Operational guide · Operational procedure

Sewage Management — MARPOL Annex IV

Sewage discharge conditions: approved plant, distances, discharge rate and additional Baltic, polar and local requirements.

MARPOL Annex IVsewageISPPsewage discharge

Operational Explanation

Annex IV applies to ships on international voyages that are at least 400 GT or certified to carry more than 15 persons, counting crew and passengers. The voyage condition applies to both branches. Regulation 9 permits an approved STP, approved comminution/disinfection with collection capacity, or a holding tank: it does not require an STP on every ship.

Under 11.1.1, sewage stored in holding tanks or originating from spaces containing live animals must be discharged at an Administration-approved moderate rate while en route at at least 4 knots. This also covers stored comminuted/disinfected sewage; it does not extend to the separate STP route under 11.1.2.

That condition does not extend to every discharge. The proviso lives inside Regulation 11.1.1 and does not touch 11.1.2.

Regulation 11.1.2 sets no fixed distance or en-route condition for effluent from an approved operating STP, without visible floating solids or water discoloration. This is not a universal permission to discharge at anchor: first check Baltic passenger-ship requirements, the Polar Code and local bans, which may prevent discharge.

Plant standards: MEPC.2(VI) for installations before 2010; MEPC.159(55) for 2010–2015; MEPC.227(64), amended by MEPC.284(70), from 2016. Check the certificate and provisions applicable to the installation rather than automatically declaring an older plant non-compliant. Under 11.1.1 distances are more than 3 nautical miles for comminuted/disinfected sewage and more than 12 nautical miles for untreated sewage, from nearest land as defined by MARPOL.

In Special Areas (e.g. the Baltic Sea), passenger ships intending to discharge treated effluent must also meet the additional standard for nitrogen and phosphorus removal, verified in the plant's Type Approval Certificate.

Regulatory Reference

Annex IV, regulations 2, 4–9 and 11: MEPC.115(51) and subsequent amendments. Rate: MEPC.157(55). Plant standards: MEPC.159(55), MEPC.227(64), MEPC.284(70). Baltic: MEPC.274(69) and MEPC.275(69).

The MEPC.157(55) method for untreated sewage uses DR_max = 0.00926 × V × D × B (m³/h), with V in knots, D draught and B breadth in metres. Average over 24 hours, or the actual discharge duration if shorter; any hour must not exceed the rate by more than 20%. The guidelines are recommendatory, but the Administration-approved rate is a mandatory condition of 11.1.1. Do not automatically apply this formula to the STP or comminuted/disinfected route.

For passenger ships in the Baltic, 11.3 permits discharge only through the 9.2.1 STP route with nitrogen/phosphorus removal; otherwise retain and deliver ashore. Operational dates were 1 June 2019 for new ships, 2021 for existing ships and 2023 only for existing ships on a direct voyage between an outside port and a port east of 28°10′ E, with no other calls in the area. All dates have passed; do not bypass the passenger regime using untreated sewage beyond 12 nautical miles.

Greywater mixed with sanitary drainage falls within the sewage definition in regulation 1.3. If the mixture contains waste or water governed by other MARPOL Annexes, their requirements also apply: sewage treatment does not authorize discharge of oil or other contaminants.

In polar waters, category A/B ships and all passenger ships constructed from 1 January 2017 may discharge only through the STP route in Part II-A, 4.2.1.3, subject to the other conditions. Polar Code 4.2.2.

2026 update: IMO PPR 13 agreed in principle on draft Sewage Record Book and Sewage Management Plan provisions, still subject to review. These are not MARPOL requirements already in force; applicable flag, local and SMS record-keeping requirements remain.

Scope of Application

Procedure / How to Complete It

First check jurisdiction, Baltic passenger-ship requirements and the Polar Code. If prohibited or conditions cannot be demonstrated, retain and deliver; do not automatically switch to the 12-mile route.
Identify the ship’s approved route: STP, comminution/disinfection or retention. Check certificate, installation-date standard, manual and remaining capacity.
For 11.1.1, establish more than 3 or more than 12 nautical miles from nearest land using GNSS position and charts/MARPOL reference lines. GPS alone does not establish the legal baseline.
For stored sewage or sewage from live-animal spaces under 11.1.1: ship en route at at least 4 knots, approved rate and no instantaneous emptying. Apply the averaging period and hourly limit of the approved method.
For STP 11.1.2 check operation and effluent; in polar waters also meet chapter 4 distance/practicability conditions. For A/B ships in prolonged ice >1/10, check Administration approval of discharge.
Record as required by local rules, flag and SMS. Do not invent a universal MARPOL sewage-discharge record book.

Practical Example

Scenario: a 500 GT ship enters Finnish territorial waters with an operating STP. Plant compliance does not authorize discharge: since 1 July 2025 the national treated-sewage ban covers ships of at least 400 GT and smaller ships designed for at least 15 persons. Close the outlet on entry and record it in the logbook as required locally; plan retention and reception capacity. Traficom — national wastewater prohibitions.

What Typically Goes Wrong

The Finnish threshold is not identical to MARPOL’s “more than 15 persons”. Do not bring forward or confuse the separate greywater timetable with the sewage ban already applicable.

Common Mistakes Mistake Library

Avoid “at least 12” instead of “more than 12”, measuring from a generic coast rather than MARPOL nearest land, treating the approved rate as optional, or using an STP as permission in every jurisdiction.

What the PSCO Checks

A PSCO may check the ISPP where required, approved equipment or retention capacity, manual, discharge conditions and available records. The Administration determines an ISPP period not exceeding five years, subject to the specific provisions of regulation 8.

Operational Tips

Prepare a route matrix covering national bans, Baltic passenger requirements and polar conditions.
Measure distances with GNSS and appropriate charts; keep an operational margin.
Verify effluent, applicable standard and approved rate, not just the “plant running” indication.

Preparation checklist

Educational checklist. This summary supports learning and preparation only. It does not replace the vessel’s approved procedures, manuals, statutory documents, company SMS, or applicable official requirements. Completing it demonstrates neither compliance nor readiness for an inspection: it shows that a list has been read, not that the ship is in order. Always verify the current documents carried on board.

Ship/voyage scope and certification checked
Local bans, Baltic passenger regime and Polar Code checked before choosing a discharge route
Plant approved for installation date and service, or adequate retention capacity
11.1.1 route: more than 3/12 nautical miles; stored/animal sewage also requires ≥4 knots and approved rate
Nearest land and ice references checked cumulatively
Required records and reception plan available

FAQ

Which distances apply under 11.1.1?

More than 3 nautical miles for comminuted/disinfected sewage and more than 12 for untreated sewage, from nearest land. Stored/animal sewage also requires en-route operation, at least 4 knots and an approved moderate rate. More restrictive regimes still apply.

Does an STP always permit discharge at anchor?

No. Regulation 11.1.2 has no en-route condition, but the Polar Code, Baltic passenger regime and national/port rules may prohibit discharge.

Does an ISPP always last five years?

No: its Administration-determined duration does not exceed five years, subject to the specific provisions of regulation 8.