International regulations for the prevention of pollution by sewage from ships enter into force on 27 September 2003 - nearly 30 years after their adoption. The regulations are contained in the optional Annex IV of the International Convention for the Prevention of Pollution from Ships, 1973, as modified by the Protocol of 1978 relating thereto (MARPOL 73/78). Annex IV is optional to Parties ratifying the MARPOL Convention, but concerns were raised at IMO during the 1990s that Annex IV was not receiving sufficient ratifications to achieve entry into force. The revised Annex IV is scheduled to be adopted at the MEPC's 51st session in March 2004 (the revised Annex can only be adopted when the existing Annex IV enters into force.)

Annex IV contains a set of regulations regarding discharge of sewage into the sea, ships' equipment and systems for the control of sewage discharge, provision of facilities at ports and terminals for the reception of sewage, and requirements for survey and certification. The revised Annex IV applies to new and existing ships engaged in international voyages, which are of 400 tons gross tonnage and above or are certified to carry more than 15 persons. Existing ships shall comply with the provisions of the revised Annex IV five years after the date of entry into force of this Annex. Every ship to which Annex IV applies shall be equipped with either a sewage treatment plant or sewage comminuting and disinfecting system or a sewage holding tank. The discharge of sewage into the sea is prohibited, except when the ship:

1) has in operation an approved sewage treatment plant; or
2) is discharging disinfected sewage using an approved system at a distance of more than three nautical miles from the nearest land; or
3) is discharging sewage which is not comminuted or disinfected at a distance of more than 12 nautical miles from the nearest land.
When a Party to Annex IV requires ships operating in the waters under its jurisdiction and visiting ships to comply with the above-mentioned discharge requirements then it shall ensure the provision of adequate facilities at ports and terminals for the reception of sewage.