Sludge Management
Storage and disposal of the oily residues generated by purifiers and separators: the point environmental inspectors scrutinise most closely.
Operational Explanation
"Sludge" is the oily residue produced mainly by fuel and lubricating oil purifiers, and to a lesser extent by drainage and leakage in the machinery space. It must be collected in dedicated tanks, recorded in the Oil Record Book (code C) and disposed of to a port reception facility or by any other approved means — an incinerator, an auxiliary boiler suitable for burning oil residues, or other acceptable means. What makes a means usable is not the list: it is being annotated in item 3.2 of the Supplement to the IOPP Certificate, Form A or B. A means that is not annotated is not used.
It is not Regulation 16 of Annex VI that governs the incineration of the ashes: Regulation 16 governs the incineration of sludge, that is the process and its emissions to air. The ashes that come out are Annex V garbage, which names them by definition.
And a finding that does not hold, stated in full. The same reviews argued that this page had inverted the direction of Regulation 12: that the Regulation does not prohibit connection to the overboard discharge, only connections to the bilge system, oily bilge water holding tanks, the tank top and the oily water separators. We reopened the text, and the finding is unfounded. There are two prohibitions, in two distinct paragraphs: 12.3.3 looks inboard, 12.3.4 looks outboard and expressly prohibits any piping with a direct overboard connection other than the standard discharge connection of Regulation 13. What the page said was true; it was half. The missing half — 12.3.3 — is now in the Regulatory reference, and it is operationally the more useful of the two, because it is the one you see when you open a valve chest.
Mind the numbering. Regulation 12 was rewritten in full by resolution MEPC.266(68), in force since 1 January 2017: the prohibition many summaries still cite as “12.2.2” is today 12.3.3. A number taken from a summary written before 2017 is the quickest way to get a citation wrong on this subject.
Regulatory Reference
MARPOL Annex I, Regulation 12 (Tanks for oil residues (sludge)): in the version replaced by resolution MEPC.266(68), in force since 1 January 2017. Oil residue tanks must be of adequate capacity (12.3.1), fitted with a designated pump able to take suction from them (12.3.2), and built so as to facilitate their cleaning (12.3.5). The two piping prohibitions are two, distinct, and must be read together. 12.3.3 looks inboard: the tanks “shall have no discharge connections to the bilge system, oily bilge water holding tank(s), tank top or oily water separators”, with two exceptions only — drains with manually operated self-closing valves and visual monitoring of the settled water, and a common piping leading to the standard discharge connection of Regulation 13, provided it does not allow the transfer of sludge to the bilge system. 12.3.4 looks outboard: the tanks “shall not be arranged with any piping that has direct connection overboard, other than the standard discharge connection referred to in regulation 13”. It is the provision a magic pipe breaches. Ships constructed before 1 January 2017 had to comply with 12.3.3 by the first renewal survey on or after that date. Regulation 17 (ORB entry code C — collection and disposal of oily residues). For the on-board incineration of sludge: Annex VI Regulation 16. The resulting ashes are not an Annex VI matter: they are garbage under Annex V, which at Regulation 1 defines “incinerator ashes” as “ash and clinkers resulting from shipboard incinerators used for the incineration of garbage” and includes them in the definition of garbage (res. MEPC.201(62)).
Scope of Application
Annex I Regulation 12, unless indicated otherwise, applies to ships of 400 gross tonnage and above fitted with combustion machinery that generates oily residues (main and auxiliary engines, HFO/MDO/LO purifiers). Below that threshold obligations may still exist, but they come from elsewhere: flag requirements, approved equipment fitted on board, the company SMS or local rules.
Procedure / How to Complete It
- Record regularly the estimated volume of sludge produced by the purifiers. The daily frequency and the monthly balance are not imposed by any IMO instrument: they are company practice and a settled expectation of PSC, USCG and vetting.
- Transfer sludge to the dedicated tank. Do not transfer or mix it with bilge water except through an arrangement expressly permitted by Regulation 12 and reflected in the ship's approved piping, the IOPP Supplement and the onboard procedures.
- For disposal in port: contact the reception facility, confirm availability, obtain the Waste Delivery Receipt.
- For on-board incineration: verify that the incinerator is approved and that the operation is permitted in the zone. ECA status does not prohibit incineration: it concerns fuel sulphur, particulate matter and NOx. The real restrictions are regional or local (Helsinki Convention in the Baltic, Californian rules, port regulations).
- Record the operation in the Oil Record Book (code C) with quantity and destination.
- Carry out a monthly mass balance (sludge balance): estimated production vs. recorded disposal.
Diagram: sludge balance (input/output)
Practical Example
Example of a monthly balance: estimated production 2.4 m³ (historical purifier data) — disposed of: 1.6 m³ at a Rotterdam reception facility + 0.7 m³ incinerated on board = 2.3 m³. A 0.1 m³ variance is justified by a tank level change and documented. The threshold beyond which a variance must be investigated is not fixed by MARPOL: it is set by the company SMS, consistently with tank calibration, the sounding method used and the characteristics of the ship.
What Typically Goes Wrong
Common Mistakes Mistake Library
| Mistake | Consequence | How to avoid it |
|---|---|---|
| Sludge balance with a large, unexplained variance | Suspicion of illegal discharge, in-depth investigation | Documented monthly balance, cross-checked against fuel consumption |
| Internal tank-to-tank transfers not recorded | Inconsistency in ORB volumes | Record every internal transfer as a code C entry |
| Disposal without a Waste Delivery Receipt | Unable to prove proper disposal | Always keep the reception facility's receipt |
What the PSCO Checks
Operational Tips
- Keep a separate sludge balance log, updated monthly and cross-checked against fuel consumption.
- Record the operation under the applicable ORB item, and do not move sludge to the bilge outside the arrangements Regulation 12 permits and the IOPP Supplement reports.
- Always keep Waste Delivery Receipts together with the ORB.
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.
- Monthly sludge balance updated and consistent
- Waste Delivery Receipts kept for every disposal in port
- No unrecorded internal transfer
- Incinerator used only in permitted zones
FAQ
Related Topics
Last substantive revision of this page: 28 August 2026 · page fingerprint 80ae6c28baee