In the case of phosphorus, the variations in performance during
the three years were smaller (Fig. 5e and f). Again, the first year
appeared to be transitional and the performance improved after
the second year. The differences in efficiency were smaller compared
to the other pollutants. TP removal showed more constant
behavior compared to OP (Fig. 3e). The comparisons of different
effluent concentrations and ALR values of each unit for each year
were found to be statistically significant (Table SM-2), implying
that the alterations in loadings affected phosphorus removal.
On the whole, the VFCW systems operated under high organic
loads. The first operation period appears to be affected by the startup
effect, as explained below; therefore, the second and the third
operation year are probably more reliable to draw conclusions. The
loading rates were gradually increased year by year and exceeded
200 g COD/m2 d during the last year, which is one of the highest
loads reported in the literature. Morris and Herbert [37] and Zhao
et al. [28] applied even higher loads (400 and 1055 g COD/m2 d,
respectively) under different configurations. The system performance
proved that it possesses a high treatment capability of strong
wastewater, rich in OM and nitrogen. The key parameters that control
the system efficiency and allow for this treatment capacity are
two: the considered parallel units and the respective duration of
the dry period. Results of the present study showed that a satisfying
decomposition of OM was achieved even with the shorter
dry period of 4 days, for an organic load up to 125 g COD/m2 d. The
dry period is crucial for the effective operation of VFCW systems,
because it allows for the oxidation of the accumulated OM inside
and on the surface area of the bed, thus avoiding clogging phenomena.
For a simultaneous effective nitrogen removal and enhanced
nitrification, a longer dry period is needed in order to restore aerobic
conditions within the bed.