The mixtures used
in the tests, in addition to 100% outlet, are the following: 25% inlet/
75% outlet, 50% inlet/50% outlet and 100% inlet (wastewater
incoming at WWTP). The ME removal evidence from wastewater
were carried out through the measurement of their concentrations
in the water samples taken from the pilot-plant with bivalves
inside; at the same time, control tests were conducted into the
pilot-plant without adhering animals. All tests were performed in
triplicate. The ME removal progress was monitored for 4 h, by
sampling the wastewaters every 30 min, which enabled to obtain
the removal slope for each ME. We chose to evaluate the ME
removal within 4 h, taking into account that the treated wastewaters
remain in the Milano-Nosedo WWTP for about 24 h; thus,
the selected time seemed to be a fair compromise in view of
integrating the conventional treatment with limited dimensional
requirements. To check the practicability of such assumptions, we
carried out further
final tests in single for a period of 24 h, taking
only two samples, one at the beginning and one at the end of the
tests. The tests were conducted with an initial
flow rate
corresponding to 3500 L/h, which would imply 18 min contact
time, recirculating the effluent in the pilot-plant 84 times to obtain
an overall 24 h contact time. After each test, the entire pilot-plant
was washed with tap water, to avoid memory-effects related to the previous tests. For this reason, to minimize this problem, as well as
to decrease the bivalve stress, the test schedule started with the
most diluted waste (100% outlet) and gradually increased its
concentration until 100% inlet. We monitored the wastewater
temperature both at the beginning and at the end of each test in
order to take into account its possible interference with the
filtration activity of zebra mussels. The wastewater temperature
within the pilot-plant during the spring season ranged from 14 to
24 C, comparable with the optimal values for D. polymorpha
filtration activity (10–20 C) [42]; we can thus exclude any negative
interference of temperature on the
filtration-removal process.
Samples were taken from the pilot-plant at the selected times by
the use of a 250 mL plastic bottles, acidified with 1% of HNO3 and
stored at 4 C at dark until analysis