Tanks and holds in ships must be cleaned after their cargo has been emptied. Regulations explain how ships holds and tanks can be cleaned, where they can be cleaned and if there is waste left over, what must happen to the waste.
Cleaning the tanks in oil tankers is dangerous as there is a chance of an explosion. Fuel vapours in an empty tank are very explosive. Oil tankers always clean their tanks at sea. The waste must be stored on the ship and taken from the ship when it is in port.
The waste is processed so that it can't harm the environment.
Cargo holds need to be cleaned between different cargoes. A hold might be transporting fertiliser and then wheat. When the hold is cleaned, dust must not be allowed to spread in the air. The waste being cleaned and collected must be disposed of so it does it does not pollute the environment. The workers that enter the holds need to be trained in safety and wear the correct safety clothing.