MARPOL Annex 1 Discharge criteria (Oil Oily mixtures)
Any discharge into the sea of oil or oily mixtures from ships to which MARPOL Annex I applies shall be prohibited except when all the following conditions are satisfied:
(a) for an oil tanker, except as provided for in subparagraph (b) of this paragraph:
(i) the tanker is not within a special area;
(ii) the tanker is more than 50 nautical miles from the nearest land;
(iii) the tanker is proceeding en route;
(iv) the instantaneous rate of discharge of oil content does not exceed 30 litres per nautical mile;
(v) the total quantity of oil discharged into the sea does not exceed for existing tankers 1/15,000 of the total quantity of the particular cargo of which the residue formed a part, and for new tankers 1/30,000 of the total quantity of the particular cargo of which the residue formed a part.
(vi) the tanker has in operation an oil discharge monitoring and control system and a slop tank arrangement as required by this Annex.
(b) From a ship of 400 tons gross tonnage and above other than an oil tanker and from machinery space bilges excluding cargo pump-room bilges of an oil tanker unless mixed with oil cargo residue:
(i) the ship is not within a special area;
(ii) the ship is proceeding en route;
(iii) the oil content of the effluent without dilution does not exceed 15 parts per million;
(iv) the ship has an operational ODMCS as required by this Annex.
The above provisions of shall not apply to the discharge of clean or segregated ballast or unprocessed oily mixtures which without dilution have an oil content not exceeding 15 parts per million and which do not originate from cargo pump-room bilges and are not mixed with oil cargo residues.
No discharge into the sea shall contain chemicals or other substances in quantities or concentrations which are hazardous to the marine environment or chemicals or other substances introduced for the purpose of circumventing the conditions of discharge specified above.
(6) The oil residues which cannot be discharged into the sea in compliance with above regulations shall be retained on board or discharged to reception facilities.
Ref : MARPOL Consolidated Edition