Oil and Water Separators

Oil and Water Separators, also called oily water separators, are extremely important in the prevention of oil pollution at sea, as stipulated in line with the MARPOL regulation, Annex I.  These Oil and Water Separators prevent ships from discharging oil overboard when pumping out their bilges, oil tanks, or other contaminated spaces.

An oily water separator consists of three sub-units, namely:

• A separator unit
• Filter unit
• OCM (Oil Content Monitor) & Control Unit

The schematic below shows the inner workings that make up an oil and water separator, with the image after that as another example of an actual separator. In the second image, the separator unit is to the right, the filter unit to the left, and the OCM and Control Unit on the top.

Separator Unit

The inner workings, and operation, of the separator unit, is as follows:

• The discharge of the bilge pumps flows directly into the separator unit.
• This separator unit consists of catch plates which are inside a coarse separating compartment and an oil collecting chamber.
• Here the oil, having a density which is 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 settle down into a fine settling compartment after passing between the catch plates.
• After a period, more oil will separate and collect in the oil collecting chamber. The oil content of the water which passes through this unit is around 100 parts per million of oil.
• A control valve, which can either be pneumatic or electronic, releases the separated oil in to the designated oil and water separator sludge tank.
• A heater may be incorporated in this unit for smooth flow and separation of oil and water.
• This stage helps in removing some physical impurities to achieve fine filtration in the later stage.

Filter Unit

The inner workings, and operation, of the filter unit, is as follows:

• This is a separate unit whose input comes from the discharge of the separator unit.
• This filter 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, a coalescer induces a coalescence process in which oil droplets are joined together to increase the size by breaking down the surface tension between oil droplets in the mixture.
• These larger oil molecules rise above the mixture in the collecting chamber and are removed when required.
• The output from this unit should be less than 15 ppm to fulfill the legal discharge criteria in accordance with MARPOL regulation.

OCM and Control Unit

The inner workings, and operation, of the OCM and Control unit, is as follows:

• This unit functions together in two parts, monitoring (OCM) and controlling (Controlling unit).
• The ppm of oil is continuously monitored by the OCM and if the ppm is high it will give alarm and feed data to the control unit.
• The control unit continuously monitors the output signal of OCM and if alarm arises, it will not allow the oily water to go overboard by means of operating 3-way solenoid valve.
• There are normally 3 solenoid valves commanded by a control unit. These are located in 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 OWS discharge, where one outlet is to overboard and a second outlet is to the OWS sludge tank.
• When OCM gives alarm, a 3-way valve discharges oily mixture in the sludge tank.

Legislation

There is legislation and regulations in place to prevent the discharge of oil into the ocean. This is as stipulated by the IMO in their MARPOL regulation Annex I – Oil. The fundamental concepts of this regulation are as follows:

• This annex mainly deals with oil pollution and oil tanker construction.
• It also stipulates that an accurate oil record book must be kept to monitor all oil that comes on-board the ship, and oil that leaves the ship.
• Annex I also stipulate that any discharge of water overboard must contain less than 15ppm oil.

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