Monitoring & Process Control of Anaerobic Digestion Plants

Abstract

Sufficient and reliable tools for process monitoring and control are important for the successful operation of any processing technology. This principle holds in particular true for the biogas process, which is a complex biological process. The process is characterized by distinct aspects such as inhomogeneous and often undefined substrates combined with a variety of process conditions. The importance of appropriate control techniques is a well-known fact in the industry. Despite this, in the biogas sector the investment in sensors, data management, and control components for the control of the biological process is far too often limited to the absolute must. The main reason is that there is no easy and cost efficient way to determine one single parameter for a comprehensive process evaluation. The available methods require a combination of sensors and many of them are either expensive, require a lot of maintenance or do not give precise results. The biogas sector is a young and quickly developed market with limited resources when it comes to available margins for extra sensor equipment. Most sensors and measurement methods have simply been taken over from other industries. Hence, the adaptation to the peculiar and demanding requirements of the biogas process is often inappropriate. Consequently, the operators experience malfunctions or implausible measurements which leads to a lack of acceptance for the control technologies. Beside this, most of the anaerobic digestion (AD) plants are integrated into agricultural practice and the procedures and the knowledge of the operators are not comparable to industry standards as for instance within the chemical industry. Furthermore, the details of the microbial interaction necessary for the degradation are still not fully understood, although there have been a variety of important findings published within the recent years. This makes it even more difficult to come up with simple and affordable monitoring and control strategies for a process, whose background is not fully comprehensible yet. Problems related to the biological process which might occur during the operation of an AD facility usually result in a negative effect on the substrate specific yield and often in an increase of concentrations of intermediates. The potential reasons for that are quite diverse and not easy to identify. They include, among others, overloading (due to changes in substrate amount or substrate characteristics), inhibition and lack of trace elements (or other nutrients). Foam formation is an issue which is often caused by the biological processes and has a direct impact on the technical processing, whilst its mechanism is hardly understood. Additionally, a variety of technical parameters need to be controlled and might influence the biology as well (temperature, mixing etc.). The development during the recent years shows a rising awareness of the operators and the plant manufacturers to the selection of appropriate and reliable equipment. This leads to the increased use of reliable monitoring tools, however, mainly only focusing on the most crucial tasks. The major future challenges of the industry comprise flexibility in regards of substrates and energy output. These tasks lead to even increased requirements on the quality of monitoring and control devices. The shortcomings of the existing solutions can be directly translated into the aims for further necessary research activities. The monitoring and control devices of the future might be based on completely new measurement principles or make existing technologies more reliable and more precise within the specific conditions of the biogas sector. Additionally, any investigations which improve the understanding of the process background will support the development of solutions solving the existing challenges. The stated topics were in the focus of the conference on “Monitoring & Process Control of Anaerobic Digestion Plants”, which was funded by the Federal Ministry for Economic Affairs and Energy within the program “Biomass energy use”. This issue of the CET presents nine selected contributions from the last conference held in March 2015 in Leipzig, Germany. They span a variety of issues from model predictive control, foam formation over microbial biosensors to spectroscopy methods and will hopefully be part of a development towards a more precise and efficient processing in the biogas sector. Hopefully you get inspiring ideas for your further activities and enjoy reading those papers. Jan Liebetrau (DBFZ Deutsches Biomasseforschungszentrum gGmbH, Head of Biochemical Conversion Department) & Fabian Jacobi (LHL Hessian State Laboratory)