What to check on a 15 ppm OWS.
Every oily water separator on a ship is designed to bring bilge water below a legal oil content limit before it reaches the overboard valve, but the "15 ppm" designation is specific: it names the separator built and certified to hold outlet oil content under 15 parts per million continuously, verified by an inline oil content monitor rather than by periodic sampling. Older 100 ppm bilge separators still exist on some vessels under grandfather clauses, but any newbuild or retrofit fitted after the relevant MEPC amendment must meet the 15…
What makes a 15 ppm unit different
Every oily water separator on a ship is designed to bring bilge water below a legal oil content limit before it reaches the overboard valve, but the "15 ppm" designation is specific: it names the separator built and certified to hold outlet oil content under 15 parts per million continuously, verified by an inline oil content monitor rather than by periodic sampling. Older 100 ppm bilge separators still exist on some vessels under grandfather clauses, but any newbuild or retrofit fitted after the relevant MEPC amendment must meet the 15 ppm standard with a monitor that stops discharge automatically, not just an alarm a watchkeeper could silence.
Main components
Gravity/coalescing first stage
Bilge water enters a settling chamber where free oil separates by density difference and rises to a collection zone. Coalescing plates or packs increase the surface area available for small droplets to combine into larger ones that rise faster, cutting the residence time needed for a given oil removal rate.
Second stage (fine filter or coalescer)
A second unit, either a stacked coalescer, a fine mechanical filter, or in some designs a chemical/absorption cartridge, removes the emulsified and dissolved fraction the first stage cannot touch. This stage decides whether the unit can reliably hit 15 ppm on real, emulsified engine room bilge water rather than clean test water.
Oil content monitor (OCM)
An in-line monitor, usually optical, continuously measures the treated water and automatically diverts the discharge back to the bilge holding tank the instant the reading approaches 15 ppm. The monitor is a separately type-approved instrument under its own certificate, distinct from the separator's own approval.
Automatic stopping/diverting device
A three-way valve or pump-stop function, driven by the OCM, is mandatory: the system must fail safe to recirculation, not to sea.
Selection and sizing
Capacity is rated in cubic metres per hour and chosen against the bilge pumping rate of the engine room bilge system, not vessel size alone. Key figures against the specification: rated throughput, maximum inlet oil concentration the unit is designed for (raw bilge runs far dirtier than the 15 ppm limit), and the type approval certificate number referencing the applicable IMO resolution. Internals in way of the bilge pump discharge are usually stainless steel or coated carbon steel, since bilge water carries detergents, fuel residue, and sometimes seawater.
Regulations and class
- MARPOL Annex I, Regulation 14, sets the 15 ppm discharge limit and requires approved oil filtering (separating) equipment with alarm.
- IMO Resolution MEPC.107(49) is the current test and approval standard for 15 ppm separators fitted from 2005 onward, superseding MEPC.60(33).
- The oil content meter is approved separately, and class checks its calibration and alarm/stop function during survey.
- Every discharge and any equipment failure must be logged in the Oil Record Book (Part I), which flag administrations and port state control check closely.
Typical faults
| Fault | Consequence |
|---|---|
| Coalescer media fouled with sludge or detergent residue | Rising differential pressure, falling separation efficiency, eventual automatic stop on every cycle |
| OCM optical cell dirty or miscalibrated | False stops (nuisance recirculation) or, worse, false-clean readings that mask an actual overboard discharge violation |
| Emulsified bilge water from heavy detergent use during tank cleaning | Unit cannot meet 15 ppm regardless of condition; this is an operational cause, not equipment failure |
| Three-way valve stuck or bypassed manually | Untreated water can reach the overboard discharge, the classic MARPOL violation scenario investigated by port state control |
What to look for in a supplier
- Current type approval certificate referencing MEPC.107(49), matched to the flag state the vessel trades under.
- Coalescer element and OCM spares reachable through a network at the ports the vessel actually calls, not just the maker's home port.
- Documented performance on real bilge water, not only on the standard test oil/water mixture used for certification.
- OCM compatibility with any bilge water monitoring system already fitted, to avoid a costly retrofit of the electronic interface.
Never accept an OCM reading at face value if the sight glass shows visible sheen; port state control inspectors know the bypass tricks, and a falsified oil record book is a career-ending finding, not a minor deficiency.