The use of ultrasound to enhance the thermophilic digestion of waste activated sludge

Thermophilic anaerobic digestion at 55 degrees C is a process which can sta bilise savage sludge chemically and achieve pasteurisation. Treatment with ultrasound has been shown to enhance the gas yields of mesophilic digestion , but there is no information about its affect on thermophilic digestion. T reatment of thickened waste activated sludge by ultrasound showed that, bas ed on the residual turbidity and the release of soluble carbohydrate, the o ptimum dose was 1.5-3.0kJ g TS-1. Five-hundred mi digesters, which were fed daily, were used to compare the gas yields obtained from sonicated and uns onicated waste activated sludges. The results showed that, with a 10 day hy draulic retention time, sonication increased the gas yield by 15%. Both dig ested sludge and waste activated sludges which had been thickened to > 15g TSl(-1), had a distinct yield stress. Sonication reduced this significantly . This suggests that sonication could be used as a pre-treatment to enhance thermophilic digestion and as a post-treatment to improve the pumping char acteristics.