of each unit for each operation year (i.e., for the three different
inlet loadings) were statistically significant for most units (p < 0.05;
Table SM-2), except of unit W9. This demonstrates that the VFCWs
are capable of receiving high organic loads when operating as the
first treatment stage.
The same comments are applicable for nitrogen pollutants.
After the first operation year, effluent concentrations dropped and
the fluctuations became moderate (Fig. 5c and d). The third year
showed the highest nitrogen removal, with a mean effluent concentration
of 14 mg/L for NH4
+-N (Table SM-1). It seems that the
longer resting period (8 days) of this year, in addition to the fact
that the plants have already reached their maximum growth after
two years, resulted in a better aeration of the bed. This is also indicated
by the highest effluent DO concentrations observed during
the third year. Fig. 3c and d confirm this remark, since nitrogen
effluent concentrations decrease as the load (and respective resting
days) increase. The ten units removed 12.1–16.4 g NH4
+-N/m2 d
(Table SM-1) during the third year, values almost double than those
of the second year (4.9–8.5 g NH4
+-N/m2 d). Similarly to OM, all
comparisons of different effluent concentrations and ALR values
of each unit and for each operation year showed statistical significance
(Table SM-2), implying a stable performance even at high
loads.