Intensification of the biogas process by improved process monitoring and biomass retention

Abstract: The utilisation of energy in the form of biogas is one of the environmentally sound alternatives available using renewable energy sources. Biogas is formed by anaerobic degradation of organic material, the main consistuent being energy-rich methane. There is a large unused potential of organic waste that is suitable for biogas production and the anaerobic digestion process has the potential of becoming an important waste treatment and bioenergy generating method in the future. Laws, government grants, taxes and an increasing environmental concern are presently directing developments in a favourable direction for the increased use of the biogas process, but the implementation of anaerobic digestion technology is not straightforward and there is a need for further efforts to develop reliable, economically feasible technology. Anaerobic degradation is performed by a well-organised community of several microbial populations, and is a complex process. Some of the microbial groups involved are slow-growing and sensitive to changes in operating conditions. This can cause instability during both the start-up and operation of the anaerobic process. To make the biogas process more attractive from a commercial point of view, and to facilitate increased integration into our energy supply systems, these instability problems must be overcome in an economically viable way. The purpose of the present work was to investigate different methods of improving the performance and efficiency of the anaerobic digestion process. Based on the knowledge concerning microbial and physical events in the anaerobic digestion process, two main strategies were applied; biomass retention and improved process monitoring. Support materials were utilised to facilitate the retention of slow-growing organisms in biofilms, thereby stabilising the process. It was shown that the protected environment of a biofilm in combination with a long adaptation period made the treatment of toxic wastewater possible. A support material was also successfully used when treating waste containing a high concentration of insoluble particles. No clogging problems occurred, and the retention of important bacteria resulted in a change in metabolic flow and increased process stability. In a two-phase process, a protected environment can be created for the sensitive methanogens. A biosensor that could be used for monitoring of the flow of organic compounds reaching the methanogens was developed. The sensor was able to measure the biochemical oxygen demand (BOD) in a few minutes, making it suitable for on-line monitoring. More efficient operation can be achieved by improving process monitoring, which allows waste treatment at a higher rate. Traditional off-line monitoring methods used in anaerobic digestion were investigated; the results indicating that the present monitoring strategy is not adequate if the process is to be operated at a higher rate. The very rapid response to overload indicates that on-line monitoring is necessary to make suitable process control possible. A new method for utilising a semiconductor sensor for the monitoring of dissolved hydrogen on-line was described. The method was evaluated together with other traditional and new monitoring methods, and proved to be a useful tool in on-line anaerobic digester process monitoring.