Answer
Under MARPOL Annex II (Regulation 13), the discharge of Noxious Liquid Substances (NLS) or ballast water/tank washings containing such substances into the sea is strictly prohibited unless specific operational criteria are met.
The criteria differ depending on the category of the substance:
NLS Cargo Categorization
Category X: Major hazard to marine resources or human health; justifies the prohibition of discharge.
Category Y: Hazards justifying a limitation on the quality and quantity of discharge.
Category Z: Minor hazards justifying less stringent restrictions.
Other Substances (OS): Evaluated and found to fall outside Categories X, Y, or Z; not subject to the strict provisions of Annex II.
Operational Discharge Criteria (Outside Special Areas)
For residues of Category X, Y, or Z, discharge into the sea is permitted only if all the following conditions are satisfied:
Vessel en route: The ship is proceeding en route at a speed of at least 7 knots (for self-propelled vessels) or 4 knots (for vessels which are towed or not self-propelled).
Discharge Location: The discharge is made below the waterline through the underwater discharge outlet(s), not exceeding the maximum design rate of the outlet.
Distance from Land: The discharge is made at a distance of not less than 12 nautical miles from the nearest land.
Water Depth: The discharge is made in a depth of water of not less than 25 meters.
Approved Arrangements: The discharge must comply with the ship’s approved Procedures and Arrangements (P&A) Manual.
Mandatory Prewash and Stripping Requirements
Category X: A mandatory prewash must be carried out before the ship leaves the port of unloading. The resulting tank washings must be discharged to a shore reception facility until the concentration in the effluent is $\le 0.1\%$ by weight. The remaining contents must be discharged to the reception facility until the tank is empty.
Category Y and Z (Stripping Standards): Ships constructed on or after 1 January 2007 must be capable of stripping tanks down to a maximum residue limit of 75 liters per tank and its associated piping. If a tank fails to meet this stripping efficiency or carries high-viscosity/solidifying substances, a mandatory prewash and shore discharge are required.
Persistent Floaters (Amendments effective 2021): For specific persistent floating substances with high viscosity or melting points (e.g., certain vegetable oils and fats) in specified areas (such as North-West European waters, the Baltic Sea, and the Norwegian Sea), a mandatory prewash and complete discharge to shore reception facilities at the unloading port are required.