Enhanced anaerobic digestion of livestock waste by ultrasonication: A tool for ammonia removal and solubilization

Ultrasonication was applied to lower the ammonia level in livestock waste to enhance the anaerobic digestion performance. In simulated waste tests, in spite of an identical temperature increase, a higher ammonia removal rate was observed at lower frequency. This could be explained by the existence of athermal effects, accounting for 64% of the total ammonia removal rate. These effects originated from various convections (micro-streaming, micro-convection, shock-waves, and micro-jets), possibly caused by stable bubbles, and this indigenous mixing ability led to a negligible effect of aeration in the ultrasound assisted ammonia stripping process. In actual waste tests, an ammonia removal rate of up to 55% was achieved with a 0.77 h^-1 mass transfer rate coefficient. After ultrasonication (28 kHz, pH 11, 15 min) of livestock waste, 58% higher CH4 yield was achieved due to the decrease of ammonia concentration (28%) and enhanced solubilization (51%).