Abstract: For treatment of ballast water, on uptake of ballast water into a ship, equilibrium peracetic acid and catalase are added to the ballast water, wherein equilibrium peracetic acid is added in an amount of from 5 to 50 mg/l peracetic acid and catalase is added in an amount which within 120 h breaks down the content of the hydrogen peroxide introduced with the equilibrium peracetic acid to less than 2 mg/l in the ballast water. For treating the ballast water, a device is suitable that comprises a ballast water tank, a conduit for filling the ballast water tank and a conduit for emptying the ballast water tank, wherein metering devices for equilibrium peracetic acid and catalase are connected to the conduit for filling the ballast water tank, and a metering device for a reducing agent is connected to the conduit for emptying the ballast water tank.