MARPOL Annex II Requirements: Discharge, Prewash, and Records

MARPOL Annex II requirements govern how ships carrying noxious liquid substances (NLS) in bulk handle, discharge, and document those cargoes, setting hazard-based categories, mandatory prewash procedures, strict sea-discharge conditions, and record-keeping duties enforced through port state inspections and, in the United States, civil penalties of up to $25,000 per violation with felony exposure for knowing offenses.¹

Who Must Comply

Annex II applies to every oceangoing chemical tanker, liquefied gas carrier, and any other vessel with tanks certified to carry noxious liquid substances in bulk.² If your ship is flagged to a MARPOL signatory state, or if it enters the port of one, you are inside the rule. In the United States, Annex II is implemented through the Act to Prevent Pollution from Ships (33 U.S.C. § 1901) and enforced by the U.S. Coast Guard.

Annex II covers only NLS carried in bulk. Oil, sewage, garbage, air emissions, and anti-fouling systems fall under other MARPOL annexes and are not addressed here.

Chemical tankers built on or after July 1, 1986 also have to comply with the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk, known as the IBC Code.³ Chapter 17 of that code is the working list: every substance covered by Annex II appears there with its category and carriage requirements.

The Four NLS Categories

Every substance covered by Annex II falls into one of four categories. The category dictates whether the residue can ever go to sea and how much cleaning is required first.

Category X — Major hazard to marine resources or human health. Discharge of the cargo or any effluent containing it is prohibited. After unloading, the tank must be prewashed and the washings sent to a shoreside reception facility until the effluent concentration drops to 0.1% by weight or below.⁴
Category Y — Hazard less severe than X, but still significant. Discharge quantity and quality are strictly limited. Prewash is often required, with washings going ashore.
Category Z — Minor hazard. Discharge conditions are somewhat less restrictive after proper stripping and cleaning.
Other Substances (OS) — Evaluated and found to pose no harm. Bilge water, ballast water, or residues containing only OS substances fall outside Annex II requirements entirely.⁵

Persistent Floaters

Amendments in force since January 1, 2021 tightened the rules for a subset of Category Y substances known as persistent floaters. These are slick-forming liquids that float, resist evaporation, and do not dissolve. When a Category Y persistent floater has a viscosity of 50 mPa·s or higher at 20°C, or a melting point at or above 0°C, mandatory prewash applies in designated waters.⁶ The designated zones include North West European waters, the Baltic Sea area, Western European waters south to Cape São Vicente, and the Norwegian Sea. In these areas the entire prewash residue-and-water mixture must go to a reception facility at the unloading port.

Residue Limits and Tank Stripping

Before any discharge or cleaning happens, the tank has to be stripped as thoroughly as the ship’s equipment allows. Tanks carrying Category Y and Z substances must have efficient stripping systems, and the piping arrangement must keep residue within the regulatory limit.⁷

For ships built on or after January 1, 2007, that limit is 75 liters per tank and associated piping, for all three of Categories X, Y, and Z.⁵ Older ships operated under looser limits of 100 or 300 liters depending on category. Compliance with the 75-liter figure is verified through tank stripping tests, and the results are documented in the ship’s Procedures and Arrangements Manual.

Prewash Rules

Prewash is the most demanding cargo-handling obligation in Annex II. After unloading a Category X substance, the tank has to be prewashed before the ship leaves the unloading port, with washings routed to a shoreside reception facility until the effluent concentration is at or below 0.1% by weight.⁴ Water added to the tank after that threshold is met may be discharged at sea if the standard discharge conditions are satisfied.

For Category Y and Z substances, prewash is required whenever the cargo has not been unloaded in accordance with the ship’s Procedures and Arrangements Manual. Washings again go to a reception facility. Persistent floaters in the designated European zones trigger prewash even when unloading followed the manual exactly.

Substances that solidify at ambient temperature or have high viscosity are treated like Category X in practice. The prewash uses hot water or chemical cleaning agents, and the entire wash is sent ashore. Cleaning additives should be selected in consultation with the facility operator.

When Sea Discharge Is Allowed

Once a tank has been properly stripped and, where required, prewashed with washings sent ashore, water used for subsequent cleaning may be discharged at sea. All of the following conditions must be met at the same time:

The ship is en route at a minimum of 7 knots (self-propelled) or 4 knots (under tow).
The discharge point is at least 12 nautical miles from the nearest land.
The water is at least 25 meters deep.
The mixture is released through a designated underwater discharge outlet at a rate within its design capacity.

For Category Y and Z substances, the concentration in the discharge stream must also stay within regulatory quality limits. Missing any single condition makes the discharge illegal; the washings have to be retained on board or sent to a reception facility at the next port.

Special Areas

Annex II designates three special areas: the Baltic Sea area, the Black Sea area, and the Antarctic area. In the Antarctic, any discharge of NLS or NLS mixtures is prohibited outright. The stricter Baltic and Black Sea rules take effect only after every bordering state certifies adequate reception facilities and the IMO sets an effective date.⁸

Required Documents

Three documents have to be kept current on every NLS-certified ship. Port state control officers check them first.

Cargo Record Book

The Cargo Record Book logs every NLS operation: loading, unloading, internal transfers, mandatory prewashes, additional cleaning, and any discharge to sea or to a reception facility. Each entry names the tank, the substance and category, the quantities, and the date and time.⁹ Missing or incomplete entries are the first thing inspectors flag, and intentional falsification is treated as a criminal offense in many jurisdictions.

Procedures and Arrangements Manual

The P&A Manual is ship-specific and approved by the flag state. It sets out exactly how the vessel handles NLS residues and washings: stripping procedures, prewash protocols, temperatures for high-viscosity cargoes, discharge arrangements, and the operation sequence for each substance category the ship is certified to carry.² Departing from the manual is what triggers additional prewash obligations for Category Y and Z cargoes.

NLS Certificate

The International Pollution Prevention Certificate for the Carriage of Noxious Liquid Substances in Bulk is issued after an initial or renewal survey confirms the ship’s tank construction, stripping capability, piping, and discharge outlets meet Annex II. An expired or missing certificate is grounds for detention at any port in a MARPOL signatory state.

Reporting Spills and Inadequate Facilities

Accidental or emergency discharge of NLS must be reported without delay by the fastest available means, usually radio. In U.S. waters, reports go to the nearest Captain of the Port or to the National Response Center at 800-424-8802.¹⁰ Each report has to include the ship’s identity, the substance, the date and time, the position, wind and sea conditions, the vessel’s condition, and an estimate of the quantity discharged or likely to be discharged. The master must supplement the initial report as circumstances develop.

When a port lacks facilities to receive NLS washings, the master is expected to file an inadequacy report using the IMO’s consolidated format, sending it to the flag state administration and, where possible, to port state authorities.¹¹ This matters because prewash residues cannot lawfully go over the side; inadequate port facilities create a real compliance trap for masters who are otherwise doing everything right.

Port State Control Inspections

Port state control officers can board any foreign-flagged vessel in port to verify MARPOL compliance. For Annex II, inspection begins with the NLS Certificate and Cargo Record Book. Officers confirm the certificate is valid and properly signed, then work through CRB entries to check whether the ship left prior ports with NLS residues on board and whether every operation was recorded.

The P&A Manual is next. Inspectors verify the ship has an efficient stripping system and that the crew followed documented procedures. Missing entries, inconsistencies, or evidence that prewash did not happen when required will escalate the inspection. Common deficiencies leading to detention include incomplete Cargo Record Books, expired or missing certificates, and stripping systems that do not perform as documented.

Penalties for Violations

MARPOL itself does not set penalties. Each signatory state enforces through its own laws. In the United States, the Act to Prevent Pollution from Ships provides both civil and criminal tools.

Civil Penalties

A violation of MARPOL, the Act, or the implementing regulations carries a civil penalty of up to $25,000 per violation, and each day a violation continues is a separate offense.¹ A false statement in any required document carries a separate civil penalty of up to $5,000 per false statement.

Criminal Penalties

A knowing violation is a Class D felony. Individuals face prison time. Corporations face a maximum fine of $500,000 or up to twice the gross gain or loss from the offense, whichever is greater.¹ The heaviest prosecutions have centered on falsified Cargo Record Books, and engineers and officers involved in falsification have individually faced criminal charges and imprisonment.¹²

Detention

Port state authorities in any signatory country can detain a vessel until deficiencies are corrected. The lost charter revenue and schedule disruption from detention often costs the owner more than the fines themselves.

1
Office of the Law Revision Counsel. 33 USC 1908 – Penalties for Violations
2
U.S. Coast Guard. Annex II
3
International Maritime Organization. IBC Code
4
BSH. Annex II Noxious Liquid Substances in Bulk
5
International Maritime Organization. Carriage of Chemicals by Ship
6
International Maritime Organization. Resolution MEPC.315(74) – Amendments to MARPOL Annex II
7
ICS Class Surveyors. MARPOL Regulations for Noxious Liquid Substances
8
eCFR. 33 CFR 151.32 – Special Areas for the Purpose of Annex II
9
AMSA. AMSA Cargo Record Book
10
eCFR. 33 CFR 151.15 – Reporting Requirements
11
International Maritime Organization. Consolidated Guidance for Port Reception Facility Providers and Users
12
United States Department of Justice. Shipping Corporations to Pay $10.4 Million for Environmental Crimes on Four Ships