A valid certificate is not the same as a compliant ship. A passenger ship can hold a current International Sewage Pollution Prevention Certificate, pass every scheduled survey, and still pump untreated sewage over the side at anchor because the duty engineer ran the discharge valve open while alongside instead of holding it for the en-route condition. The survey scheme catches the first failure; it cannot catch the second. Port State control on operational requirements is the rule built to catch the second. It lets a port State officer go past the paperwork and into the operation: ask the engineer to start and explain the sewage treatment plant, read the discharge arrangement against the Regulation 11 discharge rules, and decide whether the crew on board this week can actually run what the certificate says the ship is fitted with.
There is a numbering and provenance point to settle first, because the topic is easy to mis-cite. In the revised MARPOL Annex IV the operational-control function is Regulation 13, “Port State control on operational requirements,” which is the whole of Chapter 5, “Port State Control.” That regulation was not part of the revised Annex IV adopted by Resolution MEPC.115(51) on 1 April 2004, in force 1 August 2005. The revised Annex as first adopted ran from Regulation 1 to Regulation 12. Regulation 13 was added afterward by the amendments in Resolution MEPC.143(54), adopted on 24 March 2006 and in force from 1 August 2007. So an Annex IV that you read in its original 2004 adoption text has no Regulation 13; the consolidated Annex IV in use today does, and the regulation traces to MEPC.143(54), not to MEPC.115(51). This article uses the consolidated numbering throughout and flags the provenance where it matters.
The same operational-control function lives in every MARPOL annex, under a different number each time. It is Regulation 11 in Annex I, the oil annex; Regulation 16 in Annex II, the chemical annex; Regulation 8 in Annex III; Regulation 13 here in Annex IV; and Regulation 10 in Annex VI. The wording of the operational check, the “clear grounds” test, the familiarity requirement, and the no-sail duty, is materially the same across all of them. Only the regulation number and the pollutant change. A reference that cites “Annex IV Regulation 11” for this function is using the Annex I number by mistake, or confusing it with the Annex IV discharge regulation, which actually is Regulation 11 but governs sewage discharge, not port State control. The correct Annex IV citation for port State control on operational requirements is Regulation 13.
The two halves of MARPOL Annex IV compliance
Annex IV splits compliance into two mechanisms that are easy to confuse and worth keeping apart. The first is the survey-and-certificate scheme, set out in Regulations 4 to 8 of the revised Annex. Regulation 4 covers the surveys: an initial survey before the ship is put in service, renewal surveys at intervals the Administration sets but not exceeding five years, and an additional survey after a repair. Regulation 5 covers issue or endorsement of the certificate by the Administration. Regulation 6 covers issue or endorsement by another Government at the flag State’s request. Regulation 7 sets the form of the certificate, the International Sewage Pollution Prevention Certificate, the ISPP Certificate, and its survey scheme. Regulation 8 sets its duration and validity, up to five years, with the harmonized-survey provisions for extension and the rules that void it on a change of flag.
This half is flag-State-led, scheduled, and condition-based. It asks one question: was the ship built and equipped, and is it maintained, with a sewage system that meets the standard the Annex requires? The standard itself is in Regulation 9, “Sewage systems,” which gives the ship three ways to comply: an approved sewage treatment plant, an approved comminuting and disinfecting system with holding capability, or a holding tank. When the survey confirms one of those is fitted and working, the flag State or its recognized organization issues the ISPP Certificate, and the certificate is the portable proof of it. Annex IV applies to ships of 400 gross tonnage and above, and to smaller ships certified to carry more than 15 persons, on international voyages.
The second mechanism is port State control, and it asks a different question. Not “was the ship fitted right” but “is the crew running it right, today, in this port.” That half is port-State-led, unscheduled, and operation-based. It does not wait for a survey window; it happens whenever a foreign ship calls and an officer decides to look. It does not assume the certificate settles the matter; it treats the certificate as a starting point and tests the operation behind it. The legal home of that second mechanism in Annex IV is Regulation 13.
The distinction matters because the failure modes are different. A survey deficiency is usually a thing: a treatment plant that no longer meets its effluent standard, a macerator that has seized, a holding tank with a corroded discharge line, a standard discharge connection that does not match the dimensions in Regulation 10. An operational deficiency is usually a practice: a discharge made at anchor when the ship was not en route, a treatment plant bypassed to save running cost, a crew that has never started the plant and cannot say what its effluent standard is. The survey scheme cannot see the practice, because the practice happens between surveys, on voyages the surveyor never sails. Regulation 13 exists to put an inspector on board after that practice, close enough that the system and its records still tell the truth.
That is also why a clean ISPP Certificate does not end a Regulation 13 inspection. The certificate proves the ship was compliant at the moment of survey. It is silent on every operation since. An officer who finds the equipment in order has confirmed the survey half; the operational half is still entirely open. The two halves are deliberately separate so that neither can be used to excuse the other.
The treaty hooks: MARPOL Articles 5 and 6
Regulation 13 does not float free. It sits on two articles of the parent MARPOL Convention that give every Party the underlying power to inspect and to act. Without them, the annex regulations would be standards with no enforcement reach into a foreign ship.
Article 5 is the certificate-and-inspection article. It provides that a certificate issued under the authority of a Party shall be accepted by the other Parties and regarded as having the same validity as a certificate issued by them. That mutual recognition is the thing that makes a single survey by one flag State good in every port; without it, every port would re-survey every ship. But Article 5 carries the qualification that makes port State control possible. A ship required to hold a certificate is, in the ports of other Parties, subject to inspection by officers duly authorized by that Party. That inspection is limited to verifying that there is on board a valid certificate, unless there are clear grounds for believing that the condition of the ship or its equipment does not correspond substantially with the particulars of that certificate. The “clear grounds” phrase that governs the whole operational-control regime starts here, in the treaty, before it is repeated in the annex regulation.
Article 6 is the detection-and-enforcement article. It opens with a duty: Parties shall co-operate in the detection of violations and the enforcement of the provisions of the Convention, using all appropriate and practicable measures of detection and environmental monitoring, adequate procedures for reporting, and accumulation of evidence. It then grants the inspection power directly: a ship to which the Convention applies may, in any port or offshore terminal of a Party, be subject to inspection by officers appointed or authorized by that Party for the purpose of verifying whether the ship has discharged any harmful substances in violation of the regulations. Where the inspection turns up a violation, the Party either takes proceedings under its own law or furnishes the flag State with the evidence it has, and the flag State then investigates and may prosecute.
The two articles divide the work. Article 5 covers the standing condition of the ship against its certificate; Article 6 covers a specific suspected act of pollution and the evidence trail behind it. A Regulation 13 inspection of a passenger ship can engage both. The officer checks the ISPP Certificate and the ship’s condition under the Article 5 limb, and, if a shore report or a discharge-monitoring record suggests sewage went over the side inside the limits of Regulation 11, the Article 6 enforcement machinery opens. The no-more-favourable-treatment principle, also in the Convention, closes the obvious gap: a ship flying the flag of a State that is not a Party to MARPOL gets inspected against the same standard, so that registering under a non-Party flag buys no relief. That principle is what lets the regional MoUs hold a non-Party ship to the same Annex IV standard as a Party ship.
Regulation 13 and the “clear grounds” threshold
Regulation 13 is short, and its purpose is narrow. It establishes that a ship, when in a port or an offshore terminal of another Party, is subject to inspection by officers duly authorized by that Party concerning operational requirements under the Annex, where there are clear grounds for believing that the master or crew are not familiar with essential shipboard procedures relating to the prevention of pollution by sewage. In those circumstances, the Party shall take such steps as will ensure that the ship shall not sail until the situation has been brought to order in accordance with the requirements of the Annex. The regulation also points the inspector to the procedures the IMO has developed: the procedures relating to port State control prescribed in Article 5 of the Convention apply to the regulation. That cross-reference is the link to the free-standing IMO Procedures for Port State Control.
“Clear grounds” is the hinge, and it is not a low bar of suspicion alone. The IMO Procedures define it as evidence that the ship, its equipment, or its crew does not correspond substantially with the requirements of the relevant convention, or that the master or crew are not familiar with essential shipboard procedures relating to the safety of ships or the prevention of pollution. For an Annex IV operational check, the kinds of evidence that meet the threshold include: an ISPP Certificate that is absent, invalid, or manifestly does not match the ship; a sewage system that on a walk-through does not match the description on the certificate; a crew member responsible for the system who cannot describe how it is started, operated, and discharged; and information from a report or a complaint that the ship discharged sewage in breach of the discharge conditions. Any one of those can supply the grounds.
The threshold does real work. Below it, the officer is confined to the certificate check, because the regime deliberately protects a ship with valid papers from open-ended interrogation. Above it, the operation is open: the officer can ask the engineer to start the treatment plant, call for its type-approval certificate and effluent-standard record, walk the discharge piping from the plant or holding tank to the overboard valve and the standard discharge connection, and require the responsible crew to explain when the ship may and may not discharge. The “clear grounds” finding is what converts a routine certificate check into an operational inspection, and a careful officer records exactly which evidence supplied the grounds, because that record is what justifies any detention that follows.
The no-sail duty is the teeth. Regulation 13 does not merely permit the officer to note a problem; where the master or crew are found not familiar with the essential procedures, it requires the Party to take steps to ensure the ship does not sail until the situation is brought to order. The Procedures translate that into the detention decision: a deficiency serious enough to be detainable holds the ship until it is rectified and verified by re-inspection. A ship that is allowed to sail to a repair yard sails only to the nearest appropriate one, and only on conditions the port State sets.
What the officer actually inspects
The operational inspection of a sewage system under Regulation 13 is concrete. It is built around four things a competent crew can show and a non-compliant crew cannot: the ISPP Certificate, the sewage equipment itself, the discharge arrangement and its records, and the crew’s own familiarity with all three. Each is a separate line of evidence, and each can independently supply the clear grounds that open the rest.
The ISPP Certificate
The first document is the International Sewage Pollution Prevention Certificate. It is the certificate set out in Regulation 7 of the Annex, issued after the survey under Regulation 5, and valid for a period the Administration specifies up to five years under Regulation 8. The officer checks that it is on board, is the approved form, is in date, and matches the ship by name, IMO number and flag. The certificate also records which of the three Regulation 9 compliance options the ship is fitted with: an approved sewage treatment plant with its effluent standard, a comminuting and disinfecting system with holding, or a holding tank with its capacity. That entry is the key to the rest of the inspection, because it tells the officer what equipment should be on board and how the ship is entitled to discharge.
A certificate that is missing, expired, or for a different ship is itself a deficiency, and a serious one, because it means the ship cannot show it was ever surveyed to the standard. A certificate that is present and valid moves the inspection to the equipment: does the ship actually have what the certificate says it has, and does that equipment work? An officer who finds a treatment plant on the certificate but a holding tank in the engine room, or the reverse, has a substantial discrepancy between the ship’s condition and the particulars of its certificate, which is the exact clear-grounds trigger Article 5 names.
The sewage equipment
The Annex gives a ship three compliant configurations under Regulation 9, and the officer confirms the one the certificate records is the one fitted and working. An approved sewage treatment plant must be of a type the Administration approved against the effluent standard the IMO sets, which for plants installed on or after 1 January 2010 is the standard in Resolution MEPC.159(55) and, for ships operating in the Baltic special area, the tighter standard in Resolution MEPC.227(64) covering nitrogen and phosphorus removal. A comminuting and disinfecting system must reduce the sewage to a comminuted and disinfected state and hold it until the ship can discharge it at the permitted distance. A holding tank must have the capacity the certificate records for the retention of all sewage, with a means to show how full it is.
The officer checks the equipment by inspection and, where it is safe and practical, by operation. The treatment plant is the usual focus: is it running, does it have its type-approval plate, is the disinfection system charged, does the crew log its effluent quality? A plant that is tripped out, bypassed by a fitted bypass line, or plainly not maintained is a deficiency, because a ship certified on a treatment plant has no other lawful way to discharge inside the distances a holding-tank ship must respect. The piping and the standard discharge connection are the other equipment item: the connection to a reception facility must match the dimensions Regulation 10 specifies, so that any port’s reception facility can take the ship’s sewage.
Discharge arrangement and records
The substantive discharge rules are in Regulation 11, and the officer’s job is to test whether the ship met them, not to restate them. The short version sets the inspection’s reference points. A ship may discharge comminuted and disinfected sewage from an approved system at a distance of more than 3 nautical miles from the nearest land, and sewage that is not comminuted or disinfected at more than 12 nautical miles, in both cases only when the ship is en route and proceeding at not less than 4 knots and discharging at a moderate rate the Administration approves. Effluent from an approved treatment plant that meets the effluent standard may be discharged without those distance limits, subject to any special-area rule. Discharge inside those limits, or while stopped, or from a holding tank dumped at anchor, is a breach.
The officer tests the discharge arrangement and its records against those reference points. Where the ship keeps a sewage discharge log or records discharges in another shipboard record, the officer reads the recent entries and asks whether each discharge was made en route, at the right distance, at a moderate rate, and from the right source. The officer also looks at the physical arrangement: is the overboard discharge valve secured or fitted with a means to prevent discharge in port, and does the piping route match the certificate. A discharge logged at anchor, a holding tank that is full on arrival with no record of where it was last emptied, or a treatment plant whose effluent record does not support the discharges claimed: any of these is a deficiency, and one that goes to the heart of the discharge-control regime.
Crew familiarity
Underneath the three documents and the equipment is the thing Regulation 13 names directly: whether the master and crew are familiar with the essential shipboard procedures relating to the prevention of pollution by sewage. This is tested by conversation and demonstration, not by paperwork alone. The officer asks the responsible engineer to describe the sewage system, to start and operate the treatment plant or to show how the holding tank is monitored and emptied, to explain when the ship may discharge and when it may not, and to locate the relevant entries in whatever record the ship keeps. A crew that runs the ship competently can do this without hesitation. A crew that cannot is itself the deficiency the regulation is written to catch, regardless of how tidy the documents look, because tidy documents maintained by a crew that does not understand the system are exactly the gap between certificated condition and actual operation that Regulation 13 closes. The familiarity check ties directly to the ISM Code, which requires the company to ensure crew are familiar with their duties and the shipboard procedures, so a familiarity failure under Regulation 13 is often a safety-management failure too.
The IMO Procedures for Port State Control
Regulation 13 states the power and the threshold; it does not tell an officer in Rotterdam and an officer in Singapore how to apply them the same way. That harmonization is the job of the IMO Procedures for Port State Control, a free-standing Assembly resolution that all the annex PSC regulations point to through their cross-reference to Article 5. The current text is Resolution A.1206(34), “Procedures for Port State Control, 2025,” adopted by the IMO Assembly on 3 December 2025. It revoked the previous edition, Resolution A.1185(33), “Procedures for Port State Control, 2023,” which had itself replaced A.1155(32) of 2021, and before that A.1138(31), A.1052(27), and the long line back to A.466(XII). Each revision folds in new convention requirements and tightens the deficiency guidance; the 2025 edition is the one a current inspection runs on, and a current Annex IV inspection should be read against it rather than against the 2023 text the sibling-annex articles were written under.
The Procedures do several things the bare regulation cannot. They define the key terms, “inspection,” “more detailed inspection,” “valid certificate,” “detention,” and, importantly, “clear grounds,” so that the threshold means the same thing in every member port. They set out the inspection sequence: the initial inspection of certificates and overall condition; the move to a more detailed inspection when clear grounds appear; and the recording, in a standard report, of every deficiency and the action code attached to it. They carry, in their appendices, the operational-control guidelines that turn the abstract “essential shipboard procedures” into a checklist an officer can work through for each convention, including the Annex IV sewage-system, treatment-plant, discharge, and standard-connection items.
The 2025 edition made a structural change that matters to this topic. The detention grounds were consolidated: the effort in A.1206(34) was to gather all the items identified as clear grounds for detention into a single appendix and remove the related items scattered through other sections, so that an officer reads one list rather than hunting across the document. The 2025 text also strengthened environmental enforcement, with PSC enforcement under MARPOL Annex VI clarified and LRIT non-compliance linked clearly to detention. For Annex IV, the practical effect is that the sewage detention criteria sit in the consolidated detainable-deficiency appendix alongside the rest, applied with the same professional judgment.
That appendix defines the detainable deficiency. A detention is the most serious routine PSC outcome short of a ban: the ship is legally prohibited from sailing until every detainable deficiency has been rectified and verified by re-inspection. For operational compliance, a serious failure of an operational requirement, including a crew unable to carry out essential pollution-prevention operations, can itself ground a detention. The officer’s professional judgment governs the call, but it is judgment exercised against a documented standard, not at large. That documented standard is what lets a detention survive the appeal a shipowner is entitled to bring, and it is why the officer’s record of the clear grounds and the deficiencies is as important as the inspection itself.
The Procedures also impose discipline on the inspector. They carry guidance on the conduct of PSC officers, set proportionality limits so that an inspection does not detain a ship for trivial faults, and require that a ship released from detention be re-inspected against the deficiencies that grounded the hold. The system is built to be repeatable and reviewable, because a port State acting on a foreign ship is acting at the edge of its jurisdiction, and the legitimacy of that action rests on its being done the same way everywhere and on the record.
The regional regimes: Paris MoU and Tokyo MoU
MARPOL gives every port State the power to inspect, but it does not tell them which ships to inspect or how to share what they find. Left to itself, that produces duplication, gaps, and ships that learn which ports to avoid. The regional memoranda of understanding on port State control fix that by coordinating the targeting and pooling the results across a whole region. The two largest and most developed are the Paris MoU and the Tokyo MoU, and between them they cover most of the busy trading regions a passenger ship or a cargo ship calls at.
The Paris MoU binds 27 member maritime authorities across Europe and the North Atlantic coast of Canada. It is the older of the two and the more legally embedded, because its members that are EU states also operate inside the EU Port State Control Directive, which gives the regime statutory force, oversight by the European Maritime Safety Agency, standardized officer training, and a banning power that reaches every EU port. The Tokyo MoU binds 21 member authorities across the Asia-Pacific, the busiest region by traffic volume of any PSC regime. It operates by agreement rather than supranational directive, but its members enforce it in practice with the same machinery. Canada is a member of both, a position its geography requires.
Both regimes run on the same targeting logic, the New Inspection Regime, which the Paris MoU and the Tokyo MoU both adopted, the Tokyo MoU in January 2014. Each ship that calls is assigned a Ship Risk Profile, computed from generic and historic factors: the ship type and age, the performance record of its flag, the performance record of its recognized organization, the company’s own inspection history under the ISM document of compliance, and the number and nature of deficiencies and detentions found at recent calls. Older ships draw more risk points; ships over twelve years old carry the highest age weighting. The profile sorts ships into high, standard, and low risk, and that sorting sets how often a ship is inspected and how deeply. A low-risk ship may go 24 to 36 months between inspections; a high-risk ship faces an expanded inspection at a defined window, around five to six months.
The flag performance that feeds the profile is itself published. Both MoUs maintain White, Grey and Black lists of flag States, ranked on the detention record of their fleets over a rolling three-year window, for flags with a minimum number of inspections in the period. The calculation is statistical, not a simple ratio: the Paris MoU and the Tokyo MoU both set the allowable detention limit, the yardstick “p,” at 7%, and use a significance level of 95% in the binomial formula that sets the White-to-Grey and Grey-to-Black limits. A White-list flag signals a fleet that performs well; a Black-list flag signals one that does not. The Paris MoU’s 2024 results listed 69 flag administrations, with the poorest performers, including Cameroon at the bottom, on the Black List. A ship on a Black-list flag is inspected harder, given shorter rectification periods, and is exposed to the banning mechanism after repeated detentions. The Paris MoU issued 15 refusals of access in 2024, the bans that follow a jumped detention, a failure to call at the indicated repair yard, or repeated detention on a low-performing flag. The ban is the regime’s heaviest routine sanction, and it is the reason the New Inspection Regime’s risk targeting has real consequence rather than being a paperwork exercise.
The scale of the two regimes shows why the targeting matters. The Tokyo MoU conducted 32,054 inspections in 2024, involving 18,655 individual ships, with 1,189 ships detained for serious deficiencies, a detention rate of 3.71%. Its top deficiency categories that year were fire safety, life-saving appliances, working and living conditions, and safety of navigation. Pollution-prevention deficiencies, including Annex IV sewage items, sit below those headline categories in volume, but they are coded and counted the same way, and a serious one carries the same detention exposure. The point of the regime is that an inspection effort spread across tens of thousands of calls is concentrated, by the risk profile, on the ships most likely to be substandard.
Both regimes also run Concentrated Inspection Campaigns, periods, usually three months, in which every member authority adds a focused questionnaire on one topic to its routine inspections. The campaigns are chosen where deficiency data, new convention requirements, or accident patterns show a higher risk of non-compliance, and the Paris and Tokyo MoUs increasingly run them jointly. The 2024 joint campaign covered crew welfare and seafarers’ employment agreements under the Maritime Labour Convention, 2006, running from 1 September to 30 November 2024; of 3,863 ships inspected, 30 were detained on a campaign-related detainable deficiency. A campaign turns a diffuse inspection effort into a measurement of one thing across thousands of ships in one season, and a campaign on pollution-prevention or sewage operations is the kind of exercise that puts the Annex IV operational items, the treatment plant, the discharge records, the crew familiarity, directly in the inspector’s focus for that quarter.
Reporting and the feedback loop
A PSC inspection is not finished when the officer leaves the ship. The result is recorded in a standard inspection report, with each deficiency coded against the relevant convention and instrument and given an action code, from “rectify before departure” through “detainable” to “rectify within 14 days.” The report goes to the ship, and the data goes into the regional information system, the THETIS database for the Paris MoU and the APCIS, the Asia-Pacific Computerized Information System, for the Tokyo MoU. That database is the memory of the regime. It is what the next port’s officer reads before deciding whether to inspect, and it is what feeds the Ship Risk Profile and the flag-performance lists. A detention in any member port is visible to every other member authority at once.
The loop closes on the flag State and the recognized organization. A detention is reported to the flag State, which is obliged to investigate; a pattern of detentions on one flag moves it toward the Grey or Black list; a pattern on one recognized organization, the class society that surveyed the ship for the flag and issued the ISPP Certificate on its behalf, counts against that organization in the risk profile and can trigger review of its authorization. So an operational sewage deficiency found on one passenger ship in one port is not a local event. It changes the risk score of that ship for its next call, it counts toward the standing of its flag and its class society, and it is visible to every other member authority in the region. The system is built so that a ship cannot shed a bad operational record by sailing to a different port, which is the gap that uncoordinated national enforcement always left open.
That feedback is also the link back to the survey scheme this article is careful to keep separate. An operational deficiency repeatedly found on the ships of one recognized organization is evidence that the organization’s surveys are not catching what they should, and the PSC data is one of the inputs the flag State and the IMO use to monitor recognized-organization performance under the classification-society oversight framework. The two halves of compliance, the survey half and the port-State half, meet again in the data: the operational inspections test, in aggregate, whether the ISPP Certificates the survey scheme issues are worth what they claim.
How Regulation 13 differs from the survey regime
It helps to place Regulation 13 against its neighbors, because the operational-control function is easy to blur into the regulations it draws on. The survey-and-certificate regulations, 4 to 8, are the upstream half already described: they decide the ship’s certificated condition; Regulation 13 tests the operation behind that certificate. Regulation 9, “Sewage systems,” sets the equipment standard, the three compliant configurations; Regulation 13 is the inspection that asks whether the equipment the certificate records is actually fitted and working. Regulation 10 sets the standard discharge connection; Regulation 13 is where an officer confirms it matches the dimensions. Regulation 11 sets the discharge rule, the distances, the rates, the en-route condition; Regulation 13 is the inspection that asks whether the ship met it.
The cleanest way to hold the difference is by what each regulation produces. Regulation 9 produces a system. Regulation 10 produces a connection. Regulation 11 produces a rule. Regulations 4 to 8 produce a certificate. Regulation 13 produces a decision: inspect or not, detain or release, report and feed the result back into the regional system. The earlier regulations are the standards and the equipment and the paperwork; Regulation 13 is the enforcement act that tests them on a specific ship on a specific day. This article deliberately does not re-derive the discharge distances or the treatment-plant effluent standards, because those belong to their own regulations and articles; the focus here is the enforcement mechanism that consumes them.
The difference between survey and port State control is also a difference in who acts and when. The survey is the flag State’s act, or the recognized organization’s act on its behalf, performed on a schedule the flag sets, in a place the owner arranges. The port State control inspection is the coastal port State’s act, performed when the ship arrives, on the port State’s own initiative or under the regional targeting. A ship can pass every flag survey and still be inspected and detained by a port State on the same voyage, because the two acts answer different questions. The survey asks whether the ship was built and equipped to standard. The port State control inspection asks whether the crew is running it to standard now. Annex IV needs both, because a sewage treatment plant that passed its renewal survey six months ago tells you nothing about whether the crew bypassed it last week.
The cross-annex point bears one more mention because it is a frequent source of citation error. The function this article covers is “Port State control on operational requirements” in Annex I, where it is Regulation 11, in Annex II where it sits inside Regulation 16, in Annex III where it is Regulation 8, in Annex IV where it is Regulation 13, and in Annex VI where it is Regulation 10. The wording of the operational check, the “clear grounds,” the familiarity test, the no-sail duty, is materially the same across the annexes. A reference that cites “Annex IV Regulation 11” for port State control is wrong twice over: it is using a number that in Annex IV belongs to the sewage discharge regulation, and the port-State-control function in Annex IV is Regulation 13, added by MEPC.143(54), not part of the original revised Annex under MEPC.115(51).
A worked inspection sequence
It is worth tracing one inspection end to end, because the parts only make sense in motion. A passenger ship on a flag with a middling detention record calls at a Paris MoU port. Its Ship Risk Profile, set by its type, its age, its flag’s standing, and the deficiencies logged at its last calls in the THETIS data, comes back as high risk and due for an expanded inspection. That is the targeting step: the ship was selected by the New Inspection Regime, not at random. Passenger ships draw extra risk points by type, because the consequence of a failure is large and the number of persons on board makes the sewage load large too.
The officer boards and starts with the certificate. The International Sewage Pollution Prevention Certificate is valid, in date, and matches the ship; it records an approved sewage treatment plant. Under Article 5 and the first limb of Regulation 13, that would ordinarily be the end of it. But the officer asks the responsible engineer to start the treatment plant and describe its effluent standard, and the engineer cannot start it, says it has been “down for a while,” and cannot say what standard it was approved against. That is the “clear grounds” finding: the crew appear not familiar with an essential pollution-prevention procedure, and the ship’s condition does not correspond with the particulars of its certificate. It is recorded as the basis for going further.
The inspection now moves to the operation. The officer follows the piping and finds the treatment plant isolated, with a bypass line open to a holding tank that the certificate does not record, and the holding tank near full with no record of when it was last discharged to a reception facility. The ship’s sewage discharge entries are thin, and one recent entry logs a discharge at a position the chart shows is inside 3 nautical miles of land. Two deficiencies are now on the report: a treatment plant out of service on a ship certified to rely on it, and a discharge record that points to a breach of Regulation 11, both compounded by a crew that could not operate the system or explain its limits. The officer assesses them against the detainable-deficiency criteria in Resolution A.1206(34). A crew unable to carry out an essential pollution-prevention operation, with a certified treatment plant out of service and a record suggesting a non-compliant discharge, crosses the line.
The ship is detained. It does not sail until the deficiencies are rectified: the treatment plant is brought back into service or the ship arranges a compliant alternative the port State accepts, the holding tank is discharged to a reception facility, the crew is brought up to competence on the system, the records are corrected, and a re-inspection confirms it. The detention is reported to the flag State, which is obliged to investigate, and it is entered in THETIS, where it raises the ship’s risk profile for its next call and counts toward its flag’s standing on the White-Grey-Black list. The recognized organization that issued the ISPP Certificate sees the operational failure recorded against a ship it surveyed. Every step traces back to Regulation 13 read with Articles 5 and 6 and applied through the Procedures for Port State Control, and every step is on the record so that the detention can withstand challenge. That is the operational-control mechanism working as designed, from the targeting that put the officer on board to the feedback that follows the ship out of port.
Limitations
This article describes the enforcement mechanism, not the substantive standards it enforces. The exact discharge conditions, the distances of 3 and 12 nautical miles, the 4-knot en-route condition, the moderate-rate requirement, and the treatment-plant effluent standards are set by Regulation 11 and the related IMO resolutions, and are covered in their own articles; where they appear here it is to show what the inspection tests, not to restate the rule. The three compliant equipment configurations are the subject of Regulation 9, and the survey-and-certificate scheme is the subject of the surveys and ISPP Certificate article; both are summarized here only as far as the inspection needs them. The Baltic special-area regime for passenger ships, with its tighter nitrogen and phosphorus standards and its different discharge rule, is covered in its own article.
The regulation numbering used here is the consolidated revised Annex IV: the body adopted by Resolution MEPC.115(51), in force 1 August 2005, with Regulation 13 added by Resolution MEPC.143(54), in force 1 August 2007, and the Baltic special-area amendments added later by MEPC.200(62) and refined by MEPC.274(69) and MEPC.275(69). References that cite “Annex IV Regulation 11” for port State control on operational requirements are applying the Annex I number, or the Annex IV discharge number, to the wrong function; the correct Annex IV citation is Regulation 13. Verify the current regulation text and the current consolidated numbering against the IMO’s consolidated Annex IV before relying on a specific reference, because the IMO renumbers and amends through the MEPC cycle.
The Procedures for Port State Control are revised on a roughly biennial Assembly cycle. The current edition is Resolution A.1206(34), 2025, which revoked A.1185(33) of 2023; an inspection always runs on the edition in force at the time, and the deficiency and detention criteria can change between editions. The Paris and Tokyo MoU figures cited, the membership counts, the 2024 inspection and detention totals, the flag-list results, and the campaign topics, are current to the period stated and are revised by the regimes from year to year; the annual reports of each MoU are the authoritative source for current numbers. Nothing here is a substitute for the consolidated convention text, the current Procedures, and the relevant regional MoU procedures, read together, when a real inspection or a real detention is in question.
See also
- MARPOL Annex IV: prevention of pollution by sewage from ships
- MARPOL Annex IV Regulation 9: sewage systems
- MARPOL Annex IV Regulation 10: standard discharge connection
- MARPOL Annex IV Regulation 11: sewage discharge
- MARPOL Annex IV surveys and the ISPP Certificate
- MARPOL Annex IV Baltic special-area passenger ships
- MARPOL Annex I Regulation 11: PSC on operational requirements
- MARPOL Annex II Regulation 16: PSC measures of control
- Port State control
- Paris Memorandum of Understanding
- Tokyo MoU port State control
- MARPOL Convention: the pollution-prevention treaty
- ISM Code: safety management and crew familiarity
- Marine sewage and grey-water treatment systems