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Oily Water Separator

oily water separator


Below the water level in the deep sea, there is another world of beautiful flora and fauna, that needs oxygen for life support. We humans, are developing rapidly from the past few decades, and parallelly also responsible for various kinds of pollution and oily waste discharge from huge ships and Industries is one of its type. Oily water separator treat and separate oil and water from bilge water. These oil from the discharge forms a layers on the water surface thus preventing exchange of oxygen to take place.

The International convention for prevention of pollution from ships (MARPOL) has its Annex 1 containing regulations for prevention of pollution by oil. It says that waste water discharge from ships should have oil content less than 15ppm  (15ml/1000 litre of water). This high level filtration achieved by an equipment known as ‘Oily water separator’. It is used to treat and seperate oil and water from bilge water.

Bilge is the lower area of a ship where all the waste water from the machinery space and accommodation gets stored from where the oily water separator separates oil. The water from the machinery contains oil leaked from diesel generator, boilers, air conditioning, propulsion engine etc.

Construction of Oily Water Separator

An oily water separator has got three units.

Separator Unit

This is the Unit where the first stage of purification is done. It is basically a container where major portion of separation is done due to gravity of density difference. It consists of catch plates, baffles and orifices that traps majority of physical impurities. The oil gets decanted on the top of the tank, the control valve gets opened and the oil is skimmed off to a sludge tank. For smooth functioning, we generally install a heater in this container. The water passing out of this contains around 100 ppm of oil.

Filter Unit

This is the unit where second stage of filteration is done. It has got two parts. First one is filter which seperates impurities settled at the bottom before elemination. The second part is responsible for coalescence of oil. It is the process in which small droplets of oil suspended in water link up to form bigger droplets and decant at the top. The control valves open and oil is collected to the sludge tank. The output water from this contains less than 15 ppm of oil.

Oil Discharge and Monitoring Control unit

Oil Discharge and Monitoring Control unit of Oily Water Separator uses light and photo sensor to monitor and control water before it is discharged into the sea. It has alarms and 3-way control valve, whose output is connected to bilge tank and discharge outlet.

  • If the oil content is lesser than 15 ppm, it is sent for discharging.
  • If the oil content is above 15 ppm, alarm rings, and it is sent back to bilge tank for re-filtration.
oily water separator construction
Image Credit: Quora

Working of Oily Water Separator

It has 3 parts working on individual principles:

1. Seperator unit works on the principle of density difference or gravity.

2. Filter unit works on the principle of coalescence.

3. Monitor and control unit works on the principle of photo senser.

The dirty oily water from the bilge gets transferred to bilge water holding tank. The bilge pump is opened to intake fresh seawater to drive the air out of bilge pump through a valve called ‘Vent valve’.

The mixture is then allowed to flow through the seperator unit where most of the physical impurities gets separted by catch plates and mixture gets decanted to oil and water. The top of the container is equipped with capistance probe (a sensor which sense the oil and activates the electrically equipped ‘solenoid valve’). Solenoid valve passes the skimmed oil to a seperate storage called ‘sludge tank’. The resultant water of this tank has almost 100 ppm of oil.

The water then flows through the filter unit where coalescence take place and further separation takes place. The clean water coming out goes to 3-way valve which is connected to ‘oil discharge monitor and control unit’. It uses light and photo senser for final testing.

-If the purity is more than 15 ppm, the valve remain in open condition and water is discharged in the sea.

-If the water does not meet the condition, the valve gets closed and it is send back to the bilge tank for re-filtration.

Maintenance of Oily Water Separator

Oily water seperator seems to be very easy equipment to work on but, in reality is a very sensitive equipment and should be operated only by experts. Usually a chief engineer works on oily water separator. It should be overalled once  every two months and filters are to be cleaned at least once every months depending on the usage and type of vessel.

Essential Points

  1. The operator should wear all the safety gears and follow the protocol given in the rule book.
  2. All parts of the machine should be approved and authenticated by concerned authorities.
  3. It must be tested and additional alarms and safety equipment must be installed.
  4. The arrangement should be such as to automatically cut off the discharge if it does not meet the criteria of Annex 1. The government is very strict all over the world regarding pollution.
  5. Oil record book must be maintained for each discharge.
oily water separator

Also Read: Lube Oil Purifier

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