The invention relates to a device (1) for purifying water comprising: a reactor housing (2) having an inlet opening (13) for suppliying polluted water and an outlet opening (4) for draining the purified water from the reactor housing (2) hydrodynamic cavitation means (7) arranged in the reactor housing (2) for causing cavitation (10) in the polluted water main light means ( 9 ) for irradiating the polluted water with ultravioler light wherin the main light means provide a polychromatic and/or monochromatic pulsed ultraviolet light which is synchronized selectively with the specific cavitation event (s) in the polluted water by using senser (11) and controller (12).
