Environmental Engineering Reference
In-Depth Information
For in-line sampling the tank(s) which is(are) currently discharged when PSC
comes on board may be prioritized for sampling, to avoid that PSC has to wait many
hours until the “targeted tank” is ready to be discharged.
In general, tanks currently being discharged or those to be discharged fi rst may
be sampled fi rst as this would give the opportunity to PSC offi cers to decide for
appropriate management measures in cases of non-compliance, or in cases where an
indication of possible non-compliance is identifi ed.
Sampling for Compliance Control with the D-1 Standard
Salinity measurements of ballast water may be used to verify if the water was
exchanged according to the BWM Convention requirements (D-1 standard). Should
the measured ballast water salinity be low, e.g., below 30 psu, it can with a high
level of confi dence be assumed that the ballast water originates from coastal areas
with freshwater infl uence. This means the water was not exchanged with ocean
water as required, i.e., outside 50 or 200 nautical miles from nearest land and at
water depths higher than 200 m, because this water would clearly have a higher
salinity. Consequently, with this D-1 standard compliance control option non-
compliance would be assumed when the inspected vessel has loaded ballast in a
lower salinity or freshwater port.
The D-1 standard requires that at least 95 % of the water needs to be exchanged.
Therefore up to 5 % of water may remain in the ballast tank unexchanged. When a
vessel has taken up ballast water in a freshwater port (100 % tank volume) and 95 %
are exchanged in mid-ocean, the possibly remaining 5 % freshwater in the tank will
dilute the salinity of the ocean water taken up during the exchange. As a conse-
quence this salinity dilution may result in a false non-compliant indication in cases
when the remaining freshwater from the previous tank fi lling would be ignored.
From our on board studies we know that sometimes more than 5 % of water remain
as unpumpable ballast inside ballast tanks. This depends, e.g., on the vessels trim
and tank design. We observed salinity differences of ca. 4 psu when a ballast tank
was fi lled in Hamburg (freshwater) and the water from this tank was exchanged
with marine water according to the depth and distance requirements as stated above,
i.e. the freshwater from the previous tank fi lling “diluted” the marine water during
the water exchange.
When seasonally averaged the lowest ocean salinity is ca. 30 psu (see Fig.
1
).
Consequently, ballast water salinities below 30 psu likely indicate that the exchange
occurred less than 50 nautical miles from the nearest land with infl uence of fresh-
water from nearby rivers or estuaries, because otherwise the salinity should be
higher. Therefore a low salinity measurement indicates that the ballast water was
exchanged closer to land than required.
In an experiment we have shown that the salinity of the ballast water in a ballast
tank was not homogenous when the salinity was measured over different depths of
a sounding pipe. The deeper the salinity sensor was lowered in the sounding pipe
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