Abstract: A device for treating aqueous liquids is disclosed comprising at least 2 treatment units connected in series, each treatment unit comprising a liquid inlet, a liquid outlet, a flow compartment fluidly connecting said liquid inlet to said liquid outlet, and cavitation inducing means for inducing cavitation in an aqueous liquid to be treated, said means comprising constriction means provided at the liquid inlet and/or the liquid outlet for increasing the velocity of the aqueous liquid, thereby reducing liquid pressure.