Environmental Engineering Reference
In-Depth Information
In which,
A
i
(m
2
) represents the average underwater area of shipping category
i
, for example n length categories,
N
im
and
N
ib
represent the number of mov-
ing ships and ships moored at berth in the harbor at any time of the day,
F
i
is
the application factor expressed as the fraction of ships in category
i
treated
with a specific antifouling product, and
LR
b
and
LR
m
are the compound and
paint specific leaching rates (g, m
2
day
-1
) of ships at berth or moving. As will
be described in the next section, large uncertainties may be involved in the
proper estimation of each of the parameters in Eq. 1.
2.1
Leaching Rate
The leaching depends highly on the type of compound, characteristics and
age of the paint matrix, the velocity of the ship, and environmental factors
such as temperature and salinity [7]. Some typical leaching rates reported in
the literature are indicated in Table 1.
Manufacturers usually conduct experimental determinations of leaching
rates during the development of testing phases of new products with plates
or rotating drums coated with the product and exposed to natural or semi-
natural conditions. The results from these experimental studies cannot easily
be translated to real-life leaching rates from ships to which the product is
applied. Available ASTM (D5108-80) and ISO
DIS (15181-1,2) protocols have
been criticized by various authors [8, 9]. For many of the new products es-
tablished and certified analytical methods are hardly available [10]. Except
for the study of Steen et al. [11], hardly any field studies on actual leaching
rates from freshly painted ships have been published in the open literature.
An interesting theoretical mass-balance approach was followed in a study by
Boxall et al. [12], in which worst case leaching rates were estimated based on
lifetime of the paint and national paint usage data. Their overall estimates
were in line with the ranges reported in Table 1. In various reviews good de-
scriptions can be found of the different classes of biocidal antifouling paints
and the dependency of the biocide leaching rate on the physical and chem-
ical processes at the paint-(sea)water interface [7, 13, 14]. The following main
types of coatings were identified: soluble matrix; insoluble matrix, TBT self
polishing copolymer, and TBT-free self polishing paints, each with their spe-
cific leaching patterns and changes of the leaching rate with the age of the
paint matrix. For the soluble paint matrix, with typical in-service periods of
12-18 months, the biocide-release rate may decrease exponentially during the
lifetime of the coating. The self-polishing copolymers and paints, which both
may contain copper, have a different biocide release pattern with a rapid de-
crease in the first months to year, followed by a constant release rate during
the remaining in-service period of up to 5 years.
As demonstrated in experimental and modelling studies [7] commonly
occurring changes in speed (0-20 knots), temperature (10-30
◦
C), salinity
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