How to proceed when making modifications to installed ballast water treatment systems

Ship owners/managers may want to modify the ballast water treatment systems (BWTS) installed on their vessels, such as upgrading the BWTS or making any other changes to the BWTS. This technical news provides guidance on such modifications and outlines what is required to obtain approval of modifications to a BWTS.

Scope of modification

The scope of a BWTS modification varies. Some modifications require replacing or adding components of the BWTS, such as replacing the filter element or adding additional sensors to monitor safe operation of the BWTS within its system design limitations (SDLs). For other modifications, only the system control software is updated without any physical changes to the BWTS. Some modifications are also made to implement improved technological designs for selected BWTS components, resulting in physical changes to the earlier approved BWTS.

An example of a typical modification is the upgrade of a BWTS, which was type approved based on the IMO G8 Guidelines adopted by Resolution MEPC.174(58), to comply with a later USCG type approval and/or IMO type approval based on the BWMS Code adopted by Resolution MEPC.300(72).

How to proceed

Prior to implementing any modifications, ship owners/managers should request BWTS manufacturers to provide a clear description of the modifications, with an itemized list of the suggested changes. The BWTS manufacturer should also provide either,

a declaration that the modified BWTS still complies with the specifications of the type approval certificate (TAC) delivered with the BWTS at the time of the initial installation, or
the new TAC applicable to the modified BWTS design.

Ship owners/managers should then request approval by DNV of any BWTS modifications by submitting a request for an alteration approval project and providing the BWTS manufacturer’s description of the modifications. Depending on the description of the BWTS upgrade, DNV will provide a list of the document requirements to be submitted for approval, such as:

  • valid TAC,
  • revised type-approved documentation, i.e. Operation, Maintenance and Safety Manual (OMSM) for the BWTS,
  • component certificates,
  • a process and instrumentation diagram (P&ID) (with changes clearly marked),
  • a revised ballast water management (BWM) plan.

DNV will then evaluate the proposed BWTS modifications and inform the ship owner/manager of the requirements for class and/or statutory approval of the BWTS modifications. This includes any requirement, as applicable, for an additional survey of the modified BWTS as per Regulation E-1.1.5 of the BWM Convention.

In case the BWTS modifications are carried out while the vessel is in service, acceptance by the Flag to perform contingency measures may be required for the period where the BWTS is not opperational due to the modifications being made.

If a new TAC is required

If the BWTS modification results in a new TAC being carried on board the vessel (e.g. the earlier TAC based on the IMO G8 Guidelines is replaced with the TAC based on the BWMS Code), the modified BWTS is regarded as a newly installed BWTS in accordance with the IMO Unified Interpretation BWM.2/Circ.66/Rev.5. In such instances, an additional survey by a DNV surveyor is required to verify the modifications, and a commissioning test validating the biological performance of the modified BWTS must be conducted in accordance with Regulation E-1.1.5 of the BWM Convention and IMO guidance BWM.2/Circ.70/Rev.1 (refer also to the Technical & Regulatory News No. 14/2022). A new International Ballast Water Management Certificate (IBWMC) is then issued accordingly with a new “date installed” for the BWTS.

Upgrades to USCG TAC requirements

An upgrade of a BWTS to the requirements of the USCG TAC for the BWTS may affect the IMO type approval for the BWTS and/or the previous DNV approval of the BWTS installation. Therefore, documentation describing the modifications shall be submitted to DNV for verification and approval.

However, DNV is not authorized to provide any approval or confirmation that the upgraded BWTS complies with relevant USCG requirements. Instead, the BWTS manufacturer shall provide a declaration that the upgraded BWTS complies with the requirements of the USCG TAC, along with a new BWTS nameplate indicating the USCG approval number assigned to the BWTS.


In case a ship owner/manager chooses to modify an installed BWTS, they should request the BWTS manufacturer to provide a clear description of the modifications intended to be made to the BWTS.

We also recommend that a ship owner/manager, who intends to modify an already installed BWTS, contact DNV as early as possible to clarify the scope of the approval of the modification(s). This includes any need for an additional survey of the modified BWTS, including a biological commissioning test.
Source: DNV

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