Process for preventing contamination of the aquatic environment with organisms from ballast water

A process for treating ballast sea water capable of contaminating navigable waters with organisms of zooplankton including epifauna thereof and phytoplankton including resting stages thereof (cysts) which are taken up with ballast water of ships and transferred to another locality when the ballast water is discharged, comprising

• adding an effective amount of an agent containing at least one peroxygen compound and performic acid to the ballast water,
• allowing the agent to react until the organisms are substantially destroyed, and
• discharging the ballast water thus treated into an aquatic environment,
• wherein the agent contains, as the at least one peroxygen compound, a water-soluble percarboxylic acid with 2 to 6 carbon atoms and which is added in an amount corresponding to 0.1 to 200 mg of percarboxylic acid per liter of ballast water.