Sewage Management — MARPOL Annex IV
Discharging sewage seems simple until distance from the coast, the treatment plant's status and stricter local rules all intersect at once.
Operational Explanation
MARPOL Annex IV governs the discharge of sewage from ships. It applies to ships of 400 GT and above on international voyages, or certified to carry more than 15 persons. Every ship subject to it must be fitted with an approved treatment plant (Sewage Treatment Plant, STP), or an approved comminuting and disinfecting system, or a holding tank.
Distance is only one of the conditions, and it is the one crews remember. Regulation 11.1.1 adds, for sewage that has been stored in holding tanks and for sewage originating from spaces containing living animals, that the discharge be made not instantaneously but at a moderate rate with the ship en route and proceeding at not less than 4 knots, approved by the Administration on the basis of the IMO guidelines. A discharge from a holding tank made at anchor, or emptied in one go, is non-compliant even at the right distance, and it is exactly what a Port State Control Officer (PSCO) finds when comparing the discharge record against VDR or AIS data.
That condition does not extend to every discharge. The proviso lives inside Regulation 11.1.1 and does not touch 11.1.2.
With an approved treatment plant in operation you may discharge at anchor. Regulation 11.1.2 sets no distance, no en-route condition and no discharge rate: it asks for the plant certified under Regulation 9.1.1 and for an effluent that produces no visible floating solids and no discolouration of the surrounding water. The Special Area regime for passenger ships, any stricter requirements of the coastal State and the port's own rules all remain — but not an en-route condition the Convention does not write.
Discharge from an approved treatment plant is subject to its own conditions rather than being unconditional: the effluent must produce no visible floating solids and must not discolour the surrounding water, the plant must be of a type approved to the standard applicable to it — MEPC.227(64) for plants installed on or after 1 January 2016, earlier standards for existing plants — and the Special Area regime of Annex IV (Baltic Sea, with the stricter nitrogen and phosphorus requirements for passenger ships) and any stricter coastal State restrictions continue to apply. In the absence of an STP, discharge of comminuted and disinfected sewage is permitted beyond 3 nautical miles from the nearest land; discharge of untreated sewage requires a distance of at least 12 nautical miles.
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
MARPOL Annex IV: Regulation 1-3 (application: ships ≥400 GT on international voyages or certified for >15 persons), Regulation 9 (equipment), Regulation 11 (discharge: distances of 3 and 12 nautical miles, ship proceeding en route, discharge at a moderate rate approved by the Administration on the basis of the IMO guidelines, not less than 4 knots for ships with a holding tank; Special Area regime for the Baltic Sea), technical standard MEPC.227(64) for plants installed on or after 1 January 2016, earlier standards for existing plants. Certification via the International Sewage Pollution Prevention (ISPP) Certificate, valid for 5 years.
Regulation 11 contains three distinct discharge routes, and merging them is the error that leads to discharging wrongly. The first: sewage comminuted and disinfected using a system approved under Regulation 9.1.2, at more than 3 nautical miles from the nearest land. The second: sewage neither comminuted nor disinfected, at more than 12 nautical miles. The third has no distance: an approved sewage treatment plant in operation under Regulation 9.1.1, with effluent that “shall not produce visible floating solids nor cause discoloration of the surrounding water” (11.1.2).
The condition on discharge rate does not belong to the second route alone, and this is where almost everyone goes wrong. The text reads “provided that, in any case, the sewage that has been stored in holding tanks, or sewage originating from spaces containing living animals, shall not be discharged instantaneously but at a moderate rate when the ship is en route and proceeding at not less than 4 knots”: it applies in any case, therefore to the 3-mile route as well, and it covers sewage that has sat in a holding tank and sewage from spaces containing living animals. What a “moderate rate” is, the proviso does not quantify. A measure exists, but it must be taken for what it is: res. MEPC.157(55) — titled “Recommendation on standards for the rate of discharge of untreated sewage from ships” — proposes DRmax = 0.00926 × V × D × B, where V is average speed in knots, D draught and B breadth in metres: one two-hundred-thousandth of the swept volume, as an average over any twenty-four hour period, with individual hours allowed to exceed that average by no more than 20%.
Two caveats, both in the resolution itself. It is a recommendation, not a Convention provision: it does not turn a rate into an enforceable numerical obligation. And its scope is untreated sewage discharged beyond 12 nautical miles — neither treated by an approved plant nor comminuted and disinfected. It is therefore not the rate of the whole proviso, and in particular it does not cover the 3-mile route, which concerns comminuted and disinfected sewage. The proviso itself does apply in any case: moderate rate, ship en route, not less than 4 knots.
The Special Area regime is a different thing again and concerns passenger ships only: Regulation 11.3 prohibits discharge within the special area — for Annex IV the only one designated is the Baltic Sea — except with a treatment plant approved under Regulation 9.2.1, which is the strengthened nitrogen and phosphorus standard, not the 9.1.1 that applies elsewhere. The dates are three, not two, and they are not in Regulation 11.3: that paragraph only says “on a date determined by the Organization pursuant to regulation 13.2 of this Annex, but in no event prior to” 1 June 2019 and 1 June 2021 — two minimum thresholds. The operative dates are set by resolution MEPC.275(69) of 22 April 2016: “1 June 2019 for new passenger ships; 1 June 2021 for existing passenger ships other than those specified in paragraph 1.3 below; and 1 June 2023 for existing passenger ships en route directly to or from a port located outside the special area and to or from a port located east of longitude 28˚10' E within the special area that do not make any other port calls within the special area”. The third date is not a general extension for eastbound traffic: it is cumulative and narrow — existing passenger ship, direct voyage between a port outside the area and a port east of 28°10' E inside it, and no other port call within the special area. A single intermediate call in the Baltic voids the extension and puts the ship back on 1 June 2021. The definition of new and existing passenger ship was inserted into Regulation 1 by resolution MEPC.274(69), in force since 1 September 2017.
Scope of Application
Ships of 400 GT and above on international voyages, or ships of any tonnage certified to carry more than 15 persons.
Procedure / How to Complete It
- Verify the operational status of the treatment plant (STP) before any discharge, including compliance with the parameters of the MEPC.227(64) standard.
- If the STP is unavailable or not working, check the distance from the nearest land before proceeding: minimum 3 nautical miles for comminuted and disinfected sewage, minimum 12 nautical miles for untreated sewage.
- If the discharge is from a holding tank, or concerns sewage originating from spaces containing living animals, verify the two conditions of the Regulation 11.1.1 proviso: ship proceeding en route and discharge at a moderate rate approved by the Administration, not less than 4 knots. Record speed and position, because that is what is cross-checked against VDR/AIS. With an approved treatment plant in operation (Regulation 11.1.2) those two conditions do not apply.
- In Special Areas with additional requirements (e.g. the Baltic), verify that the plant also meets the nitrogen and phosphorus removal standard before discharging treated effluent.
- Check any local restrictions stricter than the MARPOL minimum (e.g. bans within the territorial waters of individual coastal States).
- Record the operation per the company's internal procedures, keeping evidence of position and conditions at the time of discharge.
Practical Example
Example check: before a discharge near the coast, the officer of the watch verifies that the STP is operating within the required parameters, checks the GPS position against the nearest land, and checks whether the area falls within a Special Area with additional requirements or within territorial waters with stricter local restrictions.
What Typically Goes Wrong
Common Mistakes Mistake Library
| Mistake | Consequence | How to avoid it |
|---|---|---|
| Discharge of untreated sewage with an imprecise estimate of distance from the coast | Violation of Regulation 11, possible deficiency or legal action by the coastal State | Always verify distance via GPS, never by estimate, before any untreated discharge |
| STP assumed 'always compliant' without periodic verification of effluent parameters | Non-compliant discharge even with the STP nominally running | Periodically check the actual effluent parameters, not just the plant's operating status |
| Ignoring local restrictions stricter than the MARPOL minimum (e.g. Finland from 2025) | Violation of local regulation even while fully MARPOL-compliant | Always check specific local restrictions before every call, not just the MARPOL framework |
What the PSCO Checks
Operational Tips
- Always verify position via GPS before an untreated discharge, never rely on estimates.
- Periodically check the STP's actual effluent parameters, not just the plant's operating status.
- Before every call at a new area, check whether local restrictions stricter than the MARPOL minimum exist.
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.
- ISPP Certificate valid and consistent with the plant installed on board
- STP operating status verified against the MEPC.227(64) parameters
- If discharging from a holding tank: ship en route and speed not less than 4 knots (distance alone is not enough). With an approved plant in operation the condition does not apply
- Moderate rate approved by the Administration applied for discharge from a holding tank, at a speed of not less than 4 knots
- Distance from the coast verified via GPS before untreated discharges
- Additional Special Area requirements (nitrogen/phosphorus) verified where applicable
- Specific local restrictions of the coastal State verified before the call
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Last substantive revision of this page: 19 August 2026 · page fingerprint 814096484d65