(Refer to detailed answer for 69ec4c... but give a full treatment.) The International Convention for the Control and Management of Ships' Ballast Water and Sediments (2004, in force 2017) has the purpose and objectives of preventing, minimising and eliminating the transfer of harmful aquatic organisms and pathogens through ships' ballast water and sediments, thereby protecting the marine environment, human health and biodiversity. It requires ships to have a Ballast Water Management Plan and Ballast Water Record Book, to manage their ballast water so that coastal organisms are not transported, and to comply with the D-1 (exchange) and D-2 (performance) standards, using a type-approved Ballast Water Management System where required; ships are surveyed and issued a BWM Certificate.
D-1 standard (Ballast Water Exchange Standard): ships performing ballast water exchange shall exchange so that the volumetric exchange efficiency is at least 95%; alternatively the intended "midship equivalent" may be achieved by pumping three times the tank volume (flow-through) - ships should discharge the exchanged (open-ocean) water.
D-2 standard (Ballast Water Performance Standard): after treatment/discharge the ballast water must contain:
- fewer than 10 viable organisms per cubic metre of organisms of size >= 50 micrometres;
- fewer than 10 viable organisms per ml of organisms of size 10-50 micrometres; and
- indicator microbes: toxicogenic Vibrio cholerae less than 1 cfu per 100 ml; E. coli less than 250 cfu per 100 ml; Intestinal enterococci less than 100 cfu per 100 ml.
Differences: D-1 is an operational/physical method (mid-ocean exchange to flush out coastal organisms) that does not set a discharge-quality standard; D-2 is an absolute discharge-quality standard (bioload limits) achieved by an approved treatment system. D-1 is a temporary measure for ships not yet fitted with a treatment system (phased out by the compliance dates), whereas D-2 governs the actual discharged water and is the ultimate requirement that ships meet with an approved type-approved BWMS.
(Refer to detailed answer. Methods: Sequential (empty/re-fill), Flow-through (pump 3 volumes through a partially filled tank), Dilution (mix open-ocean water with existing and discharge); precautions: exchange > 200 nm from land and depth > 200 m preferably (or at least 50 nm and 200 m where not possible), consider stability/list/trim/sloshing and longitudinal strength when emptying tanks, avoid over-pressure of the tank in flow-through, monitor and prevent sloshing, ensure pump/propulsion limitations, good weather and adequate sea room, record in the Ballast Water Record Book; limitations: exchange is a reduction not elimination, organisms may survive, restricted in ice/shallow/confined areas and bad weather, cannot always be done; treatment/D-2 is the final compliance.)