Ballast is weight added to a vessel to optimise stability, trim and safe navigability. Traditionally, ships used solid materials such as rocks or metal, but from the late 19th century onwards water became the predominant ballast because it is easier to load and discharge and offers greater operational flexibility. Ballast water is seawater or freshwater taken onboard and stored in dedicated tanks within the hull, allowing the vessel to remain within safe operating limits under different loading conditions.
Ballast is not a theoretical concept; it is an operational control mechanism. On ships, ballast is used to manage trim, draft, stability and hull stresses, ensuring the vessel maintains safe handling characteristics and that critical components—such as the propeller and rudder—remain properly immersed. This is especially relevant when a vessel is sailing unladen or with unevenly distributed cargo. In those scenarios, ballast water compensates for the absence of cargo weight and helps keep the ship within safe stability margins. A ship’s ballast system is typically composed of tanks, pumps and pipelines designed to take up, transfer and discharge water as required. The amount of ballast water carried varies depending on vessel size, loading condition and operational needs, and ballast tanks are distributed to support precise adjustments of trim and stability. The operational cycle is straightforward: ships take up ballast water when weight is needed (for example, in ballast voyages or after unloading) and discharge ballast water when cargo is loaded and cargo weight replaces the stabilising function of the water. The challenge is that ballast discharge can occur far from the original uptake point, which is why ballast water management became a global environmental and regulatory issue.
What ballast water contains and why discharge can be a problem
Ballast water is not “clean” in a biological sense. It contains the aquatic organisms, sediments and pathogens present in the water at the time it was taken onboard, including organisms at different life stages—such as phytoplankton and zooplankton—as well as bacteria and viruses. Sediments can accumulate at the bottom of ballast tanks and harbour organisms in dormant states, effectively acting as a reservoir that may be released later during discharge or tank operations. This matters because the discharge of ballast water has long been recognised as a major vector for the transfer of aquatic species into new environments. When a vessel loads ballast water in one port and discharges it in another, transported organisms may survive and establish themselves if conditions are favourable, leading to the introduction of non-native or invasive species. These species can compete with native organisms, prey on them or alter habitats, resulting in biodiversity loss and significant changes to ecosystem structure and function, often accompanied by economic impacts on fisheries, infrastructure and coastal industries.
Cases of biological invasions associated with shipping have been documented worldwide, and even targeted sampling shows how diverse ballast water can be. A study in the Port of Barcelona sampling ballast tanks from five vessels of diverse origin identified 54 different phytoplankton taxa and 69 different zooplankton taxa, highlighting the scale of potential transfer. There are also human health considerations: ballast water can transport pathogenic microorganisms. A frequently cited example is Vibrio cholerae, the bacterium responsible for cholera, which can be carried in ballast water and, after discharge, potentially contaminate coastal waters used for recreation or as a seafood source.
IMO regulatory framework: the BWM Convention and compliance standards D-1 / D-2
To address these risks, the International Maritime Organization (IMO) adopted the International Convention for the Control and Management of Ships’ Ballast Water and Sediments (BWM Convention) in 2004. The Convention entered into force on 8 September 2017 and remains the primary international legal instrument governing ballast water management. Its objective is to prevent, minimise and ultimately eliminate the transfer of harmful aquatic organisms and pathogens through ballast water and sediments, by setting enforceable operational and performance requirements and standardising documentation and control procedures across the industry.
The Convention is commonly implemented through two main standards. Regulation D-1 (Ballast Water Exchange Standard) requires ships to exchange ballast water taken up in coastal waters with oceanic water—typically far from the coast—where coastal organisms are less likely to survive (or oceanic organisms are less likely to thrive) once discharged in the next port; the expectation is that at least 95% of the ballast water volume in the tanks is exchanged. Regulation D-2 (Ballast Water Performance Standard) sets limits on the maximum number of viable organisms of different size categories and certain indicator microbes that can be discharged, which in practice usually requires the installation and operation of a ballast water treatment system.
Ballast water management plans, record keeping and BWTS technologies
The BWM Convention is not only about equipment; it is also about onboard procedures and traceability. Each ship must carry a Ballast Water and Sediments Management Plan approved by the vessel’s flag State Administration. This plan is ship-specific and typically includes operational procedures for safe ballasting and deballasting, responsibilities, contingency measures, the location of sampling points, and a description of the ballast water treatment system if installed. Ships must also maintain a Ballast Water Record Book to log ballast operations, supporting compliance verification during inspections and providing evidence that management procedures are executed consistently and in line with the approved plan.
For many vessels, especially under D-2, the practical compliance route is the installation of a Ballast Water Treatment System (BWTS). If you are assessing options for retrofit or newbuild compliance, a dedicated overview of BWTS requirements and implementation considerations is available. A BWTS processes ballast water before discharge to reduce the concentration of viable organisms to within D-2 limits; the fundamental purpose is to reduce the risk of transferring invasive species and pathogens by ensuring discharged ballast water meets performance thresholds. Most systems use a combination of physical and chemical or biological methods: filtration is commonly used as a first stage to remove larger particles and organisms, sometimes supported by physical separation technologies such as hydrocyclones, followed by a primary inactivation method. Common approaches include ultraviolet (UV) irradiation, electrochemical or electrolysis treatment (often referred to as electro-chlorination), and chemical or biocide dosing with neutralisation where needed. Other technologies include deoxygenation, heat treatment (more energy intensive), and cavitation or ultrasound mechanisms that physically damage organisms. Selecting the right BWTS is an engineering decision shaped by vessel type, available space, power capacity, trading pattern, installation complexity and lifecycle cost; in all cases, the system must follow approval requirements under IMO guidance and be operated and maintained in accordance with the ship’s management plan to ensure consistent compliance.
Ballast water management has therefore become a core compliance and operational discipline. Treating it as a structured engineering and procedural programme—rather than a documentation exercise—helps reduce regulatory risk, avoid delays during inspections, and protect vessel availability. When BWTS installation is part of a broader retrofit scope, it is typically managed within ship repair and conversion programmes, where engineering, planning and onboard execution are coordinated to minimise off-hire. For vessels requiring fast support in strategic areas, ballast-related interventions and compliance-driven repairs can also be organised in key locations such as Panama Canal, where response time is critical for ships in transit.
