33 C.F.R. § 157.37

(a) A tank vessel may not discharge an oily mixture into the sea from a cargo tank, slop tank, or cargo pump room bilge unless the vessel:

(1) Is more than 50 nautical miles from the nearest land;

(2) Is proceeding en route;

(3) Is discharging at an instantaneous rate of oil content not exceeding 30 liters per nautical mile;

(4) Is an existing vessel and the total quantity of oil discharged into the sea does not exceed 1/15,000 of the total quantity of the cargo that the discharge formed a part, or is a new vessel and the total quantity of oil discharged into the sea does not exceed 1/30,000 of the total quantity of the cargo that the discharge formed a part;

(5) Discharges:

(i) Through the above waterline discharge point described in § 157.11(b)(2);

(ii) In accordance with paragraph 5 of appendix E to this part, if the vessel is an existing vessel with a Part Flow System meeting that appendix; or

(iii) Below the waterline in accordance with paragraph (e) of this section;

(6) Has in operation an oil discharge monitoring and control system required by § 157.12 that is designed for use with the oily mixture being discharged, except that the system may be operated manually if:

(i) The automatic system fails during a ballast voyage;

(ii) The failure is recorded in the Oil Record Book;

(iii) The master ensures that the discharge is constantly monitored visually and promptly terminated when oil is detected in the discharge; and

(iv) The system is operated manually only until the ballast voyage is completed; and

(7) Is outside the “Special Areas” defined in Regulation 1.11 of Annex I to the MARPOL 73/78.

(b) A seagoing tank vessel of 150 gross tons or more that carries asphalt or other products whose physical properties inhibit effective product/water separation and monitoring must transfer all oil cargo residues and tank washings from such cargoes to a reception facility.

(c) Each oil discharge monitoring and control system must be maintained and operated in accordance with its instructions manual.

(d) All discharge data recorded by an oil discharge monitoring and control system must be retained for at least three years. The data for the most recent year must be retained on board the vessel.

(e) Ballast water containing an oily mixture may be discharged below the waterline at sea by gravity if—

(1) The ballast is not from a slop tank;

(2) Examination with an oil-water interface detector shows that oil-water separation has taken place; and

(3) The oil layer is high enough in the tank so that it will not be discharged.