Ecological Engineering 37 (2011) 128–138
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Synergism of natural and constructed wetlands in Beijing, China Honggang Zhang a , Baoshan Cui a,∗ , Jianming Hong b,1 , Kejiang Zhang c a
School of Environment, Beijing Normal University, State Key Joint Laboratory of Environmental Simulation and Pollution Control, Beijing 100875, China College of Life Sciences, Capital Normal University, Beijing 100048, China c Department of Civil Engineering, University of Calgary, Alberta, Canada T2N 1N4 b
a r t i c l e
i n f o
Article history: Received 17 January 2010 Received in revised form 4 August 2010 Accepted 7 August 2010 Available online 24 September 2010 Keywords: Integrated wetland system Constructed wetlands Natural wetlands Net water loss Water environmental capacity
a b s t r a c t An integrated wetland system (IWS) including constructed wetlands (CWs) and modiﬁed natural wetlands (NWs) for wastewater treatment to replenish water to wetlands located at the Beijing Wetland School (BWS) in Beijing, China, is presented in this paper. The synergistic effects of CWs and NWs on treated water quality are investigated. The IWS is proved to be an effective wastewater treatment technique and a better alternative to alleviate the water shortage for conservation of wetlands based on the monitoring data obtained from October 2007 to 2008. The results show that CWs and NWs play different roles in removing contaminants from wastewater. The COD removal efﬁciency in CWs is higher than that in modiﬁed NWs, whereas the modiﬁed NWs can compensate for the deﬁciency of CWs where a stable and sufﬁcient rhizosphere is not fully formed in the start-up period. All removal rates of COD, TN, and TP in CWs and modiﬁed NWs vary from 50 to 70%, while the total removal rate of COD, TN, and TP in IWS is about 85–90%. The operational results show that the maximum area loading of organic pollutants in modiﬁed NWs (65 kg/ha d) is slightly higher than the empirical one (60 kg/ha d) recommended by USEPA (2000) for free water surface wetlands. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Wetlands are fewer in Beijing due to unbalanced spatial distribution of water resources compared with other cities, such as Shanghai and Wuhan in China (Zhao et al., 2005, 2006; Gong et al., 2007; Xu et al., 2010). These limited wetlands are facing increasing degradation due to water shortage and pollution. In order to conserve existing wetlands and alleviate the degradation of wetlands, the treated domestic and agricultural wastewater can be considered water sources to replenish water to wetlands (Rios et al., 2009). However, conventional wastewater treatment plants in most rural areas of Beijing are not available due to their high costs for construction and operation (Chen et al., 2008). Constructed wetlands (CWs), which are ecologically engineered system and akin to natural wetlands (NWs), are widely used as a cost-effective approach for different kinds of wastewater treatment (Mitsch and Jorgensen, 1989; Sun et al., 2005; Prochaska and Zouboulis, 2006; Eke and Scholz, 2008). Many successful practices in USA, Finland, Italy, New Zealand, Belgium and Poland have been presented in literature (Kern and Idler, 1999; Newman et
∗ Corresponding author. Tel.: +86 010 58802079; fax: +86 010 58802079. E-mail addresses: [email protected]
, [email protected]
(B. Cui), [email protected]
(J. Hong). 1 Tel.: +86 010 68903346; fax: +86 010 68903346. 0925-8574/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2010.08.001
al., 1999; Schaafsma et al., 1999; Knight et al., 2000; Nguyen, 2000; Koskiaho et al., 2003; Mantovi et al., 2003; Poach et al., 2003; Rousseau et al., 2004a; Tuszynska and Obarska-Pempkowiak, 2008). However, a large number of reports emphasize the removal efﬁciency of pollutants or nutrients in CWs while ignoring the fact that CWs often need large areas of land and a longer retention time to achieve acceptable efﬂuent water quality (Akratos and Tsihrintzis, 2007). On the other hand, CWs have relatively low or unstable performance in the start-up period due to immature rhizosphere environments (Vymazal et al., 1998). Although CWs can simulate and enhance some functions of NWs, they cannot completely replace NWs in the removal of pollutants due to a lack of diversiﬁed plant communities, mature microbial communities, and stable rhizosphere environments. The history of NWs for wastewater treatment can be traced back to 1912 (Kadlec and Knight, 1996). Since then, wetlands have been considered alternatives for wastewater treatment due to their apparent treatment capacities. Many NWs, such as riparian wetlands, are used to reduce the nutrient load of through-ﬂowing water by removing nitrates and phosphorus from surface and subsurface runoff (Zedler, 2003; Hogan et al., 2004; Hogan and Walbridge, 2007). However, some problems of NWs are also obvious, such as lower contaminant load and limited self-purifying capacities. It is assumed that an integrated wetland system (IWS) composed of CWs and NWs based on the principle of complementary
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
Fig. 1. Decision support tree for the design of the optimum combination of constructed wetlands and modiﬁed natural wetlands.
advantages can achieve greater treatment capacity than any individual system. Thus, how to design and operate the synergistic system of CWs and NWs will be a focus in water quality improvement (Mitsch and Gosselink, 2000; Zedler and Kercher, 2005; Verhoeven et al., 2006). In this paper, an IWS is applied to purify the domestic wastewater discharged from the student dormitories. Treated water will be recharged to wetlands at the Beijing Wetland School (BWS). The objectives of this paper are to (1) develop a design method for IWS used for wetland conservation; (2) determine the efﬁciency of IWS to treat domestic wastewater; and (3) provide information and transfer knowledge to other wetland conservation projects. 2. Materials and methods 2.1. Background and study area The study IWS was built in BWS in Shunyi District located northeast of Beijing, with a latitude of 40◦ 10 N and a longitude of
116◦ 45 E. This area is located in a north temperate zone with semihumid temperate continental monsoon climate. The total area of wetlands in BWS is 6500 m2 with the open water zones of 3500 m2 . A variety of water birds are often attracted to inhabit or forage in wetlands due to higher diversity of patterns including emerged, submerged, and ﬂoating plants together with open water bodies. BWS has become the main place of environmental education for instructors and students from primary and secondary schools in Beijing. Recently, a serious degradation of wetlands has been identiﬁed due to the shortage of water sources. Restoration of the degraded wetlands was scheduled by local governments. Some approaches are limited to available water sources. In some cases, groundwater has to be used to maintain the wetlands. To ﬁx the issue of water shortage, wastewater discharged from two students’ dormitories was puriﬁed using IWS and then recharged to wetlands. The IWS is located to the north of the dormitories and its total area is approximately 2595 m2 . Wetland monitoring was carried out in July to October 2007 and 2008, when the greatest quantity of wastewa-
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
Fig. 2. The planning graph of IWS in Beijing Wetland School.
ter was discharged from the dormitories due to an increase of bath water. Fig. 1 shows a decision support tree for the design of IWS including the CWs and the modiﬁed NWs.
Open water zones are the main evaporating surfaces in wetlands and a direct means of measuring evaporation is given by: QE = Ao × E × 10−3
(2) (m2 )
2.2. Calculation for net water loss Water supplement for wetlands mainly depended on groundwater to maintain wetland ecosystem in a healthy and stable state before this project was implemented. A variety of processes such as evaporation, seepage, and direct precipitation affect water content in wetlands. The net water loss (Q) is determined based on water balance given by: Q = QE + QS − QP
where QE is the evaporation from wetlands (m3 /d), Qs is the seepage out of wetlands (m3 /d), and Qp is the direct precipitation onto wetlands (m3 /d).
where Ao is area of open water and E is capacity of water surface evaporation (mm/d). The average value of precipitation is computed for each month using data from Beijing Meteorological Bureau by: QP = Ao × P × 10−3
where Ao is area of open water (m2 ), P is precipitation direct onto wetlands (mm/d). Seepage loss is given by: QS = K × A ×
where K is the coefﬁcient of seepage (m/d), A is the area of water bed (m2 ), H is head difference between water surface and underground water table, and L is the seepage distance.
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
Table 1 The characteristics of each experimental system, HSF means horizontal subsurface ﬂow; VSF means vertical subsurface ﬂow. System
Area (m2 )
Layer thickness or water depth (cm)
Medium size (mm)
Biological Pond Pond A
15 55 20 15 55 20 Water depth 60–70 Water depth 40–50
Water depth 40–60
Gravel (5–15) Gravel (20–40) Gravel (40–50) Gravel (5–15) Gravel (20–40) Gravel (40–50) Soils Soils Soils Soils Soils Soils Soils Soils Soils Soils Soils Soils Soils
Eichhornia crassipes Phragmites australis Typha latifolia Iris tectorum Acorus calamus Scirpus validus Typha latifolia Nelumbo nucifera Scirpus triqueter Trapa incisa Nymphoides peltatum Myriophyllum verticillatum Ceratophyllum demersum
2.3. Modiﬁcation for NWs
actual available area of CWs; ALR is the area loading rates recommended by USEPA (2000) (6000 mg/m2 d).
The design of modiﬁed NWs needs to determine whether the site area can meet the requirements of CWs that are necessary to ensure that the water quality of efﬂuent is compliance with the Grade IV (MEPC, 2002). Although BOD5 and COD are widely used as indicators of organic pollutants in wastewater, two key factors must be considered in determining BOD5 : one is the longer duration for BOD5 analysis, another is the disturbance of toxic substances to the determination condition (Reynolds, 2002; Pisarevsky et al., 2005). In this paper, COD rather than BOD5 was used as design parameter to calculate the area of CWs. The area of CWs is determined by Kadlec and Knight (1996): AS = 5.2Qmax (ln C0 − ln Ce )
where AS is the surface area of CWs (m2 ), Ce
is the concentration of COD in efﬂuent (mg/L), C0 is the concentration of COD in inﬂuent (mg/L), and Qmax is the maximum inﬂow of CWs (m3 /d). If available land is smaller than the area calculated from Eq. (5), the NWs will be modiﬁed; otherwise the NWs keep the same. 2.3.1. Conﬁguration Ponds A and B were reconstructed to minimize the disturbances to wetlands by properly determine the length-to-width ratio, water depth, and plant conditions (see Fig. 2). The length-to-width ratio of ponds A and B higher than 1:1 helps to minimize short circuiting. In addition, the topographic slope ensures the water ﬂow from ponds A to B by gravity and avoid backwater in modiﬁed NWs (USEPA, 2000). The general water depth varying from 30 to 50 cm could meet the design water depth of reconstructed NWs (40–50 cm). The vegetation in ponds A and B mainly consists of Typha latifolia and Phragmites australis, which facilitate pollutant removal (Hill et al., 1997; Koottatep and Polprasert, 1997; Scholz and Xu, 2002). In modiﬁed NWs, the open water area was reserved and a variety of wetland plants were planted in shallow water and edge zones. Area loading rate method applied to estimate the least area of reconstructed NWs is given by USEPA (2000): Aw =
Q ×C ALR
2.3.2. Water controlling Water volume directly inﬂuences the detention time of wastewater in wetlands. Although the modiﬁed NWs receive continuously treated water, its volume varies from 100 to 120 m3 due to the variation of discharged wastewater from dormitories. Thus, a water dam was installed at outlet to regulate water depth and water volume in the modiﬁed NWs. The hydraulic retention time (HRT) is given by: t=
where Vw is available water volume and Qave is the average ﬂow rate. 2.3.3. Plant community To optimize the vegetation structure and enhance plant diversity, wetland plants including ﬁve rhizomatic-root plant species, i.e., T. latifolia, P. australis, Acorus calamus, Iris tectorum and Nelumbo nucifera, and two ﬁbrous-root plant species, i.e., Scirpus validus and Scirpus triqueter, were transplanted into the modiﬁed NWs. Other plants, such as Trapa incisa, Nymphoides peltatum, Myriophyllum verticillatum and Ceratophyllum demersum, were also used to improve the construction and landscape of wetlands. All plant species were collected from the nearby Han Shiqiao Wetland Nature Reserve BWS (Table 1). 2.4. Design of CWs 2.4.1. Inﬂuent and efﬂuent types CWs mainly receive domestic wastewater after preliminary treatment in septic tank. The average inﬂow concentrations of major water quality parameters are given in Table 2. Efﬂuent COD concentration obtained from Eq. (5) is used as water quality target which was determined by water environmental capacity of modiﬁed NWs.
where Aw is the total area of modiﬁed NWs (m2 ); Q is the inﬂow rate (m3 /d), the maximum monthly inﬂow (Qmax ) is calculated by 120 m3 /d; C is the inﬂuent concentration (mg/L), the COD concentration in efﬂuent from CWs is calculated by formula (5) using the
2.4.2. Constructions and conﬁgurations The CWs have two parallel pilot-scale units. Each unit encompasses storage pond, horizontal subsurface ﬂow (HSF), vertical subsurface ﬂow (VSF), biological pond, aeration ﬂume, collecting pond, inﬂow controlling pond and central controlling well (Fig. 3).
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
Table 2 Treatment performances of the IWS between 2007 and 2008. 2007
Removal (%) Max Ave Min Inﬂuent (mg/L) Max Ave Min Efﬂuent (mg/L) Max Ave Min Water quality IV (mg/L) Water quality III (mg/L)
93 91.2 86.7
93.4 90.9 88.5
96 94.6 93.3
82.7 79.1 74.2
97.1 95.1 92.9
97.9 96.5 96.1
97 84.6 69.1
5.6 4.6 3.1
99 83.4 74.1
100 89.5 81
5.3 4.6 3.4
10.7 7.6 5.3 1.5
0.12 0.23 0.17 0.3
19.1 17.1 15.6
7.1 4.4 2.6
0.29 0.1 0.2
358 306 225 29 26.5 24 30
• Storage pond: The main purpose of the storage pond is further sedimentation. The setting of the storage pond at the initial stage of CWs has the potential to extend the operational life of subsequent units in the scenario of high contents of suspended matters in inﬂow. • Aeration channel: The aeration channel is used to enhance the dissolved oxygen (DO) in water.
• Subsurface ﬂow (HSF & VSF) CWs: The main advantage of a subsurface ﬂow system is the isolation of the wastewater from animals and humans. Concerns with mosquitoes and pathogen transmission can be greatly reduced with a subsurface ﬂow system. The design parameters of each unit are given in Table 1. • Biological pond: Some ﬂoating plants such as Eichhornia crassipes with water depth of 60–70 cm are planted in the biological pond. • Collecting pond: The purposes of the collecting ponds are to collect treated efﬂuent from each group and aerate by cascading ﬂow. • Inﬂow controlling pond and central controlling well: These are used to control the distribution of inﬂuent into the two groups of CWs and regulate the pipelines used in summer, winter and emptying phase, respectively. The down-gradient location of CWs from the inlet to outlet ensures a gravity water ﬂow system. All pilot-scale units are lined with bentonite waterproof blanket with thickness of 0.8 cm. From bottom to top, gravel (size) of 40–50, 20–40, and 5–15 mm with the corresponding depth of 20, 55 and 15 cm was ﬁlled in the HSF and VSF. In order to uniformly distribute water over the bed surface and ensure that the movement of water in each cell a cross the maximum distance from inﬂow point to outﬂow one, several sizes of PVC pipes (20, 10 and 7 cm in diameter) with perforated tubes (0.8 cm in diameter) were used to distribute the wastewater ﬂow evenly into each unit. Active elbow pipes were placed at the outlet points for each cell to control water level and sample efﬂuent. Outlet gate
Fig. 3. The plan of constructed wetlands, where HSF means horizontal subsurface ﬂow; VSF means vertical subsurface ﬂow; the arrow direction is the direction of water ﬂow.
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
Table 3 Design parameters calculated by different formulas. Net water loss (m3 /d)
C0 (mg/L) 306 306
Ce (mg/L) 30 109b
As (m2 ) 1449a 595c
Q (m3 /d) 120 120
C (mg/L) 109 100d
Vw (m3 ) 975
Qave (m3 /d) 110
Qmax (m3 /d) 120 120 ALR (mg/m2 d) 6000 6000 t (d) 8.9
a b c d e
Aw (m2 ) 2180 2000e
The land area requirement of CWs calculated by formula (5). The concentration in efﬂuent from CWs with area of 595 m2 calculated by formula (5). The actual area of CWs. The COD concentration of water environmental capacity of modiﬁed NWs with area of 2000 m2 calculated by formula (5). The actual area of modiﬁed NWs.
valves located at the centre of the bottom plate for each group unit of CWs were set to adjust outﬂow and collect outﬂow sample water. During this study, all units receive the same ﬂow rates. The hydraulic loading rate (HLR) is determined by: HLR =
where Qave is the average ﬂow and As is the surface area. The 0.6-m wide footprint of the upper embankments was also designed to provide easy access for maintaining and monitoring as well as checking for any water leakage, slippage or distortion. A small pool ﬁlled with gravels (size) of 40–50 cm around the main CWs was set to collect runoff and storm water from adjacent ﬁelds. Fences were installed around CWs to restrict humans and livestock to access. 2.4.3. Planting Each subsurface ﬂow unit of CWs was planted with T. latifolia and P. australis initially with a density of eight plants (approximately 40 cm in height for each plant) per square meter in April 2007. Planting of macrophytes was conducted by hand into substrates with the depth of 10–15 cm. 2.5. Water quality monitoring The variation of discharged wastewater from the students’ dormitories was identiﬁed. The maximum and minimum occurred respectively from July to October and from November to March. During the less wastewater period, wastewater was stored in septic tanks and treated by intermittent operation of IWS. In this study, the monitoring of IWS was mainly conducted from July to October 2007 and 2008. Water quality monitoring stations were installed at the inlet and outlet of CWs. Two sampling points in modiﬁed NWs were also identiﬁed (see Fig. 2). Treated water samples were collected twice a month with two parallel samples using plastic containers. Samples were transported to the laboratory in BWS and analyzed immediately for TN, TP and COD using the method presented in the Standard Method for Examination of Water and Wastewater (MEPC and WWMAA, 2002). The treatment efﬁciency is calculated by the percent removal (R) for each parameter given by: R=
where Ci and Ce are the inﬂuent and efﬂuent concentrations of pollutants (mg/L), respectively. The mean value of two samples was
used to calculate the removal rate for each parameter to eliminate the error caused by sampling. 2.6. Statistical analyses Independent-samples t-test and Paired-samples t-test were implemented to determine any signiﬁcant difference in removal percentages between two periods (2007 and 2008) and between Pond A and Pond B at the level of p ≤ 0.05. Pearson’s correlation coefﬁcient (r) was calculated on the basis of linear dependence between two variables (the pollutant concentration in inﬂow and the removal efﬁciency) and the correlations among removal efﬁciencies for COD, TN, and TP were also analyzed. Statistical analysis was performed by using Microsoft Ofﬁce Excel and the computing package called Statistical Package for Social Science (SPSS 16.0 for Windows, SPSS Inc., IL, USA). For all tests, water monitoring data distributions were tested for normality. If data was not normally distributed, it was log-transformed and tested for normality. All data were normally distributed after transformation. 3. Results 3.1. Design parameters of IWS In this study, the average quantity of wastewater from two student dormitories (110 m3 /d) can compensate the wetland water loss (103.4 m3 /d) (Table 3). Due to the restriction of available land, the actual area of CWs (595 m2 ) is smaller than the required one (1449 m2 ) according to Eq. (5). Thus, the COD concentration in efﬂuent of CWs does not comply with the Grade IV water quality standards (30 mg/L). In order to ensure efﬂuent water quality and minimize construction costs and environmental disruption, it is necessary to transform the natural wetlands. Efﬂuent from CWs was discharged into pond A and further ﬂowed into pond B with sufﬁcient assimilative capacity. Results show that the actual area of enhanced NWs can reach 2000 m2 where the water environmental capacity of 100 mg/L is lower than the COD concentration (109 mg/L) of efﬂuent from CWs in 2007, and the area loading rates is 6540 mg/m2 /d. The HRT of modiﬁed NWs is about 6.6 d with water depth of 40–50 cm and the theoretical HRT of CWs is about 2.5 d. The HLR of CWs is about 0.24 m/d obtained from Eq. (8). On the other hand, a dozen species of water birds in wetlands, such as Gallinula chloropus, Ixobrychus sinensis, Acrocephalus orientalis and Tachybaptus novaehollandiae, were identiﬁed. Moreover, some nests of G. chloropus and A. orientalis were also found after the IWS was constructed. This indicates that the pattern of
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
Table 4 Correlation analysis between inﬂuent concentration and removal efﬁciency of each system. System COD CWs Pond A Pond B TN CWs Pond A Pond B TP CWs Pond A Pond B *
0.752* 0.574* 0.719*
0.032 0.037 0.045
0.501 0.524 0.536
0.252 0.182 0.17
−0.166 0.331 0.576
0.694 0.423 0.135
Signiﬁcant level p = 0.05, n = 32.
Table 5 Correlation matrix of removal efﬁciency between COD, TN and TP.
COD Pearson Correlation Sig. (two-tailed) TN Pearson Correlation Sig. (two-tailed) TP Pearson Correlation Sig. (two-tailed) **
Correlation is signiﬁcant at the 0.01 level (two-tailed), n = 32.
modiﬁcation, the increase of plant diversity, and the optimization of vegetation communities improve landscape heterogeneity and habitat diversity of wetlands. 3.2. Removal efﬁciency of IWS Treatment performances of IWS are summarized in Table 2. The results show that the average removal efﬁciencies of TN and TP in 2008 are higher than that in 2007. COD removal rate is about 79% which is lower than that in 2007. The average removal efﬁciencies of IWS for COD, TN and TP are 85, 93 and 95%, respectively, in monitoring period. According to EQSSWC (MEPC, 2002), the concentration of COD in efﬂuent can meet the requirement of Grade IV water quality standards (30 mg/L) and TP concentration is in compliance with the Grade III water quality standards (0.3 mg/L). For each component of IWS (CWs and NWs), the removal rate of COD shows the signiﬁcantly positive correlation (p ≤ 0.05) with the inﬂow concentration, whereas the removal rates of TN and TP are not obviously correlated with the inﬂow concentration (Table 4). The correlations among removal efﬁciencies for COD, TN, and TP are shown in Table 5. The removal efﬁciency of COD is positively correlated with that of TN with a correlation coefﬁcient of 0.561 (p ≤ 0.01). However, the correlations between the removal efﬁciencies for TN and TP, as well as COD and TP are not clear. 4. Discussion 4.1. IWS design In this research, a proper ecological engineering approach is carried out to improve the design and management of IWS. It is important to determine the area under the speciﬁed water volume and HRT, which is closely related to the removal performance of IWS (Nahlik and Mitsch, 2006; Carty et al., 2008). The overall area of IWS is determined based on the COD concentration in inﬂuent and area loading rate of COD. As the actual area of CWs is smaller
than the calculated area, water quality of efﬂuent of CWs is not in compliance with the Grade IV water quality standards recommend by EQSSWC (MEPC, 2002). The increase of the area of CWs will signiﬁcantly increase the construction costs. Moreover, increase of the area of CWs need to cut down dozens of trees with 13–18 cm diameter at breast height (DBH) or destroy natural wetlands more than 1000 m2 in BWS. These will potentially cause the deterioration of environment. Thus ﬁnal water quality mainly depends on the modiﬁcation of NWs. Although the area of both CWs and modiﬁed NWs are smaller than the calculated area, the COD concentration in efﬂuent of IWS meets the expected goal. This result reveals that the actual area of IWS is optimal than the calculated area. This also implies that the area of wetlands should be determined based on the target pollutants together with the inﬂuent and efﬂuent concentrations. In our study, organic pollutants are primary objective for the wastewater treatment. Many previous studies demonstrated that the principal design criteria leading to adequate efﬂuent water quality from wetlands is determined by target pollutants (Masi et al., 2002; Vymazal, 2005; Li et al., 2009). For example, 13 wetland systems in Waterford (Ireland) all consider the phosphorus as target pollutant (<1 mg/L in efﬂuent) (Scholz et al., 2007). The area ratio between different systems is important when various kinds of wetland systems are combined together to achieve higher treatment efﬁciency. In Italy, for example, two hybrid wetland systems with different area ratios (480 m2 :850 m2 for HSF to free water surface wetland and 180 m2 :102 m2 :148 m2 for VSF to HSF to free water surface wetland, respectively) were successfully used for concentrated winery wastewater treatment (Masi et al., 2002). Higher COD load (4044.9 and 1003.2 mg/L) was designed for each wetland system and the removal efﬁciency is more than 90% for each one. The area ratios between different systems are determined not only based on target pollutant, also depend on hydraulic loading, ﬁlling depth, and the basin slope. While in this research, in addition to these factors, the limited land and construction costs are also considered in determination of the area ratio between the CWs and NWs. HRTs for modiﬁed NWs and CWs are 6.6 and 2.5 d, respectively, which are consistent with the recommended value reported by Wood (1995), who presented that HRT for subsurface ﬂow CWs should fall into the range of 2–7 d. It should also be noted that the HLR (0.24 m/d) for this CWs is higher than the range value (0.025– 0.05 m/d) presented by Brix (1994) for vertical ﬂow wetlands. The optimization of plant communities can not only increase biodiversity in IWS, but also improve the treatment efﬁciency (Brix, 1997; Alvarez and Becares, 2008; Cheng et al., 2009). Two kinds of aquatic vascular plants (T. latifolia and P. australis) in CWs beneﬁt pollutant removal processes (Hill et al., 1997; Koottatep and Polprasert, 1997). These two species have evolved to enable their own to root in substrates with little or no available oxygen and grow vertically through the water column with most of their leaves in air. It should be emphasized that lack of proper management at the beginning of operation such as water level controlling can lead to lower aquatic plant coverage and the spread of grasses. Some researchers stated that the shallow water (<100 mm) will enhance the establishment of grasses and weeds, which can restrict the growth of the macrophytes (McCuskey et al., 1994; Scholz and Lee, 2005). Some previous studies also show that the management of water depth is particularly important to facilitate optimal water treatment (Dunne et al., 2005a,b; Scholz, 2007). 4.2. Removal efﬁciency Wastewater in wetlands is puriﬁed by complex processes including the interaction among aquatic plants, micro-organisms,
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
water and soil (Scholz and Xu, 2002; Scholz et al., 2007). The average removal efﬁciency of COD (79.1%) is lower in 2008 than that (91.2%) in 2007 (Fig. 4a) due to the relatively low inﬂuent organic loading rate of CWs (average 83.4 mg/L) (Yalcuk and Ugurlu, 2009). Lower organic loading is caused by an underground Micro-Organism Treatment system which is installed between septic tank and IWS to reduce organic concentration in wastewater to achieve the requirements of water environmental capacity of modiﬁed NWs and extend the service life of IWS in April 2008. Analysis of Pearson’s correlations shows that the relationship between inﬂuent concentration and removal efﬁciency of COD in IWS is statistically important (p < 0.05, r = 0.57–0.75) (Table 4), the result is consistent with previous studies (Korkusuz et al., 2005; Li et al., 2008). In our research, the removal rates of TN are not apparently correlated with the inﬂow concentrations (Table 4) due to the relatively higher TN inﬂuent concentration (Kadlec and Knight, 1996; Tanner et al., 1998). The lower removal rates in CWs (approximately 59% seen in Fig. 4b) are mainly due to the unstable redox process in rhizosphere and plant roots (average depth of only 10–20 cm), which are not extensive for microbial attachment and oxygen transport (Stottmeister et al., 2003; Wu et al., 2008). Moreover, the bed body with 90 cm deep in CWs results in relatively less aerobic condition. This limits the nitriﬁcation rates and leads to lower removal efﬁciency of nitrogen (Yalcuk and Ugurlu, 2009). The nitrogen removal ability is also affected by the low coverage and biomass of the plants in CWs (Wu et al., 2008). Mechanisms of phosphorus removal in wetlands include sedimentation, plant uptake, peat accretion, and sorption reactions (Reddy et al., 1999; Braskerud, 2002; Koskiaho et al., 2003). In our research, no signiﬁcant difference of the removal efﬁciencies between each system of IWS (Fig. 4c) is identiﬁed, while the removal rates are relatively stable in CWs. It is possible that the gravel selected as substrates may improve TP removal rate due to the ﬁne medium providing higher speciﬁc surface area for phosphorous adsorption (Lin et al., 2002; Van de Moortel et al., 2009). In Fig. 4c, one can observe that although CWs are still in the startup period, the TP removal efﬁciency reaches 62.4%, which is in compliance with the value of 60–70% presented by Vymazal (2007). As shown in Table 5, the relationship between the removal efﬁciency of COD and TN is statistically signiﬁcant. This might be induced by the higher loading rates of organic compounds, which lead to lower oxygen content and then the anaerobic degradation of organic compounds occurs (Cooper et al., 1996). At the same time, as carbon source, the decomposed organic compounds can provide energy for denitrifying bacteria to promote the denitriﬁcation which is one of the most important ways for nitrogen removal in wetlands (Yalcuk and Ugurlu, 2009). Whereas the removal efﬁciencies of TN and COD are not clearly correlated with TP removal rate, implying that although biological degradation plays a role in the removal of phosphorus in wetlands (Behrends et al., 2001; Sun et al., 2005), the predominant way of phosphorus removal is physical and chemical processes. It should be noted that although the monitoring is carried out from July to October in this study, the operation of IWS in the rest of the year, especially in cold weather, is also considered in system design. Subsurface ﬂow (HSF&VSF) CWs and the pipe arrangement ensure IWS can be used in cold weather. The temperature and seasonal conditions can affect both physical and biological activities within the wetland system (Werker et al., 2002). Kadlec and Knight (1996) presented that nitrogen removal in CWs is a temperature dependent process. The subsurface ﬂow CWs in IWS is more suitable for colder climate condition as the treatment occurs below the ground surface, and where bacterial communities are thereby insulated from the cold air. Higher ﬂuctuation of NH4 + –N, NO3 − –N and
Fig. 4. Correlation analysis between inﬂuent concentration and removal efﬁciency of IWS. Lines indicate removal efﬁciency and histograms indicate the inﬂuent concentration of each unit; CWc is inﬂuent concentration of CWs; pond Ac is inﬂuent concentration of pond A; pond Bc is inﬂuent concentration of pond B; CWr is removal efﬁciency of CWs; pond Ar is removal efﬁciency of pond A; pond Br is removal efﬁciency of pond B.
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
TN during different seasons were identiﬁed by Li et al. (2008). They found that the removal efﬁciencies of NH4 + –N and TN are the lowest in winter compared with that in other seasons. However, they also stated that there are no statistically signiﬁcant seasonal differences between COD removal efﬁciencies during the 12-month investigation. Whereas Maehlum and Stalnacke (1999) found that the removal efﬁciencies for N, P and COD are all lower than 10% in cold weather compared to those in warm periods. Generally, Beijing’s cold climate in winter challenges the operation of IWS because its functional maturity in pollutants removal is not well established over the ﬁrst winter, and may not be improved until the end of the ﬁrst growing season. The improvement in pollutant removal is attributed to development of a mature system, stable microbial population, and plant establishment (Wallace et al., 2001). Thus further monitoring is required in order to improve the IWS performance in future operation. 4.3. Implication for wetland conservation The IWS receives signiﬁcant wastewater. The efﬂuent from IWS is discharged into pond C and pond D for the compensation of water consumption of wetlands. Although COD concentration in efﬂuent from CWs is a little higher than water environmental capacity of the modiﬁed NWs in 2007, the ﬁnal efﬂuent meets the expected goals. The result indicates the maximum area loading of organic pollutant in modiﬁed NWs (65.4 kg/ha d) is slightly higher than that presented in USEPA (2000) (60 kg/ha/d). This might have resulted from that emerged and submerged plants, soil surface, and the water column are almost completely colonized by bioﬁlms before the start of the modiﬁcation and these conditions recover quickly after the modiﬁcation (Poe et al., 2003; Rousseau et al., 2004b). Thus, there are many favorable conditions for the removal of pollutants such as diversiﬁed plant communities and reasonable plant community structures, mature microbial communities and stable rhizosphere environments in modiﬁed NWs (USEPA, 2000; Garcia et al., 2005; Verhoeven et al., 2006; Carty et al., 2008). After the application of IWS for 2 years, water shortage and water quality in wetlands have been obviously improved. The removal performance of IWS is better than that presented in Gunes and Tuncsiper (2009). In their study, a system consisted of a combination of buried sand ﬁltration and subsurface ﬂow constructed wetlands was used to treat wastewater with 85.9 mg/L for TN and 8.94 mg/L for TP in the village of Ileydagi and the average removal values observed during a 14-month period were 85% for TN and 69% for TP. The design of IWS not only avoids the deﬁciency of using CWs or NWs solely, but also enhances the synergistic effects of constructed and natural ecosystem on wastewater treatment. The modiﬁed NWs play a crucial role in keeping better removal efﬁciency in the start of CWs where rhizosphere has been not well established. Moreover, the shallow water ﬂow with open water zones can provide aerobic conditions and sufﬁcient time for microbial metabolic activity and enhance the nitriﬁcation rates (Gearheart, 1992; USEPA, 2000). Based on the same water quality target, the IWS covering 2595 m2 can save much costs than single CWs which cover an area of 1449 m2 . This is caused by the modiﬁcation of NWs in IWS which reduces the inevitable costs of earthwork and construction. Considering the total investment divided into 20 years of lifetime, the average construction cost per unit of wastewater volume treated is 0.272 RMB/m3 for IWS and 0.661 RMB/m3 for single CWs (Table 6). The main costs of IWS are earthwork and construction, and others are planting and labor costs. The total excavation cost for 2595 m2 of IWS is about 10411.5 RMB, where CWs account for 94%, depending on the depth of CWs (about 1.5 m) and unit price of earthwork
Table 6 Cost comparison between integrated wetland system and constructed wetlands.
Area (m ) Earthwork (m3 ) Earthwork costa (RMB) Construction cost (RMB) C/Tb (RMB/m3 ) a b
2595 946.5 10411.5 208,250 0.272
1449 2173.5 23908.5 507,150 0.661
11 RMB/m3 . C/T construction cost per unit of wastewater volume treated.
(11 RMB/m3 ) (Table 6). While the earthwork cost for modiﬁed NWs is only 594 RMB, as the modiﬁcation of edge zones of NWs requires less soil to be moved. Construction costs of IWS mainly come from the construction of CWs including costs of membrane, substrate, pipes, and other facilities. The costs of planting are smaller because the plants removed from NWs being modiﬁed are replanted into the CWs. These minimize the cost of plant purchase. 5. Conclusions The performances of IWS for domestic wastewater treatment are investigated in this paper. In the IWS, wastewater discharged from septic tank ﬁrst ﬂows into the CWs and then is further treated in the modiﬁed NWs. Generally, the total removal rate of COD, TN, and TP is about 85–90% in the IWS. The IWS provides sufﬁcient puriﬁed water to replenish wetlands. The synergistic effects of the IWS are comparable to that of any single system (CWs or NWs). The results indicate that the application of the integrated treatment system is economically and environmentally feasible, which can ﬁx the issues of water shortage and wastewater problem simultaneously, implying that IWS has a signiﬁcant potential in conservation of wetlands. Acknowledgements The research was ﬁnancially supported by National Natural Science Foundation (U0833002), the program from Beijing Municipal Science & Technology Commission (D07040600770701-8), and Scientiﬁc Research Foundation of Beijing Normal University (No. 2009SD-24). The authors would like to acknowledge postgraduates Bingbing Jiang and Jingjing Ruan from the College of Life Science, Capital Normal University, for their help in data collection. The authors also acknowledge the contributions of two anonymous reviewers and the editor-in-chief for their constructive comments which signiﬁcantly improve the quality of this paper. References Akratos, C.S., Tsihrintzis, V.A., 2007. Effect of temperature, HRT, vegetation and porous media on removal efﬁciency of pilot-scale horizontal subsurface ﬂow constructed wetlands. Ecol. Eng. 29, 173–191. Alvarez, J.A., Becares, E., 2008. The effect of vegetation harvest on the operation of a surface ﬂow constructed wetland. Water SA 34, 645–649. Behrends, L., Houke, L., Bailey, E., Jansen, P., Brown, D., 2001. Reciprocating constructed wetlands for treating industrial, municipal and agricultural wastewater. Water Sci. Technol. 44, 399–405. Braskerud, B.C., 2002. Factors affecting phosphorus retention in small constructed wetlands treating agricultural non-point source pollution. Ecol. Eng. 19, 41–61. Brix, H., 1994. Use of constructed wetland inwater pollution control:historical development, present status, and future perspectives. Water Sci. Technol. 30 (8), 209–223. Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water. Sci. Technol. 35, 11–17. Carty, A., Scholz, M., Heal, K., Gouriveau, F., Mustafa, A., 2008. The universal design, operation and maintenance guidelines for farm constructed wetlands (FCW) in temperate climates. Bioresour. Technol. 99, 6780–6792. Chen, Z.M., Chen, B., Zhou, J.B., Li, Z., Zhou, Y., Xi, X.R., Lin, C., Chen, G.Q., 2008. A vertical subsurface-ﬂow constructed wetland in Beijing. Commun. Nonlinear Sci. Numer. Simul. 13, 1986–1997.
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138 Cheng, X.Y., Chen, W.Y., Gu, B.H., Liu, X.C., Chen, F., Chen, Z.H., Zhou, X.Y., Li, Y.X., Huang, H., Chen, Y.J., 2009. Morphology, ecology, and contaminant removal efﬁciency of eight wetland plants with differing root systems. Hydrobiologia 623, 77–85. Cooper, P.E., Job, G.D., Green, M.B., Shutes, R.B.E., 1996. Reed Beds and Constructed Wetlands for Wastewater Treatment. Water Research Center Publications, Medmenham, Marlow, Bucks, UK. Dunne, E.J., Culleton, N., O’Donovan, G., Harrington, R., Daly, K., 2005a. Phosphorus retention and sorption by constructed wetland soils in Southeast Ireland. Water Res. 39, 4355–4362. Dunne, E.J., Culleton, N., O’Donovan, G., Harrington, R., Olsen, A.E., 2005b. An integrated constructed wetland to treat contaminants and nutrients from dairy farmyard dirty water. Ecol. Eng. 24, 221–234. Eke, P.E., Scholz, M., 2008. Benzene removal with vertical-ﬂow constructed treatment wetlands. J. Chem. Technol. Biotechnol. 83, 55–63. Garcia, J., Aguirre, P., Barragan, J., Mujeriego, R., Matamoros, V., Bayona, J.M., 2005. Effect of key design parameters on the efﬁciency of horizontal subsurface ﬂow constructed wetlands. Ecol. Eng. 25, 405–418. Gearheart, R.A., 1992. Use of constructed wetlands to treat domestic wastewater, City of Arcata, California. Water Sci. Technol. 26, 1625–1637. Gong, Z.N., Gong, H.L., Zhao, W.J., Ll, X.J., Hu, Z.W., 2007. Using RS and GIS to monitoring Beijing wetland resources evolution. IGARSS: 2007. IEEE Int. Geosci. Remote Sensing Symp. 1/12, 4596–4599. Gunes, K., Tuncsiper, B., 2009. A serially connected sand ﬁltration and constructed wetland system for small community wastewater treatment. Ecol. Eng. 35, 1208–1215. Hill, D.T., Payne, V.W.E., Rogers, J.W., Kown, S.R., 1997. Ammonia effects on the biomass production of ﬁve constructed wetland plant species. Bioresour. Technol. 62, 109–113. Hogan, D.M., Jordan, T.E., Walbridge, M.R., 2004. Phosphorus retention and soil organic carbon in restored and natural freshwater wetlands. Wetlands 24, 573–585. Hogan, D.M., Walbridge, M.R., 2007. Urbanization and nutrient retention in freshwater riparian wetlands. Ecol. Appl. 17, 1142–1155. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands-Theory and Implementation. 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. Knight, R.L., Payne, V.W.E., Borer, R.E., Clarke, R.A., Pries, J.H., 2000. Constructed wetlands for livestock wastewater management. Ecol. Eng. 15, 41–55. Koottatep, T., Polprasert, C., 1997. Role of plant uptake on nitrogen removal in constructed wetlands located in the tropics. Water Sci. Technol. 36, 1–8. Korkusuz, E.A., Beklioglu, M., Demirer, G.N., 2005. Comparison of the treatment performances of blast furnace slag-based and gravel-based vertical ﬂow wetlands operated identically for domestic wastewater treatment in Turkey. Ecol. Eng. 24, 187–200. Koskiaho, J., Ekholm, P., Raty, M., Riihimaki, J., Puustinen, M., 2003. Retaining agricultural nutrients in constructed wetlands—experiences under boreal conditions. Ecol. Eng. 20, 89–103. Li, L.F., Li, Y.H., Biswas, D.K., Nian, Y.G., Jiang, G.M., 2008. Potential of constructed wetlands in treating the eutrophic water: evidence from Taihu Lake of China. Bioresour. Technol. 99, 1656–1663. Li, X.P., Manman, C., Anderson, B.C., 2009. Design and performance of a water quality treatment wetland in a public park in Shanghai, China. Ecol. Eng. 35, 18–24. Lin, Y.F., Jing, S.R., Lee, D.Y., Wang, T.W., 2002. Nutrient removal from aquaculture wastewater using a constructed wetlands system. Aquaculture 209, 169– 184. Maehlum, T., Stalnacke, P., 1999. Removal efﬁciency of three cold-climate constructed wetlands treating domestic wastewater: effects of temperature, seasons, loading rates and input concentrations. Water Sci. Technol. 40, 273–281. Mantovi, P., Marmiroli, M., Maestri, E., Tagliavini, S., Piccinini, S., Marmiroli, N., 2003. Application of a horizontal subsurface ﬂow constructed wetland on treatment of dairy parlor wastewater. Bioresour. Technol. 88, 85–94. Masi, F., Conte, G., Martinuzzi, N., Pucci, B., 2002. Winery high organic content wastewaters treated by constructed wetlands in Mediterranean climate. In: Proceedings of Eighth International Conference Wetland Systems for Water Pollution Control, IWA and University of Dar es Salaam, pp. 274–282. McCuskey, S.A., Conger, A.W., Hillestad, H.O., 1994. Design and implementation of functional wetland mitigation—case studies in Ohio and South Carolina. Water Air Soil Pollut. 77, 513–532. MEPC (Ministry of Environmental Protection of China), 2002. Environmental Quality Standard for Surface Water in China (GB3838-2002), Beijing, April, p. 28. MEPC (Ministry of Environmental Protection of China), WWMAA (Water and Wastewater Monitoring and Analysis Association), 2002. Standard Methods for Examination of Water and Wastewater, 4th edition. Chinese Environmental Sciences Press, Beijing, p. 28. Mitsch, W.J., Jorgensen, S.E., 1989. Classiﬁcation and samples of ecological engineering. In: Mitsch, W.J., Jorgensen, S.E. (Eds.), Ecological Engineering: An Introduction Ecotechnology. John Wiley & Sons, New York, pp. 12–19. Mitsch, W.J., Gosselink, J.G., 2000. Wetlands, vol. 254, 3rd edition. John Wiley & Son, NY, pp. 259–305. Nahlik, A.M., Mitsch, W.J., 2006. Tropical treatment wetlands dominated by freeﬂoating macrophytes for water quality improvement in Costa Rica. Ecol. Eng. 28, 246–257.
Newman, J.M., Clausen, J.C., Neafsey, J.A., 1999. Seasonal performance of a wetland constructed to process dairy milkhouse wastewater in Connecticut. Ecol. Eng. 14, 181–198. Nguyen, L.M., 2000. Organic matter composition, microbial biomass and microbial activity in gravel-bed constructed wetlands treating farm dairy wastewaters. Ecol. Eng. 16, 199–221. Pisarevsky, A.M., Polozova, I.P., Hawkridge, F.M., 2005. Chemical oxygen-demand. Russ. J. Appl. Chem. 78, 101–107. Poach, M.E., Hunt, P.G., Vanotti, M.B., Stone, K.C., Matheny, T.A., Johnson, M.H., Sadler, E.J., 2003. Improved nitrogen treatment by constructed wetlands receiving partially nitriﬁed liquid swine manure. Ecol. Eng. 20, 183–197. Poe, A.C., Pichler, M.F., Thompson, S.P., Paerl, H.W., 2003. Denitriﬁcation in a constructed wetland receiving agricultural runoff. Wetlands 23, 817– 826. Prochaska, C.A., Zouboulis, A.I., 2006. Removal of phosphates by pilot vertical-ﬂow constructed wetlands using a mixture of sand and dolomite as substrate. Ecol. Eng. 26, 293–303. Reddy, K.R., Kadlec, R.H., Flaig, E., Gale, P.M., 1999. Phosphorus retention in streams and wetlands: a review. Crit. Rev. Environ. Sci. Technol. 29, 83–146. Reynolds, D.M., 2002. The differentiation of biodegradable and non-biodegradable dissolved organic matter in wastewaters using ﬂuorescence spectroscopy. J. Chem. Technol. Biotechnol. 77, 965–972. Rios, D.A., Velez, A.E.T., Pena, M.R., Parra, C.A.M., 2009. Changes of ﬂow patterns in a horizontal subsurface ﬂow constructed wetland treating domestic wastewater in tropical regions. Ecol. Eng. 35, 274–280. Rousseau, D.P.L., Vanrolleghem, P.A., De Pauw, N., 2004a. Constructed wetlands in Flanders: a performance analysis. Ecol. Eng. 23, 151–163. Rousseau, D.P.L., Vanrolleghem, P.A., De Pauw, N., 2004b. Model-based design of horizontal subsurface ﬂow constructed treatment wetlands: a review. Water Res. 38, 1484–1493. Schaafsma, J.A., H Baldwin, A., Streb, C.A., 1999. An evaluation of a constructed wetland to treat wastewater from a dairy farm in Maryland, USA. Ecol. Eng. 14, 199–206. Scholz, M., 2007. Classiﬁcation methodology for Sustainable Flood Retention Basins. Landsc. Urban Plann. 81, 246–256. Scholz, M., Harrington, R., Carroll, P., Mustafa, A., 2007. The Integrated Constructed Wetlands (ICW) concept. Wetlands 27, 337–354. Scholz, M., Lee, B.H., 2005. Constructed wetlands: a review. Int. J. Environ. Stud. 62, 421–447. Scholz, M., Xu, J., 2002. Performance comparison of experimental constructed wetlands with different ﬁlter media and macrophytes treating industrial wastewater contaminated with lead and copper. Bioresour. Technol. 83, 71– 79. Stottmeister, U., Wiessner, A., Kuschk, P., Kappelmeyer, U., Kastner, M., Bederski, O., Muller, R.A., Moormann, H., 2003. Effects of plants and microorganisms in constructed wetlands for wastewater treatment. Biotechnol. Adv. 22, 93– 117. 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., Sukias, J.P.S., Sukias, M.P, 1998. Relationships between loading rates and pollutant removal during maturation of gravel-bed constructed wetlands. Environ. Qual. 27, 448–458. Tuszynska, A., Obarska-Pempkowiak, H., 2008. Dependence between quality and removal effectiveness of organic matter in hybrid constructed wetlands. Bioresour. Technol. 99, 6010–6016. USEPA, 2000. Constructed Wetlands Treatment of Municipal Wastewaters. Manual of EPA/625/R-99/010. U.S. Environmental Protection Agency, Cincinnati, OH. Van de Moortel, A.M.K., Rousseau, D.P.L., Tack, F.M.G., De Pauw, N., 2009. A comparative study of surface and subsurface ﬂow constructed wetlands for treatment of combined sewer overﬂows: a greenhouse experiment. Ecol. Eng. 35, 175–183. Verhoeven, J.T.A., Arheimer, B., Yin, C.Q., Hefting, M.M., 2006. Regional and global concerns over wetlands and water quality. Trends Ecol. Evol. 21, 96–103. Vymazal, J., 2005. Horizontal sub-surface ﬂow and hybrid constructed wetlands systems for wastewater treatment. Ecol. Eng. 25, 478–490. Vymazal, J., 2007. Removal of nutrients in various types of constructed wetlands. Sci. Total Environ. 380, 48–65. Vymazal, J., Brix, H., Cooper, P.F., Haberl, R., Perﬂer, R., Laber, J., 1998. Removal Mechanisms and Types of Constructed Wetlands. Backhuys Publishers, Leiden. Wallace, S., Parkin, G., Cross, C., 2001. Cold climate wetlands: design and performance. Water Sci. Technol. 44, 259–265. Werker, A.G., Dougherty, J.M., McHenry, J.L., Van Loon, W.A., 2002. Treatment variability for wetland wastewater treatment design in cold climates. Ecol. Eng. 19, 1–11. Wood, A., 1995. Constructed wetlands in water pollution control: fundamentals to their understanding. Water Sci. Technol. 32 (3), 21–29. Wu, Y., Chung, A., Tam, N.F.Y., Pi, N., Wong, M.H., 2008. Constructed mangrove wetland as secondary treatment system for municipal wastewater. Ecol. Eng. 34, 137–146. Xu, K., Kong, C.F., Liu, G., Wu, C.L., Deng, H.B., Zhang, Y., Zhuang, Q.L., 2010. Changes of urban wetlands in Wuhan, China, from 1987 to 2005. Prog. Phys. Geogr. 34, 207–220. Yalcuk, A., Ugurlu, A., 2009. Comparison of horizontal and vertical constructed wetland systems for landﬁll leachate treatment. Bioresour. Technol. 100, 2521–2526.
H. Zhang et al. / Ecological Engineering 37 (2011) 128–138
Zedler, J.B., 2003. Wetlands at your service: reducing impacts of agriculture at the watershed scale. Front. Ecol. Environ. 1, 65–72. Zedler, J.B., Kercher, S., 2005. Wetland resources: status, trends, ecosystem services, and restorability. Annu. Rev. Environ. Resour. 30, 39–74.
Zhao, B., Li, B., Zhong, Y., Nakagoshi, N., Chen, J.K., 2005. Estimation of ecological service values of wetlands in Shanghai, China. Chin. Geogr. Sci. 15, 151–156. Zhao, W.J., Gong, Z.N., Gong, H.L., Li, X.J., Zhang, S.M., Jing, L., 2006. Using remote sensing to research Beijing wetlands dynamics. Sci. China. Ser. E Technol. Sci., 97–107.