Revised Annex IV was introduced by resolution MEPC.115(51), which set its entry into force on 1 August 2005. Formal entry into force of the revised Annex IV didn’t cause any practical consequences, as the revised text was already applied from 2003.

DEFINITIONS
1. For the purposes of this Annex:
“New ship" means a ship:
- for which the building contract is placed, or in the absence of a building contract, the keel of which is laid, or which is at a similar stage of construction, on or after the date of entry into force of this Annex (i.e. on or after 2003-09-27); or
- the delivery of which is three years or more after the date of entry into force of this Annex (i.e. on or after 2006-09-27).
"Existing ship" means a ship which is not a new ship, in other words this is the ship:
- the keel of which was laid before 2003-09-27; or
- the delivery of which was before 2006-09-27.

Existing ships have to comply with the provisions five years after the date of the entry into force of the new Annex IV (i.e. by 27 September 2008).

APPLICATION
In accordance with regulation 2 of Annex IV its requirements shall apply to the following ships engaged in international voyages:
1. new ships of 400 gross tonnage and above; and
2. new ships of less than 400 gross tonnage which are certified to carry more than 15 persons; and
3. existing ships of 400 gross tonnage and above, five years after the date of entry into force of this Annex (i.e. on or after 2008-09-27); and
5. existing ships of less than 400 gross tonnage which are certified to carry more than 15 persons, five years after the date of entry into force of this Annex (i.e. on or after 2008-09-27).

GENERAL REQUIREMENTS OF ANNEX IV
Annex IV contains a set of regulations regarding:
- the discharge of sewage into the sea,
- ships' equipment and systems for the control of sewage discharge,
- the provision of facilities at ports and terminals for the reception of sewage,
- requirements for survey and certification, and
- Port State control on operational requirements.
In accordance with Annex IV requirements Governments of the MARPOL Parties are required to ensure the provision of adequate reception facilities at ports and terminals for the reception of sewage.
Annex IV also includes a model International Sewage Pollution Prevention Certificate to be issued by national shipping administrations to ships under their jurisdiction.

OPERATIONAL REQUIREMENTS
Annex IV requires ship to be equipped with:
- an approved by Administration sewage treatment plant; or
- an approved by Administration sewage comminuting and disinfecting system; or
- a sewage holding tank.
A sewage holding tank shall:
- have appropriate capacity for the retention of all sewage, having regard to the operation of the ship, the number of persons on board and other relevant factors,
- be correctly constructed and shall have a means to indicate visually the amount of its contents, and
- be equipped with the ship’s discharge line to port/terminal reception facilities fitted with the standard discharge connection.

PROVISIONS OF SEWAGE DISCHARGE
It is generally considered that seas and oceans are capable of assimilating and dealing with raw sewage through natural bacterial action. The regulations in Annex IV of MARPOL 73/78 provide the conditions of sewage discharge depending on grade of sewage treatment. The discharge of sewage into the sea is prohibited, except when:
- the ship is discharging comminuted and disinfected sewage using an approved system at a distance of more than 3 nautical miles from the nearest land; or
- the ship is discharging sewage which is not comminuted and disinfected at a distance of more than 12 nautical miles from the nearest land, provided that in any case, the sewage that has been stored in holding tanks or sewage originating from spaces containing living animals ¹, shall not be discharged instantaneously but at a moderate rate when ship is en route and proceeding at not less than 4 knots; or
- the ship is discharging sewage using an approved sewage treatment plant, the results of treatment effectiveness of the plant are laid down in the ship’s International Sewage Pollution Prevention Certificate and the effluent shall not produce visible floating solids nor cause discoloration of the surrounding water.
The discharge of sewage into the sea is allowed when:
- the discharge of sewage from a ship is necessary for the purpose of securing the safety of a ship and those on board or saving life at sea; or
- the discharge of sewage is as result from damage to a ship or its equipment if all reasonable precautions have been taken before and after the occurrence of the damage, for the purpose of preventing or minimizing the discharge.