimental conditions, the feed, and the microbial communities. Conversion of bioanodes into biocathodes was less successful and resulted in a complete transition of the microbial community on the electrode. Start-up of biocathodes with hydrogen-oxidizing enrichment cultures was a more successful strategy.