Ballast Water Management Under the Ballast Water Management Convention - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 94

high sea organisms when pumped on board during the BWE will unlikely survive
when released in coastal waters also due to possible salinity changes and the lack of
suitable habitats. Further, it is well-known that organism concentrations are much
lower in high seas compared to coastal waters which reduces the risk of species intro-
ductions. However, sampling studies on board of commercial vessels have shown
that in certain instances after BWE a higher concentration of organisms was found in
the ballast water (e.g., Macdonald and Davidson 1998 ; McCollin et al. 2001 ). This
specifi cally occurred when the BWE was undertaken in shallower seas or during
high organism concentrations, such as algal blooms, which are also known to occur
in the high seas.
Other BWE effi ciency limitations include that, due to ballast tank design, a
certain amount of unpumpable ballast water and sediments always remains inside
the tank on almost all ships (see chapter “ Vessels and Ballast Water ”). As a result
a one time BWE will not be suffi cient to reduce the organism load. IMO noted this
and therefore Regulation D-1 of the BWM Convention requires at least a 95 %
water exchange. This may be met by emptying and refi lling the tank or by pump-
ing through three times the tank volume (Rigby and Hallegraeff 1994 ). However,
when Gollasch and David conducted shipboard tests of different BWM methods
it was noticed that on vessels which were trimmed ahead, about 15 % and more of
unpumpable water remained in the tanks during the empty-refi ll (sequential)
BWE. Furthermore, a 95 % volumetric BWE is unlikely equivalent with a 95 %
organism removal because the organisms are not homogeneously distributed in a
tank (e.g., Murphy et al. 2002 ). In contrast, under certain circumstances, the 95 %
volumetric exchange may result in an even higher than 95 % organism removal.
In conclusion, pumping through less than three times the volume may also be
acceptable provided the ship can demonstrate that at least 95 % volumetric
exchange limit is met.
When conducting BWE Guidelines for Ballast Water Exchange (G6) are to be
considered. Three methods are accepted to conduct BWE and can be described as
follows (IMO 2005c ):
Sequential method - is a process by which a ballast tank is fi rst emptied and then
refi lled with replacement ballast water to achieve at least a 95 % volumetric
exchange.
Flow - through method - is a process by which replacement ballast water is pumped
into a ballast tank, allowing water to fl ow through an overfl ow on deck or other
arrangements.
Dilution method - is a process by which replacement ballast water is fi lled through
the top of a ballast tank with simultaneous discharge from the bottom at the
same fl ow rate so that a constant water level is maintained in the tank through-
out the BWE.
In addition to the requirements to be met in relation to the BWE methods used,
a ship should also consider requirements regarding where BWE shall, whenever
possible, be conducted. In the fi rst place, this is at least 200 nautical miles from
nearest land and in water depths of at least 200 m. If this is impossible, then the