The method is for treating a liquid or slurry with an ultrasonic energy. A first rotatable member being permeable to a medium and a first vibrating device are provided. The first vibrating device and the first member have a first gap formed therebetween so that the first gap represents a first distance. A guide member aligned with the first member exerts a pressure on the medium. The guide member breaks up fiber flocculation close to the upper surface of the medium. The medium is fed between the first member and the guide member. The first vibrating device generates pulses through the first member to form imploding bubbles in the medium. The bubbles have a critical diameter prior to implosion that is greater than the first distance to prevent the bubbles from growing in the first gap to a size greater than the first distance.