An oily water separator is a device that is used on marine vessels to purify bilge water prior to pumping it overboard. The primary way that these devices extract oil is through gravity separation, though a variety of other filtration methods are often included as well. These devices have been present in new ship construction since the 1970s, and are also retrofitted into older vessels. When a crew or engineer modifies an oily water separator in order to pump large quantities of untreated bilge water into the ocean, it is often referred to as a magic pipe. Dumping untreated bilge water overboard is a violation of international convention, and may be prosecuted depending on where it occurs.
The lowest compartment present in a ship is referred to as the bilge, and any water that ends up on board eventually drains down there. In order to preserve positive buoyancy and avoid sinking or capsizing, this water must be periodically emptied. Heavily contaminated bilge water may be suctioned out when a vessel is docked, though in other cases it is pumped directly into the ocean. Since bilge water is often contaminated with a variety of undesirable compounds, including oil from machinery such as the engine or hydraulics, it must first be treated.
Prior to the 1970s, bilge water was typically pumped directly into the ocean without undergoing any filtration. Marine pollution conventions in 1973 and 1978 (MARPOL 73/78) addressed a variety of different ways that ships pollute, including oily bilge water, garbage, and sewage. Regulations were created that limited the amount of oil that could be present in discharged bilge water. Since these limitations were measured in parts per million (PPM), each vessel was subsequently required to have an oily water separator.
A typical oily water separator begins by exploiting the difference in specific gravity between oil and water. Oil will tend to rise to the surface of water, leaving a limited amount of globules in suspension. This often reduces the amount of oil in the water to about 100 PPM, which is about ten times more than is allowed. A variety of other filtration systems are then used to reduce the amount of oil to an acceptable level.
Due to MARPOL regulations, ships are typically required to keep extensive logs regarding the disposal of bilge water. In some cases, a crew or an engineer will falsify these logs and modify an oily water separator so that large amounts of untreated bilge water can quickly be dumped into the ocean. This modified system is often referred to a magic pipe, the use of which has been prosecuted successfully in the United States and other countries.