Instrumentation, Control and Automation in Anaerobic Digestion

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

Abstract: Anaerobic digestion (AD) is a biochemical process in nature. It has been used to decompose organic waste in order to reduce environmental pollution, and to destroy pathogenic microorganisms for the protection of human and animal health. The process itself produces methane, which can then be used as an energy source. The use of AD in an integrated resource recovery system is one of the most important ways of achieving economic and environmental benefits. Owing to the fact that the successful anaerobic conversion of organic waste to biogas relies on a series of biochemical reactions carried out by different groups of anaerobic microorganisms that live interdependently, the process is often considered to be unstable and sensitive to environmental changes. Apart from the need to develop advanced bioreactors, close monitoring and control of the AD process has been recognized to be a necessary factor for ensuring reliable and stable operation and guarding the process against failure. One objective of the work reported in the thesis has been to investigate new analytical methods for on-line monitoring of the AD process. More specifically, the emphasis has been very much on biosensor development for analysis of intermediate fermentation products from consecutive biochemical breakdown of organic polymers to methane and carbon dioxide, as monitored by means of short-term biochemical oxygen demand (BOD). In comparison with the conventional BOD 5-day test, the biosensor was capable of estimating the BOD value rapidly (i.e. within a minute) and of providing satisfying results regarding both precision and agreement with values obtained from the 5-day test. In order to make the sensor a suitable candidate for industrial application in the field, a novel design of BOD biosensor was developed to simplify construction of the sensor and renewal of the biochemical receptor. By using a mixed culture as the bio-receptor, the sensor design allows rapid and convenient renewal of the bio-receptor on a regular basis and on-line assay of a broad range of substrates. Another aspect of the work was to investigate new control strategies for operating AD processes efficiently at high load. One successful implementation was a control system consisting of a cascade controller that was embedded into a rule-based supervisory system, based on a new principle of probing control. The control system was not only able to respond rapidly to process imbalance, but also to schedule control tasks according to different time scales. Good control performance was achieved during start-up and steady-state running operations, and also rejection of disturbances. The controller was capable of compensating for the negative effects of different types of underload and overload, as well as the temperature variability of the reactor. Furthermore, the control system attempted to steer the reactor load progressively to a maximum value while keeping the microbial ecosystem in balance. This control strategy can be considered to be an interesting and promising approach for achievement of an economically feasible anaerobic process.

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