Ultrasonic waves are acoustic waves with a frequency between 20 kHz and 10 MHz. Ultrasound causes cavitation effects in treated samples. During cavitation, an abundance of cavitation bubbles are produced. Stable cavitation bubbles last for hundreds of acoustic cycles and have almost equilibrium size during cavitation. Transient bubbles usually persist for only a few cycles and their bubble size may grow to at least two times their original size. Ultrasonication has been used to disintegrate biological cell structures since the 1950s. However, waste activated sludge ultrasonication did not flourished until the late 1990s. Hydroxyl radicals are generated during cavitation, which may chemically disintegrate the sludge. Meanwhile, local high temperature and high pressure caused by the cavitation effect also contribute to sludge disintegration. Ultrasonication can also cause water sonolysis whereby one water molecule is split into one hydrogen radical and one hydroxyl radical.