Different types of Marine Oily Water Separators

Marine Oily Water Separators


Even in the most well-maintained engine rooms, the marine bilges will gradually fill with a mixture of water, oils, and chemicals resulting from everyday activities such as maintenance, cleaning, and other sources. These other sources include the pipework of machinery that can carry hydraulic fluids, seawater, freshwater, leaking pumps and valve glands, solvents used for cleaning machinery when carrying out repairs, accidental spills, and overflowing tanks. Then there is the inevitable rust and general dirt and grime present even in the best-maintained ships. Some cleaning chemicals can act as an emulsifier, meaning that the oil in the bilges can bond with oils and greases, making later treatment more of a problem. The mix of products in the machinery space bilges can mean that the list of contaminants can be long and include some heavy metals and undesirable chemicals, although usually only in minimal amounts.

Below the engine room floor plates, many pipes serve different systems from heat exchangers, boilers, freshwater generators, and fuel lines between tanks and the engines, both main and auxiliary. Bilgewater can also accumulate in other areas of the ship, including in cargo holds and bow thrusters. At intervals, the mixture collected in the bilge wells will be pumped to a holding tank for treatment by the oily water separator (OWS). If this were not done, the bilge well would overflow into the engine room, causing a severe safety hazard. Bilgewater from cargo holds often contains nothing more than cargo residue and hold wash water but may have oil in such cases; the bilgewater will be pumped to the main bilgewater holding tank for treatment. It is recommended that there be strainers within the bilge wells to separate oil large solids that have entered the space, prevent bilgewater pumps from blockages, and, more importantly, reduce the material being sent to the oily water separators for treatment.

Before 1983, it was permissible for ships to pump bilgewater overboard directly, but with the coming into effect of MARPOL in 1983, that practice ceased. The new rule set a maximum of 15 ppm (parts per million) of oil to be permitted in overside discharges. All remaining oil and sludge were to be disposed of ashore and/or incinerated. The initial ruling set a performance standard for oily water separators that would need to be type-approved using a mix of clean water and diesel fuel alone. The requirement was changed in 2005 when a new requirement for oil water separators to be able to also deal with emulsified mixes was introduced under MEPC 107(49). This change also brought a requirement for recording and monitoring bilge water treatment by an oil content monitor.

Maintaining the bilge oily water separator has never been the easiest or most pleasant task for the engine crew to perform, but after 2005 it became even more difficult. To meet the new requirements, many new oily water separators (OWS) were introduced to the market, along with improved monitoring options for the bilge alarm.


Different types of Marine Oily Water Separators

Gravity Separation:

Early oily water separators were mostly of the gravity separation type that employs plate or filter coalescing technology to separate oil and water. The bilge water is usually heated gently to improve separation, with the oil gradually settling out above the water content. The oil is then pumped to the holding tank, and the water is discharged overboard to the sea after passing through the oil content monitor. Gravity oil water separators can have difficulty meeting the 15-ppm (parts per million) standard without further refinements, especially when the bilge water contains emulsified oils that do not separate easily.

Centrifugal Separator:


GEA Westfalia Oily Water Separator

Centrifugal separators also work using the different densities of oil and water but with the centrifuge greatly multiplying the gravity effect as the centrifuge accelerates. This type of oily water separator is more efficient and can generally deal with emulsified oils. They run continuously without significant man-hours for operation and supervision and handle varying bilgewater composition, solids loading, oil content, and the rolling and pitching motion of the ship. They are more compact than gravity-type oil water separators but have the disadvantage of requiring power to operate the centrifuge. Because of their moving parts, they often have a higher maintenance requirement. Many crew members are familiar with this type of technology and the moving parts which are also used for the treatment of fuel and lubrication oils onboard vessels.

Centrifugal oily water separators manufactured by GEA Westfalia Separator are an excellent option for new builds and retrofits. The GEA Westfalia BilgeMaster oily water separator modules are assembled to form a complete unit and can also be supplied as individual modules upon request. This means that the system can be customized to meet the local circumstances in an optimum manner, even in restricted spaces. This makes it an excellent option for replacing old Alfa Laval PureBilge oily water separators. To learn more, visit our GEA Westfalia BilgeMaster Oily Water Separator webpage.

Absorption and Adsorption:


BOSS Oily Water Separator

Absorption and adsorption are both physicochemical sorption processes that can be used to separate oil from bilgewater. Absorption is the incorporation of a substance from one physical state into another physical state (a liquid absorbed by a solid). Adsorption is the physical adherence or bonding of molecules onto the surface of another phase (reagents adsorbed from water only a solid surface). Bilgewater is pumped through the sorption media in a reactor vessel or contactor for both processes, and the oil is removed from the media. Once the capacity of the sorption media is exhausted, the reactor or contactor is removed from service, and the media is replaced. The spent media is an oily solid waste residual for all sorption processes. Certain spent media can be regenerated aboard ships, while others may be regenerated or disposed of onshore.

Organoclay is widely used to absorb oil from water. When organoclays are placed into water containing mechanically emulsified oil, greases, and large chlorinated hydrocarbons, the organophilic clay will remove these compounds by a partitioning process. Therefore, organoclay can be used to remove emulsified oil and grease, and other sparingly soluble organics. Disposal options for spent media are cement kilns, landfills, bioremediation through land farming, cement encapsulation, or incineration.

BOSS Oily Water Separators primarily utilize this technology. The oily water separator system consists of a separator unit, an organoclay media tank, an oil content monitor, and a control panel. In their second stage of the BOSS oily water separators, they use organoclay as the standard for their bulk media. This media has the advantage of being efficient and long-lasting. The bulk organoclay media is by far the lowest cost alternative for removing emulsions. Activated carbon can also be used as an option but does not last as long. To learn more, visit our BOSS Oily Water Separator webpage.

Flocculation and Coagulation:

Flocculation and coagulation use an emulsion-breaking chemical to treat emulsions after any free oil has been separated. The chemical breaks down the emulsion, and the released oil comes together to form flocks which can then be skimmed off, leaving the remaining water to go through further filtration stages. This method tends to produce large amounts of sludge and requires an outlay on the chemical reagent.


Biological treatment employs micro bacteria in a bioreactor to literally consume the organic chemicals in the oil, converting it to carbon dioxide and water. It is a slow but effective treatment for oil and emulsions and removes some of the other solvents often found in the oily bilge water. Capital outlay can be high, but operating costs are low. Care must be taken to avoid overload on the microorganisms and maintaining the operating temperature within the safe range to avoid destroying them.



Compass Oily Water Separator

Membrane technology, ultrafine filtration, and reverse osmosis are all physical means of preventing oil and other large molecules from remaining with the water that can pass through the filter barrier. They are efficient but require attention to avoid blocking the filter or membrane.

Compass Water Solutions’ Ultra-Sep Oily Water Separators utilize this technology. In their second stage of Ultra-Sep, they use their SPIR-O-LATOR® technology, providing a positive, physical barrier to break emulsions and providing continuous less than 15-ppm (or 5-ppm if required) discharge. To learn more, visit our Compass Ultra-Sep Oily Water Separator webpage.


miniBOSS Oily Water Separator

BOSS’ MiniBOSS Oily Water Separator system is an excellent option in some situations where space is limited. The MiniBOSS oily water separator system is a compact IMO MEPC 107(49) certified system utilizing easy-to-change, disposable sediment, and oil removal cartridge filters. It has a small footprint and is designed to be wall-mounted. It is designed to meet the discharge requirements on the smaller vessels that have a small amount of bilge water with a small amount of oil. To learn more, visit our MiniBOSS Oily Water Separator webpage.



We provide oil water separator equipment, consumables, spare parts, and services for coalescing filter oily water separators, centrifugal oily water separators, and oily content monitors. Failure is not an option. We are your link to IMO MEPC 107(49) compliance. To learn more, visit our Oily Water Separator webpage.


Looking for a quote? Get started below!