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 stabilise 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. Treatment of thickened waste activated sludge by ultrasound showed that, based on the residual turbidity and the release of soluble carbohydrate, the optimum 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 unsonicated waste activated sludges. The results showed that, with a 10 day hydraulic retention time, sonication increased the gas yield by 15%. Both digested 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 characteristics.