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Removal of both BOD5 and COD (Figs. 2 and 3) occurred from
the very beginning of the sampling campaign, which started about
2 months of operation. Unfortunately, the inflow concentrations
of organics decreased substantially during August 2007, after the
crushed rock was replaced with sand in the second wetland. The
average inflow BOD5 and COD concentrations dropped from 245
and 472 mg/l to only 88 and 230 mg/l, respectively (Tables 2 and 3).
Therefore, the influence of gravel replacement with sand is difficult
to evaluate. In 2008, the inflow concentrations of organics also
fluctuated but the average values were higher than in the second
part of 2007 (Tables 2 and 3). Despite the higher inflow concentrations
in 2008, the final outflow concentrations of both BOD5
and COD were comparable for the periods of steady performance
(Tables 2 and 3). In 2008, the removal of organics in the first wetland
was very high and the average removal amounted to 73.6% while
the overall removal efficiency amounted to 94.5%. Respective values
for COD removal were 64% and 84.4%. The results presented in
Figs. 2 and 3 and Tables 2 and 3 indicated that the removal of organics
in the last, horizontal-flow wetland is only very low but even
this slight decrease in concentration ofBOD5 is significant (Table 2).
However, the major role of this part was to eliminate nitrate which
are created by oxidation of ammonia in the second wetland (see
Section 3.2). The lowering water level in the first wetland caused a
slight increase in concentrations of organics (Figs. 2 and 3) after the
passage through this stage. However, this increase was eliminated
in the second and third stages so that the final BOD5 concentrations
remained low.
Fig. 4. Concentrations ofNH4-N in experimental hybrid constructed wetland during
the periods 18.6.–27.11.2007 and 26.5.–18.12.2008.
The observed treatment efficiencies for organics were comparable
with the literature data for hybrid constructed wetlands. Öövel
et al. (2007) reported BOD7 removal of 90.8% with the final concentration
of 5.5 mg/l in a VF-HF constructed wetland in Estonia.
O’Hogain (2003) reported COD and BOD5 removals of 88% and 89%
in VF-HF constructed wetland in Ireland with final COD and BOD5
outflow concentrations of 47 and 27 mg/l.
The inflow COD loadings were comparable with values
reported by Molle et al. (2008) and O’Hogain (2003) – 180
and 193 gCOD/m2 day, respectively – for the first period of
2007 (Table 4) and with values of 80 gCOD/m2 day reported by
Langergraber et al. (2008) during remaining period in 2007. In 2008,
the inflow loading (37.2 gCOD/m2 day)waslower due to lowerflow
rate. Molle et al. (2008) tested combination oftwostages of verticalflow
wetlands followed by a horizontal-flow wetland in France.
Langergraber et al. (2008) tested the combination of partially saturated
and free-drain vertical wetlands in Austria.

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