A method and device are provided for reducing the oxygen content of seawater, where the seawater is introduced into the upper part (14) of a downcomer (12), particularly for treatment of ballast water in a ballast tank (4) of a ship (1). A seawater intake pipe (6) runs from the outside of the ship to a ballast pump (8) that is designed to pump the water on through a pump pipe (10) and up to the upper part of the downcomer. The downcomer extends mainly vertically down to the ballast compartment of the ship, where the ballast water can be distributed between several ballast compartments by means of distribution headers and valves. One aim of the invention is to neutralize organisms in the ballast water.