In view of the changing climate and the finiteness of fossil resources, research into renewable energies is gaining in importance. One of the things that researchers have been looking into for quite some time is different possibilities to use organic wastewater compounds as sustainable energy sources. Carsten Meyer from the University of Stuttgart works on the generation of alternative energy sources. Together with his team of researchers, Meyer was involved in a recently finished project that looked into the biological production of hydrogen from wastewater and sewage sludge.
Hydrogen can be used as a fuel for hydrogen combustion engines or in fuel cells. Currently used methods for the production of hydrogen require a lot of energy. Moreover, the energy sources used for this purpose are primarily fossil. The biological production of hydrogen is a potential alternative to using fossil energy sources. There are many ways to generate hydrogen. For example, algae produce hydrogen in the absence of sulphur. Bacteria can also make hydrogen. Carsten Meyer, head of the Department of Wastewater Technology in the Büsnau-based Sewage Treatment Plant for Research and Training, has chosen to use bacteria in his research. Meyer and his team of researchers have been investigating the possibility of converting energy-rich organic compounds contained in wastewater and sewage sludge into hydrogen. Anaerobic wastewater and sludge digestion offers an energy- and cost-efficient way to produce hydrogen, as most of the equipment required is already available in wastewater treatment plants and the energy needed for the process is continuously delivered in the form of carbon compounds in the inflow of the treatment plant.
The “2-stage concept for the fermentative production of hydrogen and biogas through innovative gas treatment” project, which was carried out between March 2009 and February 2012, was coordinated by Iosif Mariakakis, doctoral student at the Institute for Sanitary Engineering, Water Quality and Solid Waste Management (ISWA). The researchers used a 30 l bioreactor with sewage sludge from the sewage treatment plant in Büsnau and a culture made up of different microorganisms. The bacterial culture contained hydrogen- and methane-producing bacteria. The sugar content of the used wastewater was too low for the bacteria to grow effectively, making it necessary to add saccharose. In the first stage of the two-stage process, the feed source is converted to hydrogen. Biogas-producing bacteria then convert the remaining saccharose and the resulting organic acids into methane, which can also be used as an energy source.
The experiment took 120 days and the scientists recovered a maximum of 1.72 mol hydrogen (H2) per mol substrate. According to Meyer, the result “is excellent, both in comparison with the amount of H2 other researchers around the world are able to generate and also considering the size of reactor we used.” If Mariakakis’ method with its yield of 1.72 mol hydrogen per mol substrate were to be applied to the wastewater from the entire annual production of treacle in the EU, the hydrogen generated would be sufficient to run around 58,000 cars for one year. This would of course only be possible if the cars were adapted to run on hydrogen. If all EU biowaste was used for generating hydrogen, ten times as many, i.e. around 580,000, hydrogen cars could be powered.
Although the results they achieved were excellent, Mariakakis and his team had to find solutions for quite a few problems during the time the experiment ran. “It was fairly difficult to keep the process constant over four months whilst achieving a high hydrogen yield,” Mariakakis explained. “It was also difficult to inhibit the growth of lactic acid bacteria in the sewage sludge. The bacteria consume the substrate and produce CO2, requiring a costly treatment of the gas mixture.”
As no previous results from other researchers were available with this size of reactor and with the operating parameters such as pH regulation and substrate, Meyer and his team were extremely pleased that they were able to successfully produce hydrogen under laboratory conditions.
Further improvements are necessary in order to be able to apply the process on a large scale at some stage in the future. Possible improvements include the use of lower pH values in order to further inhibit the growth of lactic acid bacteria. Another option would be to decrease the temperature in order to achieve a better energy balance. The substrate needs to be heated in order for microorganisms to be able to use it. In addition, different types of wastewater would have to be examined for their suitability for the generation of hydrogen. Any wastewater for the generation of hydrogen needs to have a relatively high sugar content, therefore not every type of wastewater is suitable. Wastewater produced by the fruit juice and sugar industry are highly promising candidates.
Mariakakis believes that it will take at least another eight to ten years before it is possible to even think about the large-scale production of hydrogen from wastewater. He also believes that it will only be possible if other technologies such as biogas processing and biogas treatment are also improved. “If fuel cells require 100 per cent pure hydrogen and we are only able to generate hydrogen with 99 per cent purity, it will not work.”
Meyer has a less technical outlook. He believes that there is a clear tendency to move away from seeing wastewater as waste and towards seeing it as a material that can be processed further, either for producing energy or recovering nutrients. “Time will tell where future demand will lie,” said Meyer, conscious of the role played by the availability of different technologies and approaches and whether it is worth optimizing them or not. “We do not yet have a clear vision as to where we are heading,” said Meyer.
The project has come to an end, but Meyer and Mariakakis are already considering submitting a proposal for another research project on hydrogen production. “Hydrogen production is a very broad field of research, and we only just scratched the surface. We are still keen to find out a lot more!”