Performance study of vertical flow constructed wetlands for phosphorus removal with water quenched slag as a substrate

Ecological Engineering 53 (2013) 39–45

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Performance study of vertical flow constructed wetlands for phosphorus removal with water quenched slag as a substrate Haibo Li a , Yinghua Li a,∗ , Zongqiang Gong b,∗∗ , Xiaodong Li c a

Key Laboratory of Regional Environment and Eco-Remediation, Ministry of Education, Shenyang University, Shenyang 110044, China Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China c Environmental Science Research Institute of Liaoning Province, Shenyang 110031, China b

a r t i c l e

i n f o

Article history: Received 13 August 2012 Received in revised form 5 January 2013 Accepted 16 January 2013 Available online 16 February 2013 Keywords: Phosphorus removal Adsorption Hydraulic retention time (HRT) Phosphorus loading Water quenched slag

a b s t r a c t Two pilot-scale vertical flow constructed wetlands (VFCW) were designed, with one system containing water quenched slag (WQS) from a steel plant as a substrate and the other one as a control without WQS. Each VFCW consisted of a down-flow unit and an up-flow unit. Both of the VFCWs were planted with Canna indica Linn. This study focused on evaluation of the ability to remove phosphorus (P) for both VFCWs. Batch P adsorption and desorption tests were conducted. The influence of hydraulic retention time (HRT) and phosphorus loadings on phosphorus removal were investigated. Results showed that the WQS had much higher P adsorption capacity than gravel. The amounts of phosphorus adsorbed on the WQS were 0.17 g/kg and 0.05 g/kg for the down-flow and up-flow units, respectively, and were 0.09 g/kg and 0.025 g/kg on the corresponding gravel, respectively. Dates from the adsorption experiment were fitted to both Freundlich and Langmuir adsorption isotherm equations. Desorption test demonstrated that the adsorption of phosphorus to the WQS was not a reversible process. An HRT of 1 d, which favored the VFCW system operation, was proposed. More than 80% of phosphorus removal was achieved for the VFCW with WQS when phosphorus loadings were in the range of 12.2–36.8 g/m2 d. The down-flow and up-flow units of the wetland system ensured sequential and efficient removal of phosphorus from wastewater, and also easy and flexible replacement of the WQS substrate in case of saturation. The WQS used in this study can be a promising substrate for the VFCW for improving phosphorus removal. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Humans have increased the input of nutrients, especially nitrogen and phosphorus, into natural ecosystems through domestic wastewater (Aslan and Kapdan, 2006). Nitrogen and phosphorus, two major nutrients, should be removed from the domestic wastewater before being discharged into water bodies, thus the application of an appropriate technology to maximum nutrient removal is necessary (Dapena-Mora et al., 2004). Constructed wetlands, which are designed to take advantage of many processes occurring in natural wetlands but within a more controlled system, have been used to remove such excess of nutrients from domestic and industrial wastewaters and agricultural runoffs for decades (Sakadevan and Bavor, 1998). The vertical flow constructed wetland (VFCW) with unsaturated flow is known to be particularly efficient in treating many types of wastewater (Molle et al.,

∗ Corresponding author. Tel.: +86 24 62266671; fax: +86 24 62266671. ∗∗ Corresponding author. Tel.: +86 24 83970367; fax: +86 24 83970367. E-mail addresses: [email protected] (Y. Li), graceli [email protected] (Z. Gong). 0925-8574/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ecoleng.2013.01.011

2005; Brix and Arias, 2005). This kind of system uses a bed of substrate for growth of rooted wetland plants. Wastewater after some pretreatment flows by gravity vertically, through the bed substrate where it contacts a mixture of microbes living in association with the substrate and plant roots (Van de Moortel et al., 2009). The vertical flow wetlands have more oxygen transport ability than the horizontal subsurface flow systems (Cooper, 2005; Kantawanichkul et al., 2009). Efficient removals of nitrogen and organic matter (measured as biological oxygen demand, BOD) have been reported for various types of wetland systems (Arias et al., 2005). Several researchers have also demonstrated that the abiotic adsorption of phosphorus (P) to the substrate is the major P-removal mechanisms in constructed wetlands, and substrate is the main sink for P in the long term (Sakadevan and Bavor, 1998). Another advantage of the sorption technique is that the media loaded with nutrients can be employed to agricultural processes as P fertilizer and soil conditioner (Hylander and Sima, 2001; Gustafsson et al., 2008). Therefore it is of utmost importance to select proper substrates with specialized physico-chemical properties to P-removal, while maintaining sufficient permeability when designing constructed wetlands (Johansson and Gustafsson, 2000). Physico-chemical properties, which may influence the adsorption

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H. Li et al. / Ecological Engineering 53 (2013) 39–45

Table 1 Chemical and physical properties of the substrates used in this study. Substrate materials

pH

General chemical composition (on the basis of oxides) (%)

SiO2

Al2 O3

MgO

Fe2 O3

CaO

P2 O5

Gravel WQS

6.82 7.54

93.06 33.24

3.19 16.95

0.17 7.62

0.63 0.68

0.19 37.41

0.001 0.02

of phosphorus (P) to the substrate, include pH, redox potential, dissolved ions, calcium, amorphous and poorly crystalline Al and Fe-oxides and organic matter (Zhu et al., 1997; Reddy et al., 1999). Sand, gravel, and peat had been commonly used as substrates at early stage of the wetland systems for treating wastewater. Nowadays, it has always been suggested that they should be replaced by materials with a higher capacity for P removal, and far more economical. Some alternatives are LECA (a reactive porous media, Zhu et al., 1997), shale (Wood and McAtamney, 1996), wollastonite (Brooks et al., 2000), zeolite (natural or artificial alumino-silicates, Sakadevan and Bavor, 1998). In recent years, slag substrate has recently been recognized as a promising technique for phosphorus removal from wastewaters. The slag from blast furnace (SFB) is a steel-making by-product, an economical media with high P-sorption capacity for wetland systems (Asuman et al., 2007). However, SFB always has high content of Fe(III), which can be reduced to Fe(II), resulting in the increase of phosphate solubility and release of phosphorus. On the other hand, the pH of the water environment around substrate bed is alkaline, which is not good for plant growth and may notably impair microbial community of the wetland system. The previous research showed that water quenched slag (WQS) from mainly the combusted coke of the steel-making process was neutral and had less Fe(III) (Tang et al., 2009). The first purpose of this study was thus to evaluate the performance of a pilot-scale VFCW with WQS as a substrate and planted with Canna indica Linn. Another one was to investigate the effect of hydraulic retention time (HRT), nutrient loading, and plant growth on phosphorus removal of VFCW using WQS as a substrate. 2. Materials and methods 2.1. Materials The gravel was obtained from a local market for building material. The particle size of gravel varies from 0.2 to 2.24 mm. The slag, WQS, was a by-product from a local steel plant. It had a particle size of 0–4 mm and was pretreated with 1% CaO to increase its alkaline reaction. For a better understanding of sorption to slag, WQS was ground into grains of less than 0.1 mm in diameter in the batch adsorption experiment. To remove residual acid and soluble compounds, the WQS was treated with hydrochloric acid (1%, v/v) and washed with distilled water. The physico-chemical characteristics of the gravel and WQS in this study are presented in Table 1. On the other hand, C. indica Linn was chosen as the plant component in the wetland system because of its tolerance to flooding and redundant elements, long root length, and landscape plant role. 2.2. Batch P adsorption and desorption tests For the batch sorption capability evaluation, potassium phosphate monobasic salt (KH2 PO4 ), which is a strong electrolyte and dissociates in solution easily, was dissolved in 0.02 M KCl to prepare stock solutions. The produced HPO4 2− , H2 PO4 − , and PO4 3− ions were available for reactions instantly. These ions are also the

Surface area (m2 /kg)

Soluble calcium (%)

186 357

0.014 0.084

main bio-available forms of phosphorus and thus contribute to eutrophication in water bodies. Before the batch adsorption test, WQS samples were dried at 105 ◦ C oven to exclude the humidity interference. Afterwards, 10 g of WQS was put into Erlenmeyer flasks (250 mL) containing 200 mL of phosphorus stock solution at concentrations of 1.0, 2.0, 5.0, 10, 20, 50, 100, 200, and 400 mg/L, respectively (concentrations were calculated based on the weight of phosphorus element). Triplicate treatments were performed for individual P-concentrations. The Erlenmeyer flasks were shaken for 24 h at 25 ◦ C and 140–150 rpm. The solution in the Erlenmeyer flask was filtered through 0.45 ␮m Millipore membrane and was ready for phosphorus measurement. The amount of P adsorbed to the WQS was calculated according to following equation. q=

(Ci − Cf ) × v M

(1)

where q is the amount of P-adsorbed (mg P/kg WQS), Ci is the initial concentration of P (mg/L), Cf is the final concentration of P or equilibrium concentration (mg/L), v is the volume of stock solution (0.2 L), and M is the mass of WQS (0.01 kg of dry weight). Data obtained from adsorption experiment were fitted to both Freundlich and Langmuir adsorption isotherm equations. The Freundlich adsorption isotherm equation is normally expressed as follows. Q = Kd Ce 1/n

(2)

where Q is adsorption density of the adsorbent (mg P/g of WQS in this study), Ce is the phosphorus equilibrium concentration in the solution, Kd and n are material characteristics and considered constant for each adsorbent. The parameter Kd is a relative indicator of adsorption capacity, representing the amount of P adsorption, while n is an indicative of reaction intensity. The Langmuir adsorption isotherm equation is expressed as follows. Q =

Qmax Ce Ce + k

(3)

where Qmax and k are material characteristics and considered constant for each adsorbent, Qmax is maximum adsorption density. The constant k is associated with binding energy. Desorption test was conducted to assess the stability of the phosphate adsorbed to the WQS. The residual WQS, which was saturated with phosphorus, was washed with deionization water twice, and was recovered immediately by centrifugation. Afterwards, 1 g sample of residual solid was put in a centrifuge tube containing 20 mL of KCl solution at a concentration of 0.02 M. The centrifuge tubes were then shaken at 140–150 rpm, 25 ◦ C ± 1 ◦ C, and water samples were withdrawn at different intervals till equilibrium. The suspension solution was filtered and subjected to analysis of desorbed phosphorus concentration. The amount of desorbed phosphorus was calculated by the amount of phosphorus in solution after the desorption experiment. 2.3. Vertical flow constructed wetland system Two pilot-scale VFCWs were designed, with one system containing WQS and the other one as a control without WQS. Each

H. Li et al. / Ecological Engineering 53 (2013) 39–45

41

6

7

4

5 3 1

4 2

6

7

6 Fig. 1. Design of vertical flow constructed wetlands.

VFCW consisted two single units, first a down-flow unit and then an up-flow unit, as shown in Fig. 1. The dimensions of the systems were 0.8 m in length, 0.4 m in width, and 0.8 m in height, with an internal barrier separating the systems into two equal size units. However, the bottoms of the two units were not separated, allowing the water to go though down-flow and up-flow units sequentially. In the system with WQS, the down-flow unit was filled with coarse gravel (5–10 mm) to a depth of 0.15 m as the bottom layer, and then was filled with fine gravel (1–2 mm) with a depth of 0.35 m at middle, and the top layer was WQS with a depth of 0.1 m. The up-flow unit of the system with WQS was nearly the same as the down-flow unit, except that the depth of fine gravel layer was only 0.25 m, thus giving a water level of 0.1 m lower than that of the down-flow unit. This wastewater flow way helped the system to draw air from the atmosphere into the substrate like a passive pump, thereby creating good aeration conditions (Tang et al., 2009). The control system was similar to the system with WQS, except that the WQS layer was replaced with fine gravel (1–2 mm). Some horizontal holes were settled in individual units of the systems for substrate sampling. These holes were at depths of 5 cm, 20 cm, and 35 cm below the surface of the inlet in the downflow unit and were at depths of 5 cm, 20 cm, and 30 cm below the surface of the outlet in the up-flow unit. They are designated as d5, d20, d35, u5, u20, u30, respectively. The domestic sewage collected from the local sewer in Shenyang University was drained to a sedimentation tank before entering the pilot-scale treatment systems.

2.4. Wetland system start-up and operation Each wetland system was planted with 32 C. indica Linn. (With similar aboveground biomass in the main layers for all the systems.) About 2 months of start-up period was allocated to allow plant growth and system acclimation to the wastewater. During the start-up period, the system was first maintained with tap water till the height of C. indica Linn. was higher than 1 m, and the root length was higher than 25 cm, then the wastewater after sedimentation was fed into the wetland system. The wastewater characteristics were: suspended solid (SS) 35–40, chemical oxygen demand (COD) 165–200, biochemical oxygen demand (BOD)

120–130, total phosphorus (TP) 5.9–7.3 and ammonia nitrogen (NH4 + –N) 49–55 mg/L. The other parameters for the start-up period were: contaminant load (calculated based on BOD) 33.6 g/m2 d, HRT 1 d, temperature 17–27 ◦ C. After completion of the start-up period, the systems were in a stable condition and were operated regularly. All the systems were watered as required depending on the weather. During the operation, the wastewater was pumped into the slag bed. The operating conditions of the systems were mainly varied by different HRT and nutrient loading. The HRT evaluated were 2, 1, and 0.5 d. The phosphorus loading considered were 12.2, 24.5, 36.8, and 48.9 g/m2 d. Influent and effluent water samples were taken for total phosphorus measurements at same intervals during the operation period. In order to measure total phosphorus in the substrates and to evaluate the effect of substrate on the distribution of the phosphorus concentration, the systems were operated under optimized parameters (contaminant load 33.6 g/m2 d and HRT 1 d) for 20 d, and then substrate samples were taken from the horizontal holes described above for phosphorus analysis. The efficiency of phosphorus removal by the VFCW with WQS as substrate was analyzed.

2.5. Analyses The percentage of phosphorus removals was calculated based on the difference between influent and effluent concentrations at the same sampling occasion. The mean P removal percentages were determined on the basis of all the removal percentages for the entire period. Total phosphorus concentration was determined using the molybdenum blue spectrophotometric method as stated in American Water Works Association (2005). Mineral compositions of the substrates were determined by X-ray diffraction (XRD). The conditions for XRD measurement were: D/max 2200PC Xray diffractometer, Ka,  0.15406 nm, Ni filtration, scanning speed 4◦ /min and scanning range 0–65◦ . The pH value was measured in a 1:2.5, soil:water slurry using a glass electrode. The soluble calcium was determined by EDTA titrimetric method (Klute, 1986). BET surface areas of the substrates were determined using N2 adsorption method. Fitting of the experimental data to the isotherm equations was performed by the use of software, Sigma Plot 10.0 (Systat Software, Inc.).

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H. Li et al. / Ecological Engineering 53 (2013) 39–45

Table 2 Parameters of the Freundlich and Langmuir isotherms for phosphorus removal. Substrate

Freundlich

Langmuir

Qmax

n

R

p

Qmax

K

R

p

Gravel WQS

34.9 146.3

2.7 2.0

0.95 0.97

0.0260 0.0388

326.8 3041.1

40.2 74.0

0.99 0.99

<0.0001 <0.0001

Table 3 Desorption of phosphorus in substrates.

3. Results and discussion

Substrate

Maximum adsorption (mg/kg)

Desorption (mg/kg)

Desorption rate (%)

A preliminary test showed that phosphors adsorption was notably affected by the particle size of slag, and adsorption mostly occurred in fine particles (Klute, 1986), thus the WQS was ground into grains of less than 0.1 mm in diameter in the batch adsorption experiment. The high levels of Al, Ca, and surface area shown in Table 1 implied that WQS had much higher potential to be a good substrate as compared to the gravel in wastewater treatment system in light of phosphorus adsorption (Grüneberg and Kern, 2001; Comeau et al., 2001). The Fe2 O3 of WQS was only 0.68% (Table 1), which might also be a good potential contribute to phosphorous removal, as high level of Fe(III) may result in release of phosphorus. To facilitate estimation of adsorption capacity, fitting of the experimental data with the two most commonly used adsorption isotherm equations: Freundlich and Langmuir was conducted. To establish a sorption isotherm with good equilibrium, some higher concentrations than those domestic wastewater concentrations reported are essential (Zhu et al., 1997). Thus some high P-concentrations (50–400 mg/L) were used in the batch adsorption test. The results of P adsorption isotherm experiments are shown in Fig. 2. The phosphorus adsorbed to the substrates drastically increased with the increases of phosphorus concentration. From the correlation coefficients in Table 2, it is shown that both the adsorption isotherms predicted well the adsorption of P to the substrates. The related constants were Kd = 34.9 and n = 2.7 for the gravel, and Kd = 146.3 and n = 2.0 for the WQS. The constant Kd is always used to assess adsorption capacity of adsorbents due to that it is considered as the adsorption density for a unit value of Ce (Mortula et al., 2007). As demonstrated by the Freundlich Kd values, the WQS had much higher P adsorption capacity than the gravel, in accordance with the fact shown in Table 1 that the WQS had higher level of surface area and pH value than gravel. The maximum adsorption density (Qmax ) of gravel was 326.8 mg/kg, which is

Gravel WQS

326.8 3041.1

20.5 22.6

6.27 0.74

Phosphorus adsorbed (mg/kg)

3.1. Batch adsorption and desorption tests

350

(a)

300

consistent with other results (Arias et al., 2001; Bubba et al., 2003). The WQS showed its Qmax was 3041.1 mg/kg, which was 10 times of that of gravel, the larger surface area (357 m2 /kg) of the WQS was a good reason to explain this. The P desorbability was termed as the percentage of the desorbed P to the total adsorbed P on the substrates. It can be seen from Table 3 that P adsorption to the slag was not completely reversible because only 0.74% adsorbed phosphorus can be desorbed. This result implied that the WQS could act as a promising adsorbent of phosphorus used in wetland system for wastewater treatment. 3.2. Effect of HRT on phosphorus removal The influent and effluent concentrations of total phosphorus (TP) during the operational period at different HRT are plotted to show a relationship between HRT and removal efficiencies of TP in Fig. 3. Percent removal efficiencies based on TP concentration were calculated according to the following equation. TP removal =

(4)

During the whole experimental period, the system with WQS performed much better than the control system without WQS, obviously the WQS increased the TP removal. The difference between the two systems increased with the decrease of HRT from 2 d to 0.5 d (Fig. 3). When HRT was 2 d, the effluent TP concentrations from both the two wetland systems were lower than 1 mg/L, with all the TP removals higher than 85%. A decrease of

(b)

2500

250

2000

200

1500

150

(Cin − Cout ) × 100 Cin

1000

100 500

50 0

0 0

50 100 150 200 250 300 350 400

0

50

100

150

200

250

300

Equlibrium concentration of phosphorus (mg/L)

Experimental data Langmuir isotherm Freundlich isotherm Fig. 2. Isothermal adsorption of phosphorus on gravel (a) and WQS (b) in the batch adsorption experiments.

90

90

HRT=1 d

80

70

60

8 7 6 5

70

2

60

1 10

50 5

10

15

20

20

30

25

30

40

8 7 6 5

70

2

60

1 10

50

35

HRT=0.5 d

80

P conc (mg/L

80

43

5

P conc (mg/L)

HRT=2 d

90

P conc (mg/L)

Total phosphorus removal (%)

H. Li et al. / Ecological Engineering 53 (2013) 39–45

10

15

20

20

30

25

40

30

35

8 7 6 5 2 1 10

50 5

10

15

20

20

30

25

30

40

35

Operation time (d) Original water Efluent (with WQS) Efluent (without WQS)

TP removal (with WQS) TP removal (without WQS) Fig. 3. Effect of HRT on phosphorus removal.

2.4–4.7% for the TP removals was observed when HRT was reduced to 1 d. When HRT was 0.5 d, TP removal decreased 5.9–9.5% compared to 2 d HRT. Phosphorus removal from wastewater in the wetland system depends on adsorption and sedimentation, and longer HRT implies longer contact time between the water and substrate, which ensured a good performance of the system. However, it should be noted that when phosphorus concentration in wastewater is lower than 2 mg/L, adsorption of phosphorus by the substrate will be influenced by much more factors (House et al., 1994). It will not contribute to phosphorus removal by the wetland notably even though HRT is increased quite a lot at this condition. The suitable HRT in this study was decided to be 1 d in the following experiments. It is reported by Lee and Lin (1999) that it is a difficult task to reduce the effluent TP concentrations to below 1.2 mg/L in a VFCW using sand and gravel as the main substrate. The wetland systems in this study showed a promising performance for TP removal. 3.3. Effect of phosphorus loading on phosphorus removal To test the capacity of the wetland system for TP removal, four phosphorus loading ranged from 12.2 g/m2 d to 48.9 g/m2 d were set for the experimental systems. The HRT was set to be 1 d, temperature was in the range of 18–26 ◦ C, and the system was operated for 40 d. It can be seen from Fig. 4 that phosphorus loadings had a remarkable impact on phosphorus removal by the wetland system, especially at high phosphorus loading (48.9 g/m2 d). When phosphorus loadings were in the range of 12.2–36.8 g/m2 d, phosphorus removals decreased only a little with the increase of phosphorus loadings. However, the system still showed promising potentials due to that 85% of TP removal was achieved for the wetland system with WQS, and even the system without WQS removed 75–80% of TP. When the phosphorus loading was increased to 48.9 g/m2 d, TP removal efficiencies decreased drastically. At this high loading, only 65% of TP was removed for the system with WQS, and less than 55% of TP removal was achieved for the system without WQS. Fig. 5 shows the relationship between TP removal and phosphorus loading. At high loading, organic materials may precipitate on the substrate surface and occupy the substrate adsorption site for phosphorus removal of the wetland system, thus weakening the

phosphorus removal efficiency (Erich et al., 2002). Another opinion is that high loadings resulted in the increase of hydraulic loadings, which facilitated plant root system development. Excessive root density in the wetland system decreased substrate porosity and clogged internal water flow, resulting in longer actual HRT. The unstable adsorbed phosphorus may also be released under the biological and chemical functions of the plant root system (Kim and Geary, 2001). If the quality of influent water is relatively stable, high nutrient loading is not a good choice for wetland system, as it, accompanied with high hydraulic loading, necessitates larger land and eventually affects technical cost (Nwoke et al., 2003; Liikanena et al., 2004; Gu, 2008). Based on above considerations, it is suggested the optimal phosphorus loading be in the range of 24.5–36.8 g/m2 d. 3.4. Vertical analysis of phosphorus removal The phosphorus concentration distribution in the down-flow and up-flow units of the two wetlands was also examined under optimal experimental conditions (HRT was 1 d, phosphorus loading was 33.6 g/m2 d) to assess the effect of WQS on longitudinal gradients of phosphorus concentration in the VFCW system after 20 d. Fig. 6 shows that phosphorus adsorbed to the substrate decreased with the increase of distance from the inlet. The great differences on phosphorus adsorption from the two wetland systems occurred in the 5 cm distance (d5 and u5) from the inlet and outlet, where the WQS existed in one system and was absent in the other. The amounts of phosphorus adsorbed for d5 and u5 were 0.17 and 0.05 mg/kg on the WQS, respectively, and were 0.09 and 0.025 mg/kg on the corresponding gravel, respectively, nearly a half reduction was observed. Whereas the other holes did not show such obvious difference, as the substrates at these holes were all gravel. According to Fig. 6, the use of WQS improved the phosphorus removal. The third substrate sample showed slightly higher phosphorus adsorption as compared to the second one did. Both the two samples were from the down-flow unit, but the third substrate sample was closer to the C. indica Linn. root system. The C. indica Linn. had well developed root system, which helped C. indica Linn. to uptake additional phosphorus and thus resulted in higher phosphorus content in the substrate. The WQS did not

H. Li et al. / Ecological Engineering 53 (2013) 39–45

Total phosphorus removal (%)

44

100

100

80

80

60

60

Phosphorus loading=12.2 g/m 2d

40

Phosphorus loading=24.5 g/m 2d

40

20

20 5

10

15

20

25

30

35

5

100

100

80

80

60

60

2 40 Phosphorus loading=36.8 g/m d

40

20

10

15

20

25

30

35

Phosphorus loading=48.9 g/m 2d

20 5

10

15

20

25

30

35

5

10

15

20

25

Operation time (d)

30

35

With WQS Without WQS

get clogged even though it was submerged into the water during the experimental period, maintaining a good condition for C. indica Linn. growth. In this study, the phosphorus removal was mainly due to calcium precipitation since the WQS had higher calcium content (37.41%) compared to its other compositions. Moreover, it was believed that Al–P and Fe–P were the less available forms when compared to the calcium bounded-P at neutral level (House et al., 1994). The substrate for phosphorus adsorption may get saturated for long time operation. In order to solve this problem, the wetland system in this study was designed in a multi-layer-vertical flow mode, with the WQS at the top layer and gravel underneath. After plant harvest, the WQS layer can be easily replaced when it gets saturated, improving performance in phosphorus removal and applicable time of the wetland systems. The top WQS layers of the down-flow and up-flow units

Phosphorus adsorbed on substrate (g/kg)

Fig. 4. Effluent concentrations of phosphorus in the wetland system at different phosphorus loadings.

.20 With WQS Without WQS

.15

.10

.05

0.00 d5

Total phosphorus removal (%)

100

d20

d3 5 u30 Sampling position

u20

u5

Fig. 6. Comparison of vertical gradients of phosphorus concentrations of the substrates after system operation.

80

removed phosphorus from water sequentially with the first acting as a priority. The WQS layer in the up-flow unit can further remove phosphorus from water if the phosphorus removal in the down-flow unit is not sufficient. The WQS replacement can also be flexible, as the WQS layers in the two units may get saturated at different time.

60

40

20

4. Conclusions

0 P loading 1

P loading 2

P loading 3

Phosphorus loading

P loading 4 With WQS Without WQS

Fig. 5. Effect of phosphorus loadings on total phosphorus removal (P loading 1 = 12.2 g/m2 d, P loading 2 = 24.5 g/m2 d, P loading 3 = 36.8 g/m2 d, P loading 4 = 48.9 g/m2 d).

Adsorption and desorption tests show that WQS can be an alternative substrate for VFCW in improving phosphorus removal. By fitting of the adsorption data with Freundlich and Langmuir equations, it was proved the WQS had advantages compared to the commonly used gravel in phosphorus adsorption. Desorption test demonstrated that adsorption of phosphorus to the WQS

H. Li et al. / Ecological Engineering 53 (2013) 39–45

was not a reversible process. An HRT of 1 d, which favored the VFCW system operation, was proposed. Phosphorus loading of the wetland system had a significant effect on phosphorus removal. More than 80% of phosphorus removal was achieved for the wetland system with WQS when phosphorus loadings were in the range of 12.2–36.8 g/m2 d. The WQS layer was set as the top layers for the down-flow and up-flow units of the wetland system, which ensured sequential and efficient removal of phosphorus from water, and also easy and flexible replacement of the WQS substrate in case of saturation. Acknowledgments The authors would like to thank the National Key Technology R&D Program (No. 2011BAJ06B02), National Natural Science Foundation of China (No. 51108275), Program for Liaoning Excellent Talents in University (LNET) (No. LJQ2012101), the Program for New Century Excellent Talents in University (No. NCET-111012), Science & Technology Program of Liaoning Province (No. 2011229002), the Program for Liaoning Excellent Talents in University (LR201028) and Dr. start-up fund of Shenyang University (No. 20210343) for the financial support of this research. References American Water Works Association, American Public Health Association, Water Environmental Federation, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed., Washington, DC, 901–914. Arias, C.A., Brix, H., Marti, E., 2005. Recycling of treated effluents enhances removal of total nitrogen in vertical flow constructed wetlands. J. Environ. Sci. Health A 40 (6–7), 1431–1443. Arias, C.A., Bubba, M.D., Brix, H., 2001. Phosphorus removal by sands for use as media in subsurface flow constructed reed beds. Water Res. 35 (5), 1159–1168. Aslan, S., Kapdan, I.K., 2006. Batch kinetics of nitrogen and phosphorus removal from synthetic wastewater by algae. Ecol. Eng. 28 (1), 64–70. Asuman, K.E., Meryem, B., Göksel, N.D., 2007. Use of blast furnace granulated slag as a substrate in vertical flow reed beds: field application. Bioresour. Technol. 98 (11), 2089–2101. Brix, H., Arias, C.A., 2005. The use of vertical flow constructed wetlands for onsite treatment of domestic wastewater: New Danish guidelines. Ecol. Eng. 25, 491–500. Brooks, A.S., Rozenwald, M.N., Geohring, L.D., Lion, L.W., Steenhuis, T.S., 2000. Phosphorus removal by wollastonite: a constructed wetland substrate. Ecol. Eng. 15 (1–2), 121–132. Bubba, M.D., Arias, C.A., Brix, H., 2003. Phosphorus adsorption maximum of sands for use medial in subsurface flow constructed reed beds as measured by the Langmuir isotherm. Water Res. 37 (2), 3390–3400. Comeau, Y., Brisson, J., Réville, J.P., Forget, C., Drizo, A., 2001. Phosphorous removal from trout farm effluents by constructed wetland. Water Sci. Technol. 44 (11–12), 55–60. Cooper, P., 2005. The performance of vertical flow constructed wetland systems with special reference to the significance of oxygen transfer and hydraulic loading rates. Water Sci. Technol. 51 (9), 81–90.

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