Environmental Engineering Reference
In-Depth Information
values and standard deviations of the aliquots from sample III and IV are shown
(72 h exposure).
The EC10 values were all below 100% and therefore estimated by interpolation.
Figure 3 shows the same tendencies as Fig. 2 , that is lower toxic effect for alumi-
num concentrations between 2 and 10 mg Al/L compared with stormwater runoff
without aluminum. As shown, highest EC50 occurs at 10 mg Al Al/L. At 20 mg Al
Al/L, the EC10 is lower than the EC10 for the aliquot without aluminum addition.
This is not consistent with the EC50 for the same aliquot, where the EC50 is indi-
cating that this aliquot has a lower toxic effect than the aliquot without aluminum
addition. In principle, the EC10 and EC50 should show the same tendency.
However, due to uncertainties in the tests and also in their interpretation, deviations
from this might happen. Due to this uncertainty, it cannot be concluded whether the
addition of 20 mg Al Al/L to stormwater runoff has a positive or negative effect on
the toxicity. It is therefore, in order to avoid toxic effects, recommended to add not
more than 10 mg Al Al/L when treating stormwater runoffs.
The tendency for the daphnia test was the same as for the algae test, that is
untreated stormwater runoff had a significant toxic effect as well as runoff samples
with addition of 30 and 40 mg Al Al/L; showing an inhibition at 25% and 95% in
the undiluted aliquots with 30 and 40 mg Al Al/L, respectively. The other aliquots
did not show any significant toxic effects. With the bacterial test, the observation of
an increasing toxicity in the aliquots with 30 and 40 mg Al Al/L was not observed
- most likely due to the relative poor sensitivity of the test.
The limit between beneficial and toxic effects caused by the addition of alumi-
num will be different at other pH-values due to the equilibrium between the specia-
tion of the aluminum hydroxides. The equilibrium influences the solubility and
flock formation and thereby the removal of toxic substances and the toxic effect of
the aluminum hydroxides itself due to different speciation hereof.
At pH 8.2, EC50 for the microalgae Selenastrum capricornutum equal to 460 mg
Al/L is reported [10] . The EC50 reported is low compared to the results in this
study where aluminum began to show toxic effects at a concentration level of
20,000-40,000 mg Al/L. The large difference is probably due to the nature of the
samples tested in this study, resulting in removal of toxic substances and phos-
phorus by reactions with aluminum as well as side reactions with organic matter.
Because of these reactions the aluminum may become unavailable to the test
organisms.
Addition of aluminum to lakes in order to obtain optimal phosphorus removal
efficiency is reported in the range from 2 to 30 mg Al/L with the most cost-effective
dosing between 2 and 5 mg Al/L [6, 11] . The most effective decrease in toxic effect
was in this study obtained in the range from 7 to 10 mg Al/L, slightly higher than
the cost-effective dose for phosphorous removal. The higher EC50 achieved
with addition of 7 to 10 mg Al/L compared to the EC50 at 5 mg Al/L should be
considered insignificant, taking the higher cost in consideration. The cost-effective
concentration of aluminum for reduction of toxic effects is therefore around 5 mg
Al/L. This correspond to a consumption of approximately 300 kg Al/year for the
stormwater pond from where the samples originated.
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