When ships carry bulk cargo, they have to clean their tanks before charging new cargo. Tank cleaning might be necessary in case of new cargo or, for example, of inspection or entering in dry dock. Liquids, residues and hydrocarbon vapours are cleaned according to strict regulations related to the cargo transported and the additives used to clean the tanks.
Wash waters can’t be discharged at sea as there are polluted. Port reception facilities make the cleaning and treat the wash waters.
This activity is related to the control of pollution by noxious liquid substances carried in bulk. 250 substances listed in the Convention are forbidden to discharge at sea. Their residues are classified into different categories according to their dangerousness. The most harmful to the less ones for the ocean.