Operation of Microbial Electrolysis Cells for Methane Production
Microbial electrolysis cells (MECs) are capable of enhancing naturally occurring methanogenesis, achieving efficient small-scale methane production from a range of organic matter feedstock when a low voltage is applied. This capacity increase is relevant considering the role of biogas in the decarbonisation of the economy, beyond large-scale projects with economies of scale advantages. Significant performance improvements have been achieved through fundamental research to understand microbiology dynamics and molecular mechanisms. Additionally, extensive material testing and design development have contributed to the understanding of the underlying phenomena and individual factors' effects. However, the variety of designs and operating conditions make the results hard to compare, and scaling up comparisons difficult, creating a body of knowledge fragmented and isolated from large scale industrial applications. It is intended here to fill those gaps, providing information regarding the individual and combined influences of alternative designs over MECs’ performance. This study focuses particularly on the influence and interaction of different anode-to-cathode relative surface area; and operational parameters such as hydraulic retention time (HRT), organic load, voltage, and hydrogen injection, over the performance of MECs from an energy-storage solution perspective. The data necessary for the discussion was obtained via sets of bio-electrochemical cultures, systematically organised as Placket-Burmann, and a complete factorial design of experiments. This allows a direct comparison of results, to identify the most influential factors and interactions that affect the overall performance. The cultures were carried out on 1L MECs with connections provided for both inlet and outlet of synthetic wastewater, H2/biogas and electrical connections for the carbon felt electrodes, using a total cell potential strategy for imposing the voltage. It was found that the organic load, voltage, and favouring the cathodic surface have a positive influence on methane production. Applying a voltage enhances the overall performance, with a positive correlation for both MPR and MCR when the applied voltage surpasses 600 mV. MEC B appears to be consistently more efficient than MEC A, regardless of the organic load and despite its smaller electrode surface area. A higher organic load increases methane production but reduces the efficiency of the overall process. 33% energy storage efficiency was achieved by MEC B when imposing 1000 mV, and 10 days of HRT. The empirical methane production rate and reference values from the literature were used to simulate the integration of MECs into household and industrial scenarios in terms of energy and mass balance. These results highlighted the feasibility of a household-scale integration, despite the carbon supply limitation. Conversely, for the industrial scenario, the current reaction rate achievable limits the contribution, although it would offset the extra energy cost required to operate a carbon capture and storage plant that reduces the CO2 emission up to 90% of a power plant, thus, hydrogenotrophic methanogenesis engagement appear crucial to converting greater amounts of vented CO2. It is expected that the results obtained in this study, especially the contour plot produced, help to operationalise the knowledge regarding MECs’ performance relationship with operational parameters, and contributes to the technology development.