What is ballast water and why is it managed?

Ballast water is seawater a ship takes into dedicated tanks, generally distributed along the double bottom, wing tanks and sometimes a forepeak and afterpeak, to control its stability, trim, draught and structural stress when it is not carrying enough cargo to sit properly in the water. An empty or lightly loaded tanker or bulk carrier would ride too high, with its propeller and rudder partly out of the water, reduced stability from a low metacentric height that makes it roll violently, and excessive hull stress from an uneven distribution of weight along its length, so ballast is used to bring the ship down to a safe working draught, correct trim by the head or stern, and keep hogging and sagging bending moments within the limits set in the loading manual. As cargo is loaded the ballast is discharged in a coordinated sequence, and the reverse happens on discharge.

The problem ballast water creates is biological rather than structural. Water taken on in one port is never sterile: it carries living organisms, larvae, plankton, bacteria, and small marine life native to that location, and when the same water is discharged in a different port, potentially on the other side of the world, those organisms can survive, establish themselves and become invasive species that outcompete native life, damage local fisheries and foul infrastructure. Well documented cases include zebra mussels spreading through the North American Great Lakes and toxic algae and cholera causing bacteria being moved between continents in ballast tanks, and the economic and ecological cost of these introductions has been substantial.

To control this, the IMO's Ballast Water Management Convention, which entered into force in 2017, requires ships to manage their ballast water rather than simply pump it in and out untreated. The older method is mid ocean ballast water exchange, where tanks are exchanged, or flushed, far from shore and in deep water so that any coastal organisms taken on are replaced with open ocean water that is far less likely to seed a new coastal population if discharged elsewhere. The convention is progressively requiring the newer D-2 standard instead, met by an approved ballast water treatment system fitted on board that filters out larger organisms and then disinfects the water, commonly by ultraviolet light or electrochlorination, before it is ever pumped out.

Every ship subject to the convention carries a ship specific ballast water management plan, a ballast water record book logging every uptake, exchange, treatment and discharge, and increasingly a certificate confirming the treatment system is approved and functioning, all of which Port State Control checks routinely. Ballast water remains essential to safe ship operation, but it now has to be managed so that moving a ship safely does not also mean moving an ecosystem.