Below the engine on ships is a contained space called a bilge. The bilge is like a giant bath collecting oil, water, dirt and anything else that might spill or leak from the machinery in a ship’s engine room. To prevent the bilge from filling up, which can be a fire hazard, it must be emptied. 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.