BioDME
Today’s conventional waste water treatment requires considerable amounts of electricity for the elimination of organic carbon compounds, while their chemical energy remains largely unused. To increase the energy efficiency of waste water treatment, the interdisciplinary collaborative project BioDME aims at using this waste water energy to produce dimethyl ether (DME) – a easy to store and transport energy carrier that can also serve as raw material for the chemical industry. Aim of the project is the development of an upscaled demonstration system to prove the stability, sustainability, and economic feasibility of this process in practice.Today’s conventional waste water treatment requires considerable amounts of electricity for the elimination of organic carbon compounds, while their chemical energy remains largely unused. To increase the energy efficiency of waste water treatment, the interdisciplinary collaborative project BioDME aims at using this waste water energy to produce dimethyl ether (DME) – a easy to store and transport energy carrier that can also serve as raw material for the chemical industry. Aim of the project is the development of an upscaled demonstration system to prove the stability, sustainability, and economic feasibility of this process in practice.
Commonly, the elimination of the organic carbon load in municipal or industrial waste water 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. Nowadays, 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 it requires less electricity, since part of the required electrical energy is supplied from the bacterial degradation of the waste water at the anode.
Schematic of the overall process to produce DME from waste water. Left: microbial electrolysis cell: At the anode acetate from waste water is oxidized by bacteria to CO, releasing protons and electrons. While the protons travel through a polymer electrolyte membrane (PEM) through the cathode, the electrons flow through an external load circuit. To enable the reduction of protons to hydrogen gas at the cathode, an additional voltage is supplied between anode and cathode. Both, hydrogen (H2) and carbon dioxide gas (CO2) from the microbial electrolysis cell are collected and fed to the DME-synthesis stage. Right: DME-synthesis: the mixture of CO2 and H2 is being compressed and fed into the synthesis reactor. There H2 and CO2 are converted to DME by using a bifunctional mixed catalyst.
While hydrogen is an attractive energy carrier, its storage and transport can be comparatively challenging. To circumvent this, the BioDME project partners follow a new concept: the hydrogen (H2) and carbon dioxide (CO2) produced from waste water in a microbial electrolysis cell are further converted into dimethyl ether using a heterogeneous catalysis process. Dimethyl ether (DME) is considered to be a renewable energy carrier and platform chemical that can easily be stored and transported. Unlike the conventional DME synthesis process based on fossil fuel resources the proposed process is efficient and sustainable, since it uses the chemical energy already contained in the waste water. Furthermore – as opposed to conventional waste water treatment – the new process does not require electricity for aeration which further increases the energy-efficiency of the overall process.
The BioDME project is a continuation of the previous project BioMethanol, which had already been founded by the German ministry of Education and Research between 2014 and 2018. During the BioMethanol project, a new process for the synthesis of methanol from industrial waste water has been developed. It has been shown, that the concept has the potential to significantly reduce both, greenhouse gas emissions as well as the cost of waste water treatment. Regarding the economic feasibility of the process, in particular the microbials electrolysis cell’s construction and materials have been identified as factors that require optimization. This is where the BioDME project picks up the lead, aiming at the up-scaling and economic as well as ecological optimization of the overall process. Thereto, an optimized microbial electrolysis cell in the m³-range will be developed, giving particular importance to the economic and ecological impact of its materials and construction. Furthermore, the performance of the MEC will be improved by using specifically enriched microbial biofilms at its anode. To increase the overall economic feasibility of the process dimethyl ether (DME) instead of methanol will be produced. The production of DME is not only favorable from a reaction engineering perspective, but also comes with higher market value compared to methanol. From the very beginning on, life cycle assessment and economic evaluation will be part of the development process. Finally, a complete up-scaled demonstrator plant will be operated and characterized using real brewery waste water to prove the stability, sustainability and economic feasibility of the novel process in practice.
