The Application of Ultrasound as a Pre-Treatment for Anaerobic Digestion

Abstract

The sonication of sewage sludge can be used as a pre-treatment to anaerobic digestion. The anaerobic digestion process has the potential to become more efficient through the break-up and solubilisation of solid sludge particles. Here, the performance of anaerobic digestion was compared when fed with unsonicated sludge and sludges sonicated at two different intensities. The tests were run as a series of batch flasks and three 100 litre anaerobic digesters. The data from the batch tests clearly indicated the positive effect on anaerobic digestion through gas production (increase by 15%) and volatile fatty acid production. The data from the anaerobic digesters was less distinct. Little difference was observed between the volumes of gas produced, however there was an increase in methane production, volatile solids reduction and the soluble COD by up to 6%, 5.5% and 15% respectively when a comparison was made between sonicated and unsonicated sludges. However, sonication was observed to have a detrimental effect on the sludge dewaterability. This effect was more acute when a higher sonication intensity was used, however anaerobic digestion did reduce this effect. Therefore, while sonication can enhance anaerobic digestion, the negative effect on the sludge properties must be considered before applying this technique to any site where dewatering processes are used. The two different sonication intensities indicated that there was a minimal difference in performance. Therefore, the application of low intensity sonication can allow both process and economic benefits.