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Removal of nutrients from wastewater with Canna indica L. under different vertical-flow constructed wetland conditions
Ecological Engineering 36 (2010) 1083–1088
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Removal of nutrients from wastewater with Canna indica L. under different vertical-flow constructed wetland conditions Lihua Cui a,∗ , Ying Ouyang b,∗ , Qian Lou a , Fengle Yang c , Ying Chen a , Wenling Zhu a , Shiming Luo d a
Department of Environment Science and Engineering, South China Agricultural University Guangzhou 510642, China Department of Water Resources, St. Johns River Water Management District, PO Box 1429, Palatka, FL 32178, USA c Yunnan Institute of Environmental Science, Wang Jiaba, Kunming 650034, China d Ecological Agriculture Key Laboratory of the Ministry of Agriculture, China, South China Agricultural University, Guangzhou 510642, China b
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 10 March 2010 Accepted 22 April 2010
Keywords: Hydraulic loading rate Substrate Wetland operational period
a b s t r a c t Constructed wetlands are becoming increasingly popular worldwide for removing contaminants from domestic wastewater. This study investigated the removal efficiency of nitrogen (N) and phosphorus (P) from wastewater with the simulated vertical-flow constructed wetlands (VFCWs) under three different substrates (i.e., BFAS or blast furnace artificial slag, CBAS or coal burn artificial slag, and MSAS or midsized sand artificial slag), hydraulic loading rates (i.e., 7, 14, and 21 cm d−1 ), and wetland operational periods (0.5, 1, and 2 years) as well as with and without planting Canna indica L. The wastewater was collected from the campus of South China Agricultural University, Guangzhou, China. Results show that the percent removal of total P (TP) and ammonium N (NH4 + -N) by the substrates was BFAS > CBAS > MSAS due to the high contents of Ca and Al in substrate BFAS. In contrast, the percent removal of total N (TN) by the substrates was CBAS > MSAS > BFAS due to the complicated nitrification/denitrification processes. The percent removal of nutrients by all of the substrates was TP > NH4 + -N > TN. About 10% more TN was removed from the wastewater after planting Canna indica L. A lower hydraulic loading rate or longer hydraulic retention time (HRT) resulted in a higher removal of TP, NH4 + -N, and TN because of more contacts and interactions among nutrients, substrates, and roots under the longer HRT. Removal of NO3 − N from the simulated VFCWs is a complex process. A high concentration of NO3 − N in the effluent was observed under the high hydraulic loading rate because more NH4 + -N and oxygen were available for nitrification and a shorter HRT was unfavorable for denitrification. In general, a longer operational period had a highest removal rate for nutrients in the VFCWs. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Constructed wetlands (CW) are engineering structures used worldwide to improve water quality. They involve a complex mixture of waters, substrates, plants, and microorganisms to produce optimal conditions for removing organic matter, nutrients, trace elements, pathogens, and other pollutants from wastewater and/or runoff water (Tang, 1993; Crites et al., 1997; Obarska and Klimkouska, 1999; Babatunde et al., 2008; Langergraber et al., 2008). With increasingly gaining acceptance, CWs are now used for many other types of wastewater treatments, including industrial and agricultural wastewaters, landfill leachate, and stormwater runoff (Vyzamal, 2005). Presently, there are primarily four types of CWs: (1) free water surface wetlands, (2) horizontal subsurface
∗ Corresponding authors. E-mail addresses: [email protected] (L. Cui), ouyangy@ufl.edu (Y. Ouyang). 0925-8574/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2010.04.026
flow wetlands, (3) vertical-flow wetlands, and (4) hybrid systems that incorporate surface and subsurface flow wetlands (Cui et al., 2009; Kadlec and Wallace, 2009). Of which, the vertical-flow constructed wetlands (VFCWs) are gaining popularity due to their greater oxygen transfer capacity and smaller size (Cooper, 1999) although pore clogging could be a problem. Many researchers report that CWs are viable alternatives for nitrogen (N) and phosphorus (P) removal from wastewater due to the functions of denitrification, uptake, and sorption that take place (Van Oostrom and Russel, 1994; Horne, 1995; Kadlec, 1997; Baker, 1998; Robins et al., 2000). A wide range of nutrient removal efficiencies with CWs has been reported but many of them fail to meet the relevant government standards, especially for total nitrogen (TN) and total phosphorus (TP) concentrations from discharged wastewater (Fraser et al., 2004; Greenway, 2005). Variations in nutrient removal efficiency among those studies are attributed to different wetland plants, substrates, and hydraulic loading rates used for different experiments.
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Table 1 Water quality in the septic tank used in this study. Parameter
The first year (2001.3–2002.3) Sample No.
COD BOD NH4 + TN TP a
12 12 12 11 12
The second year(2002.4–2003.4) −1
a
Range
Mean concentration (mg L
107–316 35–114 39.73–170.82 25.80–211.28 3.30–17.77
218 69 101.29 114.72 8.47
± ± ± ± ±
20 7 11.70 18.55 1.21
)
Sample No. 13 12 15 16 16
Range 93–277 42–139 30.08–192.34 21.22–181.30 3.23–13.25
Mean concentration (mg L−1 ) 171 76 83.82 87.97 7.28
± ± ± ± ±
16 8 10.77 10.14 0.63
The numbers after ± are standard deviations.
Wetland plants are an integral part of the CW treatment systems. They play important roles in degrading and removing nutrients and other pollutants. Wetland plants include freefloating plants, emergent plants, and submerged plants. The principle in selecting a suitable plant species for use in constructed wetland systems depends on the type of wetland design (e.g., surface or subsurface, vertical or horizontal flow), the mode of operation (e.g., continuous, batch or intermittent flow), and the loading rate and characteristics of wastewaters. Wetland plants not only take up nutrients, heavy metals and organics, but also control the ventilation and microbial conditions in the constructed wetland bed. In addition, plants can effectively filter or settle out suspended solids. Therefore, wetland plants have great influence on the purification capacity of wastewaters (Groudeva et al., 2001; Fu and Tang, 2005). An increased demand on N removal from wastewaters in 1990s has lead to more frequent uses of VFCWs that provide higher degree of filtration bed oxygenation and consequent removal of ammonia via nitrification (Vyzamal, 2005). The VFCWs have high hydraulic loading rates, less land area requirement and high oxygen transfer, and are good for nitrification because of their high oxygen transfer capability that also leads to good removal of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). The removal of N from domestic sewage in constructed wetlands mainly depends on nitrification or denitrification (Stottmeister et al., 2003), while the removal of P mainly depends on the sorption capacity of substrates (Tanner, 1999). In addition, other factors such as macrophytes, temperature, and hydro-pattern (i.e., hydraulic retention time and hydraulic loading rate) also influence the removal of nutrients. Steiner and Combs (1993) and Srinivasan et al. (2000) report that shallow depths of substrate bed (30–50 cm) are beneficial due to better aeration conditions. However, this system inherently generates high oxygen levels that result in significant nitrification but weak denitrification as well
as shorter hydraulic retention times that reduce the efficiency of nutrient removal. The purpose of this study was to investigate the removal efficiency of nutrients from the septic tank effluent by the simulated VFCWs with a substrate depth of 30 cm exposed to different conditions. Specific objectives were to evaluate the removal efficiency of nutrients by: (1) three different substrates; (2) with and without Canna indica; (3) three different hydraulic loading rates; and (4) three different wetland operation periods. 2. Materials and methods Domestic wastewater was obtained from a septic tank on the campus of South China Agricultural University (SCAU), Guangzhou, China. This wastewater was analyzed for initial contents of nutrients, COD, and BOD (Table 1) prior to its application as influent for discharging into the simulated VFCWs. The concentrations of N and P were about 2–3 times higher than the average municipal sewage in the area, while the concentrations of COD and BOD were similar to the average municipal sewage at the same location. Substrates used for the VFCWs, including midsized sand slag (MSAS), coal burn slag (CBAS), and blast furnace slag (BFAS) were made by mixing a certain amounts of coal burn slag from the student cafeteria and of soils and turf from the campus of SCAU. Physical and chemical properties of these three substrates are given in Table 2. The wetland plant species, Canna indica, was selected due to its high tolerance to pore clogging, its large biomass, and high removal of N and P (Li et al., 2008). Fig. 1 is a schematic diagram showing a simulated VFCW system designed for this study. This system was composed of a white ceramic pot with a 30-cm long and a 24.5-cm inner diameter, a glass tube, an effluent sample collector, and a growing Canna indica plant.
Table 2 Physical and chemical properties of three substrates used in this study. MSAS, CBAS, and BFAS were midsized sand artificial slag, coal burn artificial slag, and blast furnace artificial slag, respectively. D10 and D60 are the diameters of particle sizes of a substrate material at which 10% and 60% of the particles pass through the sieves based on the accumulative frequency, and K60 (=D60 /D10 ) is the uniformity coefficient. Parameter −3
Bulk density (g cm ) Specific gravity (mg cm−3 ) Porosity (%) D10 particle (mm) D60 particle (mm) K60 uniformity coefficient pH Organic carbon (g kg−1 ) TN (mg kg−1 ) TP (mg kg−1 ) Fe (mg kg−1 ) Ca (mg kg−1 ) AI (mg kg−1 )
CBAS
BFAS
MSAS
0.82 2.13 61.63 0.10 3.20 32.00 4.74 98.89 21.17 71.36 18,699.56 44,712.50 64,029.99
0.93 2.35 60.10 0.39 1.02 2.62 7.24 18.06 36.08 870.93 5089.11 307,679.17 159,209.17
1.42 2.31 38.5 0.19 0.49 2.58 6.06 12.02 29.22 87.26 5375.67 150,308.34 57,438.33
Fig. 1. A schematic diagram showing a simulated vertical-flow constructed wetland with growing Canna indica plants.
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Table 3 Treatments used for the simulated vertical-flow constructed wetland experiments. MSAS, CBAS, and BFAS were midsized sand artificial slag, coal burn artificial slag, and blast furnace artificial slag, respectively. No.
Substrates
Hydraulic loading rate (cm d−1 )
Vegetation
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
MSAS MSAS MSAS MSAS MSAS MSAS CBAS CBAS CBAS CBAS CBAS CBAS BFAS BFAS BFAS BFAS BFAS BFAS
21 21 14 14 7 7 21 21 14 14 7 7 21 21 14 14 7 7
Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted
There was a circular opening at the bottom of each pot where a glass tube was attached to empty into the effluent collector in order to control the hydraulic loading rate and water level in the VFCW. Eighteen different treatments each with triplicates were used in this study, which resulted in 54 VFCWs. Each pot was first filled with a 5-cm gravel layer and then a 20cm mixed substrate layer, which left about 5-cm free space at the top for irrigating or planting. Table 3 lists the number of treatments, types of substrates, and vegetation status. Among these treatments, the odd numbers were planted, whereas the even numbers were unplanted (control). Wastewater from the septic tank was drained to a regulating pool and was then pumped to an elevated water tank and finally was discharged into the simulated VFCWs. Depending on treatments, three different hydraulic loading rates were used in this study, namely 21, 14, and 7 cm d−1 . The VFCWs were irrigated by a dosing-cup everyday. The VFCWs started to operate in October 2000 and ended in March 2003. Three Canna indica seedlings from a greenhouse on the campus of South China Agricultural University were planted in February 2001 for the planting treatments. Three operating periods were used in this study, namely half year from October 2000 to March 2001 (no planting was done during this period even with the planting treatments), one year from April 2001 to March 2002, and two years from April 2001 to March 2003. The operation started from 8:00 am to 8:00 pm everyday, while sampling and analyzing from the inlet and outlet of the pot was accomplished every 2 weeks. TN was measured by alkaline potassium persulfate oxidationUltra spectrophotometry; NH4 + and NO3 − were measured by distilling acid titration; and TP was measured by potassium persulfate oxidation-molybdenum colorimetry (APHA, 1998). Statistical analysis with Duncan’s method at p = 0.05 was performed to compare differences in nutrient contents among those eighteen separate treatments using statistical software SAS version 8.1. 3. Results and discussion 3.1. Removal of nutrients with substrates Percent removals of nutrients by the three different substrates are presented in Fig. 2A. This figure was constructed using the
Fig. 2. Percent removal of nutrients with different substrates (A), planting styles (B), hydraulic loading rates (C), and operational times (D). These graphs were constructed using the results from the multiple comparison through statistical analysis with Duncan’s method at p = 0.05.
results from the multiple comparison through statistical analysis with Duncan’s method at p = 0.05. The percent removal of TP by the substrates was in the following order: BFAS (88.87%) > CBAS (60.07%) > MSAS (44.66%). Results indicate that BFAS had highest remedial capacity for TP in the VFCWs under varying hydraulic loading rates, wetland operational periods, and vegetation planting styles. This occurred because the substrate BFAS, which was composed of large amounts of Ca and Al (Table 2), had highest sorption capacity for TP. The substrate CBAS, which was composed of large amount of Fe and organic carbon, had moderate sorption capacity for TP, while the substrate MSAS, which was composed of sands and certain amount of Ca, had the lowest sorption capacity for TP. Physically, the porosity of BFAS and CBAS was larger than that of MSAS and therefore had higher sorption capacity for TP than that of MSAS. Although the porosity of BFAS was slightly smaller than that of CBAS, the particle size distribution of BFAS was relatively uniform as compared to that of the CBAS (i.e., larger uniformity coefficient (K60 ) as shown in Table 2). As the wastewater flowed through the CBAS substrate, the smaller particles moved downward and resulted in pore clogging and porosity reduction. As a
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result, the CBAS substrate had lower sorption capacity for TP than that of BFAS. Similar percent removal results were obtained for NH4 + -N as for the case of TP sorption. That is, the BFAS had highest sorption capacity for NH4 + -N among the three substrates. We attributed this phenomenon to the high contents of different forms of Ca and Al in BFAS. In contrast, the percentage removal of TN by the substrates was in the following order: CBAS (29.94%) > MSAS (24.1%) > BFAS (21.6%) and the result is most likely explained by the nitrification and denitrification processes. The NO3 − concentrations in effluent of the VFCWs were in the following order: BFAS (53.3 mg L−1 ) > CBAS (33.5 mg L−1 ) > MSAS (30.9 mg L−1 ). This indicates that BFAS had the lowest denitrification capacity because of the aerobic conditions under high porosity (Table 2). As a result, the BFAS had the lowest percent removal of TN. In general, the percent removal of nutrients by all of the substrates was in the following order: TP > NH4 + > TN (Fig. 2A). This suggests that TP had a higher removal efficiency than the nitrogen species by the substrates, resulting from high adsorption capacity of TP by the substrates. 3.2. Planted vs. unplanted treatments Plant uptake is one of the most important mechanisms for removal of nutrients and other contaminants from wetlands (Peterson and Teal, 1996; Tanner, 1999). In general, the roles of wetland plants are to: (1) uptake nutrients, heavy metals and toxic substances from wastewaters; (2) transfer oxygen to the rhizosphere for the growth, reproduction, and decomposition of microorganisms; and (3) enhance and stabilize the hydraulic transportation of media. Fig. 2B compares the removal of nutrients from influent (wastewater) with and without Canna indica species. In general, more nutrients were removed from the influent with the presence of the plant as compared to without this plant. For example, about 30% of TN in the influent was removed with the presence of the Canna indica plants, whereas only about 20% of TN in the influent was removed without the Canna indica plants. The former was 33% larger than the latter. This occurred primarily because of the uptake of TN from the influent by the roots. Our finding was within the range of those reported by Klomjek and Nitisoravut (2005). This author found that the nitrogen uptake rates for the wetland plants such as C. corymbosus, D. bicornis, L. fusca, B. mutica and S. patents were about 77.0%, 47.5%, 31.1%, 16.4%, and 9.8%, respectively. The NO3 − concentrations in effluent of the VFCWs for the planted and unplanted treatments were 37.71 and 40.78 mg L−1 , respectively. Results demonstrated that more NO3 − was removed with the planted treatment. 3.3. Impacts of HRT Hydraulic residential or retention time (HRT) is a measure of the average length of time that a soluble compound remains in a constructed wetland. The HRT plays an important role in the removal of nutrients by determining the time interval for interactions between the nutrients and the roots as well as by allowing times for the bacteria to flourish and transform the N. Fig. 2C compares the percentage removal of nutrients from the influent by the three different hydraulic loading rates. Normally, the higher the hydraulic loading rate is, the shorter the HRT will be. Overall, a lower hydraulic loading rate (or longer HRT) resulted in a higher removal of TP (Fig. 2C). For example, the percentage removal of TP was 71% at the hydraulic loading rate of 7 cm d−1 but was 61% at the hydraulic loading rate of 21 cm d−1 . A three-fold increase in
hydraulic loading rate resulted in 10% decreased in TP removal. This occurred because a longer HRT resulted in more contacts and interactions of TP in the influent with the substrates and roots, which in turn promoted more adsorption, transformation, and uptake of nutrient in the pot. Similar results were obtained for NH4 + and TN at low hydraulic loading rate. That is, more NH4 + and TN were removed from the influent at a low hydraulic loading rate due to the same reasons as explained above. Results suggested that impacts of HRT on removal of nutrients from the simulated VFCWs were profound. Removal of NO3 − from the simulated VFCWs is a complex process. Denitrification of NO3 − is primarily a microorganism reduction process and is dependent on anaerobic conditions and the availability of a carbon source rather than the HRT. The NO3 − concentrations in effluent of the VFCWs were 42.7, 35.3, and 39.8 mg L−1 , respectively, for high, moderate, and low hydraulic loading rates. A high concentration of NO3 − in the effluent under the high hydraulic loading rate occurred because more NH4 + and oxygen were available for nitrification. The shorter HRT and lack of anaerobic conditions made the treatments unfavorable for denitrification. A low concentration of NO3 − in the effluent under the moderate hydraulic loading rate took place presumably when the relative low NH4 + was available for nitrification and then suitable anaerobic conditions occurred for denitrification. 3.4. Impact of operational period Percent removals of nutrients for the three different operational periods were presented in Fig. 2D. The percent removal of NH4 + by the operational periods was in the following order: two year > one year > half year. Similar removal orders were obtained for TN. Results indicated that a longer operational period had a highest removal rate for nitrogen species in the VFCWs under varying hydraulic loading rates, substrates, and vegetation planting styles. This occurred because a longer operational period had a longer interaction times for nutrients and bacteria in the VFCWs. The NO3 − concentrations in effluent were 18.6, 46.5, and 42.1 mg L−1 , respectively, for half-, one-, and two-year operational periods. A low concentration of NO3 − in the effluent for the first half-year occurred because of the low number of the nitrification bacteria and low temperature during the winter at that operational period. The high concentrations of NO3 − in the effluent in oneand two-year operational periods happened due to more nitrification bacteria available for nitrification after the relatively long operations. 3.5. Mass balance Mass balance estimations of TN and TP contents for eighteen different treatments are given in Tables 4 and 5. These percentage errors were calculated as: Error % =
[(Total input of TN or TP)−(Total loss of TN or TP)] ×100 (Total loss of TN or TP) (1)
As shown in Table 4, the percentage errors of TN contents for all of the eighteen different treatments were in a range from −9.57 to 11.28%. These errors existed partially because of the experimental errors and partially because of the ammonium volatilization during the course of the experiments. Nonetheless, with about ±11% of mass balance errors, we concluded that our experimental results from this study were fairly reasonable. In contrast, the percent errors of TP contents for all of the eighteen different treatments were in a range from −6.85 to 6.47%
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Table 4 Mass balance estimation of TN for different treatments. CK denotes the control treatment. Treatment
Influent volume (m3 )
Background substrate TN (g)
Influent TN (g)
Total TN input (g)
Effluent TN (g)
Denitrification TN loss (g)
Uptake TN (g)
TN sorption by substrate (g)
Total TN loss (g)
SAS-H SAS-H-CK SAS-M SAS-M-CK SAS-L SAS-L-CK CAS-H CAS-H-CK CAS-M CAS-M-CK CAS-L CAS-L-CK BAS-H BAS-H-CK BAS-M BAS-M-CK BAS-L BAS-L-CK
5.168 5.434 3.723 3.777 2.063 2.068 4.830 5.340 3.781 3.823 2.053 2.069 5.225 5.282 3.811 3.828 2.086 2.073
0.555 0.555 0.555 0.555 0.555 0.555 0.264 0.264 0.264 0.264 0.264 0.264 0.147 0.147 0.147 0.147 0.147 0.147
520.1 547.1 371.5 375.1 205.7 206.5 494.4 535.7 380.3 382.3 204.4 206.1 528.3 533.1 379.2 380.6 208.6 206.9
520.66 547.66 372.06 375.66 206.26 207.06 494.66 535.96 380.56 382.56 204.66 206.36 528.45 533.25 379.35 380.75 208.75 207.05
406.3 476.2 278.8 267.6 123 167.5 339 455.1 251.9 285 115 143.8 373.3 492.9 269.4 324.1 158.5 150.6
65.13 51.92 68.38 52.00 42.74 32.25 67.58 56.15 76.89 58.53 46.97 35.58 42.18 32.36 44.72 33.72 27.65 24.69
39.7
13.62 10.49 15.42 17.64 11.40 6.03 30.84 27.17 26.73 31.09 24.78 25.55 23.60 14.83 28.02 18.15 7.84 14.31
524.75 538.61 399.2 337.24 212.27 205.78 481.55 538.42 395.85 374.62 222.73 204.93 474.90 540.09 381.2 375.97 230.83 189.60
−0.779 1.68 −6.80 11.39 −2.83 0.622 2.72 −0.457 −3.86 2.12 −8.11 0.698 11.28 −1.27 −0.485 1.27 −9.57 9.20
Error (%)
36.6 35.13 44.13 40.33 35.98 35.82 39.06 36.84
Error (%)
Table 5 Mass balance estimation of TP for different treatments. CK denotes the control treatment. Treatment
Influent volume (m3 )
Background TP (g)
Influent TP (g)
Total TP input (g)
Effluent TP (g)
Uptake TP (g)
TP sorption by substrate (g)
Total TP loss (g)
SAS-H SAS-H-CK SAS-M SAS-M-CK SAS-L SAS-L-CK CAS-H CAS-H-CK CAS-M CAS-M-CK CAS-L CAS-L-CK BAS-H BAS-H-CK BAS-M BAS-M-CK BAS-L BAS-L-CK
5.168 5.434 3.723 3.777 2.063 2.068 4.830 5.340 3.781 3.823 2.053 2.069 5.225 5.282 3.811 3.828 2.086 2.073
1.657 1.657 1.657 1.657 1.657 1.657 5.472 5.472 5.472 5.472 5.472 5.472 0.494 0.494 0.494 0.494 0.494 0.494
43.04 45.24 30.82 31.2 17.07 17.13 40.58 44.37 31.46 31.7 16.98 17.12 43.62 44.05 31.5 31.63 17.29 17.17
44.7 46.9 32.48 32.86 18.73 18.79 46.05 49.84 36.93 37.17 22.45 22.59 44.11 44.54 31.99 32.12 17.78 17.66
26.14 33.97 19.05 24.28 8.28 10.85 20.45 25.69 15.93 17.5 5.14 7.92 6.65 6.73 4.55 4.63 1.18 1.41
2.90
13.76 12.60 8.59 8.53 7.18 7.26 16.93 20.13 14.75 15.55 10.27 10.35 34.05 34.47 24.26 25.09 14.25 16.18
42.80 46.57 30.82 32.81 18.24 18.11 40.36 45.82 33.41 33.05 17.39 18.27 44.31 41.2 31.64 29.72 17.77 17.59
(Table 5), which was much lower than that of TN. Unlike the case of TN, there was no volatilization loss of TP during the VFCW experiments.
4. Conclusions The percent removal of TP by the substrates was BFAS (88.87%) > CBAS (60.07%) > MSAS (44.66%). Results indicated that BFAS had the highest remedial capacity for TP in the VFCWs. In contrast, the percent removal of TN by the substrates was CBAS (29.94%) > MSAS (24.1%) > BFAS (21.6%) and the result is likely best explained by the nitrification and denitrification processes. In general, the percent removal of nutrients by all of the substrates was in the following order: TP > NH4 + > TN. This suggests the VFCWs with the slag substrates removed TP more efficiently than the nitrogen species. More nutrients were removed from the influent with the presence of the Canna indica plant compared to without it. Our finding was within the range reported by Klomjek and Nitisoravut (2005). Overall, a lower hydraulic loading rate resulted in a higher removal of TP, NH4 + , and TN. Results suggested that impacts of
3.18 2.78 2.98 2.73 1.98 3.61 2.83 2.34
0.558 −2.94 0.00 −5.16 −6.85 −5.72 0.542 −3.27 −6.20 −4.26 −2.42 −6.72 −1.58 6.47 −0.445 6.04 −2.78 −2.45
HRT on removal of nutrients from the simulated VFCWs were profound. The percent removal of TN, NH4 + , and TP by the operational periods was two year > one year > half year. Results indicated that a longer operational period had a highest removal rate for nutrients in the VFCWs under varying hydraulic loading rates, substrates, and vegetation planting styles. The percent errors of TN contents for all of the eighteen treatments were about ±11% partially because of the experimental errors and partially because of the ammonium volatilization during the course of the experiments. In contrast, the percent errors of TP contents for all of the eighteen different treatments were only about ±6% because of no volatilization loss of TP during the VFCW experiments.
Acknowledgements The study was supported by the Natural Science Foundation of China (No. 40571074; No. 40871110; No. 30828005), the Key Project of Science and Technology of China (No. 2007BAD89B14), the Key Project of Science and Technology of Guangzhou city (No.
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Kadlec, R.H., Wallace, S.D., 2009. Treatment Wetlands, Second Edition. CRC Press, New York. Klomjek, P., Nitisoravut, S., 2005. Constructed treatment wetland: a study of eight plant species under saline conditions. Chemosphere 58, 585–593. Langergraber, G., Leroch, K., Pressl, A., Rohrhofer, R., Haberl, R., 2008. A two-stage subsurface vertical flow constructed wetland for high-rate nitrogen removal. Water Sci. Technol. 57, 1881–1887. Li, Z.P., Chen, J., Zhou, D., Bai, Y.L., 2008. Study on different plants growth and removal loading in artificial wetland load wipe out. J. Anhui Agric. Sci. 36, 2906–2907 (in Chinese). Obarska, H., Klimkouska, K., 1999. Distribution of nutrients and heavy metals in a constructed wetland system. Chemosphere 39, 303–312. Peterson, S.B., Teal, J.M., 1996. The role of plants in ecologically engineered wastewater treatment systems. Ecol. Eng. 6, 137–148. Robins, J.P., Rock, J., Hayes, D.F., Laquer, F.C., 2000. Nitrate removal for Platte Valley, Nebraska synthetic groundwater using a constructed wetland model. Environ. Technol. 21, 653–659. Steiner, G.R., Combs, D.W., 1993. Small constructed wetlands systems for domestic wastewater treatment and their performance. In: Moshiri, G.A. (Ed.), Constructed Wetlands for Water Quality Improvement. Lewis Publishers, Boca Raton, FL, pp. 491–498. Stottmeister, U., Wiener, A., Kuschk, P., Kappelmeyer, U., Kästner, M., Bederski, O., Müller, R.A., Moormann, H., 2003. Effects of plants and microorganisms in constructed wetlands for wastewater treatment. Biotechnol. Adv. 22, 93–117. Srinivasan, N., Richard, W.W., Bruce, J.L., Russell, A.P., 2000. Improvement of domestic wastewater quality by subsurface flow constructed wetlands. Bioresour. Technol. 75, 19–25. Tang, S., 1993. Experimental study of a constructed wetland for treatment of acidic wastewater from an iron mine in China. Ecol. Eng. 2, 253–259. Tanner, C.C., 1999. Plants for constructed wetland treatment systems—a comparison of the growth and nutrient uptake of eight emergent species. Ecol. Eng. 7, 59–83. Van Oostrom, A.J., Russel, J.M., 1994. Denitrification in constructed wastewater wetlands receiving high concentrations of nitrate. Water Sci. Technol. 29, 7–14. Vyzamal, J., 2005. Constructed wetlands for wastewater treatment. Ecol. Eng. 5, 475–477.
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Removal of nutrients from wastewater with Canna indica L. under different vertical-flow constructed wetland conditions Lihua Cui a,∗ , Ying Ouyang b,∗ , Qian Lou a , Fengle Yang c , Ying Chen a , Wenling Zhu a , Shiming Luo d a
Department of Environment Science and Engineering, South China Agricultural University Guangzhou 510642, China Department of Water Resources, St. Johns River Water Management District, PO Box 1429, Palatka, FL 32178, USA c Yunnan Institute of Environmental Science, Wang Jiaba, Kunming 650034, China d Ecological Agriculture Key Laboratory of the Ministry of Agriculture, China, South China Agricultural University, Guangzhou 510642, China b
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 10 March 2010 Accepted 22 April 2010
Keywords: Hydraulic loading rate Substrate Wetland operational period
a b s t r a c t Constructed wetlands are becoming increasingly popular worldwide for removing contaminants from domestic wastewater. This study investigated the removal efficiency of nitrogen (N) and phosphorus (P) from wastewater with the simulated vertical-flow constructed wetlands (VFCWs) under three different substrates (i.e., BFAS or blast furnace artificial slag, CBAS or coal burn artificial slag, and MSAS or midsized sand artificial slag), hydraulic loading rates (i.e., 7, 14, and 21 cm d−1 ), and wetland operational periods (0.5, 1, and 2 years) as well as with and without planting Canna indica L. The wastewater was collected from the campus of South China Agricultural University, Guangzhou, China. Results show that the percent removal of total P (TP) and ammonium N (NH4 + -N) by the substrates was BFAS > CBAS > MSAS due to the high contents of Ca and Al in substrate BFAS. In contrast, the percent removal of total N (TN) by the substrates was CBAS > MSAS > BFAS due to the complicated nitrification/denitrification processes. The percent removal of nutrients by all of the substrates was TP > NH4 + -N > TN. About 10% more TN was removed from the wastewater after planting Canna indica L. A lower hydraulic loading rate or longer hydraulic retention time (HRT) resulted in a higher removal of TP, NH4 + -N, and TN because of more contacts and interactions among nutrients, substrates, and roots under the longer HRT. Removal of NO3 − N from the simulated VFCWs is a complex process. A high concentration of NO3 − N in the effluent was observed under the high hydraulic loading rate because more NH4 + -N and oxygen were available for nitrification and a shorter HRT was unfavorable for denitrification. In general, a longer operational period had a highest removal rate for nutrients in the VFCWs. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Constructed wetlands (CW) are engineering structures used worldwide to improve water quality. They involve a complex mixture of waters, substrates, plants, and microorganisms to produce optimal conditions for removing organic matter, nutrients, trace elements, pathogens, and other pollutants from wastewater and/or runoff water (Tang, 1993; Crites et al., 1997; Obarska and Klimkouska, 1999; Babatunde et al., 2008; Langergraber et al., 2008). With increasingly gaining acceptance, CWs are now used for many other types of wastewater treatments, including industrial and agricultural wastewaters, landfill leachate, and stormwater runoff (Vyzamal, 2005). Presently, there are primarily four types of CWs: (1) free water surface wetlands, (2) horizontal subsurface
∗ Corresponding authors. E-mail addresses: [email protected] (L. Cui), ouyangy@ufl.edu (Y. Ouyang). 0925-8574/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2010.04.026
flow wetlands, (3) vertical-flow wetlands, and (4) hybrid systems that incorporate surface and subsurface flow wetlands (Cui et al., 2009; Kadlec and Wallace, 2009). Of which, the vertical-flow constructed wetlands (VFCWs) are gaining popularity due to their greater oxygen transfer capacity and smaller size (Cooper, 1999) although pore clogging could be a problem. Many researchers report that CWs are viable alternatives for nitrogen (N) and phosphorus (P) removal from wastewater due to the functions of denitrification, uptake, and sorption that take place (Van Oostrom and Russel, 1994; Horne, 1995; Kadlec, 1997; Baker, 1998; Robins et al., 2000). A wide range of nutrient removal efficiencies with CWs has been reported but many of them fail to meet the relevant government standards, especially for total nitrogen (TN) and total phosphorus (TP) concentrations from discharged wastewater (Fraser et al., 2004; Greenway, 2005). Variations in nutrient removal efficiency among those studies are attributed to different wetland plants, substrates, and hydraulic loading rates used for different experiments.
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Table 1 Water quality in the septic tank used in this study. Parameter
The first year (2001.3–2002.3) Sample No.
COD BOD NH4 + TN TP a
12 12 12 11 12
The second year(2002.4–2003.4) −1
a
Range
Mean concentration (mg L
107–316 35–114 39.73–170.82 25.80–211.28 3.30–17.77
218 69 101.29 114.72 8.47
± ± ± ± ±
20 7 11.70 18.55 1.21
)
Sample No. 13 12 15 16 16
Range 93–277 42–139 30.08–192.34 21.22–181.30 3.23–13.25
Mean concentration (mg L−1 ) 171 76 83.82 87.97 7.28
± ± ± ± ±
16 8 10.77 10.14 0.63
The numbers after ± are standard deviations.
Wetland plants are an integral part of the CW treatment systems. They play important roles in degrading and removing nutrients and other pollutants. Wetland plants include freefloating plants, emergent plants, and submerged plants. The principle in selecting a suitable plant species for use in constructed wetland systems depends on the type of wetland design (e.g., surface or subsurface, vertical or horizontal flow), the mode of operation (e.g., continuous, batch or intermittent flow), and the loading rate and characteristics of wastewaters. Wetland plants not only take up nutrients, heavy metals and organics, but also control the ventilation and microbial conditions in the constructed wetland bed. In addition, plants can effectively filter or settle out suspended solids. Therefore, wetland plants have great influence on the purification capacity of wastewaters (Groudeva et al., 2001; Fu and Tang, 2005). An increased demand on N removal from wastewaters in 1990s has lead to more frequent uses of VFCWs that provide higher degree of filtration bed oxygenation and consequent removal of ammonia via nitrification (Vyzamal, 2005). The VFCWs have high hydraulic loading rates, less land area requirement and high oxygen transfer, and are good for nitrification because of their high oxygen transfer capability that also leads to good removal of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). The removal of N from domestic sewage in constructed wetlands mainly depends on nitrification or denitrification (Stottmeister et al., 2003), while the removal of P mainly depends on the sorption capacity of substrates (Tanner, 1999). In addition, other factors such as macrophytes, temperature, and hydro-pattern (i.e., hydraulic retention time and hydraulic loading rate) also influence the removal of nutrients. Steiner and Combs (1993) and Srinivasan et al. (2000) report that shallow depths of substrate bed (30–50 cm) are beneficial due to better aeration conditions. However, this system inherently generates high oxygen levels that result in significant nitrification but weak denitrification as well
as shorter hydraulic retention times that reduce the efficiency of nutrient removal. The purpose of this study was to investigate the removal efficiency of nutrients from the septic tank effluent by the simulated VFCWs with a substrate depth of 30 cm exposed to different conditions. Specific objectives were to evaluate the removal efficiency of nutrients by: (1) three different substrates; (2) with and without Canna indica; (3) three different hydraulic loading rates; and (4) three different wetland operation periods. 2. Materials and methods Domestic wastewater was obtained from a septic tank on the campus of South China Agricultural University (SCAU), Guangzhou, China. This wastewater was analyzed for initial contents of nutrients, COD, and BOD (Table 1) prior to its application as influent for discharging into the simulated VFCWs. The concentrations of N and P were about 2–3 times higher than the average municipal sewage in the area, while the concentrations of COD and BOD were similar to the average municipal sewage at the same location. Substrates used for the VFCWs, including midsized sand slag (MSAS), coal burn slag (CBAS), and blast furnace slag (BFAS) were made by mixing a certain amounts of coal burn slag from the student cafeteria and of soils and turf from the campus of SCAU. Physical and chemical properties of these three substrates are given in Table 2. The wetland plant species, Canna indica, was selected due to its high tolerance to pore clogging, its large biomass, and high removal of N and P (Li et al., 2008). Fig. 1 is a schematic diagram showing a simulated VFCW system designed for this study. This system was composed of a white ceramic pot with a 30-cm long and a 24.5-cm inner diameter, a glass tube, an effluent sample collector, and a growing Canna indica plant.
Table 2 Physical and chemical properties of three substrates used in this study. MSAS, CBAS, and BFAS were midsized sand artificial slag, coal burn artificial slag, and blast furnace artificial slag, respectively. D10 and D60 are the diameters of particle sizes of a substrate material at which 10% and 60% of the particles pass through the sieves based on the accumulative frequency, and K60 (=D60 /D10 ) is the uniformity coefficient. Parameter −3
Bulk density (g cm ) Specific gravity (mg cm−3 ) Porosity (%) D10 particle (mm) D60 particle (mm) K60 uniformity coefficient pH Organic carbon (g kg−1 ) TN (mg kg−1 ) TP (mg kg−1 ) Fe (mg kg−1 ) Ca (mg kg−1 ) AI (mg kg−1 )
CBAS
BFAS
MSAS
0.82 2.13 61.63 0.10 3.20 32.00 4.74 98.89 21.17 71.36 18,699.56 44,712.50 64,029.99
0.93 2.35 60.10 0.39 1.02 2.62 7.24 18.06 36.08 870.93 5089.11 307,679.17 159,209.17
1.42 2.31 38.5 0.19 0.49 2.58 6.06 12.02 29.22 87.26 5375.67 150,308.34 57,438.33
Fig. 1. A schematic diagram showing a simulated vertical-flow constructed wetland with growing Canna indica plants.
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Table 3 Treatments used for the simulated vertical-flow constructed wetland experiments. MSAS, CBAS, and BFAS were midsized sand artificial slag, coal burn artificial slag, and blast furnace artificial slag, respectively. No.
Substrates
Hydraulic loading rate (cm d−1 )
Vegetation
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
MSAS MSAS MSAS MSAS MSAS MSAS CBAS CBAS CBAS CBAS CBAS CBAS BFAS BFAS BFAS BFAS BFAS BFAS
21 21 14 14 7 7 21 21 14 14 7 7 21 21 14 14 7 7
Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted Planted Unplanted
There was a circular opening at the bottom of each pot where a glass tube was attached to empty into the effluent collector in order to control the hydraulic loading rate and water level in the VFCW. Eighteen different treatments each with triplicates were used in this study, which resulted in 54 VFCWs. Each pot was first filled with a 5-cm gravel layer and then a 20cm mixed substrate layer, which left about 5-cm free space at the top for irrigating or planting. Table 3 lists the number of treatments, types of substrates, and vegetation status. Among these treatments, the odd numbers were planted, whereas the even numbers were unplanted (control). Wastewater from the septic tank was drained to a regulating pool and was then pumped to an elevated water tank and finally was discharged into the simulated VFCWs. Depending on treatments, three different hydraulic loading rates were used in this study, namely 21, 14, and 7 cm d−1 . The VFCWs were irrigated by a dosing-cup everyday. The VFCWs started to operate in October 2000 and ended in March 2003. Three Canna indica seedlings from a greenhouse on the campus of South China Agricultural University were planted in February 2001 for the planting treatments. Three operating periods were used in this study, namely half year from October 2000 to March 2001 (no planting was done during this period even with the planting treatments), one year from April 2001 to March 2002, and two years from April 2001 to March 2003. The operation started from 8:00 am to 8:00 pm everyday, while sampling and analyzing from the inlet and outlet of the pot was accomplished every 2 weeks. TN was measured by alkaline potassium persulfate oxidationUltra spectrophotometry; NH4 + and NO3 − were measured by distilling acid titration; and TP was measured by potassium persulfate oxidation-molybdenum colorimetry (APHA, 1998). Statistical analysis with Duncan’s method at p = 0.05 was performed to compare differences in nutrient contents among those eighteen separate treatments using statistical software SAS version 8.1. 3. Results and discussion 3.1. Removal of nutrients with substrates Percent removals of nutrients by the three different substrates are presented in Fig. 2A. This figure was constructed using the
Fig. 2. Percent removal of nutrients with different substrates (A), planting styles (B), hydraulic loading rates (C), and operational times (D). These graphs were constructed using the results from the multiple comparison through statistical analysis with Duncan’s method at p = 0.05.
results from the multiple comparison through statistical analysis with Duncan’s method at p = 0.05. The percent removal of TP by the substrates was in the following order: BFAS (88.87%) > CBAS (60.07%) > MSAS (44.66%). Results indicate that BFAS had highest remedial capacity for TP in the VFCWs under varying hydraulic loading rates, wetland operational periods, and vegetation planting styles. This occurred because the substrate BFAS, which was composed of large amounts of Ca and Al (Table 2), had highest sorption capacity for TP. The substrate CBAS, which was composed of large amount of Fe and organic carbon, had moderate sorption capacity for TP, while the substrate MSAS, which was composed of sands and certain amount of Ca, had the lowest sorption capacity for TP. Physically, the porosity of BFAS and CBAS was larger than that of MSAS and therefore had higher sorption capacity for TP than that of MSAS. Although the porosity of BFAS was slightly smaller than that of CBAS, the particle size distribution of BFAS was relatively uniform as compared to that of the CBAS (i.e., larger uniformity coefficient (K60 ) as shown in Table 2). As the wastewater flowed through the CBAS substrate, the smaller particles moved downward and resulted in pore clogging and porosity reduction. As a
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result, the CBAS substrate had lower sorption capacity for TP than that of BFAS. Similar percent removal results were obtained for NH4 + -N as for the case of TP sorption. That is, the BFAS had highest sorption capacity for NH4 + -N among the three substrates. We attributed this phenomenon to the high contents of different forms of Ca and Al in BFAS. In contrast, the percentage removal of TN by the substrates was in the following order: CBAS (29.94%) > MSAS (24.1%) > BFAS (21.6%) and the result is most likely explained by the nitrification and denitrification processes. The NO3 − concentrations in effluent of the VFCWs were in the following order: BFAS (53.3 mg L−1 ) > CBAS (33.5 mg L−1 ) > MSAS (30.9 mg L−1 ). This indicates that BFAS had the lowest denitrification capacity because of the aerobic conditions under high porosity (Table 2). As a result, the BFAS had the lowest percent removal of TN. In general, the percent removal of nutrients by all of the substrates was in the following order: TP > NH4 + > TN (Fig. 2A). This suggests that TP had a higher removal efficiency than the nitrogen species by the substrates, resulting from high adsorption capacity of TP by the substrates. 3.2. Planted vs. unplanted treatments Plant uptake is one of the most important mechanisms for removal of nutrients and other contaminants from wetlands (Peterson and Teal, 1996; Tanner, 1999). In general, the roles of wetland plants are to: (1) uptake nutrients, heavy metals and toxic substances from wastewaters; (2) transfer oxygen to the rhizosphere for the growth, reproduction, and decomposition of microorganisms; and (3) enhance and stabilize the hydraulic transportation of media. Fig. 2B compares the removal of nutrients from influent (wastewater) with and without Canna indica species. In general, more nutrients were removed from the influent with the presence of the plant as compared to without this plant. For example, about 30% of TN in the influent was removed with the presence of the Canna indica plants, whereas only about 20% of TN in the influent was removed without the Canna indica plants. The former was 33% larger than the latter. This occurred primarily because of the uptake of TN from the influent by the roots. Our finding was within the range of those reported by Klomjek and Nitisoravut (2005). This author found that the nitrogen uptake rates for the wetland plants such as C. corymbosus, D. bicornis, L. fusca, B. mutica and S. patents were about 77.0%, 47.5%, 31.1%, 16.4%, and 9.8%, respectively. The NO3 − concentrations in effluent of the VFCWs for the planted and unplanted treatments were 37.71 and 40.78 mg L−1 , respectively. Results demonstrated that more NO3 − was removed with the planted treatment. 3.3. Impacts of HRT Hydraulic residential or retention time (HRT) is a measure of the average length of time that a soluble compound remains in a constructed wetland. The HRT plays an important role in the removal of nutrients by determining the time interval for interactions between the nutrients and the roots as well as by allowing times for the bacteria to flourish and transform the N. Fig. 2C compares the percentage removal of nutrients from the influent by the three different hydraulic loading rates. Normally, the higher the hydraulic loading rate is, the shorter the HRT will be. Overall, a lower hydraulic loading rate (or longer HRT) resulted in a higher removal of TP (Fig. 2C). For example, the percentage removal of TP was 71% at the hydraulic loading rate of 7 cm d−1 but was 61% at the hydraulic loading rate of 21 cm d−1 . A three-fold increase in
hydraulic loading rate resulted in 10% decreased in TP removal. This occurred because a longer HRT resulted in more contacts and interactions of TP in the influent with the substrates and roots, which in turn promoted more adsorption, transformation, and uptake of nutrient in the pot. Similar results were obtained for NH4 + and TN at low hydraulic loading rate. That is, more NH4 + and TN were removed from the influent at a low hydraulic loading rate due to the same reasons as explained above. Results suggested that impacts of HRT on removal of nutrients from the simulated VFCWs were profound. Removal of NO3 − from the simulated VFCWs is a complex process. Denitrification of NO3 − is primarily a microorganism reduction process and is dependent on anaerobic conditions and the availability of a carbon source rather than the HRT. The NO3 − concentrations in effluent of the VFCWs were 42.7, 35.3, and 39.8 mg L−1 , respectively, for high, moderate, and low hydraulic loading rates. A high concentration of NO3 − in the effluent under the high hydraulic loading rate occurred because more NH4 + and oxygen were available for nitrification. The shorter HRT and lack of anaerobic conditions made the treatments unfavorable for denitrification. A low concentration of NO3 − in the effluent under the moderate hydraulic loading rate took place presumably when the relative low NH4 + was available for nitrification and then suitable anaerobic conditions occurred for denitrification. 3.4. Impact of operational period Percent removals of nutrients for the three different operational periods were presented in Fig. 2D. The percent removal of NH4 + by the operational periods was in the following order: two year > one year > half year. Similar removal orders were obtained for TN. Results indicated that a longer operational period had a highest removal rate for nitrogen species in the VFCWs under varying hydraulic loading rates, substrates, and vegetation planting styles. This occurred because a longer operational period had a longer interaction times for nutrients and bacteria in the VFCWs. The NO3 − concentrations in effluent were 18.6, 46.5, and 42.1 mg L−1 , respectively, for half-, one-, and two-year operational periods. A low concentration of NO3 − in the effluent for the first half-year occurred because of the low number of the nitrification bacteria and low temperature during the winter at that operational period. The high concentrations of NO3 − in the effluent in oneand two-year operational periods happened due to more nitrification bacteria available for nitrification after the relatively long operations. 3.5. Mass balance Mass balance estimations of TN and TP contents for eighteen different treatments are given in Tables 4 and 5. These percentage errors were calculated as: Error % =
[(Total input of TN or TP)−(Total loss of TN or TP)] ×100 (Total loss of TN or TP) (1)
As shown in Table 4, the percentage errors of TN contents for all of the eighteen different treatments were in a range from −9.57 to 11.28%. These errors existed partially because of the experimental errors and partially because of the ammonium volatilization during the course of the experiments. Nonetheless, with about ±11% of mass balance errors, we concluded that our experimental results from this study were fairly reasonable. In contrast, the percent errors of TP contents for all of the eighteen different treatments were in a range from −6.85 to 6.47%
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Table 4 Mass balance estimation of TN for different treatments. CK denotes the control treatment. Treatment
Influent volume (m3 )
Background substrate TN (g)
Influent TN (g)
Total TN input (g)
Effluent TN (g)
Denitrification TN loss (g)
Uptake TN (g)
TN sorption by substrate (g)
Total TN loss (g)
SAS-H SAS-H-CK SAS-M SAS-M-CK SAS-L SAS-L-CK CAS-H CAS-H-CK CAS-M CAS-M-CK CAS-L CAS-L-CK BAS-H BAS-H-CK BAS-M BAS-M-CK BAS-L BAS-L-CK
5.168 5.434 3.723 3.777 2.063 2.068 4.830 5.340 3.781 3.823 2.053 2.069 5.225 5.282 3.811 3.828 2.086 2.073
0.555 0.555 0.555 0.555 0.555 0.555 0.264 0.264 0.264 0.264 0.264 0.264 0.147 0.147 0.147 0.147 0.147 0.147
520.1 547.1 371.5 375.1 205.7 206.5 494.4 535.7 380.3 382.3 204.4 206.1 528.3 533.1 379.2 380.6 208.6 206.9
520.66 547.66 372.06 375.66 206.26 207.06 494.66 535.96 380.56 382.56 204.66 206.36 528.45 533.25 379.35 380.75 208.75 207.05
406.3 476.2 278.8 267.6 123 167.5 339 455.1 251.9 285 115 143.8 373.3 492.9 269.4 324.1 158.5 150.6
65.13 51.92 68.38 52.00 42.74 32.25 67.58 56.15 76.89 58.53 46.97 35.58 42.18 32.36 44.72 33.72 27.65 24.69
39.7
13.62 10.49 15.42 17.64 11.40 6.03 30.84 27.17 26.73 31.09 24.78 25.55 23.60 14.83 28.02 18.15 7.84 14.31
524.75 538.61 399.2 337.24 212.27 205.78 481.55 538.42 395.85 374.62 222.73 204.93 474.90 540.09 381.2 375.97 230.83 189.60
−0.779 1.68 −6.80 11.39 −2.83 0.622 2.72 −0.457 −3.86 2.12 −8.11 0.698 11.28 −1.27 −0.485 1.27 −9.57 9.20
Error (%)
36.6 35.13 44.13 40.33 35.98 35.82 39.06 36.84
Error (%)
Table 5 Mass balance estimation of TP for different treatments. CK denotes the control treatment. Treatment
Influent volume (m3 )
Background TP (g)
Influent TP (g)
Total TP input (g)
Effluent TP (g)
Uptake TP (g)
TP sorption by substrate (g)
Total TP loss (g)
SAS-H SAS-H-CK SAS-M SAS-M-CK SAS-L SAS-L-CK CAS-H CAS-H-CK CAS-M CAS-M-CK CAS-L CAS-L-CK BAS-H BAS-H-CK BAS-M BAS-M-CK BAS-L BAS-L-CK
5.168 5.434 3.723 3.777 2.063 2.068 4.830 5.340 3.781 3.823 2.053 2.069 5.225 5.282 3.811 3.828 2.086 2.073
1.657 1.657 1.657 1.657 1.657 1.657 5.472 5.472 5.472 5.472 5.472 5.472 0.494 0.494 0.494 0.494 0.494 0.494
43.04 45.24 30.82 31.2 17.07 17.13 40.58 44.37 31.46 31.7 16.98 17.12 43.62 44.05 31.5 31.63 17.29 17.17
44.7 46.9 32.48 32.86 18.73 18.79 46.05 49.84 36.93 37.17 22.45 22.59 44.11 44.54 31.99 32.12 17.78 17.66
26.14 33.97 19.05 24.28 8.28 10.85 20.45 25.69 15.93 17.5 5.14 7.92 6.65 6.73 4.55 4.63 1.18 1.41
2.90
13.76 12.60 8.59 8.53 7.18 7.26 16.93 20.13 14.75 15.55 10.27 10.35 34.05 34.47 24.26 25.09 14.25 16.18
42.80 46.57 30.82 32.81 18.24 18.11 40.36 45.82 33.41 33.05 17.39 18.27 44.31 41.2 31.64 29.72 17.77 17.59
(Table 5), which was much lower than that of TN. Unlike the case of TN, there was no volatilization loss of TP during the VFCW experiments.
4. Conclusions The percent removal of TP by the substrates was BFAS (88.87%) > CBAS (60.07%) > MSAS (44.66%). Results indicated that BFAS had the highest remedial capacity for TP in the VFCWs. In contrast, the percent removal of TN by the substrates was CBAS (29.94%) > MSAS (24.1%) > BFAS (21.6%) and the result is likely best explained by the nitrification and denitrification processes. In general, the percent removal of nutrients by all of the substrates was in the following order: TP > NH4 + > TN. This suggests the VFCWs with the slag substrates removed TP more efficiently than the nitrogen species. More nutrients were removed from the influent with the presence of the Canna indica plant compared to without it. Our finding was within the range reported by Klomjek and Nitisoravut (2005). Overall, a lower hydraulic loading rate resulted in a higher removal of TP, NH4 + , and TN. Results suggested that impacts of
3.18 2.78 2.98 2.73 1.98 3.61 2.83 2.34
0.558 −2.94 0.00 −5.16 −6.85 −5.72 0.542 −3.27 −6.20 −4.26 −2.42 −6.72 −1.58 6.47 −0.445 6.04 −2.78 −2.45
HRT on removal of nutrients from the simulated VFCWs were profound. The percent removal of TN, NH4 + , and TP by the operational periods was two year > one year > half year. Results indicated that a longer operational period had a highest removal rate for nutrients in the VFCWs under varying hydraulic loading rates, substrates, and vegetation planting styles. The percent errors of TN contents for all of the eighteen treatments were about ±11% partially because of the experimental errors and partially because of the ammonium volatilization during the course of the experiments. In contrast, the percent errors of TP contents for all of the eighteen different treatments were only about ±6% because of no volatilization loss of TP during the VFCW experiments.
Acknowledgements The study was supported by the Natural Science Foundation of China (No. 40571074; No. 40871110; No. 30828005), the Key Project of Science and Technology of China (No. 2007BAD89B14), the Key Project of Science and Technology of Guangzhou city (No.
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