Bilge Water Management
Discharging bilge water through the oily water separator (OWS): the procedure most exposed to illegal tampering.
Operational Explanation
Bilge water collected in the machinery space contains traces of oil from leaks, condensation and washings. Before discharge at sea it must pass through the oil filtering equipment (commonly called the Oily Water Separator, OWS), which must ensure an oil content not exceeding 15 ppm (parts per million).
The equipment fitted depends on tonnage, and the split is finer than the usual summary. For ships of 400 GT and above but below 10,000 GT, Regulation 14.6 asks only for equipment “of a design approved by the Administration” ensuring that the effluent does not exceed 15 ppm: that paragraph mentions neither the alarm nor the automatic stopping arrangement. Alarm and automatic stopping are both in Regulation 14.7, which applies to ships of 10,000 GT and above and which adds to the text of 14.6 an “alarm arrangement to indicate when this level cannot be maintained” together with the automatic stopping provisions.
The Convention does not prescribe that ships of 400 to 10,000 GT “must have 15 ppm oil filtering equipment with an alarm”, nor does it instruct the reader to discharge “under way and outside Special Areas”. Neither is in the text: Regulation 14.6 does not name the alarm, and Regulation 15.3 does permit discharge inside a Special Area.
So why is there an alarm on smaller ships too? Because it reaches them by another route. Regulation 14.6 itself requires the Administration to have regard to the “specification recommended by the Organization”, and that specification — resolution MEPC.107(49) — defines the pollution prevention equipment as comprising a “15 ppm Bilge Separator; 15 ppm Bilge Alarm; and automatic stopping device”, the last of these qualified “where applicable”. The alarm on a ship below 10,000 GT therefore follows from type approval, not from the text of the Convention. The distinction is not academic: it matters when you have to say which provision the requirement rests on, and it matters because the automatic stopping device — unlike the alarm — really is absent on that ship.
Equipment is only half the condition. Regulation 15 also requires the ship to be proceeding en route: a discharge carried out at anchor or alongside is not compliant even if the effluent is below 15 ppm, and it is the condition a Port State Control Officer (PSCO) checks by comparing the ORB against VDR or AIS data.
And discharging inside a Special Area is not prohibited. Regulation 15.3 permits it on five cumulative conditions, only one of which differs from the open sea: the equipment must meet Regulation 14.7, that is, have both alarm and automatic stopping. The practical consequence inverts the tonnage reading: a ship between 400 and 10,000 GT fitted only with 14.6 equipment cannot discharge in a Special Area, while the same ship with a 14.7-compliant installation can. A prohibition without exceptions does exist, but it covers other water: Regulation 15.4 reserves it to the Antarctic area.
Regulatory Reference
MARPOL Annex I, Regulation 14 (Oil filtering equipment — the wording «oil/water separating equipment» belongs to the text in force before 2004), in the revised version adopted by resolution MEPC.117(52): paragraph 14.6 prescribes for ships of 400 GT to below 10,000 GT only the 15 ppm performance with equipment of approved design; paragraph 14.7 adds, for ships of 10,000 GT and above, the alarm and the automatic stopping arrangement. Type-approval specifications are in resolution MEPC.107(49), applicable to installations on ships the keels of which were laid on or after 1 January 2005. Regulation 15 (control of operational discharges of oil): paragraph 15.2 governs discharges outside Special Areas and refers generically to Regulation 14; paragraph 15.3 governs those inside Special Areas and refers specifically to Regulation 14.7; paragraph 15.4 prohibits any discharge whatsoever in the Antarctic area. In both cases the discharge requires the ship to be proceeding en route, oil content not exceeding 15 ppm, effluent processed through the filtering equipment and not originating from cargo pump-room bilges nor mixed with cargo oil residues. The exceptions in Regulation 4 (safety of the ship, saving life at sea, damage) remain available. Corresponding ORB codes: D (non-automatic discharge) and E (automatic discharge).
Scope of Application
Ships of 400 GT and above must be fitted with type-approved oil filtering equipment (Regulation 14.6); from 10,000 GT the text of the Convention adds the alarm and the automatic stopping arrangement (Regulation 14.7). Anyone intending to discharge inside a Special Area needs 14.7 equipment regardless of tonnage: it is Regulation 15.3 that requires it. Ships below 400 GT that have such equipment on board remain subject to the same discharge limits, including the «en route» condition.
Procedure / How to Complete It
- Verify that the OWS is operating and correctly calibrated before every discharge.
- Start the discharge only while the ship is under way. Inside a Special Area the discharge is permitted (Regulation 15.3), but only with equipment meeting Regulation 14.7: check first that the installation on board does meet it, because on a ship below 10,000 GT it may not. In the Antarctic area there is no discharge in any case (Regulation 15.4). Regulation 15 sets no minimum distance from land for machinery space bilge water: the distance criterion belongs to Regulation 34, which covers the cargo area of oil tankers.
- Continuously monitor the oil filtering equipment with its 15 ppm alarm during the discharge. The Oil Discharge Monitoring and Control System (ODMCS) is a different thing: it belongs to Regulation 31, covers the cargo area of oil tankers and is recorded in Part II of the ORB.
- If the alarm activates: check on the approved configuration of the installed equipment, and on the onboard procedures, whether the discharge stops by itself. Do not presume either the presence or the absence of automatic stopping from tonnage alone: Regulation 14.7 prescribes it above 10,000 GT, but below that threshold an approved installation may have it all the same. Where the approved installation does not provide it, attendance and manual stopping are handled according to the manual and the SMS. In no case force a bypass.
- Record the operation in the ORB, choosing the code according to what actually happened. D — discharge or other disposal of accumulated bilge water: enter the particulars required by the applicable items, including quantity and position where required. E — automatic discharge or condition of the oily filtering equipment: record items 16-18 as set out in the format. Code E does not call for a volume: inventing one produces a non-compliant entry.
Practical Example
Code E has no volumes. In the Oil Record Book Part I, in the sections replaced by resolution MEPC.187(59), code (E) is titled “Automatic starting of discharge overboard, transfer or disposal otherwise of bilge water which has accumulated in machinery spaces” and has three items only, all of them times: 16 “Time and position of ship at which the system has been put into automatic mode of operation for discharge overboard, through 15 ppm equipment”; 17 the time the system was put into automatic mode for transfer to a holding tank, with the tank identified; 18 the time it was put back into manual operation.
The quantity lives under code (D), item 13: “Quantity discharged, transferred or disposed of, in m3”, with item 14 for the start and stop times and item 15 for the method — and 15.1, where the discharge is through the 15 ppm equipment, asks for the position at start and end. The titles of the two sections are also more precise than the usual shorthand: they speak of starting, that is the non-automatic or automatic initiation, not of the discharge itself.
Example entry, rewritten on two rows. Code E, item 16: "2026-08-03 22:10, system put into automatic mode of operation for discharge overboard through the 15 ppm equipment, position 41°12'N 010°05'E"; item 18: "2026-08-04 01:35, system put back into manual operation". Code D, for a manually started discharge: item 13 "4.2 m³"; item 14 "start 22:10, stop 01:35"; item 15.1 "through 15 ppm equipment, position at start 41°12'N 010°05'E, at end 41°44'N 010°31'E". Chief Engineer's signature on each entry.
What Typically Goes Wrong
Common Mistakes Mistake Library
| Mistake | Consequence | How to avoid it |
|---|---|---|
| Manual bypass of the alarm system | Environmental offence, detention, criminal proceedings | No exceptions: in case of a fault, stop the discharge and repair the OWS |
| OWS never calibrated or tested | PSC deficiency, out-of-spec discharges | Calibration and maintenance per the approved plan |
| Discharge position not recorded | Unable to demonstrate compliance with Special Area limits | Always record position and distance from the coast |
What the PSCO Checks
Operational Tips
- Keep a preventive maintenance plan dedicated to the OWS, separate from the general engine maintenance plan.
- Never ignore a 15 ppm alarm, even if it looks like a false positive. If the ship is below 10,000 GT, remember that no device will stop the discharge for you.
- Document every maintenance intervention or component replacement on the OWS.
Preparation checklist
Educational checklist. This summary supports learning and preparation only. It does not replace the vessel’s approved procedures, manuals, statutory documents, company SMS, or applicable official requirements. Completing it demonstrates neither compliance nor readiness for an inspection: it shows that a list has been read, not that the ship is in order. Always verify the current documents carried on board.
- OWS calibrated and certified
- No unauthorised piping or flange near the system
- Oil filtering equipment (15 ppm) alarm log consistent with ORB entries
- Tonnage threshold checked: alarm and automatic stopping are prescribed by Regulation 14.7 for ships of 10,000 GT and above; below that, Regulation 14.6 asks only for the 15 ppm performance and stopping is manual
- Before any discharge in a Special Area: confirmed that the equipment meets Regulation 14.7 (Regulation 15.3)
- Discharge position always recorded
FAQ
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Last substantive revision of this page: 28 August 2026 · page fingerprint cf22681c91ba