Oily Water Separator (OWS): Working Principle, Components, Operation and MARPOL Regulations
An Oily Water Separator (OWS) is an essential piece of marine engineering equipment used onboard ships to separate oil from oily bilge water before any permitted discharge into the sea.
During normal engine room operations, bilge water accumulates from machinery leaks, condensation, equipment drains and cleaning activities. This water may contain fuel oil, lubricating oil and other contaminants that can cause marine pollution.
An OWS helps control this pollution by treating the oily mixture, monitoring its oil content and preventing non-compliant discharge.
For marine engineering cadets and seafarers, understanding the OWS is important for onboard operations, examinations and compliance with the International Convention for the Prevention of Pollution from Ships (MARPOL).
What Is an Oily Water Separator?
An OWS is a pollution-prevention system designed to separate oil from water collected in a ship's machinery-space bilges.
Key points to remember:
OWS: Oily Water Separator.
Primary function: Remove oil from oily bilge water.
Common designation: 15 PPM Bilge Separator.
Separation methods: Gravity separation, filtration and coalescence, depending on the system design.
Monitoring: A 15 PPM Bilge Alarm monitors the oil content of treated water.
Pollution control: An automatic stopping device prevents non-compliant effluent from being discharged overboard where required.
What Does 15 PPM Mean?
PPM stands for parts per million.
15 PPM means 15 parts of oil per million parts of water by volume.
It is the applicable maximum oil-content limit for relevant effluent under MARPOL requirements.
The oil content must be measured without dilution.
Why Is an OWS Required on Ships?
Oily bilge water is generated during routine ship operations and cannot simply be discharged into the sea without meeting the applicable requirements.
Common sources of bilge water include:
Condensation: Water from coolers, intercoolers and other machinery.
Machinery drains: Drains from boilers, air compressors and oil purification equipment.
Engine room cleaning: Wash-down water containing oil and dirt.
Leakages: Oil and water from pipes, pumps, glands and machinery.
Other contaminants: Fuel oil, lubricating oil, soot, cleaning agents and suspended solids.
An OWS helps by:
Separating oil from the bilge-water mixture.
Collecting separated oil for appropriate disposal.
Monitoring the oil content of treated water.
Preventing excessive oil discharge into the marine environment.
Supporting compliance with applicable pollution-prevention regulations.
MARPOL Annex I Regulations for OWS
MARPOL Annex I establishes requirements for the prevention of pollution by oil from ships.
Important regulatory points:
Ships of 400 gross tonnage and above are generally required to carry approved oil-filtering equipment, subject to applicable exceptions.
The oil content of relevant effluent must not exceed 15 PPM.
An appropriate oil-content alarm must indicate when the prescribed limit cannot be maintained.
An automatic stopping device must prevent overboard discharge when the oil content exceeds 15 PPM, where required.
Relevant machinery-space bilge operations must be recorded in the Oil Record Book as applicable.
Discharge must satisfy all applicable operational and geographical restrictions.
Discharge of oil or oily mixtures into the sea is prohibited in the Antarctic area under MARPOL Annex I.
Meeting the 15 PPM limit does not automatically make a discharge legal. The vessel must also satisfy the applicable conditions regarding its operation, location and equipment.
For further information, refer to the International Maritime Organization's MARPOL guidance.
Working Principle of an Oily Water Separator
A conventional two-stage OWS works on the principle of gravity separation followed by filtration and coalescence.
Stage 1: Gravity Separation
Oily bilge water enters the first-stage chamber.
Oil is generally less dense than water and rises towards the surface.
Baffle plates help control the flow and support separation.
Free oil accumulates in the upper oil collection space.
The collected oil is drained into the designated oil tank.
Some smaller oil droplets remain in the water and pass into the second stage.
Stage 2: Filtration and Coalescence
Partially treated water enters the second-stage chamber.
Filter elements remove suspended solids and assist oil separation.
The coalescing element combines small oil droplets into larger droplets.
The larger droplets rise towards the oil collection space.
Treated water passes through the oil-content monitoring arrangement.
If the monitored oil content exceeds 15 PPM, the alarm and automatic stopping arrangement respond as required to prevent non-compliant discharge.
OWS process flow:
Oily bilge water → Gravity separation → Filtration and coalescence → 15 PPM monitoring → Permitted discharge or diversion to the holding tank.
Main Components of an OWS
Component | Function |
Feed pump | Transfers oily bilge water into the separator. |
First-stage chamber | Separates free oil through gravity. |
Baffle plates | Guide the flow and support separation. |
Filter elements | Remove suspended particles and assist oil removal. |
Coalescing filter | Combines small oil droplets into larger droplets. |
Oil collection space | Collects separated oil. |
Oil drain valve | Transfers collected oil to the designated tank. |
15 PPM Bilge Alarm | Monitors the oil content of treated water. |
Automatic stopping device | Diverts non-compliant effluent away from overboard discharge where required. |
Step-by-Step OWS Operation
The exact operating procedure varies by manufacturer and equipment design. The following describes the general sequence for a conventional OWS.
Step 1: Prepare the system
Inspect the separator, feed pump, piping, valves and filters.
Check the alarm and automatic stopping arrangement.
Confirm that the holding-tank arrangements are ready.
Step 2: Fill and vent the separator
Introduce clean water as specified in the operating manual.
Remove trapped air through the designated vent.
Close the vent according to the approved procedure.
Step 3: Start the feed pump
Start the designated pump.
Introduce oily bilge water at the prescribed flow rate.
Step 4: Separate oil by gravity
Allow free oil droplets to rise in the first chamber.
Collect and drain the separated oil as required.
Step 5: Filter the water
Pass partially treated water through the filter and coalescing elements.
Remove suspended particles and separate the remaining oil droplets.
Step 6: Monitor the effluent
Pass the treated water through the approved sampling arrangement.
Monitor its oil content using the 15 PPM Bilge Alarm.
Step 7: Control the discharge
Discharge overboard only when the equipment is operating correctly and all applicable discharge conditions are met.
If the oil content exceeds 15 PPM, the automatic stopping arrangement must prevent overboard discharge as required.
Step 8: Complete the records
Record the required details of the operation in the Oil Record Book.
Follow the vessel's shutdown and post-operation procedures.
Factors Affecting OWS Separation Efficiency
Several factors influence how efficiently oil separates from water.
Density difference: A suitable difference between the densities of oil and water supports gravity separation.
Temperature: Affects the density and viscosity of the fluids. Heating may improve separation where provided and permitted by the equipment design.
Oil droplet size: Larger droplets generally rise more readily than smaller droplets.
Viscosity: High-viscosity oil can be more difficult to separate.
Emulsification: Detergents and cleaning agents may create stable oil-water emulsions that are difficult to separate.
Filter condition: Clogged or deteriorated filter elements can reduce separation performance.
When an OWS struggles to maintain the required oil-content limit, investigate the bilge-water condition, flow rate, filters and monitoring system according to the approved troubleshooting procedure.
15 PPM Bilge Alarm and Automatic Stopping Device
The 15 PPM Bilge Alarm is a critical part of the OWS pollution-control system.
Main Functions
Measures the oil content of treated water.
Activates an audible and visual alarm when required.
Signals the automatic stopping or diversion arrangement.
Helps prevent excessive oil discharge into the sea.
Records relevant alarm and operating information where required.
Depending on the equipment design, oil-content monitoring may use optical or other approved measurement technology.
What Happens When Oil Content Exceeds 15 PPM?
The monitoring system detects excessive oil content.
The alarm activates.
The automatic stopping device prevents the non-compliant effluent from reaching the overboard line where required.
A three-way valve or equivalent arrangement diverts the flow to the designated bilge or holding tank.
For equipment approved under IMO Resolution MEPC.107(49), the specified 15 PPM Bilge Alarm response time must not exceed five seconds, with the required recording and data-storage functions.
Never bypass or manipulate the alarm or automatic stopping device to allow an unauthorized discharge.
Regular maintenance is essential for reliable operation and pollution prevention.
Essential Maintenance Checklist
Inspect the separator and associated piping for leaks or damage.
Check filters and coalescing elements for clogging or deterioration.
Clean or replace filter elements according to the manufacturer's instructions.
Check the oil-content monitor and alarm system.
Verify the operation of the automatic stopping device and three-way valve.
Ensure the sampling arrangement provides a representative sample.
Arrange calibration and servicing by authorized personnel as required.
Keep operating and maintenance manuals available onboard.
Ensure engine-room personnel are familiar with the equipment and approved procedures.
Common OWS Problems
Problem | Possible cause or action |
Oil content remains above 15 PPM | Investigate emulsification, flow conditions and filter condition. |
Frequent alarms | Check the actual oil content, sampling arrangement and monitoring equipment. |
Poor separation | Inspect filters, coalescing elements and operating conditions. |
Automatic valve fails to divert flow | Treat as a pollution-prevention fault and follow the approved response procedure. |
If the equipment is defective, do not discharge overboard unless the system and all applicable requirements permit it. Follow the vessel's procedures for reporting and correcting the fault.
Oil Record Book (ORB) Requirements
The Oil Record Book provides an official record of applicable oil-related operations onboard.
For ships required to maintain Oil Record Book Part I, relevant machinery-space operations must be recorded according to MARPOL requirements.
Important Points for ORB Entries
Record the quantity of bilge water discharged, transferred or disposed of, as applicable.
Enter the required start and stop times.
Specify the method of disposal, such as approved oil-filtering equipment or a shore reception facility.
Record relevant equipment failures as required.
Complete entries accurately, in chronological order and within the required time.
Ensure the appropriate officers complete the required signatures.
During a Port State Control inspection, officers may compare the OWS monitoring data with the Oil Record Book entries.
Inconsistent records or missing information can raise serious compliance concerns. Accurate documentation is therefore as important as correct equipment operation.
Consequences of Improper OWS Operation
Incorrect operation, equipment failure or deliberate bypassing of the OWS can have serious consequences.
Marine pollution: Oil can contaminate seawater and harm marine life.
Regulatory penalties: Violations can result in fines and other enforcement action.
Vessel detention: Serious deficiencies may lead to detention during inspections.
Professional consequences: Responsible personnel may face disciplinary or certificate-related action, depending on the circumstances.
Operational disruption: Repairs, investigations and inspections can delay vessel operations.
Correct operation, proper maintenance and accurate records help protect the marine environment and maintain regulatory compliance.
Important OWS Questions for Marine Engineering Students
For examinations, oral assessments and onboard familiarisation, remember these key points:
What is OWS? Oily Water Separator.
What is 15 PPM? The applicable maximum oil-content limit for relevant effluent.
Which MARPOL annex covers oil pollution? MARPOL Annex I.
What is the first stage of a conventional OWS? Gravity separation.
What is coalescence? The combining of small oil droplets into larger droplets.
What does the 15 PPM Bilge Alarm do? Monitors oil content and activates an alarm when required.
What is the function of the automatic stopping device? Prevents non-compliant effluent from being discharged overboard where required.
Why is the Oil Record Book important? It documents applicable oil-related operations and disposal activities.
Why is regular maintenance necessary? To maintain separation efficiency, reliable monitoring and effective pollution prevention.
Frequently Asked Questions (FAQs)
1. What is the main purpose of an OWS on a ship?
To separate oil from oily bilge water and help prevent marine pollution.
2. What does 15 PPM mean in an OWS?
It means 15 parts of oil per million parts of water by volume.
3. What is the working principle of an OWS?
A conventional two-stage OWS uses gravity separation followed by filtration and coalescence.
4. What happens if the oil content exceeds 15 PPM?
The alarm activates, and the automatic stopping arrangement prevents overboard discharge as required.
5. Why is a coalescing filter used?
It combines small oil droplets into larger droplets, making separation easier.
6. Can bilge water below 15 PPM always be discharged into the sea?
No. All applicable MARPOL discharge conditions must also be satisfied.
7. Why is the Oil Record Book important?
It provides an official record of applicable oil-related operations and helps demonstrate compliance during inspections.
Conclusion: The Role of OWS in Cleaner Seas
The Oily Water Separator is an essential pollution-prevention system in a ship's engine room. By combining oil-water separation, filtration, monitoring and discharge control, it helps prevent oil pollution and supports compliance with MARPOL Annex I.
For marine engineers, understanding the OWS means knowing more than its components. It also requires correct operation, routine maintenance, effective troubleshooting and accurate recordkeeping.
Safe machinery. Responsible discharge. Cleaner seas
Disclaimer: This article is for educational purposes only and is not personalised financial, investment, insurance, or tax advice. All investments involve risks..
