Revised international regulations for the prevention of pollution of the sea by sewage from ships (revised MARPOL Annex IV) have entered into force on 1 August 2005. Under current thinking it is assumed that the oceans are capable of assimilating and dealing with raw sewage through natural bacterial action. However, the regulations prohibit the discharge of sewage by ships within a specified distance of the nearest land, unless they have an approved sewage treatment plant or system in operation.

The revised MARPOL Annex IV will apply to new and existing ships of 400 gross tonnage and above or ships which are certified to carry more than 15 persons, engaged in international voyages. Existing ships will be required to comply with the provisions by 27 September 2008 (five years after the entry into force of MARPOL Annex IV). The Annex requires ships to be equipped with either a sewage treatment plant, a sewage comminuting and disinfecting system or a sewage holding tank. The discharge of sewage into the sea will be prohibited at a distance of 12 nautical miles, or less, from the nearest land. Exceptions apply when the ship has an approved sewage treatment plant in operation or when discharging comminuted and disinfected sewage using an approved system at a distance of more than three nautical miles from the nearest land.When a Party to Annex IV requires ships under its jurisdiction, i.e. ships under its flag, and other ships operating in its waters, to comply with the discharge requirements, then it must ensure adequate facilities at ports and terminals for the reception of sewage are provided.