Fresh or salt water, sometimes containing sediments, held in tanks and cargo holds of ships to increase stability and maneuverability during transit.
Ballast water taken into a tank from one body of water and discharged in another body of water can introduce invasive species of aquatic life. The discharge of water from ballast tanks has been responsible for the introduction of species that cause environmental and economic damage.
The International convention for the control and management of ships' ballast water and sediments in 2004 tries to deal with this environmental problem, by regulating the discharge and charge of ballast water. Boats must comply with the regulations in effect and change their ballast water when they sail in high sea. The exchange should be made by high depth (more than 2000 meters) so far as possible, during the day and as far away from the littoral as possible.
To avoid stability or mechanical stress problems, there are two options for the replacement of ballast water:
- empty the whole tank and fill it,
- without emptying the tank, proceed with the filling by allowing the overflow to come out by the air outflow, in which case three times the volume of the tank has to be exchanged.
All the discharges and charges have to be logged in a register. Studies are also carried about a chemical and mechanical treatment of discharged ballast water for boats only travelling in low seas (for instance boats from Spain to the Netherlands which sail through Spanish, Portuguese, French, British, Belgian and Dutch waters without being able to discharge their ballast water in high seas).