Abstract: A reactor configured to clean ballast water on ships includes a reactor housing having an inlet and an outlet and a reactor tube in the reactor housing providing a fluid connection between the inlet and the outlet. The reactor tube includes an inner wall having a plurality of rounded recesses extending longitudinally from a first end of the reactor tube to a second end of the reactor tube, and the rounded recesses each have a substantially constant cross section from the first end of the reactor tube to the second end of the reactor tube. A radially inwardly projecting ridge extends between each circumferentially adjacent pair of the rounded recesses.