Cavitation field analysis for an increased efficiency of ultrasonic sludge pre-treatment using a novel hydrophone system.

•A novel hydrophone system was applied to study the efficiency of sludge pre-treatment.•The cavitation noise level was measured in water and sludge using a hydrophone.•Increasing the pressure to 1 bar increased the cavitation in waste activated sludge from 5 to 18 dB.•A reduction of the reactor gap to 40 mm increased the cavitation intensity in waste activated sludge.•The efficiency of ultrasonic sludge pre-treatment can be improved by an optimized reactor design.