Ballast water: D-1, D-2 and the BWMS Code

Which ships, which documents

The Convention applies to ships that carry ballast water. Ships of 400 GT and above (other than floating platforms, FSUs and FPSOs) are surveyed and carry the International Ballast Water Management (IBWM) Certificate.¹ Every ship must carry an approved BWMP and keep a BWRB; BWRB entries stay on board for at least two years after the last entry and then in the company’s control for at least three more (regulation B-2).¹ From 1 February 2025 the BWRB follows the new format adopted by resolution MEPC.369(80), with a ballast tank diagram and coded entries.⁵

D-1 and D-2

D-1 requires at least 95% volumetric exchange; pumping through three times the volume of each tank is accepted as meeting it (regulation D-1). Under regulation B-4 the exchange should be done at least 200 nautical miles from the nearest land in water at least 200 metres deep, or failing that at least 50 nautical miles from land in 200 metres of water, and never at the expense of the ship’s safety.¹

D-2 limits what is discharged to:¹

The B-3 schedule: why everyone is on D-2 now

Regulation B-3, replaced by resolution MEPC.297(72) from 13 October 2019, phased ships from D-1 to D-2 at their first or second IMO renewal survey after 8 September 2017, depending on when that survey fell. Ships built on or after 8 September 2017 had to meet D-2 from delivery, and any ship to which the renewal-survey trigger did not apply had to meet D-2 by a date set by the Administration but “not later than 8 September 2024”.² As of today all ships subject to the Convention must meet D-2; the MEPC 84 amendments will remove D-1 as a principal method from the certificate form.⁸

Approval of the system: G8, the BWMS Code and active substances

Regulation D-3, as amended by MEPC.296(72), requires a BWMS installed on or after 28 October 2020 to be approved under the BWMS Code (resolution MEPC.300(72), mandatory from 13 October 2019). Systems installed before that date may be approved under the earlier G8 guidelines or the Code.³ “Installed” means the contractual date of delivery of the system to the ship.⁴ MEPC 84 approved a unified interpretation confirming that systems approved to the 2016 G8 Guidelines may remain installed after 28 October 2020, to end confusion seen in recent PSC campaigns; it is to be added to BWM.2/Circ.66/Rev.6.⁸ Systems that use active substances (chlorine from electrolysis, for example) also need IMO approval of the substance under regulation D-3.2.¹

The BWMS Code adds requirements that matter to the people running the system:⁴

• the type approval certificate lists the system design limitations (SDL): the water quality and operating parameters the system was validated for;
• control and monitoring equipment records proper functioning or failure, stores data for at least 24 months, and can display, print or export a report for official inspections (paragraphs 4.17 to 4.20);
• bypasses or overrides for safety must activate an alarm, and every bypass is recorded by the control system and in the BWRB (paragraph 7.2);
• sampling facilities are installed so that representative samples of the discharge can be taken (paragraph 7.1); and
• the type approval certificate, operation, maintenance and safety manual (OMSM), BWMP, installation drawings and commissioning procedures must be on board (paragraph 8.2).

Lesson 5 covers the commissioning test and the surveys.

When the system cannot treat: CWQ, contingency and the experience-building phase

Resolution MEPC.387(81) (March 2024) is interim guidance for ships in challenging water quality (CWQ): high sediment, high organic load or very turbid water. Its key points: build ship-specific CWQ triggers and procedures into the BWMP; a BWMS reducing flow or alarming in CWQ is working as designed, not failing; bypass is “the last resort”; after a bypass, plan decontamination before the next discharge; tell the next State that will receive the affected water; and record it in the BWRB.⁷ Where a system fails for other reasons, the ship’s flag and the port States concerned agree a contingency measure case by case.⁷

The experience-building phase set up by resolution MEPC.290(71) says a ship should not be penalised solely because a discharge exceeds D-2, provided the BWMS is approved, correctly installed, maintained to the manufacturer’s instructions, operated to the BWMP, and either its self-monitoring shows proper operation or the port State was told it was defective before discharge.⁶ It does not protect a ship whose system is simply not used or not maintained.

What is coming: the convention review

MEPC 80 approved a review plan, and MEPC 84 (April to May 2026) finalised amendments to the Convention for adoption at MEPC 85, with the revised BWMS Code to follow so that both enter into force together and close the experience-building phase.⁸ As approved, the amendments would:⁸

• give regulation A-3 a legal basis for discharging unmanaged or partly managed water after CWQ or a contingency measure;
• require the BWMP to state the BWMS approval status, summarise maintenance, and include CWQ and contingency procedures, with Administration re-approval of mandatory changes;
• add a BWMS maintenance log to the BWRB (regulation B-2) and evidence of crew familiarisation (B-6);
• make the maximum allowable discharge concentration (MADC) of active substances mandatory under D-2;
• extend surveys under E-1: renewal and intermediate surveys to confirm the BWMS achieves D-2, annual surveys to verify maintenance and include annual sampling of residual active substances; and
• require a time-bound repair plan to flag and destination port States after a defect.

The new 2026 G4 Guidelines on BWMPs (resolution MEPC.409(84)) were adopted at the same session; the old G4 is revoked only when the amendments enter into force.⁹ The entry-into-force date will be set when the amendments are adopted.