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Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions
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Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions María E. Hernández ∗ , Michelli Galindo-Zetina, Juan Carlos Hernández-Hernández Biotechnological Management of Resources Network, Institute of Ecology, Carretera Antigua a Coatepec # 351, El Haya, Xalaoa, C.P. 91070, Veracruz, Mexico
a r t i c l e
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Article history: Received 20 March 2017 Received in revised form 31 May 2017 Accepted 1 June 2017 Available online xxx Keywords: Methane Nitrous oxide Wastewater treatment Carbon Nitrogen
a b s t r a c t Methane (CH4 ) and nitrous oxide (N2 O) emissions, and pollutant removal were investigated in constructed wetland (CW) mesocosms planted with the ornamental plant Zantedeschia aethiopica to treat polluted river water. We used two types of CWs, surface flow (SF) and subsurface flow (SSF), and two plant densities, high density HD (32 plants m−2 ) and low density LD (16 plants m−2 ). We also compared CH4 and N2 O emissions in zones planted with macrophytes (Typha sp and Cyperus papyrus) versus zones planted with Zantedeschia aethiopica in a pilot scaled CW treating municipal wastewater. In the mesocosms, average CH4 emissions were significantly higher in SFCW (436 ± 32 and 518 ± 46 mg m−2 d−1 ). than SSFCW (319 ± 65 and 210 ± 74 mg m−2 d−1 ), and plant density did not affect such emissions. SSFCW showed higher ammonia and nitrate removal efficiencies than SFCW and also showed higher N2 O emissions (17 ± 3 and 23 ± 5 mg m−2 d−1 ). Phosphate removal efficiencies were significantly higher in SFCW than SSFCW. In the pilot scale CW, no nitrous oxide emissions were observed and average CH4 emission (11,000 ± 930 mg m−2 d−2 ) was higher in the zones near the outflow planted with Zantedeschia aethiopica than in the zones near the inflow planted with Typha sp (4500 ± 800 mg m−2 d−2 ) and Cyperus papyrus (5500 ± 600 mg m−2 d−2 ), although TOC was higher in the zones near the inflow. We concluded that substrate and water flow are important factors controlling greenhouse gas emissions in CW and the amount of Zantedeschia aethiopica plants did not influence the emissions. Differences in methane emissions in the zones planted with native wetland plants in comparison with zones planted with Zantedeschia aethiopica might indicate that methane production and consumption in CW is influenced differently by the ornamental plants than by native wetland plants. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Constructed wetlands CW are designed to treat wastewater using the same natural processes that occur in natural wetlands but in a controlled manner. They provide an excellent alternative to conventional wastewater treatment such as active sludge systems, because constructed wetlands are low in cost and maintenance, offer good performance, and provide a natural appearance to the residents. Removal of organic matter in treatment wetlands is mediated by several microbial reactions such as aerobic respiration, denitrification, sulphate reduction, fermentation processes and methanogenesis (García et al., 2005). The products of some of these reactions include greenhouse gases such as methane (CH4 ), carbon dioxide (CO2 ) and nitrous oxide (N2 O), which are released
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (M.E. Hernández).
to the atmosphere. Greenhouse gases contribute to the radiative forcing in the atmosphere and consequently affect Climate Change, thus, their quantification in CW needs to be addressed before a greater implementation of this technology. The utilization of ornamental plants in CW is a topic that has been investigated for the last 13 years in several countries especially near the tropics (Belmont and Metcalfe, 2003; Zhang et al., 2006; Konnerup et al., 2009; Calheiros et al., 2015; Zurita et al., 2006, 2009, 2016). In México the strategy of using ornamental plants in constructed wetlands has been proposed to facilitate the introduction of this technology in small communities. Flowers produced in the constructed wetlands can be commercialized by the community as an economical incentive in exchange for providing maintenance to the treatment wetlands (Hernández, 2016). The majority of studies regarding the use of ornamental plants have been focused on the treatment efficiency and plant growth (Konnerup et al., 2009; Méndez-Mendoza et al., 2015; Saumya et al., 2015). However, little is known about the effects of ornamental plants on the biogeochemical process that occur in constructed wetlands. The objective of this study was
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Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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to investigate greenhouse gas GHG emissions in different types of CW with ornamental plants at different scales. We investigated the effect of water flow, substrate type and plant density on GHG emission, and pollutant removal in CW mesocosms with ornamental plants to treat polluted river water. We also compared GHG emissions in areas with native wetland plants versus zones planted with ornamental plants; in a pilot scaled subsurface constructed wetland, for treating municipal wastewater. Our hypotheses were: (1) surface flow CW mesocosms with soil would have more anaerobic conditions than subsurface flow CW with gravel, therefore higher methane emissions. (2) High plant density would provide more oxygen to the substrate, decreasing methane emissions. (3) In the pilot scale treatment wetland; our hypothesis was that areas with ornamental plants would have lower GHG because they were near the outflow and therefore would have less nutrient availability, as compared to areas with native wetlands plants near the inflow. 2. Material and methods 2.1. Study sites This study was conducted in the central part of Veracruz State in Southeastern Mexico. The CW mesocosms were in a glass greenhouse (without temperature or humidity control) at the botanical garden “Francisco Javier Clavijero” in Xalapa city (96◦ 56 24 W 19◦ 30 54 N) at 1560 m AMSL. Weather in the region is humid subtropical with an annual precipitation of 1509.1 mm and an annual average temperature of 18◦ C. CW mesocosms were supplied with water from the Sordo River. This river is a third order stream that originates in a tropical mountain rainforest upstream from the botanical garden; downstream, it joins the Pixquiac River and finally it merges to La Antigua River which flows into the Gulf of Mexico. The pilot scale treatment wetland was constructed in the community of Pinoltepec municipality, of Emiliano Zapata, Veracruz (96◦ 45 18 W 19◦ 26 45 N) at 780 AMSL. Weather in the region is humid tropical with an annual precipitation of 2779.1 mm and annual average temperature of 25.2◦ C. The pilot treatment wetland was supplied with settled municipal wastewater. Population in Pinoltepec is 649 inhabitants and has 168 residences. 2.1.1. Mesocosm units The experimental mesocosms consisted of cells built using fiber glass (1.5 m length, 0.25 m wide and 0.6 m depth). Four cells were set up for free surface flow wetland SFCW mesocosm using upland soil (C = 13.8%, TN = 0.93%, N-NO3 = 83.28 mg Kg−1 , pH = 5.8) as substrate (0.4 m deep, free water surface flow column of 10 cm), and four cells for subsurface flow wetland SSFCW mesocosm using volcanic gravel as substrate (0.04 m diameter, 0.5 m depth, water flow 10 cm below surface). Two cells of each type of flow were planted with Zantedeschia aethiopica at low density (16 plants m−2 ) and two cells with high density (32 plants m−2 plants). Mesocosms were supplied with water from the Sordo River, which receives untreated urban wastewater from Xalapa city. River water was pumped into a 500 L distribution tank and continuously discharged into each mesocosm trough a PVC pipe. Water flow rate was adjusted for each cell to have a Hydraulic Retention Time (HRT) of 3 days (Fig. 1). 2.1.2. Pilot scale treatment wetland The treatment wetland consisted of two parallel concrete cells (20 m length, 1 m wide and 0.6 m depth) with subsurface water flow (volcanic gravel 0.05 m diameter, 0.5 m depth, water flow 10 cm below surface). Both cells were planted with the same array of vegetation and received the same flow of wastewater. Vegetation in the cells were distributed from the inflow to the outflow as
follows: 0–5 m Typha sp, 5–12 m Cyperus papyrus, 12–20 m Zantedeschia aethiopica. This arrangement was established to enhance flower production of Zantedeschia aethiopica; since previous studies demonstrated that a high nutrient load stimulated the growth of Zantedeschia aethiopica, but no flower production (unpublished data). On the other hand, wetland plant species such as Typha sp and Cyperus papyrus, are tolerant to high nutrient loads, therefore they were planted near the inflow where nutrient concentration was higher. Wastewater flow rates were adjusted for each cell to have a Hydraulic Retention Time (HRT) of 40 h. 2.1.3. Sampling and analytical methods Water samples (200 ml) were taken from the influent and effluent of each CW mesocosm. Influent samples were taken beyond the distribution tank and from each individual hose that fed river water to the mesocosms and, effluent samples were taken at the exit of each cell. Water sampling was performed twice a week from May to November. In the pilot scale treatment wetland, water samples (200 ml) were also taken from influent and effluent of each cell every other week, from June to October. Additionally, PVC wells were installed next to the gas sampling chamber and water samples from these wells were taken each time that GHG were measured. The samples were analyzed for Chemical Oxygen Demand (COD), ammonia nitrogen (N-NH3 ), nitrate (N-NO3 −1 ), orthophosphate (P-PO4 3− ), Total Phosphorus (TP), Total Nitrogen (TN), Total Organic Carbon, Total Solids (TS) and Total Suspended Solids (TSS). COD was measured using the oxidation of K2 Cr2 O7 micro-method (APHA, 1998); N-NH3 was analyzed by the Nessler method; Total Nitrogen TN was analyzed by Kjeldahl method (APHA, 1998); PPO4 were quantified using the ascorbic acid method according to Sandell and Onish (1978); Total Phosphorus was analyzed by persulfate digestion followed by ascorbic acid method; N-NO3 were quantified with the salicylic acid method, according to Rofarge and Jonson (1983). All these colorimetric analyses were performed using a UV–vis Jenway-Genova spectrophotometer (Jenway-Essex, UK). Total Organic Carbon in water was analyzed in a Total Organic Carbon analyzer (Torch, Teledyne Tekmar). TSS and TS were analyzed by gravimetric methods (APHA, 1988) 2.1.4. Gas flux measurements In the mesocosms, gas fluxes were measured twice a month from June to November 2012, and in the pilot scale treatment wetland, from July to October 2013. In both cases the closed chamber technique was used for the measurement of greenhouse gas emissions (Hernández and Mitch, 2006; Altor and Mitsch, 2008). In the mescosms, chambers consist of a permanent base and a removable cover. The base was a PVC pipe (0.40 m height, 0.05 m ID) with a plastic collar to create a water seal with the cover. They were inserted permanently 0.12 m into the wetland substrate. The cover was 0.10 m height × 0.075 m ID and had a gas sampling port and thermometer. Two chambers were installed in each wetland messocosm, one chamber was placed near the water inlet and one near the outlet. In the pilot scale treatment wetland, the chamber bases were a PVC pipe (0.25 m height, 0.10 m ID) with a cover (0.10 m height x 0.30 m ID). Chambers were placed in both cells, at 3, 9 and 15 m from inflow to outflow in Thypha sp, Cyperus papyrus and Zantedeschia aethiopica vegetation zones respectively. When the gas fluxes were measured, the cover was placed on top and sealed to the base using water in the collar. Internal gas samples were collected from the chamber every 5 min, during a 40-min period by using a syringe fitted with a Luer stopcock. Gas samples were stored into 10 ml vials previously vacuum evacuated and kept at 4 ◦ C until their analysis (within three days after they were collected). The gas samples were analyzed for their CH4 and N2 O
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 1. Schematic layout of the CW mesocosms with different water flow and substrate planted with the ornamental plant Zhantedeshia aethiopica to treat plluted river water.
concentrations simultaneously using a gas chromatograph (Perkin Elmer Clarus 500) equipped with a 2 m Porapak Q, 80/100 column, a flame ionization detector-FID and Electron Capture Detector- ECD. Operation conditions were: 13 ml min-1 of nitrogen as the carrier gas, temperature of the FID, ECD, injector and oven was set at 150, 360, 95 and 40 ◦ C, respectively. The gas fluxes were calculated from its concentration increase in the chamber, according to the following equation. F = V/A ∗
dC dt
mg/m3 d
Treatment
Removal% N-NH4
HD-SSFCW LD-SSFCW HD-SFCW LD-SFCW
N-NO3
96 ± 4 44 ± 9 b 5±6 c 0c a
55 ± 9 35 ± 7 b 23 ± 5 c 12 ± 4 d a
P-PO4
TOC
26 ± 4 33 ± 6 69 ± 10 62 ± 12
53 ± 6 a 51 ± 7 a 48 ± 4 a 31 ± 5 b
HD-SSFCW-High density subsurface flow constructed wetland, LD-SSFCW Low density subsurface flow constructed wetland, HD-SFCW- High density surface flow constructed wetland, LD-SFCW- Low density subsurface flow constructed wetland.
Where: F, is the gas flux (mg m−2 d), V is the chamber volume (m3 ), A is the area enclosed by the chamber, (m2 ) and dC is the gas concentration gradient dt
Table 1 Removal efficiency of pollutants in CW messocosms with different water flow planted with the ornamental plant Zhantedeshia aethiopica using high density (16 plants m−2 ) and low density (32 plants m−2 ).
.
2.1.5. Statistical analysis Statistical analyses were performed with SPSS version 14 for Windows. Kolmogrov-Smirnov, Lilliefors’ and Shapiro-Wilk’s tests were used to check normality. The data fit normal distributions. Two-way analysis of variance (ANOVA) with Tukey HSD multiple comparison tests were used to investigate the effect of water flow and plant density on pollutant removal and GHG emissions in mesocosms CW. Pearson correlation coefficient were performed between GHG emissions and water quality parameter at the inflow of each mesocosm in the same day that GHG were measured. One-way ANOVA was used to investigate differences on methane emissions, TOC and ammonia concentration in the different zones of pilot CW. A 5% significance level was used to determine differences among treatments. 3. Results and discussion 3.1. Pollutant concentration and removal in CW mesocosms N-NH3 in the inflow of CW mesocosms ranged from 3 to 20 mg L−1 during the study period (Fig. 2a). SSFCW mesocosms showed significantly higher average ammonia nitrogen removal efficiency than SFCW mesocosms; high plant density increased
the removal efficiency but only in SSFCW mesocosmsm (Table 1). This was probably due the fact that SSFCW has gravel as a substrate, which does not provide enough nutrients to the plants, therefore they have to uptake nitrogen from the wastewater. Contrarily, SFCW has soil as a substrate, which could have provided nutrients to the plants, therefore plants did not uptake nitrogen from the wastewater. N-NH3 , concentrations found in the Sordo river are within the range reported for polluted rivers (5.4–14.5 mg L−1 ) in Taiwan (Jing et al., 2001; Huang et al., 2007) and China (0.16–12.8 mg L−1 ) (Ruan et al., 2006; Tang et al., 2009). The Taiwanese Environmental Protection Agency determined that rivers with N-NH3 concentrations higher than 3 mg L−1 are considered heavily polluted. The recommended maximum concentration of N-NH3 in water bodies to protect aquatic life is 0.5 mg L-1 (Wetzel, 1981). According to this, the Sordo River is heavily polluted and the aquatic life in the sampling station might be endangered. CW mesocosms, especially SSFCW with high density of Zhantdeshia aethiopica were very efficient to remove ammonia nitrogen from polluted river water. These removal efficiencies were higher than the reported efficiencies (0–85%) by Dong et al., 2012 in constructed wetlands treating polluted river water with N-NH3 concentrations of 3.5–10.6 mg L−1 in China. N-NO3 in the inflow of CW mesocosms ranged from 1–10 mg L−1 (Fig. 2-b). SSFCW mesocosms showed significantly (P ≥ 0.05) higher average nitrate removal efficiencies than SFCW mesocosms, and high plant density increased the removal efficiency in both types
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 2. Pollutant concentration at the inflow and outflow of CW mesocosms with different water flow and planted with low and high plant density of Zhantedeshia aethiopica, a) ammonia nitrogen, b) nitrates, c) Phosphates and d) Total organic carbon. HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland.
of CW mesocosms (Table 1). This indicated that SSFCW had aerobic microsites that allowed nitrification and anoxic microsites in which nitrates were denitrified. The positive effect of high plant density on nitrate removal efficiency could be related with the root carbon exudates that enhance denitrification (Morgan et al., 2008). Phosphates in the inflow of CW mesocosms ranged from 1–6 mg L−1 . (Fig. 2c) We found significantly higher (P ≤ 0.05) phosphate removal efficiencies in the SFCW than SSFCW mesocosms and no effect due to plant density on phosphate removal efficiencies being observed (Table 1). Average phosphate concentrations observed in this study were much higher than in comparison to a polluted river in China 0.3 mg L−1 (Tang et al., 2009). A maximum concentration of 0.005 mg L−1 of phosphates is recommended to
avoid eutrophication in bodies of water (Wetzel, 1981). Phosphate concentration in the Sordo River is thousand times higher than the recommended limit, and CWs were able to decrease the concentration to acceptable levels. Removal efficiencies were similar in high plant density and low plant density, indicating that phosphates were removed by physical methods such as adsorption and the fact that SFCW showed better phosphorus removal efficiencies, may be explained by the clay content in the soils that provide surface for P-PO4 adsorption (Mitsch and Gosselink, 2007). TOC in the inflow of CW mesocosms ranged from 40 to 350 mg L−1 (Fig. 2d). On average CW mesocosms removed approximately half of TOC, only SFCW with low plant density showed significantly(P ≤ 0.05) lower TOC removal efficiency (31%). Organic
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 3. Methane emissions in CW mesocosms with different water flow and planted with low and high plant density of Zhantedeshia aethiopica, a) temporal emissions, b) average of emissions during the study period. HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland.
carbon in constructed wetlands is removed by microorganisms that use it as an energy source. Vast arrays of microorganisms are adapted to aerobic surface waters or anaerobic soils. The aerobic microorganisms consume oxygen to breakdown organics which provides them with energy. Anaerobic bacteria breakdown organic matter using different electron acceptors such as CO2 to produce methane. Anoxic-anaerobic reactions are less efficient than aerobic reactions. Our results indicated that in SFCW more anaerobic conditions prevailed than in SSFCW, therefore less efficient carbon breakdown occurred in these mesocosms producing methane. We found that high plant density in SFCW improved carbon removal which might be related to the presence of more roots that served as a support for microorganisms (Brix, 1997)
3.2. Greenhouse gas emissions in CW mesocosms Methane emission in CW mesocosms ranged from 0 to 2100 mg m−2 d−1 , with higher emissions in SFCW than SSFCW (Fig. 3). For the majority of treatments, the peaks of methane emission were observed in summer (July-August), except for SSFCW where the peak was observed in late September. The average methane emissions were significantly (P ≤ 0.05) higher in SFCW (436 ± 32 and 518 ± 46 mg m−2 d−1 ) than SSFCW (319 ± 65 and 210 ± 74 mg m−2 d−1 ), and no effect on methane emissions by plant density was observed. N2 O emissions ranged from −3 to 150 mg, m−2 d−1 with the peak of emission during October for all treatments (Fig. 4a). The average of nitrous oxide emissions was significantly higher (P ≤ 0.05) in SSFCW (17 ± 3 and 23 ± 5 mg m−2 d−1 ) than SFCW
(11 ± 2 and 6 ± 4) mesocosms and no effect on nitrous oxide emissions by plant density was observed (Fig. 4b). SSFCW were more efficient for nitrogen (ammonia and nitrate) removal than SFCW and these mesocosms also had high nitrous oxide emissions. Nitrous oxide is produced by nitrification in the aerobic zones and by denitrification in the anoxic zones (Hernández and Mitsch, 2006; Mander et al., 2008, 2014). SSFCW mesocosms had a gravel substrate, which probably enhanced aerobic zones that promoted ammonia oxidation into nitrates which were then denitrified in the anoxic zones in the deeper layers. In contrast, SFCW had a soil substrate and a water column which could have created anoxic and anaerobic conditions that did not allow ammonia oxidation therefore; nitrites where not available for denitrification, resulting in low nitrogen removal and nitrous oxide emissions. SFCW also had higher methane emissions than SSFCW, even though they had similar carbon removal efficiencies. This also indicates that SFCW had more anaerobic conditions than SSFCW, resulting in inefficient anaerobic carbon breakdown reactions with methane. Besides the reduced conditions, in SFCW the soil used as a substrate had organic carbon, which could be used as source of methane, while in the SSFCW, gravel lack of organic carbon before the biofilm formation, therefore we accepted our hypothesis no. 1. However, GHG emissions were not affected by plant density, thus we rejected our hypothesis no. 2. The radial oxygen loss ROL from plant roots that create microaerobic zones in the rhizosphere have been described for several native wetlands plants (Wiessner et al., 2002) and has been documented that it varies between plant species (Mei et al., 2014). In this study, we used the ornamental plant Zantedeschia aethiopica, and for this specie, ROL has not been described;
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 4. Nitrous emissions in CW mesocosms with different water flow and planted with low and high plant density of Zhantedeshia aethiopica, a) temporal emissions, b) average emissions during the study period. HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland.
Table 2 Pearson correlation coefficients between water quality parameter and CH4 and N2 O emissions in CW mesocosms with different water flow and substrate. Treatment HDSSF-CW LDSSF-CW HDSF-CW LDSF-CW
N2 O CH4 N2 O CH4 N2 O CH4 N2 O CH4
N-NO3
N-NH4
TOC
−0.4013 0.3605 −0.7240* −0.3727 0.3040 0.1583 −0.2146 0.1167
−0.3923 −0.0793 −0.6090* −0.4906 −0.1894 −0.3329 −0.2690 0.6326
−0.1016 −0.1018 −0.0366 −0.2569 −0.3514 −0.2229 0.0396 −0.0482
HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland. * = indicates significant correlation at 0.05 probability level.
so our results suggested that ROL by this ornamental plant in SFCW was not enough to stimulate microaerobic zones near the rhizosphere that mitigate methane emissions. When we correlated N2 O and CH4 emissions with water quality parameters (N-NO3 , N-NH4 and TOC) we only found significant negative correlations between N2 O emissions in LD-SSCW and N-NO3 (P = 0.0023) and N-NH4 (P = 0.016) (Table 2). The negative correlation observed only in the low density SSFCW could be related to the
less amount carbon exudates provided by root plants that caused low C/N ratios and low C/N ratios has been described that decrease N2 O emissions in CW (Li et al., 2017).
3.3. Pollutant concentration and removal GHG emissions in the pilot scale CW During the first five months of operation, the pilot scale CW with a polyculture of macrophytes and ornamental plants showed a decrease in the pollutant concentrations from inflow to outflow with high removal efficiencies for COD, and suspended solids and low removal efficiencies for TP, P-PO4 , N-NH3 and total nitrogen (Table 3). Average methane emissions in the zones planted with Typha sp were 4500 ± 800 mg m−2 d−2 , which were not significantly different (p ≥ 0.05) from the average methane emissions in the zones planted with Cyperus papyrus (5500 ± 600 mg m−2 d−2 ) (Fig. 5). However, these emissions were significantly lower than those observed in the zones planted with the ornamental plant Zhathedeshia aethiopica (11,000 ± 930 mg m−2 d−2 ). The average of organic carbon at 3 m, and 6 m from the inflow were 225 ± 15 and 230 ± 17 mg L−1 respectively, which were significantly higher than the observed at 15 m (187 ± 11 mg L−1 ). Ammonia concentra-
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 5. Average methane emissions in a pilot scale CW a), Average TOC concentrations and c) Average ammonia nitrogen concentrations in a CW pilot treatment wetlands planted with a polyculture of macrophytes and ornamental plants.
Table 3Average concentrations of pollutants at the inflow and outflow of a pilot scale CW planted with polyculture of macrophytes and ornamental plants, and the removal percentage achieved during the first 5 months of operation (June 2013–October 2013) with a HRT of 40 hrs in Pinoltepec, Veracruz. Parameter mg L−1
Inflow
Outflow
% Removal
Total Nitrogen TN Total Phosphorus TP O-phosphates O-P Amonia Nitrogen N-NH3 Chemical Oxygen Demand COD Total Suspended Solids TSS Total Solids TS
119 ± 20 12 ± 3 8±1 33 ± 4 378 ± 31 91 ± 13 720 ± 52
72 ± 9 8±2 6 ± 0.4 24 ± 2 124 ± 20 15 ± 5 474 ± 35
47 ± 12 33 ± 6 25 ± 2 27 ± 3 67 ± 14 67 ± 9 34 ± 9
tion decreased from inflow towards the outflow from 45 mg L−1 to 40 mg L−1 but it was not significantly different. No nitrous oxide emissions were observed during the study period in this pilot scale CW. This finding might be related to the low ammonia removal efficiencies observed, which indicated poor nitrification and denitrification activity had occurred during the study period. In general, the low pollutant removal efficiencies could be explained by the short HRT (40 hrs) in which CW was operated during the first 5 months. The short HRT was chosen to treat most wastewater produced in the community per area of the CW. However after these results, a longer HRT was implemented to increase pollutant removal efficiency. The higher CH4 emissions in the Zhantedeshia aethiopica zones compared with the zones near the inflow planted with Typha sp and Cyperus gigantus was contrary to our hypothesis, therefore hypothesis no. 3 was rejected. This phenomenon might be related with
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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differences in ROL between the plant species (Mei et al., 20014) that causes differences in the methane production and consumption within the CW. However, more studies on this are needed. 4. Conclusions Greenhouse gas emissions in CW mesocosms with ornamental plants were affected by the type of substrate and water flow. SFCW with soil substrate had higher CH4 emissions than SSFCW with gravel substrate. However, SSFCW microcosms with gravel were more efficient to remove nitrogen (ammonia nitrogen and nitrates) and had higher nitrous oxide emissions, indicating that they had less anaerobic conditions. High plant density enhanced ammonia and nitrate removal in SSFCW, and carbon removal in SFCW. However, plant density did not have any effect on greenhouse gas emissions in CW mesocosms planted with the ornamental plant Zhantedeshia aethiopica. In the pilot scale CW with a polyculture of macrophytes and ornamental plants; methane emissions were higher in the zones near the outflow planted with the ornamental plant Zhantedeshia aethiopica as compared to the zones near the inflow planted with native wetlands plants (Thypha sp and Cyperus papyrus). Near the outflow TOC concentrations were lower than near the inflow, which might indicate that methane production and consumption in the zones with ornamental plants probably is different from zones planted with native wetland plants. Aknowlegments Funding for this project was provided by the Mexican National Council for Science and Technology-CONACYT- basic science project No. 081942 References APHA, American public Health Association, 1998. Standard Methods for the Examination of Water and Wastewater, 3rd edition. APHA–AWWA–WPCF, Washington. DC. Altor, A.E., Mitsch, W.J., 2008. Pulsing hydrology, methane emissions, and carbon dioxide fluxes in created marshes: a 2-year ecosystem study. Wetlands 28, 423–438. Belmont, M.A., Metcalfe, C.D., 2003. Feasibility of using ornamental plants (Zantedeschia, aethiopica) in subsurface flow treatment wetlands to remove nitrogen, chemical oxygen demand and nonyphenol ethoxylate surfactants a laboratory scale study. Ecol. Eng. 21, 233–247. Brix, H., 1997. Do macrophytes play a role in constructed wetlands treatments. Water Sci. Technol. 5 (5), 11–17. Calheiros, C.S.C., Bessa, V.S., Mesquita, B.R., Brix, H., Rangel, A.O.S.S., Castro, P.M.L., 2015. Constructed wetland with a polyculture of ornamental plants for wastewater treatment at a rural tourism facility. Ecol. Eng. 79 (2015), 1–7. Dong, H., Quiang, Z., Li, T., Jin, H., Chen, W., 2012. Effect of artificial aeration on the performance of vertical-flow constructed wetland treating heavily polluted river water. J. Environ. Sci. 24 (4), 596–601. García, J., Barragán, Aguirre P., Mujeriego, J., Matamoros, R., Bayona, J., 2005. Effect of key design parameters on the efficiency of horizontal subsurface flow constructed wetlands. Ecol. Eng. 25 (4), 405–418.
Hernández, M.E., Mitsch, W.J., 2006. Influence of hydrologic pulses and vegetation on nitrous oxide emissions from created riparian marshes in Midwestern USA. Wetlands 26 (3), 862–877. Hernández, M.E., 2016. Humedales ornamentales con participación comunitaria para el saneamiento de aguas municipales en México. Revista Internacional de Desarrollo Regional Sustentable 1 (2), 1–12. Jing, S.R., Lin, Y.F., Lee, D.Y., Wang, T.W., 2001. Nutrient removal from polluted river water by using constructed wetlands. Bioresour. Technol. 76, 131–135. Konnerup, D., Koottatep, T., Brix, H., 2009. Treatment of domestic wastewater in tropical, subsurface flow constructed wetlands planted with Canna and Heliconia. Ecol. Eng. 3 (5), 248–257. Li, M., Wu, Haiming, Zhang, J., Ngo, H.H., Gao, W., Kong, Q., 2017. Nitrogen removal and nitrous oxide emmisions in surface flow constructed wetlands for treating sewage treatment plant effluents: effect of C/N ratios. Bioresour. Technol., http://dx.doi.org/10.1016/j.biortech.2017.02.054. Méndez-Mendoza, A.S., Bello-Mendoza, R., Herrera-López, D., Mejía-González, G., Calixto-Romoa, A., 2015. Performance of constructed wetlands with ornamental plants in the treatment of domestic wastewater under the tropical climate of South Mexico. Water Prac. Technol. 10 (1), 1–10. Mander, U., Krista Lõhmus, K., Teiter, S., Mauring, T., Nurk, K., Augustin, J., 2008. Gaseous fluxes in the nitrogen and carbon budgets of subsurface flow constructed wetlands. Sci. Total Environ. 404, 343–353. Mander, U., Dotro, G., Ebie, Y., Towprayoon, S., Chiemchaisri, C., Nogueira, S.F., Jamsranjav, B., Kasak, K., Truu, J., Tournebize, J., Mitsch, W.J., 2014. Greenhouse gas emission in constructed wetlands for wastewater treatment: a review. Ecol. Eng. 66, 19–35. Mei, X., Yang, Y., Tam, N.F., Wang, Y.W., Li, L., 2014. Roles of root porosity, radial oxygen loss, Fe plaque formation on nutrient removal and tolerance of wetland plants to domestic wastewater. Water Res. 50, 147–159. Mitsch, W.J., Gosselink, J., 2007. Wetlands, 4th edition. John Wiley and Sons Inc, Nueva York (581 pp.). Morgan, J.A., Martin, J.F., Bouchard, V., 2008. Identifying plant species with root associated bacteria that promote nitrification and denitrification in ecological treatment systems. Wetlands 28, 220–231. Rofarge, E., Jonson, B., 1983. Water and waste water analysis for nitrate via nitratation of salicylic acid. Commun. Soil Sci. Plant Anal. 14 (12), 1207–1215. Ruan, X., Xue, Y., Wu, J., Ni, L., Sun, M., Zhang, X., 2006. Treatment of polluted river water using pilot-scale constructed wetlands. Bull. Environ. Contam. Toxicol. 76, 90–97. Sandell, E., Onish, H., 1978. Photometric Determination of Traces of Metals. John Wiley and Sons Inc., USA, pp. p. 249–250. Saumya, S., Akansha, S., Rinaldo, J., Jayasri, M.A., Suthindhiran, K., 2015. Construction and evaluation of prototype subsurface flow wetland planted with Heliconiaangusta for the treatment of synthetic greywater. J. Clean. Prod. 91, 235–240. Tang, X., Huang, S., Scholz, M., 2009. Nutrient removal in pilot-scale constructed wetlands treating eutrophic river water assessment of plants, intermittent artificial aereation and polyhedron hollow polypropylene balls. Water Air Soil Pollut. 197, 61–63. ˜ 679 pp. Wetzel, R., 1981. Limnología Omega, 1st edition. Barcelona, Espana, Wiessner, A., Kuschk, P., Stottmeister, U., 2002. Oxygen release by roots of Typha latifolia and Juncus effusus in laboratory hydroponic systems. Eng. Life Sci. 22, 209–216. Zhang, X.B., Liu, P., Yang, Y.S., Chen, W.R., 2006. Phytoremediation of urban wastewater by model wetlands with ornamental hydrophytes. J. Environ. Sci. 19, 902–909. Zurita, F., De Anda, J., Belmont, M.A., 2006. Performance of laboratory-scale wetlands planted with tropical ornamental plants to treat domestic wastewater. Water Qual. 41 (4), 410–417. Zurita, F., De Anda, J., Belmont, M.A., 2009. Treatment of domestic wastewater and production of commercial flowers in vertical and horizontal subsurface-flow constructed wetlands. Ecol. Eng. 35, 861–869. Zurita, F., Belmont, M.A., De Anda, J., White, J.R., 2011. Seeking a way to promote the use of constructed wetlands for domestic wastewater treatment in developing countries. Water Sci. Technol. 63 (4), 654–659.
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Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions María E. Hernández ∗ , Michelli Galindo-Zetina, Juan Carlos Hernández-Hernández Biotechnological Management of Resources Network, Institute of Ecology, Carretera Antigua a Coatepec # 351, El Haya, Xalaoa, C.P. 91070, Veracruz, Mexico
a r t i c l e
i n f o
Article history: Received 20 March 2017 Received in revised form 31 May 2017 Accepted 1 June 2017 Available online xxx Keywords: Methane Nitrous oxide Wastewater treatment Carbon Nitrogen
a b s t r a c t Methane (CH4 ) and nitrous oxide (N2 O) emissions, and pollutant removal were investigated in constructed wetland (CW) mesocosms planted with the ornamental plant Zantedeschia aethiopica to treat polluted river water. We used two types of CWs, surface flow (SF) and subsurface flow (SSF), and two plant densities, high density HD (32 plants m−2 ) and low density LD (16 plants m−2 ). We also compared CH4 and N2 O emissions in zones planted with macrophytes (Typha sp and Cyperus papyrus) versus zones planted with Zantedeschia aethiopica in a pilot scaled CW treating municipal wastewater. In the mesocosms, average CH4 emissions were significantly higher in SFCW (436 ± 32 and 518 ± 46 mg m−2 d−1 ). than SSFCW (319 ± 65 and 210 ± 74 mg m−2 d−1 ), and plant density did not affect such emissions. SSFCW showed higher ammonia and nitrate removal efficiencies than SFCW and also showed higher N2 O emissions (17 ± 3 and 23 ± 5 mg m−2 d−1 ). Phosphate removal efficiencies were significantly higher in SFCW than SSFCW. In the pilot scale CW, no nitrous oxide emissions were observed and average CH4 emission (11,000 ± 930 mg m−2 d−2 ) was higher in the zones near the outflow planted with Zantedeschia aethiopica than in the zones near the inflow planted with Typha sp (4500 ± 800 mg m−2 d−2 ) and Cyperus papyrus (5500 ± 600 mg m−2 d−2 ), although TOC was higher in the zones near the inflow. We concluded that substrate and water flow are important factors controlling greenhouse gas emissions in CW and the amount of Zantedeschia aethiopica plants did not influence the emissions. Differences in methane emissions in the zones planted with native wetland plants in comparison with zones planted with Zantedeschia aethiopica might indicate that methane production and consumption in CW is influenced differently by the ornamental plants than by native wetland plants. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Constructed wetlands CW are designed to treat wastewater using the same natural processes that occur in natural wetlands but in a controlled manner. They provide an excellent alternative to conventional wastewater treatment such as active sludge systems, because constructed wetlands are low in cost and maintenance, offer good performance, and provide a natural appearance to the residents. Removal of organic matter in treatment wetlands is mediated by several microbial reactions such as aerobic respiration, denitrification, sulphate reduction, fermentation processes and methanogenesis (García et al., 2005). The products of some of these reactions include greenhouse gases such as methane (CH4 ), carbon dioxide (CO2 ) and nitrous oxide (N2 O), which are released
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (M.E. Hernández).
to the atmosphere. Greenhouse gases contribute to the radiative forcing in the atmosphere and consequently affect Climate Change, thus, their quantification in CW needs to be addressed before a greater implementation of this technology. The utilization of ornamental plants in CW is a topic that has been investigated for the last 13 years in several countries especially near the tropics (Belmont and Metcalfe, 2003; Zhang et al., 2006; Konnerup et al., 2009; Calheiros et al., 2015; Zurita et al., 2006, 2009, 2016). In México the strategy of using ornamental plants in constructed wetlands has been proposed to facilitate the introduction of this technology in small communities. Flowers produced in the constructed wetlands can be commercialized by the community as an economical incentive in exchange for providing maintenance to the treatment wetlands (Hernández, 2016). The majority of studies regarding the use of ornamental plants have been focused on the treatment efficiency and plant growth (Konnerup et al., 2009; Méndez-Mendoza et al., 2015; Saumya et al., 2015). However, little is known about the effects of ornamental plants on the biogeochemical process that occur in constructed wetlands. The objective of this study was
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to investigate greenhouse gas GHG emissions in different types of CW with ornamental plants at different scales. We investigated the effect of water flow, substrate type and plant density on GHG emission, and pollutant removal in CW mesocosms with ornamental plants to treat polluted river water. We also compared GHG emissions in areas with native wetland plants versus zones planted with ornamental plants; in a pilot scaled subsurface constructed wetland, for treating municipal wastewater. Our hypotheses were: (1) surface flow CW mesocosms with soil would have more anaerobic conditions than subsurface flow CW with gravel, therefore higher methane emissions. (2) High plant density would provide more oxygen to the substrate, decreasing methane emissions. (3) In the pilot scale treatment wetland; our hypothesis was that areas with ornamental plants would have lower GHG because they were near the outflow and therefore would have less nutrient availability, as compared to areas with native wetlands plants near the inflow. 2. Material and methods 2.1. Study sites This study was conducted in the central part of Veracruz State in Southeastern Mexico. The CW mesocosms were in a glass greenhouse (without temperature or humidity control) at the botanical garden “Francisco Javier Clavijero” in Xalapa city (96◦ 56 24 W 19◦ 30 54 N) at 1560 m AMSL. Weather in the region is humid subtropical with an annual precipitation of 1509.1 mm and an annual average temperature of 18◦ C. CW mesocosms were supplied with water from the Sordo River. This river is a third order stream that originates in a tropical mountain rainforest upstream from the botanical garden; downstream, it joins the Pixquiac River and finally it merges to La Antigua River which flows into the Gulf of Mexico. The pilot scale treatment wetland was constructed in the community of Pinoltepec municipality, of Emiliano Zapata, Veracruz (96◦ 45 18 W 19◦ 26 45 N) at 780 AMSL. Weather in the region is humid tropical with an annual precipitation of 2779.1 mm and annual average temperature of 25.2◦ C. The pilot treatment wetland was supplied with settled municipal wastewater. Population in Pinoltepec is 649 inhabitants and has 168 residences. 2.1.1. Mesocosm units The experimental mesocosms consisted of cells built using fiber glass (1.5 m length, 0.25 m wide and 0.6 m depth). Four cells were set up for free surface flow wetland SFCW mesocosm using upland soil (C = 13.8%, TN = 0.93%, N-NO3 = 83.28 mg Kg−1 , pH = 5.8) as substrate (0.4 m deep, free water surface flow column of 10 cm), and four cells for subsurface flow wetland SSFCW mesocosm using volcanic gravel as substrate (0.04 m diameter, 0.5 m depth, water flow 10 cm below surface). Two cells of each type of flow were planted with Zantedeschia aethiopica at low density (16 plants m−2 ) and two cells with high density (32 plants m−2 plants). Mesocosms were supplied with water from the Sordo River, which receives untreated urban wastewater from Xalapa city. River water was pumped into a 500 L distribution tank and continuously discharged into each mesocosm trough a PVC pipe. Water flow rate was adjusted for each cell to have a Hydraulic Retention Time (HRT) of 3 days (Fig. 1). 2.1.2. Pilot scale treatment wetland The treatment wetland consisted of two parallel concrete cells (20 m length, 1 m wide and 0.6 m depth) with subsurface water flow (volcanic gravel 0.05 m diameter, 0.5 m depth, water flow 10 cm below surface). Both cells were planted with the same array of vegetation and received the same flow of wastewater. Vegetation in the cells were distributed from the inflow to the outflow as
follows: 0–5 m Typha sp, 5–12 m Cyperus papyrus, 12–20 m Zantedeschia aethiopica. This arrangement was established to enhance flower production of Zantedeschia aethiopica; since previous studies demonstrated that a high nutrient load stimulated the growth of Zantedeschia aethiopica, but no flower production (unpublished data). On the other hand, wetland plant species such as Typha sp and Cyperus papyrus, are tolerant to high nutrient loads, therefore they were planted near the inflow where nutrient concentration was higher. Wastewater flow rates were adjusted for each cell to have a Hydraulic Retention Time (HRT) of 40 h. 2.1.3. Sampling and analytical methods Water samples (200 ml) were taken from the influent and effluent of each CW mesocosm. Influent samples were taken beyond the distribution tank and from each individual hose that fed river water to the mesocosms and, effluent samples were taken at the exit of each cell. Water sampling was performed twice a week from May to November. In the pilot scale treatment wetland, water samples (200 ml) were also taken from influent and effluent of each cell every other week, from June to October. Additionally, PVC wells were installed next to the gas sampling chamber and water samples from these wells were taken each time that GHG were measured. The samples were analyzed for Chemical Oxygen Demand (COD), ammonia nitrogen (N-NH3 ), nitrate (N-NO3 −1 ), orthophosphate (P-PO4 3− ), Total Phosphorus (TP), Total Nitrogen (TN), Total Organic Carbon, Total Solids (TS) and Total Suspended Solids (TSS). COD was measured using the oxidation of K2 Cr2 O7 micro-method (APHA, 1998); N-NH3 was analyzed by the Nessler method; Total Nitrogen TN was analyzed by Kjeldahl method (APHA, 1998); PPO4 were quantified using the ascorbic acid method according to Sandell and Onish (1978); Total Phosphorus was analyzed by persulfate digestion followed by ascorbic acid method; N-NO3 were quantified with the salicylic acid method, according to Rofarge and Jonson (1983). All these colorimetric analyses were performed using a UV–vis Jenway-Genova spectrophotometer (Jenway-Essex, UK). Total Organic Carbon in water was analyzed in a Total Organic Carbon analyzer (Torch, Teledyne Tekmar). TSS and TS were analyzed by gravimetric methods (APHA, 1988) 2.1.4. Gas flux measurements In the mesocosms, gas fluxes were measured twice a month from June to November 2012, and in the pilot scale treatment wetland, from July to October 2013. In both cases the closed chamber technique was used for the measurement of greenhouse gas emissions (Hernández and Mitch, 2006; Altor and Mitsch, 2008). In the mescosms, chambers consist of a permanent base and a removable cover. The base was a PVC pipe (0.40 m height, 0.05 m ID) with a plastic collar to create a water seal with the cover. They were inserted permanently 0.12 m into the wetland substrate. The cover was 0.10 m height × 0.075 m ID and had a gas sampling port and thermometer. Two chambers were installed in each wetland messocosm, one chamber was placed near the water inlet and one near the outlet. In the pilot scale treatment wetland, the chamber bases were a PVC pipe (0.25 m height, 0.10 m ID) with a cover (0.10 m height x 0.30 m ID). Chambers were placed in both cells, at 3, 9 and 15 m from inflow to outflow in Thypha sp, Cyperus papyrus and Zantedeschia aethiopica vegetation zones respectively. When the gas fluxes were measured, the cover was placed on top and sealed to the base using water in the collar. Internal gas samples were collected from the chamber every 5 min, during a 40-min period by using a syringe fitted with a Luer stopcock. Gas samples were stored into 10 ml vials previously vacuum evacuated and kept at 4 ◦ C until their analysis (within three days after they were collected). The gas samples were analyzed for their CH4 and N2 O
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 1. Schematic layout of the CW mesocosms with different water flow and substrate planted with the ornamental plant Zhantedeshia aethiopica to treat plluted river water.
concentrations simultaneously using a gas chromatograph (Perkin Elmer Clarus 500) equipped with a 2 m Porapak Q, 80/100 column, a flame ionization detector-FID and Electron Capture Detector- ECD. Operation conditions were: 13 ml min-1 of nitrogen as the carrier gas, temperature of the FID, ECD, injector and oven was set at 150, 360, 95 and 40 ◦ C, respectively. The gas fluxes were calculated from its concentration increase in the chamber, according to the following equation. F = V/A ∗
dC dt
mg/m3 d
Treatment
Removal% N-NH4
HD-SSFCW LD-SSFCW HD-SFCW LD-SFCW
N-NO3
96 ± 4 44 ± 9 b 5±6 c 0c a
55 ± 9 35 ± 7 b 23 ± 5 c 12 ± 4 d a
P-PO4
TOC
26 ± 4 33 ± 6 69 ± 10 62 ± 12
53 ± 6 a 51 ± 7 a 48 ± 4 a 31 ± 5 b
HD-SSFCW-High density subsurface flow constructed wetland, LD-SSFCW Low density subsurface flow constructed wetland, HD-SFCW- High density surface flow constructed wetland, LD-SFCW- Low density subsurface flow constructed wetland.
Where: F, is the gas flux (mg m−2 d), V is the chamber volume (m3 ), A is the area enclosed by the chamber, (m2 ) and dC is the gas concentration gradient dt
Table 1 Removal efficiency of pollutants in CW messocosms with different water flow planted with the ornamental plant Zhantedeshia aethiopica using high density (16 plants m−2 ) and low density (32 plants m−2 ).
.
2.1.5. Statistical analysis Statistical analyses were performed with SPSS version 14 for Windows. Kolmogrov-Smirnov, Lilliefors’ and Shapiro-Wilk’s tests were used to check normality. The data fit normal distributions. Two-way analysis of variance (ANOVA) with Tukey HSD multiple comparison tests were used to investigate the effect of water flow and plant density on pollutant removal and GHG emissions in mesocosms CW. Pearson correlation coefficient were performed between GHG emissions and water quality parameter at the inflow of each mesocosm in the same day that GHG were measured. One-way ANOVA was used to investigate differences on methane emissions, TOC and ammonia concentration in the different zones of pilot CW. A 5% significance level was used to determine differences among treatments. 3. Results and discussion 3.1. Pollutant concentration and removal in CW mesocosms N-NH3 in the inflow of CW mesocosms ranged from 3 to 20 mg L−1 during the study period (Fig. 2a). SSFCW mesocosms showed significantly higher average ammonia nitrogen removal efficiency than SFCW mesocosms; high plant density increased
the removal efficiency but only in SSFCW mesocosmsm (Table 1). This was probably due the fact that SSFCW has gravel as a substrate, which does not provide enough nutrients to the plants, therefore they have to uptake nitrogen from the wastewater. Contrarily, SFCW has soil as a substrate, which could have provided nutrients to the plants, therefore plants did not uptake nitrogen from the wastewater. N-NH3 , concentrations found in the Sordo river are within the range reported for polluted rivers (5.4–14.5 mg L−1 ) in Taiwan (Jing et al., 2001; Huang et al., 2007) and China (0.16–12.8 mg L−1 ) (Ruan et al., 2006; Tang et al., 2009). The Taiwanese Environmental Protection Agency determined that rivers with N-NH3 concentrations higher than 3 mg L−1 are considered heavily polluted. The recommended maximum concentration of N-NH3 in water bodies to protect aquatic life is 0.5 mg L-1 (Wetzel, 1981). According to this, the Sordo River is heavily polluted and the aquatic life in the sampling station might be endangered. CW mesocosms, especially SSFCW with high density of Zhantdeshia aethiopica were very efficient to remove ammonia nitrogen from polluted river water. These removal efficiencies were higher than the reported efficiencies (0–85%) by Dong et al., 2012 in constructed wetlands treating polluted river water with N-NH3 concentrations of 3.5–10.6 mg L−1 in China. N-NO3 in the inflow of CW mesocosms ranged from 1–10 mg L−1 (Fig. 2-b). SSFCW mesocosms showed significantly (P ≥ 0.05) higher average nitrate removal efficiencies than SFCW mesocosms, and high plant density increased the removal efficiency in both types
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 2. Pollutant concentration at the inflow and outflow of CW mesocosms with different water flow and planted with low and high plant density of Zhantedeshia aethiopica, a) ammonia nitrogen, b) nitrates, c) Phosphates and d) Total organic carbon. HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland.
of CW mesocosms (Table 1). This indicated that SSFCW had aerobic microsites that allowed nitrification and anoxic microsites in which nitrates were denitrified. The positive effect of high plant density on nitrate removal efficiency could be related with the root carbon exudates that enhance denitrification (Morgan et al., 2008). Phosphates in the inflow of CW mesocosms ranged from 1–6 mg L−1 . (Fig. 2c) We found significantly higher (P ≤ 0.05) phosphate removal efficiencies in the SFCW than SSFCW mesocosms and no effect due to plant density on phosphate removal efficiencies being observed (Table 1). Average phosphate concentrations observed in this study were much higher than in comparison to a polluted river in China 0.3 mg L−1 (Tang et al., 2009). A maximum concentration of 0.005 mg L−1 of phosphates is recommended to
avoid eutrophication in bodies of water (Wetzel, 1981). Phosphate concentration in the Sordo River is thousand times higher than the recommended limit, and CWs were able to decrease the concentration to acceptable levels. Removal efficiencies were similar in high plant density and low plant density, indicating that phosphates were removed by physical methods such as adsorption and the fact that SFCW showed better phosphorus removal efficiencies, may be explained by the clay content in the soils that provide surface for P-PO4 adsorption (Mitsch and Gosselink, 2007). TOC in the inflow of CW mesocosms ranged from 40 to 350 mg L−1 (Fig. 2d). On average CW mesocosms removed approximately half of TOC, only SFCW with low plant density showed significantly(P ≤ 0.05) lower TOC removal efficiency (31%). Organic
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Fig. 3. Methane emissions in CW mesocosms with different water flow and planted with low and high plant density of Zhantedeshia aethiopica, a) temporal emissions, b) average of emissions during the study period. HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland.
carbon in constructed wetlands is removed by microorganisms that use it as an energy source. Vast arrays of microorganisms are adapted to aerobic surface waters or anaerobic soils. The aerobic microorganisms consume oxygen to breakdown organics which provides them with energy. Anaerobic bacteria breakdown organic matter using different electron acceptors such as CO2 to produce methane. Anoxic-anaerobic reactions are less efficient than aerobic reactions. Our results indicated that in SFCW more anaerobic conditions prevailed than in SSFCW, therefore less efficient carbon breakdown occurred in these mesocosms producing methane. We found that high plant density in SFCW improved carbon removal which might be related to the presence of more roots that served as a support for microorganisms (Brix, 1997)
3.2. Greenhouse gas emissions in CW mesocosms Methane emission in CW mesocosms ranged from 0 to 2100 mg m−2 d−1 , with higher emissions in SFCW than SSFCW (Fig. 3). For the majority of treatments, the peaks of methane emission were observed in summer (July-August), except for SSFCW where the peak was observed in late September. The average methane emissions were significantly (P ≤ 0.05) higher in SFCW (436 ± 32 and 518 ± 46 mg m−2 d−1 ) than SSFCW (319 ± 65 and 210 ± 74 mg m−2 d−1 ), and no effect on methane emissions by plant density was observed. N2 O emissions ranged from −3 to 150 mg, m−2 d−1 with the peak of emission during October for all treatments (Fig. 4a). The average of nitrous oxide emissions was significantly higher (P ≤ 0.05) in SSFCW (17 ± 3 and 23 ± 5 mg m−2 d−1 ) than SFCW
(11 ± 2 and 6 ± 4) mesocosms and no effect on nitrous oxide emissions by plant density was observed (Fig. 4b). SSFCW were more efficient for nitrogen (ammonia and nitrate) removal than SFCW and these mesocosms also had high nitrous oxide emissions. Nitrous oxide is produced by nitrification in the aerobic zones and by denitrification in the anoxic zones (Hernández and Mitsch, 2006; Mander et al., 2008, 2014). SSFCW mesocosms had a gravel substrate, which probably enhanced aerobic zones that promoted ammonia oxidation into nitrates which were then denitrified in the anoxic zones in the deeper layers. In contrast, SFCW had a soil substrate and a water column which could have created anoxic and anaerobic conditions that did not allow ammonia oxidation therefore; nitrites where not available for denitrification, resulting in low nitrogen removal and nitrous oxide emissions. SFCW also had higher methane emissions than SSFCW, even though they had similar carbon removal efficiencies. This also indicates that SFCW had more anaerobic conditions than SSFCW, resulting in inefficient anaerobic carbon breakdown reactions with methane. Besides the reduced conditions, in SFCW the soil used as a substrate had organic carbon, which could be used as source of methane, while in the SSFCW, gravel lack of organic carbon before the biofilm formation, therefore we accepted our hypothesis no. 1. However, GHG emissions were not affected by plant density, thus we rejected our hypothesis no. 2. The radial oxygen loss ROL from plant roots that create microaerobic zones in the rhizosphere have been described for several native wetlands plants (Wiessner et al., 2002) and has been documented that it varies between plant species (Mei et al., 2014). In this study, we used the ornamental plant Zantedeschia aethiopica, and for this specie, ROL has not been described;
Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001
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Fig. 4. Nitrous emissions in CW mesocosms with different water flow and planted with low and high plant density of Zhantedeshia aethiopica, a) temporal emissions, b) average emissions during the study period. HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland.
Table 2 Pearson correlation coefficients between water quality parameter and CH4 and N2 O emissions in CW mesocosms with different water flow and substrate. Treatment HDSSF-CW LDSSF-CW HDSF-CW LDSF-CW
N2 O CH4 N2 O CH4 N2 O CH4 N2 O CH4
N-NO3
N-NH4
TOC
−0.4013 0.3605 −0.7240* −0.3727 0.3040 0.1583 −0.2146 0.1167
−0.3923 −0.0793 −0.6090* −0.4906 −0.1894 −0.3329 −0.2690 0.6326
−0.1016 −0.1018 −0.0366 −0.2569 −0.3514 −0.2229 0.0396 −0.0482
HD-SSCW = High density subsurface constructed wetland, HD-SFCW = High density surface flow constructed wetland, LD-SSCW = Low density subsurface constructed wetland, LD-SFCW = Low density surface flow constructed wetland. * = indicates significant correlation at 0.05 probability level.
so our results suggested that ROL by this ornamental plant in SFCW was not enough to stimulate microaerobic zones near the rhizosphere that mitigate methane emissions. When we correlated N2 O and CH4 emissions with water quality parameters (N-NO3 , N-NH4 and TOC) we only found significant negative correlations between N2 O emissions in LD-SSCW and N-NO3 (P = 0.0023) and N-NH4 (P = 0.016) (Table 2). The negative correlation observed only in the low density SSFCW could be related to the
less amount carbon exudates provided by root plants that caused low C/N ratios and low C/N ratios has been described that decrease N2 O emissions in CW (Li et al., 2017).
3.3. Pollutant concentration and removal GHG emissions in the pilot scale CW During the first five months of operation, the pilot scale CW with a polyculture of macrophytes and ornamental plants showed a decrease in the pollutant concentrations from inflow to outflow with high removal efficiencies for COD, and suspended solids and low removal efficiencies for TP, P-PO4 , N-NH3 and total nitrogen (Table 3). Average methane emissions in the zones planted with Typha sp were 4500 ± 800 mg m−2 d−2 , which were not significantly different (p ≥ 0.05) from the average methane emissions in the zones planted with Cyperus papyrus (5500 ± 600 mg m−2 d−2 ) (Fig. 5). However, these emissions were significantly lower than those observed in the zones planted with the ornamental plant Zhathedeshia aethiopica (11,000 ± 930 mg m−2 d−2 ). The average of organic carbon at 3 m, and 6 m from the inflow were 225 ± 15 and 230 ± 17 mg L−1 respectively, which were significantly higher than the observed at 15 m (187 ± 11 mg L−1 ). Ammonia concentra-
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Fig. 5. Average methane emissions in a pilot scale CW a), Average TOC concentrations and c) Average ammonia nitrogen concentrations in a CW pilot treatment wetlands planted with a polyculture of macrophytes and ornamental plants.
Table 3Average concentrations of pollutants at the inflow and outflow of a pilot scale CW planted with polyculture of macrophytes and ornamental plants, and the removal percentage achieved during the first 5 months of operation (June 2013–October 2013) with a HRT of 40 hrs in Pinoltepec, Veracruz. Parameter mg L−1
Inflow
Outflow
% Removal
Total Nitrogen TN Total Phosphorus TP O-phosphates O-P Amonia Nitrogen N-NH3 Chemical Oxygen Demand COD Total Suspended Solids TSS Total Solids TS
119 ± 20 12 ± 3 8±1 33 ± 4 378 ± 31 91 ± 13 720 ± 52
72 ± 9 8±2 6 ± 0.4 24 ± 2 124 ± 20 15 ± 5 474 ± 35
47 ± 12 33 ± 6 25 ± 2 27 ± 3 67 ± 14 67 ± 9 34 ± 9
tion decreased from inflow towards the outflow from 45 mg L−1 to 40 mg L−1 but it was not significantly different. No nitrous oxide emissions were observed during the study period in this pilot scale CW. This finding might be related to the low ammonia removal efficiencies observed, which indicated poor nitrification and denitrification activity had occurred during the study period. In general, the low pollutant removal efficiencies could be explained by the short HRT (40 hrs) in which CW was operated during the first 5 months. The short HRT was chosen to treat most wastewater produced in the community per area of the CW. However after these results, a longer HRT was implemented to increase pollutant removal efficiency. The higher CH4 emissions in the Zhantedeshia aethiopica zones compared with the zones near the inflow planted with Typha sp and Cyperus gigantus was contrary to our hypothesis, therefore hypothesis no. 3 was rejected. This phenomenon might be related with
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differences in ROL between the plant species (Mei et al., 20014) that causes differences in the methane production and consumption within the CW. However, more studies on this are needed. 4. Conclusions Greenhouse gas emissions in CW mesocosms with ornamental plants were affected by the type of substrate and water flow. SFCW with soil substrate had higher CH4 emissions than SSFCW with gravel substrate. However, SSFCW microcosms with gravel were more efficient to remove nitrogen (ammonia nitrogen and nitrates) and had higher nitrous oxide emissions, indicating that they had less anaerobic conditions. High plant density enhanced ammonia and nitrate removal in SSFCW, and carbon removal in SFCW. However, plant density did not have any effect on greenhouse gas emissions in CW mesocosms planted with the ornamental plant Zhantedeshia aethiopica. In the pilot scale CW with a polyculture of macrophytes and ornamental plants; methane emissions were higher in the zones near the outflow planted with the ornamental plant Zhantedeshia aethiopica as compared to the zones near the inflow planted with native wetlands plants (Thypha sp and Cyperus papyrus). Near the outflow TOC concentrations were lower than near the inflow, which might indicate that methane production and consumption in the zones with ornamental plants probably is different from zones planted with native wetland plants. Aknowlegments Funding for this project was provided by the Mexican National Council for Science and Technology-CONACYT- basic science project No. 081942 References APHA, American public Health Association, 1998. Standard Methods for the Examination of Water and Wastewater, 3rd edition. APHA–AWWA–WPCF, Washington. DC. Altor, A.E., Mitsch, W.J., 2008. Pulsing hydrology, methane emissions, and carbon dioxide fluxes in created marshes: a 2-year ecosystem study. Wetlands 28, 423–438. Belmont, M.A., Metcalfe, C.D., 2003. Feasibility of using ornamental plants (Zantedeschia, aethiopica) in subsurface flow treatment wetlands to remove nitrogen, chemical oxygen demand and nonyphenol ethoxylate surfactants a laboratory scale study. Ecol. Eng. 21, 233–247. Brix, H., 1997. Do macrophytes play a role in constructed wetlands treatments. Water Sci. Technol. 5 (5), 11–17. Calheiros, C.S.C., Bessa, V.S., Mesquita, B.R., Brix, H., Rangel, A.O.S.S., Castro, P.M.L., 2015. Constructed wetland with a polyculture of ornamental plants for wastewater treatment at a rural tourism facility. Ecol. Eng. 79 (2015), 1–7. Dong, H., Quiang, Z., Li, T., Jin, H., Chen, W., 2012. Effect of artificial aeration on the performance of vertical-flow constructed wetland treating heavily polluted river water. J. Environ. Sci. 24 (4), 596–601. García, J., Barragán, Aguirre P., Mujeriego, J., Matamoros, R., Bayona, J., 2005. Effect of key design parameters on the efficiency of horizontal subsurface flow constructed wetlands. Ecol. Eng. 25 (4), 405–418.
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Please cite this article in press as: Hernández, M.E., et al., Greenhouse gas emissions and pollutant removal in treatment wetlands with ornamental plants under subtropical conditions. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.001