Apparatus and method for treating water

An apparatus and method for treating water to be supplied to a ballast tank uses a water pathway having a main inlet for connection to a body of navigable open raw water containing sediment and marine organisms, and a main outlet for connection to the vessel's ballast tank(s). A centrifugal separator is provided in the pathway and has a separator inlet for receiving the raw water. The centrifugal separator separates sediment with some water from the raw water to produce initially processed water which contains at least some marine organisms. The separator has a first outlet for returning the sediment with some water to the body of raw water and a second outlet for supplying the initially processed water to the pathway. A UV or chemical biocidal mechanism is connected to the second outlet and is in the pathway downstream of the centrifugal separator from the main inlet for receiving the initially processed water and for killing and/or altering the DNA of the marine organisms in the initially processed water to produce treated ballast water. A ballast pump is connected in the pathway, in line with the biocidal mechanism for pumping water along the pathway between the main inlet and the main outlet, and through the centrifugal separator and the biocidal mechanism.