Have you ever looked under a car bonnet at the engine? As the car gets older, more and more oil and grime coats the engine. This also happens inside a ship's engine room and other places where there is mechanical equipment.
Below the ship's engine room is a space that collects the oily waste from the ship. This is called the engine room bilge.
To prevent the bilge from filling up, which can be a fire hazard; it must be emptied almost on a daily basis. If the liquid in the bilge is pumped overboard without treatment, the sea will be polluted.
Ships are permitted to pump water from their bilges providing this is done through a special oily water filter or separator.
This avoids oil polluting the sea. The oily water separator ensures the water pumped overboard contains less than 15 parts per million of oily substance. This is such a small amount that you could not see it. Oil can only be seen in the sea when it is 50 parts per million.
Ships must not pump their bilges when sailing through designated sensitive areas like the Great Barrier Reef.
The oily waste from the bilge that remains in the ship, together with other polluting substances like chemicals used for cleaning the machinery, must be carefully removed in port so that it can be treated and disposed of in an environmentally safe way.