Heavy metal removal within pilot-scale constructed wetlands receiving river water contaminated by confined swine operations

Heavy metal removal within pilot-scale constructed wetlands receiving river water contaminated by confined swine operations

Desalination 249 (2009) 368–373 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m ...

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Desalination 249 (2009) 368–373

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Heavy metal removal within pilot-scale constructed wetlands receiving river water contaminated by confined swine operations T.Y. Yeh ⁎, C.C. Chou, C.T. Pan Department of Civil and Environmental Engineering, National University of Kaohsiung, Taiwan

a r t i c l e

i n f o

Article history: Accepted 24 November 2008 Available online 1 October 2009 Keywords: Constructed wetland Heavy metals Translocation Bioavailability

a b s t r a c t Three parallel pilot-scale surface flow constructed wetlands were employed to investigate heavy metal removal receiving river water contaminated by swine confined-housing operations in Taiwan. Wastewater from swine operation contained elevated levels of copper and zinc due to their abundance in feed. Two macrophytes, namely cattail (Typha latifolia) and reed (Phragmites australis), were planted to observe their heavy metal removal efficiency. Significant total recoverable copper and zinc reduction for three tested wetlands were 80 and 91% for unplanted control, 83 and 92% for cattail, and 83 and 92% for reed wetland systems. Acid-soluble forms were 56 and 86% of total recoverable influent metals for copper and zinc, respectively. More bioavailable zinc was subjected to releasing back to aqueous environment. Heavy metals entering the studied systems as insoluble forms were settling from water column. Concentrations of metals were higher in the vegetated sediments than in the non-vegetated sediments. The sequential extraction results of sediments indicated that most retained metals were in less mobile fractions. Most of metal uptake by vegetation remained in root portions. Translocations of both copper and zinc for tested macrophytes were not prominent. The metal species in incoming water and metal fractionations in sediment were demonstrated as the major factors to influence plant metal levels. The performance of the studied wetland systems can comply with local water criteria rendering for further water reuse. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Constructed wetlands are engineered systems that have been designed to employ natural processes including vegetation, soil, and microbial activity to treat contaminated water. Constructed wetlands possess the merits of low-cost and low-maintenance, and are capable of removing various pollutants including heavy metals, nutrients, organic matters, and micropollutants [1,2]. In addition, constructed wetlands are recently used for treating various wastewater types including point source domestic sewage, acid main drainage, agricultural wastewater, landfill leachate, and non-point source storm water runoff [3–7]. In Taiwan, 63 newly constructed full-scale wetland water purification systems were in operation to treat 338,000 m3 per day contaminated surface water [8]. Heavy metal contamination is one of the most serious environment problems throughout the world. The wastewater generated from confined swine operations is one of the primary pollution sources in Taiwan [9]. The effluent is discharged in the surrounding waterways containing significant amounts of heavy metals such as copper and zinc. These metals are intentionally added in fodder to ⁎ Corresponding author. National University of Kaohsiung, Department of Civil and Environmental Engineering, Kaohsiung 811, Taiwan. Tel.: +886 7 591 9536; fax: +886 7 591 9376. E-mail address: [email protected] (T.Y. Yeh). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2008.11.025

prevent diarrhea and to enhance immune systems of swine. The onfarm treatment facilities are not properly operated or even not in place for the swine operation. The manures of swine operation were visibly observed in the river water in this study area. The remediation options to treat large volume of low contaminated river are inevitable. It might not be economical to build a concrete treatment plant to mitigate metal polluted river water. Recently, green remediation approaches, constructed wetlands, have gained drastically attention due to their pollution removal, recreational assets, and landscape aesthetic values in Taiwan. The potential of employing constructed wetlands to treat metal-containing wastewater has also received increasing attention worldwide [10,11]. Wetland soils characterized by their reduced condition and high organic matter content can accumulate heavy metals [12]. Metals introduced into wetlands might exist in various particulate or dissolved forms while dissolved metals can adsorb onto particles, exist as complexes with inorganic and organic ligands, or be remained as the free ion state in solution. Metals are trapped within wetland compartments via numbers of mechanisms including plant uptake, cation exchange with soils, and particulate settling. Copper and other divalent metal cations are known to bind strongly to peats and humics, therefore wetlands are employed to trap metals before they reach receiving water [13]. The chemical forms of heavy metals associated with the settled solids can influence their fate. Many studies dealing with metals in

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the sediments focused on total concentration with no attempt to distinguish various forms in which metal might exist. In order to provide a complete picture for predicting the metal distribution, mobility, and bioavailability in settled solids, the employment of sequential extraction may be of great value [14]. Many schemes have been proposed for the sequential extraction techniques [15,16]. Some of these schemes may be variants of one another with minor variations in the chemical extractants and operating conditions. The sequential extraction technique might not provide the precise separation of metals in settled solids into all specific chemical forms. Nonetheless, broad categories of heavy metals in sediments still can be generated by the sequential extraction. The major sink for metal removal within constructed wetlands was in sediment while plants might also assist the metal reduction. The emergent macrophytes developed their roots in the reduced soil matrix where metals can be immobilized. The study also demonstrated that metal concentration in plant tissue where higher in roots than in stems and leaves [17]. The objectives of this research were to examine metal forms relative to bioavailability and to measure the partitioning of metals within the wetland compartments. Treatment performance of pilotscale wetland systems was characterized by investigating changes in aqueous constituent concentrations from inflow to outflow. In addition, the fraction of metals in sediments and the role of plants in heavy metal decontamination within tested wetland systems were studied. The evaluation of the extent of metals accumulated in different parts of tested macrophytes was also conducted.

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as control, II) planting cattail (Typha latifolia), and III) planting reed (Phragmites australis), were operated in continuous flow mode. The inflow water was pumped from the river that was contaminated by swine wastewater. The copper and zinc levels were elevated in comparison to the metal levels of most surface water in Taiwan. Three pilot-scale wetland units made of acrylic plates with dimension of 180 × 50 × 50 cm were built. The schematic of the wetland modules is shown in Fig. 1. To support the growth of emergent macrophytes, one layer of gravel and one layer of coarse sand, were laid at the bottom of each wetland unit at depth of 10 and 20 cm, respectively. Specifically, the support media consisted of (8–16 mm) gravel at the bottom layer and (0–4 mm) sand at the top layer. The unplanted system as control had the same setup as the one used in the planted system. The water surface was maintained around 18 cm above the top of the media. One month old cattail and reed macrophytes were transferred and planted in June 2007 in tested wetland modules. Two planted wetland systems received the secondary treated sewage for a month to make sure the growth of macrophytes. Two units were planted with cattail and reed at a density of 20/m2. The experiments were carried out for four months (i.e. from the beginning of July 2007 up to the end of October 2007). All wetland units were fed with contaminated water with the elevated zinc and copper concentrations at a constant flow rate of 0.144 m3 per day. The wetland systems (planted cattail, planted reed, and unplanted) were all operating with hydraulic retention time of 1.25 days and hydraulic loading rate of 16 cm/d. 2.2. Sample collection and laboratory analysis

2. Material and methods 2.1. Experimental set up and operation conditions of wetlands Three surface flow wetland systems were constructed and located outdoors at the university campus. The experiment was performed at Kaohsiung University in tropical southern Taiwan (22°73′N, 120°28′E). Three pilot-scale wetland systems, including I) without vegetation

Influent and effluent samples were collected from each wetland module daily for four months. Water samples were measured for BOD, COD, SS, TP, and PO3− 4 . All analyses were done according to the standard methods [18]. Water quality parameters such as pH, temperature, and dissolved oxygen were recorded while taking water samples. A HACH sensION2 pH meter and an YSI 550A portable dissolved oxygen meter were used to determine pH and DO. In addition to water samples,

Fig. 1. Schematic of the experimental pilot wetlands.

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Table 1 Influent water quality parameters of the pilot-scale constructed wetland. Analysis items

Average (± SD)

Temperature (°C) DO (mg/L) pH BOD (mg/L) COD (mg/L) SS (mg/L) TP (mg/L) PO3− (mg/L) 4

21.6 ± 4.9 2.71 ± 0.81 7.66 ± 0.36 20.1 ± 11.1 26.2 ± 11.9 38.0 ± 17.2 1.26 ± 0.38 0.53 ± 0.23

2.3. Plant uptake data analysis In order to evaluate the phytoextraction potential of plants, two metal uptake parameters were calculated to enable comparisons of metal uptake/distribution and comparisons among metals. Bioconcentration factor (BCF) was then calculated as the ratio of root metal to sediment metal concentrations (Croots/Csediments) while translocation factor (TF) was expressed as leaf and stem metal to root metal concentration (Cstem + leaf/Croot) [20]. 2.4. Statistical analyses

sediment samples (0–5 cm depth) were collected from each wetland toward the end of experiment in October 2007. Sediment samples were collected from a midpoint within each wetland. The sediment was defined as the soil-like material that was present in the pore of the gravel filter medium. The sediment was referred to as anything that was not belowground biomass or gravel. The sediment originated mainly from solids in the inflow water. At the end of the experiment, plant samples were harvested from two planted wetland systems and analyzed to determine for the metal uptake. The collected 77 water samples and 38 sediment samples of each wetland, and 18 plant samples of two planted wetlands were analyzed for metal contents. Water samples were taken daily from the inflow and outflow for heavy metal determination. Soluble metals were determined after that samples were filtrated through 0.45 μm membrane filters and subsequently acidified to pH below 2. The soluble metals are most likely to be the most bioavailable form. For acid-soluble metals, the samples are acidified to pH 2 and filtered. Total recoverable metal pretreatment involves vigorous digestion via microwave apparatus (Perkin Elmer MW 3000). The filtrate and pretreated samples were followed by the measurement of a Perkin Elmer AAnalyst 200 atomic adsorption spectrophotometer (AA). The detection limits of copper and zinc were 0.008 and 0.005 mg/L, respectively. Water samples were analyzed for soluble, total, and acid extractable copper and zinc contents. The macrophytes were first rinsed with DI water to remove attached sediment, separated into roots, stems, and leaves, shredded and dried. The vegetation and sediments were dried in an oven at 103 °C and digested via a microwave digestion apparatus. The metal contents of various parts of vegetation were then measured by AA. The distribution and chemical fraction of heavy metals retained in pilot-scale wetland sediments were examined to investigate the bioavailability of deposited heavy metals. The fractionation of heavy metals was investigated by the sequential extraction techniques. Five grams of air-dried sediment samples was placed in centrifuge tubes and subjected to a six-step serial extraction procedure. The procedure of sequential chemical extraction used in this study includes a series of reagents. They are depicted as exchangeable (1 M KNO3), inorganically bound (0.5 M KF), organically bound (0.1 M Na4P2O7), Feand Mn-oxide bound (0.3 M Na3C6H5O7, 1 M NaHCO3 and 0.5 g Na2S2O4), and sulfide (6 M HNO3) forms, respectively [19]. The metal concentrations in each fraction were determined by AA.

Statistical significance among metal contents was assessed using mean comparison test. Differences between treatment concentration means of metals were determined by Student's t tests. A level of p < 0.05 considered statistically significant was used in all comparisons. Means are reported ± standard deviation. In addition, differences among five fractions in the sequential extraction experiment were tested with F statistics. All statistical analyses were performed with Microsoft Office EXCEL 2003. 3. Results and discussions The water parameters of inflow contaminated river water are shown in Table 1. The average inflow suspended solid concentration was 38.0 mg/L while BOD and COD concentrations were 20.1 and 26.2 mg/L, respectively. This result indicated the level of particulate organic pollution. The dissolved oxygen varied around 2.7 mg/L revealed that the river water was polluted and was not in compliance with local water quality criteria. The influent water pH was approximately neutral while the temperature was 22 °C. Though the redox was not monitored in this study, this parameter might be useful to explain the metal behavior in the wetland sediment. Nevertheless, the pH and oxidizing conditions favored partitioning of soluble metals to the solid phase, by precipitation and sorption to solids such as organic matter [21]. The inflow organic matter and suspended solid, though the concentrations varied during the study period, did not notably affect the growth of the plant species within the wetland systems. 3.1. Metal species monitoring The influent and effluent of copper and zinc metal species for three tested pilot-scale wetland systems are shown in Table 2. The influent total metals were significantly higher than soluble and acid-soluble for both copper (p < 0.001) and zinc (p < 0.03). Acid-soluble forms were 56 and 86% of total recoverable influent metals for copper and zinc, respectively. The results indicated that more bioavailable zinc was subjected to releasing back to aqueous environment. Soluble fractions of both metals only accounted for small amounts of total influent metals (p < 0.001). Particulate copper and zinc were the predominate forms which were responsible for metal settling. Average total recoverable copper concentration was 0.064 mg/L in the influent while the effluent

Table 2 Metal species concentrations of influent and effluent unit: mg/L. Constituents

Metal species

Influent

I — control

II — cattail

III — reed

Copper

Soluble Acid-soluble Total

0.011 ± 0.009 0.036 ± 0.019 0.064 ± 0.037

Zinc

Soluble Acid-soluble Total

0.055 ± 0.022 0.316 ± 0.132 0.366 ± 0.137

0.010 ± 0.008 0.012 ± 0.007 0.013 ± 0.007 (80%) 0.016 ± 0.010 0.025 ± 0.007 0.034 ± 0.013 (91%)

0.010 ± 0.007 0.011 ± 0.005 0.011 ± 0.007 (83%) 0.019 ± 0.012 0.029 ± 0.016 0.030 ± 0.014 (92%)

0.010 ± 0.006 0.010 ± 0.007 0.011 ± 0.008 (83%) 0.016 ± 0.008 0.025 ± 0.008 0.031 ± 0.014 (92%)

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copper concentrations were 0.013, 0.011, and 0.011 mg/L for the system of control, planting cattail, and planting reed, respectively. The results indicated 80, 83, and 83% decrease in total recoverable copper for the control, cattail, and reed system, respectively. The vegetated wetland systems demonstrated only slightly better copper removal. The effluent total copper concentrations were not significantly different for planted cattail (p = 0.078) and planted reed (p = 0.101) system when compared with the unplanted control system. These results might be due to the initial stage of wetland system operation. The initial growth phase of vegetated system only exhibited slightly better metal reduction. Soluble copper concentrations were similar for both influent and effluent. The soluble copper from the water column was not removed effectively within wetland systems. Average total zinc concentration was 0.366 mg/L in the influent while the effluent zinc concentrations were 0.034, 0.030, and 0.031 mg/L for the system of control, planting cattail, and planting reed, respectively. The t-test statistical analyses were conducted to observe the differences among the effluent zinc levels. The results indicated 91, 92, and 92% decrease in total zinc for the control, cattail (p = 0.068), and reed (p = 0.170) system, respectively. The vegetated wetlands did not demonstrate significant zinc reduction compared to the non-vegetated control wetland system. However, the significant zinc removal efficiency of all tested wetland systems still illustrated the ability of constructed wetland to function in transforming zinc associated with the water column. For all three tested wetland systems, the influent soluble forms of metals were only accounted for 17% (p < 0.001) and 15% (p < 0.001) of the total recoverable metals for copper and zinc, respectively. The major amounts of inflow heavy metals were present in suspended particulate forms or metal species complexed/adsorbed onto particles. Zinc entered the wetland in a predominantly loosely bound form due to 86% inflow zinc presented as acid-soluble forms. The results suggested that the primary mechanisms of zinc removal might be metal species sorption on the settling solids. The declined levels of total metals were consistent with the decrease of suspended solids. Similar research results indicated that more than 50% of the heavy metals can be easily adsorbed onto particulate matter in the wetland and thus be removed from the water column by sedimentation [11]. Copper in general tends to be adsorbed most strongly and zinc is usually held weakly. The behavior of zinc is more liable and is more likely caused by adverse impact to surrounding biota [21]. In this study, zinc was probably associated with solids such as clay particles or organic matter. The removal mechanism of copper might be different from that of zinc within the studied wetland systems. The concentrations of total heavy metals do not adequately reflect the degree of adsorption such as loosely bound or tightly bound. The acidsoluble heavy metal content has been demonstrated as a more direct indication to the availability of the adsorbed metals and the possibility of remobilization. In this study, the bulk of incoming metals was mainly immobilized via settlement into the bottom media and only slightly removed via

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emergent vegetation through complexation or chelation, leaving a small percentage of the metal in the aqueous phase. Heavy metals could be readily removed by constructed wetland systems, though the removal efficiency might be influenced by the type of wetlands, the age of wetlands, and the types of wastewater to be treated [10]. Effective metal removal can be enhanced by providing feasible conditions to promote settlement and to prevent resuspension. Effluent concentrations of copper and zinc for these pilot-scale wetland systems declined significantly. Sedimentation is believed as the primary sinks or reservoirs for metals. In this study, heavy metals entering the studied systems as insoluble forms were settling from water column in the similar removal mechanism as suspended solids. Heavy metal levels were reduced effectively due to settling processes in these wetland systems. However, removal efficiencies are strongly dependent on influent concentration and hydraulic loading rate. The effluent copper and zinc concentration met the local regulatory criteria of 0.03 mg/L. The polished water can be rendering for further water reuse. 3.2. Sequential extraction analysis of sediments Additional total metal extraction results by concentrated HNO3 extraction revealed that copper retained in sediments were 31.53, 36.27, and 36.33 mg/kg, while zinc retained in sediments were 141.80, 166.21, and 156.13 mg/ kg for control, cattail, and reed wetland systems, respectively. Concentrations of metals were higher in the vegetated sediments than in the non-vegetated sediments. Metals in sediments exist in several forms including ionic or complex species, complexed with organic matter, embedded in oxides and hydroxides of iron and manganese, and entrapped in primary and secondary minerals. The latter fractions are more inert, permanently bound to the sediments, and less bioavailable [22]. Heavy metal associated with different fractions has varied impacts on the environment. The sequential extraction results of sediments within three tested wetlands are listed in Table 3. Differences in averaged concentrations among different fractions were tested with F statistics. p values were all < 0.001 except for the copper in the planted reed system (p = 0.043), indicating significant differences among different fractions. Copper and zinc in sediments of wetland systems were mainly organically, Fe- and Mn-oxide bound, and sulfide bound. These results indicated that metals were immobilized and unavailable to the surrounding biota. The relatively stable fractionation also makes both metals unavailable for plant uptake. More copper than zinc presented as the inorganically bound fraction (p < 0.001) in the unplanted control system. This result revealed that more copper within the wetlands sediments was weakly adsorbed. The loosely retained copper might become a secondary source of metal pollution in the unplanted wetland system. The availability of heavy metals within sediments can change under variable redox, pH, and salinity conditions. Within the reduced

Table 3 Sequential extraction analysis of soil within tested wetlands unit: mg/kg.

Cu

Zn

I Control II Cattail III Reed I Control I Cattail III Reed

Exchangeable

Inorganic bound

Organic bound

Fe- and Mn-oxide bound

Sulfide bound

2.35 ± 1.47 (6.9%)

2.01 ± 0.66 (5.9%)

5.27 ± 1.98 (15.4%)

9.27 ± 1.74 (27.1%)

15.34 ± 4.69 (44.8%)

2.21 ± 1.61 (5.4%)

1.79 ± 0.59 (4.4%)

9.55 ± 2.04 (23.3%)

12.27 ± 1.53 (29.9%)

15.24 ± 5.28 (37.1%)

2.17 ± 0.69 (6.1%)

2.10 ± 0.97 (5.9%)

7.88 ± 4.05 (22.1%)

9.41 ± 3.76 (26.4%)

14.07 ± 3.87 (39.5%)

1.97 ± 0.59 (1.5%)

1.03 ± 0.61 (0.8%)

8.93 ± 4.92 (6.6%)

40.28 ± 9.47 (29.7%)

83.24 ± 15.39 (61.5%)

1.89 ± 0.71 (1.3%)

2.06 ± 0.57 (1.5%)

10.27 ± 4.01 (7.3%)

31.56 ± 19.38 (22.3%)

95.51 ± 10.17 (67.6%)

3.37 ± 0.65 (2.3%)

2.12 ± 0.62 (1.4%)

13.75 ± 7.02 (9.2%)

34.04 ± 11.27 (22.7%)

96.58 ± 14.62 (64.5%)

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soil, matrix metals are mainly immobilized and metal accumulation in the emergent macrophytes is slow. The elevated copper and zinc concentration in cattail and reed might be the result of an increased bioavailability of two metals. 3.3. Metal accumulation of macrophytes The metal uptake results of cattail and reed are shown in Fig. 2. The metal concentrations in plants were analyzed before starting to feed metal-laden wastewater. The copper concentrations for cattail and reed were 10.5 and 3.42 mg/kg, respectively, while zinc concentrations for cattail and reed were 30.1 and 20.6 mg/kg, respectively. The macrophytes were harvested for the metal uptake investigation toward the end of experiments in October 2007. The growth phase of plant was in the initial stage of wetland system operation. Metal concentrations in the plant tissue were higher in roots than in both stems and leaves. For two vegetation systems, both metal concentrations in above- and belowground biomass were in the order of root > stem > leave. Regarding metal vegetation uptake, roots are the primary site of metal uptake. This result revealed that the significant translocation did not occur for both copper and zinc. Harvesting might not be viable metal removal options for these studied wetland systems. These results are consistent with following studies. Swedish investigation revealed that copper and zinc were taken by plants with the highest accumulation found in the roots [22]. Study assessed the feasibility of treating industrial wastewater within pilot-scale wetland

Table 4 Bioconcentration factor and translocation factor of macrophytes. BCF

Cattail Reed

TF

Copper

Zinc

Copper

Zinc

1.66 ± 0.17 1.23 ± 0.39

1.09 ± 0.23 1.05 ± 0.28

0.41 ± 0.07 0.45 ± 0.05

0.36 ± 0.05 0.27 ± 0.03

in Argentina. Average metal removal efficiencies were: Fe (83%), Cr (82%), Ni (69%), and Zn (55%). Metal concentration in macrophyte tissues increased significantly where metal concentration in the roots was 2–3 times higher than in the stems and leaves. However, a small fraction of metal retained (7%, 2%, and 4% of the Cr, Ni, and Zn, respectively) in the wetland was stored in the macrophyte tissue [23]. Similar research results indicated that less than 2% of the annually removed mass was accumulated in the aboveground reed biomass while the rest was retained in the sediment or belowground reed biomass [24]. The relative contribution of the aboveground vegetation biomass in the overall metal accumulation was generally very low. The sediment is the most important sink for metal accumulation. Contradictory research investigated the metal accumulation by different parts of wetland plants and concluded no clear trend [25]. Nevertheless, translocation of metal within emergent macrophyte is dependent on the plant and metal species. BCF and TF values for both vegetated system are listed in Table 4. Comparisons between planted cattail and reed for copper and zinc BCFs and TFs were conducted with t-test. Except for zinc BCF (p = 0.49), all p values were <0.001 indicating significant differences between two different macrophytes. BCFs were depicted to assess concentrations in roots to environmental loading. In this study, metals were accumulated in roots greater than concentration in adjacent sediments with BCF of ≧1. TFs were calculated to enable the assessment of transport of accumulated metal from root to shoot. Metals in leaves and stems were lower than half that of roots. In this study, the underground organs of plant species are mainly responsible for heavy metal phytoextraction. Researches have demonstrated that metal concentrations in macrophytes vary considerably according to the plant part and to the type of metals [22], though previous research results were reported that zinc and copper accumulation in plant tissue is often higher compared to other nonessential heavy metals [26]. The low plant metal levels might be due that most copper and zinc were tightly bound according to aforementioned sequential extraction analysis of sediment. Besides direct vegetation uptake, emergent plants may contribute to metal reduction via processes such as favoring the settlement of particulate metals and promoting biochemical reactions which enhance metal retention by the sediment. The role of vegetation in metal retention includes serving as sites for metal precipitation and sedimentation. In addition, previous research has demonstrated that organic matter in the form of fallen leaf and stem debris from emergent vegetation plays a significant role in the immobilization of metals within wetland systems [27]. Plant metal concentrations might not be directly correlated with sediment or water metal concentration. The availability of metals to aquatic plants is complex and is dependent on factors associated with the metal and substrate [28]. Effluent plant tolerance is an important factor in the efficiency of wetlands and design. Due the initial stage of wetland module operation, the significant toxic symptoms of macrophytes were not detected within the duration of the study. In addition, the metal levels in the incoming water and the sediments might still be tolerated by the macrophytes. The metal concentrations in plant tissue measured in this study were low when compared with the reported toxicity thresholds in the literature [29]. 4. Conclusions

Fig. 2. Metal concentrations of various portions of macrophytes.

Prominent metal reduction has been demonstrated in three pilotscale surface flow wetland systems receiving contaminated water

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from confined swine operations. Swine wastewater contains significant amounts of copper and zinc due to their abundance in feed. The bulk of incoming metals was mainly immobilized via settlement into the bottom media and only slightly removed by emergent plants. The vegetated wetland systems only demonstrated slightly better metal removal compared to the unplanted control system due to the early growth phase of macrophytes. Evapotranspiration can have a big effect on effluent metal concentrations in the outdoor study. The emergent plants within surface flow wetland modules might lessen the evapotranspiration. Nevertheless, the effect of evapotranspiration on the water metal concentration needs to be further investigated. Further study is required to scrutinize the water balance and metal mass balance in the different compartments of wetland systems. Significant translocation did not occur for both copper and zinc within the wetland systems while metals were retained in roots of vegetation. The immobilization of metals in sediment was not effectively translocated by emergent macrophytes. The sediment excavation rather than macrophyte harvesting is inevitable to be employed if the permanent metal removal from wetland systems is mandatory. References [1] R.H Kadlec, R.L. Knight, Treatment Wetlands, Lewis Publishers, Boca Raton, FL, 1996. [2] C. Keffala, A. Ghrabi, Nitrogen and bacterial removal in constructed wetlands treating domestic waste water, Desalination 185 (2005) 383–389. [3] A.K. Kivaisi, The potential for constructed wetlands for wastewater treatment and reuse in developing countries: a review, Ecol. Eng. 16 (2001) 545–560. [4] A. Al-Omari, M. Fayyad, Treatment of domestic wastewater by subsurface flow constructed wetlands in Jordan, Desalination 155 (2003) 27–39. [5] E. Huertas, M. Folch, M. Salgot, I. Gonzalvo, C. Passarell, Constructed wetlands effluent for streamflow augmentation in the Besos River (Spain), Desalination 188 (2006) 141–147. [6] B.E. Hamouri, J. Nazih, J. Lahjouj, Subsurface-horizontal flow constructed wetland for sewage treatment under Moroccan climate conditions, Desalination 215 (2007) 153–158. [7] D.P.L. Rousseau, E. Lesage, A. Story, P.A. Vanrolleghem, N.D. Pauw, Constructed wetlands for water reclamation, Desalination 218 (2008) 181–189. [8] Natural purification systems website, Taiwan Environmental Protection Administrate, http://wqp.epa.gov.tw/ecological/. Accessed 17 July, 2008. [9] J.H. Hsu, S.L Lo, Effect of composting of characterization and leaching of copper, manganese, and zinc from swine manure, Environmental Pollution 114 (2001), 119–127.

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