What’s an oily water separator?

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An oily water separator is a device used on marine vessels to purify bilge water before it is pumped overboard. The primary way these devices extract oil is through gravity separation, although a variety of other filtration methods are often included as well. These devices have been present in new ship construction since the 1970s, and were also retrofitted to older ships. When a crew or engineer modifies an oily water separator to pump large amounts of raw bilge water into the ocean, it is often referred to as a magic pipeline. Dumping untreated bilge water overboard is a violation of international convention and may be prosecuted depending on where it occurs.

The lowest compartment present on a ship is known as the bilge, and the water that ends up on board is ultimately drained there. To preserve positive buoyancy and avoid sinking or capsizing, this water must be emptied periodically. Heavily polluted bilge water can be sucked out when a ship is docked, although 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 hydraulic system, it must first be treated.

Before the 1970s, bilge water was pumped directly into the ocean without any type of filtration. The 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 the discharged bilge water. Since these limitations were measured in parts per million (PPM), each vessel subsequently had to have an oily water separator.

A typical oil-water separator begins by exploiting the difference in specific gravity between oil and water. The oil will tend to rise to the surface of the 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 what 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 generally required to keep extensive records on the disposal of bilge water. In some cases, a crew or engineer will falsify these records and modify an oily water separator so that large amounts of untreated bilge water can be quickly dumped into the ocean. This modified system is often referred to as a magic tube, the use of which has been successfully prosecuted in the United States and other countries.

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