Understanding the Effects of Kinetic Limitations on Degradation Rates for Different Substrates in MFCs and the Impact on Trophic Layers on these Rates

Increasing global demand for clean energy and water has sparked recent interest in Microbial Fuel Cells (MFCs). Despite significant progress in improving this technology over the last 25 years, there remains a lack of understanding of the processes occurring within MFCs, such as degradation rates of complex organic compounds. Limited literature on what these rates are under realistic conditions hinders this technology's optimisation. This first phase of this research project aims to help fill this research gap by investigating the degradation rates of different substrates (acetate, glucose and starch) in kinetic and non-kinetic limited systems and at varying temperatures.

Previous research has shown that power outputs when using simpler compounds (acetate) are significantly higher when compared to complex compounds (starch) due to acetate not needing to be broken down by fermentation, indicating hydrolysis is the rate-limiting step in these systems. However, this was under fully stirred conditions. In reality, many MFCs operate in batch mode, and those which are continuous, have such low flow rates that turbulence does not create fully stirred conditions. If MFCs are to be used for real wastewater treatment, the degradation rates and limitations need to be fully understood. Such rates are needed to model and engineer MFC reactors effectively.

To determine whether the systems are kinetically limited, two bench-scale reactor configurations will be used; one that is fully stirred, allowing the basic limitations of the main degradation pathways to be determined; and one that is not stirred, giving an indication as to whether the processes within the system are taking place in the bulk liquid or on the biofilm. Stirring and non-stirring conditions will be used to determine the mass transfer limitations within the MFCs, and ultimately help design reactors and operational conditions which maximise their efficacy for treating complex wastes.

The second phase of this research project will involve repeating these initial experiments but with larger MFC configurations that simulate decentralised treatment scales. This will allow for more realistic conditions to be tested.
Since wastewater is comprised of highly multifarious substrates, the breakdown of complex compounds needs to be optimised. A potential solution to this is problem is the incorporation of plants as well as a soil based medium to the MFC system. Plant MFCs (P-MFCs), a modification of MFCs, have recently emerged and show promising results for MFC performance. Studies have demonstrated that plant based MFCs are successful. However, work still needs to be done to improve the output of these systems in winter months. It is the hope that the introduction of soil and plants to an MFC system will give the further benefit of trophic levels within the metabolic food web. The soil connecting the plants to the cathode will be full of organisms such as worms and many different microbiota, all of which could possibly aid the digestion/degradation pathways within these systems and ultimately increase their performance and subsequent wastewater treatment and energy recovery. Several studies have demonstrated the effectiveness of using worms to treat and transform wastewater, sewage, and wet sludge at both household and large-scale.

The final phase of this research project will therefore investigate the impact of trophic layers on the rates of degradation in L-scale MFCs. The reactors will be designed and built-in triplicate and will undergo preliminary tests under variable conditions. Initially, tests will be carried out in the laboratory, and eventually will be tested outside to enable us to determine if this technology is viable and if it can be implemented into real life set ups.