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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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