Oil pollution is one of the most significant concerns for seafarers and ship managers. Ships generate oil-water mixtures daily, which must be separated to comply with environmental regulations before discharging water overboard. MARPOL Annex I sets strict guidelines, limiting the oil content in bilge water discharged into the sea to less than 15 parts per million (ppm).
To meet these regulations, vessels are required to have an oil discharge monitoring and control system equipped with an Oily Water Separator (OWS). This equipment is crucial for preventing pollution and is frequently inspected by Port State Control (PSC) officers and surveyors. Ship engineers often encounter various OWS models during their careers, highlighting the importance of understanding their design and operation. The primary function of an OWS is to separate oil from water, ensuring the treated water discharged from engine rooms, bilges, or oil-contaminated spaces meets regulatory standards.
MARPOL Regulations for Oily Water Separators
Under Annex I of MARPOL, Regulation 4 explicitly prohibits the direct discharge of oil or oily water mixtures into the sea. The regulation provides conditions under which treated oily water may be discharged:
Oil Water Discharge Conditions (For Vessels 400 GT and Above):
- The vessel must be en route.
- The oily mixture must pass through an OWS compliant with Regulation 14.
- The treated effluent must not exceed 15 ppm of oil content without dilution.
- Discharge must not originate from cargo pump-room bilges on oil tankers.
- Oil-water mixtures must not be combined with cargo residues in tankers.
- Discharge of oil or oily water mixtures is strictly prohibited in Antarctic waters.
Key Requirements for Oily Water Separators
To ensure compliance and safe operations, the following requirements for OWS systems must be adhered to:
- Certified Monitoring Equipment: The bilge alarm or Oil Content Monitor (OCM) must comply with MEPC 107(49) and be certified by an authorized organization.
- Tamper-Proof Design: The OCM must prevent unauthorized adjustments and activate alarms if freshwater is used for cleaning or zeroing.
- Separation Standards: The OWS must consistently achieve a discharge limit of 15 ppm, even with challenging type C emulsions.
- Sensor and Alarm Placement: Sensors and alarms should be positioned to ensure reliable monitoring, even during periods without constant oversight.
- Authorized Operation: Only trained officers, supervised by the Chief Engineer, should operate the OWS. The Chief Engineer is also responsible for locking out the equipment when not in use and safeguarding the keys.
- Readiness and Maintenance: It is illegal to sail without a functional OWS. Ships must carry adequate spare parts to ensure the system remains operational.
By adhering to these regulations and maintaining proper OWS operations, ship operators can ensure compliance with international laws, prevent oil pollution, and contribute to the protection of marine environments.
Construction and Operation of Oily Water Separator (OWS)
An Oily Water Separator (OWS) comprises three primary segments, each playing a critical role in achieving effective separation of oil from water. Below is an overview of each segment and its functionality:
1. Separator Unit
- Design and Functionality:
The separator unit contains catch plates within a coarse separating compartment and an oil-collecting chamber. Due to its lower density, oil rises into the oil-collecting chamber, while the remaining oil-water mixture flows into a fine settling compartment after passing through the catch plates. - Efficiency of Separation:
Over time, additional oil separates and collects in the chamber. Water exiting this stage typically contains about 100 ppm of oil. A control valve (pneumatic or electronic) directs the separated oil into the designated OWS sludge tank. - Heater Integration:
A heater, installed in the middle or bottom of the unit, facilitates smoother oil and water separation by maintaining optimal flow properties. Heater placement depends on the separator's operational capacity and area of use. - Initial Filtration:
This stage also removes some physical impurities to enhance the performance of the subsequent filtration stages.
2. Filter Unit
- Structure and Components:
The filter unit receives input from the separator unit and operates in three stages: - Filtration Stage: Removes impurities and particles, which settle at the bottom for removal.
- Coalescer Stage: Induces coalescence, merging smaller oil droplets into larger ones by reducing surface tension.
- Collecting Chamber: Larger oil molecules rise and are removed as needed.
- Output Quality:
This unit ensures the effluent contains less than 15 ppm of oil to meet legal discharge requirements. If oil content exceeds this threshold, maintenance such as filter cleaning or replacement is necessary. - Maintenance Features:
A freshwater inlet connection is provided for cleaning and flushing the filter. This process is typically performed before and after operating the OWS.
By combining efficient design and robust functionality, the separator and filter units work cohesively to ensure effective oil removal from bilge water, meeting stringent regulatory standards and supporting environmental compliance. Regular maintenance and adherence to operational best practices are essential for optimal performance.
3. Oil Content Monitor and Control Unit
The Oil Content Monitor (OCM) and Control Unit play an integral role in the operation of an Oily Water Separator (OWS). Their main functions are as follows:
Key Functions:
- Monitoring:
The OCM continuously monitors the oil content in the effluent. If the oil content exceeds permissible levels, the OCM triggers an alarm and sends data to the control unit. - Controlling:
The control unit responds to OCM signals by operating a three-way solenoid valve to redirect the effluent. When an alarm arises: - Effluent discharge overboard is stopped.
- The oily mixture is redirected to the OWS sludge tank for safe storage.
- Solenoid Valves:
Three solenoid valves are typically commanded by the control unit:
1. Located in the first unit\u2019s oil collecting chamber.
2. Located in the second unit\u2019s oil collecting chamber.
3. A three-way valve on the OWS discharge, directing the flow either overboard or to the sludge tank.
- Freshwater Flushing:
The OCM has a freshwater connection for flushing. When this line is in use, an alarm is sounded and recorded in the OCM log to ensure compliance and confirm that the discharge valve remains closed.
Crew Training and Operation:
OWS systems are primarily operated by the Chief Engineer, who ensures maintenance and oversees proper use. Due to typically low training levels for other crew members, ship operators should incorporate OWS operation and safety into onboard training programs.
Factors Affecting Oil Separation from Bilge Water
Several factors influence the efficiency of oil-water separation in OWS systems:
- Density Differences:
Water has a higher density than oil, causing oil to rise to the surface. Seawater\u2019s higher density compared to freshwater enhances separation rates. - Fluid Viscosity:
Lower viscosity in the continuous fluid (e.g., freshwater) allows oil to separate more easily. Dense, viscous fluids hinder oil movement. - Temperature:
- Higher temperatures reduce viscosity and improve separation.
- Lower temperatures increase fluid viscosity, reducing efficiency.
- Particle Size:
Larger oil particles separate more effectively than smaller ones, which may remain suspended in the mixture.
Dismantling Procedure for Cleaning the OWS
Regular maintenance, including cleaning, is essential to ensure optimal OWS performance. Follow these steps to dismantle and clean the OWS:
- Preparation:
- Stop the OWS bilge pump and steam flow to heating coils.
- Shut the main overboard valve.
- Open vents for the separation and filtration chambers.
- Drainage:
- Slowly open the drain valves of each section at the bottom to remove oil and water.
- Disassembly:
- Disconnect all electric and pipe connections.
- Unscrew nuts and bolts from the top cover of the separation chamber.
- Remove and clean baffle plates with a brush and oil.
- Open the coalescer filters and inspect their condition.
- Filter Maintenance:
- Replace filters if necessary.
- Ensure all components are cleaned thoroughly.
- Reassembly and Testing:
- Assemble the system carefully, ensuring all components are securely in place.
- Fill the OWS with fresh, clean water and check for leaks.
By maintaining proper cleaning routines and addressing operational challenges, the OWS can operate effectively, ensuring compliance with environmental regulations and avoiding operational downtime.
Causes of Oily Water Separator Malfunction
1. Oil-to-Water Gravity Gradient
The OWS relies on the difference in density between oil and water for separation. Factors affecting this include:
- Oil Globule Size: Larger oil droplets separate more easily due to reduced resistance in water. Smaller droplets are harder to isolate.
- System Temperature: Higher temperatures lower water viscosity and increase the density difference between oil and water, improving separation efficiency.
- Use of Saltwater: Saltwaters higher density compared to freshwater can further enhance separation.
2. Pumping Considerations
The type and operation of pumps significantly impact OWS performance:
- Disintegration of Oil Droplets: Centrifugal pumps, often used for bilge pumping, create turbulence that breaks oil into smaller droplets, reducing separation efficiency.
- Pump Type: Positive displacement pumps (e.g., screw, reciprocating, or gear pumps) minimize turbulence and maintain larger droplet sizes.
- High Pumping Rates: Excessive flow rates cause turbulence and emulsification, hindering separation.
- Throughput and Motion: High throughput or extreme ship movements (e.g., rolling and pitching) can disrupt separation processes.
Common Malfunction Causes
- Overloading the separator with high throughput.
- Excessive turbulence from incompatible pump ratings.
- Inability to process stable emulsions or colloidal particles.
- Blockages from excessive solids or debris.
- Chemical emulsions formed by cleaning fluids with surfactants or detergents.
Mitigation Strategies
- Install recirculation systems to test the 15 ppm alarm and automated stopping mechanism without discharging overboard.
- Avoid using surfactant-based cleaning chemicals in bilge water to reduce chemically stabilized emulsions.
Maintenance of Oily Water Separator
Routine maintenance is critical to keeping an OWS in excellent operating condition and ensuring it meets effluent limitations.
Maintenance Tips
- Accessibility:
Above-ground separators are easier to maintain as all components are accessible for cleaning and replacement. Below-ground units require stricter adherence to maintenance schedules to avoid neglect. - Monthly Checks:
- Turn off the water supply to the unit.
- Open the cover and remove any collected oil, disposing of it per regulations.
- Drain water from the separator and measure the depth of residual solids.
- Record solid depths for future maintenance planning and remove solids if necessary.
- Cleaning:
- Regularly clean plates, filters, and inlet areas to prevent clogging.
- Inspect for damage and ensure the internal coating is intact to maintain performance.
By performing these routine checks and addressing potential causes of malfunction, ship operators can ensure their OWS operates efficiently, remains compliant with regulations, and minimizes the risk of environmental contamination.
Cleaning and Maintenance of OWS Media Plates
Proper cleaning of media plates is essential to maintain the efficiency of the Oily Water Separator (OWS). Follow these steps for effective cleaning:
- In-Situ Cleaning:
- Insert a low-pressure hose between the plate gaps to clean without dismantling.
- Drain debris from the plates through the hopper outlet.
- External Cleaning:
- Remove the plates from the unit and rinse them with a low-pressure hose.
- Avoid discharging rinse water onto the ground to prevent groundwater contamination.
- Sludge Removal:
- Dispose of accumulated sludge and oil as required.
- Inspect the tank for damage and repair the internal coating if necessary.
- Reassembly:
- Reinsert the plate packs in the same order and ensure they are securely attached to prevent dislodging during operation.
- Scheduled Maintenance:
- Maintenance frequency depends on the unit's usage and the volume of collected oil and debris. A skilled specialist should handle regular inspections and cleanings.
Factors Affecting Separation in Oily Water Separators
The efficiency of OWS systems is influenced by various operational and environmental factors. Understanding these factors can help optimize performance and maintain compliance with regulations.
Common Challenges
OWS systems are often underutilized or poorly maintained due to insufficient training, operational negligence, or apathy. Strict inspections and severe penalties make it vital for marine engineers to keep these systems in optimal condition.
Contaminants in Bilge Water
Bilge water contains complex mixtures of various substances:
- Oil (of different grades).
- Suspended solids, such as soot, rust, paint chips, and cargo dust.
- Chemicals and detergents that stabilize emulsions.
These contaminants pose challenges to the efficient operation of OWS systems.
Key Factors Influencing OWS Performance
- Design Factors:
- Understanding the basic working principles and marine-specific enhancements of OWS systems.
- Operational Knowledge:
- Familiarity with generic operational procedures is crucial for proper usage.
- Bilge Management:
- Efficient bilge management involves controlling what enters the bilge to minimize contamination.
- Care and Maintenance:
- Regular cleaning, inspections, and adherence to maintenance schedules are essential.
- Equipment-Specific Knowledge:
- Each OWS model has unique features and potential issues. Knowledge of the installed system is beneficial for optimal performance.
- Crew Attitude and Training:
- Proper training and a proactive approach are critical for running OWS systems effectively.
- Good Housekeeping:
- Maintaining a clean engine room and minimizing oily water production reduces the burden on the OWS.
Optimizing OWS Performance
By addressing the design, operational, and environmental factors, ship operators can ensure the effective functioning of their OWS systems. This involves:
- Adopting efficient bilge management practices.
- Providing proper training to crew members.
- Conducting regular maintenance and inspections.
Implementing these measures will help improve OWS reliability, simplify PSC inspections, and support environmental compliance.
Design Factors for Oily Water Separator (OWS)
The Oily Water Separator (OWS) is a critical component used to remove oil from bilge water before overboard discharge, ensuring compliance with environmental regulations. Its operation is based on Stokes Law, which leverages the difference in specific gravity between oil and water to facilitate separation. Below are the key design considerations and their implications for OWS performance.
Stokes Law and Separation Efficiency
According to Stokes Law, the rate of rise (velocity) of oil droplets is influenced by several factors:
- Density of Oil:
- Lighter oils rise faster than heavier oils, making separation easier.
- For heavier oil contamination, adjusting the OWS flow rate can improve performance.
- Density of Continuous Fluid:
- Seawater, being denser than freshwater, enhances the separation rate.
- Discharging condensate drains into dedicated clean drain tanks instead of bilges can help maintain optimal conditions.
- Viscosity of Continuous Fluid:
- Lower viscosity fluids (e.g., freshwater) improve the rate of rise, enhancing OWS performance.
- This creates a trade-off when working with seawater, as its higher density aids separation but its higher viscosity may hinder it.
- Size of Oil Droplets:
- Larger oil droplets separate more efficiently.
- Avoiding mechanical agitation and emulsification reduces the formation of smaller oil droplets, aiding separation.
- Temperature Effects:
- Low Temperatures:
Increased fluid viscosity at low temperatures impedes separation. Slightly warming bilge water, using steam coils or preheating holding tanks, can improve efficiency. - High Temperatures:
Excessive heating can promote emulsification, especially during ship rolling or mechanical agitation. It is important to balance temperature adjustments carefully.
Operational Considerations for OWS Performance
Space Optimization for Marine Use:
OWS units for ships are compactly designed to fit within space constraints. Additional features, such as coalescers or heating elements, are incorporated to enhance separation within limited dimensions.
Key Design Enhancements:
- Steam coils or preheating mechanisms to maintain optimal temperature conditions.
- Separation compartments and catch plates designed to reduce turbulence and support efficient droplet rise.
- Filters to remove particulate matter that could hinder oil-water separation.
Important Factors for OWS Operation on Ships
Efficient operation of OWS systems relies on an understanding of various interrelated factors, including:
- Design Knowledge:
Understanding the working principles and limitations of the installed OWS model. - Operational Practices:
Properly following operational guidelines ensures optimal performance. This includes maintaining laminar flow and avoiding conditions that promote turbulence or emulsification. - Bilge Management:
Keeping bilge water free from unnecessary contaminants, such as detergents and particulates, reduces the burden on the OWS. - Temperature Control:
Managing temperature levels in bilge water improves separation efficiency while preventing emulsification. - Maintenance and Training:
Regular cleaning and inspections, combined with crew training, ensure the OWS operates effectively and complies with regulatory requirements.
Conclusion
By leveraging Stokes Law and understanding the design and operational factors affecting OWS performance, ship operators can optimize separation efficiency and maintain regulatory compliance. Regular maintenance and thoughtful bilge management are essential for sustaining reliable operation and ensuring environmental protection.
Operational Factors for Efficient Oily Water Separator (OWS) Performance
Correct operational knowledge is critical to the effective functioning of all machinery on ships, including Oily Water Separators (OWS). Marine engineers encounter various OWS types throughout their careers, each with unique features. While specific knowledge from onboard manuals is essential, understanding general operational principles and challenges is equally important. Below are key operational factors to optimize OWS performance:
Key Operational Practices
- Avoid Emulsions:
- Emulsions form when interfacial tension between liquids is reduced, causing oil droplets to disperse in water. Factors like mechanical agitation, shearing forces, chemicals, surfactants, and particulate matter contribute to emulsions.
- Minimize Chemical Emulsions:
- Chemicals, including detergents and alkaline agents used for cleaning, act as surfactants and stabilize emulsions. Avoid introducing these chemicals into bilge water.
- Prevent Secondary Dispersion:
- Primary emulsions (larger oil droplets) separate naturally within 24 hours. However, turbulent conditions can create secondary emulsions, consisting of fine, stable oil droplets that resist separation.
- Control Suspended Solids:
- Particulate matter, such as soot, rust, and cargo residues, stabilizes emulsions. Neutrally buoyant solids, which neither rise nor settle, are particularly challenging. Remove solids to prevent high ppm alarms.
- Avoid Turbulence:
- Laminar flow is essential for optimal OWS operation. Heavy rolling, retrofitted pipelines, or partially opened valves create turbulence that disrupts separation, often causing emulsions or hindering oil droplet rise.
- Limit Particulate Matter:
- Fine particles (e.g., soot or microbial contaminants) act as emulsifying agents. Regular cleaning of bilge tanks and avoiding contamination helps reduce their impact.
- Use Emulsion Breakers Carefully:
- When necessary, use emulsion breakers as instructed by the manufacturer. Overuse or misuse of these chemicals can exacerbate emulsification.
- Restrict Chemical Drainage:
- Properly collect and dispose of chemicals used for water conditioning, corrosion inhibition, or cleaning. Allowing free drainage into bilges causes emulsification and operational inefficiency.
- Dispose of Detergents Separately:
- Mop water and detergent residues can stabilize emulsions. Use quick-break detergents or dispose of such waste separately to improve OWS efficiency.
- Avoid Prolonged Storage of Bilge Water:
- Long storage times oxidize free oil and alter its properties, making it harder to separate. Regularly process bilge water to maintain separation efficiency.
Maintenance and Preparation Tips
- Proper Filtration:
- Use strainers to remove large solid particles and floating debris before bilge water enters the OWS.
- Collect Leakages:
- Limit oil reaching bilge wells. If large oil volumes are present, transfer them to a dedicated oil tank instead of processing through the OWS.
- Heat the Influent:
- Warming the bilge water reduces viscosity, aiding oil separation. Avoid overheating during turbulent conditions to prevent emulsification.
- Segregate Wastes:
- Do not mix sludge, treated sewage, or other contaminants with bilge water. Such mixtures complicate OWS operation and may require tank cleaning before further use.
- Fill OWS Before Use:
- Ensure the OWS is filled with clean water and free from air pockets before operation. Air pockets can interfere with sensor readings and disrupt automation.
- Backflush Regularly:
- Backflush filters per manufacturer recommendations to extend filter life and maintain system efficiency.
- Clean Sensors Frequently:
- Regular cleaning of interface sensors ensures accurate readings, effective oil removal, and reduced water contamination.
- Remove Accumulated Oil:
- Beyond automated removal, manually clear any residual oil in OWS chambers to maintain operational capacity.
General Best Practices
- Follow Proper Operating Procedures:
- Adhere to a systematic, step-by-step operating procedure for the OWS. Review the manufacturer's guidelines regularly for optimal performance.
- Maintain the OWS:
- Routine maintenance, as per the manufacturer\u2019s instructions, is vital to keeping the OWS in top condition. Inspections, cleaning, and timely repairs prevent equipment failures and non-compliance.
By addressing these operational factors and following maintenance best practices, marine engineers can ensure OWS systems function efficiently, meeting regulatory requirements and minimizing environmental risks.