Cruise Ship Ballast Water & Invasive Species


One of the operations that involves the most water going in and out of a ship is ballasting. Ballast is sea water used to fill tanks in the bottom of a ship, to help ensure that the structure and stability of the vessel remains within set operating limits. This water can represent thousands of tonnes of weight, and the ship carries this weight around everywhere it goes.

Why Do Cruise Ships Need Ballast?

Ballast water can help a ship by:

  • Filling a tank on one side of the ship, to correct List (an imbalance in the weights on wither side of the ship) or Heel (an outside force like the wind causing the ship to lean over).
  • Filling a tank in the forward (front) or aft (back) part of the ship, to ensure an efficient Trim (the level of the ship’s deck in a forward-aft direction).
  • Replacing the weight that would normally be taken up by cargo. If a ship is too light, it is in danger of capsizing (tipping over) and it may even have the propeller out of the water!
  • Ensuring an even spread of weight throughout the vessel.

While not so much of a problem on cruise ships, on cargo ships the cargo hold is usually designed to be filled on both sides of the bulkhead (wall). The cargo in the adjacent hold actually helps to support the structure of the ship! If one hold is empty, and the next is full, it can put too much strain on the vessel’s steel work. By filling a hold with water, the extra weight and pressure on the hold’s bulkhead helps to support the cargo on the other side.

Ballast Operations
Ballasting operations can carry invasive species thousands of kilometers!

It is this transport of sea water that poses a risk to the sometimes delicate maritime environments around the world. When the sea water is taken up in the ocean, and mud, silt or organisms present will also be sucked in. Should these ‘stowaway’ organisms be introduced to another part of the world, they can take over the local ecosystems. These invasive species will affect the local fishing, commercial operations such as power plants and water treatment plants, or simply the enjoyment of local waterways.

These tanks aren’t friendly places at the best of times – both to humans and these organisms. They are dark, are not ventilated and as such many of the organisms will die after a short period. However, some are more hardy creatures, and can survive many days travel to a new breeding ground on the other side of the world. Each ship will have a unique plan to conduct a ‘mid-ocean ballast exchange’, and new regulations introduce a requirement for ships to have a ballast water treatment system.

Cleaning Ballast Tanks

A ballast exchange is when each tank is emptied and refilled several times, flushing out any organisms and sediment that may have been collected. Creatures that were picked up in the shallow coastal water can’t survive in the open ocean, and similarly any brought on board in the middle of the ocean stand little chance of surviving at the ship’s destination. The vessel can be sure that any water taken in is ‘mid-ocean’ water by doing this during an ocean crossing, usually more than 200 Nautical Miles from shore and in water more than 2000 meters deep.

The International Maritime Organisation (IMO) is a division of the United Nations, and works to ensure international shipping operations are effectively controlled worldwide. This organisation has recently decided on requirements for Ballast Water Treatment Systems. More information about their work to control this problem can be found on their website:

These systems take an extra step towards eliminating the risk of organisms being moved between ocean areas, by processing and treating the ballast water. There are many methods that can be used, including:

  • Chemical treatment
  • Ultra Violet (UV) light
  • Filtration of the ballast water
  • Heat treatment

Many companies have been reluctant to purchase and install these systems, as although the IMO have set their requirements several years ago, the United States of America have their own regulations that are more stringent. Manufacturers are only just getting their systems approved by the USA, so over the coming years we can expect to see shipping companies investing in these systems.

By using these systems, ships will no longer be constrained to ballast water managed mid-ocean, but will be able to take up, treat and discharge ballast water almost without limitation. This will improve both the safety of ship board operations, and the environment we operate in.

Every seafarer’s goal should be to leave every country, port and ocean in the same condition as what we found it in – or better! The careful management of ballast waster is a big step towards achieving this goal.

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