An invention relates to cleaning of a fluid, especially cleaning of drinking water or waste water by using ultrasonic waves and a cavitation. The inventions concerns an arrangement (101) for removing a harmful substance from a fluid (102), wherein a tubular space (103) has an inlet (104) for receiving the fluid and an outlet (105) for ejecting the fluid. The arrangement comprises a cavitation element (106) that is configured to rotate inside the tubular space to cause a cavitation in the fluid, the cavitation element tapering towards its top (109) so that a bottom (110) of the cavitation element is wider than the top and an axel intended for rotating the cavitation element is located at a line (111) that penetrates the top and the bottom's midpoint (112). The arrangement further comprises at least one acoustic power transducer (107) for generating sound waves (108), said at least one acoustic power transducer to be located such that the sound waves are targeted to the fluid under an influence of cavitation, and a whirl element (113) for driving the fluid inside the tubular space to a spiral movement (114) towards the top of the cavitation element and the cavitation element's surface from the top to the bottom, the spiral movement contributing the breaking of the molecules.
