常弘： 冬季满堂河小流域人工湿地对污染物消减研究
similar result obtained for a wastewater treatment study involving T. latifolia (Ciria et al. 2005).
The COD removal rate was higher in aerated compared to non-aerated wetlands. In comparison to the non-aerated wetland F, intermittent artificial aeration caused a significant COD increase of 4.36, 5.58, 10.38 and 8.94 g/m2 day for the aerated wetlands A, B, C and D, respectively (Fig. 1). Vymazal (1999) reported that aerobic degradation was the predominant process responsible for COD removal. Dissolved oxygen availability was the key limiting corresponding process variable.
Findings of this study showed that an increase in oxygen availability due to artificial aeration enhances the removal of COD in constructed wetlands confirm previous research by Ouellet-Plamondon et al. (2006) and Nivala et al. (2007). Compared to wetland F, the presence of PHPB resulted in an additional COD removal of 6.62 g/m2 day for wetland E. Furthermore, the highest COD mean removal of 36.76 g/m2 day) was found for wetland C (Fig. 1). Results indicated that a combination of bottom aeration and PHPB presence enhances COD removal.
3.2 Nitrogen Removal
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Fig. 2 a Ammonia–nitrogen (NH4–N), b nitrate–nitrogen (NO3–N), and total nitrogen (TN) removal in different constructed wetlands during the Typha latifolia L. (cattail) growing season between June and November 2006. Max, Min Mean and SD represent the maximal, minimal, mean values and standard deviation of the removal rate. Box charts with differen letters are significantly (p<0.05) different from each other according to Duncan’s multiple range tests.
Table 3 Dissolved oxygen (DO) concentrations and pH values in tested constructed wetlands at different distances during the period of aeration between June and November 2006
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常弘： 冬季满堂河小流域人工湿地对污染物消减研究
A, B, C, D, E, F and G represent constructed wetlands with different operations. Water samples were taken at a frequency of once per week; data are shown as means±SD.
a
0.3 m below the surface of the inlet
0.7 m below the surface of the inlet
b c d
0.6 m below the surface of the outlet
0.3 m below the surface of the outlet
With respect to NH4–N removal, the planted wetland F showed higher removal performances than the unplanted wetland G (Fig. 2a). Moreover, the improvement was significant and accounted for a 17.18% increase in NH4–N mean removal efficiency (p<0.05). Insufficient microbial activity in unplanted wetland substrate is likely to limit NH4–N removal. Oxygen availability is the main reason for insufficient NH4–N removal. Moreover, nitrification can be encouraged at greater oxygen availability (Bezbaruah and Zhang 2003; De Renzo 1978; Grady and Lim 1980; Nivala et al. 2007). According to Table 3, oxygen concentrations at each sampling depth were higher (>2 mg/l) in the aerated compared to the non-aerated wetlands. Increased oxygen availability resulted in an improvement of the mean NH4–N removal by 0.76, 1.29, 1.34 and 0.89 g/m2 day for the aerated wetlands A, B, C and D in comparison to the non-aerated wetland F (Fig. 2a). High NH4–N removal occurred in aerated wetlands confirming the positive effect of aeration on nitrifying bacteria(Ouellet-Plamondon et al. 2006). Further NH4–N mean removal of 0.34 g/m2 day was obtained for wetland E containing PHPB compared to wetland F,which did not contain PHPB (Fig. 2a). However, reduced increases in NH4–N mean removal of 0.05 and 0.13 g/m2 day were obtained for the aerated wetlands C and D
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containing PHPB compared to the corresponding wetlands A and B without PHPB(Fig. 2a), respectively. It follows that supplementary aeration weakened the positive effect of PHPB on NH4–N removal.
As can be seen in Fig. 2b, the planted wetland F removed more NO3–N in comparison to the unplanted wetland G. Typha latifolia significantly (p<0.05) contributed to a 23.68% increase in NO3–N mean removal efficiency. However, the aerated wetlands A and B had significantly lower NO3–N removal in comparison to the non-aerated wetland F. The lowest NO3–N mean removal of 0.45 g/m2 day was obtained in the aerated wetland B, while the highest rate of 0.99 g/m2 day was observed in the non-aerated wetland F (Fig. 2b). This was likely due to the supply of oxygen to the substrate, which subsequently increased the DO concentrations in the aerated wetlands (Table 3). However, anaerobic conditions,which are necessary for denitrification, are suppressed at the same time (Ciria et al. 2005). The pH may impact on NO3–N removal as shown by Vymazal (2007) and Li et al. (2008). The mean values for pH were between 7.58 and 8.00 (Table 3), which is well within the optimal range (6.6 to 8.3) for denitrification, as discussed by ?imek et al. (2002). In planted constructed wetlands, most NO3–N is removed through biomass uptake and denitrification in the root zone (Brix 1997; Scholz 2006). The presence of PHPB provided suitable habitat for microorganisms and subsequently improved biological nitrogen removal (Chen et al. 2006; Yu et al. 2005). However,several studies have indicated that bacteria prefer the autotrophic uptake of NH4–N compared to NO3–N(Bigambo and Mayo 2005; Metcalf and Eddy 1995).In this study, NH4–N is the main component of the influent TN, and is therefore rarely in short supply(Table 1 and Fig. 2a). This might be the reason for the lack of improvement concerning NO3–N removal by using PHPB as a wetland substrate.
The TN removal (Fig. 2c) predominantly comprised of NH4–N (Fig. 2a) and NO3–N (Fig. 2b) removal. According to Fig. 2c, the planted wetland F showed a better TN removal than the unplanted wetland G. The presence of T. latifolia resulted in a significant (p<0.05) additional TN removal of 21.78%. This observation verified earlier findings showing that wetland plants can make a significant contribution to TN removal (Brix 1997; Scholz 2006;Vymazal 2007). Complete nitrification followed by denitrification was the most important process in total nitrogen removal (Nivala et al. 2007; Vymazal 2007).Intermittent artificial aeration allowed
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常弘： 冬季满堂河小流域人工湿地对污染物消减研究
nitrification and denitrification to occur due to the chosen switching on and off pattern of the compressed air pump. In this study, no significant difference was observed between each aerated wetlands (Fig. 2c). In contrast to the non-aerated wetland F, intermittent artificial aeration led to an increase of 0.57, 0.80, 1.05 and 0.74 g/m2 day in TN mean removal for the aerated wetlands A, B, C and D, respectively (Fig. 2c).
A good performance concerning TN removal was observed in wetland E containing PHPB. The presence of PHPB resulted in an additional TN removal of 0.32 g/m2 day, if compared to the wetland F, which did not contain PHPB. Furthermore, for wetland C,the highest TN mean removal (5.04 g/m2 day) was observed for the combined effect between bottom aeration and PHPB presence (Fig. 2c). The processes facilitating TN removal in wetland systems are nitrification-denitrification and uptake by plants and microbes (Bigambo and Mayo 2005; Brix et al. 1997; Vymazal 2007), which can be enhanced by the presence of the PHPB.
3.3 Phosphorus Removal
According to Fig. 3, the TP removal pattern is similar to the SRP removal pattern. The amount of organic phosphorus, which is equal to TP minus SRP in the wetland systems, is relatively low, because of the high SRP/TP ratio in the influent (Table 1). Hence, organic phosphorus mineralization can be neglected for the experimental wetlands. The SRP and TP removal performed much better in the planted wetland F than in the unplanted wetland G (Fig. 3). Typha latifolia caused significant (p<0.05) increases in mean SRP and TP removal efficiencies of 18.28% and 17.81%,respectively. Findings showing that plants can significantly reduce phosphorus loading rates were also reported by Yang et al. (2001) and Fraser et al. (2004).
The SRP and TP removal rates were better in aerated than non-aerated constructed wetlands(Fig. 3). In comparison to wetland F, aeration led to SRP and TP mean removal rate increases of 0.06 and 0.09 g/m2 day for aerated wetlands A and B,respectively (Fig. 3).
The good performance of phosphorus removal in aerated wetlands can be interpreted as follows:artificial aeration increased the dissolved oxygen concentrations (Table 3), and thus improved chemical precipitation of phosphorus (Kadlec and Knight 1996;Scholz 2006). Furthermore, the removal of phosphorus by adsorption can be encouraged through ehanced contact between substrate and phosphorus during intermittent artificial aeration. This assumption
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