Noxious liquid substances shall be divided into four categories as follows:

Category X

Noxious liquid substances (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 therefore, justify the prohibition of the discharge into the marine environment.

Category Y

Noxious liquid substances (NLS) 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 or cause harm to amenities or other legitimate use of the sea and therefore, justify a limitation on the quality and quantity of the discharge into the marine environment.

Category Z

Noxious liquid substances (NLS) 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 and therefore, justify less stringent restrictions on thequality and quantity of the discharge into the marine environment.

Other Substances (OS)

Substances indicated as OS (Other Substances) in the pollution category column of chapter 18 of the IBC Code which have been evaluated and found to fall outside categories X, Y or Z as defined in regulation 6.1 of Annexe 2 because, they are at present, 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 or deballasting operations. The discharge of bilge or ballast water or other residues or mixtures containing only substances referred to as ‘Other Substances’ shall not be subject to any requirements of this Annexe.

 

Reference

“SHIP SAFETY AND ENVIRONMENTAL PROTECTION” BY GAUTAM SEN