Optimisation of the anaerobic digestion of agricultural resources

Abstract

It is in the interest of operators of anaerobic digestion plants to maximise methane production whilst concomitantly reducing the chemical oxygen demand of the digested material. Although the production of biogas through anaerobic digestion is not a new idea, commercial anaerobic digestion processes are often operated at well below their optimal performance due to a variety of factors. This paper reviews current optimisation techniques associated with anaerobic digestion and suggests possible areas where improvements could be made, including the basic design considerations of a single or multi-stage reactor configuration, the type, power and duration of the mixing regime and the retention of active microbial biomass within the reactor. Optimisation of environmental conditions within the digester such as temperature, pH, buffering capacity and fatty acid concentrations is also discussed. The methane-producing potential of various agriculturally sourced feedstocks has been examined, as has the advantages of co-digestion to improve carbon-to-nitrogen ratios and the use of pre-treatments and additives to improve hydrolysis rates or supplement essential nutrients which may be limiting. However, perhaps the greatest shortfall in biogas production is the lack of reliable sensory equipment to monitor key parameters and suitable, parallelised control systems to ensure that the process continually operates at optimal performance. Modern techniques such as software sensors and powerful, flexible controllers are capable of solving these problems. A direct comparison can be made here with, for instance, oil refineries where a more mature technology uses continuous in situ monitoring and associated feedback procedures to routinely deliver continuous, optimal performance.