MARPOL Annex IV: Sewage Discharge Tiers, Special Areas, and Certificates

Under the MARPOL Annex IV sewage discharge rules, a ship at sea has three options depending on how the waste has been treated: effluent from an approved treatment plant may be released with no distance limit, comminuted and disinfected sewage may be discharged more than three nautical miles from the nearest land, and untreated sewage may only be discharged more than twelve nautical miles from land while the ship is underway at no less than four knots.¹ The Baltic Sea is a designated Special Area with tighter rules for passenger ships. Annex IV has been in force internationally since September 27, 2003.²

What Annex IV Treats as Sewage

The discharge rules only apply to waste that meets the Annex IV definition of sewage. That covers drainage from toilets and urinals, drainage from medical spaces such as sick bays and dispensaries, drainage from spaces holding live animals, and any other waste water mixed with those sources.¹

Grey water from showers, laundry, dishwashers, and galley sinks is not sewage under Annex IV, and there are no international rules on discharging grey water on its own. It only becomes regulated waste if it is combined with black water. That is why ships that want maximum discharge flexibility keep the two plumbing systems separate.

Which Ships the Rules Apply To

Annex IV binds two categories of ships on international voyages: vessels of 400 gross tonnage or more, and smaller vessels certified to carry more than 15 people, counting passengers and crew together.³ The second category sweeps in many charter and excursion vessels that would otherwise fall below the tonnage threshold. A dive boat certified for 20 people is covered regardless of its size.

Warships and government ships used exclusively for non-commercial purposes are exempt under MARPOL’s sovereign immunity provisions. Naval forces commonly follow Annex IV standards as a matter of policy, but cannot be inspected or detained the way commercial vessels can.

The Three Discharge Tiers

Regulation 11 organizes discharge into three tiers. The cleaner the sewage before it leaves the ship, the closer to shore it can be released.

Treated Effluent From an Approved Plant

A ship operating a certified sewage treatment plant that meets the IMO effluent standards under Resolution MEPC.227(64) can discharge treated effluent with no minimum distance from shore, no speed requirement, and no need to be underway.¹ Two conditions still apply: the plant’s test results must be recorded on the ship’s International Sewage Pollution Prevention Certificate, and the effluent cannot produce visible floating solids or discolor the surrounding water.

The performance benchmarks the plant must meet during type-approval testing are:

Thermotolerant coliforms: no more than 100 per 100 milliliters (geometric mean)
Total suspended solids: no more than 35 mg/l (geometric mean, adjusted for dilution)
BOD5: no more than 25 mg/l (geometric mean)
COD: no more than 125 mg/l (geometric mean)
pH: between 6 and 8.5⁴

A plant that cannot hit these numbers consistently will not receive type approval, and discharge from it is illegal regardless of distance from shore.

Comminuted and Disinfected Sewage

Sewage that has been ground up and chemically disinfected but not fully treated may be discharged more than three nautical miles from the nearest land.¹ If the waste has been held in tanks before release, discharge must occur at a moderate rate while the ship is en route at no less than four knots. Dumping the contents of a holding tank all at once is prohibited.

Untreated Sewage

Raw sewage may only be discharged more than 12 nautical miles from the nearest land. The ship must be en route at a minimum speed of four knots, and the discharge rate must follow standards approved by the flag administration.¹ Speed and distance work together so that waste disperses across a wide area rather than forming concentrated plumes.

The rate is not a matter of judgment. IMO Resolution MEPC.157(55) sets a formula:

DR_max = 0.00926 × V × D × B

DR_max is the maximum permissible discharge rate in cubic meters per hour, V is the ship’s average speed in knots, D is the draft in meters, and B is the beam in meters.⁵ The rate is averaged over 24 hours or the total discharge duration, whichever is shorter, and it may be exceeded by no more than 20 percent in any single hour.

Stricter Rules in the Baltic Sea Special Area

The Baltic Sea is currently the only Special Area designated under Annex IV.² Inside it, passenger ships (those carrying more than 12 passengers) face a near-complete ban on sewage discharge. The only exception is for ships operating an approved treatment plant that meets the standard MEPC.227(64) limits and additionally removes nitrogen and phosphorus: total nitrogen below 20 mg/l (or a 70 percent reduction) and total phosphorus below 1.0 mg/l (or an 80 percent reduction).⁴

The ban was phased in. New passenger ships have had to comply since June 1, 2019, most existing passenger ships since June 1, 2021, and vessels transiting directly through the eastern Baltic since June 1, 2023.² Passenger ships that cannot meet the enhanced standard must land all sewage at port reception facilities.

Onboard Equipment Determines What You Can Do

Regulation 9 requires every ship covered by Annex IV to carry at least one of three systems, and the system dictates which of the discharge tiers is available.⁶

A sewage treatment plant that processes waste to IMO effluent standards. This gives the ship the most flexibility because discharge is allowed with no distance limit.
A comminuting and disinfecting system that grinds and chemically treats sewage without fully purifying it. Discharge requires at least three nautical miles from land.
A holding tank sized to hold waste for the expected number of people and voyage length, with a visual indicator so crew can monitor its level. The tank contents go to a shore reception facility.⁷

Non-functional equipment found during a port state inspection can trigger immediate detention.

The Certificate That Proves Compliance

Compliance is documented through the International Sewage Pollution Prevention Certificate (ISPP). An initial survey before the ship enters service confirms that the sewage equipment matches the approved plans and works correctly.⁸ Renewal surveys follow at intervals of no more than five years, carried out by the flag state or an authorized classification society. Major repairs or modifications to the sewage system trigger an additional survey. A valid ISPP certificate is a prerequisite for entering most international ports.

Enforcement and Penalties

Flag states carry the primary enforcement duty at sea, but port state control authorities in any country can board and inspect foreign-flagged vessels calling at their ports. A deficiency finding can range from a note requiring correction by the next port to a detention order that keeps the ship idle until repairs are complete.

In the United States, MARPOL violations are enforced under the Act to Prevent Pollution from Ships. Civil penalties can reach $25,000 per violation, with false statements carrying a separate penalty of up to $5,000 each.⁹ A knowing violation is a Class D felony, which carries a possible prison sentence of five to ten years.¹⁰ The Coast Guard has authority to board and inspect any non-public vessel in U.S. navigable waters and can detain vessels believed to be operating in violation.¹¹

The United States also imposes a domestic layer on top of MARPOL through the Clean Water Act. Vessels 65 feet (19.7 meters) or shorter may use a Type I, Type II, or Type III marine sanitation device; larger vessels must use a Type II or Type III device.¹² States can petition the EPA to designate No-Discharge Zones where no sewage discharge is permitted at all, even fully treated effluent.¹¹ Other countries layer their own national rules on top of the MARPOL baseline, so operators in multiple jurisdictions need to check local requirements as well as the international ones.

1
Bahamas Maritime Authority. MARPOL Annex IV Sewage Pollution Prevention
2
International Maritime Organization. Prevention of Pollution by Sewage from Ships
3
Parliamentary Monitoring Group. MARPOL Annex IV Regulations for the Prevention of Pollution by Sewage
4
International Maritime Organization. Resolution MEPC.227(64) – 2012 Guidelines on Effluent Standards and Performance Tests for Sewage Treatment Plants
5
International Maritime Organization. Recommendation on Standards for the Rate of Discharge of Untreated Sewage from Ships – MEPC.157(55)
6
Bahamas Maritime Authority. MN059 MARPOL Annex IV Sewage Pollution Prevention
7
Paris MoU. MARPOL Annex IV Inspection Guidelines
8
Clasification Society Rulefinder. Guidelines for Surveys for the International Sewage Pollution Prevention Certificate
9
Office of the Law Revision Counsel. 33 USC 1908 – Penalties for Violations
10
Office of the Law Revision Counsel. 18 USC 3559 – Sentencing Classification of Offenses
11
Office of the Law Revision Counsel. 33 USC 1322 – Marine Sanitation Devices
12
eCFR. Marine Sanitation Devices – 33 CFR Part 159