Use of duckweed-based constructed wetlands for nutrient recovery and pollutant reduction from dairy wastewater

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Use of duckweed-based constructed wetlands for nutrient recovery and pollutant reduction from dairy wastewater Umesh Adhikari, Timothy Harrigan, Dawn M. Reinhold ∗ Department of Biosystems and Agricultural Engineering, Michigan State University, 524 S. Shaw Lane, East Lansing, MI 48824, United States

a r t i c l e

i n f o

Article history: Received 1 November 2013 Received in revised form 12 May 2014 Accepted 21 May 2014 Available online xxx Keywords: Lemna minor Stabilization ponds Nutrient uptake Wetland modeling Nitrogen Phosphorus E. coli

a b s t r a c t Over the last few decades, constructed wetlands have increasingly been designed and implemented to treat agricultural wastewaters. However, while treatment of manure-containing wastewaters protects water quality, it also eliminates a valuable source of nutrient-laden soil amendments. The goal of this research is to investigate the ability of wetlands to simultaneously treat high-strength manure-containing wastewaters while recovering nutrients from the manures in plant biomass. Diluted raw dairy waste was fed to a combination of duckweed-based surface flow and subsurface flow wetlands. Nutrient recovery through duckweed harvesting and waste treatment, characterized by chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and E. coli removal, were assessed under steady-state conditions for three strengths of waste. Duckweed-based wetlands used for primary treatment had higher removal rates of COD and TN than those used for secondary treatment; however, no significant difference was observed for TP removal rates. COD removal ranged from 3 to 81% in primary duckweed wetlands and from −35% to 38% in secondary duckweed wetlands. Areal removal rates for nutrients in primary duckweed wetlands were 194.9 ± 18.9 g TN/m2 /yr and 13.0 ± 3.0 g TP/m2 /yr, while removal rates in secondary duckweed wetlands were 104.1 ± 13.1 g TN/m2 /yr and 9.3 ± 2.1 g TP/m2 /yr. Removal of COD, TN, and TP more closely followed first-order removal to a background concentration than first-order removal or a previously published model for duckweed ponds. Mean log E. coli reduction of 0.30 obtained in this experiment was within the range reported in literature. Duckweed production in the wetlands was satisfactorily described as a second-order polynomial function of influent TN concentration. More N and P was recovered from primary wetlands as compared to secondary wetlands. Average N and P recovered by harvesting duckweed across all the wetlands were 22.4 g N/m2 /yr and 7.4 P/m2 /yr, respectively. © 2014 Published by Elsevier B.V.

1. Introduction Large quantities of manure-containing wastewater, wash water, and runoff are produced from animal agriculture (USEPA, 2001). Storage and treatment of manure-containing wastewaters, while necessary to improve the environmental sustainability of animal production, substantially contribute to the cost of animal production. However, manure-containing wastewaters contain two valuable resources: water and nutrients. Land application is one approach to utilizing the water and nutrient resources of manurecontaining wastewaters. However, land application of manure also introduces nutrients, salts, pathogens, antibiotics, and hormones into fields, increasing the risk of surface and groundwater

∗ Corresponding author. Tel.: +1 517 432 7732. E-mail address: [email protected] (D.M. Reinhold).

contamination during runoff or leaching events (Muchovej and Pacosvsky, 1997; Morgenroth, 2000; USEPA, 2001). As manure slurries enhance the survival and transport of pathogens in soils, pollution of water bodies with pathogens from land application is expected to be widespread (Bradford et al., 2008). Additionally, the ratio of nitrogen (N) to phosphorus (P) present in manurecontaining wastewaters rarely matches crop needs, leading to excess application of N or P and increasing the risk of eutrophication of water bodies from nutrient-rich runoff or leachate. Consequently, utilization of water available in manure-containing wastewaters is limited in land application, as addition of water is inherently associated with addition of nutrients. A variety of management practices, such as oxidation ponds, facultative lagoons, vegetated buffer strips, constructed wetlands, storage ponds, composting, and aerobic and anaerobic digestion are used to treat excess nutrients and pathogens from animal manure (NRCS, 1999; Rogers and Haines, 2005). These practices can be used to treat

http://dx.doi.org/10.1016/j.ecoleng.2014.05.024 0925-8574/© 2014 Published by Elsevier B.V.

Please cite this article in press as: Adhikari, U., et al., Use of duckweed-based constructed wetlands for nutrient recovery and pollutant reduction from dairy wastewater. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.05.024

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Fig. 1. Schematics of the constructed wetland system. Inf: Influent, SF: Surface flow wetlands, SSF: subsurface flow, P: Pump, C: collection tank). Arrows denote direction of flow and * denotes sampling locations.

wastewater prior to land application or, less frequently, surface water discharge. Duckweed, a small free-floating aquatic macrophyte from the family Lemnaceae, grows throughout much of the world. There are four genera and 40 species of duckweed (Rusoff et al., 1980). Propagating primarily through vegetative reproduction, duckweed produces biomass rapidly and can double its mass every 5 or 6 days (Lukina, 1977; Hillman and Culley, 1978). Apart from high productivity, cultivation of duckweed is advantageous due to duckweed’s high nutritive value and ease of harvesting and stocking (Boyd, 1974). Harvested biomass can be used for composting and soil amendments, digested anaerobically for biogas production, processed for animal feed, or mixed with solid manure to adjust the N:P ratio (Sooknah and Wilkie, 2004; Henderson et al., 2012). There is an immediate need to recover nutrients from wastewaters and macrophyte ponds, including duckweed-based surface wetlands, are a promising technology for nutrient recovery (Shilton et al., 2012). Constructed wetlands are known for their effectiveness in removing a variety of pollutants, including organic carbon, P, N and pathogens (Lee et al., 2009). The biological removal of carbon in wetlands is primarily due to degradation of organic carbon by microorganism; in wetlands, duckweed plants can provide additional surface area for bacterial growth and oxygenate the water, enhancing degradation of organic carbon (Korner et al., 2003). Major processes affecting the fate of N in constructed wetlands are ammonia volatilization, nitrification, denitrification, plant and microbial uptake, anaerobic ammonia oxidation, and burial (Vymazal, 2007). P is removed primarily by wetlands through sorption, precipitation, plant and microbial uptake, and peat or soil accretion (Vymazal, 2007). Pathogen removal mechanisms include natural die-off, attack by lytic bacterial and bacteriophages, predation, filtration, adsorption and subsequent sedimentation, chemical oxidation, and inactivation by UV radiation (Vymazal et al., 2006). In the past, researchers have attempted to quantify nutrient reduction and recovery potential of floating aquatic macrophytebased systems from anaerobically-digested flushed dairy wastewater (Sooknah and Wilkie, 2004) and from dairy lagoon wastewater (DeBusk et al., 1995; Tanner, 1996; Tripathi and Upadhyay, 2003). Tripathi and Upadhyay (2003) reported that duckweed-based treatment ponds could remove approximately 60% of N and 56% of P from pretreated dairy wastewater. Duckweed preferentially uptakes ammonium over nitrate (Cedergreen and Madsen, 2002), which may sustain its growth in dairy lagoon wastewaters where ammonium is the dominant form of N. However, DeBusk et al. (1995) grew duckweed in 1:1 diluted dairy lagoon effluent and reported a relatively low productivity of 2.1 g dry wt/m2 /d. Sooknah and Wilkie (2004) reported that pennywort and water lettuce failed to grow in undiluted anaerobically-digested flushed dairy manure wastewater, while the growth of water hyacinth was inhibited. With 1:1 diluted wastewater, the authors reported 51.9, 2.6 and 1.0 g dry wt/m2 /d of productivity of water hyacinth, pennyworth and water lettuce, respectively. While these studies document substantial capabilities for aquatic plants to accumulate nutrients,

pathogen reduction during nutrient accumulation was not characterized. In previous studies, researchers have focused on either anaerobically digested dairy wastewater or lagoon effluent and not on raw dairy wastewater. Additionally, models to predict pollutant removal and macrophyte growth under multiple loadings were not developed. The present study was designed to investigate steady-state pollutant reduction and nutrient recovery potential of duckweedbased constructed wetlands as components of a hybrid constructed treatment wetland system. The experimental system included duckweed-based surface flow wetlands preceding and following subsurface flow wetlands, allowing for evaluation of the effects of sequencing on pollutant treatment and nutrient recovery. Specific objectives were to: (1) Quantify chemical oxygen demand (COD), total N (TN), total P (TP), and E. coli removal in tubscale duckweed-based constructed wetlands used for primary and secondary treatment of dairy wastewaters; (2) Compare the descriptive ability of three common models for COD, TN, and TP removal in the duckweed-based constructed wetlands; and (3) Quantify nutrient recovery potential of duckweed. 2. Materials and methods 2.1. Experimental setup The experimental system consisted of connected surface flow and subsurface flow wetlands (Fig. 1). Each wetland was constructed from oval-shaped, opaque, low-density polyethylene tanks with overall dimensions of 0.84 m × 0.66 m × 0.30 m and volume of 138 L. Surface flow wetlands were planted with duckweed (Lemna minor) collected from natural wetlands near East Lansing, MI. Subsurface-flow wetlands utilized pea gravel as the subsurface media and were planted with bulrush (Schoenoplectus tabernaemontani) and begger-tick (Bidens comosa) grown from seed (Cardno, Indianapolis, IN). Plants in the subsurface wetlands were established one year prior to experimentation. The wetlands were maintained inside a research bay that was heated in winter and cooled in summer to minimize seasonal variation. The temperature ranged from 15 ◦ C to 22 ◦ C for the duration of this study. Light was provided by fluorescent lights with a light intensity of 2000 lx operated 16 h daily. Flow rate was maintained to achieve a nominal hydraulic residence time (HRT) of 9 days for each of the surface flow wetlands, resulting in a nominal HRT of 22.5 days for the entire wetland system. Diluted manure slurry was collected from a dairy farm in Webberville, MI after straining of large solid particles but prior to discharge into the lagoon. Dairy manures were diluted to approximately three strengths of wastewater: 250 mg/L COD (low), 500 mg/L COD (medium) and 1500 mg/L COD (high). Manure slurry was collected approximately every three months from a dairy farm in Webberville, MI. After straining of large solid particles, COD concentration in the manure slurry was measured and tap water was added in order to achieve desired COD concentration. The diluted manure slurry was then added to the influent tank (150 L) and pumped from the influent tank to the primary

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surface flow (SF) wetlands continuously using three separate peristaltic pumps operated by a single variable speed motor drive (Masterflex, Cole-Parmer, Vernon Hills, IL). Primary SF wetlands were elevated, so that gravity maintained the flow from the primary SF wetlands through the final subsurface flow (SSF) wetland (Fig. 1). At the end of the final SSF wetland, effluent was collected and pumped to the secondary SF wetlands. Effluent from the final SF wetlands was discarded and not reused. 2.2. Sampling and analysis For each strength of wastewater, sufficient time was allowed for the system to attain steady-state concentrations of COD prior to assessment of pollutant removal. When influent and effluent COD concentrations reached steady state concentrations, samples were collected from eight locations to yield three replicates (Fig. 1). Sample collection and analysis for each COD level lasted about a month. Samples were collected during 3/31/2011 to 4/29/2011, 10/5/2012 to 11/2/2012 and 8/29/2011 to 9/23/2011 for low, medium and high concentrations of COD, respectively. Samples were mixed by agitation but were not filtered. COD was measured using high range HACH COD digestion vials (Hach Company, Loveland, CO). E. coli was analyzed using membrane filtration with modified-MTEC agar (USEPA, 2009) purchased from VWR (VWR, Radnor, PA). TN and TP were analyzed using acid digestion and subsequent analysis with ion chromatography. A modified persulfate digestion method (Ebina et al., 1983) was used to convert N compounds to nitrate and P compounds to phosphate. For TN analysis, 5 mL of 0.148 M potassium persulfate and 250 ␮L of 0.1 M sodium hydroxide solutions were added to 15 mL of sample and autoclaved for 30 min at 110 ◦ C. After digestion, 250 ␮L of 0.1 M sodium hydroxide solution was added and the sample was filtered through 0.2 ␮m syringe filter, and analyzed with ion chromatography (IC). For TP analysis, 5 mL of digestion reagent was added to 10 mL of sample and autoclaved for 30 min at 110 ◦ C. After digestion, 1 mL of borate buffer solution was added and the sample was filtered through 0.2 ␮m syringe filter and analyzed with IC.A Dionex ICS 5000 chromatography system with an Ionpac AS22 column for anion separation was used to analyze digested samples. Anions were eluted with 4.5 mM sodium carbonate/1.4 mM sodium bicarbonate solution at a flow rate of 1.2 mL/min and quantified with a conductivity detector. Prior to analysis, calibration curves were prepared using standard solutions. Each week, 50% of the duckweed was harvested. Harvested duckweed was drained for 20 min and fresh mass was measured. Additionally, a sample was taken from harvested duckweed, dried at 65 ◦ C for 48 h, and weighed. Dry duckweed was ground in mortar and pestle. Deionized water was added to a known weight of ground duckweed, digested, and analyzed for N and P using the same procedure described previously. 2.3. Curve fitting Three modeling approaches were evaluated for their capabilities to describe COD, TN and TP removal by the experimental system. Initially, a first-order reaction model was used to describe pollutant removal in the constructed wetlands (Kadlec et al., 1993), ln

Ce = −kt Co

(1)

where Ce is the effluent concentration (mg/L), Co is the influent concentration (mg/L), k is the first-order rate constant (1/day), t is the hydraulic residence time (day). The second approach, perhaps the most well-known approach for treatment wetland modeling (Kadlec and Wallace, 2009),

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incorporates a first-order reaction model with background concentration as shown in Eq. (2). ln

Ce − C∗ = −kt Co − C∗

(2)

where C* is the background concentration (mg/L). Other parameters are the same as previously defined for Eq. (1). Lastly, the third model, or the duckweed-based wastewater treatment model, developed by Krishna and Polprasert (2008), describes effluent concentration in terms of influent concentration, organic loading rate and duckweed density as shown in Eq. (3). ln

Ce = −kx ˇy  (T −20) t Co

(3)

where  is the OLR or organic loading rate (kg COD/ha/d), ˇ is the SD or stocking density (kg/m2 ), T is the temperature (◦ C); , x, y are the coefficients; the value of  is 1.05 (Polprasert and Agarwalla, 1995), k and t are the same as previously defined for Eq. (1). Models were fitted to the experimental data using MATLAB R2009b. Goodness of fit of the models were evaluated using significance level (p-value), coefficient of determination (adjusted R2 ), and Akaike information criterion (AIC). Significance of each parameter was evaluated using Student’s t-test at a 5% significance level. Values are presented as mean plus/minus standard error. 3. Results 3.1. COD, TN, TP and E. coli removal The wetland systems were allowed to attain a steady-state condition of COD before the samples were analyzed for nutrients and E. coli removal. On average, 76% of COD was removed by the entire experimental system, including all surface flow and subsurface flow wetlands (Table 1). Removal of COD in the primary wetlands ranged from 3 to 81%, while the removal ranged from −35% to 38% in secondary wetlands for the entire duration of the experiment (Fig. 2a). Based on both percentage removal rates and areal removal rates, COD removal in primary wetlands was significantly higher than COD removal in secondary wetlands. Averaged over all strengths of wastewater, primary wetlands removed approximately 43% of the COD, or 3.5 times the COD removed by the secondary wetlands. The annual areal removal rates of COD were 3869 ± 352 and 405 ± 50 g COD/m2 /yr for primary and secondary wetlands, respectively (Fig. 2b). Based on percentage removal rates, no significant difference was observed in N removal in primary and secondary wetlands. Averaged over all strengths of influent, primary wetlands removed 29.3 ± 3.9% of TN and secondary wetlands removed 27.3 ± 1.8% of TN (Fig. 2c). However, areal removal rates were significantly higher in primary wetlands than secondary wetlands, due to higher concentrations of influent N fed to the primary wetlands. When averaged over all strengths of influent, primary and secondary wetlands removed 194.9 ± 18.9 and 104.1 ± 13.1 g TN/m2 /yr, respectively (Fig. 2d). Due to technical problems, TP in primary wetlands at high loading (when influent COD was near 1500 mg/L) was not measured, and hence was not included in analysis. Thus, the range of TP observed during the whole experimental period in secondary wetlands appeared to be higher than the TP in primary wetlands (Fig. 2e and f). Percentage TP removal obtained in primary wetlands was higher than that of the secondary wetlands; however, no significant difference was observed in areal removal rates of P between primary and secondary wetlands. On average, primary wetlands removed 31.7 ± 9.4% TP, while secondary wetlands removed 6.6 ± 3.5% of TP for all strengths of wastewater. Average annual areal TP removal in the both wetlands was 10.4 ± 1.7 g TP/m2 /yr,

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Table 1 Influent and effluent concentrations of COD, TN, TP and E. coli in primary and secondary wetlands during low, medium and high COD concentration (mean ± standard error). Loading

Primary wetlands

Secondary wetlands

Influent COD (mg/L)

TN (mg/L)

TP (mg/L)

E. coli (CFU/mL)

Low Medium High Low Medium High Low Medium High Low Medium High

268.8 542.3 1544.0 30.9 35.6 118.9 2.9 1.6 25.2 38.8 0.0 265.0

± ± ± ± ± ± ± ± ± ± ± ±

Effluent 44.1 42.6 190.3 0.5 4.3 3.0 0.5 0.9 2.8 33.3 0.0 86.2

with average annual areal TP removal rates of 13.0 ± 3.0 and 9.3 ± 2.1 g TP/m2 /yr in the primary and secondary wetlands, respectively. Percentage removal of COD increased with increasing COD loading under low COD loadings, but plateaued under high COD loadings. Unlike COD, percentage removal of TN and TP did not show any clear trend with respect to loading concentration. Areal removal rates for COD, TN and TP all increased with increasing loading concentration over entire experimental range. E. coli was not detected in many samples and consequently fewer data were collected for E. coli than for TN, TP, and COD. E. coli counts in influent ranged from non-detectable to 440 CFU/mL, with a detection limit of 0.02 CFU/mL. Log E. coli removal obtained in the wetlands ranged from −0.57 to 1.11 with a geometric mean of 0.30. Similar to TN and TP removal, percentage E. coli removal rates did not show any clear relationship with the loading concentrations (Fig. 2g). The three models mentioned previously were fitted to COD, TN, and TP removal data (Fig. 3 and Table 2). During the highest loading, duckweed did not grow in primary SF wetlands. Since duckweed density is a predictor variable in DUBWAT (duckweed-based wastewater treatment) model, COD, TN and TP data collected during the experiment period from primary surface flow wetlands under high loading were excluded from model fitting so that comparison between models could be performed. Only the first-order model was significant (p < 0.05) for all parameters and yielded coefficients with small standard errors (≤10% of parameter value). First-order removal rates were statistically significant with low error for the first-order removal with background concentration approach (Table 2). However, the fitted values for background concentrations of TN and TP were not statistically significant (p > 0.16). The first-order removal rates with or without background concentrations were similar for TN and TP, but differed for COD. The DUBWAT model yielded at least one parameter that was not significant for each pollutant. Additionally, negative parameters and large standard errors were also observed within the DUBWAT model results. Based on AIC, DUBWAT model appears to be the best fit to the data as compared to other two models. However, as can be seen from MSE (mean squared error) (Table 2), DUBWAT model improvements over other two models were not dramatic. Moreover, many fitting parameters in DUBWAT model were not significant (Table 2). Based on AIC and MSE, first order models with and without background appear to be similar except for COD removal. Background concentration for TN and TP were statistically not different from zero, while background concentration for COD did differ from zero (Table 2). Hence, the first order model with background concentration appeared to perform better than first order model for COD, while the first

130.5 309.8 879.4 13.4 33.8 98.0 2.1 2.7 14.9 19.7 0.0 96.7

± ± ± ± ± ± ± ± ± ± ± ±

Influent 2.0 8.8 31.6 0.8 1.0 1.7 0.2 1.0 0.9 12.4 0.0 23.6

86.2 131.8 373.8 4.6 23.4 63.8 2.4 1.7 11.7 0.0 0.0 21.7

± ± ± ± ± ± ± ± ± ± ± ±

Effluent 5.7 11.5 9.6 0.4 3.5 1.7 0.5 1.5 0.9 0.0 0.0 9.8

89.5 105.0 257.3 3.9 13.9 45.8 2.3 1.3 9.8 0.0 0.0 14.8

± ± ± ± ± ± ± ± ± ± ± ±

3.0 3.4 5.7 0.2 1.6 0.9 0.3 0.5 0.4 0.0 0.0 3.9

order model without a background concentration (i.e., or with a background concentration of 0 mg/L) performed best for TN and TP. 3.2. Duckweed production and nutrient recovery Weekly duckweed production was correlated with the influent TN concentration, but was not correlated with influent TP or COD concentrations. The relationship between duckweed production and influent TN concentration was satisfactorily described with a second-order polynomial equation. DW = p1 + p2 × X + p3 × X 2

(4)

where DW is the weekly duckweed production (g DW/week), X is the influent TN concentration (mg/L), p1 , p2 , p3 are the coefficients Table 3 shows the fitted values along with their 95% confidence interval and Fig. 4 shows the fitting. Based on the fitted model, the highest amount of duckweed can be produced when the influent TN concentration is approximately 32 mg/L. The data were also fitted to the model developed by Driever et al. (2005), which describes duckweed growth in terms of temperature, biomass and nutrients content of the media:

 dB   1  dt

B



=r

T − Tmin Topt − Tmin



N N + hN



P P + hP

 h  B B + hB

−l (5)

where B, hB are the biomass surface coverage and half-saturation biomass coverage (g DW/m2 ), r is the maximum growth rate (1/wk), l is the loss (1/wk), T is the temperature (◦ C), Tmin , Topt are the minimum and optimum temperature for duckweed growth (◦ C), N, hN is the N concentration and half-saturation N concentration (mg/L), P, hP are the P concentration and half-saturation P concentration (mg/L), t is the time step (wk). The values of hB , Tmin , and Topt were 26 g dry mass/m2 , 5 ◦ C and 26 ◦ C, respectively (Driever et al., 2005). The values for T, N, P and B were measured, s and r, l, hN , and hP were estimated from the observed data. Table 3 shows the fitted parameters. Based on AIC and adjusted R2 values, the second order polynomial function performed better than the model developed by Driever et al. (2005). Duckweed N content was not strongly related to influent TN concentration (Fig. 5). However, duckweed P content, especially in the secondary wetlands, was positively correlated to influent TP concentrations (Fig. 5). Mass of N recovered through duckweed harvesting in experimental wetlands ranged from 0.3 to 20.6% of the influent N concentration. Mass recovery of N depended on influent concentration and decreased linearly with the increase in influent TN concentration. P recovery by duckweed through harvesting ranged from 0.6 to 45%, depending on the influent TP concentration.

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Fig. 2. COD, TN, TP and E. coli loading rate and respective percentage removal rates along with COD, TN and TP loading rates and respective areal removal rates.

P recovery by duckweed also showed decreasing trend; however, the relationship was not as strong as that for the TN. 4. Discussion Kadlec and Wallace (2009) reported removal rates of 2325 to 9975 g BOD/m2 /yr with a median value of 3427 g BOD/m2 /yr for the surface flow wetlands loaded with >200 mg/L of influent BOD. Moreover, data presented by the authors indicate BOD removal rate increases with increase in loading rate. IDF (1981) reported

that the COD/BOD ratio of dairy effluent from production of liquid milk, butter or cheese ranged from 1.16 to 1.57, with a mean value of 1.45. In this experiment, influent COD concentrations in primary wetlands ranged from 159 to 2089 mg/L and COD removal rate was within the reported by Kadlec and Wallace (2009) when the COD/BOD conversion factor was considered. However, COD removals obtained in the secondary wetlands were well below the 2325 to 9975 g BOD/m2 /yr reported by Kadlec and Wallace (2009) for wetlands with influent BOD concentrations of >200 mg/L. COD removal rates were instead similar to the 69 to 3230 g BOD/m2 /yr

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Table 2 Estimated values and other fitting parameters for COD, TN, TP and E. coli removal data fitted with different models. Model

First order

Influent

COD TN TP E. coli COD

First order model with background concentration

TN TP E. coli COD

TN DUBWAT model TP

E. coli

Parameter

k k k k k Cb k Cb k Cb k Cb k x y k x y k x y k x y

Estimate

0.06 0.04 0.02 0.11 0.08 84 0.04 −2.7 0.019 −1.14 0.11 5.59 0.01 0.37 0.25 1.91 −0.80 0.02 0.00 2.05 0.76 2.41 −0.79 0.02

95% confidence interval Lower

Upper

0.05 0.03 0.02 0.09 0.07 60 0.025 −13.8 0.013 −3.43 0.09 −17.6 −0.004 0.12 0.15 −1.37 −1.19 −0.15 0.00 1.27 0.58 −1.54 −1.12 −0.05

0.06 0.05 0.03 0.13 0.09 109 0.05 8.37 0.026 1.14 0.13 28.73 0.03 0.61 0.35 5.18 −0.41 0.19 0.00 2.83 0.94 6.36 −0.46 0.09

Std. error

p-Value

Adjusted R2

AIC

MSE

RMSE as % of Y-scale

0.003 0.003 0.002 0.009 0.006 12.27 0.006 5.56 0.003 1.14 0.01 11.39 0.008 0.12 0.05 1.64 0.19 0.09 0.00 0.39 0.09 1.94 0.16 0.04

<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.31 <0.01 0.16 <0.01 0.31 0.06 <0.01 <0.01 0.12 <0.01 0.39 0.30 <0.01 <0.01 0.11 <0.01 0.32

0.78 0.83 0.96 0.71

518 264 −20 258

1749 44.25 0.66 1204

14.47 13.97 6.71 15.8

0.84

495

1235

12.16

0.83

266

44.73

14.05

0.96

−19

0.65

6.69

0.71

260

1231

15.9

0.87

485

1052

11.22

0.86

254

36.9

12.76

0.98

−70

0.26

4.18

0.86

235

594

11.1

p-Value

Adjusted R2

AIC

MSE

0.62

138

2.35

0.17

169

20.5

Table 3 Second order polynomial and Driever et al. (2005) derived model fitting parameters. Model

Second order polynomial

Driever et al. (2005)

Parameter

p1 p2 p3 R l hN hP

Estimate

1.31 0.297 −0.0046 0.102 0.226 −0.878 −0.136

95% Confidence interval Lower

Upper

0.33 0.230 −0.0054 −0.032 0.118 −7.12 −0.228

2.30 0.360 −0.0037 0.237 0.335 5.36 −0.043

reported by Kadlec and Wallace (2009) for BOD loadings of 30 to 100 mg/L. In this experiment, COD influent concentrations in the secondary wetlands ranged from 64 to 395 mg/L, with 65% of values exceeding 200 mg/L. However, wastewater received by secondary surface flow wetlands in this experiment was already treated by the

Std. error

0.49 0.032 0.0040 0.067 0.054 3.11 0.046

<0.01 <0.01 <0.01 0.06 <0.01 0.39 <0.01

primary surface flow wetlands and three in-series SSF wetlands. As easily degradable organic matter and suspended solids were likely preferably removed by the primary SF and SSF wetlands, secondary SF wetlands received partially treated wastewater containing more recalcitrant organic matter. Presence of more recalcitrant organic

Fig. 3. Comparison between observed influent and effluent concentrations and model predictions for COD removal (Vobs : observed data, Vpred : model prediction, CB: 95% confidence band, PB: 95% prediction band).

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Fig. 4. Observed and second order polynomial fitted data for duckweed growth (Vobs : observed data, Vpred : model prediction, CB: 95% confidence band).

matter could explain why secondary treatment wetlands had relatively low COD removal rates, behaving as lightly loaded wetlands despite higher influent COD concentrations. Similar to Tanner et al. (1995a), COD load removal increased with increase in influent loading. Initially, percentage COD removal increased with increase in influent concentration. However, at higher loading rates, no duckweed grew in the primary wetlands, and hence, lack of vegetation likely caused the removal rate to plateau and decrease below the expected trend. Removal of TN (28.3% for primary and 27.3% for secondary wetlands) and TP (31.7% for primary and 27.3% for secondary wetlands) obtained in this study were lower than the 40 to 55% removal of TN and 40 to 60% removal of total TP reported by Vymazal (2007). Higher loading rates of TN and TP could explain the lower percentage removal. In terms of annual load removal, TN removal rates obtained in this study (194 g/m2 /yr for primary and 104 g/m2 /yr for secondary wetlands) were lower than the removal rate reported by Vymazal (2007), but were comparable to the median removal

rate reported by Kadlec and Wallace (2009). A wide range of TN removal rates, ranging from 6 to 4683 g TKN/m2 /year with a median removal rate of 207 g TKN/m2 /yr, was reported by Kadlec and Wallace (2009), while Vymazal (2007) reported 247 g N/m2 /yr. Likewise, annual TP load removal rates obtained in this study (13.0 g/m2 /yr for primary and 9.3 g/m2 /yr for secondary wetlands) were lower than the removal rates reported by Vymazal (2007), but were comparable to the median removal rate reported by Kadlec and Wallace (2009). Kadlec and Wallace (2009) reported annual P removal of −31 to 474 g P/m2 /yr, with a median value of 6 g P/m2 /yr, while Vymazal (2007) reported annual TP load removal of 70 g P/m2 /yr. The loading rates in this study (24 to 132 mg/L TN and 0.3 to 4 mg/L TP for primary wetlands; 4 to 68 mg/L TN and 0.05 to 14 mg/L TP for secondary wetlands) were higher than the loading rate of 14.3 g/L of TN and 4.2 g/L of TP reported by Vymazal (2007). Hence, the differences in removal rates are likely due to the differences in loading rates. Similar to the findings by Tanner et al. (1995b), annual mass removal rates of TN and

Fig. 5. Relationships of influent nitrogen and phosphorus with duckweed nutrient concentrations and nutrient recovery by harvest of duckweed.

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TP increased as the loading rate increased. Percentage removal rates did not show any clear relationship with the loading rate; however, the removal rates were low at higher loading rates. These findings suggest that COD, TN and TP removal kinetics in duckweed-based constructed wetlands subjected to dairy wastewater did not differ from other constructed wetlands found in literature. In a review of various fecal coliform removal data, Kadlec and Wallace (2009) reported −2.79 to 3.16 log reduction in constructed wetlands. Mean log removal obtained in this study and the range was well within the range reported by the authors. Moreover, influent and effluent data satisfactorily fitted first order removal kinetics. These results suggest that bacterial removal kinetics in duckweed-based constructed wetlands subjected to dairy wastewater is similar to the other treatment wetlands found in literature. Duckweed production in this experiment was a function of influent TN. Landesman et al. (2005) observed a similar relationship between duckweed growth and influent N content and modeled duckweed growth in terms of influent N concentration, temperature and solar radiation. However, in their experiment, Landesman et al. (2005) observed peak duckweed production at about 8 mg/L of influent N. In this experiment, raw manure was used and a fraction of the measured TN could have been unavailable for plant uptake. As a result, duckweed production peaked near 30 mg/L of influent TN. As influent TN further increased, there was sharp decline in duckweed production, potentially due to toxicity. This observation was different from the results obtained by Lasfar et al. (2007) who reported no significant increase in duckweed intrinsic growth rate beyond 3 mg/L of N concentration in the media. The discrepancy could be due to the difference in TN and available N in synthesized media versus diluted dairy manures. Moreover, Lasfar et al. (2007) conducted the experiments under optimum temperature (27 ◦ C) conditions, and thus, the difference in temperature in this experiment and aforementioned experiment also could have contributed to the difference. Attempts were made to fit the model developed by Driever et al. (2005) to the observed data; however, the modeling produced a poor fitting (p = 0.16). The poor fitting could be due to the difference in duckweed growth media. Driever et al. (2005) validated their model with the duckweed grown under synthetic nutrient media. This experiment was conducted using dairy wastewater as the growth media and the results were similar to the findings reported by Landesman et al. (2005) who used cattle feedlot runoff water as the growth media. Edwards et al. (1992) reported positive correlation between N and P concentrations in water and N and P concentrations in duckweed grown in septage-loaded ponds. However, the maximum N and P content used by the authors were about 12 mg/L of TKN and 1.4 mg/L of TP. Leng et al. (1995) found that crude protein content of duckweed increased with increasing aqueous N from traces concentrations to 15 mg/L. Additionally, P content of duckweed increased with increasing aqueous P up to 1.5 mg/L. Thus, the absence of relationship between influent N content and duckweed N content, as well as influent P content and duckweed P content, could be due to the saturation level of nutrients present in the media. The observed decrease in the percentage of N and P recovered by harvesting duckweed with increasing influent N and P concentration could be attributed to limitations in duckweed growth rates due to crowding that were previously described by Driever et al. (2005). Frederic et al. (2006) reported that up to 176 g/m2 /yr of N and 47 g/m2 /yr of P can be removed by harvesting duckweed under optimum growing conditions. Kadlec and Wallace (2009) reported that 50 to 150 g N/m2 /yr could be recovered by harvesting duckweed. In dairy lagoon wastewater, DeBusk et al. (1995) reported up to 32 g/m2 /yr of N and 7.3 g/m2 /yr of P could be recovered by harvesting duckweed. Overall average N and P recovery obtained in this study were below the values

reported by Frederic et al. (2006) and Kadlec and Wallace (2009), but were comparable with the values reported by DeBusk et al. (1995). Plants play a significant role in overall N and P removal, especially under higher loading rates (Tanner et al., 1995b). Since no unplanted control was used in the study, a conclusive discussion on the effect of duckweed in TN and TP removal is not warranted. Nonetheless, when no duckweed growth occurred in primary wetlands, COD removal did not follow the previous pattern (Fig. 2a). Duckweed growth peaked when influent contained 32 mg/L TN, indicating a maximum influent loading rate to optimize duckweed production. TN in harvested duckweed ranged from 1.7% to 6.6% by mass while TP ranged from 0.4% to 2.7% by mass. These findings suggest that duckweed with high nutrient content can be harvested sustainably from surface flow wetlands. IPNI (2008) reported that N:P ratio required for corn grain and silage is 5.4 and 7.2, respectively, while N:P ratio required for soybean grain and hay is 10.4 and 9.4, respectively. Average TN:TP ratio in the harvested duckweed was 3.0 ± 0.15, while the TN:TP ratio of the influent dairy wastewater was 46.6 ± 13.7. The result suggests that N:P ratio in harvested duckweed is much closer to the actual N:P ratio required by these two common crops, making duckweed a potential balanced nutrient source for fertilization. Additionally, potential use of duckweed as animal feed or energy production makes duckweed suitable candidate of wetland plant in surface flow wetlands.

5. Conclusions This experiment examined the organic carbon, nutrient, and E. coli reduction potential of duckweed-based constructed wetlands and evaluated three modeling approaches to best describe the obtained data. The experiment demonstrated that with appropriate dilution, constructed wetlands can serve dual purpose of treating raw dairy wastewater and producing duckweed biomass. Results showed that duckweed-based constructed wetlands removed COD, TN, and TP from dairy wastewater; however, removal rates were variable and dependent on influent concentrations. Moreover, higher per unit load reduction in primary treatment wetlands as compared to the secondary treatment wetlands implies that constructed wetland area is better utilized if duckweed-based treatment wetlands are used for primary treatment of the dairy wastewater than for secondary treatment, as long as manure is diluted to below 1000 mg/L COD. Comparisons with previous studies utilizing different wastewater or media sources indicated that pollutant reduction of duckweed-based wetlands was within the range expected for surface flow wetlands. This has important implications for future design of very low-cost wetland systems, given the simplicity of seeding duckweed as compared to planting emergent plant species. Compared to first order removal and DUBWAT model, first order removal with a background concentration best predicted effluent COD, TN, TP and E. coli in dairy wastewater treatment wetlands, with the background concentrations not significantly different than zero for TN or TP under the experimental conditions. Duckweed production was highest when influent TN concentration was about 32 mg/L, which can be a basis of constructed wetland design to optimize duckweed production. The concentration of TN at which peak duckweed production was observed differed from that previously reported, most likely because a substantial portion of TN in raw dairy manure was unavailable for uptake. The ratio of N:P in harvested duckweed was similar to that required for crop growth, indicating that duckweed grown in raw dairy wastewaters has value as a environmentally-friendly, lowcost fertilizer. Hence, duckweed-based treatment wetlands provide

Please cite this article in press as: Adhikari, U., et al., Use of duckweed-based constructed wetlands for nutrient recovery and pollutant reduction from dairy wastewater. Ecol. Eng. (2014), http://dx.doi.org/10.1016/j.ecoleng.2014.05.024

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the additional benefit of growing value-added biomass while producing similar treatment as traditional treatment wetlands. As the experiments were conducted under a single hydraulic loading and harvesting rate, further experiments with multiple hydraulic loading and harvesting rates are needed to optimize the duckweed production from duckweed-based constructed wetlands. Acknowledgements The study was supported by the Michigan Animal Agriculture Initiative. Additionally, the authors would like to thank S. Devin McLennan for his assistance in design and construction of the wetland system. References Boyd, C.E., 1974. Utilization of aquatic plants. In: Mitchell, D.S. (Ed.), Aquatic Vegetation and Its Use and Control. United Nations Educational, Scientific and Cultural Organization (UNESCO), Paris, France, pp. 107–115. Bradford, S.A., Segal, E., Zheng, W., Wang, Q., Hutchins, S.R., 2008. Reuse of concentrated animal feeding operation wastewater on agricultural lands. J. Environ. Qual. 37 (5), S97–S115. Cedergreen, N., Madsen, T.V., 2002. Nitrogen uptake by the floating macrophyte Lemna minor. New Phytol. 155 (2), 285–292. DeBusk, T.A., Peterson, J.E., Reddy, K.R., 1995. Use of aquatic and terrestrial plants for removing phosphorus from dairy wastewaters. Ecol. Eng. 5 (2-3), 371–390. Driever, S.M., Nes, E.H.V., Roijackers, R.M., 2005. Growth limitation of Lemna minor due to high plant density. Aquat. Bot. 81 (3), 245–251. Ebina, J., Tsutsui, T., Shirai, T., 1983. Simultaneous determination of total nitrogen and total phosphorus in water using peroxodisulfate oxidation. Water Res. 17 (12), 1721–1726. Edwards, P., Hassan, M.S., Chao, C.H., Pacharaprakiti, C., 1992. Cultivation of duckweeds in septage-loaded earthen ponds. Bioresour. Technol. 40 (2), 109–117. Frederic, M., Samir, L., Louise, M., Abdelkrim, A., 2006. Comprehensive modeling of mat density effect on duckweed (Lemna minor) growth under controlled eutrophication. Water Res. 40 (15), 2901–2910. Henderson, S., Triscari, P., Reinhold, D., 2012. Anaerobic digestion of dairy manure with duckweed (Lemnaceae). Biol. Eng. Trans. 5 (3), 147–157. Hillman, W.S., Culley, D.D., 1978. The uses of duckweed: the rapid growth, nutritional value, and high biomass productivity of these floating plants suggest their use in water treatment, as feed crops, and in energy-efficient farming. Am. Sci. 66 (4), 442–451. IDF, 1981. Dairy effluent. In: Bulletin of the IDF No. 138/1981. International Dairy Federation, Brussels, Belgium. IPNI, 2008. Average nutrient removal rates for crops in the Northcentral region. In: Plant Nutrient Today, No. 4. International Plant Nutrition Institute, Norcross, GA. Kadlec, R.H., Wallace, S., 2009. Treatment Wetlands. CRC Press, Boca Raton, FL. Kadlec, R.H., Bastiaens, W., Urba, D.T., 1993. Hydrological design of free water surface treatment wetlands. In: Constructed Wetlands for Water Quality Improvement. CRC Press, Boca Raton, FL. Korner, S., Vermaat, J.E., Veenstra, S., 2003. The capacity of duckweed to treat wastewater: ecological considerations for a sound design. J. Environ. Qual. 32 (5), 1583–1590. Krishna, K.C.B., Polprasert, C., 2008. An integrated kinetic model for organic and nutrient removal by duckweed-based wastewater treatment (DUBWAT) system. Ecol. Eng. 34 (3), 243–250. Landesman, L., Parker, N.C., Fedler, C.B., Konikoff, M., 2005. Modeling duckweed growth in wastewater treatment systems. Livest. Res. Rural Dev. 17 (6.).

9

Lasfar, S., Monette, F., Millette, L., Azzouz, A., 2007. Intrinsic growth rate: a new approach to evaluate the effects of temperature, photoperiod and phosphorus–nitrogen concentrations on duckweed growth under controlled eutrophication. Water Res. 41 (11), 2333–2340. Lee, C.G., Fletcher, T.D., Sun, G., 2009. Nitrogen removal in constructed wetland systems. Eng. Life Sci. 9 (1), 11–22. Leng, R.A., Stambolie, J.H., Bell, R., 1995. Duckweed—a potential high-protein feed resource for domestic animals and fish. Livest. Res. Rural Dev. 7 (1), 36. Lukina, G.A., 1977. Biology of the Duckweed Family. In: I: Vsesoyuznaya konferentsiya po vysshim vodnym i pribrezhnovodnym rasteniyam (I AllUnion Conf. on Higher Aquatic and Riparian–Aquatic Plants), Borok, Russia, pp. 21–23. Morgenroth, E., 2000. Opportunities for nutrient recovery in handling of animal residuals. In: Proceedings of the Animals Residuals Management Conference. Kansas City, Missouri, Nov 12–14,Water Environment Federation, Alexandria, VA. Muchovej, R.M.C., Pacosvsky, R.S., 1997. Future directions of by-products and wastes in agriculture. In: Rechcigl, J.E., MacKinnon, H.C. (Eds.), Agricultural Uses of by-Products and Wastes, ACS Symp. Ser. 668. American Chemical Society, Washington, DC, pp. 1–19. NRCS, 1999. National Engineering Handbook Part 651: Agricultural Waste Management Field Handbook. United States Department of Agriculture, Natural Resources Conservation Service, Washington, DC. Polprasert, C., Agarwalla, B.K., 1995. Significance of biofilm activity in facultative pond design and performance. Water Sci. Technol. 31 (12), 119–128. Rogers, S., Haines, J., 2005. Detecting and Mitigating the Environmental Impact of Fecal Pathogens Originating from Confined Animal Feeding Operations: Review. United States Environmental Protection Agency, Cincinnati, OH. Rusoff, L.L., Blakeney, E.W., Culley, D.D., 1980. Duckweeds (Lemnaceae family)—a potential source of protein and amino-acids. J. Agric. Food Chem. 28 (4), 848–850. Shilton, A.N., Powell, N., Guieysse, B., 2012. Plant based phosphorus recovery from wastewater via algae and macrophytes. Curr. Opin. Biotechnol. 23 (6), 884–889. Sooknah, R.D., Wilkie, A.C., 2004. Nutrient removal by floating aquatic macrophytes cultured in anaerobically digested flushed dairy manure wastewater. Ecol. Eng. 22 (1), 27–42. Tanner, C.C., 1996. Plants for constructed wetland treatment systems—a comparison of the growth and nutrient uptake of eight emergent species. Ecol. Eng. 7 (1), 59–83. Tanner, C.C., Clayton, J.S., Upsdell, M.P., 1995a. Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands—I. Removal of oxygen demand, suspended solids and faecal coliforms. Water Res. 29 (1), 17–26. Tanner, C.C., Clayton, J.S., Upsdell, M.P., 1995b. Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands—II. Removal of nitrogen and phosphorus. Water Res. 29 (1), 27–34. Tripathi, B.D., Upadhyay, A.R., 2003. Dairy effluent polishing by aquatic macrophytes. Water Air Soil Pollut. 143 (1-4), 377–385. USEPA, 2009. Method 1603: Escherichia coli (E. coli) in Water by Membrane Filtration Using Modified Membrane-Thermotolerant Escherichia coli Agar (Modified mTEC). United States Environmental Protection Agency, Office of Water, Washington, DC. USEPA, 2001. Development document for the proposed revisions to the national pollutant discharge elimination system permit regulation and effluent limitation guidelines and standards for concentrated animal feeding operations (CAFOs). In: Rep. 821-R-01-003. USEPA, Washington, DC. Vymazal, J., 2007. Removal of nutrients in various types of constructed wetlands. Sci. Total Environ. 380 (1-3), 48–65. Vymazal, J., Greenway, M., Tonderski, K., Brix, H., Mander, Ü., 2006. Constructed wetlands for wastewater treatment. In: Caldwell, M.M., Heldmaier, G., Jackson, R.B., Lange, O.L., Mooney, H.A., Schulze, E.D., Sommer, U. (Eds.), Wetlands and Natural Resource Management. Springer, Berlin Heidelberg, pp. 69–96.

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