Ballast Water Management Systems for Vessels - Global Maritime Transport and Ballast Water Management: Issues and Solutions - page 114

Some pre-treatment technology is used by 80 BWMS, of these 70 use fi ltration,
three use a combination of hydrocyclone and fi ltration (nr. 16, 28 and 32), one uses
a combination of fl occulation and fi ltration (nr. 39), four use a hydrocyclone (nr. 35,
90, 95, and 97), and the remaining two use different other methods (nr. 5 and 17). It
is interesting to note that 24 systems do not have a pre-treatment separation step.
Most of BWMS identifi ed are regarded as BWMS that make use of an active sub-
stance (58). The most frequently used technique seems to be electrolysis/electrochlo-
rination (35), and is applied as stand-alone treatment method by 28 BWMS, and by
seven in combination with other techniques. The remaining 24 BWMS use dosing of
different active substances, e.g., chlorine, PeraClean, SeaKleen and Akrolein.
In the second place is UV treatment with 34 BWMS, 24 of these use UV as
stand-alone treatment method, while ten systems use UV in combination with one
or more other techniques, i.e., TiO 2 , ultrasound, ozonation, electrolysis, plasma.
In total 26 BWMS use two or more treatment techniques in combination as the
main treatment method, while 75 rely on one treatment technique, no information
was available for three BWMS (see Table 2 ).
One BWMS (Table 2 , nr. 74) is the only system which makes use of vacuum de-
oxygenation and bioaugmentation. Bioaugmentation is a mechanism to, e.g., start
activated sludge bioreactors in municipal wastewater treatment plants. In this
BWMS microorganisms will be used to treat living organisms.
The application of BWMS that make use of active substances may result in resid-
ual active substances above the maximum allowable level (TRO 1 0.2 mg L −1 ) when
this is to be discharged into the surrounding waters, hence they need to neutralise
these before the discharge. The BWMS without neutralization will depend on a
longer holding time of ballast water in the tanks during which the chlorine will
breakdown to uncritical substances. Chlorine dioxide has a half-life of approxi-
mately 6-12 h (according to the suppliers and Olivieri et al. 1986 ), but at the con-
centrations at which it is employed it can be safely discharged after a maximum of
24 h. However, this relates also to water salinity and temperature and both should be
taken into account when evaluating the minimum retention time before discharge.
Thirty-four BWMS that make use of active substances have included also an
obligatory neutralisation process at discharge, and further three have this as an
option. The most frequently used neutralisation is by Sodium Thiosulphate (24
BWMS), Sodium Sulphite use fi ve BWMS, three use Sodium Biosulphite, one uses
Activated Carbon, one uses Thiosulphate, and for three BWMS the substance is
unknown (see Table 2 ). Most chlorination systems are applying a dose which results
in approx. 10 mg L −1 chlorine during treatment, which has proven to be effective to
kill organisms, but less than 0.2 mg L −1 residual chlorine in the ballast water dis-
charges has proven to be environmentally acceptable to the recipient waters (see
various references of Final Approvals of BWMS and GESAMP BWWG reports
(IMO 2005-2012)). Most ozonation suppliers are using an ozone dose of 1-2 mg
L −1 which has proven to be effective (Lloyds Register 2011a ).
1 TRO = total residual oxidants