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Aeration and plant coverage influence floating treatment wetland remediation efficacy
Ecological Engineering 122 (2018) 62–68
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Aeration and plant coverage influence floating treatment wetland remediation efficacy Lauren M. Garcia Chancea, Sarah A. Whiteb, a b
T
⁎
Environmental Toxicology Graduate Program, Clemson University, 509 Westinghouse Rd, Pendleton, SC 29670, USA Department of Plant and Environmental Sciences, Clemson University, E-143 Poole Agricultural Center, Clemson, SC 29634, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphorus Nitrification Denitrification Canna flaccida Juncus effusus
Nutrient contamination of waterways is a growing concern, instigating the emergence of floating treatment wetland systems (FTWs) to remove nutrients from water. To determine if aeration of water systems enhances nutrient removal efficacy or nutrient fixation within plant tissues, aeration in combination with FTW installation techniques were investigated. During two separate trials with Juncus effusus and Canna flaccida, treatments consisted of either aerated or non-aerated mesocosms, with varying coverage and planting density combinations. Aeration increased the dissolved oxygen levels within each mesocosm, but reduced the removal of nitrogen and phosphorus from the water column in comparison to non-aerated systems. Plant samples collected from 100% planting density treatments that were aerated had a greater nitrogen uptake than non-aerated by as much as 55% or 13.5 g/m2 at harvest. Some discrepancies between plant uptake and water column nutrient levels can be attributed to microbially-mediated nitrogen losses (e.g., denitrification).
1. Introduction As water quality degradation concerns grow in public and private sectors, evaluation of remediation technologies to treat contaminants has increased. One such remediation technique is the use of floating treatment wetlands (FTWs) to remove excess levels of nitrogen (N) and phosphorous (P) from water systems. Floating treatment wetlands are often installed in existing storm water retention ponds. In many cases, especially when found in neighborhoods or other public spaces, these retention ponds have pre-existing fountains and other aeration methods. Aeration has proven to affect the speciation and remediation of contaminants in many constructed wetlands, however no previous literature has considered the impact upon FTWs (Dong et al., 2012; Ong et al., 2010; Zhang et al., 2010). The effect of aeration has been evaluated multiple times for both surface and subsurface constructed wetlands (Bowmer, 1987; MaltaisLandry et al., 2007, 2009; Zhang et al., 2010). The effect of aeration on P removal has varied across studies (Dong et al., 2012; Zhang et al., 2010). Remediation of ammonium (NH4+-N) has been reported to consistently increase with aeration (Butterworth et al., 2013; Dong et al., 2012; Liu et al., 2013; Ong et al., 2010); however, nitrate (NO3N) removal was higher in either non-aerated or intermittently aerated systems (Butterworth et al., 2013; Fan et al., 2013). This transformation of N correlates with the anaerobic and aerobic conditions supporting ⁎
nitrification and denitrification (Fig. 1). Nitrification is the oxidation of ammonium or ammonia (NH3) to nitrite and then to nitrate (Tanner et al., 2002; Wu et al., 2009). Nitrosomonas bacteria first convert ammonium to nitrite. Then nitrobacter convert nitrite to nitrate, both forms of N that are readily absorbed by plants. Denitrification is the microbial-mediated conversion of nitrate into nitrogen gas (N2) via nitrite, nitric oxide (NO), and nitrous oxide (N2O) (Tallec et al., 2008; Wu et al., 2009). Dissolved oxygen (DO) concentration plays an important role in nitrification and denitrification because nitrification is strictly aerobic (DO > 2.0 mg/L) while denitrification is strictly anoxic (DO < 1.0 mg/L) (DeBusk, 1999; Tallec et al., 2008). Nitrification and denitrification are the main pathways for N removal in CWs, but they usually do not occur simultaneously in a single wetland cell due to conflicting oxygen demands (Liu et al., 2013). Floating treatment wetlands can be compared with CWs in this scenario because if fountain and aeration methods are installed within retention ponds, they are likely to be continuously used rather than intermittently turned on and off, possibly homogenizing the pathway for N removal. Within retention and urban ponds, water column stratification is typical with great heterogeneity of thermo-chemical indices (McEnroe et al., 2013). For example, DO concentrations increase at the surface of ponds during the daytime, leading to possible supersaturation, and then decrease during the evening hours (Wetzel, 2001). However, FTWs stabilize and decrease DO levels beneath the floating mat, possibly
Corresponding author. E-mail address: [email protected] (S.A. White).
https://doi.org/10.1016/j.ecoleng.2018.07.011 Received 8 May 2018; Received in revised form 9 July 2018; Accepted 13 July 2018 0925-8574/ © 2018 Elsevier B.V. All rights reserved.
Ecological Engineering 122 (2018) 62–68
L.M. Garcia Chance, S.A. White
Fig. 1. Reactions of the microbial nitrogen cycle. (1) Nitrogen gas fixation; (2) aerobic ammonium oxidation by bacteria and archaea; (3) aerobic nitrite oxidation; (4) denitrification; (5) anaerobic ammonium oxidation; and (6) dissimilatory nitrate and nitrite reduction to ammonium (Jetten, 2008).
Fig. 3. Experimental layout of the planting coverage and planting density with circles representing plants and inner rectangle the floating mat.
of 12 EUs to provide direct air circulation (Fig. 2C). Two experiments were conducted, each with a different plant species. The first experiment was conducted with Juncus effusus L. (soft rush). On April 21, 2009, 10 cm Juncus liners were inserted into aerator cups, and EUs were planted with appropriate number of plants according to treatment design. The experiment was concluded October 29, 2009; Juncus plants were harvested March 8 and 9, 2010. The second experiment was conducted with Canna flaccida L. (golden canna). On March 23, 2010, Canna bare root liners (roots ≈ 10 cm and shoots ≈ 15 cm) were wrapped with coconut coir mat pieces (10 cm × 20 cm), inserted into aerator cups, and each EU was planted with the appropriate number of plants per the experimental design. Canna plants were harvested August 31 and September 2, 2010. Due to trials occurring separately, in different years, statistical comparisons between the two species were not conducted; instead analysis were conducted by species by year. Overall experimental design by year was 1 plant species * 2 aeration levels * 3 plant coverage levels * 3 plant density levels * 4 replicates, with year 1 (2009) conducted using Juncus and year 2 (2010) conducted using Canna.
changing water column stratification and thus the effect of aeration on nutrient removal in comparison to CWs (Wang and Sample, 2013; White and Cousins, 2013). Zhang et al. (2010) determined that with aeration, less surface area was needed to remediate organic matter and N compared to a nonaerated CW system. To better employ FTWs within surface water bodies, additional knowledge related to economics of installation and maintenance, specifically the percent of pond surface area covered and planting density, are needed. The objective of this work was to quantify the effect of percent surface area covered, planting density, and aeration on FTW efficacy for treating nutrients in surface runoff.
2. Materials & methods 2.1. Experimental design Experiments were carried out over the spring-fall seasons of 2009 and 2010. An experimental system was assembled in Pendleton, SC (34.640, −82.773) consisting of twenty-four 378.5 L structural foam stock tanks (Rubbermaid, Atlanta, GA; Fig. 2). Each stock tank or experimental unit (EU) had a surface area of 1.15 m2 and a volume of 0.38 m3. Holes were drilled 6 cm from the rim at one end of each EU to regulate overflow and release of water. Floating mats, 1 cm think and cut to 60 cm × 30 cm, were supplied by Beemats (New Smyrna Beach, FL). The mats are buoyant, interlocking solid-core foam mats joined with 10 cm nylon connectors. Each section of mat has ten (7.5 cm) pre-cut holes spaced 12 cm on center (Fig. 3). Holes allow insertion of specially designed plastic aerator cups in which to place plants (Fig. 2B). Treatments consisted of 50% and 100% surface coverage using mats with plant densities of either 10 plants (50% or 100% coverage) or 20 plants (100% coverage) (Fig. 3). Twelve of the twenty-four EUs were continuously aerated and twelve had no supplemental aeration. Aeration was controlled by individual aquarium bubblers placed within each
2.2. Runoff simulation Six holding tanks, ranging in volume from 795 L (1 tank), 1135 L (4 tanks) to 1230 L (1 tank) were used to feed EUs. Solutions in the holding tanks were made by mixing water (municipal source) and a water-soluble fertilizer (20N-2P-20K Nitrate Special Soluble Fertilizer, Southern Agricultural Insecticides, Inc., Hendersonville, NC). Solutions flowing from the holding tanks averaged concentrations of 34.6 ± 6.4 mg/L N and 3.8 ± 0.5 mg/L P, representative of agricultural surface runoff (Prystay and Lo, 2001; White, 2013). The watersoluble fertilizer was completely dissolved in water prior to addition to the stock tanks to ensure uniform distribution. Treatments were randomly assigned to groups of four EUs per holding tank. Water distribution lines were plumbed so that water flowed continuously into
Fig. 2. Experimental setup for floating treatment wetland experiments including: (A) six holding tanks and 24 mesocosms with 378.5 L, (B) aerator cups into which plants are inserted, and (C) the aeration system. 63
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Fig. 4. Average dissolved oxygen concentration in floating treatment wetland experiments established with Canna flaccida (2010, n = 56/treatment) and Juncus effusus (2009, n = 92/treatment) to determine as influenced by treatment factors: aeration, planting density, and percent coverage.
100 mg of each tissue and assayed using an Elementar Vario Macro Nitrogen combustion analyzer (Mt. Laurel, NJ), and P was assayed by wet acid digestion procedure using the nitric acid and hydrogen peroxide method (Huang et al., 2004). Additionally, P, K, Ca, Mg, Zn, Cu, Mn, Fe, S, Na, B, and Al concentrations in plant tissues were determined by ICP-ES. Only results for nitrogen and phosphorus are discussed. During the Canna experiment, the coconut coir mat pieces promoted a low level of weed growth. To determine the role of weed growth within the system, weeds were permitted to grow as they emerged and were harvested and separated from the trial plants during the Canna harvest dates. Harvested weeds were identified and treated using the same methods outlined above to determine nutrient concentrations.
each EU. Flow from each tank was calibrated to 65 mL/min for a uniform 2-day hydraulic retention time in each EU (Fig. 2). Daily load was calculated using tank flow rates and nutrient concentrations. Rainfall was not measured. 2.3. Water sampling and analysis Water samples were collected every 2 weeks from each EU and holding tank to permit calculations of concentration and loading from both influent and effluent. Water samples were processed and evaluated using three separate analyses: inductively coupled plasma emission spectrophotometer (ICP-ES), ion chromatography (IC), and total organic carbon analyzer (TOC). Trace elements including: P, K, Ca, Mg, Zn, Cu, Mn, Mo, Ni, Fe, S, Na, B, and Al were analyzed via ICP-EC (61E Thermo Jarrell Ash, Franklin, MA). Lower detection limits were 5 µg/L. Anions including: chloride, nitrate, nitrite, phosphate, and sulfate analysis were detected using a Dionex AS10 ion chromatograph with AS50 auto-sampler (Dionex Corp., Sunnyvale, CA, USA). Lower detection limits were 0.2 mg/L. Analysis for dissolved (non-purgeable) organic carbon (DOC) was performed using a Shimadzu TOC-VCPH total organic carbon analyzer (Shimadzu Scientific Instruments, Kyoto, Japan). All analyses were conducted to US EPA protocol methods 6010B, 9056A, and 9060A and calibration standards were instituted for quality assurance and control (USEPA, 1997; USEPA, 2004; USEPA, 2007). Water quality parameters including pH, dissolved oxygen (DO, mg/L), and temperature (°C), were collected every 14 days using a handheld probe (YSI, Yellow Springs, OH). The probe was inserted until it touched the bottom of each EU at the effluent end.
2.5. Data analysis All data presented represent the average value for each sampling event ± the standard error of the mean. When appropriate, statistical analysis were conducted using the Analysis of Variance procedure within JMP v13 (SAS Institute Inc. Cary, NC). Where main effects were significant (α < 0.05), a Student’s t-test was conducted to determine the influence of the treatment on nutrient removal. 3. Results and discussion 3.1. Dissolved oxygen In the absence of aeration, DO levels were maintained at an average 2.9 mg/L for Canna and 5.14 mg/L for Juncus (Fig. 4). The DO concentrations for aerated systems averaged 6.94 mg/L for Canna and 9.75 mg/L for Juncus. Despite differences in DO concentrations between non-aerated and aerated systems (p < 0.001 for both Canna and Juncus), the concentration of DO in non-aerated systems never declined to < 1 mg/L DO (never became anaerobic). While depletion of DO is possible in other forms of CWs (Butterworth et al., 2013; Fan et al., 2013), FTWs permit gas exchange across the water surface and maintain acceptable aeration levels for nitrification processes without additional artificial aeration (Nowak, 2000). Additionally, DO levels were impacted by planting density and mat coverage in the EU (Fig. 4). Nonaerated mesocosms covered at 100% planting coverage had a lower DO level than those with 50% coverage, signifying gas exchange most likely
2.4. Plant sampling and analysis At the initiation of each experiment, three plants from each EU were randomly selected for measurement. Monthly measurements were performed on the same plants within each EU (repeated measure). Root and shoot growth were measured to monitor establishment of the FTWs. The same 3 plants from each EU were selected for harvest. Plant root and shoot tissue samples were weighed (fresh weight, g) and dried at 80 °C, re-weighed (dry weight, g), and then ground in a Wiley mill (Swedesboro, NJ) to pass through a 40-mesh (0.425-mm) screen. Nitrogen concentration was determined for root and shoot tissues using 64
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Fig. 5. Dissolved oxygen concentration over time of influent to floating treatment wetland experimental units that were either aerated or non-aerated, contrasted with water temperature over the course of the experiments in 2010 Canna flaccida and 2009 Juncus effusus.
(Table 2). Overall, no difference between aerated and non-aerated system concentrations for ammonia and nitrite were detected, apart from Canna ammonia. Conversely, the concentration of nitrate varied between aerated and non-aerated systems (p < 0.001), with non-aerated systems containing less nitrate than aerated for both Juncus and Canna. The higher levels of nitrate within aerated systems could be partially attributed to differences in microbial activity between the aerated and non-aerated systems. Typically, aerated systems encourage aerobic nitrification processes that result in greater production of nitrate for plant uptake, while non-aerated systems produce less nitrate and greater amounts of nitrous oxide in a gaseous form due to anaerobic processes (DeBusk, 1999; Tallec et al., 2008). Within this system, anaerobic conditions were not attained. The aerated systems had greater plant uptake of N than non-aerated, precluding the possibility that nitrate was removed from the system more quickly by plants in the non-aerated mesocosms. This disparity in described mechanisms of N removal vs. actual within these experiments can partially be attributed to the location of the DO sampling within each EU (the bottom of the mesocosm). Because DO sampling occurred below the root systems, it is possible that the DO measurement recorded did not reflect the oxygen levels within the root system itself, though the depth of the system may negate any significant DO differences. Future DO measurements should consistently be taken within the root zone to better inform researchers of likely microbial activity and aerobic or anaerobic status. If anaerobic conditions were of impact for N removal via FTWs, concerns as to the release of nitrous oxide into the atmosphere may steer individuals toward less efficient aerated system with greater plant uptake and removal. No differences were found in ionN speciation due to planting density and coverage.
occurs in the uncovered portions of the system (p = 0.0002 for Canna and p = 0.002 for Juncus). This difference was less extreme in aerated systems, but still existed. Of note, DO levels within aerated systems closely followed those of the inflow, while non-aerated systems appeared to have stabilized independent of the influent DO over time (Fig. 5). Potentially indicating that DO saturation levels of the aerated systems were regulated by temperature. Additionally, as temperature increased, DO levels decreased and vice-versa (r = 0.66 Canna, r = 0.73 Juncus). Non-aerated systems appear less influenced by temperature, save for Juncus effluent when air temperatures began to decline at the end of the experiment (r = 0.39 Canna, r = 0.46 Juncus).
3.2. Nitrogen Ionic nitrogen [ionN = NH3-N (ammonia) + NO2-N (nitrite) + NO3-N (nitrate)] concentrations were reduced by 22.6% in the presence of Juncus and 67.0% in the case of Canna. Mass balance calculations were made and reductions in daily load determined that incorporated plant contribution to ionN removal (Table 1). On average, the daily loading rate ionN was reduced by 18.9% in the presence of Juncus and 63.9% in the presence of Canna. Plant tissue analyses indicate that plant associated ionN was responsible for only 24.9% ionN removal in Juncus and 12.9% in Canna. This leaves 67.3 g m−2 experiment−1 of ionN removal unaccounted for in the presence of Juncus and 306 g m−2 experiment−1 of ionN removal unaccounted for in the presence of Canna. Mass balance determinations were made based on the experimental factors of aeration, planting density, and coverage to parse out the effects of treatment factors (Table 1). Aerated systems had a higher average effluent ionN concentrations than non-aerated systems (p < 0.0001); non-aerated systems better reduced total average ionN concentrations. The same trend held true for reduction in daily loading rates of ionN in systems established with Canna (p < 0.0001). Plant uptake, however, was greater in aerated systems than non-aerated systems for Juncus (p < 0.0001), but no difference in ionN uptake was found for Canna. Additionally, in both Canna and Juncus, full density plantings had greater individual plant uptake than those with half density plantings (p < 0.0001 for Juncus and p = 0.0016 for Canna). Daily load reductions were only influenced by coverage and planting density for Juncus (p = 0.0002), with full coverage, half density systems removing the most ionN. Planting density and coverage did not influence either average reduction in concentration or reduction in daily load for Canna (p = 0.2975). These results led to the question, why did non-aerated and half density planting systems remove more ionN than aerated systems, yet have less plant uptake? To address this question, ionN speciation analyses were conducted to determine where within the N cycle, N fate was impacted the most
3.3. Phosphorus Phosphorus concentrations were reduced in the presence of both Juncus (32.3%) and Canna (49.5%), at similar levels documented by previous FTW phosphorus studies (Wang and Sample, 2013; White and Cousins, 2013). As with other reports of P remediation within constructed wetlands, no differences were found between aerated and nonaerated systems regarding phosphorus remediation (Table 1). Some differences were noted in planting density and coverage of both Juncus and Canna. Full coverage, half density systems removed a greater amount of daily phosphorus load than full coverage, full density (p = 0.0214) or half coverage, full density systems (p < 0.001). Juncus effusus removed only 3.85 g m−2 experiment−1 or 11.9% of phosphorus, and Canna removed only 7.62 g m−2 experiment−1 or 13.5% (Table 1). Like N, plant uptake contributed less phosphorus removal in the full coverage, half density systems than full density systems (p = 0.001). Within the current FTW experiments, plant uptake percentages were lower than expected and accounted for only a small 65
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Table 1 Nitrogen and phosphorus mass balance calculations for Juncus effusus and Canna flaccida after season-long exposure to nutrients in floating treatment wetlands. Values presented are the means (standard error) of the mean. Total Nitrogen
Total Phosphorus
Juncus
Canna
Average Concentration
(mg L−1)
Influent Effluent
19.2 (1.98) 14.8 (1.60) Aerated Non-Aerated Full coverage, full density Half coverage, full density
16.3 13.4 14.7 15.3
(1.27) (1.93) (1.82) (1.60)
21.0 (2.53) 6.92 (1.58) a b a a
4.34 22.6%
Daily Load
(g m−2 day−1)
Influent Effluent
3.22 (0.31) 2.61 (0.27) 2.76 2.46 2.79 2.59 2.46
Canna
(mg L−1)
Reduction in Concentration % Concentration Reduction
Aerated Non-Aerated Full coverage, full density Half coverage, full density Full coverage, half density
Juncus
8.59 5.25 6.77 7.32
(1.60) (1.56) (1.85) (1.66)
3.22 (0.27) 2.18 (0.26) a b a a
2.31 2.04 2.04 2.40
14.0 67.0%
(0.22) (0.33) (0.31) (0.27) (0.23)
(0.23) (0.29) (0.29) (0.27)
4.44 (0.53) 2.24 (0.41) a a a a
1.04 32.3%
2.36 2.11 2.47 2.19
(0.48) (0.35) (0.43) (0.53)
2.20 49.5%
(g m−2 day−1) 3.41 (0.40) 1.23 (0.25)
a a a ab b
1.51 0.95 1.17 1.33 1.20
(0.26) (0.25) (0.30) (0.27) (0.19)
0.60 (0.05) 0.38 (0.04) a b a a a
0.39 0.37 0.39 0.41 0.35
a a a a b
(0.08) (0.07) (0.07) (0.08) (0.04)
Mass Balance
(g m−2 experiment−1)
Total Influent Load Total Effluent Load Total Load Reduction
473 (45.6) 384 (36.8) 89.6
549 (64.4) 198 (40.3) 351
88.2 (7.40) 55.9 (5.90) 32.3
119 (14.5) 62.8 (11.3) 56.3
Plant Uptake % Plant Uptake
22.3 (1.21) 24.9%
45.4 (4.02) 12.9%
3.85 (0.23) 11.9%
7.62 (0.67) 13.5%
Other Removal Processes
(1.93) (1.40) (1.88) (1.08) (1.76)
a b a a b
a a a a b
0.35 47.3%
(g m−2 experiment−1)
50.7 40.1 62.9 50.7 22.6
67.3
0.22 36.7%
0.41 0.36 0.37 0.43 0.33
0.61 18.9%
24.1 20.4 25.6 26.6 14.5
2.18 63.9%
(0.04) (0.05) (0.05) (0.05) (0.04)
0.74 (0.09) 0.39 (0.07)
Reduction in Daily Load % Daily Load Reduction
Aerated Non-Aerated Full coverage, full density Half coverage, full density Full coverage, half density
a a a a
(6.71) (4.36) (7.94) (6.66) (2.34)
a a a a b
4.49 3.21 4.40 4.67 2.49
306
(0.35) (0.25) (0.41) (0.32) (0.19)
a b a a b
28.5
8.01 7.23 10.9 8.25 3.72
(6.66) (4.49) (1.36) (0.99) (0.36)
a a a a b
48.7
particulates in sediment and precipitation from the water column (Appan and Wang, 2000; Wang et al., 2003). Therefore, other processes most likely were responsible for this difference in P remediation in the water column compared to plant uptake.
proportion of the phosphorus removed in these experiments. These results were similar to those found by Tanner and Headley (2011), who reported that uptake of P into plant tissues accounted for only a small fraction of P removal in their experiments. Tanner and Headley (2011) compared living plant roots with artificial roots and determined that release of a bioactive compound from the rhizosphere or changes in the physico-chemical conditions of the water column enhanced remediation processes (Tanner and Headley, 2011). This would further explain the greater P uptake seen in full density planting systems (Table 1). The fate of phosphorus in constructed wetlands is typically sorption to
3.4. Weeds Within the Canna study, any weeds within the systems were harvested and processed according to similar protocols as the Canna. Primula spp. and Carex spp. accounted for a total 17.1 ± 1.02 g N in
Table 2 Nitrogen speciation within water samples collected from floating treatment wetlands established with Juncus effusus (2009) and Canna flaccida (2010) as influenced by aeration, planting density, and percent cover. Average Concentration
Total NH3+
Total NO2
Juncus (mg L Aerated Non-Aerated Full coverage, full density Half coverage, full density Full coverage, half density
0.53 0.54 0.43 0.62 0.54
(0.04) (0.03) (0.05) (0.05) (0.06)
−1
a a a a a
)
Canna (mg L 0.52 0.38 0.40 0.52 0.44
(0.05) (0.05) (0.06) (0.06) (0.05)
−1
a b a a a
)
Total NO3 −1
Juncus (mg L 0.87 0.82 0.77 0.89 0.87
66
(0.23) (0.18) (0.11) (0.20) (0.16)
a a a a a
)
Canna (mg L 0.07 0.11 0.08 0.11 0.08
(0.02) (0.01) (0.02) (0.02) (0.01)
−1
a a a a a
)
Juncus (mg L−1)
Canna (mg L−1)
15.5 13.0 14.3 14.6 13.9
7.99 4.76 6.29 6.69 6.15
(0.28) (0.39) (0.34) (0.48) (0.41)
a b a a a
(0.40) (0.56) (0.50) (0.43) (0.27)
a b a a a
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Fig. 6. Average dissolved organic carbon measured in water in floating treatment wetland experimental units (n = 56/treatment) planted with Canna flaccida as influenced by aeration, planting coverage, and planting density.
and daily reduction in P load ranged from 37% to 47%. Plant uptake accounted for < 25% of P and ionN removal from the systems. Aeration enhanced N and P uptake within Juncus. Some nutrient removal was explained via weed uptake and increased concentrations of DOC. Ultimately, FTW systems can operate under aerobic or anaerobic conditions.
aerated systems and 34.8 ± 1.43 g N in non-aerated systems; they also accounted for total 2.83 ± 0.17 g P in aerated systems and 4.65 ± 0.19 g P in non-aerated systems. These values varied within an EU and depended upon the number of weeds within each EU. While these values are low in comparison to the total N and P removed in both the Juncus and Canna experiments, accounting for weedy species contributions to both ionN and P remediation help to further explain the remediation of within the system and should be considered in future experiments.
Acknowledgements The authors wish to thank Elizabeth Nyberg, Brandon C. Seda and J. Brad Glenn for contributions in sampling and laboratory work. This research was financed by the Horticulture Research Institute. Publication of this material is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2014-51181-22372. This material is based upon work supported by NIFA/USDA, under project number SC-1700536. Technical contribution no. 6670, of the Clemson University Experiment Station.
3.5. Dissolved organic carbon Carbon availability and denitrification have been shown to closely correlate in previous studies (Chen et al., 2011; Wen et al., 2010). To expand potential explanations for nitrogen and phosphorus remediation results in this study, the DOC concentrations measured during the Canna experiment were modeled with aeration and planting density and found to correlate (p < 0.001) (Fig. 6). Availability of DOC increased in aerated systems compared to influent (p = 0.0026), and even greater concentrations of DOC were available in non-aerated systems (p < 0.001). The constituents of DOC could include: decaying organic matter, algae, and metabolites sloughed from plant roots systems. Studies indicate organic carbon can act as a nutrient sink and external carbon sources stimulate nitrate removal in CWs (Chen et al., 2011; Gebremariam and Beutel, 2008; Lin et al., 2002). The greater DOC concentrations found in non-aerated systems may have enhanced removal of phosphorus and nitrate unaccounted for in water and plant samples.
References Appan, A., Wang, H., 2000. Sorption isotherms and kinetics of sediment phosphorus in a tropical reservoir. J. Environ. Eng. 126, 993–998. Bowmer, K.H., 1987. Nutrient removal from effluents by an artificial wetland: influence of rhizosphere aeration and preferential flow studied using bromide and dye tracers. Water Res. 21, 591–599. Butterworth, E., Dotro, G., Jones, M., Richards, A., Onunkwo, P., Narroway, Y., Jefferson, B., 2013. Effect of artificial aeration on tertiary nitrification in a full-scale subsurface horizontal flow constructed wetland. Ecol. Eng. 54, 236–244. Chen, Y., Wen, Y., Cheng, J., Xue, C., Yang, D., Zhou, Q., 2011. Effects of dissolved oxygen on extracellular enzymes activities and transformation of carbon sources from plant biomass: implications for denitrification in constructed wetlands. Bioresour. Technol. 102, 2433–2440. DeBusk, W.F., 1999. Wastewater Treatment Wetlands: Contaminant Removal Processes. SL155. University of Florida Institute of Food and Agricultural Sciences Cooperative Extension Service, Gainesville, FL. Dong, H., Qiang, 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, 596–601. Fan, J., Wang, W., Zhang, B., Guo, Y., Ngo, H.H., Guo, W., Zhang, J., Wu, H., 2013. Nitrogen removal in intermittently aerated vertical flow constructed wetlands: impact of influent COD/N ratios. Bioresour. Technol. 143, 461–466.
4. Conclusions Floating treatment wetlands behave differently than CWs when exposed to aeration. Aerobic conditions were maintained within the experimental FTW systems, even in EUs without aeration. Water effluent N remediation was higher within non-aerated systems than aerated systems. Daily reduction in ionN load ranged from 19% to 64%, 67
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341–353. Tallec, G., Garnier, J., Billen, G., Gousailles, M., 2008. Nitrous oxide emissions from denitrifying activated sludge of urban wastewater treatment plants, under anoxia and low oxygenation. Bioresour. Technol. 99, 2200–2209. Tanner, C.C., Headley, T.R., 2011. Components of floating emergent macrophyte treatment wetlands influencing removal of stormwater pollutants. Ecol. Eng. 37, 474–486. Tanner, C.C., Kadlec, R.H., Givvs, M.M., Sukias, J.P.S., Nguyen, M.L., 2002. Nitrogen processing gradients in subsurface-flow treatment wetlands-influence of wastewater characteristics. Ecol. Eng. 18, 499–520. Wang, C.-Y., Sample, D.J., 2013. Assessing floating treatment wetlands nutrient removal performance through a first order kinetics model and statistical inference. Ecol. Eng. 61 (Part A), 292–302. Wang, H., Appan, A., Gulliver, J.S., 2003. Modeling of phosphorus dynamics in aquatic sediments: I – model development. Water Res. 37, 3928–3938. Wen, Y., Chen, Y., Zheng, N., Yang, D., Zhou, Q., 2010. Effects of plant biomass on nitrate removal and transformation of carbon sources in subsurface-flow constructed wetlands. Bioresour. Technol. 101, 7286–7292. Wetzel, R.G., 2001. Limnology: Lake and River Ecosystems. Academic Press, San Diego. White, S.A., 2013. Wetland technologies for nursery and greenhouse compliance with nutrient regulations. HortScience 48, 1103–1108. White, S.A., Cousins, M.M., 2013. Floating treatment wetland aided remediation of nitrogen and phosphorus from simulated stormwater runoff. Ecol. Eng. 61 (Part A), 207–215. Wu, J., Zhang, J., Jia, W., Xie, H., Gu, R.R., Li, C., Gao, B., 2009. Impact of COD/N ratio on nitrous oxide emission from microcosm wetlands and their performance in removing nitrogen from wastewater. Bioresour. Technol. 100, 2910–2917. Zhang, L.-Y., Zhang, L., Liu, Y.-D., Shen, Y.-W., Liu, H., Xiong, Y., 2010. Effect of limited artificial aeration on constructed wetland treatment of domestic wastewater. Desalination 250, 915–920.
Gebremariam, S.Y., Beutel, M.W., 2008. Nitrate removal and DO levels in batch wetland mesocosms: Cattail (Typha spp.) versus bulrush (Scirpus spp.). Ecol. Eng. 34, 1–6. Huang, L., Bell, R.W., Dell, B., Woodward, J., 2004. Rapid nitric acid digestion of plant material with an open-vessel microwave system. Commun. Soil Sci. Plant Anal. 35, 427–440. Jetten, M.S.M., 2008. The microbial nitrogen cycle. Environ. Microbiol. 10, 2903–2909. Lin, Y.F., Jing, S.-R., Wang, T.W., Lee, D.Y., 2002. Effects of macrophytes and external carbon sources on nitrate removal from groundwater in constructed wetlands. Environ. Pollut. 119, 413–420. Liu, L., Zhao, X., Zhao, N., Shen, Z., Wang, M., Guo, Y., Xu, Y., 2013. Effect of aeration modes and influent COD/N ratios on the nitrogen removal performance of vertical flow constructed wetland. Ecol. Eng. 57, 10–16. Maltais-Landry, G., Chazarenc, F., Comeau, Y., Troesch, S., Brisson, J., 2007. Effects of artificial aeration, macrophyte species, and loading rate on removal efficiency in constructed wetland mesocosms treating fish farm wastewater. J. Environ. Eng. Sci. 6, 409–414. Maltais-Landry, G., Maranger, R., Brisson, J., 2009. Effect of artificial aeration and macrophyte species on nitrogen cycling and gas flux in constructed wetlands. Ecol. Eng. 35, 221–229. McEnroe, N.A., Buttle, J.M., Marsalek, J., Pick, F.R., et al., 2013. Thermal and chemical stratification of urban ponds: Are they 'completely mixed reactors'? Urban Ecosyst. 16, 327–339. Nowak, O., 2000. Upgrading of wastewater treatment plants equipped with rotating biological contactors to nitrification and P removal. Water Sci. Technol. 41, 145–153. Ong, S.-A., Uchiyama, K., Inadama, D., Ishida, Y., Yamagiwa, K., 2010. Performance evaluation of laboratory scale up-flow constructed wetlands with different designs and emergent plants. Bioresour. Technol. 101, 7239–7244. Prystay, W., Lo, K.V., 2001. Treatment of greenhouse wastewater using constructed wetlands. J. Environ. Sci. Health Part B: Pestic. Food Contamin. Agric. Wastes 36,
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Aeration and plant coverage influence floating treatment wetland remediation efficacy Lauren M. Garcia Chancea, Sarah A. Whiteb, a b
T
⁎
Environmental Toxicology Graduate Program, Clemson University, 509 Westinghouse Rd, Pendleton, SC 29670, USA Department of Plant and Environmental Sciences, Clemson University, E-143 Poole Agricultural Center, Clemson, SC 29634, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Phosphorus Nitrification Denitrification Canna flaccida Juncus effusus
Nutrient contamination of waterways is a growing concern, instigating the emergence of floating treatment wetland systems (FTWs) to remove nutrients from water. To determine if aeration of water systems enhances nutrient removal efficacy or nutrient fixation within plant tissues, aeration in combination with FTW installation techniques were investigated. During two separate trials with Juncus effusus and Canna flaccida, treatments consisted of either aerated or non-aerated mesocosms, with varying coverage and planting density combinations. Aeration increased the dissolved oxygen levels within each mesocosm, but reduced the removal of nitrogen and phosphorus from the water column in comparison to non-aerated systems. Plant samples collected from 100% planting density treatments that were aerated had a greater nitrogen uptake than non-aerated by as much as 55% or 13.5 g/m2 at harvest. Some discrepancies between plant uptake and water column nutrient levels can be attributed to microbially-mediated nitrogen losses (e.g., denitrification).
1. Introduction As water quality degradation concerns grow in public and private sectors, evaluation of remediation technologies to treat contaminants has increased. One such remediation technique is the use of floating treatment wetlands (FTWs) to remove excess levels of nitrogen (N) and phosphorous (P) from water systems. Floating treatment wetlands are often installed in existing storm water retention ponds. In many cases, especially when found in neighborhoods or other public spaces, these retention ponds have pre-existing fountains and other aeration methods. Aeration has proven to affect the speciation and remediation of contaminants in many constructed wetlands, however no previous literature has considered the impact upon FTWs (Dong et al., 2012; Ong et al., 2010; Zhang et al., 2010). The effect of aeration has been evaluated multiple times for both surface and subsurface constructed wetlands (Bowmer, 1987; MaltaisLandry et al., 2007, 2009; Zhang et al., 2010). The effect of aeration on P removal has varied across studies (Dong et al., 2012; Zhang et al., 2010). Remediation of ammonium (NH4+-N) has been reported to consistently increase with aeration (Butterworth et al., 2013; Dong et al., 2012; Liu et al., 2013; Ong et al., 2010); however, nitrate (NO3N) removal was higher in either non-aerated or intermittently aerated systems (Butterworth et al., 2013; Fan et al., 2013). This transformation of N correlates with the anaerobic and aerobic conditions supporting ⁎
nitrification and denitrification (Fig. 1). Nitrification is the oxidation of ammonium or ammonia (NH3) to nitrite and then to nitrate (Tanner et al., 2002; Wu et al., 2009). Nitrosomonas bacteria first convert ammonium to nitrite. Then nitrobacter convert nitrite to nitrate, both forms of N that are readily absorbed by plants. Denitrification is the microbial-mediated conversion of nitrate into nitrogen gas (N2) via nitrite, nitric oxide (NO), and nitrous oxide (N2O) (Tallec et al., 2008; Wu et al., 2009). Dissolved oxygen (DO) concentration plays an important role in nitrification and denitrification because nitrification is strictly aerobic (DO > 2.0 mg/L) while denitrification is strictly anoxic (DO < 1.0 mg/L) (DeBusk, 1999; Tallec et al., 2008). Nitrification and denitrification are the main pathways for N removal in CWs, but they usually do not occur simultaneously in a single wetland cell due to conflicting oxygen demands (Liu et al., 2013). Floating treatment wetlands can be compared with CWs in this scenario because if fountain and aeration methods are installed within retention ponds, they are likely to be continuously used rather than intermittently turned on and off, possibly homogenizing the pathway for N removal. Within retention and urban ponds, water column stratification is typical with great heterogeneity of thermo-chemical indices (McEnroe et al., 2013). For example, DO concentrations increase at the surface of ponds during the daytime, leading to possible supersaturation, and then decrease during the evening hours (Wetzel, 2001). However, FTWs stabilize and decrease DO levels beneath the floating mat, possibly
Corresponding author. E-mail address: [email protected] (S.A. White).
https://doi.org/10.1016/j.ecoleng.2018.07.011 Received 8 May 2018; Received in revised form 9 July 2018; Accepted 13 July 2018 0925-8574/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Reactions of the microbial nitrogen cycle. (1) Nitrogen gas fixation; (2) aerobic ammonium oxidation by bacteria and archaea; (3) aerobic nitrite oxidation; (4) denitrification; (5) anaerobic ammonium oxidation; and (6) dissimilatory nitrate and nitrite reduction to ammonium (Jetten, 2008).
Fig. 3. Experimental layout of the planting coverage and planting density with circles representing plants and inner rectangle the floating mat.
of 12 EUs to provide direct air circulation (Fig. 2C). Two experiments were conducted, each with a different plant species. The first experiment was conducted with Juncus effusus L. (soft rush). On April 21, 2009, 10 cm Juncus liners were inserted into aerator cups, and EUs were planted with appropriate number of plants according to treatment design. The experiment was concluded October 29, 2009; Juncus plants were harvested March 8 and 9, 2010. The second experiment was conducted with Canna flaccida L. (golden canna). On March 23, 2010, Canna bare root liners (roots ≈ 10 cm and shoots ≈ 15 cm) were wrapped with coconut coir mat pieces (10 cm × 20 cm), inserted into aerator cups, and each EU was planted with the appropriate number of plants per the experimental design. Canna plants were harvested August 31 and September 2, 2010. Due to trials occurring separately, in different years, statistical comparisons between the two species were not conducted; instead analysis were conducted by species by year. Overall experimental design by year was 1 plant species * 2 aeration levels * 3 plant coverage levels * 3 plant density levels * 4 replicates, with year 1 (2009) conducted using Juncus and year 2 (2010) conducted using Canna.
changing water column stratification and thus the effect of aeration on nutrient removal in comparison to CWs (Wang and Sample, 2013; White and Cousins, 2013). Zhang et al. (2010) determined that with aeration, less surface area was needed to remediate organic matter and N compared to a nonaerated CW system. To better employ FTWs within surface water bodies, additional knowledge related to economics of installation and maintenance, specifically the percent of pond surface area covered and planting density, are needed. The objective of this work was to quantify the effect of percent surface area covered, planting density, and aeration on FTW efficacy for treating nutrients in surface runoff.
2. Materials & methods 2.1. Experimental design Experiments were carried out over the spring-fall seasons of 2009 and 2010. An experimental system was assembled in Pendleton, SC (34.640, −82.773) consisting of twenty-four 378.5 L structural foam stock tanks (Rubbermaid, Atlanta, GA; Fig. 2). Each stock tank or experimental unit (EU) had a surface area of 1.15 m2 and a volume of 0.38 m3. Holes were drilled 6 cm from the rim at one end of each EU to regulate overflow and release of water. Floating mats, 1 cm think and cut to 60 cm × 30 cm, were supplied by Beemats (New Smyrna Beach, FL). The mats are buoyant, interlocking solid-core foam mats joined with 10 cm nylon connectors. Each section of mat has ten (7.5 cm) pre-cut holes spaced 12 cm on center (Fig. 3). Holes allow insertion of specially designed plastic aerator cups in which to place plants (Fig. 2B). Treatments consisted of 50% and 100% surface coverage using mats with plant densities of either 10 plants (50% or 100% coverage) or 20 plants (100% coverage) (Fig. 3). Twelve of the twenty-four EUs were continuously aerated and twelve had no supplemental aeration. Aeration was controlled by individual aquarium bubblers placed within each
2.2. Runoff simulation Six holding tanks, ranging in volume from 795 L (1 tank), 1135 L (4 tanks) to 1230 L (1 tank) were used to feed EUs. Solutions in the holding tanks were made by mixing water (municipal source) and a water-soluble fertilizer (20N-2P-20K Nitrate Special Soluble Fertilizer, Southern Agricultural Insecticides, Inc., Hendersonville, NC). Solutions flowing from the holding tanks averaged concentrations of 34.6 ± 6.4 mg/L N and 3.8 ± 0.5 mg/L P, representative of agricultural surface runoff (Prystay and Lo, 2001; White, 2013). The watersoluble fertilizer was completely dissolved in water prior to addition to the stock tanks to ensure uniform distribution. Treatments were randomly assigned to groups of four EUs per holding tank. Water distribution lines were plumbed so that water flowed continuously into
Fig. 2. Experimental setup for floating treatment wetland experiments including: (A) six holding tanks and 24 mesocosms with 378.5 L, (B) aerator cups into which plants are inserted, and (C) the aeration system. 63
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Fig. 4. Average dissolved oxygen concentration in floating treatment wetland experiments established with Canna flaccida (2010, n = 56/treatment) and Juncus effusus (2009, n = 92/treatment) to determine as influenced by treatment factors: aeration, planting density, and percent coverage.
100 mg of each tissue and assayed using an Elementar Vario Macro Nitrogen combustion analyzer (Mt. Laurel, NJ), and P was assayed by wet acid digestion procedure using the nitric acid and hydrogen peroxide method (Huang et al., 2004). Additionally, P, K, Ca, Mg, Zn, Cu, Mn, Fe, S, Na, B, and Al concentrations in plant tissues were determined by ICP-ES. Only results for nitrogen and phosphorus are discussed. During the Canna experiment, the coconut coir mat pieces promoted a low level of weed growth. To determine the role of weed growth within the system, weeds were permitted to grow as they emerged and were harvested and separated from the trial plants during the Canna harvest dates. Harvested weeds were identified and treated using the same methods outlined above to determine nutrient concentrations.
each EU. Flow from each tank was calibrated to 65 mL/min for a uniform 2-day hydraulic retention time in each EU (Fig. 2). Daily load was calculated using tank flow rates and nutrient concentrations. Rainfall was not measured. 2.3. Water sampling and analysis Water samples were collected every 2 weeks from each EU and holding tank to permit calculations of concentration and loading from both influent and effluent. Water samples were processed and evaluated using three separate analyses: inductively coupled plasma emission spectrophotometer (ICP-ES), ion chromatography (IC), and total organic carbon analyzer (TOC). Trace elements including: P, K, Ca, Mg, Zn, Cu, Mn, Mo, Ni, Fe, S, Na, B, and Al were analyzed via ICP-EC (61E Thermo Jarrell Ash, Franklin, MA). Lower detection limits were 5 µg/L. Anions including: chloride, nitrate, nitrite, phosphate, and sulfate analysis were detected using a Dionex AS10 ion chromatograph with AS50 auto-sampler (Dionex Corp., Sunnyvale, CA, USA). Lower detection limits were 0.2 mg/L. Analysis for dissolved (non-purgeable) organic carbon (DOC) was performed using a Shimadzu TOC-VCPH total organic carbon analyzer (Shimadzu Scientific Instruments, Kyoto, Japan). All analyses were conducted to US EPA protocol methods 6010B, 9056A, and 9060A and calibration standards were instituted for quality assurance and control (USEPA, 1997; USEPA, 2004; USEPA, 2007). Water quality parameters including pH, dissolved oxygen (DO, mg/L), and temperature (°C), were collected every 14 days using a handheld probe (YSI, Yellow Springs, OH). The probe was inserted until it touched the bottom of each EU at the effluent end.
2.5. Data analysis All data presented represent the average value for each sampling event ± the standard error of the mean. When appropriate, statistical analysis were conducted using the Analysis of Variance procedure within JMP v13 (SAS Institute Inc. Cary, NC). Where main effects were significant (α < 0.05), a Student’s t-test was conducted to determine the influence of the treatment on nutrient removal. 3. Results and discussion 3.1. Dissolved oxygen In the absence of aeration, DO levels were maintained at an average 2.9 mg/L for Canna and 5.14 mg/L for Juncus (Fig. 4). The DO concentrations for aerated systems averaged 6.94 mg/L for Canna and 9.75 mg/L for Juncus. Despite differences in DO concentrations between non-aerated and aerated systems (p < 0.001 for both Canna and Juncus), the concentration of DO in non-aerated systems never declined to < 1 mg/L DO (never became anaerobic). While depletion of DO is possible in other forms of CWs (Butterworth et al., 2013; Fan et al., 2013), FTWs permit gas exchange across the water surface and maintain acceptable aeration levels for nitrification processes without additional artificial aeration (Nowak, 2000). Additionally, DO levels were impacted by planting density and mat coverage in the EU (Fig. 4). Nonaerated mesocosms covered at 100% planting coverage had a lower DO level than those with 50% coverage, signifying gas exchange most likely
2.4. Plant sampling and analysis At the initiation of each experiment, three plants from each EU were randomly selected for measurement. Monthly measurements were performed on the same plants within each EU (repeated measure). Root and shoot growth were measured to monitor establishment of the FTWs. The same 3 plants from each EU were selected for harvest. Plant root and shoot tissue samples were weighed (fresh weight, g) and dried at 80 °C, re-weighed (dry weight, g), and then ground in a Wiley mill (Swedesboro, NJ) to pass through a 40-mesh (0.425-mm) screen. Nitrogen concentration was determined for root and shoot tissues using 64
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Fig. 5. Dissolved oxygen concentration over time of influent to floating treatment wetland experimental units that were either aerated or non-aerated, contrasted with water temperature over the course of the experiments in 2010 Canna flaccida and 2009 Juncus effusus.
(Table 2). Overall, no difference between aerated and non-aerated system concentrations for ammonia and nitrite were detected, apart from Canna ammonia. Conversely, the concentration of nitrate varied between aerated and non-aerated systems (p < 0.001), with non-aerated systems containing less nitrate than aerated for both Juncus and Canna. The higher levels of nitrate within aerated systems could be partially attributed to differences in microbial activity between the aerated and non-aerated systems. Typically, aerated systems encourage aerobic nitrification processes that result in greater production of nitrate for plant uptake, while non-aerated systems produce less nitrate and greater amounts of nitrous oxide in a gaseous form due to anaerobic processes (DeBusk, 1999; Tallec et al., 2008). Within this system, anaerobic conditions were not attained. The aerated systems had greater plant uptake of N than non-aerated, precluding the possibility that nitrate was removed from the system more quickly by plants in the non-aerated mesocosms. This disparity in described mechanisms of N removal vs. actual within these experiments can partially be attributed to the location of the DO sampling within each EU (the bottom of the mesocosm). Because DO sampling occurred below the root systems, it is possible that the DO measurement recorded did not reflect the oxygen levels within the root system itself, though the depth of the system may negate any significant DO differences. Future DO measurements should consistently be taken within the root zone to better inform researchers of likely microbial activity and aerobic or anaerobic status. If anaerobic conditions were of impact for N removal via FTWs, concerns as to the release of nitrous oxide into the atmosphere may steer individuals toward less efficient aerated system with greater plant uptake and removal. No differences were found in ionN speciation due to planting density and coverage.
occurs in the uncovered portions of the system (p = 0.0002 for Canna and p = 0.002 for Juncus). This difference was less extreme in aerated systems, but still existed. Of note, DO levels within aerated systems closely followed those of the inflow, while non-aerated systems appeared to have stabilized independent of the influent DO over time (Fig. 5). Potentially indicating that DO saturation levels of the aerated systems were regulated by temperature. Additionally, as temperature increased, DO levels decreased and vice-versa (r = 0.66 Canna, r = 0.73 Juncus). Non-aerated systems appear less influenced by temperature, save for Juncus effluent when air temperatures began to decline at the end of the experiment (r = 0.39 Canna, r = 0.46 Juncus).
3.2. Nitrogen Ionic nitrogen [ionN = NH3-N (ammonia) + NO2-N (nitrite) + NO3-N (nitrate)] concentrations were reduced by 22.6% in the presence of Juncus and 67.0% in the case of Canna. Mass balance calculations were made and reductions in daily load determined that incorporated plant contribution to ionN removal (Table 1). On average, the daily loading rate ionN was reduced by 18.9% in the presence of Juncus and 63.9% in the presence of Canna. Plant tissue analyses indicate that plant associated ionN was responsible for only 24.9% ionN removal in Juncus and 12.9% in Canna. This leaves 67.3 g m−2 experiment−1 of ionN removal unaccounted for in the presence of Juncus and 306 g m−2 experiment−1 of ionN removal unaccounted for in the presence of Canna. Mass balance determinations were made based on the experimental factors of aeration, planting density, and coverage to parse out the effects of treatment factors (Table 1). Aerated systems had a higher average effluent ionN concentrations than non-aerated systems (p < 0.0001); non-aerated systems better reduced total average ionN concentrations. The same trend held true for reduction in daily loading rates of ionN in systems established with Canna (p < 0.0001). Plant uptake, however, was greater in aerated systems than non-aerated systems for Juncus (p < 0.0001), but no difference in ionN uptake was found for Canna. Additionally, in both Canna and Juncus, full density plantings had greater individual plant uptake than those with half density plantings (p < 0.0001 for Juncus and p = 0.0016 for Canna). Daily load reductions were only influenced by coverage and planting density for Juncus (p = 0.0002), with full coverage, half density systems removing the most ionN. Planting density and coverage did not influence either average reduction in concentration or reduction in daily load for Canna (p = 0.2975). These results led to the question, why did non-aerated and half density planting systems remove more ionN than aerated systems, yet have less plant uptake? To address this question, ionN speciation analyses were conducted to determine where within the N cycle, N fate was impacted the most
3.3. Phosphorus Phosphorus concentrations were reduced in the presence of both Juncus (32.3%) and Canna (49.5%), at similar levels documented by previous FTW phosphorus studies (Wang and Sample, 2013; White and Cousins, 2013). As with other reports of P remediation within constructed wetlands, no differences were found between aerated and nonaerated systems regarding phosphorus remediation (Table 1). Some differences were noted in planting density and coverage of both Juncus and Canna. Full coverage, half density systems removed a greater amount of daily phosphorus load than full coverage, full density (p = 0.0214) or half coverage, full density systems (p < 0.001). Juncus effusus removed only 3.85 g m−2 experiment−1 or 11.9% of phosphorus, and Canna removed only 7.62 g m−2 experiment−1 or 13.5% (Table 1). Like N, plant uptake contributed less phosphorus removal in the full coverage, half density systems than full density systems (p = 0.001). Within the current FTW experiments, plant uptake percentages were lower than expected and accounted for only a small 65
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Table 1 Nitrogen and phosphorus mass balance calculations for Juncus effusus and Canna flaccida after season-long exposure to nutrients in floating treatment wetlands. Values presented are the means (standard error) of the mean. Total Nitrogen
Total Phosphorus
Juncus
Canna
Average Concentration
(mg L−1)
Influent Effluent
19.2 (1.98) 14.8 (1.60) Aerated Non-Aerated Full coverage, full density Half coverage, full density
16.3 13.4 14.7 15.3
(1.27) (1.93) (1.82) (1.60)
21.0 (2.53) 6.92 (1.58) a b a a
4.34 22.6%
Daily Load
(g m−2 day−1)
Influent Effluent
3.22 (0.31) 2.61 (0.27) 2.76 2.46 2.79 2.59 2.46
Canna
(mg L−1)
Reduction in Concentration % Concentration Reduction
Aerated Non-Aerated Full coverage, full density Half coverage, full density Full coverage, half density
Juncus
8.59 5.25 6.77 7.32
(1.60) (1.56) (1.85) (1.66)
3.22 (0.27) 2.18 (0.26) a b a a
2.31 2.04 2.04 2.40
14.0 67.0%
(0.22) (0.33) (0.31) (0.27) (0.23)
(0.23) (0.29) (0.29) (0.27)
4.44 (0.53) 2.24 (0.41) a a a a
1.04 32.3%
2.36 2.11 2.47 2.19
(0.48) (0.35) (0.43) (0.53)
2.20 49.5%
(g m−2 day−1) 3.41 (0.40) 1.23 (0.25)
a a a ab b
1.51 0.95 1.17 1.33 1.20
(0.26) (0.25) (0.30) (0.27) (0.19)
0.60 (0.05) 0.38 (0.04) a b a a a
0.39 0.37 0.39 0.41 0.35
a a a a b
(0.08) (0.07) (0.07) (0.08) (0.04)
Mass Balance
(g m−2 experiment−1)
Total Influent Load Total Effluent Load Total Load Reduction
473 (45.6) 384 (36.8) 89.6
549 (64.4) 198 (40.3) 351
88.2 (7.40) 55.9 (5.90) 32.3
119 (14.5) 62.8 (11.3) 56.3
Plant Uptake % Plant Uptake
22.3 (1.21) 24.9%
45.4 (4.02) 12.9%
3.85 (0.23) 11.9%
7.62 (0.67) 13.5%
Other Removal Processes
(1.93) (1.40) (1.88) (1.08) (1.76)
a b a a b
a a a a b
0.35 47.3%
(g m−2 experiment−1)
50.7 40.1 62.9 50.7 22.6
67.3
0.22 36.7%
0.41 0.36 0.37 0.43 0.33
0.61 18.9%
24.1 20.4 25.6 26.6 14.5
2.18 63.9%
(0.04) (0.05) (0.05) (0.05) (0.04)
0.74 (0.09) 0.39 (0.07)
Reduction in Daily Load % Daily Load Reduction
Aerated Non-Aerated Full coverage, full density Half coverage, full density Full coverage, half density
a a a a
(6.71) (4.36) (7.94) (6.66) (2.34)
a a a a b
4.49 3.21 4.40 4.67 2.49
306
(0.35) (0.25) (0.41) (0.32) (0.19)
a b a a b
28.5
8.01 7.23 10.9 8.25 3.72
(6.66) (4.49) (1.36) (0.99) (0.36)
a a a a b
48.7
particulates in sediment and precipitation from the water column (Appan and Wang, 2000; Wang et al., 2003). Therefore, other processes most likely were responsible for this difference in P remediation in the water column compared to plant uptake.
proportion of the phosphorus removed in these experiments. These results were similar to those found by Tanner and Headley (2011), who reported that uptake of P into plant tissues accounted for only a small fraction of P removal in their experiments. Tanner and Headley (2011) compared living plant roots with artificial roots and determined that release of a bioactive compound from the rhizosphere or changes in the physico-chemical conditions of the water column enhanced remediation processes (Tanner and Headley, 2011). This would further explain the greater P uptake seen in full density planting systems (Table 1). The fate of phosphorus in constructed wetlands is typically sorption to
3.4. Weeds Within the Canna study, any weeds within the systems were harvested and processed according to similar protocols as the Canna. Primula spp. and Carex spp. accounted for a total 17.1 ± 1.02 g N in
Table 2 Nitrogen speciation within water samples collected from floating treatment wetlands established with Juncus effusus (2009) and Canna flaccida (2010) as influenced by aeration, planting density, and percent cover. Average Concentration
Total NH3+
Total NO2
Juncus (mg L Aerated Non-Aerated Full coverage, full density Half coverage, full density Full coverage, half density
0.53 0.54 0.43 0.62 0.54
(0.04) (0.03) (0.05) (0.05) (0.06)
−1
a a a a a
)
Canna (mg L 0.52 0.38 0.40 0.52 0.44
(0.05) (0.05) (0.06) (0.06) (0.05)
−1
a b a a a
)
Total NO3 −1
Juncus (mg L 0.87 0.82 0.77 0.89 0.87
66
(0.23) (0.18) (0.11) (0.20) (0.16)
a a a a a
)
Canna (mg L 0.07 0.11 0.08 0.11 0.08
(0.02) (0.01) (0.02) (0.02) (0.01)
−1
a a a a a
)
Juncus (mg L−1)
Canna (mg L−1)
15.5 13.0 14.3 14.6 13.9
7.99 4.76 6.29 6.69 6.15
(0.28) (0.39) (0.34) (0.48) (0.41)
a b a a a
(0.40) (0.56) (0.50) (0.43) (0.27)
a b a a a
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Fig. 6. Average dissolved organic carbon measured in water in floating treatment wetland experimental units (n = 56/treatment) planted with Canna flaccida as influenced by aeration, planting coverage, and planting density.
and daily reduction in P load ranged from 37% to 47%. Plant uptake accounted for < 25% of P and ionN removal from the systems. Aeration enhanced N and P uptake within Juncus. Some nutrient removal was explained via weed uptake and increased concentrations of DOC. Ultimately, FTW systems can operate under aerobic or anaerobic conditions.
aerated systems and 34.8 ± 1.43 g N in non-aerated systems; they also accounted for total 2.83 ± 0.17 g P in aerated systems and 4.65 ± 0.19 g P in non-aerated systems. These values varied within an EU and depended upon the number of weeds within each EU. While these values are low in comparison to the total N and P removed in both the Juncus and Canna experiments, accounting for weedy species contributions to both ionN and P remediation help to further explain the remediation of within the system and should be considered in future experiments.
Acknowledgements The authors wish to thank Elizabeth Nyberg, Brandon C. Seda and J. Brad Glenn for contributions in sampling and laboratory work. This research was financed by the Horticulture Research Institute. Publication of this material is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 2014-51181-22372. This material is based upon work supported by NIFA/USDA, under project number SC-1700536. Technical contribution no. 6670, of the Clemson University Experiment Station.
3.5. Dissolved organic carbon Carbon availability and denitrification have been shown to closely correlate in previous studies (Chen et al., 2011; Wen et al., 2010). To expand potential explanations for nitrogen and phosphorus remediation results in this study, the DOC concentrations measured during the Canna experiment were modeled with aeration and planting density and found to correlate (p < 0.001) (Fig. 6). Availability of DOC increased in aerated systems compared to influent (p = 0.0026), and even greater concentrations of DOC were available in non-aerated systems (p < 0.001). The constituents of DOC could include: decaying organic matter, algae, and metabolites sloughed from plant roots systems. Studies indicate organic carbon can act as a nutrient sink and external carbon sources stimulate nitrate removal in CWs (Chen et al., 2011; Gebremariam and Beutel, 2008; Lin et al., 2002). The greater DOC concentrations found in non-aerated systems may have enhanced removal of phosphorus and nitrate unaccounted for in water and plant samples.
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