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Detailed Guide for Oily Water Separator

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Marine Engineer
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Oily water separator is one of the most important but perhaps least used equipment on ships. An oily water separator provides filtration to separate water and oily mixtures. But even most experienced engineers often avoid utilizing this equipment and prefer to keep it clean.

Since the document covers a broad subject, all of them are briefly mentioned in this training. You can learn about that subject in more detail from the relevant links under the topics.

Oily Water Separators

As a result of different operations on ships, unwanted water, oil, and water + oil components are formed. Although these unwanted products vary according to the ship’s design, they are usually stored in the Bilge Tank. If the bilge tank is complete, it must be discharged from the vessel.

We have several options for discharging oily water from the ship:

  • Ship to Barge
  • Ship to Shore
  • Ship to Sea

The first two methods discharge unwanted bilge water directly to the barge or shore via the bilge line ready on board.

Therefore, the separation process does not need to be done on board. In the ship-to-sea method, this bilge water is discharged into the sea. So, we have some responsibilities due to Marpol regulations.

Marpol sets the following conditions for us:

  • In what situations can we use or not use this equipment?
  • Minimum specifications that the equipment must have

Marpol informs us of many more rules and points to be considered. To read in detail, you can find the official website here.

Go to Marpol’s official website.

Oily Water Separator Working Principle

Oily bilge water separators generally consist of 2 stages. The first stage removes large oil particles directly from the system. The second stage is designed to separate smaller oil particles.

Stage 1

Consider a separation tank, this tank takes some mixture from the wise tank. Since water is denser than oil as a gravity feature, the water tends to stay at the bottom of the tank and the oil at the top. Here, cross plates are used to accelerate their separation.

When the mixture hits these plates, thanks to the channels that narrow upwards, the oil particles will merge into larger particles, and the split will take place more effectively. The oil sensing probe mounted on the top of the tank will open the valve to the sludge tank, and the oil will go towards the sludge tank. You will send the remaining water and a small fat to the 2nd stage.

Stage 2

The oily water that reaches this stage is passed through specially designed filters. The oily water mixture from stage 1 is pressurized by a pump and forced to pass through these filters.

It is designed to hold oil due to its filter feature. Thus, only water can pass through the filters. Then the water separated from the oil is ready to be discharged from the ship.

I explained the working principle briefly, but it is another big topic. So the below link, you can learn everything about ” How to work oily water separators “.

Go to >> How to works Marine Oil Water Separators

Types of Oily water separator

Gravity Separation

This type of oil separators perform separation processes by taking advantage of the density difference between oil and water. Gravity pulls the heavy water to the bottom of the tank and the oil stays on top of the tank. Usually the mixture is heated to make the separation better.

This system is not used on ships because the processing time is long and it is difficult to meet the 15ppm rule. But we can think of the same logic as the first stage of ows in ship types.

We prepared a wide topic about gravity separation. Avaliable here

Go to >> Gravity Separation tutorial

Centrifugal Separation

Although centrifugal type oil separators are generally used to separate main engine oil on ships, this technology can be used for separating bilge water. These separators, which use centrifugal force as their working principle, rotate at high speeds and separate water and oil at high rates in oily water mixtures. In order not to miss this separator type, we wanted to mention it under this topic. You can use the link below to learn more in detail.

We prepared a wide topic about Centrifugal separation. Avaliable here

Go to >> Marine Separation tutorial


It is a method generally used in oily wise water separators on ships. The oily water mixture is easily reduced below 15ppm by going through several stages. It allows us to use the oily water separated from large oil particles continuously under 5ppm with reverse osmosis, heavy filtering or spirolater technology.

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Marine Engineer
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