Commonly, the elimination of the organic carbon load in municipal or industrial wastewater is achieved by O2-respiring microorganisms in an activated sludge process. To fully degrade the organic carbon fraction to CO2 the wastewater must be actively aerated, which requires a considerable amount of electrical energy. At present, only part of the waste water’s chemical energy content is recovered in the subsequent anaerobic digestion of the sewage sludge, which yields methane-rich biogas. As an alternative technology for a more energy-efficient waste water treatment microbial fuel cells are currently being developed. In microbial fuel cells, so-called exoelectrogenic bacteria use the anode of a fuel cell as terminal electron acceptor instead of oxygen simultaneously producing CO2 and protons from organic carbon (Fig.1). At the cathode, oxygen reduction completes the electron cycle. This way, electricity can be directly generated from the energy-rich organic carbon fraction of the waste water while at the same time the energy-intensive aeration can be circumvented. If the cathode is placed in an oxygen-free environment protons instead of oxygen are reduced at the cathode and hydrogen gas is generated. Thereto an additional small voltage has to be applied between the anode and cathode of the fuel cell. This process is called microbial electrolysis. Compared to conventional electrolysis this process requires less electricity, since part of the required electrical energy is supplied from the bacterial degradation of the waste water at the anode.
Fig. 1: Schematic representation of renewable methanol production from industrial or municipal waste water. Microbial electrolysis cell: At the anode the organic load of the wastewater is oxidized to CO2 by exoelectrogenic bacteria, releasing protons and electrons. Between anode and cathode a voltage is applied by an external power source and the electrons travel through an external circuit to the cathode. The protons migrate through the polymer electrolyte membrane (PEM) to the cathode where they are reduced to H2. Both CO2 and H2 from the microbial electrolysis cell are collected and delivered to the methanol synthesis stage. Methanol synthesis: The mixture of CO2 and H2 is compressed and fed into the reactor and through a catalytic reaction gaseous methanol and water are produced.
Overall aim of this interdisciplinary and collaborative project is to develop a laboratory scale process for the production of methanol from waste water. Methanol is considered to be a renewable energy carrier that can easily be stored and transported. Unlike the conventional methanol synthesis process based on fossil fuel resources the proposed process is efficient and sustainable: carbon dioxide (CO2) and hydrogen (H2) are produced from municipal or industrial waste water in a microbial electrolysis cell and further converted into methanol by a downstream heterogeneous catalysis process. For this purpose, an optimized microbial electrolysis cell for the production of H2 and CO2 will be developed. At the same time, new catalysts for the methanol synthesis reaction are investigated and optimized towards improved tolerance against catalyst poisons and impurities (H2S, NH3, ...) that may be present as contaminants in the product gases from the microbial electrolysis cell. In the third year of the project, a complete laboratory scale system for the production of methanol from wastewater will be operated and characterized under application-relevant conditions with real waste water (Fig. 1). This will provide data for the ecological and economic evaluation of the overall concept "Sustainable methanol from waste water" and serve as a basis for future pilot plants. In addition, different process configurations will be compared and possibilities for the integration of the system into a larger context will be evaluated (Fig. 2).
The central work package is the development of a microbial electrolysis cell for operation with real waste waters and its optimization and characterization under application-relevant conditions. The goal is to elucidate the influence of the construction and operation of the microbial electrolysis cell on the product gas composition and the efficiency of the process. In the third year of the project, an optimized microbial electrolysis cell stack will be operated continuously with real waste waters and coupled to the methanol synthesis stage. In addition, the performance of the microbial anode will be improved by optimizing microbial colonization and therefore the utilization of the available anode surface area by microorganisms.
To successfully realize the concept "Sustainable methanol from waste water" it will be necessary to optimize the methanol synthesis stage towards operation with the gas mixture produced in microbial electrolysis cell. In order to develop robust catalysts it will be of particular importance to identify and quantify the impurities and potential catalysts poisons. In addition, the catalysts for methanol synthesis will be developed towards lower reaction temperatures to further increase the yield and energy efficiency of the methanol synthesis process. These improvements will have a direct impact on the economic viability of the concept.
The project will be accompanied by an ecological and economic evaluation of the overall concept "Sustainable methanol from waste water". Thus, already at an early stage of process development existing risks related to the commercialization can be detected and the development and research activities can be adjusted accordingly. The evaluation will also consider different scenarios of application, such as the use of photovoltaics to provide electrical energy for microbial electrolysis cells and the use of combined heat and power (CHP) exhaust gases as an additional source of CO2 (Fig. 2). It will also be investigated whether the concept can serve as a buffer for the surplus of renewable electricity at certain times during the day. Furthermore, both conventional and microbial electrolysis will be compared in terms of technological and economic aspects.
Fig. 2: Possible scenarios of application for the concept "Sustainable methanol from waste water"