Under the Convention, ships are required, according to a timetable of implementation, to comply with the D1 or D2 standards.[1] The D1 standard requires ships to carry out a ballast water exchange, and specifies the volume of water that must be replaced.[17] This standard involves exchanging the uptaken discharge water from the last port, with new sea water; it must occur at a minimum of 200 nautical miles from shore. The D2 standard is more stringent and requires the use of an approved ballast water treatment system.[18] The system must ensure that only small levels of viable organisms remain left in water after treatment so as to minimise the environmental impact of shipping.[17]