Ballast Water Management Systems for Vessels - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 111

Table 1 Generi c treatment process and some main BWMS technologies

Treatment

Pre-treatment Chemical Physical Biological Residual control

Filtration Chlorination UV radiation Bioaugmentation

with microorganisms

Chemical
reduction
(Neutralisation)

Hydrocyclone Electrochlorination Deoxygenation

Coagulation Ozonation Inert gas or

Nitrogen
injection

Flocculation Chlorine dioxide Ultrasonic

treatment

Peracetic acid Cavitation

Other active
substances

Fine fi ltration
Heat

in three stages, i.e., pre-treatment, treatment and residual control (neutralisation). In
the pre-treatment stage the main focus is to exclude as much as possible solid material
and bigger organisms, and with this helping the other treatment process(es) to be more
effective, e.g., UV effi cacy is limited if there are many solid particles in suspension
because organisms survive when being in shadows of such particles, and the more
solids and organisms are in the water, the more active substances are needed to achieve
the same lethal effect. The residual control stage (neutralisation) is needed if there are
any substances left in the ballast water after the treatment process is completed that
could cause harm when being discharged from a vessel, e.g., residual toxicity from the
use of active substances and their by-products (see Table 1 ).
In the following paragraphs we describe some of the main working principles of
BWMS components.

Filtration

Filtration of ballast water seems to be the most environmentally sound method, but
the amounts of ballast water that have to be treated are immense. Different fi lter
technologies are in use, e.g., disk fi lters, mesh and wedge-wire fi lters. Ultra-fi ltration
methods have not yet been tested or proven to work with large volumes of ballast
water and high loads of sediments. The effi cacy of removing particles larger than the
mesh-size of these fi lter units is with 95-98 % very effective. In addition some per-
centage of the smaller particles may also be removed. Some systems use a combina-
tion of two fi lters where the fi rst removes very large particles, which enhances the
effi ciency of the second fi ner fi lter. However, although the organism removal rate is
high the D-2 standard is unlikely to be met with fi ltration as a stand-alone technol-
ogy. Most fi lters used include an automatic backwash mechanisms for self-cleaning
to ensure continuous operation. Overboard disposal of the collected residues as fi lter
backwash would occur at the location of ballasting rather than at the destination port,
thereby avoiding the transfer of non-native species with the fi lter backwash.