Marine bilge water treatment reference — oily-water separator (OWS) design, IMO MEPC.107(49) 15 ppm discharge standard, MARPOL Annex I logbook requirements, and the interaction between OWS, 15 ppm bilge alarm and 3-way discharge valve. Covers coalescing plate, gravity settling, and hollow-fibre membrane OWS technology.
MARPOL Annex I regulates discharge of oily bilge water from ships. The core rule: discharge is only permitted if the oily-water separator (OWS) delivers effluent with an oil content ≤ 15 ppm, and a 15 ppm bilge alarm continuously monitors the discharge. If the alarm exceeds 15 ppm, an automatic 3-way valve diverts the flow back to the bilge holding tank. The system's type approval is issued to IMO MEPC.107(49) which prescribes the OWS + alarm + valve integration test.
| Component | Function | Type-approval scope |
|---|---|---|
| Bilge well pumps | Draw oily water from engine-room bilges | Not covered by 107(49) |
| Bilge holding tank | Buffer + primary gravity separation, oil layer pumped ashore | Not covered |
| OWS feed pump | Feeds OWS from the holding tank at low, controlled flow | Part of 107(49) integrated system |
| Oily-water separator | Two-stage: gravity + coalescing / membrane; separates oil to ≤ 15 ppm | Yes — 107(49) type-approved unit |
| 15 ppm bilge alarm | Continuously measures effluent oil content; alarms if > 15 ppm | Yes — 107(49) type-approved |
| 3-way discharge valve | Fail-safe divert to holding tank on alarm; overboard when clean | Yes — 107(49) actuator + fail-safe response |
| Overboard discharge line | Above waterline, sample point ahead of valve | Not covered |
| Technology | Working principle | Effluent | Typical use |
|---|---|---|---|
| Coalescing plate | Corrugated plates in laminar flow — oil droplets coalesce and rise | 15 ppm at design flow | General merchant fleet, legacy standard |
| Gravity + coalescing filter | Gravity primary + polypropylene coalescing filter secondary | 15 ppm | Standard modern OWS |
| Hollow-fibre membrane | Ultrafiltration through 0.01 μm hollow fibre; oil retained on membrane | < 5 ppm | Emulsified bilge water, cruise, tanker |
| Chemical + gravity | De-emulsifier chemical dosing + gravity | 15 ppm | Highly emulsified bilge, small workboat |
MARPOL Annex I requires an Oil Record Book Part I entry for every: bilge holding tank transfer, OWS operation start / stop, overboard discharge (start volume + duration), and any 15 ppm alarm event. Entries are signed by the officer in charge and countersigned by the master; the ORB is inspected by port state control and any discrepancy between entries and the OWS totaliser is a MARPOL violation.