What you need to know about Oily Water Separator (OWS)

The effective cleaning of engine room bilge water is essential to protect the environment and ensure that the ship meets its regulatory obligations. The aim is to remove all of the pollutants from the engine room bilge water, so that only clean water goes into the sea.

The condensate and leakage water collected in the engine room bilges is a mixture of seawater, freshwater, heavy fuel oil, lube oil and other residues.

Although the ways in which various bilge separators work get put in separate categories, in reality of the same processes go on to varying degrees in most types of separators. These ways are:

      • Coalescence – When particles or oil droplets “coalesce” it means that they come together and form one larger droplet which is more easy to separate from water;
      • Flocculation – is a process by which two or more particles aggregate (stick together) without losing their individual boundaries. This can be achieved by using chemical dosing to raise pH as high alkaline promotes flocculation;
      • Gravity – is the process where heavy particles, sludge and dirt, sink to the bottom whilst lighter fractions (oil and scum) float to the top;
      • Centrifugal Force.

The bilge water separators, generally combine these mentioned ways in stages. For example, The TURBULO – MPB Oily water separator operates as a pressure system. The system functions according to the principle of  gravitation supported by oleophilic coalescer inserts called HEC (High Efficiency Coalescer) in the first stage. These coalescer inserts are corrosion resistant and offer a very large surface area at a high free volume. The oily water mixture is passed through the separator by means of a dedicated pump mounted on the first stage. The separated oil is drained out of the collecting space automatically by means of a level control. If the separator is required to process heavy oil, a heating coil is installed in the oil collecting space to support the operation.

Example of coalescer filters inside OWS

The second ‘breaking’ stage utilises mechanically working ‘Hyca Sep’ elements (Hydro Carbon Separation) to separate mechanical emulsions in accordance with IMO-Resolution MEPC 107(49). The ‘Hyca Sep’ elements function by the principle of coalescence.

The bilge separator operates automatically and discharges water overboard or back to the bilge water holding tank depending on the oil content of the discharged liquid and separated oil to the waste oil drain tank. Bilge water is drawn from the bilge main by the attached pump and into the bilge separator where it passes, usually through a two-stage separation process. The separator uses the difference in density and surface tension between oil and water in usually two stages that are housed separately or in the same compartment.

The separator is initially filled with clean water before admitting bilge water.

OWS filled with fresh water after maintenance

The pump supplies the oil water mixture to the first stage where most of the oil is retained. Oil droplets are attracted to the coalescer surface or gravity plates, forming into increasingly larger drops until they float. The coalescer has a very large open pore surface area and a very low pressure loss and is stable against suspended matter found in bilge water, hence these particles have no detrimental effect on the coalescer. This means that the coalescer will still continue to operate effectively even with considerable fouling.
Following separation in the first stage, the water, now with a very low oil content, is passed into the second stage chamber, which contains, usually, a second coalescer filter to separate out any remaining oil particles, leaving water that may now be discharged overboard.
A conductive oil/air sensing probe at the top of the first stage (HEC) chamber constantly monitors the oil level in the separator, the length of the probe’s electrode determining the operating range. When oil (or air) is detected, the valve to the oil drain tank opens and the valve to the second stage chamber closes and the oil is discharged to the oil drain tank. The supply pump remains
running during the oil discharge. When most of the oil has been displaced, the oil sensing probe is again immersed in water and activates the control system to resume the separating operation.
The separator works automatically and will operate as long as there is water in the bilge water holding tank. Heating may be applied to improve separation, but the heater will only operate when the separator is full of liquid. The separator is fitted with sampling valves which allow oil samples to be drawn and enable the oil/water interface level to be determined.

The Oil Content Discharge monitor samples the bilge water as it passes out of the separator. Should the oil content exceed maximum 15ppm, the three-way valve changes the output flow from the overboard discharge to discharge to the bilge water holding tank. An audible alarm sounds to warn the operator of the alarm condition. The 15ppm device setting can be adjusted from 1ppm up to the maximum 15ppm, but cannot be set higher. The monitor sensing element may be, normally flushed through with fresh water when in operation by moving the supply lever from the SAMPLING to the FLUSHING position.
This action automatically operates the three-way valve on the discharge line and returns the water to the bilge holding tank. Nowadays, the monitor contains a memory card recording the monitor readings for a period of 18 months, after which the data is automatically overwritten. The card is not to be removed from the instrument as it records the following information:

      • Time;
      • Date;
      • Oil content greater than 15 ppm;
      • Separator status

Example of an oil monitoring device

The oil content monitor must be checked each month and must be flushed through in order to remove any debris which could influence the reading.

The maximum flow capacity should not be exceeded, as excess flow will prevent effective separation. The bilge pump suction strainer should be kept clean in order to avoid large solid particles entering the separator, as these will have a detrimental effect on the separation process.

It is important to notice that the oily water separator is designed to separate oil from water, not water from oil. Therefore, if the bilge water supply to the separator contains excessive amounts of oil it will render the equipment inoperable and result in unnecessary maintenance.

Same, if the separator uses flocculation chemicals, great  care must be taken when handling the treatment chemicals, as these substances are caustic and can cause chemical burning on contact with skin and will cause severe damage to eyes. The appropriate protective clothing, including eye protection, must be worn when handling the chemicals.

When operating the oily bilge water separator and the overboard oil monitoring
system, the date, quantity and location of the discharge overboard is to be recorded in the Oil Record Book. All pumping operations and discharges are also to be in accordance with the latest MARPOL Regulations, Annex I, Regulations 9, 10, 11 and 16.

Example of an Oil Record Book

The date, operational code and item number needs to be written in appropriate columns and the required particulars should be recorded chronologically in the blank space. For discharges overboard, the ship’s position at the start and end of the discharge should be entered. Each completed operation shall be signed for and dated by the officer or officers in charge of the operation and each completed page must be countersigned by the master of the vessel.

Example on how to record the operation in ORB

Failure to fill in the oil record book in a proper way has led to those onboard and companies being prosecuted.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.

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Source and Bibliography:

  • You tube video source and credit: Marine NAV & TECH and Victor Marine

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