MARPOL Annex II Regulations for the control of pollution by noxious liquid substances in bulk sets out a pollution categorization system for noxious and liquid substances(entered into force on 6 April 1987)
Noxious liquid substances: Means any substance indicated in the pollution category column of chapter 17 or 18 of the Int. bulk chemical code or provisionally assessed under the provisions of regulations as falling into category X ,Y, or Z.
International bulk chemical code means the international code for the construction and equipment of ships carrying dangerous chemicals in bulk adopted by the Marine Environment Protection Committee of the organization.
En route means that the ship is underway at sea on a course or courses, including deviation from the shortest direct route, which as for as practicable for navigational purposes, will cause any discharge to be spread over as great an area of the sea as is reasonable and practicable.
The four categories are:
Noxious liquid substances which it discharged into the sea from tank cleaning or deballasting operation are deemed to present a major hazards to either marine sources or human health, therefore justify the prohibition of the discharge into the marine environment.
Noxious liquid substances which if discharged into the sea from tank cleaning or deballasting operations are deemed to present a hazard to either marine resources or human health. Therefore Justify the limitation on the quality and quantity of the discharge into marine environment.
Noxious liquid substances which if discharged into the sea from tank cleaning or deballasting operations are deemed to present a minor hazard to either marine resources or human health therefore justify less stringent restrictions on the quantity & quality of the discharge into marine environment.
Other Substances: substances which have been evaluated and found to fall outside Category X, Y or Z because they are considered to present no harm to marine resources, human health, amenities or other legitimate uses of the sea when discharged into the sea from tank cleaning of deballasting operations. The discharge of bilge or ballast water or other residues or mixtures containing these substances are not subject to any requirements of MARPOL Annex II.