In the earlier times, solid ballast such as sandbags, rocks, iron blocks were used to load onto the vessels once the cargo loading was completed. This method helped to maintain the stability of the ship and its seaworthiness. In the nineteenth century, cargo boats returning from Europe to North America would carry quarried stone as ballast. They contribute to the architectural heritage of some east coast cities (for example Montreal), where this stone was used in building.
As soon steam-powered ships appeared during the modern industrial revolution, their design changed to a metal-hulled type. They started to carry water for the steam machine but also developed the use of water as stabilization ballast.
Today cargo vessels, such as tankers, bulk carriers, or container ships carry liquid ballast. It includes freshwater, saltwater, or brackish water in various ballast tanks. If the cargo vessel wishes to travel empty or partially empty to collect cargo, it must travel in ballast. It keeps the vessel in trim and keeps the propeller and rudder submerged. Typically, being “in ballast” will mean flooding the ballast tanks with seawater as ships get bigger in size. The cargo carried by the vessels varies from one port to another. Water ballast tanks are used to compensate for maintaining the trim and stability of the vessel for a safe sea passage.
The process of loading and unloading untreated ballast water poses a major threat to the environment. Ships become a medium for the transfer of organisms between ecosystems from one part of the world to another.
When ballast water is pumped into a ship many microscopic organisms and sediments can be introduced into a ship’s ballast tanks. These organisms include bacteria, microbes, small invertebrates, eggs, cysts, and larvae of various species. Many of these organisms can survive in a ship’s ballast tanks. When the ballast water is discharged, the organisms are released into new environments. If suitable conditions exist in the new environment into which they are released, these species can survive, reproduce, and become aquatic invasive species.
Ballast water treatment systems are used to maintain the balance of the marine ecosystem by filtering and disinfecting ballast water. Several technologies are used in ballast water treatment systems to avoid polluting seawater. As time has passed, companies are favoring chemical-free ways of treating ballast water. Thus, physical treatment is gaining importance.
The International Marine Organization (IMO) has imposed strict regulations to check potential harm to seawater. By checking on foreign toxic species such as pathogens, these regulations and their strict enforcement are making owners install ballast water treatment systems on ships.
A number of factors are taken into account for choosing a ballast water treatment system for a ship.
Some of the main factors taken into consideration are