Green Methane Wastewater Treatment Plants as Drivers for Energy Transition
By implementing an innovative technology, any wastewater treatment plant would be able to produce methane from carbon dioxide in an environmentally friendly way.
When some microorganisms in wastewater treatment plants are fed hydrogen and carbon dioxide, they produce pure methane. Heating systems and vehicles running on natural gas can use this methane without any technical modifications. By combining the expertise of the research groups of Dr. Tito Gehring / Professor Marc Wichern and Professor Ulf-Peter Apfel, both at the Ruhr-Universität Bochum, a technical add-on module was developed that can, in principle, turn any wastewater treatment plant into a CO2 sink and decentralized methane production plant in an environmentally friendly way.
Methane has a bad reputation as a climate-damaging gas. However, it has several good properties that mean it could contribute to the energy transition: it is easier to handle and store than hydrogen because its molecules are larger, making it less volatile. Its energy density is four times that of hydrogen, and it can be fed into the existing natural gas infrastructure without additional modifications. “Vehicles and heaters that run on natural gas can easily run on methane,” says Tito Gehring of the Department of Urban Water Management and Environmental Engineering. He cites another advantage of the gas over hydrogen, which is produced in southern regions where water is scarce: “If these regions export and use methane, they are effectively exporting water. This is greatly mitigated by using methane as an energy carrier, which reduces water losses by about half.”
Methane can be produced very efficiently by methanogenic organisms and is, for example, produced in wastewater treatment plants as a component of biogas. “As a result, some wastewater treatment plants cover their own energy needs and are energy self-sufficient,” says Tito Gehring. However, only 60 percent of biogas is methane, the rest is mainly CO2. In order to produce highly concentrated methane, the methanogenic biomass needs CO2 as well as hydrogen, which has to be added to the system. In order to produce this hydrogen, Ulf-Peter Apfel's group from the Technical Electrochemistry and Electrosynthesis departments at Fraunhofer Umsicht developed a special electrolyzer with a catalyst that is free of precious metals and ensures a long-lasting and energy-efficient supply of hydrogen. The implementation and testing of this system was carried out by Ramineh Rad, the first author of the publication.
Replacing Part of the Natural Gas
Fed with this in-situ produced H2, the microorganisms release about one molecule of methane per molecule of carbon dioxide consumed in an auxiliary module that can in principle be installed in any wastewater treatment plant. They also metabolize various compounds in the wastewater without requiring additional nutrients. “Many wastewater treatment plants are connected to the natural gas grid and could easily feed it with the methane produced in this way,” says Tito Gehring.
He sees green methane from wastewater treatment plants as one of several building blocks in the energy transition: “Initial estimates have shown that about 20 liters of methane per day and per inhabitant could be obtained by capturing CO2 from the waste gases of sludge treatment in wastewater treatment plants alone. This would also ensure that less methane is released into the atmosphere as a climate-changing gas. That's because the release of methane during the extraction of natural gas, oil and coal is a major source of emissions of this greenhouse gas.”
Original Publication: Ramineh Rad, Tito Gehring, Kevinjeorjios Pellumbi, Daniel Siegmund, Edith Nettmann, Marc Wichern, Ulf-Peter Apfel: A hybrid bioelectrochemical system coupling a zero-gap cell and a methanogenic reactor for carbon dioxide reduction using a wastewater-derived catholyte, in: Cell Reports Physical Science, 2023, DOI: 10.1016/j.xcrp.2023.101526