What to check on a Oily Water Separator (15ppm).
An Oily Water Separator (OWS) processes bilge water collected from the engine room bilge wells, separating free and emulsified oil from water before any discharge overboard. The 15ppm designation refers to the discharge standard it must meet under MARPOL Annex I: treated effluent may contain no more than 15 parts of oil per million parts of water. The separator itself only does part of the job — it is always fitted with a downstream Oil Content Meter/Monitor (OCM) that continuously samples the outlet and automatically closes a three-way valve to…
What defines a 15ppm Oily Water Separator
An Oily Water Separator (OWS) processes bilge water collected from the engine room bilge wells, separating free and emulsified oil from water before any discharge overboard. The 15ppm designation refers to the discharge standard it must meet under MARPOL Annex I: treated effluent may contain no more than 15 parts of oil per million parts of water. The separator itself only does part of the job — it is always fitted with a downstream Oil Content Meter/Monitor (OCM) that continuously samples the outlet and automatically closes a three-way valve to divert flow back to the bilge holding tank the moment the reading approaches the limit. Without a working, calibrated OCM, an OWS has no legal discharge path at all.
Main components
Separation stages
Most designs use two or three stages: a first gravity or coalescing stage that removes the bulk of free oil, followed by a fine coalescing or membrane/absorption stage that reduces emulsified oil to meet the 15ppm limit. Some systems add a final polishing filter cartridge.
Oil Content Monitor (OCM)
A separate, IMO-type-approved instrument sampling the treated outlet continuously, alarming and automatically closing the overboard valve on any reading approaching 15ppm, and logging every discharge event.
Three-way automatic valve
Directs treated water either overboard or back to a bilge holding tank, actuated automatically by the OCM and also operable manually for testing.
Sludge and oil collection
Separated oil is drained to a sludge or waste oil tank for later incineration or shore disposal; this stream, and the volumes logged, are cross-checked against the Oil Record Book.
Selection and sizing
Capacity is set by expected bilge water generation rate for the engine room size and machinery type, expressed in cubic metres per hour, with common sizes from around 0.25 to 5 m3/h for merchant vessels. Selection also depends on the range of oil types and emulsifying agents expected — heavy fuel residues, lubricating oil, detergents used in tank cleaning — since some separator technologies handle stable emulsions poorly. Material selection for wetted parts needs to tolerate the corrosive, contaminated nature of bilge water over years of intermittent running.
Regulations and class
MARPOL Annex I and IMO Resolution MEPC.107(49) set the type-approval standard for 15ppm bilge separators and their oil content monitors; a unit must carry an IMO type approval certificate referencing this resolution to be fitted. The OCM must be calibrated to a recognised standard and both instruments are checked at every Port State Control inspection likely to review the engine room, since bilge water discharge is one of the most heavily enforced pollution offences in shipping. Class surveys verify the automatic stopping device is operative and that Oil Record Book entries for every discharge, transfer or internal transfer of bilge water match the OWS operating log.
| Requirement | Reference |
|---|---|
| Discharge limit | 15 ppm oil content, MARPOL Annex I |
| Type approval standard | IMO Resolution MEPC.107(49) |
| Automatic stopping device | Mandatory, tied to OCM reading |
| Record keeping | Oil Record Book Part I, checked against OWS log |
Typical faults
- Coalescing media fouling — heavy sludge or detergent residue blinds the media, reducing separation efficiency and forcing frequent OCM trips.
- OCM sensor drift or fouling — a dirty sample cell gives false readings, either nuisance-tripping the discharge valve or, more seriously, under-reading actual oil content.
- Three-way valve seizure — a valve stuck in the overboard position defeats the automatic stopping function entirely and is treated as a serious deficiency.
- Bypass piping — illegal or undocumented piping around the OWS, historically the cause of the largest MARPOL prosecutions, remains one of the first things a Port State Control officer looks for.
What to look for in a supplier
- Current IMO type approval certificate for both the separator and the OCM, matching the specific model to be installed.
- Demonstrated performance on the emulsion types actually generated onboard, not only on clean test oil.
- Availability of spare coalescing media and OCM sample cells through a global service network, since these are consumables with limited life.
- Clear commissioning documentation package for class approval and Port State Control file.
Never let anyone touch the OWS piping without an entry in the Oil Record Book to match: an unexplained valve position or an undocumented pipe run is the single fastest way to turn a minor equipment fault into a detention.