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Phosphorus removal from agricultural runoff by constructed wetland
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 402–409
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Phosphorus removal from agricultural runoff by constructed wetland S.Y. Lu a,b , F.C. Wu a,∗ , Y.F. Lu b , C.S. Xiang b , P.Y. Zhang b , C.X. Jin a a
State Environmental Protection Key Laboratory for Lake Pollution Control, Research Center of Lake Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China b Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
A free-water surface wetland covering an area of 2800 m2 was operated from March 2002 to
Received 13 February 2008
June 2004 for agricultural runoff treatment in the Dianchi Valley in China. In the wetland
Received in revised form
were grown Zizania Caduciflora Turez Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud.
3 October 2008
The instantaneous inflow rate was measured and the integrated flux was recorded by an
Accepted 13 October 2008
ultrasonic flow instrument all year round. The average inflow rate, hydraulic loading rate (HLR) and hydraulic retention time (HRT) were kept at 242 m3 d−1 , 12.7 cm d−1 and 2.0 d, respectively. The annual average total phosphorus (TP) in the inflow was 0.87 mg L−1 , and
Keywords:
the corresponding removal efficiency was calculated to be 59.0%. Biannual plant uptake and
Phosphorus uptake by plants
removal by harvesting and seed transport was the main pathway for TP removal, while the
Ultrasonic flow instrument
influent TP load was 12.9 g m−2 year−1 . Hydraulic retention time had a significant positive
Phosphorus removal
correlation with the removal of P (r2 = 0.88). Water temperature, inflow phosphorus load,
Correlation
inflow and hydraulic load rates were positively correlated with the removal of P. Inflow phosphorus concentrations were negatively correlated with the removal of P. It is shown that the free-water surface wetland was an effective and economical system for agricultural runoff treatment in lake regions. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
A significant amount of non-point phosphorus (P) entering surface waters, including Lake Dianchi in the Midwestern part of China comes from agricultural activities (Kovacic et al., 2000; Lu et al., 2007). Also, P in agricultural runoff may cause eutrophication of recipient waterbodies (Liikanen et al., 2003; de-Bashan and Bashan, 2004). As one of the key waterbodies to be protected in China, Lake Dianchi in Yunnan Province is highly sensitive to pollution sources, and agricultural runoff in the region must be treated before it is discharged into the lake. Wetlands are considered to be a low-cost alternative for treatment of agricultural effluents. Constructed wetlands (CWs)
∗
Corresponding author. Tel.: +86 10 84935064; fax: +86 10 84915190. E-mail address: [email protected] (F.C. Wu). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.10.002
are preferred because they have more engineering systems and they are easier to control (Ayaz and Akc¸a, 2001; Diemont, 2006). Reddy studied the first CW for agricultural runoff treatment in 1982, while Higgins reported the first full-scale CW for agricultural runoff treatment in 1993 (Vymazal, 1998). These CWs showed a very good capacity for removal of contaminants such as nitrogen and phosphorus. Four restored wetlands covering areas of 9313, 10,328, 10,351, and 5456 m2 , respectively, were used to improve the quality of agricultural runoff in the Delta of the Ebro River in 1993 (Romero et al., 1999). Submerged aquatic vegetation mesocosms (3.7 m2 ) received agricultural runoff from June 1998 to February 2000. The mean total phosphorus (TP) loading rates were 19.7, 8.3 and 4.5 g m−2 year−1 ,
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 402–409
respectively. Mean inflow TP concentrations (107 g L−1 ) were reduced to 52, 29, and 23 g L−1 while the hydraulic retention times (HRTs) were 1.5, 3.5, and 7.0 d, respectively (Dierberg et al., 2002). Scholz et al. (2007) reported the concept of integrated free surface flow integrated constructed wetlands (ICW). This explicitly combines the objectives of cleansing and managing water flow from farmyards with that of integrating the wetland infrastructure into the landscape and enhancing its biological diversity. In China, CW systems are increasingly being used to control non-point source wastewater pollution from small towns and villages for the protection of vulnerable waterbodies. Liu (1997) reported a CW with an area of 1257 m2 for agricultural runoff treatment in China. The removal efficiencies of total phosphorus (TP) and total dissolved phosphorus (TDP) were calculated to be 24.4% and 9.8%, respectively. Phragmites australis (Cav.) Trin.ex Steud. was used as a dominant macrophyte in wetlands for agricultural runoff treatment (Liu, 1997; Romero et al., 1999). Jiang et al. (2007) reported on ditch wetlands for removal of agricultural non-point source pollutants in Lake Taihu for eutrophication control. There are many mechanisms that interact with P in constructed wetland: (1) sedimentation, means gravitational settling of solids, (2) precipitation, means formation of or co-precipitation with insoluble compounds, (3) adsorption, means adsorption on substrate and plant surfaces, (4) plant uptake, means under proper conditions significant quantities of these contaminants will be taken by plants. For many constructed wetland treating domestic wastewater and industrial wastewater, mechanisms 2 and 3 are primary effect, and mechanism 1 is incidental effect. During mechanism 3, CEC adsorption is important (Vymazal et al., 1998). The most commonly used design model currently used for constructed wetlands is the areal k-C* model (Kadlec and Knight, 1996; IWA, 2000). This study reports on the performance of a free-water surface (FWS) wetland for agricultural runoff treatment with an operating period of 27 months. This work differs from other investigations in the following aspects: (1) the instantaneous delivery inflow rate was measured and the integrated flux was recorded by an ultrasonic flow instrument all year round; (2) most previous studies on treatment of agricultural runoff by CWs focused on agricultural regions without plastic shed mulch. The contaminant source was a non-point pollution source from arable land with high plastic shed coverage (85%), and the land was cultivated all the year round. Thus, it had its unique P release mechanism compared to farmlands without plastic shed cover. The area also had a high rate of fertilizer application compared to farmlands in other regions with lower temperatures and without plastic shed cover. The P discharge regularity of this farmland was summarized in detail in our previous paper (Gui et al., 2003); (3) there was gravity flow of water into the wetland; (4) the monitoring frequency was 5–6 times per month as compared to the commonly used 1–2 times per month in previous studies; (5) seasonal performance of the wetland with intermittent wetting and drying was investigated; and (6) the accumulative inflow rate of this wetland was accurately recorded by an ultrasonic flow instrument. This study focuses on the effects of HRT and temperature on P retention. The results provided successful cases and
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fundamental data for the effective treatment of agricultural runoff in arable watersheds.
2.
Materials and methods
2.1.
Season division
In this study, the seasons were divided as follows: spring was from March to May; summer was from June to August; autumn was from September to November; and winter was from December to February.
2.2.
Site description and wetland design
The free water surface wetland was built on the eastern side of Lake Dianchi, which is a potential drinking water source for Kunming City (Fig. 1). Thus, the water quality of Lake Dianchi is closely correlated to the health of people in Kumming. The area is located in a northern subtropical zone with an average annual rainfall (797–1007 mm), of which 85% occurs in the summer. The constructed wetland covers an area of approximately 2800 m2 , and the inflow consists of agricultural runoff coming from upstream farmland with a watershed area of 0.23 km2 (Fig. 1). The CW effluent discharge modes are various in the rainy season (from May to September) and dry season (from December to next April). During the rainy season, the water level of Lake Dianchi was lower than that of the CW effluent, the CW effluent was directly discharged into Lake Dianchi. During the dry season; the water level of Lake Dianchi was higher than that of the CW effluent; therefore, the CW effluent was discharged into the surface drain before being pumped into the lake. The land selected for the CW is a bottomland instead of farmland, because the latter is too expensive to rent. As the CW is narrow in its southern part, four measurements were taken during our design to ensure an appropriate flow path (Fig. 1): (1) the influent weir ended at the half of the southern inlet boundary; (2) a guide wall (20 m × 0.6 m × 0.7 m) was built in the southern part of the wetland; (3) the distance between the southern-most points of this wall to the southern-most borders was 6 m; and (4) no weir crest zone (which 26 m in length) was established in the south of the effluent weir.
2.3.
Sample analysis
The monitoring and surveying of CW was conducted from May 2002 to June 2004. Water and plant samples were analyzed periodically.
2.4.
Analysis of water samples
The daily water inflow rate of this wetland was recorded by an ultrasonic flow instrument (HBML-3, Beijing Huanke Environ. Port. Tech. CO.) in the inflow ditch. Temperature and pH were measured on the site when water samples were taken. Water was collected for analysis of TP five to six times per month. Two mixed samples were attained. One was attained by sampling at 3 points 0.5 meter following the inflow weir, and mixing them evenly. The other was attained
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Fig. 1 – Schematic plan and site of the constructed wetland. (1) The effluent pathway of wetland during the rainy season (from May to October); (2) the effluent pathway of wetland during the dry season (from November to April); (3) the influent sampling point; and (4) the effluent sampling point.
by sampling at 3 points 0.5 meter in front of the outflow weir, and then mixing them evenly. The water samples were analyzed using protocols described in the Standard Methods (State Environmental Protection Administration of China, 2002). The water samples were digested, but did not use any filtering. The TP was measured by ammonium molybdate-antimony potassium tartrate-ascorbic acid spectrophotometry. Because of the important function of plants in the wetland (Vymazal et al., 1998; Serra et al., 2004), the biomass, height, water contents (WC) and nutrient contents of plants were measured. The macrophytes were observed throughout the experimental period for general appearance and health. Plant tissues were sampled within 2 m2 of the sampling plot in November 2002 and July 2003. Plant materials were chopped and dried at 65 ◦ C for 30 min and 105 ◦ C for 24 h before analysis for phosphorus contents (Lu, 2000). In November 2002 and July 2003, plants including all the Zizania Caduciflora Turez
Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud of the constructed wetland were harvested.
2.5.
Statistical analyses
All data analyses were performed by analysis of variance in Excel sheet. The correlation analyses between influent characteristics and operation parameters were made in Excel sheet.
3.
Results and discussion
3.1.
Water inflow rate and temperature variations
The seasonal variation of water inflow rate is illustrated in Fig. 2. The average annual inflow rate was measured to be 242 m3 d−1 .
Data from average North American surface wetlands (summarized by Kadlec and Knight, 1996) (Nelson et al., 2003). Data from four surface flow constructed wetlands (CW-1, CW-2, CW-3, CW-4) in the cold temperate climate of Norway for 3 to 7 years of operation (Braskerud, 2002). This calculation was based on 138 water samples during the 27-month operating period. HLR means the hydraulic loading rate, in cm d−1 . HRT means hydraulic retention time, in d; TPinf means the influent concentrations of total phosphorus; TPeff means the effluent concentrations of TP, TP means the removal efficiency of TP; L-TPinf means influent TP load, in kg ha−1 d−1 . ##
#
1992–1999 1996–1999 1992–1999 1992–1999
1996–1999
21–24 0.11–3.0 0.08–2.1 0.12–1.95 0.07–1.45 0.02–0.32 0.01–0.13
0.2–0.90 0.12–0.48
70–510 5–330 30–390
12.7 2.0 0.75 0.87 0.36 59.0 138 04 6.3 3.8 0.34 0.76 0.33 56.1 18 20 11.3 2.2 0.79 1.00 0.50 49.7 20 – 12.6 2.0 0.74 0.91 0.26 71.3 53 20 HLR (cm d ) HRT (d) L-Tpinf (kg ha−1 d−1 ) TPinf (mg L−1 ) TPeff (mg L−1 ) TP (%) n Period
Spring
16.0 1.7 0.92 0.82 0.43 47.8 47 02
Winter
−1
Summer
Autumn
All-phase
– – 0.5 3.78 1.62 57.0
CW-3## CW-2## CW-1## CW-0# This study Constructed wetland
Table 1 – Phosphorus concentrations, load and removal efficiencies of agricultural runoff treated by constructed wetlands in various seasons.
CW-4##
2–130
CW-5##
0.71–1.95
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 402–409
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Fig. 2 – Seasonal variations of average inflow rate. Spring-02 means spring in 2002. Summer-02 means summer in 2002. Autumn-02 means autumn in 2002. Winter-02 means winter in 2002. Spring-03 means spring in 2003. Summer-03 means summer in 2003. Autumn-03 means autumn in 2003. Winter-03 means winter in 2003. Spring-04 means spring in 2004. Summer-04 means summer in 2004.
Water inflow rates for various seasons varied in the ascending order of winter, spring, autumn and summer. The average water temperature was 18.7 ◦ C all year round, while the average temperatures in spring, summer, autumn and winter were 18.9, 23.1, 19.8, and 12.7 ◦ C, respectively. The lowest average monthly water temperature was above 10 ◦ C. This temperature condition is advantageous for application of CW for wastewater treatment because the plants and microbes in the CW were able to maintain growth activity all year round. Inconstant inflow rates during the various seasons had an impact on the reduction of contaminants in the wetland.
3.2.
Phosphorus removal efficiency and its prediction
3.2.1.
Phosphorus removal efficiency
The average HRT, HLR, seasonal water quality and contaminant removal efficiency, and the influent load and removal efficiency of TP were compared with the average North American surface flow wetland systems (FWSaNA ) (Kadlec and Knight, 1996; Nelson et al., 2003) and wetlands in the cold temperate climate of Norway (Braskerud, 2002) (Table 1). The average pH values of influents and effluents were 7.0 and 7.4, respectively. For a FWS (free-water surface wetland) system there was a good relationship between the load applied and the removal efficiency. The load increased, as did the removal efficiency. This is in good agreement with what was reported by Tanner et al. (1995) and Tanner et al. (1998). Although the TP load rate of this wetland was only 0.23, like that of the FWSaNA , the present CW has slightly higher TP removal efficiency (59%) than that of the FWSaNA (57%). This can be attributed to the
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Fig. 3 – Average influent (-inf) load and removal (-rem) load (in kg per 3 months) of TP (total phosphorus) in spring and autumn.
Fig. 4 – Average influent (-inf) load and removal (-rem) load (in kg per 3 months) of TP (total phosphorus) in summer and winter.
following two factors. Firstly, the soil has a better capability of P absorption, and according to our studies the P absorption capability was 1.21 mg P per gram of soil. This lies within the range of values reported, i.e., 0.032–5.2 (Gale et al., 1994; Sakadevan and Bavor, 1998). Secondly, the intermittent inflow of the present wetland as a result of receiving agricultural runoff all the year round was beneficial to the removal of P (Brix, 1994; Verhoeven and Meuleman, 1999; Sun et al., 2005). In view of the different structural and loading characteristics of natural and artificial systems treating point and non-point source pollution, it is not surprising that the reported retention efficiencies vary widely (Reinhardt et al., 2005). To explain variations in phosphorus retention efficiency it is essential to make long-term and highly temporal resolution investigations that allow the comparison of different system states. In the wetlands (CW-area, 350–900 m2 ) reported by Braskerud (2002), the phosphorus removal rates varied from 21% to 24%, while in this research the phosphorus removal rate was 57%. The difference can be explained by the fact that the HLR in this research was higher than in research performed by Braskerud. Also in our research, the temperature was higher and the phosphorus loading rate was lower. High temperature was beneficial for the growth of macrophytes and the activity of microbes, which was beneficial for the phosphorus absorption by the macrophytes. To understand the performance of the CW in different seasons and under different conditions, we compared the CW performance in spring with that in autumn (Fig. 3), since May and September are generally rainy seasons. The CW had a higher HRT, higher average water temperature (Fig. 2), lower HLR and lower inflow load (except the P load) (Table 1) in autumn than in spring. But in spring the CW had a better TP removal efficiency than in autumn. In spring macrophytes grew and assimilated P rapidly, while from mid to late autumn the macrophytes withered away and released P to the wetland.
In summer the TP removal efficiency was lower than in winter (Fig. 4) due to the following reasons. Firstly, summer had less HRT (1.7 d) than winter (3.8 d). It should be noted that Tanner et al. (1995) and Sakadevan and Bavor (1999) reported that the greater retention time positively enhanced the removal of P from wastewater in the constructed wetland systems. Secondly, the capability of soil to adsorb P in CWs tends to decrease gradually. However, the amount of total P removal was 22.4 kg in summer, much higher than 9.4 kg in winter. It indicated that temperature had no important effect on the removal of P, and this is in agreement with the results reported by Dahab and Surampalli (2001), and Vanier and Dahab (2001). On the other hand, it was found that HRT and exposure time of sediments to water had a significant effect on the removal of P. Removal efficiency of P in winter was not much lower than what was observed in the other seasons (Table 1). A similar result was also reported by Maehlum and Stalnacke (1999), in which they found that for the contaminants, there was less than 10% difference in removal efficiency between the warm and cold periods. Data from the constructed wetlands in Denmark, Sweden and North America showed that winter performance was not significantly reduced as compared to that in the other seasons (Jessen et al., 1993). Furthermore, Kern and Idler (1999) stated that the P removal efficiencies of a reed bed did not show any seasonal variation for domestic and agricultural wastewater. In the present work, the good performance of the CW in winter was mainly due to three factors. Firstly, the first harvest in November 2002 avoided P release caused by the decomposition of wizened plants (Lu et al., 2005; Kroger et al., 2007) and strengthened oxygen diffusion from the atmosphere. Secondly, the average water temperature in winter was higher than 5 ◦ C. Thirdly, the intermittent inflow for this CW, caused by receiving agricultural runoff all year round (especially during
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water balance and plug flow).
Table 2 – Correlation analysis between influent characteristics and operation parameters. Removal efficiency (%) Inflow (m3 d−1 ) P concentration (mg/L) Water temperature (◦ C) Hydraulic load rate (cm/d) Hydraulic retention time (d) P load (kg ha−1 d−1 ) Removal efficiency (%)
Q ln k
C − C∗ 0 Ci − C∗
Temperature effects on k are most important for removal of the various forms of nitrogen, and are generally summarized using the following modified Arrhenius equation in which kt is the rate constant at temperature T (◦ C) and k20 is the rate constant at 20 ◦ C:
0.0028 −0.794 0.0087 0 0.879* 0.0889 1
kt = K20 T−20
Note: *Significant level: ˛ = 0.05, **significant level: ˛ = 0.01, n = 5.
the dry period) was beneficial to phosphorus removal (Verhoeven and Meuleman, 1999; Sun et al., 2005). Correlation analysis between inflow characteristics and operational characteristics is given in Table 2. Hydraulic retention time was related closely to P removal efficiency (Table 2). This is in agreement with the results obtained by Tanner (1994). Water temperature, inflow P load, inflow, inflow P concentrations, and hydraulic load rate had a slight influence on P removal (Table 2). Tanner et al. (1998) determined that the mean annual mass removal of TP was in the range of 15–38% with removal efficiencies being inversely related to loadings in five pilot-scale constructed wetlands in their 4th and 5th years of operation. This may be caused by the difference between HRT and HLR. These two parameters for the five pilot-scale constructed wetlands (which are 1.5–7.0 cm d−1 and 2–9.5 d) are much lower than the corresponding values reported in this study (which are 7.0 cm d−1 and 2 d).
3.2.2.
A=−
Phosphorus removal efficiency prediction
The most commonly used design model currently used for constructed wetlands is the areal k-C* model (Kadlec and Knight, 1996; IWA, 2000), which accounts for temperaturebased, first-order reduction to background levels (C*). C* accounts for substances generated in the wetland by biological activity, sediment release, etc., as well as the non-degradable fraction of the contaminant. Knowing the average wastewater flow (Q, m3 d−1 ), the appropriate area based concentration (Ci ), the wetland areas (A, m2 ) required to meet the target outflow concentration (C0 ) can then be calculated (assuming neutral
Use first-order design model to calculate the outflow phosphorus concentration (C0 ). First, the average temperature is 18.7 ◦ C. Calculate the temperature-corrected first-order rate constants, kt using equation suggested by Kadlec and Knight (1996), e.g., for phosphorus, where K20 = 12 m year−1 and = 1.00, kt = K20 T−20 = 12 Then calculate the predicted outflow phosphorus concentration (C0 ) using the following equation (assuming neutral water balance and plug flow), A=−
Q ln k
2800 = −
C − C∗ 0 Ci − C∗
Q ln k
C − C∗ 0 Ci − C∗
=
242 × 365 ln 12
C − 0.02 0 0.87 − 0.02
C0 = 0.60 mg L−1 The predicted C0 value (0.60 mg L−1 ) is far larger than the measured value (0.36 mg L−1 ). Mainly because the temperature variation of this wetland was smaller than many other constructed wetlands, macrophytes had large biomass all the year round and the soil of this wetland had strong phosphorus adsorption capacity. These reasons explain the better phosphorus removal rate in the wetland.
3.3.
Phosphorus distribution pathway
To understand the contaminant removal mechanisms, the P distribution pathway was studied. Biomass, water contents and nutrient contents of macrophytes are listed in Table 3. Table 3 shows that Zizania Caduciflora Turez Hand-mazt has higher water contents than Phragmites australis (Cav.) Trin.ex
Table 3 – Biomass, water contents and nutrient contents of Zizania Caduciflora Turez Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud. Time
Plant
Water content (%)
Biomass (kg m−2 )
mg P g−1
Nov. 2002
ZC PA
64.0 54.3
6.81 3.59
3.2 1.2
July 2003
ZC PA Emergent species#
74.8 58.2
4.16 3.05
1.5 0.8 7∼9.4
ZC: Zizania Caduciflora Turez Hand-mazt PA: Phragmites australis (Cav.) Trin.ex Steud. #
Data from Greenway (1997). The phosphorus contents here mean the highest value of 4 kinds of emergent species, including Eleocharis sphacelata, Baumea articulata, Typha domingensis, and Cyperus involucratus.
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Steud., mainly because the former has a higher ratio of leaf to stem (in weight) than the latter. Table 3 indicates that Zizania Caduciflora Turez Hand-mazt has a larger biomass (in wet weight) than Phragmites australis (Cav.) Trin.ex Steud., mainly because the former has a larger growth density than the latter. The P contents of Zizania Caduciflora Turez Hand-mazt were higher than those of Phragmites australis (Cav.) Trin.ex Steud. The major P distribution pathways in CWs include adsorption, desorption, precipitation, exchange with groundwater, plant harvest and discharge in CWs (Vymazal et al., 1998; Lu et al., 2004). In the period August 2002 to July 2003, two harvests were performed: one in November 2002 and the other in July 2003. It was then known that the corresponding P loads of input (recorded as 100%), discharge (measured value is 38%), plant uptake and removal by harvesting and seed transport (measured value is 36%) can be calculated from the data we have collected. Thus, the P load due to adsorption/desorption/precipitation/and exchange with groundwater (26%) can be calculated as P-input load minus P-discharge load, as well as plant uptake and removal by harvesting and seed transport P load. It should be noted that plant harvest is the major P removal pathway. This is similar to the result of research performed by Tanner (1996), who stated that in 0.238 m2 × 0.6 m deep gravel-bed wetland mesocosms fed with dairy farm wastewaters pre-treated in an anaerobic lagoon, there was a maximum plant accumulation of 18.5 g P m−2 year−1 . This accounts for approximately 30% of the levels supplied in the wastewater. The important role of macrophytes in contaminant removal in this wetland is due to the following three factors. Firstly, good growing seasons provided good conditions for the growth of Zizania Caduciflora Turez Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud., and these plants had large biomass and good P absorption capability. Secondly, plant harvests conducted in July 2003 and December 2003 not only effectively avoided P release from the macrophytes, but also caused the removal of P out of the wetland in large amounts. Thirdly, the influent P load was at low level (TP, 12.9 g m−2 year−1 ), lower than that of municipal wastewater (TP, 66.7 g m−2 year−1 ). Thus, the amount of P removal through the wetland was lower than that for the same wetland used for municipal wastewater treatment.
3.4.
Practicalities and economics of plant harvesting
Because of the lower labour cost, the harvest and carriage expense standard of the emergent macrophytes is 36.9 $US per 1000 m2 . The macrophytes were harvested and then were carried to their homes by some farmers hired by us to their home, because these macrophytes can be used as fuel and feed. Thus, we paid 206.5 $US twice for the harvest and carriage of the macrophytes twice. The amount of phosphorus removal by the two harvests and carriages is 14 kg. We also provided 8600 kg (in dry weight) of fuel and feed for the farmers. The third advantage is that the harvest during winter can effectively inhibit the phosphorus release. The fourth advantage is that the harvests provide some temporary jobs per year. Thus, the harvests have the advantages of promoting pollutant removal (phosphorus load cut off) and gaining considerable economic and social benefits.
4.
Conclusions
This CW was favorable to the removal of TP all year round. When the inflow TP concentrations were 0.87 mg L−1 , the corresponding removal efficiency measurement was 59.0%. Plant harvest had a critical contribution to the treatment of agricultural runoff by CW with a low P load. The removal efficiencies of P by two harvests were about 58% of the total removal load when the influent TP load was 12.9 g m−2 year−1 . This suggested that macrophytes may be critical in achieving low residual phosphorus concentrations in final effluents from such systems. Hydraulic retention time (1.7–3.8 d) had a significant correlation with the P removal efficiency (a = 0.05), whereas water temperature, inflow P load, inflow, inflow P concentrations, and the hydraulic loading rate had a slight influence on P removal.
Acknowledgements We acknowledge the Major Special Program of the Ministry of Science and Technology (863 Program) (K99-05-35-02), (2005AA60101005), China’s national basic research program (2008CB418204). We also thank Mr. Deyi Hou from Stanford University of America and Prof. Yonghui Song from Chinese Research Academy of Environmental Sciences of China for their suggestions.
references
Ayaz, S.C¸., Akc¸a, L., 2001. Treatment of wastewater by natural systems. Environ. Int. 26 (3), 189–195. Braskerud, B.C., 2002. Factors affecting phosphorus retention in small constructed wetlands treating agricultural non-point source pollution. Ecol. Eng. 19 (1), 41–61. Brix, H., 1994. Use of constructed wetlands in water-pollution control-historical development, present status, and future perspectives. Water Sci. Technol. 30 (8), 209–223. Dahab, M.F., Surampalli, R.Y., 2001. Subsurface-flow constructed wetlands treatment in the plains: five years of experience. Water Sci. Technol. 44 (11–12), 375–380. de-Bashan, L.E., Bashan, Y., 2004. Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997–2003). Water Res. 38, 4222–4246. Diemont, S.A.W., 2006. Mosquito larvae density and pollutant removal in tropical wetland treatment systems in Honduras. Environ. Int. 32 (3), 332–341. Dierberg, F.E., De Busk, T.A., Jackson, S.D., et al., 2002. Submerged aquatic vegetation-based treatment wetlands for removing phosphorus from agricultural runoff: response to hydraulic and nutrient loading. Water Res. 36 (6), 1409–1422. Gale, P.M., Reddy, K.R., Graetz, D.A., 1994. Phosphorus retention by wetland soils used for treated waste-water disposal. J. Environ. Quality 23 (2), 370–377. Greenway, M., 1997. Nutrient content of wetland plants in constructed wetlands receiving municipal effluent in tropical Australia. Water Sci. Technol. 35 (5), 135–142. Gui, M., Zhu, W.P., Yu, G., Zhang, P.Y., 2003. Release regularity of agricultural non-point pollution in drainage area in Dianchi region (in Chinese). J. Agro-Environ. Sci. 22 (1), 1–5. IWA, 2000. Constructed wetlands for pollution control: process, design and operation. International Water Association Scientific and Technical report no. 8. IWA Publishing, London, UK.
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Jessen, P.D., Maehlum, T., Krogstad, T., 1993. Potential use of constructed wetlands for wastewater treatment in northern environments. Water Sci. Technol. 28 (10), 149–157. Jiang, C.L., Fan, X.Q., Cui, G.B., Zhang, Y.B., 2007. Removal of agricultural non-point source pollutants by ditch wetlands: implications for lake eutrophication control. Hydrobiologia 581, 319–327. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. Lewis Publishers, Boca Raton, FL. Kern, J., Idler, C., 1999. Treatment of domestic and agricultural wastewater by reed bed systems. Ecol. Eng. 12, 13–25. Kovacic, D.A., David, M.B., Gentry, L.E., Starks, K.M., Cooke, R.A., 2000. Effectiveness of constructed wetlands in reducing nitrogen and phosphorus export from agricultural tile drainage. J. Environ. Quality 29 (4), 1262–1274. Kroger, R., Holland, M.M., Moore, M.T., Cooper, C.M., 2007. Plant senescence: a mechanism for nutrient release in temperate agricultural wetlands. Environ. Pollut. 146 (1), 114–119. Liikanen, A., Puustinen, M., Koskiaho, J., Väisänen, T., Martikainen, P., Hartikainen, H., 2003. Phosphorus removal in a wetland constructed on Former Arable Land. J. Environ. Quality 33, 1124–1132. Liu, W.X., 1997. Study on the application of artificial wetland in agriculture non point source pollution (in Chinese). Res. Environ. Sci. 10 (4), 14–18. Lu, R.K., 2000. Analytic Method of Soil and Agricultural Chemistry (in Chinese). Agricultural Scientific and Technical Press of China, Beijing, pp. 1–338. Lu, S.Y., Zhang, P.Y., Yu, G., Zhu, W.P., Xiang, C.S., 2004. Stabilization pond-plant bed composite system treatment of farmland irrigation and drainage water. China Environ. Sci. 24 (5), 605–609. Lu, S.Y., Zhang, P.Y., Yu, G., Zhu, W.P., Xiang, C.S., 2005. The contaminants release rule of Zizania Caduciflora, Phragmites australis and Eichhornia Crassipes (in Chinese). China Environ. Sci. 25 (5), 554–557. Lu, S.Y., Yu, G., Zhang, P.Y., Zhu, W.P., Gui, M., Xiang, C.S., 2007. Agricultural runoff contaminant discharge in Dianchi Valley agricultural district of China. Int. J. Environ. Pollut. 31 (3/4), 394–414. Maehlum, T., Stalnacke, P., 1999. Removal efficiency of three cold climate constructed wetlands treating domestic wastewater: effects of temperature, seasons, loading rates and input concentration. Water Sci. Technol., 273–281. Nelson, M., Alling, A., Dempster, W.F., van Thillo, M., Allen, J., 2003. Advantages of using subsurface flow constructed wetlands for wastewater treatment in space applications: ground based Mars base prototype. Adv. Space Res. 31 (7), 1799–1804. Reinhardt, M., Gächter, R., Wehrli, B., Müller, B., 2005. Phosphorus retention in small constructed wetlands treating agricultural drainage water. J. Environ. Quality 34 (4), 1251–1259.
409
Romero, J.A., Comín, F.A., García, C., 1999. Restored wetlands as filters to remove nitrogen. Chemosphere 39 (2), 323–332. Sakadevan, K., Bavor, H.J., 1998. Phosphate adsorption characteristics of soils, slags and zeolite to be used as substrates in constructed wetland systems. Water Res. 32 (2), 393–399. Sakadevan, K., Bavor, H.J., 1999. Nutrient removal mechanisms in constructed wetlands and sustainable water management. Water Sci. Technol. 40 (2), 121–128. Scholz, M., Harrington, R., Carroll, P., et al., 2007. The integrated constructed wetlands (ICW) concept. Wetlands 27 (2), 337–354. Serra, T., Fernando, H.J.S., Rodríguez, R.V., 2004. Effects of emergent vegetation on lateral diffusion in wetlands. Water Res. 38 (1), 139–147. State Environmental Protection Administration of China, 2002. Monitoring and Analytical Method of Water and Wastewater (4th edition) (in Chinese). Environmental and Scientific Press of China, Beijing. Sun, G.Z., Zhao, Y.Q., Allen, S., 2005. Enhanced removal of organic matter and ammoniacal-nitrogen in a column experiment of tidal flow constructed wetland system. J. Biotechnol. 115, 189–197. Tanner, C.C., 1994. Treatment of dairy farm wastewaters in Horizontal and up-flow gravel-bed constructed wetlands. Water Sci. Technol. 29 (4), 85–93. Tanner, C.C., 1996. Plants for constructed wetland treatment systems—a comparison of the growth and nutrient uptake of eight emergent species. Ecol. Eng. 7 (1), 59–83. Tanner, C.C., Clayton, J.S., Upsdell, M.P., 1995. Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands. 2. Removal of nitrogen and phosphorus. Water Res. 29 (1), 27–34. Tanner, C.C., Sukias, J.P.S., Upsdell, M.P., 1998. Relationships between loading rates and pollutant removal during maturation of gravel-bed constructed wetlands. J. Environ. Quality 27 (2), 448–458. Vanier, S.M., Dahab, M.F., 2001. Start-up performance of a sub surface-flow constructed wetland for domestic wastewater treatment. Environ. Technol. 22 (5), 587–596. Verhoeven, J.T.A., Meuleman, A.F.M., 1999. Wetlands for wastewater treatment: opportunities and limitations. Ecol. Eng. 12, 5–12. Vymazal, J., 1998. Introduction. In: Vymazal, J., Brix, H., Cooper, P.F., et al. (Eds.), Constructed Wetlands for Wastewater Treatment in Europe. Backhuys Publishers, Leiden, pp. 1–15. Vymazal, J., Brix, H., Cooper, P.F., et al., 1998. Removal mechanisms and types of constructed wetlands. In: Vymazal, J., Brix, H., Cooper, P.F., et al. (Eds.), Constructed Wetlands for Wastewater Treatment in Europe. Backhuys Publishers, Leiden, pp. 17–66.
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Phosphorus removal from agricultural runoff by constructed wetland S.Y. Lu a,b , F.C. Wu a,∗ , Y.F. Lu b , C.S. Xiang b , P.Y. Zhang b , C.X. Jin a a
State Environmental Protection Key Laboratory for Lake Pollution Control, Research Center of Lake Environment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China b Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
A free-water surface wetland covering an area of 2800 m2 was operated from March 2002 to
Received 13 February 2008
June 2004 for agricultural runoff treatment in the Dianchi Valley in China. In the wetland
Received in revised form
were grown Zizania Caduciflora Turez Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud.
3 October 2008
The instantaneous inflow rate was measured and the integrated flux was recorded by an
Accepted 13 October 2008
ultrasonic flow instrument all year round. The average inflow rate, hydraulic loading rate (HLR) and hydraulic retention time (HRT) were kept at 242 m3 d−1 , 12.7 cm d−1 and 2.0 d, respectively. The annual average total phosphorus (TP) in the inflow was 0.87 mg L−1 , and
Keywords:
the corresponding removal efficiency was calculated to be 59.0%. Biannual plant uptake and
Phosphorus uptake by plants
removal by harvesting and seed transport was the main pathway for TP removal, while the
Ultrasonic flow instrument
influent TP load was 12.9 g m−2 year−1 . Hydraulic retention time had a significant positive
Phosphorus removal
correlation with the removal of P (r2 = 0.88). Water temperature, inflow phosphorus load,
Correlation
inflow and hydraulic load rates were positively correlated with the removal of P. Inflow phosphorus concentrations were negatively correlated with the removal of P. It is shown that the free-water surface wetland was an effective and economical system for agricultural runoff treatment in lake regions. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
A significant amount of non-point phosphorus (P) entering surface waters, including Lake Dianchi in the Midwestern part of China comes from agricultural activities (Kovacic et al., 2000; Lu et al., 2007). Also, P in agricultural runoff may cause eutrophication of recipient waterbodies (Liikanen et al., 2003; de-Bashan and Bashan, 2004). As one of the key waterbodies to be protected in China, Lake Dianchi in Yunnan Province is highly sensitive to pollution sources, and agricultural runoff in the region must be treated before it is discharged into the lake. Wetlands are considered to be a low-cost alternative for treatment of agricultural effluents. Constructed wetlands (CWs)
∗
Corresponding author. Tel.: +86 10 84935064; fax: +86 10 84915190. E-mail address: [email protected] (F.C. Wu). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.10.002
are preferred because they have more engineering systems and they are easier to control (Ayaz and Akc¸a, 2001; Diemont, 2006). Reddy studied the first CW for agricultural runoff treatment in 1982, while Higgins reported the first full-scale CW for agricultural runoff treatment in 1993 (Vymazal, 1998). These CWs showed a very good capacity for removal of contaminants such as nitrogen and phosphorus. Four restored wetlands covering areas of 9313, 10,328, 10,351, and 5456 m2 , respectively, were used to improve the quality of agricultural runoff in the Delta of the Ebro River in 1993 (Romero et al., 1999). Submerged aquatic vegetation mesocosms (3.7 m2 ) received agricultural runoff from June 1998 to February 2000. The mean total phosphorus (TP) loading rates were 19.7, 8.3 and 4.5 g m−2 year−1 ,
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 402–409
respectively. Mean inflow TP concentrations (107 g L−1 ) were reduced to 52, 29, and 23 g L−1 while the hydraulic retention times (HRTs) were 1.5, 3.5, and 7.0 d, respectively (Dierberg et al., 2002). Scholz et al. (2007) reported the concept of integrated free surface flow integrated constructed wetlands (ICW). This explicitly combines the objectives of cleansing and managing water flow from farmyards with that of integrating the wetland infrastructure into the landscape and enhancing its biological diversity. In China, CW systems are increasingly being used to control non-point source wastewater pollution from small towns and villages for the protection of vulnerable waterbodies. Liu (1997) reported a CW with an area of 1257 m2 for agricultural runoff treatment in China. The removal efficiencies of total phosphorus (TP) and total dissolved phosphorus (TDP) were calculated to be 24.4% and 9.8%, respectively. Phragmites australis (Cav.) Trin.ex Steud. was used as a dominant macrophyte in wetlands for agricultural runoff treatment (Liu, 1997; Romero et al., 1999). Jiang et al. (2007) reported on ditch wetlands for removal of agricultural non-point source pollutants in Lake Taihu for eutrophication control. There are many mechanisms that interact with P in constructed wetland: (1) sedimentation, means gravitational settling of solids, (2) precipitation, means formation of or co-precipitation with insoluble compounds, (3) adsorption, means adsorption on substrate and plant surfaces, (4) plant uptake, means under proper conditions significant quantities of these contaminants will be taken by plants. For many constructed wetland treating domestic wastewater and industrial wastewater, mechanisms 2 and 3 are primary effect, and mechanism 1 is incidental effect. During mechanism 3, CEC adsorption is important (Vymazal et al., 1998). The most commonly used design model currently used for constructed wetlands is the areal k-C* model (Kadlec and Knight, 1996; IWA, 2000). This study reports on the performance of a free-water surface (FWS) wetland for agricultural runoff treatment with an operating period of 27 months. This work differs from other investigations in the following aspects: (1) the instantaneous delivery inflow rate was measured and the integrated flux was recorded by an ultrasonic flow instrument all year round; (2) most previous studies on treatment of agricultural runoff by CWs focused on agricultural regions without plastic shed mulch. The contaminant source was a non-point pollution source from arable land with high plastic shed coverage (85%), and the land was cultivated all the year round. Thus, it had its unique P release mechanism compared to farmlands without plastic shed cover. The area also had a high rate of fertilizer application compared to farmlands in other regions with lower temperatures and without plastic shed cover. The P discharge regularity of this farmland was summarized in detail in our previous paper (Gui et al., 2003); (3) there was gravity flow of water into the wetland; (4) the monitoring frequency was 5–6 times per month as compared to the commonly used 1–2 times per month in previous studies; (5) seasonal performance of the wetland with intermittent wetting and drying was investigated; and (6) the accumulative inflow rate of this wetland was accurately recorded by an ultrasonic flow instrument. This study focuses on the effects of HRT and temperature on P retention. The results provided successful cases and
403
fundamental data for the effective treatment of agricultural runoff in arable watersheds.
2.
Materials and methods
2.1.
Season division
In this study, the seasons were divided as follows: spring was from March to May; summer was from June to August; autumn was from September to November; and winter was from December to February.
2.2.
Site description and wetland design
The free water surface wetland was built on the eastern side of Lake Dianchi, which is a potential drinking water source for Kunming City (Fig. 1). Thus, the water quality of Lake Dianchi is closely correlated to the health of people in Kumming. The area is located in a northern subtropical zone with an average annual rainfall (797–1007 mm), of which 85% occurs in the summer. The constructed wetland covers an area of approximately 2800 m2 , and the inflow consists of agricultural runoff coming from upstream farmland with a watershed area of 0.23 km2 (Fig. 1). The CW effluent discharge modes are various in the rainy season (from May to September) and dry season (from December to next April). During the rainy season, the water level of Lake Dianchi was lower than that of the CW effluent, the CW effluent was directly discharged into Lake Dianchi. During the dry season; the water level of Lake Dianchi was higher than that of the CW effluent; therefore, the CW effluent was discharged into the surface drain before being pumped into the lake. The land selected for the CW is a bottomland instead of farmland, because the latter is too expensive to rent. As the CW is narrow in its southern part, four measurements were taken during our design to ensure an appropriate flow path (Fig. 1): (1) the influent weir ended at the half of the southern inlet boundary; (2) a guide wall (20 m × 0.6 m × 0.7 m) was built in the southern part of the wetland; (3) the distance between the southern-most points of this wall to the southern-most borders was 6 m; and (4) no weir crest zone (which 26 m in length) was established in the south of the effluent weir.
2.3.
Sample analysis
The monitoring and surveying of CW was conducted from May 2002 to June 2004. Water and plant samples were analyzed periodically.
2.4.
Analysis of water samples
The daily water inflow rate of this wetland was recorded by an ultrasonic flow instrument (HBML-3, Beijing Huanke Environ. Port. Tech. CO.) in the inflow ditch. Temperature and pH were measured on the site when water samples were taken. Water was collected for analysis of TP five to six times per month. Two mixed samples were attained. One was attained by sampling at 3 points 0.5 meter following the inflow weir, and mixing them evenly. The other was attained
404
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 402–409
Fig. 1 – Schematic plan and site of the constructed wetland. (1) The effluent pathway of wetland during the rainy season (from May to October); (2) the effluent pathway of wetland during the dry season (from November to April); (3) the influent sampling point; and (4) the effluent sampling point.
by sampling at 3 points 0.5 meter in front of the outflow weir, and then mixing them evenly. The water samples were analyzed using protocols described in the Standard Methods (State Environmental Protection Administration of China, 2002). The water samples were digested, but did not use any filtering. The TP was measured by ammonium molybdate-antimony potassium tartrate-ascorbic acid spectrophotometry. Because of the important function of plants in the wetland (Vymazal et al., 1998; Serra et al., 2004), the biomass, height, water contents (WC) and nutrient contents of plants were measured. The macrophytes were observed throughout the experimental period for general appearance and health. Plant tissues were sampled within 2 m2 of the sampling plot in November 2002 and July 2003. Plant materials were chopped and dried at 65 ◦ C for 30 min and 105 ◦ C for 24 h before analysis for phosphorus contents (Lu, 2000). In November 2002 and July 2003, plants including all the Zizania Caduciflora Turez
Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud of the constructed wetland were harvested.
2.5.
Statistical analyses
All data analyses were performed by analysis of variance in Excel sheet. The correlation analyses between influent characteristics and operation parameters were made in Excel sheet.
3.
Results and discussion
3.1.
Water inflow rate and temperature variations
The seasonal variation of water inflow rate is illustrated in Fig. 2. The average annual inflow rate was measured to be 242 m3 d−1 .
Data from average North American surface wetlands (summarized by Kadlec and Knight, 1996) (Nelson et al., 2003). Data from four surface flow constructed wetlands (CW-1, CW-2, CW-3, CW-4) in the cold temperate climate of Norway for 3 to 7 years of operation (Braskerud, 2002). This calculation was based on 138 water samples during the 27-month operating period. HLR means the hydraulic loading rate, in cm d−1 . HRT means hydraulic retention time, in d; TPinf means the influent concentrations of total phosphorus; TPeff means the effluent concentrations of TP, TP means the removal efficiency of TP; L-TPinf means influent TP load, in kg ha−1 d−1 . ##
#
1992–1999 1996–1999 1992–1999 1992–1999
1996–1999
21–24 0.11–3.0 0.08–2.1 0.12–1.95 0.07–1.45 0.02–0.32 0.01–0.13
0.2–0.90 0.12–0.48
70–510 5–330 30–390
12.7 2.0 0.75 0.87 0.36 59.0 138 04 6.3 3.8 0.34 0.76 0.33 56.1 18 20 11.3 2.2 0.79 1.00 0.50 49.7 20 – 12.6 2.0 0.74 0.91 0.26 71.3 53 20 HLR (cm d ) HRT (d) L-Tpinf (kg ha−1 d−1 ) TPinf (mg L−1 ) TPeff (mg L−1 ) TP (%) n Period
Spring
16.0 1.7 0.92 0.82 0.43 47.8 47 02
Winter
−1
Summer
Autumn
All-phase
– – 0.5 3.78 1.62 57.0
CW-3## CW-2## CW-1## CW-0# This study Constructed wetland
Table 1 – Phosphorus concentrations, load and removal efficiencies of agricultural runoff treated by constructed wetlands in various seasons.
CW-4##
2–130
CW-5##
0.71–1.95
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 402–409
405
Fig. 2 – Seasonal variations of average inflow rate. Spring-02 means spring in 2002. Summer-02 means summer in 2002. Autumn-02 means autumn in 2002. Winter-02 means winter in 2002. Spring-03 means spring in 2003. Summer-03 means summer in 2003. Autumn-03 means autumn in 2003. Winter-03 means winter in 2003. Spring-04 means spring in 2004. Summer-04 means summer in 2004.
Water inflow rates for various seasons varied in the ascending order of winter, spring, autumn and summer. The average water temperature was 18.7 ◦ C all year round, while the average temperatures in spring, summer, autumn and winter were 18.9, 23.1, 19.8, and 12.7 ◦ C, respectively. The lowest average monthly water temperature was above 10 ◦ C. This temperature condition is advantageous for application of CW for wastewater treatment because the plants and microbes in the CW were able to maintain growth activity all year round. Inconstant inflow rates during the various seasons had an impact on the reduction of contaminants in the wetland.
3.2.
Phosphorus removal efficiency and its prediction
3.2.1.
Phosphorus removal efficiency
The average HRT, HLR, seasonal water quality and contaminant removal efficiency, and the influent load and removal efficiency of TP were compared with the average North American surface flow wetland systems (FWSaNA ) (Kadlec and Knight, 1996; Nelson et al., 2003) and wetlands in the cold temperate climate of Norway (Braskerud, 2002) (Table 1). The average pH values of influents and effluents were 7.0 and 7.4, respectively. For a FWS (free-water surface wetland) system there was a good relationship between the load applied and the removal efficiency. The load increased, as did the removal efficiency. This is in good agreement with what was reported by Tanner et al. (1995) and Tanner et al. (1998). Although the TP load rate of this wetland was only 0.23, like that of the FWSaNA , the present CW has slightly higher TP removal efficiency (59%) than that of the FWSaNA (57%). This can be attributed to the
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Fig. 3 – Average influent (-inf) load and removal (-rem) load (in kg per 3 months) of TP (total phosphorus) in spring and autumn.
Fig. 4 – Average influent (-inf) load and removal (-rem) load (in kg per 3 months) of TP (total phosphorus) in summer and winter.
following two factors. Firstly, the soil has a better capability of P absorption, and according to our studies the P absorption capability was 1.21 mg P per gram of soil. This lies within the range of values reported, i.e., 0.032–5.2 (Gale et al., 1994; Sakadevan and Bavor, 1998). Secondly, the intermittent inflow of the present wetland as a result of receiving agricultural runoff all the year round was beneficial to the removal of P (Brix, 1994; Verhoeven and Meuleman, 1999; Sun et al., 2005). In view of the different structural and loading characteristics of natural and artificial systems treating point and non-point source pollution, it is not surprising that the reported retention efficiencies vary widely (Reinhardt et al., 2005). To explain variations in phosphorus retention efficiency it is essential to make long-term and highly temporal resolution investigations that allow the comparison of different system states. In the wetlands (CW-area, 350–900 m2 ) reported by Braskerud (2002), the phosphorus removal rates varied from 21% to 24%, while in this research the phosphorus removal rate was 57%. The difference can be explained by the fact that the HLR in this research was higher than in research performed by Braskerud. Also in our research, the temperature was higher and the phosphorus loading rate was lower. High temperature was beneficial for the growth of macrophytes and the activity of microbes, which was beneficial for the phosphorus absorption by the macrophytes. To understand the performance of the CW in different seasons and under different conditions, we compared the CW performance in spring with that in autumn (Fig. 3), since May and September are generally rainy seasons. The CW had a higher HRT, higher average water temperature (Fig. 2), lower HLR and lower inflow load (except the P load) (Table 1) in autumn than in spring. But in spring the CW had a better TP removal efficiency than in autumn. In spring macrophytes grew and assimilated P rapidly, while from mid to late autumn the macrophytes withered away and released P to the wetland.
In summer the TP removal efficiency was lower than in winter (Fig. 4) due to the following reasons. Firstly, summer had less HRT (1.7 d) than winter (3.8 d). It should be noted that Tanner et al. (1995) and Sakadevan and Bavor (1999) reported that the greater retention time positively enhanced the removal of P from wastewater in the constructed wetland systems. Secondly, the capability of soil to adsorb P in CWs tends to decrease gradually. However, the amount of total P removal was 22.4 kg in summer, much higher than 9.4 kg in winter. It indicated that temperature had no important effect on the removal of P, and this is in agreement with the results reported by Dahab and Surampalli (2001), and Vanier and Dahab (2001). On the other hand, it was found that HRT and exposure time of sediments to water had a significant effect on the removal of P. Removal efficiency of P in winter was not much lower than what was observed in the other seasons (Table 1). A similar result was also reported by Maehlum and Stalnacke (1999), in which they found that for the contaminants, there was less than 10% difference in removal efficiency between the warm and cold periods. Data from the constructed wetlands in Denmark, Sweden and North America showed that winter performance was not significantly reduced as compared to that in the other seasons (Jessen et al., 1993). Furthermore, Kern and Idler (1999) stated that the P removal efficiencies of a reed bed did not show any seasonal variation for domestic and agricultural wastewater. In the present work, the good performance of the CW in winter was mainly due to three factors. Firstly, the first harvest in November 2002 avoided P release caused by the decomposition of wizened plants (Lu et al., 2005; Kroger et al., 2007) and strengthened oxygen diffusion from the atmosphere. Secondly, the average water temperature in winter was higher than 5 ◦ C. Thirdly, the intermittent inflow for this CW, caused by receiving agricultural runoff all year round (especially during
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water balance and plug flow).
Table 2 – Correlation analysis between influent characteristics and operation parameters. Removal efficiency (%) Inflow (m3 d−1 ) P concentration (mg/L) Water temperature (◦ C) Hydraulic load rate (cm/d) Hydraulic retention time (d) P load (kg ha−1 d−1 ) Removal efficiency (%)
Q ln k
C − C∗ 0 Ci − C∗
Temperature effects on k are most important for removal of the various forms of nitrogen, and are generally summarized using the following modified Arrhenius equation in which kt is the rate constant at temperature T (◦ C) and k20 is the rate constant at 20 ◦ C:
0.0028 −0.794 0.0087 0 0.879* 0.0889 1
kt = K20 T−20
Note: *Significant level: ˛ = 0.05, **significant level: ˛ = 0.01, n = 5.
the dry period) was beneficial to phosphorus removal (Verhoeven and Meuleman, 1999; Sun et al., 2005). Correlation analysis between inflow characteristics and operational characteristics is given in Table 2. Hydraulic retention time was related closely to P removal efficiency (Table 2). This is in agreement with the results obtained by Tanner (1994). Water temperature, inflow P load, inflow, inflow P concentrations, and hydraulic load rate had a slight influence on P removal (Table 2). Tanner et al. (1998) determined that the mean annual mass removal of TP was in the range of 15–38% with removal efficiencies being inversely related to loadings in five pilot-scale constructed wetlands in their 4th and 5th years of operation. This may be caused by the difference between HRT and HLR. These two parameters for the five pilot-scale constructed wetlands (which are 1.5–7.0 cm d−1 and 2–9.5 d) are much lower than the corresponding values reported in this study (which are 7.0 cm d−1 and 2 d).
3.2.2.
A=−
Phosphorus removal efficiency prediction
The most commonly used design model currently used for constructed wetlands is the areal k-C* model (Kadlec and Knight, 1996; IWA, 2000), which accounts for temperaturebased, first-order reduction to background levels (C*). C* accounts for substances generated in the wetland by biological activity, sediment release, etc., as well as the non-degradable fraction of the contaminant. Knowing the average wastewater flow (Q, m3 d−1 ), the appropriate area based concentration (Ci ), the wetland areas (A, m2 ) required to meet the target outflow concentration (C0 ) can then be calculated (assuming neutral
Use first-order design model to calculate the outflow phosphorus concentration (C0 ). First, the average temperature is 18.7 ◦ C. Calculate the temperature-corrected first-order rate constants, kt using equation suggested by Kadlec and Knight (1996), e.g., for phosphorus, where K20 = 12 m year−1 and = 1.00, kt = K20 T−20 = 12 Then calculate the predicted outflow phosphorus concentration (C0 ) using the following equation (assuming neutral water balance and plug flow), A=−
Q ln k
2800 = −
C − C∗ 0 Ci − C∗
Q ln k
C − C∗ 0 Ci − C∗
=
242 × 365 ln 12
C − 0.02 0 0.87 − 0.02
C0 = 0.60 mg L−1 The predicted C0 value (0.60 mg L−1 ) is far larger than the measured value (0.36 mg L−1 ). Mainly because the temperature variation of this wetland was smaller than many other constructed wetlands, macrophytes had large biomass all the year round and the soil of this wetland had strong phosphorus adsorption capacity. These reasons explain the better phosphorus removal rate in the wetland.
3.3.
Phosphorus distribution pathway
To understand the contaminant removal mechanisms, the P distribution pathway was studied. Biomass, water contents and nutrient contents of macrophytes are listed in Table 3. Table 3 shows that Zizania Caduciflora Turez Hand-mazt has higher water contents than Phragmites australis (Cav.) Trin.ex
Table 3 – Biomass, water contents and nutrient contents of Zizania Caduciflora Turez Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud. Time
Plant
Water content (%)
Biomass (kg m−2 )
mg P g−1
Nov. 2002
ZC PA
64.0 54.3
6.81 3.59
3.2 1.2
July 2003
ZC PA Emergent species#
74.8 58.2
4.16 3.05
1.5 0.8 7∼9.4
ZC: Zizania Caduciflora Turez Hand-mazt PA: Phragmites australis (Cav.) Trin.ex Steud. #
Data from Greenway (1997). The phosphorus contents here mean the highest value of 4 kinds of emergent species, including Eleocharis sphacelata, Baumea articulata, Typha domingensis, and Cyperus involucratus.
408
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Steud., mainly because the former has a higher ratio of leaf to stem (in weight) than the latter. Table 3 indicates that Zizania Caduciflora Turez Hand-mazt has a larger biomass (in wet weight) than Phragmites australis (Cav.) Trin.ex Steud., mainly because the former has a larger growth density than the latter. The P contents of Zizania Caduciflora Turez Hand-mazt were higher than those of Phragmites australis (Cav.) Trin.ex Steud. The major P distribution pathways in CWs include adsorption, desorption, precipitation, exchange with groundwater, plant harvest and discharge in CWs (Vymazal et al., 1998; Lu et al., 2004). In the period August 2002 to July 2003, two harvests were performed: one in November 2002 and the other in July 2003. It was then known that the corresponding P loads of input (recorded as 100%), discharge (measured value is 38%), plant uptake and removal by harvesting and seed transport (measured value is 36%) can be calculated from the data we have collected. Thus, the P load due to adsorption/desorption/precipitation/and exchange with groundwater (26%) can be calculated as P-input load minus P-discharge load, as well as plant uptake and removal by harvesting and seed transport P load. It should be noted that plant harvest is the major P removal pathway. This is similar to the result of research performed by Tanner (1996), who stated that in 0.238 m2 × 0.6 m deep gravel-bed wetland mesocosms fed with dairy farm wastewaters pre-treated in an anaerobic lagoon, there was a maximum plant accumulation of 18.5 g P m−2 year−1 . This accounts for approximately 30% of the levels supplied in the wastewater. The important role of macrophytes in contaminant removal in this wetland is due to the following three factors. Firstly, good growing seasons provided good conditions for the growth of Zizania Caduciflora Turez Hand-mazt and Phragmites australis (Cav.) Trin.ex Steud., and these plants had large biomass and good P absorption capability. Secondly, plant harvests conducted in July 2003 and December 2003 not only effectively avoided P release from the macrophytes, but also caused the removal of P out of the wetland in large amounts. Thirdly, the influent P load was at low level (TP, 12.9 g m−2 year−1 ), lower than that of municipal wastewater (TP, 66.7 g m−2 year−1 ). Thus, the amount of P removal through the wetland was lower than that for the same wetland used for municipal wastewater treatment.
3.4.
Practicalities and economics of plant harvesting
Because of the lower labour cost, the harvest and carriage expense standard of the emergent macrophytes is 36.9 $US per 1000 m2 . The macrophytes were harvested and then were carried to their homes by some farmers hired by us to their home, because these macrophytes can be used as fuel and feed. Thus, we paid 206.5 $US twice for the harvest and carriage of the macrophytes twice. The amount of phosphorus removal by the two harvests and carriages is 14 kg. We also provided 8600 kg (in dry weight) of fuel and feed for the farmers. The third advantage is that the harvest during winter can effectively inhibit the phosphorus release. The fourth advantage is that the harvests provide some temporary jobs per year. Thus, the harvests have the advantages of promoting pollutant removal (phosphorus load cut off) and gaining considerable economic and social benefits.
4.
Conclusions
This CW was favorable to the removal of TP all year round. When the inflow TP concentrations were 0.87 mg L−1 , the corresponding removal efficiency measurement was 59.0%. Plant harvest had a critical contribution to the treatment of agricultural runoff by CW with a low P load. The removal efficiencies of P by two harvests were about 58% of the total removal load when the influent TP load was 12.9 g m−2 year−1 . This suggested that macrophytes may be critical in achieving low residual phosphorus concentrations in final effluents from such systems. Hydraulic retention time (1.7–3.8 d) had a significant correlation with the P removal efficiency (a = 0.05), whereas water temperature, inflow P load, inflow, inflow P concentrations, and the hydraulic loading rate had a slight influence on P removal.
Acknowledgements We acknowledge the Major Special Program of the Ministry of Science and Technology (863 Program) (K99-05-35-02), (2005AA60101005), China’s national basic research program (2008CB418204). We also thank Mr. Deyi Hou from Stanford University of America and Prof. Yonghui Song from Chinese Research Academy of Environmental Sciences of China for their suggestions.
references
Ayaz, S.C¸., Akc¸a, L., 2001. Treatment of wastewater by natural systems. Environ. Int. 26 (3), 189–195. Braskerud, B.C., 2002. Factors affecting phosphorus retention in small constructed wetlands treating agricultural non-point source pollution. Ecol. Eng. 19 (1), 41–61. Brix, H., 1994. Use of constructed wetlands in water-pollution control-historical development, present status, and future perspectives. Water Sci. Technol. 30 (8), 209–223. Dahab, M.F., Surampalli, R.Y., 2001. Subsurface-flow constructed wetlands treatment in the plains: five years of experience. Water Sci. Technol. 44 (11–12), 375–380. de-Bashan, L.E., Bashan, Y., 2004. Recent advances in removing phosphorus from wastewater and its future use as fertilizer (1997–2003). Water Res. 38, 4222–4246. Diemont, S.A.W., 2006. Mosquito larvae density and pollutant removal in tropical wetland treatment systems in Honduras. Environ. Int. 32 (3), 332–341. Dierberg, F.E., De Busk, T.A., Jackson, S.D., et al., 2002. Submerged aquatic vegetation-based treatment wetlands for removing phosphorus from agricultural runoff: response to hydraulic and nutrient loading. Water Res. 36 (6), 1409–1422. Gale, P.M., Reddy, K.R., Graetz, D.A., 1994. Phosphorus retention by wetland soils used for treated waste-water disposal. J. Environ. Quality 23 (2), 370–377. Greenway, M., 1997. Nutrient content of wetland plants in constructed wetlands receiving municipal effluent in tropical Australia. Water Sci. Technol. 35 (5), 135–142. Gui, M., Zhu, W.P., Yu, G., Zhang, P.Y., 2003. Release regularity of agricultural non-point pollution in drainage area in Dianchi region (in Chinese). J. Agro-Environ. Sci. 22 (1), 1–5. IWA, 2000. Constructed wetlands for pollution control: process, design and operation. International Water Association Scientific and Technical report no. 8. IWA Publishing, London, UK.
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 402–409
Jessen, P.D., Maehlum, T., Krogstad, T., 1993. Potential use of constructed wetlands for wastewater treatment in northern environments. Water Sci. Technol. 28 (10), 149–157. Jiang, C.L., Fan, X.Q., Cui, G.B., Zhang, Y.B., 2007. Removal of agricultural non-point source pollutants by ditch wetlands: implications for lake eutrophication control. Hydrobiologia 581, 319–327. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. Lewis Publishers, Boca Raton, FL. Kern, J., Idler, C., 1999. Treatment of domestic and agricultural wastewater by reed bed systems. Ecol. Eng. 12, 13–25. Kovacic, D.A., David, M.B., Gentry, L.E., Starks, K.M., Cooke, R.A., 2000. Effectiveness of constructed wetlands in reducing nitrogen and phosphorus export from agricultural tile drainage. J. Environ. Quality 29 (4), 1262–1274. Kroger, R., Holland, M.M., Moore, M.T., Cooper, C.M., 2007. Plant senescence: a mechanism for nutrient release in temperate agricultural wetlands. Environ. Pollut. 146 (1), 114–119. Liikanen, A., Puustinen, M., Koskiaho, J., Väisänen, T., Martikainen, P., Hartikainen, H., 2003. Phosphorus removal in a wetland constructed on Former Arable Land. J. Environ. Quality 33, 1124–1132. Liu, W.X., 1997. Study on the application of artificial wetland in agriculture non point source pollution (in Chinese). Res. Environ. Sci. 10 (4), 14–18. Lu, R.K., 2000. Analytic Method of Soil and Agricultural Chemistry (in Chinese). Agricultural Scientific and Technical Press of China, Beijing, pp. 1–338. Lu, S.Y., Zhang, P.Y., Yu, G., Zhu, W.P., Xiang, C.S., 2004. Stabilization pond-plant bed composite system treatment of farmland irrigation and drainage water. China Environ. Sci. 24 (5), 605–609. Lu, S.Y., Zhang, P.Y., Yu, G., Zhu, W.P., Xiang, C.S., 2005. The contaminants release rule of Zizania Caduciflora, Phragmites australis and Eichhornia Crassipes (in Chinese). China Environ. Sci. 25 (5), 554–557. Lu, S.Y., Yu, G., Zhang, P.Y., Zhu, W.P., Gui, M., Xiang, C.S., 2007. Agricultural runoff contaminant discharge in Dianchi Valley agricultural district of China. Int. J. Environ. Pollut. 31 (3/4), 394–414. Maehlum, T., Stalnacke, P., 1999. Removal efficiency of three cold climate constructed wetlands treating domestic wastewater: effects of temperature, seasons, loading rates and input concentration. Water Sci. Technol., 273–281. Nelson, M., Alling, A., Dempster, W.F., van Thillo, M., Allen, J., 2003. Advantages of using subsurface flow constructed wetlands for wastewater treatment in space applications: ground based Mars base prototype. Adv. Space Res. 31 (7), 1799–1804. Reinhardt, M., Gächter, R., Wehrli, B., Müller, B., 2005. Phosphorus retention in small constructed wetlands treating agricultural drainage water. J. Environ. Quality 34 (4), 1251–1259.
409
Romero, J.A., Comín, F.A., García, C., 1999. Restored wetlands as filters to remove nitrogen. Chemosphere 39 (2), 323–332. Sakadevan, K., Bavor, H.J., 1998. Phosphate adsorption characteristics of soils, slags and zeolite to be used as substrates in constructed wetland systems. Water Res. 32 (2), 393–399. Sakadevan, K., Bavor, H.J., 1999. Nutrient removal mechanisms in constructed wetlands and sustainable water management. Water Sci. Technol. 40 (2), 121–128. Scholz, M., Harrington, R., Carroll, P., et al., 2007. The integrated constructed wetlands (ICW) concept. Wetlands 27 (2), 337–354. Serra, T., Fernando, H.J.S., Rodríguez, R.V., 2004. Effects of emergent vegetation on lateral diffusion in wetlands. Water Res. 38 (1), 139–147. State Environmental Protection Administration of China, 2002. Monitoring and Analytical Method of Water and Wastewater (4th edition) (in Chinese). Environmental and Scientific Press of China, Beijing. Sun, G.Z., Zhao, Y.Q., Allen, S., 2005. Enhanced removal of organic matter and ammoniacal-nitrogen in a column experiment of tidal flow constructed wetland system. J. Biotechnol. 115, 189–197. Tanner, C.C., 1994. Treatment of dairy farm wastewaters in Horizontal and up-flow gravel-bed constructed wetlands. Water Sci. Technol. 29 (4), 85–93. Tanner, C.C., 1996. Plants for constructed wetland treatment systems—a comparison of the growth and nutrient uptake of eight emergent species. Ecol. Eng. 7 (1), 59–83. Tanner, C.C., Clayton, J.S., Upsdell, M.P., 1995. Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands. 2. Removal of nitrogen and phosphorus. Water Res. 29 (1), 27–34. Tanner, C.C., Sukias, J.P.S., Upsdell, M.P., 1998. Relationships between loading rates and pollutant removal during maturation of gravel-bed constructed wetlands. J. Environ. Quality 27 (2), 448–458. Vanier, S.M., Dahab, M.F., 2001. Start-up performance of a sub surface-flow constructed wetland for domestic wastewater treatment. Environ. Technol. 22 (5), 587–596. Verhoeven, J.T.A., Meuleman, A.F.M., 1999. Wetlands for wastewater treatment: opportunities and limitations. Ecol. Eng. 12, 5–12. Vymazal, J., 1998. Introduction. In: Vymazal, J., Brix, H., Cooper, P.F., et al. (Eds.), Constructed Wetlands for Wastewater Treatment in Europe. Backhuys Publishers, Leiden, pp. 1–15. Vymazal, J., Brix, H., Cooper, P.F., et al., 1998. Removal mechanisms and types of constructed wetlands. In: Vymazal, J., Brix, H., Cooper, P.F., et al. (Eds.), Constructed Wetlands for Wastewater Treatment in Europe. Backhuys Publishers, Leiden, pp. 17–66.