What to check on a Bilge Water Separator (OWS 15ppm).
A 15 ppm oily water separator treats bilge water collected in machinery spaces so that any overboard discharge contains no more than 15 parts per million of oil, the limit set by MARPOL Annex I for discharge outside special areas. It differs from a fuel or lube oil purifier in both purpose and design: an OWS handles a dirtier, more variable feed — bilge water mixed with detergents, rust, and emulsified oil — and must prove compliance continuously via an in-line oil content monitor, not just produce a generally cleaner…
What this type is
A 15 ppm oily water separator treats bilge water collected in machinery spaces so that any overboard discharge contains no more than 15 parts per million of oil, the limit set by MARPOL Annex I for discharge outside special areas. It differs from a fuel or lube oil purifier in both purpose and design: an OWS handles a dirtier, more variable feed — bilge water mixed with detergents, rust, and emulsified oil — and must prove compliance continuously via an in-line oil content monitor, not just produce a generally cleaner output.
Main components
- First-stage separator — typically gravity/coalescing separation that removes the bulk of free oil before the water reaches finer treatment.
- Second-stage / fine separation — coalescer plates, membrane, or flotation stage that brings oil content down toward the 15 ppm target.
- Oil content monitor (OCM/ODM) — a continuous in-line analyser, separately certified, that measures the treated water and triggers the automatic stopping device if the limit is exceeded.
- Automatic stopping device — stops the overboard discharge (usually by diverting to a holding tank) automatically the moment the OCM reads over 15 ppm, without needing operator action.
- Bilge alarm and sample valves — support monitoring and periodic verification of the system's performance.
Selection and sizing
Capacity is set by the machinery space bilge volume and the engine room's typical oil-in-water challenge, expressed as throughput in cubic metres per hour. The separator technology (gravity/coalescer, membrane, or combined) affects how well it copes with emulsified oil from detergents and cleaning chemicals, which is often the harder problem than free oil.
| Separator type | Handles emulsified oil | Typical fit |
|---|---|---|
| Gravity/coalescer | Limited | Older/simple installations |
| Coalescer + fine filtration | Moderate | Common modern retrofit |
| Membrane-based | Good | Where emulsified challenge is high |
Regulations and class
MARPOL Annex I, Regulation 14, sets the 15 ppm discharge limit and requires the OWS, OCM and automatic stopping device combination on ships of 400 GT and above, built to the standards in IMO resolution MEPC.107(49) for equipment installed on or after 2005 (earlier equipment may remain under MEPC.60(33)). Class and flag surveys check type approval certificates for both the separator and the OCM, verify the automatic stopping/diverting function actually operates, and review the Oil Record Book Part I entries against the equipment's logged performance.
Typical faults
- OCM sensor fouled by oil film or scale, giving a falsely low reading that lets an over-limit discharge pass undetected.
- Automatic stopping device bypassed or manually overridden — a serious deficiency and, if done to defeat the system, a MARPOL violation with direct exposure for those involved.
- Coalescer media saturated or channelled, so separation efficiency drops even though the unit still runs and passes water.
- Detergent or cleaning chemical use in the bilge creating an emulsion the separator was not designed to break, overwhelming the system intermittently.
What to look for in a supplier
- Type approval for both the separator and the OCM to the applicable MEPC resolution, matched to the vessel's build/retrofit date.
- Demonstrated performance against emulsified oil, not just free oil, if the vessel uses bilge cleaning chemicals regularly.
- Spare parts and coalescer media availability on the vessel's trading routes.
- Clear commissioning test documentation for the automatic stopping device function specifically.
Never treat the automatic stopping device as an inconvenience to be worked around — it is the component port state control and flag investigators check first after any oil discharge finding, and its integrity is what separates a deficiency from a prosecution.