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3544
dx.doi.org/10.1021/es102790d |
Environ. Sci. Technol.
2011, 45,
3539–3546
Environmental Science & Technology
ARTICLE
sample volume per trial is set at 7 m
3
, then compliant and
noncompliant tests will often be apparent after a single test. In
cases where results are very close to compliance thresholds,
multiple trials may be necessary before success or failure can be
fully assessed.
When the summed Poisson method was applied to test data
from three di
ff
erent BW treatment systems, results were readily
interpreted at the per-trial and multiple trial levels. Although two
systems yielded mixed results in which some trials positively
rejected the null hypothesis and others did not, when summed
Poisson was applied, noncompliance with the discharge standard
was unequivocal (Table 2).
In addition to land-based testing currently underway, BW
treatment systems on ships will require shipboard evaluations to
(1) verify initial performance, and (2) ensure that treatment
consistently meets the standard throughout the vessels
’
lifetime.
15
The summed Poisson method permits rapid and
robust analyses of results and can, in some instances, provide
extremely prompt performance feedback.
In all probability, identical or similar systems will be installed
on multiple vessels. Because discharge standards are concentra-
tion-based, they apply to all vessel types, regardless of the
environmental conditions of operation. If sampling protocols
are standardized across vessels and meet the assumptions
described above, then results from multiple vessels might also
be considered as independent tests of the same treatment system.
Under these circumstances the summed Poisson approach allows
individual installations to be assessed separately, thereby provid-
ing speci
fi
c information about the performance of speci
fi
c
installations (single vessels) across time. Alternatively, the
fl
eet
can be assessed as a whole, yielding more generalized perfor-
mance information on the treatment system across platforms.
Although detailed sampling and analysis may not be feasible
for frequent, routine, or continuous compliance monitoring of
operational BW treatments systems,
15
there will likely be a need
for targeted, comprehensive biological assessments of high-risk
vessels entering ports. Results from the present analyses indicate
that 7-m
3
time-integrated samples may provide a reasonable
balance of statistical power and logistic achievability when
applied to zooplankton discharge. When applied to actual BW
treatment test facility results, the summed Poisson approach
provided clear-cut results, even at sample volumes of 5 m
3
. Given
the apparent power of this testing protocol, one course of action
would be to conduct selected but infrequent biological assess-
ments of BW interspersed with continuous, automated monitor-
ing of treatment system mechanical operations and indirect
measures of treatment performance, such as changes in BW
physical or chemical conditions.
15
Our approach is well suited for
discharge testing of zooplankton (biota
g
50
μ
m in dimension)
at the IMO discharge standard. In theory, the same basic
statistical treatment should apply to organisms in the regulatory
size class (
g
10 and <50
μ
m in minimum dimension
—
but
admonishments concerning colonial or chain-forming phyto-
plankton on aggregation must be considered, see Table S1 and
references therein), with sample volume and threshold lowered
to account for higher concentration allowed in the discharge
standard (<10 viable organisms
3
mL
1
), and assuming viable
organisms can be as readily detected and di
ff
erentiated from
dead.
31,32
Phase 2 discharge standards proposed by USCG are
e
ff
ectively up to 1000 times more stringent than phase 1, and if
implemented, will clearly require protocols (and sample
volumes) that di
ff
er from what is presented here. Indeed, more
sophisticated technologies for use in BW sampling, biological
detection, biological viability analysis, and enumeration may be
necessary for compliance testing at the USCG phase 2 standard
level. Furthermore, while other sources of error must be ad-
dressed to identify proper sample volume thresholds (see Sup-
porting Information) regardless of the discharge standard, this
likely becomes even more important as the discharge standard
becomes more stringent. There are various criteria that must
be considered in establishing robust sampling protocols and
methods. However, the statistical approach that is ultimately
used to enforce ballast water discharge standards will in
fl
uence
Figure 3.
Power analysis of the summed Poisson method for identifying
BW concentrations that exceed a discharge standard of 10 zooplankton
3
m
3
using multiple, 7-m
3
sample volumes from independent trials,
R
= 0.05.
Table 2. Summed Poisson Analysis Applied to Three Treatment Technologies
a
a
All trials employed 5-m
3
time-integrated sampling from discharge pipe. All technologies were evaluated based on individual trial results and the
combined trial results. Red shading indicates noncompliance and green indicates compliance with IMO discharge standard for zooplankton (
R
= 0.05).