Environmental Engineering Reference
In-Depth Information
through a 0.45 mm filter prior to toxicity tests. All four samples were analyzed for
toxic effects using different toxicity tests. The last two samples were treated by
aluminum addition to investigate its potential influence on the toxic effect.
The samples collected in April were divided into aliquots of 320 mL in 500
mL glass bottles. Aluminum sulfate (Al
2
(SO
4
)
3
16H
2
O) was added to a final
aluminum concentration of 0, 2, 5, 7 and 10 mg Al/L (sample III) and a concen-
tration of 0, 10, 20, 30 and 40 mg Al/L (sample IV). The aliquots were placed
on a shaker for 1 h to ensure thoroughly mixing and afterwards left to settle for
3 days at 5°C. Three days was chosen as this is the minimum hydraulic retention
time for a typical Danish wet detention pond for stormwater treatment
[8]
. After
the 3 days, the supernatant was extracted without disturbing the settled precipi-
tate. The supernatant was tested for toxic effects by three different toxicity tests.
The natural pH-value for sample III and IV prior to aluminum addition was 8.76
and 8.78, respectively. The products of the dissociation and hydration of the
aluminum salt at these pH-values is mainly aluminum hydroxide (Al(OH)
3
) and
to a lesser extent aluminate ion (Al(OH)
4-
). Aluminum hydroxide could be
observed as visible flocks in the aliquots with an aluminum concentration equal
to 5 mg Al/L and above.
Toxicity Tests
Toxicity tests were chosen on the basis the following criteria: The tests should be
validated and comply with a standard; an existing protocol should be available; the
tests should use organisms on different trophic levels; and the tests should be cost-
time efficient. These criteria led to three different tests, using bacteria (Vibrio fis-
cheri), algae (Selenastrum capricornutum) and crustaceans (Daphnia magna) as test
organisms. Vibrio fisheri is a marine bacterium and will therefore not be found
naturally in wet detention ponds. However, the test is commonly used and relative
fast, giving the result after only 30 min and was on this basis included in the test
battery. The last two tests use organisms which are indigenous to freshwater. All
three tests are designed to measure acute toxic effects.
The test using Vibrio fisheri is based on the capacity of the bacteria to emit
luminescence, meaning that the less luminescence the bacteria produce, the
more stressed are the bacteria due to exposure to toxic substances. Specific
volumes of the samples and different dilutions hereof were mixed with bacteria
suspension (duplicate for each dilution). The luminescence was measured after
0, 5, 15 and 30 min of exposure to the sample on a M500 analyzer from SDI
in the Microtox
®
Acute mode. Decrease in luminescence relative to a control
gives an expression for the toxic effect and EC-values can be estimated. As Vibrio
fischeri is a marine bacterium, the runoff samples were adjusted to a salinity
of 2% prior to analysis. An advantage of this test is that for each dilution, a
very large number of test individuals are applied, minimizing statistical fluctua-
tions. The whole test was carried out according to the international standard
DS/EN ISO 11348-1.
