International sewage discharge is regulated under the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex IV, yet not all wastewater is treated equally.
While discharge of black water is regulated, grey water is not. To understand the legislation, there is a need to differentiate between black water and grey water.
What is black water?
While black water lacks an official regulatory definition, it is commonly understood to correspond with MARPOL’s definition of sewage. It includes drainage and waste from toilets, urinals, medical facilities, and areas housing animals. In its untreated state, black water may contain harmful bacteria, viruses, and other substances that can negatively affect marine ecosystems.
MARPOL Annex IV sewage discharge requirements (Black water)
Black water is subject to strict international discharge regulations. Ships ≥400 GT or carrying ≥15 persons must be equipped with:
- An approved sewage treatment plant, or
- A sewage comminuting and disinfecting system with a holding tank, or
- A holding tank with capacity to retain all sewage.
Discharge Conditions under MARPOL:
- Discharge of untreated sewage is only allowed ≥12 nautical miles from the nearest land and must be performed at an approved rate of discharge whilst proceeding en route at not less than 4 knots.
- Discharge is permitted ≥3 nautical miles from land when a sewage comminuting and disinfecting system approved by the administration is used.
- Discharge is permitted at all times if the ship has in operation an approved sewage treatment plant which has been certified by the administration to meet the technical specifications and testing outlined in MEPC.227(64).
The Baltic Sea is a MARPOL Annex IV special area; therefore, stricter sewage discharge requirements apply in this area. It applies only to passenger ships and prohibits the discharge of all sewage, regardless of distance, unless the ship is adhering to point 3 above.
The Polar code sets out sewage restrictions within polar waters, outlined in Part II-A, Chapter 4, which prohibits the discharge of any untreated sewage, regardless of distance, from category A, B and all passenger ships.
What is grey water?
Grey water is the wastewater generated from sinks, showers, laundry, dishwashers, and galleys. While often perceived as cleaner, it can still carry pollutants like detergents, traces of oils, food particles, pathogens, and microplastics.
Grey water, unlike black wateris not directly regulated by MARPOL Annex IV. This means, in most cases, ships have been permitted to discharge grey water overboard without treatment, even in nearshore areas.
However, as environmental research evolves, and coastal states observe the cumulative impact of grey water discharge, local legislation is going above and beyond the MARPOL requirements.
Where local laws restrict the discharge of grey water, vessel crews must actively manage grey water production and storage by maintaining sufficient retention capacity for the expected port stay or arranging for the use of approved shore reception facilities.
Crews should be fully familiar with the ship’s grey water systems, ensuring that all grey water is directed to the sewage holding tank. Additionally, the overboard discharge valve must be kept closed, sealed, and properly documented to comply with port state control inspections.
Associations, like BIMCO, are supporting harmonised international standards for handling grey water. Regional organisations, such as the Helsinki Commission (HELCOM), the Baltic Sea Action Group (BSAG) and the US Environmental Protection Agency (EPA) actively reports and analyses sewage discharge quantities and its effects.
Local restrictions on sewage discharge
- Black water: Prohibits the discharge of even treated sewage within their waters.
- Grey water: None
Norway
- Black water: Prohibits all sewage discharge in its World Heritage fjords, outlined in section 10a of the environmental safety for ships and mobile offshore units.
- Grey water: Norway prohibits the discharge of grey water within its waterways if it is untreated, requiring grey water to be processed through an approved sewage treatment plant (point 3 above). In addition, within the world heritage fjords, passenger ships ≥ 2500 GT and carrying more than 100 persons, are prohibited of discharging grey water. These requirements are outlined in section 10 and 10a of the environmental safety for ships and mobile offshore units.
Finland
- Black water: Prohibits the discharge of treated sewage within its territorial waters effective since 1 July 2025. Outlined within their Maritime Environmental Protection Act proposal under chapter 5.
- Grey water: Finland prohibits the discharge of grey water into their territorial waters effective from January 1, 2030. This applies to all vessels above 400GT or vessels permitted to carry more than 15 people and have a hull length of more than 24 meters. This is outlined within their Maritime Environmental Protection Act proposal, under chapter 5.
Türkiye
- Black water: Türkiye prohibits the discharge of all sewage within its coastal waters. Violating this regulation may result in fines based on the vessel’s gross tonnage.
- Grey water: Turkeys restriction of all sewage includes grey water, defined as domestic and washing waters, within its coastal waters.
United States (US)
- Black water: The US, under their Clean Water Act (CWA) have their own compliance requirements which stipulate the required standard of sewage plants within 3nm of their territorial waters. The US also maintain a list of no discharge zones (NDZ) where all sewage, including treated, cannot be discharged.
- Grey water: The requirements for grey water discharge are found in 40 C.F.R. 139.21, which establishes discharge standards for releases occurring within 3 nautical miles.
Canada
- Black water: The marine transportation department of Canada have enforced increased restrictions for cruise ships. Within 3nm from shore, all sewage, including grey water, cannot be discharged. Between 3nm and 12nm, all sewage, including grey water must be processed through an approved treatment plant.
- Grey water: Grey water is included in all sewage discharge restrictions for cruise ships.
China
- Black water: China has their own sewage discharge plant requirements,with limits depending on when the sewage plant was installed. If meeting the limits, there is no restrictions, but unlike MARPOL the vessel must always be proceeding en route for any discharge to take place. Certain areas restrict the discharge of all sewage, such as Huangpu and Suzhou River.
- Grey water: No restrictions, but provide guidance for managing the quantity and quality.