Shipboard Oily Water Separators - How Do They Work?

Marpol regulations regarding discharge of oil in sea water are becoming stringent day by day. For this purpose Oily water seperators have become compulsory on all types of ships. OWS is used to treat the oil-water mixture from bilge spaces (read about bilge water here), oil tanks or any compartment in the ship which has accumulated water, before discharging it to the sea. According to one of the annexure of Marpol, water that has to be discharged should contain less than 15 parts per million of oil. OWS is used to attain this.

Almost all OWS use gravity system for the separation of water from oil. This method allows the system to bring down the PPM (parts per million) number to 100, which is not enough. For this purpose, all OWS are equipped with a filtration system to bring down the PPM level. The PPM is measured with an instrument which is similar in working to the Salinometer used to check salt ppm.

In this article we will learn about the working of such an oily water separator which has two units- A separator unit and a filtrating unit.

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The Separator unit itself consists of two parts. The first part consists of a coarse separating compartment with an inlet for the oil-water mixture. The oily water mixture enters the compartment through the inlet pipe.Some of the oil molecules due to their lower density will separate and rise to the top of the mixture into an oil collecting compartment.

The remaining oil leaves the compartment, from an outlet provided at the bottom of the chamber to the second part which is a fine separating compartment. This compartment has stationary catch plates arrangement fixed at the center of the compartment as shown in the figure. The oil mixture flows slowly between the catch plates. The oil from the mixture will stick to the underside of the catch plates and slowly move outwards to rise at the top of the mixture and gets collected in an oil collecting space.

The clean water which has PPM of around 100, flushes out of the compartment through a pipe attached at the center of the compartment. All the oil collecting compartments are provided with heating coils or a steam inlets for heating the oil to facilitate a free flow. The air produced is released through a vent provided at the top of the compartment. The mixture is then passed to the filteration unit.

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The water that comes out of the separating unit generally has a PPM level of 100 or less. To reduce the PPM level, it is passed to the filtrating unit. The filtrating unit also consists of two stages.

When the water enters the first stage, solid particles and physical impurities are removed with the help of filter units. These impurities settle at the bottom of the compartment and are removed later. The water then passes through the second stage for complete removal of oil. The second stage uses coalescer inserts to achieve this. Coaslescer induces coalescence process between the oil particles. Due to this process the surface tension between the oil molecules in the mixture breaks down which makes them to stick together to form larger molecules. This large oil molecules then rise to the top of the mixture to get collected in the collecting chamber.

The collecting chamber is emptied as and when require. The filter units are to be changed generally once a week. The OSW system is fixed with a PPM monitoring device which keeps a continuous check on the discharged water's PPM level. The system raises an alarm, in case the PPM level goes above 15.

According to Marpol, this PPM monitoring device is also compulsory on ship along with the OWS.

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Introduction to Marine Engineering by D.A Taylor

Image Credits

Velcon Filtration Unit

Oily Water Separator

Whole Unit OWS

OIl Water Separator: Introduction to Marine Engineering by D.A Taylor