What are Categories or types of Noxious Liquid substance or NLS as per MARPOL annex II ?

Category X

NLS which, if discharged into the sea from tank cleaning or deballasting operations, are deemed to present a major hazard to either marine resources or human health and which therefore justify the prohibition of the discharge into the marine environment;

Category Y

NLS which, if discharged into the sea from tank cleaning or ballasting operations, are deemed to present a hazard to either marine resources or human health or cause harm to amenities or other legitimate uses of the sea and therefore justify a limitation on the quality and quantity of the discharge into the marine environment;

 Category Z

NLS which, if discharged into the sea from tank cleaning or ballasting operations, are deemed to present a minor hazard to either marine resources or human health, and therefore justify less stringent restrictions on the quality and quantity of the discharge into the marine environment; and

Other substances (OS)

Substances found to fall outside of category X, Y or Z, as defined in the MARPOL Annex II regulations, and 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 and ballasting operations.

Note:

• Category X residues cannot be discharged overboard. On completion of discharge, the tank is washed and the tank washings pumped ashore.
• Category Y residues remaining in the tank after stripping in accordance with the P&A Manual can be discharged to the sea while tank cleaning provided that the conditions stipulated by MARPOL annex II are met.
• Category Z residues remaining in the tank after stripping in accordance with the P&A Manual canbe discharged to the sea while tank cleaning, provided that the conditions stipulated by MARPOL annex II are met.