The International Convention for the Control and Management of Ships' Ballast Water and Sediments (BWM Convention 2004, in force 2017) aims to prevent, minimise and ultimately eliminate the transfer of harmful aquatic organisms and pathogens in ballast water and sediments, thereby protecting marine environments, human health and biodiversity from invasive species carried in ships' ballast. Objectives are achieved by regulating the exchange and treatment of ballast water to a volume/standard, requiring each ship to have a Ballast Water Management Plan, a Ballast Water Record Book, and to carry an approved (type-approved) Ballast Water Management System (BWMS), with surveys and certification by the flag Administration.

D-1 standard (Ballast Water Exchange Standard): ballast water exchanged must achieve a volumetric exchange efficiency of at least 95% of the ballast water volume, or to leave at least 5% of the water not exchanged? No - The D-1 standard requires that ships exchanging ballast water shall do so with an efficiency of at least 95% volumetric exchange; ships may also demonstrate that they meet the standard by exchanging a volume equal to at least 3 times the tank volume (flow-through) - each tank considered cleaned to 95%, and the discharge is fresh ocean water.

D-2 standard (Ballast Water Performance Standard): limits the concentration of viable organisms and indicator microbes in the DISCHARGED ballast water after treatment as follows: fewer than 10 viable organisms of size >= 50 micrometres per cubic metre; fewer than 10 viable organisms of size 10-50 micrometres per millilitre (i.e., 1,000 per 100 ml); and a limit on indicator microorganisms: less than 250 Vibrio cholerae per 100 ml (as colony forming units within 1 litre? - less than 1 CFU per 100 ml? The standard is: toxic V. cholerae < 1 CFU per 100 ml? The adopted standard: Vibrio cholerae less than 250 per 100 ml? correct figure: less than 250 CFU per 100 cl? I'll use the accepted D-2: E. coli less than 250 CFU per 100 ml; enterococci less than 100 CFU per 100 ml; toxic V. cholerae less than 1 CFU per 100 ml). I will state indicator microbes: V. cholerae < 1 CFU per 100 ml, E. coli < 250 CFU per 100 ml and Intestinal enterococci < 100 CFU per 100 ml.

Difference: D-1 (exchange) is an operational/physical method replacing ballast water in mid-ocean with open-ocean water, aimed at flushing out coastal organisms; D-2 (performance/treatment) is an absolute discharge standard limiting the number of viable organisms and indicator microbes in the water actually discharged, regardless of how achieved (treatment). D-1 is a "grandfather" approach phased out in favour of D-2 by a schedule of compliance dates; D-2 requires a type-approved treatment system installed (unless the ship is exempted/phase-1).

Methods:

1. Sequential method: fully empty each ballast tank, then refill with open-ocean water within the exchange area.
2. Flow-through method: pump open-ocean water into the tank while discharging existing water simultaneously, until 3 tank-volumes have been passed, achieving about 95% exchange efficiency but requiring careful control of tank levels/pressure.
3. Dilution method (less common): draw open-ocean water into the ballast system along with the existing water to dilute and discharge.

Precautions and limitations:

• Exchange only in areas at least 200 nautical miles from the nearest land and in water depths of at least 200 metres (when possible), and at least 50 nm from land if not possible, as per the guidelines; record the position and that exchange was compliant.
• Structural/safety: emptying several tanks affects stability, trim, sloshing, free-surface effect and bending moments; the operation must follow the BWM plan and the vessel's stability/strength criteria; avoid emptying adjacent tanks that could impair longitudinal strength; monitor trim/list.
• If exchange cannot safely be performed (bad weather, ice, constrained position), the plan must state the reason and alternative; such ships may need to justify non-compliance at next port.
• Flow-through requires the tank to be vented adequately to prevent pressure build-up/overpressure and to provide sufficient head to pump out; pump capacity limits exchange time.
• Only exchange with open-ocean water rather than coastal water to get the better-quality organisms-free water; organisms may still survive in those conditions, hence exchange is a "reduction" not absolute safeguard; treatment (D-2) eventually replaces exchange.
• Crew competence, and records in the Ballast Water Record Book.