Abstract

Constructed wetlands (CWs) are an aesthetic and sustainable form to treat wastewater, however, their performance can be increased by improving a number of factors. The pilot-scale hybrid constructed wetland (CW) system was the combination of constructed floating treatment wetlands (CFWs) and horizontal subsurface flow constructed wetlands (HSFCWs); operated for a year and covered all seasons. The research was conducted to investigate the performance of the CW system regarding water depth, spatial, and seasonal removal of pollutants. Nine economical plants species were selected and divided into four groups to grow in CW-I to CW-IV, respectively. Removal increased along the bed and most of the total phosphorus (TP) removal occurred in the second bed, whereas total nitrogen (TN) and ammonium (NH4) removal were associated with the plant root system and biomass. Optimum removal of nutrients with respect to water depth was at 35 cm. TN and NH4 removal patterns were similar in different CWs. TN and NH4 removal were higher during summer compared to winter; only CW-IV showed the opposite trend.

INTRODUCTION

Constructed wetlands (CWs) for wastewater treatment are a natural, simple, low construction cost, low maintenance cost, and environmentally friendly technique, representing alternative and promising solutions for environmental protection and restoration, most suitable for small and rural communities (Álvarez et al. 2008; Kadlec 2009; Vymazal & Kröpfelová 2011). CWs have been successfully used in removing a wide variety of pollutants from wastewater (Haberl et al. 1995; Matamoros & Bayona 2006; Kadlec & Wallace 2008; Ranieri et al. 2011, 2013; Gikas et al. 2013; Zhang et al. 2014). Water and nutrient recycling is an emerging and integral part of water demand management (Niemczynowicz 1999). Properly managing wastewater is an issue of increasing importance in sustainable development and design due to growing populations, whereas global climatic changes have developed water stress conditions, especially in regions where water resources are limited (Vörösmarty et al. 2000; Barbagallo et al. 2014; Lavrnić & Mancini 2016). In constructed wetland systems, plants can play an important role in the removal of nutrients from wastewater, compared to unplanted wetlands (Bachand & Horne 1999; Lin et al. 2002). Macrophytes can improve wastewater quality by helping to settle suspended solids, directly uptake nutrients, and provide support for microbial flora (Brix 1997; Chen 2011; Taylor et al. 2011). The effect of the treatment efficiency of planted CWs is often documented (Fraser et al. 2004; García et al. 2008; Maltais-Landry et al. 2009). On the other hand, some previous studies showed a contradictory role of macrophytes in constructed wetlands, such as plants that do not contribute or may have negative effects on the purification process (Hansson & Granéli 1984; Sartoris et al. 1999; Mitsch et al. 2005), whereas more recent studies have found that the plants play a generally positive role (Gottschall et al. 2007; García et al. 2008; Mander & Mitsch 2009). These contradictions are probably because of variations of pollutant removal efficiencies within the CWs (Zhu et al. 2012). Although macrophyte in CWs can play different roles for water-purification mechanisms, a re-release of nutrients from decomposing vegetation is inevitable. During the growing stage, plants need nutrients for growth and incorporate them from the environment into new tissues. However, upon senescence, they release the nutrients back to the environment if they are not harvested (Kröger et al. 2007; Hu et al. 2010). Nutrient removal mechanisms in CWs have been fully investigated. Nutrient removal in constructed wetlands is complex, especially the removal of P, which is variable and dependent mainly on the nature of the bed media which is the major sink for P in wetlands. P removal involves both biotic processes (uptake by vegetation, microbes and periphyton, mineralization of plant litter, and soil organic phosphorous) and abiotic processes (burial and sedimentation, precipitation and adsorption, exchanges between soil and the overlying water column) (Reddy & D'angelo 1997). N removal can occur through denitrification, ammonia volatilization, ammonia adsorption, plant uptake, ANAMMOX, and organic nitrogen burial, whereas phosphorus removal occurs through sorption, plant uptake, precipitation, burial, and peat accretion (Vymazal 2007). It is expected that the primary removal mechanism of nutrients changes over time, N removal increases with establishing vegetation and enough carbon builds up for denitrification, whereas phosphorous removal drops with the saturation of the substrate sorption capacity (Gottschall et al. 2007). However, nutrients removal processes from wastewater in constructed wetlands are also greatly affected by environmental (Yates & Prasher 2009) and operational factors (Moustafa et al. 2012). Generally, these factors are interrelated and can be affected by other factors, such as the redox potential, greatly influenced by operational conditions, vegetation, and the design of CWs (White et al. 2004; Faulwetter et al. 2009). Therefore, seasonal selection of plants for constructed wetlands should be an important part of wastewater treatment management in a CW system. Most of the previous research on constructed wetlands covered one plant season, but the period of vegetative growth is followed, culminating in death. After completion of a life cycle, plants leave and roots begin to die and if plants remain in the CW, decay from the plant material will release nutrients back to the CW. Therefore, appropriate plants for the specific season will optimize the function of the constructed wetland.

The CW's performance with the presence of plants has been evaluated by different authors (Tanner et al. 1995; Allen et al. 2002; Riley et al. 2005; Akratos & Tsihrintzis 2007; Iamchaturapatr et al. 2007). Plants may alter the biogeochemistry of wetland systems by oxidation of the rhizosphere and excretion of organic acids, H+, and CO2 (Coleman et al. 2001; Jahangir et al. 2016; Maucieri et al. 2017). However, there is limited literature available for the removal performance of constructed floating wetlands (CFW) containing hybrid systems. The purpose of this study was to evaluate nutrient removal efficiency and overall function of a pilot-scale hybrid constructed wetland by growing economical vegetation. The efforts were made to improve the understanding of nutrient removal regarding spatial and seasonal variation, depth, and vegetation types in a constructed wetland. Nine economical plant species were grown seasonally and the impact of nutrients was monitored.

MATERIALS AND METHODS

Experimental site and system configuration

The study was conducted at the Southeast University campus, New District, Wuxi, China. Wuxi is located on the north shore of Taihu Lake and in the heart of the Yangtze Delta. The city has four distinct seasons and exists in a north subtropical humid climate zone. The average 30 years (1981–2010) perennial temperature is 16.2 °C (Summer and Winter average temperature is 29 °C and 2.8 °C, respectively), the average precipitation is 1,121.7 mm with 123 rainy days and 1,924.3 h of sunshine time (Zhai et al. 2016). The total covered area of the constructed wetland was 100 m2. Four pilot-scale hybrid constructed wetland systems were established for experimental plants. The hybrid system was the combination of CFW and horizontal flow constructed wetlands (HFCW). Each bed unit was 2.5 m × 0.3 m × 0.5 m (length × width × height) made of concrete and lined with epoxy. The first beds were designed for CFW, whereas the second beds of each unit were packed with a 10 cm supporting layer of large gravel at the bottom (30–40 mm), 25 cm of ceramsite in the middle (10–20 mm), and 10 cm of small gravel (20–20 mm) (Figure 1). The ceramsite is artificial sand with a well-developed porous structure and is favorable for the development of microbial communities. At the end of each bed equipped was an outlet for flow from different heights and stoppers were used to control the flow, whereas the HFCW bed was also equipped with 25 mm diameter PVC pipes at different distances along the bed for sampling. The CFW bed directly received wastewater from the storage tank and flow was controlled by valves and at the end of the CFW, 0.2 m distribution channel for the HFCW.

Figure 1

Schematic diagram of hybrid constructed wetland; (1) constructed floating treatment wetlands (CFW); (2) distribution channel for HFCW; (3) horizontal subsurface flow constructed wetlands (HFCW) packed with large gravel (bottom), ceramsite (middle), and small gravel (top); (4) PVC (polyvinyl chloride) pipes along the bed for sampling; and (5) the outlet to control water depth; sp1 to sp6 are the sampling positions.

Figure 1

Schematic diagram of hybrid constructed wetland; (1) constructed floating treatment wetlands (CFW); (2) distribution channel for HFCW; (3) horizontal subsurface flow constructed wetlands (HFCW) packed with large gravel (bottom), ceramsite (middle), and small gravel (top); (4) PVC (polyvinyl chloride) pipes along the bed for sampling; and (5) the outlet to control water depth; sp1 to sp6 are the sampling positions.

Plant material

Nine different species of economical plants were selected (Table 1) to grow in the constructed wetland, seasonally, and the selection was made to keep the guidelines presented by Tanner (1996) in mind, the economic value of vegetables, the easy availability in the local market, aesthetic worth, and adaptation to existing climatic conditions. All plants species (15 cm to 28 cm high) were obtained from a local nursery and transplanted into the CWs. Polyethylene foam boards 0.04 m thick were used as floating mats for planting in the CFW bed, perforated with 2 cm holes for each plant.

Table 1

Selected plant species for experiments

S. no. Common name Scientific name Used name Density (plants/m2
Leek Allium porrum L. A. porrum 150 
Mizuna Brassica juncea L. B. juncea 100 
Water spinach Ipomoea aquatica Forsk. I. aquatica 100 
Hot pepper Capsicum annuum L. C. annuum L. 20 
Manchurian wildrice Zizania latifolia Z. latifolia 100 
Crown Daisy Glebionis coronaria G. coronaria 50 
Lettuce Lactuca sativa L. sativa 50 
Chinese cabbage Brassica rapa B. rapa 50 
Chinese celery Oenanthe javanica DC. O. javanica 50 
S. no. Common name Scientific name Used name Density (plants/m2
Leek Allium porrum L. A. porrum 150 
Mizuna Brassica juncea L. B. juncea 100 
Water spinach Ipomoea aquatica Forsk. I. aquatica 100 
Hot pepper Capsicum annuum L. C. annuum L. 20 
Manchurian wildrice Zizania latifolia Z. latifolia 100 
Crown Daisy Glebionis coronaria G. coronaria 50 
Lettuce Lactuca sativa L. sativa 50 
Chinese cabbage Brassica rapa B. rapa 50 
Chinese celery Oenanthe javanica DC. O. javanica 50 

Experimental design

The experiments were carried out under natural conditions in the continuous operation mode. The wastewater originated from dormitories, restaurants, and laboratories of the Southeast University campus. Sewage contained 30–80 mg·L−1 chemical oxygen demand (COD), 0.95–4.3 mg·L−1 dissolved oxygen (DO), 21.49–34.27 mg·L−1 total nitrogen (TN), 7.14–20.13 mg·L−1 NH4-N, 7.29–22.91 mg·L−1 NO3-N, and 1.23–3.20 mg·L−1 total phosphorus (TP). Selected plants were divided into four groups to grow in four CW systems, seasonally (Table 2). The hydraulic loading was maintained at 0.2 m3·m−2·day−1 during the whole study period. The water depth in the CFW and HFCW beds were maintained at 10 cm and 45 cm, respectively, except during the water depth study, when the water depth of HFCW beds was 30 cm, 35 cm, 40 cm, and 45 cm and fluctuate after every three weeks. The wastewater samples were collected from each sampling point on a routine basis. To investigate nutrients concentrations along the bed samples were collected twice per week for two months (August to October; n = 16, average water temperature was 18.2–27.6 °C), and during the water depth study, samples were collected three times per week (July to September; n = 9, average water temperature was 24–29 °C), whereas for the seasonal variation study samples were taken once a week from July to April next year (n = 40). The wastewater flow in the system was inspected on a daily basis, whereas overall function was conducted on a weekly basis. Special attention was given to the inlet and outlet flow, as suspended solids present in wastewater can cause obstruction of the pipes.

Table 2

Selection of plant species grown in four constructed wetlands, seasonally

CW 1st crop 2nd crop 
 CFW HFCW Planting time CFW HFCW Planting time 
I. aquatica Z. latifolia 7-Nov G. coronaria L. sativa 7-Nov 
II I. aquatica A. porrum 8-Nov B. rapa A. porrum 8-Nov 
III A. porrum I. aquatica 12-Nov A. porrum B. juncea 12-Nov 
IV A. porrum C. annuum L. 15-Nov O. javanica G coronaria 15-Nov 
CW 1st crop 2nd crop 
 CFW HFCW Planting time CFW HFCW Planting time 
I. aquatica Z. latifolia 7-Nov G. coronaria L. sativa 7-Nov 
II I. aquatica A. porrum 8-Nov B. rapa A. porrum 8-Nov 
III A. porrum I. aquatica 12-Nov A. porrum B. juncea 12-Nov 
IV A. porrum C. annuum L. 15-Nov O. javanica G coronaria 15-Nov 

Analytical methods

Samples were taken and transported to the laboratory in 250 mL glass bottles and analysis were done immediately after sample collection, otherwise properly stored in refrigerated until analysis (sample holding time was less than 24 h). Standard methods (APHA 2005) were used to analyze TP (molybdenum antimony anti points spectrophotometry), TN (potassium persulfate spectrophotometry), and NH4-N (salicylic acid spectrophotometry). The analyses were done immediately after sample collection; otherwise, they were properly stored.

Data analysis

Initially, data were recorded in MS Excel (Microsoft Office package 16) and then software SPSS version 18.0 (SPSS Inc., Chicago, IL, USA) was used to perform one-way analysis of variance (ANOVA), whereas Duncan Multiple Range test was used to find the mean difference among various treated groups.

RESULTS

Nutrient removal along the CW beds

The concentration and removal rate of TP, NH4, and TN along the CW beds are shown in Figure 2. In all cases the concentration of TP, NH4, and TN decrease and removal rate increase along the bed. The mean TP removal rate from influent to SP6 were between 5.7–34%, 9.6–88%, 57–90% and 60–81%, whereas NH4 and TN removal were 5.8–44.4%, 9.8–85.7%, 14.2–78.19%, 13.13–80.32% and 3.4–35.36%, 5.6–66.8%, 3.1–69.35%, 6.7–62.70% in CW-I, CW-II, CW-III, and CW-IV, respectively. SP1 to SP3 showed lower TP removal as compared to SP4 to SP6, and most of TP removal occurred in matrix beds. Overall TP removal was lower than 40% in CW-I. CW-I and CW-III showed relatively low removal efficiency for NH4-N as compared to CW-II and CW-IV, whereas CW-II and CW-III showed higher removal for TN. CW-I showed poor removal efficiency for all three nutrients.

Figure 2

Nutrients concentration and removal along the constructed wetland beds planted with different vegetation. CW-I CW-II CW-III CW-IV .

Figure 2

Nutrients concentration and removal along the constructed wetland beds planted with different vegetation. CW-I CW-II CW-III CW-IV .

Effect of water depth on the removal of nutrients

The results for nutrient concentrations at different depths in CW are shown in Figure 3. There are variations in concentrations of nutrients at the same depth in different CWs. General trends showed higher concentrations with increased depth. All CWs showed a similar behavior of TP concentration, and the lowest concentration was observed in CW-II at 35 cm. Similar to TP, the lowest concentration was in CW-II and CW-IV at a depth of 35 cm, whereas the lowest TN concentrations were observed at a depth of 35 cm in all CWs.

Figure 3

Variation in nutrients concentration with water depth. CW-I CW-II CW-III CW-IV .

Figure 3

Variation in nutrients concentration with water depth. CW-I CW-II CW-III CW-IV .

Seasonal variation of nutrients

The seasonal variation of TP, NH4, and TN concentrations in influent and effluent during the whole study period in CW-I to CW-IV are shown in Figure 4. The average effluent TP concentrations were between 0.05–1.12 mg·L−1, 0–1.84 mg·L−1, 0.4–1.54 mg·L−1 and 0.48–1.85 mg·L−1 in CW-I, CW-II, CW-III, and CW-IV, respectively (Table 3). CW-I and CW-II showed the higher removal of TP as compared to CW-III and CW-IV. Moreover, TP removal increased during later months of study after the change of crop in CW-IV. Most of the TP removal occurred in matrix beds. The average effluent concentrations of NH4 were 2.92–9.18 mg·L−1, 4.53–14.15 mg·L−1, 3.93–11.51 mg·L−1, and 2.24–14.05 mg·L−1, whereas the average effluent concentrations of TN were 7.23–24.05 mg·L−1, 10.21–24.91 mg·L−1, 11.69–23.42 mg·L−1, and 9.09–24.3 mg·L−1 in CW-I to CW-IV, respectively. TN and NH4 removal patterns were similar in different CWs. TN and NH4 removal were higher during the start of the study period, especially in CW-I, but later removal efficiency decreased, whereas CW-IV showed the opposite, and in later months it showed a higher removal rate compared to all other CWs.

Table 3

Statistical analysis values of nutrients in different CWs

Parameters Units HCW effluent
 
Effluent
 
Removal rate
 
Min Max Mean ± sd** Min Max Mean ± sd** Min Max Mean± sd** 
(mg·L−1(mg·L−1
TP CW1 0.8531 1.7974 1.306 ± 0.265 0.0517 1.1274 0.6520 ± 0.3433 0.462 0.979 0.696 ± 0.174b 
CW2 0.9614 2.2962 1.7535 ± 0.376 0.00 1.843 1.037 ± 0.499 0.344 1.00 0.536 ± 0.216a 
CW3 0.7504 2.0025 1.3577 ± 0.3156 0.4033 1.5436 0.8979 ± 0.9271 0.347 0.847 0.598 ± 0.134a,b 
CW4 0.73 2.485 1.6 ± 0.509 0.4865 1.8565 1.1007 ± 0.3946 0.35 0.71 0.533 ± 0.104a 
NH4 CW1 6.062 14.205 9.00 ± 2.293 2.922 9.183 5.256 ± 1.478 0.292 0.759 0.531 ± 0.159b 
CW2 6.015 19.195 10.104 ± 3.722 4.531 14.151 7.241 ± 3.039 0.25 0.609 0.403 ± 0.110a 
CW3 5.279 14.651 8.878 ± 2.595 3.937 11.514 6.736 ± 2.141 0.274 0.67 0.434 ± 0.091a 
CW4 5.235 16.946 9.349 ± 3.555 2.246 14.045 7.088 ± 3.703 0.239 0.731 0.440 ± 0.153a 
TN CW1 14.021 29.643 21.211 ± 3.511 7.236 24.05 14.776 ± 3.877 0.254 0.648 0.451 ± 0.132c 
CW2 16.254 30.276 23.074 ± 3.802 10.211 24.917 17.988 ± 3.740 0.238 0.503 0.336 ± 0.087a 
CW3 14.884 27.396 21.100 ± 3.223 11.69 23.425 17.150 ± 2.778 0.219 0.5 0.363 ± 0.071a,b 
CW4 15.172 27.253 20.160 ± 4.320 9.09 24.3 15.77 ± 4.92 0.24 0.659 0.418 ± 0.144a,c 
Parameters Units HCW effluent
 
Effluent
 
Removal rate
 
Min Max Mean ± sd** Min Max Mean ± sd** Min Max Mean± sd** 
(mg·L−1(mg·L−1
TP CW1 0.8531 1.7974 1.306 ± 0.265 0.0517 1.1274 0.6520 ± 0.3433 0.462 0.979 0.696 ± 0.174b 
CW2 0.9614 2.2962 1.7535 ± 0.376 0.00 1.843 1.037 ± 0.499 0.344 1.00 0.536 ± 0.216a 
CW3 0.7504 2.0025 1.3577 ± 0.3156 0.4033 1.5436 0.8979 ± 0.9271 0.347 0.847 0.598 ± 0.134a,b 
CW4 0.73 2.485 1.6 ± 0.509 0.4865 1.8565 1.1007 ± 0.3946 0.35 0.71 0.533 ± 0.104a 
NH4 CW1 6.062 14.205 9.00 ± 2.293 2.922 9.183 5.256 ± 1.478 0.292 0.759 0.531 ± 0.159b 
CW2 6.015 19.195 10.104 ± 3.722 4.531 14.151 7.241 ± 3.039 0.25 0.609 0.403 ± 0.110a 
CW3 5.279 14.651 8.878 ± 2.595 3.937 11.514 6.736 ± 2.141 0.274 0.67 0.434 ± 0.091a 
CW4 5.235 16.946 9.349 ± 3.555 2.246 14.045 7.088 ± 3.703 0.239 0.731 0.440 ± 0.153a 
TN CW1 14.021 29.643 21.211 ± 3.511 7.236 24.05 14.776 ± 3.877 0.254 0.648 0.451 ± 0.132c 
CW2 16.254 30.276 23.074 ± 3.802 10.211 24.917 17.988 ± 3.740 0.238 0.503 0.336 ± 0.087a 
CW3 14.884 27.396 21.100 ± 3.223 11.69 23.425 17.150 ± 2.778 0.219 0.5 0.363 ± 0.071a,b 
CW4 15.172 27.253 20.160 ± 4.320 9.09 24.3 15.77 ± 4.92 0.24 0.659 0.418 ± 0.144a,c 

Here, the superscripts a,b,c are the mean difference among various plants treated values according to Duncan Multiple Range test, whereas ** is the P value <0.001. TN = total nitrogen; NH4–N = ammonium; TP = total phosphorus.

Figure 4

Seasonal variation of nutrient concentrations in influent and effluent in different CWs. Influent CFW effluent effluent removal efficiency .

Figure 4

Seasonal variation of nutrient concentrations in influent and effluent in different CWs. Influent CFW effluent effluent removal efficiency .

DISCUSSION

In the current study, four hybrid constructed wetland pilot scale systems were used by growing seasonal economical plants and the variation in concentrations of nutrients were monitored with respect to spatial variation, depth, and seasons. Nutrient removal occurred to appropriate extents and was higher than that reported by other authors (Mantovi et al. 2003; García et al. 2005; Calheiros et al. 2007). The sampling points from substrate beds showed higher removal compared to sampling points from CFW beds. CFW beds of CW3 and CW4 were planted with I. aquatica, which has a good ability to uptake P (Abbasi et al. 2018); therefore, in those beds, P removal occurred by plant uptake (Kahiluoto & Vestberg 1998; Perner et al. 2006). The matrix in CWs is the major storage pool for phosphorous, although under less reducing conditions some may eventually be released to the water column (Geary & Moore 1999). Meanwhile, the fate of P is liberated in the treatment system and removal may be more difficult to predict (Frazer-Williams 2010). Stone et al. (Stone et al. 2004) stated that P removal is not optimally effective in constructed wetlands. Hunt et al. (Hunt et al. 2006) found total P removal efficiency ranging from 65% to 240%, and P concentration in the effluent was higher compared to influent. The low P removal efficiency of a system is often attributed to the substrate (Brooks et al. 2000; Ballantine & Tanner 2010). There was little variation between the removal of NH4 and TN in different CW, and removal increased along the CW beds. NH4 removal was more noticeable in the first bed and, additionally, in latter sampling points, in the matrix bed, the DO concentration was low and, ultimately, removal of NH4+-N was restrained, and most of the NH4 removal occurred through nitrification. Chemical properties of the bed substrate determine the microbial and other living communities in CWs system, as well as the capacity to absorb nutrients and contaminants (Bradshaw et al. 2006). Substrates in CW systems offer a rooting medium and habitat for plants and microbial communities (Burchell et al. 2007). In turn, plants and microbes contribute to substrate aggregate formation and organic carbon production. Therefore, the composition of the substrate in CW systems directly influences nutrient removal (Cui et al. 2008).

Water depth is an important parameter in the construction and operation of constructed wetlands that not only affect the hydraulic retention time, but also the physical and chemical environment of wetlands. Water depth can influence the removal of nutrients by affecting the growth of plants and microbes in the constructed wetland system. The literature on the impact of water depth in the constructed wetland is limited to a few papers (Coleman et al. 2001; García et al. 2004; Aguirre et al. 2005). Water depth influences the coefficient of the mass transfer of oxygen from the atmosphere to the water (Burgoon et al. 1995) and determines the fraction of water volume in contact with the underground biomass of the macrophytes. In the current study, results indicate that the mean efficiency of P removal increased with the increase of water depth in some instances, but at too high a water depth, P removal decreased and the removal of TP by plant uptake in CWs is limited. It is believed that most of the TP removal occurred through adsorption and retention by the substrate (Burgoon et al. 1995), whereas more developed roots and the abundance of DO in shallower water were more favorable for NH4+-N removal. The matrix function of adsorption and retention did not dominate for NH4+-N removal, and ammonia-oxidizing bacteria in the rhizosphere are mainly responsible for ammonium removal.

Seasonal variation in the removal of nutrients in wastewater has often been reported (Kuschk et al. 2003; Vohla et al. 2007). In this study, removal efficiencies of TP, NH4, and TN were significantly higher during the summer season in CW1, CW2, and CW3, whereas CW4 shows higher removal for all three nutrients in later months. Removal efficiencies can be associated with temperature (Vohla et al. 2007). In summer, lack of oxygen and high temperature can cause significantly higher nutrients in constructed wetland system, which has a negative effect on nutrient removal (Hu et al. 2010) and higher removal during winter was probably due to the higher solubility of oxygen in the cooler water, resulting in enhanced nitrification and phosphorus removal processes (Vohla et al. 2007; Hu et al. 2010). Moreover, decomposition of dead plants material in summer is higher compared to winter (Chimney & Pietro 2006; Longhi et al. 2008), and can elevate nutrient concentration in the wetland. From Figure 4, a sharp decrease in the removal rate during the plant harvesting period in all types of constructed wetlands can be observed. CW4 showed lower uptake of nutrients during the first season and this may be because of lower uptake of nutrients by green pepper (Malkanthi et al. 1995). NH4 removal was strong during summer because of good growth and optimum temperature for microbial growth. The basic paths for N removal in CW are nitrification followed by denitrification (Spieles & Mitsch 1999). Dissolved oxygen in the water can limit the enzyme system required for denitrification. Many factors can influence the denitrification processes, such as nitrate concentration, organic carbon source type, microbial flora, dissolved oxygen, plant residues, redox potential, pH, temperature, and water level (Sirivedhin & Gray 2006). Temperature is a key factor related to activities of nitrifying bacteria and the effect on microorganism activities and diffusion of oxygen in CW systems. Previous studies have shown that the denitrifying rate was higher during summer compared to winter. According to Vymazal (2007), the optimum temperature range is 30–40 °C for nitrification and 40–60 °C for ammonification with an optimum pH of 6.5–8.5. The plant roots provide aerobic zone and surface area for microbial growth, and many physio-biochemical processes take place in root zones by interactions between plants, microorganisms, and nutrients (Lee et al. 2009).

CONCLUSIONS

The main findings are the following:

  • The removal of nutrients increased along the bed, and most of the TP removal occurred in the matrix bed, whereas N removal was associated with plant root systems and biomass.

  • The mean efficiency of nutrient removal increased with the increase of the water depth at some instance, but too high of a water depth is not associated with a higher removal of nutrients. Moreover, the lowest concentrations of nutrients were observed at a water depth of 35 cm.

  • Seasonal variation of nutrient removal is associated with temperature and growth stages of vegetation. Seasonal vegetation in CW can improve the removal efficiency of the system.

ACKNOWLEDGEMENTS

The authors are thankful to the Ministry of Environment, People's Republic of China for providing funding for this project. This work was financially supported by the ‘Major Science and Technology Project of Water Pollution Control and Management in China’ grant number 2012ZX07101-005.

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