Ballast Water Management Systems for Vessels - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 112

Hydrocyclone
Cyclonic separation has been proposed as a relatively simple and inexpensive way
of removing larger particles and organisms from ballast water. Water and particles
enter the hydrocyclone tangentially, thus setting up a circular fl ow. They are then
drawn through tangential slots and are accelerated into the separation chamber.
Centrifugal action tosses particles heavier than the water to the perimeter of the
separation chamber. The solids gently drop along the perimeter and end up in the
calm collection chamber of the separator. Solids may be periodically purged or
continuously extracted from the separator. However, cyclonic separation of organ-
isms with a specifi c gravity similar to that of water is limited which is valid for
many plankton taxa. Therefore, some BWMS use the hydrocyclone as a pre-
separator which is followed by a fi ltration unit thereby enhancing the performance
of the fi ltration unit.
Ultraviolet Radiation
Ultraviolet (UV) radiation is commonly used for sterilising potable or waste water
and for the purifi cation in aquaculture and fi sheries. UV radiation operates by caus-
ing photochemical reactions of biological components such as nucleic acids (DNA
and RNA) and proteins. The lower UV wavelengths are generally more effective.
However, radiation at these wavelengths shows a lower transmission in water. It's
performance may further be affected by organic material, particles or bubbles. The
effectiveness of UV treatment depends also largely upon the pigmentation, size,
morphology of organisms (surface/volume ratio). Viruses require similar dosages to
bacteria. Algae require larger dosages than bacteria due to their size and their pig-
mentation. Disadvantaging is the effect that some smaller organisms could pass the
UV unit in the shadow of larger organisms/particles with reduced treatment and the
reduced transmission of UV-radiation in turbid waters. It was observed in tests that
some organisms have a self-repair mechanism so that re-growth of organisms after
UV treatment occurred. This is (partly) overcome by applying the UV exposure also
during ballast water discharge. Another and unsolved problem is that the UV effect
on organisms is not immediately observed (Liebich et al. 2012 ; Martínez et al.
2012 ) so that compliance with the D-2 standard is diffi cult to show when the water
is treated during discharge.
Electrochlorination
The use of electrochlorination as a means of preventing marine growth is well
known. Electrochlorination is used on board so that the active substances are gener-
ated from the ballast water taken on board (no storage tank of chemicals) and this
may either be done in a side stream or in the full ballast water stream of a vessel.
Electrolyzers usually consist of a number of reactor cells arranged in series.