Anaerobic Digestion – Microbial Ecology, Improved Operational Design and Process Monitoring

University dissertation from Siv Holmqvist, Department of Biotechnology, Lund University

Abstract: The implementation of anaerobic digestion is important in the concept of sustainable development, especially regarding the environmental benefits. Biogas is produced when organic matter is degraded by microorganisms under oxygen-free (anaerobic) conditions. Biogas production occurs in nature where oxygen is absent; in rice fields, landfills, sediments, the intestinal tract of animals, etc. The advantage of degrading organic matter and at the same time producing renewable energy in the form of energy-rich methane, as well as other environmental benefits, has made anaerobic digestion interesting in industrial applications. Anaerobic digestion has been exploited as an effective biotechnological method for organic waste and wastewater treatment. Current national environmental regulations and other policies governing land use and waste disposal have increased interest in anaerobic digestion. However, the anaerobic degradation of organic matter is a complex process involving microorganisms in an advanced interaction. Our knowledge about the anaerobic digestion process is increasing, but to increase the efficiency of the process and to make it more economically beneficial there is a need for improvements in the technology in many areas. Studying the microbial ecology is important for in-depth understanding of the anaerobic degradation process. Fluorescence in situ hybridisation (FISH) was employed to identify microorganisms in the anaerobic process with the focus on methanogens. Probes commonly used in FISH for detecting methanogens were re-evaluated and redesigned, giving better coverage of the methanogens, and the experimental conditions were optimised for use of the probes in FISH. Using FISH, changes in microbial composition could be detected when changing feed composition in anaerobic reactors, which could be correlated to changes in process conditions. A new application for partitioning in aqueous two-phase systems was developed. The mixed culture from an anaerobic digestion process was multi-step partitioned in an aqueous two-phase system in order to obtain a “fingerprint” of the culture. Differences in operational conditions could be reflected in the distribution profiles obtained. Process control and monitoring are of great importance in improving the efficiency of the anaerobic degradation process. Traditional and new methods were evaluated for the monitoring of anaerobic digestion processes. Since the anaerobic process is so complex, one parameter is seldom sufficient to monitor the conditions in a reactor, and a combination of parameters should be used for reliable process monitoring and control. With a good monitoring and control strategy it is possible to run the digesters closer to their maximum capacity thereby improving the treatment capacity and the process economy. The development of process designs and configurations has improved treatment rates making the process more cost-efficient. The successful co-digestion of agricultural waste with sewage sludge and pig manure was reported. Mixing of waste must be done with care as unsuitable mixtures can lead to unstable process conditions resulting in failure of the anaerobic digestion process. The development of new high-rate reactor designs has increased the efficiency and stability of the biogas process compared with the conventional reactor configuration. Three different high-rate reactor designs were evaluated and successfully applied to the treatment of solid agricultural waste.

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