A ballast water treatment apparatus including a ballast water transport line configured to transport a ballast water main stream, a plasma generation device configured to generate an activated gas from a feed gas; and a mixing device. The plasma generation device includes a feed gas inlet for a feed gas, a first electrode and a second electrode, for activating the feed gas by an electric discharge between the first electrode and the second electrode thereby generating the activated gas, a cooling fluid passage for cooling the plasma generation device, the cooling fluid passage having a cooling fluid inlet and a cooling fluid outlet, the cooling fluid passage being fed with a cooling fluid, and a combining section being configured to combine the activated gas with the cooling fluid from the cooling fluid outlet to generate an activated fluid product. The mixing device is configured to mix the activated fluid product and the ballast water main stream.