Nutrient and metal removal in a constructed wetland for wastewater treatment from a metallurgic industry

e c o l o g i c a l e n g i n e e r i n g 2 6 ( 2 0 0 6 ) 341–347

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Nutrient and metal removal in a constructed wetland forwastewater treatment from a metallurgic industry

M.A. Mainea,∗, N. Sunea, H. Hadada, G. Sancheza, C. Bonettob

a Quımica Analıtica, Facultad de Ingenierıa Quımica, Universidad Nacional del Litoral,Santiago del Estero 2829 (3000), Santa Fe, Argentinab Instituto de Limnologıa “Dr. Ringuelet”, Av. Calchaqui km 23.5 (1888), Florencio Varela, Buenos Aires, Argentina

a r t i c l e i n f o

Article history:

Received 1 April 2005

Received in revised form 9

a b s t r a c t

This contribution summarizes the nutrient and metal removal of a free water surface con-

structed wetland, compares it with the previous small-scale prototype and discusses the

December 2005

Accepted 28 December 2005

Keywords:

Metal

Nutrient

Removal

Wastewater

Wetland

observed differences. Several locally available macrophyte species were transplanted into

the wetland. Eichhornia crassipes (water hyacinth) showed a fast growth and it soon became

dominant, attaining 80% cover of the wetland surface. Typha domingensis (cattail) and Pan-

icum elephantipes (elephant panicgrass) developed as accompanying species attaining 14 and

4% cover. The wetland removed 86% of Cr and 67% of Ni. Zn concentrations were below

50 �g l−1 in most samplings. The FeS precipitation probably caused the high retention of

Fe (95%). The outcoming water was anoxic in most samplings. Phosphate and ammonium

were not retained within the wetland while 70% and 60% of the incoming nitrate and nitrite

were removed. Large denitrification losses are suggested. Cr, Ni and Zn were retained by the

macrophytes in the larger wetland and in sediment in the small-scale one. Differences in

the retention mechanism of the two wetlands are discussed.

© 2006 Elsevier B.V. All rights reserved.

1. Introduction

The construction of artificial wetlands for wastewater treat-ment is now a widely accepted and increasingly commontreatment alternative. Constructed wetlands were initiallyutilized for nutrient removal in residential and municipalsewage, storm water and agricultural runoff displaying a widerange of removal efficiencies. The accelerating industrializa-tion in developing countries with an enormous consumptionof metals constitutes an environmental contamination haz-ard. The application of wetlands for industrial wastewatertreatment is a promising alternative. However, current experi-ence in Argentina remains largely unreported. Conditions arefavourable since there is a large availability of marginal landaround most cities with a low population density. The centraland northern areas of the country have mild winters, allow-

∗ Corresponding author. Tel.: +54 342 4571164; fax: +54 342 4571162.E-mail address: [email protected] (M.A. Maine).

ing extended growing periods for plants. Macrophytes are themain biological component of wetlands. They not only assim-ilate pollutants directly into their tissues, but they also actas catalysts for purification reactions by increasing the envi-ronment diversity in the root zone and promoting a varietyof chemical and biochemical reactions which enhance purifi-cation (Jenssen et al., 1993). Water hyacinth (Eichhornia cras-sipes) is one of the most commonly used plants in constructedwetlands because of its fast growth rate and large uptake ofnutrients and contaminants. It attains dense stands in thefloodplain wetlands of the Middle Parana River close to thestudy site.

Bahco S.A., a metallurgic factory, constructed a small-scaleexperimental free water surface wetland to assess the feasi-bility of treating wastewater from the Santo Tome (Argentina)tool manufacturing plant. The wetland removed 81%, 66% and

0925-8574/$ – see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.ecoleng.2005.12.004

342 e c o l o g i c a l e n g i n e e r i n g 2 6 ( 2 0 0 6 ) 341–347

59% of the incoming Cr, Ni and Zn, respectively, and 84% and75% of the inorganic N and soluble reactive P (SRP) from theincoming wastewater (Maine et al., 2005). Because of the highremoval efficiency attained in the small-scale experimentalwetland, a large-scale facility was constructed for wastewa-ter treatment of the whole factory. The present contributionassesses the removal efficiency of the larger wetland, com-pares it with the previous small-scale prototype and discussesthe observed differences.

2. Materials and methods

2.1. Design of the small-scale and the largerconstructed wetlands

The wetlands were constructed at Bahco Argentina metal-lurgic plant, located in Santo Tome, Argentina (S31◦40′01.9′′;W60◦47′06.9′′).

The small-scale experimental wetland was 6 m length, 3 mwide and 0.4 m deep (Fig. 1A). A polyethylene impermeablefilm was placed at the bottom and a soil layer of 30 cm wasadded. The influent entered the wetland through a PVC tube(diameter: 63 mm) with a perpendicular drip dispersion tubewith aligned holes to produce a laminar flow. The inflowdischarge was 1000 l d−1 and the approximate hydraulic res-idence time was 7 days. Three floating (Pistia stratiotes, E.

to 12 days. The wetland was rendered impermeable by meansof a bentonite layer covered with a 1 m-layer of the excavatedsoil. Several locally available macrophytes were transplantedinto the wetland, being E. crassipes, T. domingensis and P. cor-data those of the greatest cover. After crossing the wetland, theeffluent followed an excavated channel to a 1.5 ha pond. Bothwastewater from the industrial processes and sewage fromthe factory were treated together. It was expected that highnutrient concentrations could increase the toxicity toleranceof the macrophytes (Manios et al., 2003). Effluents reached thewetland after a primary treatment. During the first 3 monthsof operation only sewage entered the wetland. Later, industrialwaste plus sewage were treated.

2.2. Sampling

Twenty-four water samplings were performed at the inlet andoutlet of the larger free water surface wetland from Novem-ber 2002 through February 2004. Samples were taken monthlyuntil March 2003, and every 2 weeks afterwards.

Sediment total P (TP), Ni, Cr and Zn concentrations weredetermined twice in a month at the inlet and outlet of the wet-land. Sediment samples were collected using a 4 cm-diameterPVC corer. All the samples were transported to the laboratoryat 4 ◦C.

In order to estimate the biomass, emergent and floatingmacrophyte stands were sampled with a 0.50 m × 0.50 m sam-

crassipes and Salvinia rotundifolia) and eight emergent (Cyperusalternifolius, P. elephantipes, Thalia geniculata, Polygonum puncta-tum, Pontederia cordata, Pontederia rotundifolia, Typha domingen-sis and Aechmea distichantia) macrophytes were transplanted(Maine et al., 2005).

The larger free water surface wetland was 50 m lengthby 40 m wide and 0.5–0.8 m deep, with a central baffle(Fig. 1B). The baffle doubled the flowpath and resulted in a 5:1length–width ratio. The wetland received wastewater througha PVC pipe provided with a perpendicular distribution pipewith holes at regular distances in order to allow uniform dis-tribution of flow. Wastewater discharge was approximately100 m3 d−1 and the hydraulic residence time ranged from 7

Fig. 1 – Scheme of the (A) small-scale wetland and (B)large-scale wetland.

pler following Vesk and Allaway (1997). Four replicates weretaken. Macrophytes were then harvested and sorted by speciesat the laboratory, washed, separated between aboveground(shoots and leaves) and belowground (roots and rhizomes)parts, and dried at 105 ◦C until constant weight was reached(APHA, 1998). Plant cover was estimated measuring the areaoccupied by each stand within the wetland.

2.3. Chemical analysis

Conductivity was measured with a 33 model YSI conduc-timeter, O2 with a Horiba OM-14 portable meter and pHwith an Orion pH-meter. Water samples were filtered throughMillipore membrane filters (0.45 �m) for P and N determi-nations. Chemical analysis was performed following APHA(1998); NO2

− was determined by coupling diazotation followedby a colorimetric technique, NH4

+ and NO3− by potentiome-

try (Orion ion selective electrodes, sensitivity: 0.01 mg l−1 ofN, reproducibility: ±2%). Soluble reactive phosphate (SRP) wasdetermined by the colorimetric molybdenum blue method(Murphy and Riley, 1962). Ca2+ and Mg2+ were determined byEDTA titration. Na+ and K+ were determined by flame emis-sion photometry. Alkalinity (CO3

2− and HCO3−) was measured

by HCl titration. Cl− was determined by the argentometricmethod. SO4

2− was assessed by turbidimetry. COD was deter-mined by the open reflux method and BOD by the 5-day BODtest (APHA, 1998). Fe, Cr, Ni and Zn concentrations were deter-mined in water samples by atomic absorption spectrometry(by flame or electrothermal atomization, according to the sam-ple concentration, Perkin-Elmer 5000), following APHA (1998).

Total phosphorous (TP) in sediment was determined afteran acid digestion with an HClO4/HNO3/HCl (7:5:2) mixture fol-lowed by SRP determination in the digested samples (Murphy

e c o l o g i c a l e n g i n e e r i n g 2 6 ( 2 0 0 6 ) 341–347 343

Table 1 – Measured variables in the constructed wetland during the study period

Parameter Inlet Outlet surface Outlet bottom

Mean Range Mean Range Mean Range Mean removal (%)

Temperature (◦C) 16.9 8.0–27 16.9 8.0–27 16.9 8.0–27Conductivity (mS cm−1) 2.9 0.4–8.5 1.3 0.47–2.6 1.8 1.2–2.9O2 (mg l−1) 2.4 0–7.1 1.5 0–5.6 0.5 0–7.5pH 8.7 7.2–10.8 7.2 6.9–8.1 7.5 7.0–8.3Alkalinity (mg l−1 CaCO3) 463 205–1187 237 95–475 296 195–595 37Ca (mg l−1) 156 27.0–651 43 22.3–61.2 53 36.1–77.2 65SO4

2− (mg l−1) 957 98.1–2506 395 159–855 538 158–950 44N–NO3

− (mg l−1) 16 1.6–68 3.0 0.8–7.9 4.5 1.0–17 70N–NO2

− (mg l−1) 0.79 0.021–3.4 0.15 0.01–0.99 0.27 0.02–1.3 60N–NH4

+ (mg l−1) 1.9 0.1–12 1.6 0.12–6.7 4.2 0.13–18 −49Inorganic N (mg l−1) 19 1.8–74 5.0 1.2–10 6.5 2.0–21 53Fe (mg l−1) 13.7 0.16–74 0.38 0.05–1.2 0.67 0.11–3.2 95Cr (�g l−1) 22 3.3–150 3.6 2.8–5.3 3.0 1–3.5 86Ni (�g l−1) 17 6.1–60 9.0 3.9–27 6.1 3.5–8.1 67Zn (�g l−1) <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 –SRP (mg l−1) 0.20 0.003–0.51 0.19 0.02–0.43 0.25 0.03–1.1 −19COD (mg l−1) 204 22–430 39 15–103 45 11–64 78BOD (mg l−1) 41 7–89 15 5–32 10 7–30 77

and Riley, 1962). Cr, Ni and Zn sediment concentrations weredetermined in the digests by atomic absorption spectrometry(Perkin-Elmer 5000).

Above and belowground macrophyte tissues were groundfor P and metal determination. TP, Cr, Ni and Zn in planttissues were determined in the same way as in sediment sam-ples. Chlorophyll was extracted with acetone for 48 h underdark and cold conditions (3–5 ◦C). The transmittance percent-age of the extracts was read to 645 and 665 nm using a UV–visspectrophotometer following Vollenweider (1974) in order tocalculate chlorophyll a concentration. A nearby undisturbedfloodplain wetland of the Parana river was also sampled tocompare plant height, chlorophyll and nutrient contents inplants with those of the wastewater wetland.

2.4. Statistical analysis

Statistical significance between inlet and outlet water concen-trations was assessed using a paired-sample comparison test(p < 0.05). Differences in the concentrations of TP and met-als between inlet and outlet sediment and between initialand final concentrations were determined by a mean com-parison test (p < 0.05). Calculations were carried out with theStatgraphics Plus 3.0 program.

3. Results and discussion

ToCintcavg

to 0–2 until August, and remained anoxic (<1 mg l−1) for theremainder of the study. The lower depths of the outlet wereanoxic in most samplings. Oxygen concentrations induceddifferences between surface and bottom in most other param-eters. Since the water released from the wetland was takenclose to the bottom, removal efficiencies shown in Table 1 cor-responded to the bottom samples. The design of the outletfacility is therefore important. Outcoming water quality wouldbe improved just by taking it from the surface.

BOD and COD were reduced by 77% and 78% at the outlet,suggesting a large decomposition and metabolism of incom-ing organic matter. Nitrate, nitrite and sulphate were reducedby 70%, 60% and 44%. On the contrary, ammonium concentra-tion nearly doubled. Organic matter mineralization representsan important source of ammonium, which is not nitrifiedbecause low oxygen concentration limited nitrification. Due tonitrate in the incoming water was much greater than ammo-nium, the overall inorganic N balance showed a net reduc-tion of 53% of the incoming inorganic N. Attained macrophytebiomass and tissue N concentrations suggest the biomass Npool represented <10% of the N removed from the incomingwastewater. Given the observed oxygen depletion it seemslikely that denitrification is the major removal process.

Several different studies have consistently shown denitri-fication to be a major pathway in wetlands. D’Angelo andReddy (1993) determined that most of the 15N-nitrate (roughly90%) applied to sediment-water cores was lost by denitrifica-

able 1 summarizes the variables measured at the inlet andutlet of the wetland and the estimated removal efficiency.omparing the concentration of variables measured at the

nlet and the outlet in the different sampling, there are sig-ificantly statistical differences, being the concentrations athe outlet significantly lower than at the inlet, except in thease of SRP and NH4

+. O2 concentration at the inlet showedlarge variability, being anoxic in several samplings. Vertical

ariations were observed at the outlet. On the surface, oxy-en concentrations decreased from 4–5 mg l−1 until May 2003,

tion. Reddy et al. (1989) measured large denitrification rates inthe rhizosphere of emergent macrophytes of deltaic wetlands.Matheson et al. (2002) performed 15N balances in wetlandmicrocosms estimating that denitrification accounted for 61%of the nitrate load, 25% was retained in the soil while only14% was assimilated by the vegetation. Minzoni et al. (1988)measured large N losses through denitrification in enclosuresinstalled on rice fields, and Golterman et al. (1988) confirmedthe results by mass balances performed in experimental plots.

Organic matter mineralization increased CO2 concentra-tion in water, which, in turn, decreased water pH from 7.2–10.2

344 e c o l o g i c a l e n g i n e e r i n g 2 6 ( 2 0 0 6 ) 341–347

Fig. 2 – pH, O2, calcium, and SRP concentrations in the large-scale wetland during the study period.

at the inlet to 7.0–8.3 at the outlet. Mean calcium concen-trations decreased 65% and alkalinity 37%. Removal percent-ages of calcium and alkalinity were greater when pH of theincoming water was higher (9.2–10.2). On the contrary, con-centrations were slightly greater at the outlet when the pH ofthe incoming water was lower (7.2–7.6), suggesting that cal-cium carbonate precipitation within the wetland representsan important pathway governed by the incoming water pH.SRP concentrations at the inlet had greater standard devi-ations (Table 1). SRP concentrations at the inlet were lowerwhen water pH was higher, above 9. At the outlet large differ-ences between surface and bottom SRP concentrations wereoften observed, being greater at the bottom. Since the waterreleased from the wetland was taken from the bottom, SRPconcentration of the outcoming water had an overall meanconcentration 19% greater than at the inlet. Mineralizationof incoming organic matter seemed to be the main contri-bution to the observed increased concentration at the out-let. The low SRP concentrations at the inlet coincidentallywith the samplings of high pH, calcium and carbonate mightbe caused by phosphorous coprecipitation with calcium car-bonate (Fig. 2). As pH decreases, SRP sorption to carbon-ates decreases while adsorption to iron increases (Golterman,1995). However, oxygen depletion prevented adsorption toiron, causing the observed large SRP concentrations in the out-let waters (Fig. 2).

The wetland showed high Fe, Cr and Ni retention (Table 1).The overall mean throughout the study period was 95%,86% and 67% retention for Fe, Cr and Ni, respectively. Theremoval percentages of each metal remained almost con-stant during the experimental period, in consequence thehighest the incoming concentrations, the largest the removedmetal amounts. Zn concentration was below 50 �g l−1 (detec-tion limit of the analytical method) both in inlet and outletthroughout the study period. Simultaneous sulphate and Feremoval and oxygen depletion in the water column suggestinsoluble FeS formation. Because of the high sulphate con-centration in the incoming wastewater, most of the organicmatter mineralization took place at the expense of biologicalsulphate reduction as observed in coastal marine sedimentswhere sulphate reduction is responsible for 25–79% of the totalorganic matter mineralization. Hydrogen sulphide, liberatedby sulphate reduction subsequently reacts with iron to formiron sulphide minerals (Giblin, 1988).

Table 2 shows TP and metal concentrations in the bottomsediment. TP showed spatial and temporal variations. Afterthe first year of operation, TP showed a significant increase atthe inlet while at the outlet TP concentration was not signif-icantly different comparing the beginning and the end of thestudy period. Greater sediment concentrations were consis-tent with the hypothesized SRP coprecipitation with carbon-ates at the inlet. Cr, Ni and Zn concentration in the bottom

e c o l o g i c a l e n g i n e e r i n g 2 6 ( 2 0 0 6 ) 341–347 345

Table 2 – Mean metal and TP concentration in thebottom sediment of the large-scale constructed wetland(units are �g g−1)

Initial Final

Inlet Outlet

Cr 30 ± 5 25 ± 3 21 ± 5Ni 19 ± 4 17 ± 2 16 ± 4Zn 60 ± 7 53 ± 5 59 ± 5TP 343 ± 12 426 ± 15 361 ± 17

sediment did not increase significantly throughout the studyperiod.

Several macrophytes were transplanted into the wetland.E. crassipes (water hyacinth) became dominant and coveredabout 80% of the water surface until January 2004, when thewetland water was emptied out for 5 days for maintenanceactivities. Although some water remained at the bottom andthe plants were anchored in the mud. Afterwards plant coverof water hyacinth decreased to 49% while plant height andchlorophyll concentration significantly decreased and becamesignificantly smaller than in a nearby undisturbed floodplainwetland.

After the maintenance activities, P. elephantipes (elephantpanicgrass) grew among the water hyacinth plants and P.cordata (pickerelweed) decreased in cover until its total dis-appearance of the wetland. T. domingensis (cattail) increasedprogressively from 4 to 14% on the wetland surface and P. ele-phantipes remained covered about 2–4% along the study period.T. domingensis shoot density was lower while shoot height washigher than in the undisturbed floodplain wetland along mostof 2003. However, shoot density and height did not show sig-nificant differences from the floodplain wetland in the late2003–2004 growing period (December 2003–March 2004). Metalcontent in plant tissue increased significantly in E. crassipesand T. domingensis roots, but not in shoots (Table 3). Cr, Ni andZn concentration in cattail roots were significantly greater atthe end of the first year compared to the initial concentration.Csi0

cattail. To ensure long-term removal, periodic harvests arebeing performed twice a year. Metal concentrations in the har-vested plants are below the admissible limits for the nationalregulation for hazardous waste and fertilizers. Therefore theseresidues are being used as compost for ornamental plants cul-tivated in a greenhouse which was constructed in the samefactory. Metals in ornamental plant tissues will be monitoredto ensure maximum safety conditions.

The large-scale wetland showed retention efficiency formetals similar to that of the small-scale prototype stud-ied earlier (Table 4). The incoming wastewater was differ-ent; the influent of the small-scale wetland presented greaterpH, conductivity, nutrient and metal concentrations than thelarger wetland (Table 4). Three floating and eight emergentmacrophytes were transplanted to the small-scale wetland.After some initial growth, the small-scale wetland became amonospecific stand of cattail with a biomass similar to undis-turbed environments (Maine et al., 2005). Metal concentrationin both, incoming wastewater and plant tissues, were lowerthan the metal thresholds reported in literature (Gibson andPollard, 1988; Delgado et al., 1993; Sen and Bhattacharyya,1994; Selvaphathy et al., 1997; Cardwell et al., 2002; Fakayodeand Onianwa, 2002; Ingole and Bhole, 2003; Manios et al., 2003;Soltan and Rashed, 2003, Maine et al., 2004). Hadad et al. (inpress) reported pH toxicity thresholds of 10, 11 and 11 for E.crassipes, P. stratiotes and Salvinia herzogii, respectively, and con-ductivity toxicity thresholds of 4 and 8 mS cm−1 for E. crassipes

r and Ni concentrations in water hyacinth roots increasedignificantly, while Zn concentrations did not increase signif-cantly by the end of the first year. Plant biomass attained.7–1.2 kg dw m−2 for water hyacinth and 1.9–4.0 kg dw m−2 for

Table 3 – Metals and TP in plant tissue at the inlet of thelarge-scale constructed wetland after 1 year operation(units are �g g−1)

Initial Final

Roots Leaves Roots Leaves

E. crassipes (water hyacinth)Cr 16 12 78 9Ni 28 9 42 21Zn 43 35 24 15TP 1920 4190 1560 3020

T. domingensis (cattail)Cr 20 13 139 12Ni 24 5 198 2Zn 60 45 129 18TP 1230 2980 1560 1980

and P. stratiotes, respectively. In most of the samplings in thesmall-scale wetland conductivity and pH were higher than thetolerance thresholds (Maine et al., 2005). In consequence, con-ductivity and pH were probably the main cause of the progres-sive disappearance of most transplanted species. As regardsemergent species, Klomjek and Nitisoravut (2005) studied theresponse given by eight species used in a constructed wet-land, similar to the one studied at a pilot scale which receivedsaline wastewaters (14–16 mS cm−1). T. angustifolia, among oth-ers, proved to be tolerant and showed a satisfactory growth.

These conductivity values are higher than those recordedfor the studied wetland, being this the reason why T. domingen-sis adapted to the conditions of the system, presenting a posi-tive growth rate. It seems therefore not surprising that floatingmacrophytes disappeared comparatively earlier than emer-gent ones. In consequence, the floating macrophytes couldnot develop within the pH and conductivity prevailing in theincoming wastewater of the small-scale wetland even in theabsence of metals. Increased water depth and decreased pH,conductivity and metal concentration in the large-scale wet-land resulted in the dominance of water hyacinth.

Oxygen concentrations in the large-scale constructed wet-land were consistently lower than those of the small-scalewetland, in spite of lower COD and BOD loads in the for-mer. Extensive development of water hyacinth causes oxy-gen depletion irrespective of external loadings as observedin many natural undisturbed floodplain environments of theParana River (Pedrozo et al., 1992). Oxygen concentrationseems to be the cause of different retention mechanismsbetween the two wetlands: while Cr, Ni, and Zn were mainlystored in the bottom sediment in the small-scale wetland,in the large-scale facility metals were retained in the vege-tation. Guo et al. (1997a,b) studied metal speciation at differ-

346 e c o l o g i c a l e n g i n e e r i n g 2 6 ( 2 0 0 6 ) 341–347

Table 4 – Water composition at the inlet and outlet and removal efficiency of the small-scale wetland (Maine et al., 2005)

Inlet Outlet % Removal

Mean Range Mean Range

Temperature (◦C) 18.4 11–26 18.4 11–26 –Conductivity (mS cm−1) 5.13 3.3–8.5 3.1 0.7–5.4 –O2 (mg l−1) 6.18 0–10.6 3.15 0–5.8 –pH 10 8.0–12.5 7.9 7.0–9.0 –Alkalinity as CaCO3 (mg l−1) 303 179–539 344 166–665 –Ca (mg l−1) 171 43–292 97 22–238 41SO4

2− (mg l−1) 1444 298–2322 929 204–1690 35N–NO3

− (mg l−1) 20 5–86 1.5 0.2–3.9 88N–NO2

− (mg l−1) 2.25 0–12 0.15 0.001–0.89 85N–NH4

+ (mg l−1) 2.2 0.3–4.9 2.2 0.006–9.5 13Inorganic N (mg l−1) 24.8 10–95 3.91 0.31–14.4 84Fe (mg l−1) 9.1 0.05–32 0.21 0.05–0.7 82Cr (�g l−1) 127 5–589 13 1–105 81Ni (�g l−1) 181 3–750 51 4–190 66Zn (�g l−1) 60 40–210 40 10–69 59SRP (�g l−1) 550 30–2600 141 10–572 75COD (mg l−1) 276 57–583 40 22–85 86BOD (mg l−1) 136 17–400 15 2–38 89

ent redox potentials in sediment-water suspensions. At highredox potentials (430 mV) Cr, Ni, and Zn are adsorbed ontoFe and Mn colloids. With the oxygen concentrations that pre-vailed in the small-scale wetland coprecipitation with irondetermined the metal retention within the bottom sediment.On the contrary, almost permanent oxygen depletion in thelarge-scale wetland prevented precipitation to the wetlandbottom. Macrophyte roots release oxygen to the rhizosphere(Reddy et al., 1989) and produce the precipitation of Fe toform the so-called “iron-plaque” (Otte et al., 1995). Metal bind-ing affinity to iron oxyhydroxides causes metal accumulationinto the iron-plaque (Otte et al., 1995). Thus, metals mighteither be directly sorbed by the macrophytes from the solutionor co-precipitate with Fe onto the macrophyte roots. Oxygendepletion also seemed to be the cause of the increased SRPand ammonium at the outlet while the small-scale wetlandefficiently removed SRP.

4. Conclusions

• Both, the preliminary small-scale and the subsequent large-scale facility efficiently removed metals from the effluentof a metallurgic plant. The small-scale and the larger wet-lands reduced Cr, Ni, Fe concentrations by 81%, 66%, 82%,and 86%, 67%, 95%, respectively. However, the small-scale

• Decreasing water level will favour the dominance of emer-gent macrophytes as evidenced by the changes producedwhen the wetland was emptied for maintenance. Further-more, reducing water depth would contribute to increasingoxygen concentration.

Acknowledgements

The authors thank Consejo Nacional de InvestigacionesCientıficas y Tecnicas (CONICET) from Argentina and Univer-sidad Nacional del Litoral, CAI + D Project for providing fundsfor this work.

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