What to check on a Oily Water Separator (15 ppm).
Every ship generating oily bilge water needs some means of cleaning it before overboard discharge, but the term "15 ppm separator" refers specifically to a unit type-approved against the IMO discharge standard of 15 parts per million oil content, tested and certified under MEPC.107(49) (or the earlier MEPC.60(33) for older units still in service). What distinguishes this type is the combination of a mechanical treatment stage with a certified 15 ppm bilge alarm and automatic stopping device: the separator alone only removes oil, the complete system also monitors the outlet…
What sets the 15 ppm type apart
Every ship generating oily bilge water needs some means of cleaning it before overboard discharge, but the term "15 ppm separator" refers specifically to a unit type-approved against the IMO discharge standard of 15 parts per million oil content, tested and certified under MEPC.107(49) (or the earlier MEPC.60(33) for older units still in service). What distinguishes this type is the combination of a mechanical treatment stage with a certified 15 ppm bilge alarm and automatic stopping device: the separator alone only removes oil, the complete system also monitors the outlet continuously and shuts discharge to the bilge holding tank the moment the reading rises toward the limit. A separator without a certified monitor and shutdown fitted is not a compliant 15 ppm unit, whatever its treatment performance on paper.
Main components
Gravity / coalescing separation stage
Bilge water enters a first chamber where free oil rises by gravity, then passes through a coalescing stage of plates or a filter/coalescer pack that merges small oil droplets into larger ones that can also rise out.
Fine filter or second-stage polisher
Many designs add a second treatment stage, often a replaceable cartridge filter, membrane or coalescer, to bring the outlet reliably below 15 ppm even with emulsified oil that gravity alone cannot separate.
15 ppm bilge alarm and monitor
An oil content meter, sampling the treated water continuously, wired to sound an alarm and automatically stop overboard discharge (diverting flow back to the bilge holding tank) if the reading approaches 15 ppm. This unit carries its own separate type approval certificate.
Sludge and waste oil handling
Separated oil drains to a sludge tank; the separator's own performance depends on this tank not being allowed to back up into the unit.
Selection and sizing
| Parameter | Typical basis |
|---|---|
| Rated capacity | sized to the vessel's expected bilge water generation rate, commonly 0.5 - 10 m3/h |
| Inlet oil content design basis | up to several thousand ppm depending on approval class |
| Outlet requirement | 15 ppm continuous, monitored |
| Materials | bronze, stainless or coated steel wetted parts for seawater/bilge service life |
Undersizing against actual bilge generation is a common yard mistake; a unit rated for the design flow rarely copes once engine room leakage exceeds the original estimate.
Regulations and class
Governed by MARPOL Annex I, Regulation 14, which requires ships to discharge oily mixtures only through equipment meeting the 15 ppm standard, with special area restrictions (Antarctic area: no discharge at all) layered on top. The equipment and its alarm/monitor both need individual type approval certificates referencing the applicable MEPC resolution, and both are checked at survey together with the associated Oil Record Book entries and IOPP Certificate. Class surveyors check the automatic stopping function actually operates, not just that the alarm sounds.
Typical faults
- Coalescer fouling - sludge and detergent residue block the coalescing media, and treated output quality drops even though the pump still runs normally.
- Monitor calibration drift - an uncalibrated oil content meter reads falsely low, allowing out-of-spec water overboard undetected until the next survey catches it.
- Emulsified oil bypass - detergents or cleaning chemicals in the bilge break oil into an emulsion that gravity separation cannot remove, overloading the polishing stage.
- Sludge tank back-pressure - a full sludge tank prevents separated oil from draining away, and it re-mixes into the treated stream.
What to look for in a supplier
- Type approval certificate for the complete system, separator and monitor together, matching the resolution actually in force for the ship's flag and build date.
- Spare parts and calibration service availability for the specific oil content monitor model, since these need periodic recalibration to stay certified.
- Documented performance at realistic inlet oil concentrations, not only the clean-water test condition.
A 15 ppm unit that passes its annual survey with clean test water can still fail in service the moment someone runs degreaser through the bilges - emulsified oil is the real test of a separator, not the paperwork test.