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The layer effect in nutrient removal by two indigenous plant species in horizontal flow constructed wetlands
Ecological Engineering 37 (2011) 2101–2104
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Short communication
The layer effect in nutrient removal by two indigenous plant species in horizontal flow constructed wetlands Shuyuan Liu a,b,c , Baixing Yan a,∗ , Lixia Wang a a
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, China School of Civil Engineering and Architecture, Taizhou University, Taizhou 317000, China c Graduate University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 24 January 2011 Received in revised form 8 June 2011 Accepted 29 June 2011 Available online 19 August 2011 Keywords: Nitrogen removal Layer effect Oxidation–reduction potential pH Constructed wetland Wetland plants
a b s t r a c t A layer effect study was conducted to investigate the transformation of nitrogenous pollutants in two batch subsurface horizontal constructed wetlands. Artificial drainage water containing a low concentration of chemical oxygen demand (CODCr ), but high concentration of ammonia and nitrate, was treated in two batch wetland cuboids. The nitrogen removal rates were found to be significantly affected by the characters of the layer as well as the biomass and roots of different plant species (P < 0.05). Correlations between pH, oxidation–reduction potential, and retention time indicated that nitrogen removal rates under study conditions mainly depended on the location of the layer and the plant species in the constructed wetland. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitrification–denitrification is widely accepted as the main process for nitrogen removal (Vymazal, 2005; Tanner et al., 2002; Tjasa, 2006). Plants are biologically considered the indispensable component in constructed wetland systems. Numerous studies have reported that plants stimulate nitrogen removal in these treatment wetlands (Kadlec, 2008; Lin et al., 2002). Under the effect of a wetland plant in a horizontal subsurface flow constructed wetland by intermission operation, the media layer in the vertical direction with enriched external carbon sources is considered suitable for denitrification as its anoxic and anaerobic habitats (Hume et al., 2002; Oskar et al., 2007). In the same manner, a significantly greater oxygen transport capacity provides much better conditions for nitrification (Brix, 1997; Gebremariam and Beutel, 2008; Langergraber, 2005). However, the lack of information on the contributing layers in different locations in the constructed wetland, as well as the effects of these layers on the nitrogen removal efficiency inside the wetland systems, lead to unstable permanence for purification capacity.
∗ Corresponding author. E-mail address: [email protected] (B. Yan). 0925-8574/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2011.06.040
The objective of the present study was to determine the layer effect, that is, the difference in the nitrogen removal capacity at the top and bottom layers in the subsurface horizontal constructed wetland. 2. Materials and methods 2.1. Treatment wetland and experimental design The system consisted of two contrasting outdoor horizontal subsurface flow-constructed wetlands (W1 and W2) that were operated with the intermittent strategy. Each wetland was in triplicate. W1 was planted with Calamagrostis angustifolia, whereas W2 with Phragmites australis. The wetlands were constructed with identical dimensions. The wetland beds were PVC cuboids (12 mm thick), each measuring 2 m long, 0.5 m wide and 0.65 m high. These cuboids contained a 40 cm layer of slag (2.74 ± 0.57 mm in diameter) at the bottom and 10 cm layer of soil above the slag layer, giving an average porosity value of 0.35 in the media. Each cuboid included the inlet (0.2 m long), work (1.6 m long), and outlet sections (0.2 m long). Sewage flowed into or out from the wetland through a water distribution area filled with gravel (24.8 ± 5.6 mm in diameter). Undisturbed soil with 50 C. angustifolia and 25 young P. australis from a local natural wetland was selected to implant in the substrates of W1 and W2, respectively.
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NH4 -N removal efficiencies (%)
b
80
b
60 40 20
NO3 -N removal efficiencies (%)
a
a 100
0
40 20
b
0.4
b
0.3 0.2 0.1 0.0
-2
0.5
a b
-1
a
NO3-N removal load (g m d )
-2
b 60
0.6
a
-1
NH4-N removal load (g m d )
80
1 2 2 W1 of W of W of W r of ayer ayer ayer l laye l l p p m m o o o o T T Bott Bott
0.6
To
b a
0
W1 W2 W1 W2 r of r of r of r of laye laye laye laye p p m m o o o o T T Bott Bott
er of p lay
a
100
0.5
a 0.4
b 0.3 0.2 0.1 0.0
W1
of ayer om l Bott
W1
r of laye Top
W2
of ayer om l Bott
W2
1 2 1 2 fW of W of W of W yer yer o yer a a a ayer l l l l p p om om To To Bott Bott
Fig. 1. Removal efficiencies of NH4 -N and NO3 -N in the bottom and top layers. Identical letters above bars indicate groups of means with no statistically significant differences at the 95% confidence level.
Experiments lasted 100 days (from June to September 2009). During the first 90 days, the main objective was to promote bacteria communities in the treatment wetlands. W1 functioned in the same way as W2, 10 days’ feeding with more than 2 days’ interval. The average value of the hydraulic loads of the 10-day experiment in these wetlands was 0.018 m d−1 . In order to ensure the same influent concentration in each cuboid, the artificial drainage water was prepared in a feed tank by dissolving ammonium nitrate and sodium dihydrogen phosphate with tap water. 2.2. Pollutant sampling and analysis Water samples were collected from each cuboid by perforated tubes and inverted siphon method. The length of the perforated parts for each sampling tube embedded into the substrates of the wetland system was 20 cm. Three sampling tubes whose centers of perforated parts were 15 cm apart from the surface of substrates layers were employed to collect water samples from the top layer. Another three tubes whose centers of perforated parts were 15 cm apart from the bottom of the cuboid were in charge of the bottom layer. The water samples were analyzed for total nitrogen (TN), (NH4 -N), ammoniacal-nitrogen nitrite-nitrogen (NO2 -N), nitrate-nitrogen (NO3 -N), chemical oxygen demand (CODCr ), oxidation–reduction potential (ORP), dissolved oxygen (DO), phosphate-phosphorus (PO4 -P) and pH. Test-in-tube persulfate digestion method for TN (i.e., the persulfate digestion
oxidizes all nitrogen into nitrate, which is then analyzed by a colorimetric reagent), ferrous sulfate method for NO2 -N, and cadmium reduction method for NO3 -N were performed. A Piccolo pH meter was used to measure pH. ORP and DO were determined using a Sension ORP and DO electrode. The remaining parameters were tested in accordance with the Standard Methods (APHA, 1995). At the end of the experimental period, the plant roots in each wetland were collected and dried at 80 ◦ C for 24 h, and the dry weight was measured. The influent wastewater characteristics were NH4 -N, 28.67 ± 1.33 mg/L; NO2 -N, 0.24 ± 0.04 mg/L; NO3 -N, 32.45 ± 1.44 mg/L; TN, 62.85 ± 2.39 mg/L; CODCr , 12.64 ± 6.42 mg/L; total phosphorus, 12.45 ± 1.30 mg/L; PO4 -P, 8.74 ± 0.83 mg/L; DO, 7.22 ± 0.51 mg/L; and pH range, 7.62–8.23. 3. Results and discussion 3.1. Layer effect of nitrogen removal The NH4 -N and NO3 -N removal efficiencies in the bottom and top layers of W1 and W2 after the 10-day retention period are displayed in Fig. 1. On the whole, higher nitrogen removal rates were obtained in the top layers comparing with those in the bottom layers (P < 0.05). In an aqueous environment, nitrate can be removed via the microbial denitrification and direct uptake by plants and microbes (Greenan et al., 2006; Lin et al., 2002; Robertson et al., 2008). The
S. Liu et al. / Ecological Engineering 37 (2011) 2101–2104
2103
7.8 7.3
Top layer Bottom layer Linear fit of top layer Linear fit of bottom layer
7.7 7.6 7.5
7.1 2
R =0.8970
7.3
2
R =0.6741
7.1 7.0 6.9
pH of W2
pH of W1
7.4
7.2
Top layer Bottom layer Linear fit of top layer Linear fit of bottom layer
7.2
7.0
2
R =0.8041 2
R =0.6755 6.9 6.8
6.8 6.7
0
50
100
150
200
250
Retention time (h)
6.7
0
50
100
150
200
250
Retention time (h)
Fig. 2. The pH values in the bottom and top layers of W1 and W2 with retention time.
condition of bottom layer, based on the anoxic and anaerobic zones, is more suitable for NO3 -N removal. However, the results from the data analyses in the current experiment contrast with the anticipated results. This phenomenon can be explained by the different external carbon sources between the top and bottom layers. The low CODCr value in raw water indicates very limited biodegradable organics, which is unfavorable to the progress of heterotrophic denitrification in a constructed wetland (Hume et al., 2002). Under these circumstances, bacteria tend to use plant productivity, either from biomass litter or root release, as the source of organic carbon to fuel denitrification (Ingersoll and Baker, 1998). Unexpected organic nitrogen is evident from the significant difference (46.37% in W1 and 62.54% in W2, P < 0.05) between the average values of persulfate TN (34.38 mg/L in W1 and 14.32 in W2, P < 0.05) and the sum (N, 23.42 mg/L in W1 and 8.81 mg/L in W2, P < 0.05) of ammoniacal-, nitrite-, and nitrate-nitrogen treatment in wetlands. Moreover, as one of the indispensable conditions for denitrification, anoxic zones could still coexist with aerobic zones in the top layer. Furthermore, the top layer, as the part enriched by plant roots, provides the extra plant uptake for nitrogen by the roots. Evidence has proven that the main pathway of ammonia removal is directly affected by ORP (Sun and Austin, 2007). In this study, the ORP from the top and bottom layers decreased in W1 and W2. The concentrations of DO in the top layer of each wetland were mainly higher than 2.0 mg/L during the first 5 days in the experiment, whereas those in the bottom layer were less than 1.0 mg/L. These results imply that the top layer provides a more aerobic condition due to the additional chances of exposure of water to environment and receipt of oxygen released by the plant rhizosphere, which is beneficial to nitrification for ammonia removal. On the contrary, an anoxic condition was present in the bottom layer, which probably hindered nitrification in this section. 3.2. Layer effects of pH and ORP The variations in the pH and ORP with retention time are shown in Figs. 2 and 3. The pH values in the top layer were mostly above 6.8 (in W1) and 6.7 (in W2) and did not decrease, which were lower than those in the bottom layer, which increased to above 7.2 (in W1) and 6.8 (in W2) (P < 0.01). Only a weak trend line between pH, ORP, and retention time existed in the bottom layer, whereas evident trends were evident in the top layer (Figs. 2 and 3). The trend lines (Fig. 2) between pH and retention time are consistent with the implications of pH variations in autotrophic nitrification, that is, ammonia oxidation into nitrite,
followed by further oxidation of nitrite into nitrate (Metcalf and Eddy, 2003; Gray, 2004). The widely accepted classic two-step route of autotrophic nitrification pathway, CANON (Completely Autotrophic Nitrogen-removal Over Nitrite), discovered recently that all consumes alkalinity (Bishay and Kadlec, 2005; Metcalf and Eddy, 2003). Meanwhile, NH4 -N transformed into NO2 -N or NO3 -N, and hydrogen ions are generated during the first step of the nitrification process (Metcalf and Eddy, 2003; Gray, 2004). Therefore, in the current study, increasing retention time should be clearly correlated with a decrease in pH (Fig. 2). The orderly appearance of aerobic (ORP >200 mV), anoxic (200 to −200 mV), and anaerobic (ORP <−200 mV) zones around the plant roots is a result of the oxygen released from the plant rhizosphere (Kadlec and Knight, 1996). This phenomenon is highlighted by the higher ORP which could be generated in the top layer enriched by plant roots. As a result, partial-nitrification mainly transpires in the top layer. Hence, the main reason for the lower pH in the top layer than the bottom layer can be easily elucidated.
3.3. Decreasing gradient in the layer effects and ORPs between W1 and W2 As shown in Fig. 1, the difference values of the removal capacities between the top layer and the bottom layer in W1 was higher than these in W2 (P < 0.05), which have the same media materials and operation under similar loadings except for the diverse vegetation planted. For NH4 -N in W1, the difference values in the removal efficiency and removal load were 27.24% and 0.13 g m−2 d−1 , respectively, whereas 13.86% and 0.07 g m−2 d−1 in W2. In the rhizosphere of the vegetations (macrophyte), complex interactions transpire between microorganisms and pollutants, and pollutant removal could be enhanced due to intensified microbial activities stimulated by material release and oxygen transfer via the roots (Edwards et al., 2006; Stottmeister et al., 2003). In the current study, the weight of the dry roots of plants was measured—that of P. australis in W1 (1.26 kg) was heavier than C. angustifolia in W2 (0.88 kg). The larger root mass and deeper root growth in the top layer in W2, a result of various species, may have benefited the microbial activities and stimulated the nutrient removal process in the deeper layer. As a result, this condition gives the removal capacity of each layer a tendency to be average. However, this convergent inclination is weak if the mass of plant root is small and its growth is low, as shown in W1.
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400
400 ORP in upper layer ORP in bottom layer Linear fit of ORP in upper layer Linear fit of ORP in bottom layer
300
200
ORP of W2 (mv)
200
ORP of W1 (mv)
ORP in upper layer ORP in bottom layer Linear fit of ORP in upper layer Linear fit of ORP in bottom layer
300
100 0 R2=0.656
-100 -200 -300
R2=0.901
100 0 -100 -200 R2=0.0113
-300
R2=0.06
-400
-400 0
50
100
150
200
250
Retention time (h)
0
50
100
150
200
250
Retention time (h)
Fig. 3. The variation in the ORP in the bottom and top layers of W1 and W2 with retention time.
The effect of plant root was also demonstrated with the ORP variation in W1 and W2. As shown in Fig. 3, the ORP in the top layer is evidently correlated with retention time in each wetland due to the effect of the abundant roots. However, the ORP in the bottom layer does not coincide with the increasing of retention time resulting from the anaerobic condition unqualified for nitrification. Nevertheless, due to the smaller amount of reed roots which were deep enough to transfer oxygen into the bottom layer, a more intense variation in ORP was observed in W2. 4. Conclusion The spatial variation of removal efficiencies of nitrogen and the values of ORP and pH demonstrate that the layer effect transpires in subsurface horizontal flow wetlands treating artificial drainage water containing high ammonia and nitrate but low COD by intermittent operation. The top layer exhibits a higher capacity for the removal of nitrogenous pollutants than the bottom layer (P < 0.05). The pH values in the top layer are mainly lower than the bottom layer due to ammonia degradation (P < 0.05). Generally, the ORP of the top layer is higher than the bottom layer due to its location and the effect of wetland plants. The decreasing gradient in removal capacity between the top and bottom layer mainly results from the mass and depth of plant roots from different species. Acknowledgements The authors wish to express their gratitude to the Knowledge Innovation Project of the Chinese Academy of Sciences (KZCX2YW-Q06-03) and the National Natural Science Foundation of China (Grant No. 40901128) for funding the present work. References APHA (American Public Health Association), 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. APHA, Washington, DC. Bishay, F., Kadlec, R.H., 2005. Wetland treatment at Musselwhite mine. In: Vymazal, J. (Ed.), Nutrient Cycling and Retention in Natural and Constructed Wetlands. Backhuys Publishers, Leiden, Netherlands, pp. 176–198.
Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water Sci. Technol. 35 (5), 11–17. Edwards, K.R., Cizkova, H., Zemanova, K., Santruckova, H., 2006. Plant growth and microbial processes in a constructed wetland planted with Phalaris arundinacea. Ecol. Eng. 27 (2), 153–165. Gebremariam, S.Y., Beutel, M.W., 2008. Nitrate removal and DO levels in batch wetland mesocosms: cattail (Typha spp.) versus bulrush (Scirpus spp.). Ecol. Eng. 34, 1–6. Gray, N.F., 2004. Biology of Wastewater Treatment, 2nd ed. Imperial College Press, London, UK, pp. 282–290. Greenan, C.M., Moorman, T.B., Kaspar, T.C., Parkin, T.B., Jaynes, D.B., 2006. Comparing carbon substrates for denitrification of subsurface drainage water. J. Environ. Qual. 35 (3), 824–829. Hume, N.P., Fleming, M.S., Horne, A.J., 2002. Plant carbohydrate limitation on nitrate reduction in wetland microcosms. Water Res. 36, 577–584. Ingersoll, T.L., Baker, L.A., 1998. Nitrate removal in wetland microcosms. Water Res. 32 (3), 677–684. Kadlec, R.H., 2008. The effects of wetland vegetation and morphology on nitrogen processing. Ecol. Eng. 33 (2), 126–141. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. Lewis Publishers, Boca Raton, FL. Langergraber, G., 2005. The role of plant uptake on the removal of organic matter and nutrients in subsurface flow constructed wetlands: a simulation study. Water Sci. Technol. 51 (9), 213–223. Lin, Y.F., Jing, S.R., Wang, T.W., Lee, D.Y., 2002. Effects of macrophytes and external carbon sources on nitrate removal from groundwater in constructed wetlands. Environ. Pollut. 119 (3), 413–420. Metcalf and Eddy, revised by Tchobanoglous, G., Burton, F.L., Stensel, H.D., 2003. Wastewater Engineering: Treatment and Reuse, 4th ed. McGraw-Hill, Boston, USA, pp. 611–616. Oskar, M., Kensuke, F., Kazuo, Y., 2007. Denitrification with methane as external carbon source. Water Res. 41 (12), 2726–2738. Robertson, W.D., Vogan, J.L., Lombardo, P.S., 2008. Nitrate removal rates in a 15-yearold permeable reactive barrier treating septic system nitrate. Ground Water Monit. Remediation 28 (3), 65–72. Stottmeister, U., Wiessner, A., Kuschk, P., Kappelmeyer, U., Kastner, M., Bederski, O., Muller, R.A., Moormann, H., 2003. Effects of plants and microorganisms in constructed wetlands for wastewater treatment. Biotechnol. Adv. 22 (1–2), 93–117. Sun, G.Z., Austin, D., 2007. Completely autotrophic nitrogen-removal over nitrite in lab-scale constructed wetlands: evidence from a mass balance study. Chemosphere 68 (6), 1120–1128. Tanner, C.C., Kadlec, R.H., Gibbs, M.M., Sukias, J.P.S.M., Long, N., 2002. Nitrogen processing gradients in subsurface-flow treatment wetlands—influence of wastewater characteristics. Ecol. Eng. 18 (4), 499–520. Tjasa, G.B., 2006. Long term performance of a constructed wetland for landfill leachate treatment. Ecol. Eng. 26 (4), 365–374. Vymazal, J., 2005. Horizontal sub-surface flow and hybrid constructed wetlands systems for wastewater treatment. Ecol. Eng. 25 (5), 478–490.
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Short communication
The layer effect in nutrient removal by two indigenous plant species in horizontal flow constructed wetlands Shuyuan Liu a,b,c , Baixing Yan a,∗ , Lixia Wang a a
Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130012, China School of Civil Engineering and Architecture, Taizhou University, Taizhou 317000, China c Graduate University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 24 January 2011 Received in revised form 8 June 2011 Accepted 29 June 2011 Available online 19 August 2011 Keywords: Nitrogen removal Layer effect Oxidation–reduction potential pH Constructed wetland Wetland plants
a b s t r a c t A layer effect study was conducted to investigate the transformation of nitrogenous pollutants in two batch subsurface horizontal constructed wetlands. Artificial drainage water containing a low concentration of chemical oxygen demand (CODCr ), but high concentration of ammonia and nitrate, was treated in two batch wetland cuboids. The nitrogen removal rates were found to be significantly affected by the characters of the layer as well as the biomass and roots of different plant species (P < 0.05). Correlations between pH, oxidation–reduction potential, and retention time indicated that nitrogen removal rates under study conditions mainly depended on the location of the layer and the plant species in the constructed wetland. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitrification–denitrification is widely accepted as the main process for nitrogen removal (Vymazal, 2005; Tanner et al., 2002; Tjasa, 2006). Plants are biologically considered the indispensable component in constructed wetland systems. Numerous studies have reported that plants stimulate nitrogen removal in these treatment wetlands (Kadlec, 2008; Lin et al., 2002). Under the effect of a wetland plant in a horizontal subsurface flow constructed wetland by intermission operation, the media layer in the vertical direction with enriched external carbon sources is considered suitable for denitrification as its anoxic and anaerobic habitats (Hume et al., 2002; Oskar et al., 2007). In the same manner, a significantly greater oxygen transport capacity provides much better conditions for nitrification (Brix, 1997; Gebremariam and Beutel, 2008; Langergraber, 2005). However, the lack of information on the contributing layers in different locations in the constructed wetland, as well as the effects of these layers on the nitrogen removal efficiency inside the wetland systems, lead to unstable permanence for purification capacity.
∗ Corresponding author. E-mail address: [email protected] (B. Yan). 0925-8574/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2011.06.040
The objective of the present study was to determine the layer effect, that is, the difference in the nitrogen removal capacity at the top and bottom layers in the subsurface horizontal constructed wetland. 2. Materials and methods 2.1. Treatment wetland and experimental design The system consisted of two contrasting outdoor horizontal subsurface flow-constructed wetlands (W1 and W2) that were operated with the intermittent strategy. Each wetland was in triplicate. W1 was planted with Calamagrostis angustifolia, whereas W2 with Phragmites australis. The wetlands were constructed with identical dimensions. The wetland beds were PVC cuboids (12 mm thick), each measuring 2 m long, 0.5 m wide and 0.65 m high. These cuboids contained a 40 cm layer of slag (2.74 ± 0.57 mm in diameter) at the bottom and 10 cm layer of soil above the slag layer, giving an average porosity value of 0.35 in the media. Each cuboid included the inlet (0.2 m long), work (1.6 m long), and outlet sections (0.2 m long). Sewage flowed into or out from the wetland through a water distribution area filled with gravel (24.8 ± 5.6 mm in diameter). Undisturbed soil with 50 C. angustifolia and 25 young P. australis from a local natural wetland was selected to implant in the substrates of W1 and W2, respectively.
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NH4 -N removal efficiencies (%)
b
80
b
60 40 20
NO3 -N removal efficiencies (%)
a
a 100
0
40 20
b
0.4
b
0.3 0.2 0.1 0.0
-2
0.5
a b
-1
a
NO3-N removal load (g m d )
-2
b 60
0.6
a
-1
NH4-N removal load (g m d )
80
1 2 2 W1 of W of W of W r of ayer ayer ayer l laye l l p p m m o o o o T T Bott Bott
0.6
To
b a
0
W1 W2 W1 W2 r of r of r of r of laye laye laye laye p p m m o o o o T T Bott Bott
er of p lay
a
100
0.5
a 0.4
b 0.3 0.2 0.1 0.0
W1
of ayer om l Bott
W1
r of laye Top
W2
of ayer om l Bott
W2
1 2 1 2 fW of W of W of W yer yer o yer a a a ayer l l l l p p om om To To Bott Bott
Fig. 1. Removal efficiencies of NH4 -N and NO3 -N in the bottom and top layers. Identical letters above bars indicate groups of means with no statistically significant differences at the 95% confidence level.
Experiments lasted 100 days (from June to September 2009). During the first 90 days, the main objective was to promote bacteria communities in the treatment wetlands. W1 functioned in the same way as W2, 10 days’ feeding with more than 2 days’ interval. The average value of the hydraulic loads of the 10-day experiment in these wetlands was 0.018 m d−1 . In order to ensure the same influent concentration in each cuboid, the artificial drainage water was prepared in a feed tank by dissolving ammonium nitrate and sodium dihydrogen phosphate with tap water. 2.2. Pollutant sampling and analysis Water samples were collected from each cuboid by perforated tubes and inverted siphon method. The length of the perforated parts for each sampling tube embedded into the substrates of the wetland system was 20 cm. Three sampling tubes whose centers of perforated parts were 15 cm apart from the surface of substrates layers were employed to collect water samples from the top layer. Another three tubes whose centers of perforated parts were 15 cm apart from the bottom of the cuboid were in charge of the bottom layer. The water samples were analyzed for total nitrogen (TN), (NH4 -N), ammoniacal-nitrogen nitrite-nitrogen (NO2 -N), nitrate-nitrogen (NO3 -N), chemical oxygen demand (CODCr ), oxidation–reduction potential (ORP), dissolved oxygen (DO), phosphate-phosphorus (PO4 -P) and pH. Test-in-tube persulfate digestion method for TN (i.e., the persulfate digestion
oxidizes all nitrogen into nitrate, which is then analyzed by a colorimetric reagent), ferrous sulfate method for NO2 -N, and cadmium reduction method for NO3 -N were performed. A Piccolo pH meter was used to measure pH. ORP and DO were determined using a Sension ORP and DO electrode. The remaining parameters were tested in accordance with the Standard Methods (APHA, 1995). At the end of the experimental period, the plant roots in each wetland were collected and dried at 80 ◦ C for 24 h, and the dry weight was measured. The influent wastewater characteristics were NH4 -N, 28.67 ± 1.33 mg/L; NO2 -N, 0.24 ± 0.04 mg/L; NO3 -N, 32.45 ± 1.44 mg/L; TN, 62.85 ± 2.39 mg/L; CODCr , 12.64 ± 6.42 mg/L; total phosphorus, 12.45 ± 1.30 mg/L; PO4 -P, 8.74 ± 0.83 mg/L; DO, 7.22 ± 0.51 mg/L; and pH range, 7.62–8.23. 3. Results and discussion 3.1. Layer effect of nitrogen removal The NH4 -N and NO3 -N removal efficiencies in the bottom and top layers of W1 and W2 after the 10-day retention period are displayed in Fig. 1. On the whole, higher nitrogen removal rates were obtained in the top layers comparing with those in the bottom layers (P < 0.05). In an aqueous environment, nitrate can be removed via the microbial denitrification and direct uptake by plants and microbes (Greenan et al., 2006; Lin et al., 2002; Robertson et al., 2008). The
S. Liu et al. / Ecological Engineering 37 (2011) 2101–2104
2103
7.8 7.3
Top layer Bottom layer Linear fit of top layer Linear fit of bottom layer
7.7 7.6 7.5
7.1 2
R =0.8970
7.3
2
R =0.6741
7.1 7.0 6.9
pH of W2
pH of W1
7.4
7.2
Top layer Bottom layer Linear fit of top layer Linear fit of bottom layer
7.2
7.0
2
R =0.8041 2
R =0.6755 6.9 6.8
6.8 6.7
0
50
100
150
200
250
Retention time (h)
6.7
0
50
100
150
200
250
Retention time (h)
Fig. 2. The pH values in the bottom and top layers of W1 and W2 with retention time.
condition of bottom layer, based on the anoxic and anaerobic zones, is more suitable for NO3 -N removal. However, the results from the data analyses in the current experiment contrast with the anticipated results. This phenomenon can be explained by the different external carbon sources between the top and bottom layers. The low CODCr value in raw water indicates very limited biodegradable organics, which is unfavorable to the progress of heterotrophic denitrification in a constructed wetland (Hume et al., 2002). Under these circumstances, bacteria tend to use plant productivity, either from biomass litter or root release, as the source of organic carbon to fuel denitrification (Ingersoll and Baker, 1998). Unexpected organic nitrogen is evident from the significant difference (46.37% in W1 and 62.54% in W2, P < 0.05) between the average values of persulfate TN (34.38 mg/L in W1 and 14.32 in W2, P < 0.05) and the sum (N, 23.42 mg/L in W1 and 8.81 mg/L in W2, P < 0.05) of ammoniacal-, nitrite-, and nitrate-nitrogen treatment in wetlands. Moreover, as one of the indispensable conditions for denitrification, anoxic zones could still coexist with aerobic zones in the top layer. Furthermore, the top layer, as the part enriched by plant roots, provides the extra plant uptake for nitrogen by the roots. Evidence has proven that the main pathway of ammonia removal is directly affected by ORP (Sun and Austin, 2007). In this study, the ORP from the top and bottom layers decreased in W1 and W2. The concentrations of DO in the top layer of each wetland were mainly higher than 2.0 mg/L during the first 5 days in the experiment, whereas those in the bottom layer were less than 1.0 mg/L. These results imply that the top layer provides a more aerobic condition due to the additional chances of exposure of water to environment and receipt of oxygen released by the plant rhizosphere, which is beneficial to nitrification for ammonia removal. On the contrary, an anoxic condition was present in the bottom layer, which probably hindered nitrification in this section. 3.2. Layer effects of pH and ORP The variations in the pH and ORP with retention time are shown in Figs. 2 and 3. The pH values in the top layer were mostly above 6.8 (in W1) and 6.7 (in W2) and did not decrease, which were lower than those in the bottom layer, which increased to above 7.2 (in W1) and 6.8 (in W2) (P < 0.01). Only a weak trend line between pH, ORP, and retention time existed in the bottom layer, whereas evident trends were evident in the top layer (Figs. 2 and 3). The trend lines (Fig. 2) between pH and retention time are consistent with the implications of pH variations in autotrophic nitrification, that is, ammonia oxidation into nitrite,
followed by further oxidation of nitrite into nitrate (Metcalf and Eddy, 2003; Gray, 2004). The widely accepted classic two-step route of autotrophic nitrification pathway, CANON (Completely Autotrophic Nitrogen-removal Over Nitrite), discovered recently that all consumes alkalinity (Bishay and Kadlec, 2005; Metcalf and Eddy, 2003). Meanwhile, NH4 -N transformed into NO2 -N or NO3 -N, and hydrogen ions are generated during the first step of the nitrification process (Metcalf and Eddy, 2003; Gray, 2004). Therefore, in the current study, increasing retention time should be clearly correlated with a decrease in pH (Fig. 2). The orderly appearance of aerobic (ORP >200 mV), anoxic (200 to −200 mV), and anaerobic (ORP <−200 mV) zones around the plant roots is a result of the oxygen released from the plant rhizosphere (Kadlec and Knight, 1996). This phenomenon is highlighted by the higher ORP which could be generated in the top layer enriched by plant roots. As a result, partial-nitrification mainly transpires in the top layer. Hence, the main reason for the lower pH in the top layer than the bottom layer can be easily elucidated.
3.3. Decreasing gradient in the layer effects and ORPs between W1 and W2 As shown in Fig. 1, the difference values of the removal capacities between the top layer and the bottom layer in W1 was higher than these in W2 (P < 0.05), which have the same media materials and operation under similar loadings except for the diverse vegetation planted. For NH4 -N in W1, the difference values in the removal efficiency and removal load were 27.24% and 0.13 g m−2 d−1 , respectively, whereas 13.86% and 0.07 g m−2 d−1 in W2. In the rhizosphere of the vegetations (macrophyte), complex interactions transpire between microorganisms and pollutants, and pollutant removal could be enhanced due to intensified microbial activities stimulated by material release and oxygen transfer via the roots (Edwards et al., 2006; Stottmeister et al., 2003). In the current study, the weight of the dry roots of plants was measured—that of P. australis in W1 (1.26 kg) was heavier than C. angustifolia in W2 (0.88 kg). The larger root mass and deeper root growth in the top layer in W2, a result of various species, may have benefited the microbial activities and stimulated the nutrient removal process in the deeper layer. As a result, this condition gives the removal capacity of each layer a tendency to be average. However, this convergent inclination is weak if the mass of plant root is small and its growth is low, as shown in W1.
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400 ORP in upper layer ORP in bottom layer Linear fit of ORP in upper layer Linear fit of ORP in bottom layer
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100 0 R2=0.656
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Fig. 3. The variation in the ORP in the bottom and top layers of W1 and W2 with retention time.
The effect of plant root was also demonstrated with the ORP variation in W1 and W2. As shown in Fig. 3, the ORP in the top layer is evidently correlated with retention time in each wetland due to the effect of the abundant roots. However, the ORP in the bottom layer does not coincide with the increasing of retention time resulting from the anaerobic condition unqualified for nitrification. Nevertheless, due to the smaller amount of reed roots which were deep enough to transfer oxygen into the bottom layer, a more intense variation in ORP was observed in W2. 4. Conclusion The spatial variation of removal efficiencies of nitrogen and the values of ORP and pH demonstrate that the layer effect transpires in subsurface horizontal flow wetlands treating artificial drainage water containing high ammonia and nitrate but low COD by intermittent operation. The top layer exhibits a higher capacity for the removal of nitrogenous pollutants than the bottom layer (P < 0.05). The pH values in the top layer are mainly lower than the bottom layer due to ammonia degradation (P < 0.05). Generally, the ORP of the top layer is higher than the bottom layer due to its location and the effect of wetland plants. The decreasing gradient in removal capacity between the top and bottom layer mainly results from the mass and depth of plant roots from different species. Acknowledgements The authors wish to express their gratitude to the Knowledge Innovation Project of the Chinese Academy of Sciences (KZCX2YW-Q06-03) and the National Natural Science Foundation of China (Grant No. 40901128) for funding the present work. References APHA (American Public Health Association), 1995. Standard Methods for the Examination of Water and Wastewater, 19th ed. APHA, Washington, DC. Bishay, F., Kadlec, R.H., 2005. Wetland treatment at Musselwhite mine. In: Vymazal, J. (Ed.), Nutrient Cycling and Retention in Natural and Constructed Wetlands. Backhuys Publishers, Leiden, Netherlands, pp. 176–198.
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