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PBT in BWMS (Ballast Water Management System)

 In the context of BWMS, PBT stands for:)

Persistent, Bioaccumulative, and Toxic

It is a key environmental assessment criterion used by the International Maritime Organization when approving BWMS that use active substances under the Ballast Water Management Convention.

Meaning of PBT

  • Persistent (P):
    Substance does not break down easily in the marine environment.

  • Bioaccumulative (B):
    Substance accumulates in living organisms and increases up the food chain.

  • Toxic (T):
    Substance is harmful to marine life or humans.

Why PBT is Important in BWMS?

BWMS using active substances (e.g. electrochlorination, chemicals) may release residual chemicals or by-products into the sea.

👉 IMO requires that:

  • Active substances and by-products must NOT be PBT

  • Otherwise, the system will not get G9 approval

Relation to G9 Approval

During the G9 procedure:

  • Active substances are evaluated for:

    • Toxicity

    • Persistence

    • Bioaccumulation

  • Assessment is done by GESAMP-BWWG

If substance is PBT → approval may be denied or restricted.

Practical Shipboard Example

  • Chlorine-based BWMS:

    • Chlorine itself is not persistent

    • But some by-products (e.g. brominated compounds) are checked for PBT properties

  • Neutralization is required before discharge to keep values within limits.


In BWMS, PBT refers to Persistent, Bioaccumulative, and Toxic properties of active substances or their by-products. IMO evaluates these properties during G9 approval to ensure ballast water treatment does not harm the marine environment.

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