Ballast Water Management Convention
Why there is a need to manage the ballast water?
While ballast water is essential for safe and efficient modern shipping operations, it may pose serious ecological, economic, and health problems due to the multitude of marine species carried in ships’ ballast water. The transferred species may survive to establish a reproductive population in the host environment, becoming invasive, out-competing native species and multiplying into pest proportions. The effects in many areas of the world have been devastating
Ballast Water Standards
There are two types of ballast water standards in the convention. Their differences are significant and they should not be directly compared.
The first, Ballast Water Exchange, is based on specified distances and depths where a vessel may discharge.
Ballast Water Performance is a standard based on the number of viable organisms allowed per unit of treated water.
Some areas are instituting standards which exceed the IMO guidelines. Both California and the Great Lakes region of the United States have adopted stringent local guidelines.
Different types of Ballast water exchange
Before entering into the waters of any coastal state, ballast water exchange has to be done in mid-ocean or at least 200 nautical miles from shore and at a location where the water depth is at least 200 meters or more.
If neither of these scenarios is possible then a vessel may exchange “in an area designated by the port state”. While undertaking ballast water exchange a port state must not cause deviation or delay of a ship.
The efficiency of exchange should be at least 95% of volume and must be repeated three times. Vessels that demonstrate high-efficiency exchanges may reduce the number of exchange cycles.
Ballast Water Exchange should only be undertaken when the safety of the vessel and crew are guaranteed.
There are three different methods of ballast water exchange:
The sequential method entails completely emptying ballast tanks of the coastal waters and refilling with open-ocean water. Emptying of certain tanks may lead to significantly reduced stability, higher vessel structural stresses, high sloshing pressures, and/or reduced forward drafts which may then increase the probability of bow slamming. Margins are to be provided for stability and strength for all seagoing conditions, as specified in the vessel’s approved trim and stability booklet and the loading manual.
2. Flow-through method:
The flow-through method involves pumping replacement ballast water into the bottom of a full ballast tank, forcing existing ballast water out through an overflow or other arrangement. Ballast water equal to approximately three times the tank capacity must be pumped through the tank to achieve 95% effectiveness in eliminating aquatic organisms. The flow-through method does not typically alter stability, hull girder stress, and vessel attitude.
In the dilution method, replacement ballast water is filled through the top of the ballast tank and simultaneously discharged from the bottom at the same flow rate while maintaining a constant level in the tank throughout the ballast exchange operation. As with the flow-through method, ballast equal to approximately three times the tank capacity must be pumped through the tank to achieve 95% effectiveness in eliminating aquatic organisms. The dilution method has the advantages of the flow-through method with regard to maintaining stability and strength and other similar benefits. By discharging water from the bottom of the ballast tanks, sediments are more easily removed. This method avoids the use of air vent pipes and the removal of manhole covers to discharge water over the deck.
Ballast Water Performance
In the case of Ballast Water Exchange, ship operators are flushing untreated ballast out of the tanks. This is a practical if not perfect way of allowing older vessels to operate without the expense and logistical problems of ballast water treatment retrofits.
New and retrofitted vessels are much less likely to transport unwanted species because the ballast water treatment systems eliminate a large proportion of the viable organisms from the ballast tanks before discharge.
Systems like these significantly reduce the chances of unwanted species being introduced by inefficient exchange practices or in the event of an untreated near-shore discharge for safety reasons.
The IMO uses the following guidelines for the Ballast Water Exchange standard in regulation D-2.
Ballast water must contain less than 10 viable organisms which are greater than or equal to 50 microns in size per cubic meter (264 US Gallons). For reference, 50 microns is about half the thickness of an average human hair.
In addition, ballast water must contain less than 10 viable organisms that are less than 50 microns but greater than or equal to 10 microns per milliliter(1/1000th of a liter).
Indicator Microbes may not exceed the following amounts; Vibrio Cholerae - less than 1 CFU (colony forming unit) per 100 Ml, E. Coli – less than 250 CFU per 100Ml, Enterococci – less than 100 CFU per 100Ml.
Water treated to this standard is considered pure enough to discharge in most ports. These steps to re-mediate ballast water are only effective in the removal of unwanted organisms. It is still possible to carry toxins like copper and heavy metals often found in ports to other destinations in ballast water and these pollutants may concentrate in ballast tank sediment. Radioactive substances can also be transported in ballast but any serious cases would likely be found quickly by monitoring personnel.