Ballast water

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This article discusses ballast water regulations to prevent marine organisms from spreading worldwide beyond their native habitats. An overview is given of various ballast water management systems to comply with these regulations and some pros and cons. This article is largely based on the publication of Lakshmi et al. (2021[1]), the IMO website on Ballast Water Management and the 1996 NRC report Stemming the Tide[2].


Ballast loading is necessary to stabilize ships at sea. To this end, water is collected in special ballast tanks before departure from the port. This reduces the load on the hull, provides lateral stability, improves propulsion and maneuverability and compensates for weight changes at different load levels and due to fuel and water consumption. Large tankers can carry in excess of 200,000 m3 of ballast water. When pumping up ballast water, local marine organisms will inevitably also be included and probably also some sediment with adsorbed marine organisms.

The amount of ballast water depends on the sea conditions. Therefore, ballast water must be regularly taken in or discharged during the journey. Without special precautions, this practice causes a massive spread of marine organisms from their native habitats to areas where they do not naturally occur. Ballast water is therefore widely regarded as the most important vector for the spread of potentially invasive alien species. Human health is affected by invasions through spread of diseases like paralytic poisoning, cholera outbreak etc. Ballast water discharge typically contains a variety of biological materials, including plants, animals, viruses and bacteria. There are hundreds of organisms carried in ballast water that cause ecological effects outside their natural environment. The negative impact of the worldwide spread of alien species is considerable, as discussed more in detail in the article Non-native species invasions.

Shipping traffic has increased sharply in recent decades and transit time has decreased. Hence, more ballast water is being pumped up and discharged, resulting in an increase in the spread of non-native species. Over ten billion tons of ballast water is transferred annually from one place to another[3][4]. The global economic loss because of alien invasive species has been estimated to tens of billions US dollars per year[5]. Internationally accepted measures to combat the spread of non-native species via ballast water and their enforcement therefore deserve high priority.

Examples of ship-borne introductions of invasive alien species worldwide since the 1980s[2].
Species Origin Location
Dinoflagellate Gymnodinium catenatum Japan Australia
American Comb Jellyfish Mnemiopsis leidyi North America Black and Azov seas
Polychaete worm Marenzelleria viridis North America Western and Northern Europe
American razor clam Ensis americanus North America Western and Northern Europe
Japanese mussel Musculista senhousia Japan New Zealand
Swimming crab Charybdis helleri Mediterranean Colombia, Venezuela, Cuba, United States
Seastar Asterias amurensis Japan Australia
Mitten crab Eriocheir sinensis China Western and Northern Europe, North America

The BWM convention

The International Convention for the Control and Management of Ships' Ballast Water and Sediments, requiring all ships to implement a ballast water management plan (BWM Convention), was adopted in 2004. Parties to the Convention are given the option to take additional measures which are subject to criteria set out in the Convention and to IMO guidelines. The BWM Convention entered into force on 8 September 2017. It has been signed by 81 countries as of January 2020. From the date of entry into force, ships in international traffic are required to manage their ballast water and sediments according to the BWM convention. Ships have to carry:

  • A ship-specific ballast water management plan,
  • A ballast water record book,
  • An International Ballast Water Management Certificate (ships of 400 gt and above) issued by or on behalf of the Administration (flag State).

There are two types of sampling required for checking compliance with the Ballast water convention. One is to verify compliance with Regulation D-1 (BW exchange standard) and the other is to verify compliance with Regulation D-2 (BW performance standard). To check with the D-1 compliance is relatively easy as it needs to check with whether the ship’s ballast water is being exchanged in the mid sea before entering the port. These can be checked with a salinometer. The D-2 regulation sets performance standards limiting the concentrations of live organisms allowed to be released: 1) Fewer than 10 organisms per m3 ≥ 50 μm size. 2) Fewer than 10 organisms per ml between <50 μm and ≥10 μm. 3) Fewer than set concentrations for harmful or indicator organisms (E.coli, Vibrio cholera). All ships must meet the D2 standards by 2024. The compliance of the ship with the D-2 regulation is difficult as it needs more complex and precise testing to evaluate the complex organisms in the ballast water.

Several articles and regulations of the Ballast Water Management Convention refer to guidelines developed by the International Maritime Organization IMO to facilitate global and uniform implementation of the instrument. See annex A: Guidelines.

Ballast water exchange

Ships without proper Ballast Water Management System (BWMS), use ballast water exchange methods - especially for short voyages. Mid-sea exchange of ballast water is the regulatory method of ballast water management. Deep sea ballast water exchange is based on a theory that freshwater, estuarine and coastal water organisms (from the port of departure) generally cannot survive in the deep sea environment, while, on the other hand, the deep sea marine organisms cannot survive in the coastal environment of the destination port. Ships thus have to replace their ballast water when sailing in the deep sea at depths of more than 200 meters and 200 nautical miles offshore. Ballast water exchange methods can be divided into emptying, overflowing and dilution method according to their different operation.

Emptying Method

The ship has to discharge all the ballast water which is pumped in at the coastal port, and clean the residue at the tank bilge, and then inject deep sea water into the ballast tank. Using this method the coastal ballast water can be 100% replaced by deep sea water, so it is recognized as the most effective and most practical method to prevent the spread of harmful aquatic organisms and pathogens. During the process of replacing the ballast water, the loading condition and stability of the ship will change significantly, so the ship officers must calculate the stability, trim, strength of the ship at each step of replacing process in advance so as to ensure the safe navigation. The emptying method is not suitable when encountering bad weather, because the strength and stability of the ship cannot be ensured.

Overflowing Method

This method is to pump the deep sea water into the full ballast tank from the tank bottom, so that the original coastal ballast water will overflow from the holes on top of the deck. The ship has to pump in three times the amount of ballast water in order to replace 90% of the coastal ballast water. This method does not change the stability, strength and the trim of the ship, so it can be performed during adverse weather conditions. But as the ballast water cannot be replaced completely, the validity of this method is challenged.

Dilution Method

This method uses three times of deep sea water to replace the amount of the coastal ballast water from the top edge of the ballast tank, while at the same time discharging the original ballast from the bottom of ballast tanks. The dilution method is safer than the overflowing method, but the effectiveness is less than the emptying method.

Ballast Water Management Systems (BWMS)

Since the importance of ballast water management was recognized, various systems have been developed to prevent marine organisms from being taken in with the ballast water or to prevent their survival in the ballast tanks. BWM systems usually consist of several treatment steps, but at least of a primary and a secondary step. A number of systems, some of which are in use and others still in development, are briefly described below.

Hydrocyclone used in a Ballast Water Management System.

Primary treatment

  • Filtration. Usually filters with mesh size of ~50 μm in close proximity to the ballast pump.
  • Hydrocyclone. Particle separation based on density difference with water.

Secondary mechanical treatment

  • Hydrodynamic cavitation. Destruction of microorganisms and micro pollutants by collapsing cavitation bubbles.
  • Ultrasound. Destruction of plankton by ultrasound waves.
  • Micro wave. Destruction of microorganisms by heating with micro wave technology.
  • Electric pulse. Destruction of microorganisms by electrocution and associated chlorine production and heat shock.
  • Heat treatment. Ballast water heating by a boiler system using waste heat from ships main engine.
  • Ultra violet radiation. Destruction of bacteria and viruses by causing photochemical reactions with nucleic acids and protein of organisms.
  • Electro-ionization magnetic separation. Coagulation and flocculation of contaminants by ionized oxygen and nitrogen and subsequent removal by magnetic separation filtration.

Secondary chemical treatment

  • Chlorine dioxide – biocide added into the ballast water. Oxidizing agent destroying all organisms, provided sufficient concentration and contact time. The biocide Vibrex is based on chlorine dioxide. Requires later dechlorination by sulphur dioxide.
  • On board electrolytic generation of hypochlorite - biocide added into the ballast water. Generation of sodium hypochlorite, sodium hydroxide, hydrogen and other chemical compounds by passing an electric current through a ballast water flow chamber.
BWMS based on hypochlorite generation. Image from
  • Peraclean – biocide added into the ballast water. Peroxyacetic acid with hydrogen peroxide as the secondary active ingredient.
  • Sea Kleen – biocide added into the ballast water. Mixture of naphthoquinone, menadione and its bisulfate.
  • Glutaraldehyde. Biocide added into the ballast water that can be deactivated by nitrogen containing compounds, such as amines, amino acids or amino alcohols.
  • Acrolein – biocide added into the ballast water. Highly toxic contact herbicide that polymerizes over time into hard, porous plastic.
  • De-oxygenation. Biocide method by bubbling nitrogen or other inert gases into ballast water to reduce the oxygen content.
  • Strong ionization discharge – hydroxyl radical (OH). Creation of hydroxyl radicals by application of a strong ionizing high-frequency voltage to gaseous seawater and subsequent injection into the inflowing ballast water.

List of approved BWM systems

Evaluation of BWM systems

Chemical biocides like chlorine dioxide, Peraclean, Seakleen, Vibrex, etc. can destroy all organisms in the ballast water if used at high concentration and after sufficient exposure time. However, high concentration of biocides and their byproducts are toxic to organisms when discharged into the sea. Unless rapidly degradable, they therefore have to be chemically deactivated prior to ballast water discharge.

BWMS based on hydroxyl generation. Image from OceanGuard.

Besides environmental considerations, other important factors for the suitability of BWM systems are the costs, the time it takes to effectively remove all marine organisms and the speed at which ballast water can be taken in. The evaluation by Lakshmi et al., 2021[1] suggests that filtration primary treatment followed by secondary treatment with hydroxyl radicals produced by a strong ionization discharge, is the most effective method among the treatment systems considering the cost, power consumption, effectiveness in inactivating the organisms, physico-chemical parameters, in managing high flow rates and low production of toxic byproducts. It also increases the dissolved oxygen in the ballast water tank while discharging ballast water with high dissolved oxygen is beneficial for the marine waters.

Enforcement of the BWM guidelines is a matter of concern.

  • Some flag states and ship owners claim that there is no need for sampling and quality analysis if the ship is operationally compliant with the Type approved BWMS and correctly maintained and operated;
  • There is no uniform ballast water sampling and analysis methodology currently established worldwide;
  • According to the IMO guidelines, the time required to analyze the ballast water samples shall not be used as a basis for unduly delaying the operation, movement or departure of the ship.

Annex A: Guidelines for Ballast Water Management

The following is the up-to-date list of Guidelines relating to the uniform implementation of the BWM Convention that have been developed, adopted and, in some cases, revised since MEPC 53:

  • Guidelines for sediment reception facilities (G1) (resolution MEPC.152(55));
  • Guidelines for ballast water sampling (G2) (resolution MEPC.173(58));
  • Guidelines for ballast water management equivalent compliance (G3) (resolution MEPC.123(53));
  • Guidelines for ballast water management and development of ballast water management plans (G4) (resolution MEPC.127(53));
  • Guidelines for ballast water reception facilities (G5) (resolution MEPC.153(55));
  • 2017 Guidelines for ballast water exchange (G6) (resolution MEPC.288(71));
  • 2017 Guidelines for risk assessment under regulation A-4 of the BWM Convention (G7) (resolution MEPC.289(71));
  • 2016 Guidelines for approval of ballast water management systems (G8) (resolution MEPC.279(70)) (this will be superseded by the BWMS Code (resolution.300(72)) in October 2019);
  • Procedure for approval of ballast water management systems that make use of Active Substances (G9) (resolution MEPC.169(57));
  • Guidelines for approval and oversight of prototype ballast water treatment technology programmes (G10) (resolution MEPC.140(54));
  • Guidelines for ballast water exchange design and construction standards (G11) (resolution MEPC.149(55));
  • 2012 Guidelines on design and construction to facilitate sediment control on ships (G12) (resolution MEPC.209(63));
  • Guidelines for additional measures regarding ballast water management including emergency situations (G13) (resolution MEPC.161(56));
  • Guidelines on designation of areas for ballast water exchange (G14) (resolution MEPC.151(55));
  • Guidelines for ballast water exchange in the Antarctic treaty area (resolution MEPC.163(56)); and
  • Guidelines for port State control under the BWM Convention (resolution MEPC.252(67)).

Related articles

Non-native species invasions
Legislation for the sea


  1. 1.0 1.1 Lakshmi, E., Priya, M. and Sivanandan Achari, V. 2021. An overview on the treatment of ballast water in ships. Ocean and Coastal Management 199, 105296
  2. 2.0 2.1 National Research Council. 1996. Stemming the Tide: Controlling Introductions of Nonindigenous Species by Ships' Ballast Water. Washington, DC: The National Academies Press., free reading on
  3. Jingguo, Y. 2016. The Pollution of Ships' Ballast Water to the Marine Environment and Countermeasures. 4th International Conference on Machinery, Materials and Computing Technology.
  4. David, M. and Perkovic, M. 2004. Ballast water sampling as a critical component of biological invasions risk management. Mar. Pollut. Bull. 49: 313-318
  5. Marbuah, G., Gren, I.M. and McKie, B. 2014. Economics of harmful invasive species: a review. Diversity 6: 500–523

The main author of this article is Job Dronkers
Please note that others may also have edited the contents of this article.

Citation: Job Dronkers (2021): Ballast water. Available from [accessed on 12-04-2021]