Enhanced anaerobic digestion by ultrasonic pretreatment of organic residues for energy production

This study is aimed at assessing the improvement of anaerobic digestion yields produced by the ultrasonic pretreatment of solid organic substrates. The possibility of enhancing methane production from anaerobic digestion of organic residues is particularly interesting. To this end, application of pretreatments represents one of the most suitable options. Among pretreatments, sonolysis arises as a novel technology, whose effectiveness is related to the occurrence of cavitation phenomena. They promote the solubilisation of organic matter, thus increasing the amount of substrate that can be converted into methane. The application of ultrasound as pretreatment of sewage sludge for biological processing has been extensively studied and full-scale facilities have already been established. However, ultrasonic pretreatment of solid organic substrates has not yet been actioned as its effectiveness is highly influenced by process conditions. Experimental results show that ultrasonic energy inputs in the range 31e93 W h/L enhanced biogas production from differently composed substrates up to 71%. The highest increase was found for lignocellulosic-based materials and was related to improvements in solubilisation. Conversely, a lower biogas enhancement, in the range 3e23%, was found for protein-rich substrates. In this case, any relevant variation in soluble COD was observed after ultrasonic pretreatment. Thus, dependent upon the specific substrate composition, the relationship between solubilisation and anaerobic biodegradability was found to differ significantly and this evidence represents a key point for scaling up of the combined ultrasonic/anaerobic digestion process.