the comparison with a respective study on horizontal subsurface
flow (HSF) systems [23] showed that NH4
+-N removal was about
20% greater in the VFCW system than in a planted HSF unit with
the same carbonate medium gravel. Fig. 2c and d shows a gradual
decrease in nitrogen effluent concentrations with time. As plants
grow, their expanded root system contribute to nitrogen removal
at a higher rate. In addition, increased nitrite effluent concentrations
(Fig. 2f) indicate the presence of anoxic micro-zones within
the substrate, which favors denitrification.
Phosphorus removal efficiency was lower than that for the other
constituents. Mean TP and OP removal for all units was 36.5%
and 36.8%, respectively (Table 3). As before, the highest reduction
was observed in unit W10 (more than 45% for both constituents;
Table 3) and the lowest in the unplanted unit W3 and the Type
III unit W9 (ca. 30%). However, removal values obtained here are
higher than those reported in the literature (i.e., 20–30%) [5]. It
seems that the relative short contact time between wastewater and
porous media did not favor phosphorus adsorption [6], as implied
by the small range of effluent concentrations during the entire
operation period (Fig. 2g and h).
All VFCW units exhibited pH values ranging from slightly acidic
(6.5–7) to slightly alcalic (7–7.5; Table 3), with small differentiations
among the VFCW units. Fig. 2h reveals a weak annual cycle
for effluent pH values. Variations during the first operation year
should be attributed to the start-up phase. The elevated effluent
values compared to influent values should be mainly attributed
to the interactions between the substrate and the biofilm, and
secondarily to the plant presence [6]. The similarity of effluent
pH values between planted and unplanted units confirms this
assumption.