MARPOL's six annexes

The Port State Control officer comes up the gangway, introduces himself, and asks for three things within the first five minutes: the oil record book, the IOPP certificate and access to the engine room to look at the 15 ppm separator. He has not inspected anything yet, but he already knows where to look: year after year, deficiencies linked to the MARPOL Convention and its annexes sit among the top categories in the Paris MoU and Tokyo MoU statistics, alongside fire safety and life-saving appliances.

MARPOL is probably the most quoted and least read convention in shipping. The 1973 text, amended by the 1978 protocol — hence MARPOL 73/78 —, is long, technical and continuously amended by the IMO's MEPC committee. Yet for the person who actually operates the vessel, the convention comes down to something very concrete: each annex corresponds to specific equipment on board that has to work, be maintained, and leave a written trace. A separator, a sewage treatment plant, an incinerator, holding tanks, record books, certificates with due dates.

That is what this article offers: the six annexes taken one by one, not from the lawyer's point of view, but from that of the chief engineer, the master or the fleet manager who has to keep the vessel compliant between two port calls — and prove it in thirty seconds when the inspector asks for the register.

What do the MARPOL Convention and its six annexes cover?

The MARPOL Convention (MARine POLlution) is the IMO instrument governing the prevention of pollution from ships. It consists of six technical annexes: oil (I), noxious liquid substances carried in bulk (II), harmful substances in packaged form (III), sewage (IV), garbage (V) and air pollution (VI). Annexes I and II are mandatory for every signatory state; the other four are optional at ratification but so widely ratified that in practice they apply to almost the entire world fleet.

Every annex follows the same logic: discharge rules (what may be discharged, where, how far from the coast, in what condition), equipment required to meet those rules, a certificate issued after survey, and one or more record books proving day after day that the rules are being followed. The table below sums up the mechanism.

The application threshold varies from one annex to another: many documentary obligations start at 400 GT, others as soon as a vessel is certified to carry more than 15 persons. Even below the thresholds, the discharge rules themselves apply to everyone. Check the exact applicability with your flag state or classification society — and, for US inland and coastal operators, remember that the USCG enforces MARPOL through its own regulations, alongside frameworks such as Subchapter M for towing vessels. That applicability matrix is the first line of your compliance file.

Annex I: oil — the oldest and the most inspected

Annex I is the historical core of the MARPOL Convention and remains the most inspected annex. In the engine room it takes the shape of a well-known trio: the oily water separator with its 15 ppm alarm, the oil record book (part I for machinery spaces), and the discharge line with its valves — which some operators have sealed to remove any ambiguity during an inspection.

The 15 ppm separator: critical equipment in its own right

The rule is simple: no discharge of machinery space bilge water above 15 ppm of oil content, vessel en route, outside special areas where restrictions are tighter. Everything therefore rests on one piece of equipment: the separator and its oil content meter. And it is a machine that ages badly when neglected: sensing cells to replace on schedule, an alarm to test periodically, a recirculation valve to check, coalescing filters to change. It deserves a full job plan in the planned maintenance system, exactly like a propulsion auxiliary: a separator found out of service during an inspection is an almost automatic deficiency, and often the start of a more detailed inspection.

The oil record book: the most read document on board

Every fuel transfer, every bilge pumping, every landing of sludge ashore must be recorded, dated and signed, with quantities and positions. The inspector cross-checks the record book against shore reception receipts, sludge tank soundings and the theoretical capacity of the separator. An inconsistency — tanks that never fill, discharges that are miraculously regular — weighs far more than a crossed-out line. The "magic pipe" cases that end up in US federal courts almost always start with an inconsistent record book. Daily writing discipline, the kind that a digital engine room logbook structures, is the best protection there is.

Add to this the IOPP certificate, issued after survey for vessels of 400 GT and above, and the shipboard oil pollution emergency plan (SOPEP), with its drills to be recorded and its contact list to be kept up to date.

Annexes II and III: noxious liquid substances and packaged goods

These two annexes primarily concern specialised trades. Annex II governs the carriage of noxious liquid substances in bulk — the world of chemical tankers: product categorisation (X, Y, Z), mandatory prewash for the most hazardous substances, cargo record book, procedures and arrangements (P&A) manual. Annex III deals with harmful substances carried in packaged form and largely refers to the IMDG Code: packing, marking, labelling, documentation and stowage.

For a fishing, service or passenger fleet, direct exposure remains limited. Two points still deserve attention. First, the technical products carried on board — paints, solvents, degreasers, cleaning acids — do fall under the logic of Annex III as soon as they travel in packaged form: safety data sheets available, correct storage, and a trace of what comes on board, which proper onboard stock management provides naturally. Second, a service vessel that occasionally loads drums or IBCs for an offshore site falls squarely within Annex III: documenting those packages is not optional.

Annex IV: sewage — the equipment everyone forgets

Annex IV typically applies to vessels of 400 GT and above, or certified to carry more than 15 persons — which automatically brings in ferries, passenger vessels, crewed yachts and expedition ships. Two technical options: an approved sewage treatment plant, or holding tanks with discharge ashore or at sea under the prescribed conditions (untreated sewage discharged well clear of the coast, vessel en route at sufficient speed; stricter rules exist, notably for passenger ships in the Baltic special area). The whole arrangement is covered by the ISPP certificate.

The point that most often goes unnoticed: the sewage treatment plant is ordinary maintenance equipment, with its membranes, its chlorine dosing or electrolytic cells, its transfer pumps, its float switches and level alarms. A plant that has been out of service for months, discovered during a survey, puts the operator in front of an uncomfortable choice: emergency repair or operating restriction. It deserves its equipment card, its history and its preventive job plans like any other machine — dosing to check, periodic samples, spare parts identified. On passenger vessels, where the volumes bear no comparison with a cargo ship, it is a machine whose failure shows up in operations immediately, as our article on ferry and passenger vessel maintenance points out.

Annex V: garbage — the annex of quick checks

Annex V is the one that applies to the largest number of vessels: the principle is a general prohibition on discharge at sea, with narrow exceptions (food waste at a distance from the coast, in particular). The garbage management plan is required from 100 GT or 15 persons on board, the garbage record book from 400 GT or for vessels certified to carry 15 persons or more, and placards are mandatory for almost everyone.

This is the annex of quick checks: a garbage record book can be read in thirty seconds. Waste categories correctly coded, plausible quantities, shore reception receipts filed, signatures. A poorly kept register does not cost much in itself, but it immediately sets the tone for the rest of the inspection: the officer who finds three weeks of gaps in the garbage record book will then go through the oil record book in a very different mood. Conversely, a vessel whose daily documentation is beyond reproach reverses the dynamic. That documentary discipline is built over time; it sits at the heart of preparing for Port State Control inspections.

Annex VI: emissions — now the central annex of the MARPOL Convention

If Annex I remains the most inspected, Annex VI has become the one that shapes operations the most. It covers three families of requirements.

Sulphur and emission control areas

Fuel sulphur content capped at 0.50% worldwide since 2020, and at 0.10% in emission control areas (ECAs) — the Baltic, the North Sea, North America, the US Caribbean, and now the Mediterranean, a 0.10% SECA since 1 May 2025: a major change for every fleet trading there. In practice: bunker delivery notes (BDNs) kept on board for three years, sealed samples, fuel changeover procedures recorded when approaching the zones, or a scrubber with its own maintenance and record-keeping burden.

NOx and engine certificates

Each diesel engine in scope carries its EIAPP certificate and its technical file, which locks down the components that influence emissions: injectors, camshafts, turbocharger, settings. Any replaced part must remain consistent with the file — one more reason to record exact part references in the work history. The vessel itself carries the IAPP certificate, issued after survey from 400 GT.

Energy efficiency: EEXI, CII, SEEMP

Since 2023, cargo and passenger ships of 400 GT and above must hold their EEXI (attained technical efficiency, often addressed through engine power limitation), and those of 5,000 GT and above receive an annual CII rating from A to E, calculated on their actual emissions relative to transport work. This is where maintenance changes status: a clean hull and propeller, correctly tuned engines and monitored consumption are no longer good housekeeping but compliance. A poorly maintained ship drifts towards D and E ratings, with a mandatory corrective action plan at the end of the road. We covered this mechanism in detail in our article on CII, EEXI and the decisive role of maintenance. And the input feeding the whole edifice is consumption: fuel consumption monitoring, reading by reading, feeds the calculation of the indicator. Per-vessel bunker and tank data is no longer a management convenience: it has become regulatory data.

Which MARPOL certificates and records must be kept on board?

The short answer: three main certificates (IOPP for Annex I, ISPP for Annex IV, IAPP for Annex VI, plus the EIAPP for each engine), two or three living record books (oil, garbage, and cargo where applicable), and the associated plans (SOPEP, garbage management plan, SEEMP). The certificates follow the classic survey cycle — initial, annual, intermediate, five-year renewal — harmonised with the vessel's other statutory certificates.

Seen from the office, that adds up, for a ten-vessel fleet, to several dozen MARPOL due dates running in parallel with all the other certificates. This is exactly the problem centralised management solves: certificate due dates with alerts across the fleet, every document digitised and accessible from on board, including offline. The expired certificate discovered by the inspector is one of the most avoidable deficiencies there is; our guide to managing ship certificates and the survey calendar sets out the method.

The common thread: producing the evidence, annex by annex

Read the six annexes of the MARPOL Convention again through the operator's eyes and one pattern repeats: a piece of equipment that has to work, a record book that has to prove it, a certificate that has to be current. The separator and its record book, the treatment plant and its ISPP, the incinerator and the garbage record book, the engines and their NOx files, the bunkers and their BDNs.

The practical consequence is direct: MARPOL compliance is not decided in a binder at the office, it is decided in the maintenance plan. Three habits give it structure:

• Every MARPOL-related item has its equipment card and its job plans: separator, sewage treatment plant, incinerator, scrubber where fitted, with their critical spares identified in stock — 15 ppm cell, membranes, dosing consumables.
• Every job leaves a dated history: it is this history that, in an ISM audit as in a PSC inspection, proves the equipment is not compliant by accident but maintained by system. The ISM Code requires precisely that the ship be maintained in conformity with the applicable rules — MARPOL included — and that the company be able to demonstrate it, as our article on the ISM Code and maintenance compliance explains.
• Every due date is visible before it is missed: certificate surveys, 15 ppm alarm tests, cell replacements, SOPEP drills, sampling campaigns — all of it gets planned like the rest of the maintenance, not in reaction to a letter from the flag state.

Environmental compliance and maintenance management have become inseparable: it is one of the threads running through our complete maritime CMMS guide. An operator who treats MARPOL as a paperwork subject ends up chasing registers; one who treats it as a maintenance subject produces the evidence with no extra effort, because it is the natural by-product of a well-run planned maintenance system.

Key takeaways

The MARPOL Convention and its six annexes do not require the operator to become a lawyer: they require equipment that works and evidence that exists. Annex I: a maintained separator and a consistent oil record book. Annexes II and III: limited exposure outside specialised trades, but vigilance on packaged products. Annex IV: a treatment plant treated as a real machine. Annex V: daily documentary discipline that sets the tone in inspections. Annex VI: compliant fuel, certified engines and monitored consumption, which the CII has turned into regulatory data.

The common denominator is traceability: job plans executed on time, dated histories, certificates under watch, record books consistent with one another. That is exactly what a maritime CMMS — ship maintenance software built for operations at sea — brings to a fleet, from the 15 ppm separator to the CII calculation.

Smart Sailors centralises equipment cards, maintenance job plans, certificate due dates and consumption data for your whole fleet, with a mobile app that works offline and synchronises at the port call. Designed in Marseille by seafarers and deployed on more than 700 vessels, it turns MARPOL compliance into a by-product of your daily maintenance. Explore fleet management in Smart Sailors, request a demonstration on your own equipment, or take a look at our plans — free 30-day trial.