Optimizing anaerobic digestion, a two-strategy approach

Anaerobic digestion (AD) for biogas production is attractive to complement other renewable energy sources. The process is independent of climate and geography, and biogas can be utilized manifold: (i) to generate electricity and heat in a combined heat-and-power plant, (ii) feed the upgraded methane into the public gas grid, or (iii) use it as biofuel for vehicles. Biogas can be stored easily, which is a major advantage compared to other renewable energy sources. Furthermore, a wide variety of biodegradable wastes can be processed avoiding disposal or incineration. Typical substrates for biogas production are domestic organic wastes, byproducts from agriculture and industry, and sewage sludge. Despite these advantages, biogas plant operators are often under pressure due to volatile feed-in tariffs, inconsistent political backing and a competition for substrates. Thus, the basic optimization in plant operation and process control, to maintain reactor stability and increase gas yields at the same time, is a hot topic and is crucial for the future continuation of the biogas technology. This project aims at contributing to the foundations of a next-generation AD system, exhausting the full potential for flexible energy production and using the existing infrastructure. This project will focus on two promising strategies: serial AD and triggering a priming effect. Converting the process from parallel to serial operation is promising to increase biogas production. In this cascade, the first reactor receives the double amount of fresh material and the second reactor receives pre-digested sludge material from the first one. This setup guarantees for a higher substrate utilization and offers increased methane yields and reduced amounts of digestate. The addition of an easily degradable co-substrate in order to trigger a priming effect will be another way to improve process efficiency. It is supposed to lead to an activation of the involved microorganisms and to increase their growth rates in general. Using the priming effect shall help facilitating the consumption of hardly degradable substances in particular. Experiments will be set up in bioreactors with a working volume of 6.5 L. Researchers of the Department of Waste and Resource Management at the University of Innsbruck will monitor classical process parameters, while researchers of the Department of Microbiology will investigate the involved microbiome, both from a taxonomical and functional point of view. The research consortium is complemented by scientists of the University of Magdeburg who will analyze the microbial proteome. The results of this basic research project shall contribute towards a better understanding of the biomethanisation process and shall allow for a precise process control. The project helps to reach the 20-20-20 goals of the European Union, which define the current EU-27 energy policy.

Althoug a niche technology in the energy industry, anaerobic digestion based on renewable resources is characterized by several ecological and economic advantages over other technologies. Therefore, expanding this technology makes sense in the future and should definitely be pursued further. Anaerobic digestion is independent of climate and geography and can be applied globally. In addition, a wide range of municipal, agricultural or industrial organic wastes can be used as a substrate, thus avoiding incineration or landfill. The biogas produced is versatile and can be converted into heat and electricity, fed into the public gas grid after purification or be used as biofuel for vehicles. The gas can also be stored easily and efficiently, which makes it a perfect complement to other renewable energy sources. Despite these advantages, operators are often under enormous pressure when it comes to the profitability of their plant. Organic waste is now also in demand from other consumers and feed-in tariffs are falling. It is precisely this pressure that requires new, innovative strategies that start directly at the base. This involves increasing the efficiency of the degradation process to obtain more biogas while at the same time reducing the amount of residual sludge at the end of the process. This project investigated two promising strategies for increasing efficiency, namely serial biogas production and the use of the so-called priming effect. When switching from parallel to serial operating mode, two reactors are connected in series in a cascade. This enables the establishment of specially adapted microbial communities in the primary and secondary reactors, leading to improved degradation of recalcitrant substrates. The second strategy dealt with the introduction of easily degradable substances (in this case glycerol), which can be utilized immediately by the microorganisms involved and triggers an effect known as priming, which can lead to increased biogas yield. At the Universität Innsbruck, the experiments were carried out in 6.5 L bioreactors, with the classical process parameters being recorded at the Department of Environmental Technology and the microbiome being examined on both a phylogenetic and functional basis at the Department of Microbiology. In addition, the proteome was examined by scientists from the Hochschule Anhalt. The results from this basic research project demonstrate advantages both of the cascade digestion approach and the supplementation with glycerol. They also provide the basis for a better understanding of anaerobic digestion in general and for the optimal control of process sequences. The project thus contributed to achieving the 20-20-20 targets of the European Union, which define the current climate policy of the EU-27. It is hoped that the results of this project will help to increase the acceptance of biogas as an important supplement to other renewable energy sources.