Oily Water Separator (OWS) is of utmost importance on board ship. They are used to separate oil from oily wastewater I.e., bilge water before discharging this wastewater into the environment. These waste discharge water must comply with the requirements laid out in Marpol 73/78. The International Maritime Organization (IMO) has published regulations through the Marine Environment Protection Committee (MEPC). On July 18, 2003, the MEPC issued new regulations that each vessel built after this date had to follow. Each OWS must be able to get clean bilge water under 15 ppm of type C oil or heavily emulsified oil and other contaminants also if present. Oil Water Separator must be able to flush out contaminants like lubricating oil, cleaning product, soots, fuel oil, rust, sewage, and other things that can be harmful to the ocean environment as well as oil.

Source : Internet, Courtesy : www.redsharkmarine.com


An Oily Water Separator consists of three main parts; separator, filter, and control unit. The whole arrangement is separated into two-compartment one for the separator and the other for the filter. A separator unit consists of a course separator with a catch or baffles plates to collect small oil particles with different parts such as test cock, collection chamber, drain, oil heater, riser pipe, and separating sheet to assist in this process.
The other compartment with a filtering unit then filters and removes these accumulated oils from the separator. The unit consists of two or three-stage coalescer filters; where the impurities separated are then later removed manually. On other hand, a control unit consists of two separate monitoring and control devices. They consist of a mixing pump, test chamber, source of illumination, and a controller with inputs from discharge rate, ship speed, and oil content.

Line Diagram OWS
Source: Internet, Courtesy: www.dieselduck.info
OWS Diagram
Source: Internet, Courtesy: marineexam.blogspot.com


  1. Separator Unit

The Separator unit has catch plates inside a coarse separating compartment and an oil collecting chamber. The oil which has a density lower than that of the water, which makes the former rise into the oil collecting compartment and the rest of the non-flowing oil mixture settles down in the fine settling compartment after passing the catch plates. After a period of your time, more oil will separate and collect within the oil collecting chamber. Water with oil content passing through this unit is about 100 parts per million of oil. Then a control valve releases the separated oil into the Oily Water Separator sludge tank. The heater could also be incorporated in this unit for smooth flow and separation of oil and water. The first stage helps in removing some physical impurities to attain fine filtration in the later stage.

  1. Filter Unit

This is a separate unit whose input comes from the discharge of the primary unit. This unit consists of three stages – the filter stage, coalescer stage, and collecting chamber. The impurities and particles are separated by the filter and are settled at the bottom for removal. In the second stage, coalescer induces the coalescence process during which oil droplets are joined to increase the dimensions by breaking down the surface tension between oil droplets within the mixture. These large oil molecules rise above the mixture within the collecting chamber and are removed when required. The output from this unit should be less than 15 ppm to fulfill legal discharge criteria.
If the oil content in water is over 15 ppm then maintenance work like filter cleaning or renewal of filters is to be done as needed.

  1. Oil content monitoring unit

This unit functions together in two parts – monitoring and controlling. The ppm of oil is continuously monitored by Oil Content Monitor (OCM); if the ppm is high it’ll give the alarm and feed data to the control unit. The control unit continuously monitors the output of OCM and if alarm arises, it’ll not allow the oily water to travel overboard by means of operating a 3 way solenoid valve. There are normally 3 solenoid valves commanded by the control unit. These are located within the first unit oil collecting chamber, second unit oil collecting chamber, and one in the discharge side of the oily water separator which is a 3-way valve. The 3-way valve inlet is from the Oily Water Separator discharge, where one outlet is to overboard and the second outlet is to OWS sludge tank. When OCM gives an alarm, the 3-way valve discharges oily mixture in the sludge tank.

Authored By:- Cdt. Aditee, TMI

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