Methane has a bad reputation as a gas that has a harmful effect on the climate, but it could be seen as one of the building blocks of energy transition, say researchers in Germany.
Methane can be produced very efficiently by methanogenic organisms, which reduce the biological oxygen demand of wastewater, while producing renewable methane as biogas. This gas can be captured in anaerobic digesters and used as fuel.
“Some wastewater treatment plants thus cover their own energy needs and are self-sufficient in terms of energy,” points out Tito Gehring from the faculty of civil and environmental engineering at Ruhr University Bochum in Germany.
While hydrogen has been hailed as a potential gamechanger in accelerating energy transition from fossil fuels to renewables, methane is less celebrated, but has several advantageous properties over hydrogen. Firstly, it is easier to handle and store, because its molecules are larger, making it less volatile.
“Some wastewater treatment plants thus cover their own energy needs and are self-sufficient in terms of energy."
In addition, the energy density of methane is four times higher than that of hydrogen, and it can be fed into existing natural gas infrastructure without any additional modifications. Another advantage of methane over hydrogen is particularly pertinent in southern regions where water is scarce.
Exporting hydrogen effectively means exporting water. Where methane is used as the energy carrier, this is strongly mitigated, reducing water losses by about half.
However, only 60% of biogas from wastewater is methane, the rest is mainly carbon dioxide (CO2), which means there is an opportunity to produce a highly concentrated form of methane using CO2 as well as hydrogen, which must be added to the system. A dedicated electrolyser that can ensure a long-lasting and energy-efficient supply of hydrogen has been developed by a group led by Ulf-Peter Apfel at the Fraunhofer Institute for Environmental Safety & Energy Technology (UMSICHT) in Germany.
This in situ hydrogen production module ensures that the microorganisms release about one molecule of methane per molecule of carbon dioxide consumed - metabolising various wastewater compounds. They can be installed at any wastewater treatment plant.
“Initial estimates have shown that about 20 litres of methane per day and per inhabitant could be obtained from CO2 capture from the waste gases of sludge treatment in wastewater treatment plants alone,” says Gehring. “Many wastewater treatment plants are connected to the natural gas grid and could simply feed it with the methane produced in this way.”
Doing so would also ensure that less methane is released into the atmosphere as a harmful climate gas and green methane from wastewater treatment plants could be one of several building blocks of the energy transition.