As per Annex 1 of MARPOL under regulation 4 specified under paragraphs 2, 3, and 6, any direct discharge of oil or oily water mixture into the sea shall be prohibited.
The regulation further explains how an oily water mixture can be treated onboard and can be discharged out at sea:
Oil Water Discharge Regulation:
For a ship with 400 GT and above, discharge of oil mixture can be done under the following conditions:
1. The ship is en route;
2. The oily mixture is processed through an oil-water separator filter meeting the requirements of regulation 14 of this Annex;
3. After passing the oil-water separator system, the oil content of the effluent without dilution does not exceed 15 parts per million;
4. The oily mixture does not originate from cargo pump-room bilges on oil tankers
5. In an oil tanker ship, the oil-water mixture is not mixed with oil cargo residues
When the ship is plying in the Antarctic area, any discharge into the sea of oil or oily mixtures from any vessel shall be prohibited.
Important Oily Water Separator (OWS) requirement: