Ships have carried solid ballast, in the form of rocks, sand or metal, for thousands of years. In modern times, ships use water as ballast. It is much easier to load on and off a ship, and is therefore more efficient and economical than solid ballast. When a ship is empty of cargo, it fills with ballast water. When it loads cargo, the ballast water is discharged.
Ship without cargo:
1. At source port: Loading ballast water.
2. During voyage: Cargo hold empty
Ship with cargo:
3. At destination port: Discharging ballast water.
4. During voyage: Ballast tanks empty.