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. At the time of writing, discharge of black water is regulated, whilst grey water is not. To understand the legislation, we need to differentiate between black water and grey 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.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.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, orA sewage comminuting and disinfecting system with a holding tank, orA 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.Local authorities may impose stricter discharge regulations that apply even to treated sewage, as outlined below.Grey WaterUnlike black water, grey water is 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 dischargeThe table below outlines our understanding of the positions taken by local authorities that have or will prohibit the discharge of sewage. This information is to the best of our know