MARPOL Annex II is the International Maritime Organization regime governing pollution by noxious liquid substances carried in bulk at sea. It classifies bulk liquid chemicals into four categories. X, Y, Z, and OS, based on environmental hazard, and prescribes ship-type requirements, tank cleaning standards, and discharge restrictions per category. Annex II applies to bulk shipping in ISO tanks, parcel tankers, and dedicated chemical tankers. It does not apply to packaged liquid cargo (drums, IBCs), that cargo is governed by IMDG.
The four categories
| Category | Environmental hazard | Ship-type requirement | Discharge restriction |
|---|---|---|---|
| X | Major hazard. Discharge prohibited. | Type 1 (most stringent construction) | None to sea. Tank washings to reception facility only. |
| Y | Hazard. Limited operational discharge allowed. | Type 2 | Discharge under specific conditions (distance from coast, vessel speed, depth) |
| Z | Minor hazard. Less restrictive discharge. | Type 3 | Discharge under conditions less strict than Y |
| OS | Other Substances. No environmental hazard. | No specific type | No restrictions beyond MARPOL general rules |
The category for each substance is published in the IBC Code (International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk), Chapter 17.
Common Chinese-export bulk chemicals by category
The category drives the ship type the cargo can ride on, which in turn drives the freight rate. A few representative examples:
- Category X (most restricted): certain chlorinated solvents, some specific phenolic compounds, certain pesticide actives. Limited bulk shipping; usually ships in IBCs or drums to avoid the bulk constraint.
- Category Y: most acrylate monomers, methanol (when carried in dedicated tank ships), most amines, sulphuric acid, sodium hydroxide solution at elevated concentrations, MEG, propylene glycol, BTX aromatics
- Category Z: ethylene glycol when shipped at lower concentrations, some glycol ether solvents, several lower-toxicity organic acids, dilute caustic solutions
- OS: pure water (deionised water shipped in bulk), refined glycerine in some grades, certain food-grade products
The Y-vs-Z split for some products depends on concentration and on the specific tabulation in IBC Chapter 17, always verify before booking.
Ship type requirements
| Ship type | Construction | Cargo allowed |
|---|---|---|
| Type 1 | Highest standard: double bottom, double sides, separate cargo systems | Category X cargoes |
| Type 2 | Stricter than general chemical tanker | Category Y cargoes |
| Type 3 | Standard chemical tanker | Category Z and OS cargoes |
For ISO-tank cargo on a container vessel, the ship-type requirement is met by the container vessel itself plus the certification of the ISO tank. For parcel tanker cargo (multiple parcels of different chemicals on a chemical tanker), the ship type must match the most restrictive cargo in the parcel mix.
Tank cleaning and prewash
The Annex II tank-cleaning regime is the part that affects buyers most directly. After discharge, tanks carrying Category X, Y, or Z cargoes may require:
- Mandatory prewash before next cargo loading (Category X always; Category Y in some cases; Category Z in fewer)
- Discharge of washings to a port reception facility (Category X; Category Y in some cases)
- Documented prewash certificate showing residue removal to specified levels
For a buyer using ISO tanks for caustic soda solution shipments, the tank operator runs a prewash between cargoes. The cost is typically USD 800 to USD 2,500 per tank depending on the cargo and the destination port’s reception facility charges. This is built into the per-tank lease cost in most operating models, but for spot bookings it can show as a separate line.
The prewash discipline is what keeps category-Y-and-X bulk chemical tanks from cross-contaminating subsequent cargoes. A tank that carried MEG (Y) and is then loaded with deionised water (OS) without proper prewash arrives with MEG residue. The buyer’s process is contaminated. The tank operator owns the failure but the buyer carries the loss.
Annex II vs IMDG marine pollutant
Both regimes cover environmental hazard from chemical cargoes, but they apply to different cargo modes:
| Regime | Applies to | Key marker |
|---|---|---|
| MARPOL Annex II | Bulk liquid cargo (ISO tank, parcel tanker, chemical tanker) | Category X/Y/Z/OS |
| IMDG marine pollutant | Packaged cargo (drums, IBCs, containers) | Fish-and-tree mark |
A substance can be Annex II Category Y when shipped in bulk AND IMDG marine pollutant when shipped in packaged form. The two regimes coexist on the same substance in different cargo modes.
Documentation chain
For Annex II bulk cargo:
- Cargo loading documents declare the Annex II category
- Procedures and Arrangements (P&A) Manual of the tank or vessel must show approval to carry that category
- Cargo record book entries for loading, discharge, and any tank cleaning
- For Category X discharge: a Pollution Prevention Officer signed entry plus port reception facility receipt for the washings
A buyer chartering an ISO tank for first-time bulk Category Y shipment from China should request the tank’s P&A Manual approval and the prewash certificate from the tank operator. The carrier confirms acceptance based on these documents.
Operator note: the parcel tanker book is shrinking
Parcel-tanker capacity for moving Chinese chemicals to Western destinations has tightened since 2022. Several operators retired older Type 2 vessels, and new-build Type 1 capacity is limited. For Category X cargoes the spot market can be very thin, with lead times running 6 to 10 weeks even for major Chinese ports. ISO-tank container shipping has absorbed some of this displaced volume, but for true bulk parcel cargoes (1,000+ tonnes per parcel) ISO-tank is not economic. Build longer planning horizons into Category X procurement.
Related terms
IMDG is the umbrella for packaged DG. ISO tank is the most common Annex II cargo mode for chemical buyers. Marine pollutant is the parallel IMDG designation for packaged cargo.