Oily Water Separator (OWS): Construction and Working
Why Do We Need an OWS Onboard?
Modern ships generate bilge water during normal operation—water that is contaminated with oil, fuel, lubricants, and cleaning agents from machinery spaces. Simply pumping this overboard is illegal and environmentally disastrous.
Enter the oily water separator. Its primary function is to separate oil from bilge water, ensuring that only clean water—with an oil content below the legal limit of 15 parts per million (ppm)—is discharged into the sea. The system must meet the standards laid out in MARPOL Annex I, Regulation 14.
Construction of an Oily Water Separator
A typical OWS system is composed of the following key components:
1. Bilge Water Inlet and Feed Pump
The system starts with a feed pump—most commonly a screw-type pump—that supplies oily bilge water to the separator. The pump needs to provide steady pressure and accommodate a mix of fluids, including viscous oils.
2. Primary Separation Chamber
This is the first stage where the basic separation occurs via gravity. Since oil is lighter than water, it rises to the top and is collected, while the cleaner water moves to the next stage. Larger oil droplets are easily separated here.
3. Coalescer Unit (Second Stage)
In the second stage, the coalescer plays its role. It combines smaller oil droplets into larger ones—a process known as coalescing—making them easier to separate. This stage is critical for dealing with emulsified mixtures and reducing oil content further.
4. Filter or Polishing Stage (Optional)
Some advanced OWS units may include a final filter stage to remove fine contaminants or emulsified traces of oil before discharge.
5. Oil Content Monitor (OCM) or 15 PPM Bilge Alarm
Before any water is discharged overboard, it passes through an Oil Content Monitor. If the oil concentration exceeds 15 ppm, the system automatically stops discharge and diverts the flow back to the bilge holding tank.
6. Control Valves and Flow Regulator
These ensure controlled flow and automate system responses based on the oil content readings. Any abnormal condition, such as high oil content or flow irregularities, is flagged immediately.
Working Principle of an OWS
The process is straightforward but depends heavily on precise flow control and regular maintenance. Here’s a step-by-step breakdown of the OWS operation:
- Bilge Water Collection: Mixture from the engine room bilges is collected in a bilge holding tank.
- Feeding the Separator: The bilge water is pumped into the OWS using a positive displacement pump.
- Primary Gravity Separation: Larger oil droplets are separated due to the density difference between oil and water.
- Coalescing Process: The remaining water passes through the coalescer, which helps smaller oil droplets bind and rise to the surface.
- Oil Content Monitoring: The nearly clean water flows through the oil content monitor. If readings are below 15 ppm, discharge is allowed. If not, the system halts and returns the flow to the bilge tank.
- Automatic Control & Logging: Most modern OWS units include automatic logging and alarms, with a bypass prevention feature.
Regulations and Compliance
The use of OWS systems is tightly regulated by MARPOL:
- MARPOL Annex I governs the discharge of oil and oily mixtures.
- Vessels over 400 GT must be fitted with type-approved OWS equipment.
- All operations involving bilge water treatment and discharge must be recorded in the Oil Record Book (Part I).
- The 15 ppm Bilge Alarm must be calibrated annually and tamper-proof.
Any intentional or accidental bypass of the OWS system can lead to serious legal consequences, including detention, fines, and criminal prosecution.
Common Issues and Maintenance Tips
Despite its design, the OWS can become inefficient or inoperative due to:
- Emulsified Bilge Water (from detergents or cleaning agents)
- Clogged Coalescer Filters
- Malfunctioning OCM
- Improper Flow Rates
- Lack of Training or Supervision
To avoid these, regular cleaning of the coalescer unit, routine checks on the OCM, and strict adherence to maintenance schedules are essential.
Also, remember that only a certified engineer should supervise the operation and maintenance of the OWS. The system should never be bypassed, and all alarms or abnormalities must be investigated and documented.
