The removal of bacterial biofilm from water-filled tubes using axially propagated ultrasound

Axially propagated ultrasound (APU) has been used to remove mineralized Proteus mirabilis biofilms from water-filled glass tubes. Pulses (30 s) of ultrasound at frequencies ranging from 350 to 20 kHz were applied to one end of the tubes and the extent of biofilm removal was assessed by infra-red absorptiometry. The removal of biofilm from 7 cm tubes by two 30 s pulses of APU at 150 kHz (54.8%) was comparable to thar (60.9%) achieved by sonication at 33 kHz in a conventional sonic cleaning bath. In 50 cm tubes, three 30 s pulses of APU from the 20 kHz transducer removed 87.5% of the biofilm. The equivalent results for APU at 150 kHz and 350 kHz were 66.8% and 31.3%, respectively. These observations suggest that the axial propagation of ultrasound along lengths of water-filled tubing offers the prospect of on-line cleaning of systems such as heat-exchanger pipework.