Article pubs.acs.org/IECR
Electrochemical Deammonification of Synthetic Swine Wastewater Luis A. Diaz and Gerardine G. Botte* Center for Electrochemical Engineering Research, Chemical and Biomolecular Engineering Department, Ohio University 165 Stocker Center, Athens, Ohio 45701, United States S Supporting Information *
ABSTRACT: The ammonia electrolysis process, which presents as only products nitrogen and hydrogen fuel, was evaluated for the deammonification of wastewater. The effects of cell voltage, pH, and flow rate were analyzed for single pass ammonia electrooxidation of synthetic swine wastewater. A surface response analysis was performed to obtain quadratic models that predict the NH3 conversion and hydrogen (H2) production where pH was identified as the variable with the most statistic significance to explain changes in ammonia conversion with hydrogen production. NH3 conversions above 80% and specific energy consumption under 3 kW·h kg−1 of NH3, without considering the energy that can be obtained from the H2 produced, show that the ammonia electrolysis technology has promising potential for the deammonification of wastewater with less energy consumption than the conventional nitrification/denitrification process.
1. INTRODUCTION The reduction of ammonia emissions in wastewater constitutes a major issue since ammonia is one of the main contaminants in industrial, residential, and agricultural wastewaters.1−3 Beyond the effects of the presence of NH3 in wastewaters, which causes the eutrophication of rivers, lakes, and seas, NH3 is a gas at ambient pressure and temperature, and its presence in the atmosphere leads to the formation of particulate matter (ammonia aerosols), affects human health, and determines the formation of acid rain, which causes soils acidification.2,4 With ca. 80% of the global NH3 emissions,2,5 the most significant source of atmospheric ammonia comes from domestic animals and livestock wastes,4 which are currently disposed in open lagoons or land sprayed.2,4 Traditional methods, such as biological nitrification/denitrification, chemical precipitation, ammonia stripping, and evaporation have been developed to reduce the ammonia content in wastewater.6−9 However, these methods do not approach the problem holistically. Aerobic treatments and ammonia stripping processes have low efficiency due to the high amount of energy required.2,8 Although anaerobic processes are claimed to be the most cost efficient methods,2 those are just suitable for effluents with low ammonia concentration,8 while methods for high ammonia concentration are required.10 On the other hand, chemical precipitation requires the addition of other chemicals, such as MgCl2 and Na2HPO4,6,11 which could end in new water pollutants.8,12 Among the water remediation technologies, the electrochemical conversion of NH3 (ammonia electrolysis technology) is then presented as a great alternative to reduce NH3 emissions in wastewaters, with minimal byproduct generation and ease of control and operation.3,12 The ammonia electro-oxidation reaction (eq 1), can be coupled with the hydrogen evolution reaction (eq 2), in an alkaline electrolytic cell with less than 5% of the energy required for the electrolysis of water. The process described is known as ammonia electrolysis, and it was proposed by Botte’s group:13−18 2NH3(aq) + 6OH− → N2(g) + 6H 2O + 6e− © 2012 American Chemical Society
6H 2O + 6e− → 3H 2(g) + 6OH−
(2)
2NH3(aq) → N2(g) + 3H 2(g)
(3)
The overall reaction in eq 3 shows that three molecules of hydrogen can be obtained from two ammonia molecules. The beauty of the process is that eq 3 takes place at low cell voltages (e.g., 0.057 V thermodynamics potential), much lower than water electrolysis (over 1.23 V). Therefore, when ammonia is present in water only the oxidation of ammonia takes place (according to eq 1) as long as the cell voltage is kept under 1.2 V, consequently removing the ammonia from the water. Since hydrogen and nitrogen are separately produced in the cathode and the anode of the cell, respectively, pure nitrogen and fuel grade hydrogen are produced as the main byproducts.18 Based on this concept, members of the Center of Electrochemical Engineering Research at Ohio University (CEER) have designed and built an ammonia electrolytic cell stack (AEC), which uses carbon paper based electrodes with Pt and Ir as active catalysts assembled in such a way that a permeable membrane prevents the mixing of gases, while the cathode and anode streams flow through nine electrolytic cells connected in parallel. In this paper, a surface response analysis was used to investigate the ammonia removal rate from synthetic wastewater in the ammonia electrochemical flow system (AEC) built by Biradar.19 Three levels in the experimental factors, applied voltage, wastewater pH, and wastewater flow rate, were tested to obtain the statistical significance of the experimental factors over NH3 fractional conversion as a measure of the ammonia removal capacity. Hydrogen production and power consumption during the removal of ammonia were also considered as response variables based on their importance to the net energy consumption of the process. Received: Revised: Accepted: Published:
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Figure 1. Ammonia electrolyzer setup. The cathode side was kept in a close recirculation loop, while the anode side stream went through a single pass in the electrolyzer.
2. MATERIALS AND METHODS 2.1. Materials. Ammonium chloride, Alfa Aesar 99.5%, was used as the ammonia source for the preparation of the synthetic wastewater solution after dilution in ultrapure water (>18 MΩ). Synthetic swine wastewater was prepared from ACS certified reagents NH4Cl (Alpha Aesar 99.5%), KH2PO4 (Fisher Scientific), MgSO4·7H2O (Acros Organics 98%+), and CaCl2·2H2O (Fisher scientific), according to the concentration ranges presented by Ye et al.20 Analytical grade KOH (Fisher Scientific ≥85%) was used to prepare the electrolyte solution for the process. 2.2. Ammonia Electrolyzer. The ammonia electrolyzer is composed of a set of nine ammonia electrolytic cells (AECs). The electrodes for each AEC (anode and cathode) were prepared by electroplating of Pt−Ir on Toray TGP-H-030 (0.11 μm thick and 80% porosity) carbon fiber paper (CFP).21,22 The anode and cathode compartment were separated with a W. L. Gore Associates proprietary Teflon membrane to avoid the mixture of the gas products (H2 and N2). A detailed description of the nine AECs stack is presented elsewhere.19,23 An ARBIN BT-2000 battery test equipment was used to operate the AEC at the voltage conditions required for the experimental matrix. The experimental set up is presented in Figure 1. The anode and cathode streams were fed to the AECs stack by Cole Parmer gear pumps, and the flows were controlled with analogue Omega rotameters for corrosive applications (±10% full scale accuracy). Both AECs stack outlet streams were separated in splitter columns (0.0254 m of diameter and 0.18 m height), and the gases were conducted to water displacement collectors, where the height of the displaced water was used to calculate the mass of gases produced. The liquid stream leaving the cathode side splitter was sent again to the ACE in a closed loop while the anode side liquid stream constituted the process effluent. 2.3. Experimental Methods. The pH of the synthetic wastewater solution was adjusted with the addition of a concentrated KOH solution (8.15 M). Before each run, the experimental setup was rinsed thoroughly with ultrapure water,
followed by the solution to be tested. The adjusted pH solution was fed both to the cathode and the anode side. The cathode side stream was kept in a closed loop at 120 mL·min−1, whereas the anode side stream was adjusted to the desired flow rate in a single pass configuration through the electrolyzer. Once the flows were stabilized, a constant voltage was applied. The experiment was allowed to reach a steady state behavior, which was determined by no major changes in the current profile. During the steady state operation, the amount of hydrogen produced was reported and the effluent was collected to measure ammonia concentration. Hydrogen and nitrogen gas production were measured by the water displacement method. Ammonia concentration in the synthetic wastewater and deammonified synthetic wastewater was measured with a high performance ammonia ion selective electrode (ISE) from Thermo Scientific. Hence, the ammonia fractional conversion was calculated using eq 4. The pH and the conductivity of the wastewater were measured with an OAKTON Acorn Meter pH 5 (pH and temperature meter) and a Mettler Toledo MC226 Conductivity Meter, respectively. x=
[NH3]INLET − [NH3]EFLUENT [NH3]INLET
(4)
2.4. Experimental Design and Statistical Analysis. A 1624 ppm solution of NH4Cl was prepared in ultrapure water. The pH of the solution was adjusted to 9, 11, and 13 after the addition of KOH 8.15 M solution in different amounts. Ammonia in aqueous media can be found as ammonia (NH3(aq) in alkaline media) or ammonium (NH4+ in acid media). However, the electro-oxidation in acid media has been shown to be slow and almost nonactive.24−28 Hence, ammonia is recognized as the active molecule toward electro-oxidation. The equilibrium concentration of NH4+ and NH3 versus the pH was calculated using the electrolyte nonrandom two liquid (NRTL) model,29 with the Redlich−Kwong equation of state in Aspen Plus and 12168
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summarized in Table 2. The factors under study were selected based on their influence in the operational costs of the process, since voltage and flow rate are directly associated with energy requirements. On the other hand, pH was used as a control variable for the amount of electrolyte added in the process. The experimental data were analyzed by Minitab 15, where an analysis of variance (ANOVA) with a 95% of confidence limit (p ≤ 0.05) was performed to identify the statistical significance of each one of the factors presented in Table 1 with the changes presented in the response variables (Table 2). Moreover, surface response analysis was completed to obtain quadratic models that describe the ammonia conversion and hydrogen production in terms of the factors and their interactions. The accuracy of the empirical model was tested by means of cross validation, with a test set of four points. Finally, a composite optimization (including the energy consumption per kg of NH3 removed) was performed. 2.5. Synthetic Wastewater Analysis. Synthetic swine wastewater was prepared to observe if the presence of ions such as Mg2+, Ca2+, PO43−, and SO42−, which can be found in swine wastewater,20 could significantly affect the performance of ammonia electrolysis. Two levels of concentrations reported by Ye et al.,20 which cover the range of composition of these ions in swine wastewater, were prepared, while the ammonia concentration was kept constant at 1624 ppm NH4Cl. The ion compositions used for the synthetic swine wastewater test are presented in Table 3.
presented in Figure 2. The measured dependence of the conductivity media with the pH has been also included in
Figure 2. Effects of pH in ammonia−ammonium equilibrium and synthetic swine wastewater conductivity.
Figure 2, which shows that the pH of the solution (wastewater) affects the NH3/NH4+ ratio along with the conductivity of the media. A randomized Box−Behnken experimental design with three factors, three central point repetitions, and three levels was used to analyze the effects of cell voltage in V (X1), pH (X2), and flow rate of wastewater in mL·min−1 (X3), over the ammonia fractional conversion (Y1) and hydrogen production (Y2) in standard litters per minute (SLPM). A description of the experimental design is presented in Table 1, where levels −1, 0, Table 1. Experimental Conditions for the Surface Response Analysis
Table 3. Synthetic Swine Wastewater Test Conditions and Results
factors levels
applied cell voltage (V)
pH [KOH concn., M]
flow rate (mL·min−1) X1 X2 X3
−1 0 1
0.6 0.7 0.8
9a [0.04]b 11a [0.09]b 13a [0.56]b
0.6 1.8 3.6
a
Ca2+ concn. (ppm)a Mg2+ concn. (ppm)a PO43− concn. (ppm)a SO42− concn. (ppm)a NH3 fractional conversionb H2 production (SLPM)b
pH. bKOH concentration, M.
low concn.
high concn.
42 94 79 371 0.8 0.006
158 246 241 978 0.78 0.005
a
Concentration of ions in the synthetic swine wastewater. bExperimental results.
and 1 represent the lower, medium, and higher levels for each factor. The complete set of experiments performed is Table 2. Experimental Matrix Results factors run order
X1
X2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0.8 0.7 0.8 0.7 0.7 0.6 0.6 0.8 0.8 0.6 0.7 0.6 0.7 0.7 0.7
11 13 11 11 9 11 11 13 9 13 13 9 11 9 11
responses X3
fractional NH3 conversion (Y1)
H2 produced (SLPM) (Y2)
energy consumption (kWh·kgNH3−1)
Faraday’s efficiency in terms of NH3 consumption, η
0.6 3.1 3.1 1.85 0.6 0.6 3.1 1.85 1.85 1.85 0.6 1.85 1.85 3.1 1.85
0.31 0.37 0.16 0.13 0.19 0.09 0.1 0.7 0.11 0.37 0.72 0.08 0.15 0.10 0.15
0.0020 0.0070 0.0060 0.0015 0.0014 0.0009 0.0001 0.0102 0.0020 0.0060 0.0088 0.0010 0.0013 0.0014 0.0017
12.58 4.22 11.09 5.13 10.53 16.78 3.76 5.92 4.14 7.00 19.91 3.49 3.06 4.99 2.98
30.11 78.50 34.14 64.61 31.47 16.94 75.54 79.36 67.28 40.60 16.65 81.36 72.74 66.42 51.98
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3. RESULTS AND DISCUSSION 3.1. Response Surface Analysis. Ammonia Conversion. ANOVA results indicate a statistically significant correlation among the NH3 conversion and the three factors studied with a confidence level >95% (see Table S1, Supporting Information). The pH of the solution was shown to be the most significant factor (p = 0.000). This means that any change in the conversion value can be explained by alterations of the three factors. Hence, the surface response analysis (Figure S1, Supporting Information) provided a quadratic regression model that takes into account binary interactions among the experimental factors with a model regression coefficient R2 = 97.89%. The coefficients of the quadratic model for each one of the factors in coded units to predict ammonia conversion in the nine cell AECs are presented in the following equation:
The validity of the model presented in eq 6 was tested by means of cross validation with a test set composed of four points, which are presented in Figure 4, obtaining a mean normalized
Y1 = 5.651 − 2.149X1 − 1.076X 2 + 0.391X3 − 0.417X12 + 0.0439X 22 + 0.0165X32 + 0.375X1X 2 − 0.32X1X3 − 0.026X 2X3
Figure 4. Cross validation test for the quadratic prediction of H2 production in SLPM at 0.8 V. Test set points are presented with a confidence interval of 95% of three sample repetitions.
(5)
The validity of the model presented in eq 5 was tested by means of cross validation with a test set composed of four points, which are presented in Figure 3. The quadratic model obtained
error MNE = 1.29. The low lack of fit observed for hydrogen production can be attributed to the presence of dead volume inside the electrolyzer, which trapped the gases produced. 3.2. Synthetic Wastewater Analysis. Table 3 presents the results of the ammonia conversion and hydrogen production obtained at the conditions of maximum conversion presented in Figure 3 (0.8 V, pH = 13, and 0.6 mL·min−1). Although the ammonia conversion seems to decrease as long as the concentrations of Mg2+, Ca2+, PO43−, and SO42− increase, the effect can be considered as insignificant, since the values of conversion obtained lay inside the confidence interval of 95% presented in Figure 3. The same conclusion can be obtained from the hydrogen production of synthetic swine wastewater in presence of the different ions tested. 3.3. Composite Optimization. Along with the ammonia conversion and hydrogen production, the energy required for the ammonia electrolysis constitutes a relevant factor to analyze the feasible implementation of the technology. The specific energy consumption (kWh·kg−1 of NH3 removed) was calculated by measuring the current passing through the AEC. Columns 2 and 3 in Table 2 show the measured specific energy required for the process as well as the faradaic efficiency (η). The faradaic efficiency (eq 7) indicates what fraction of the current flowing through the AEC is being used for the ammonia electro-oxidation (eq 1). As it has been reported by other authors,16,17,30−32 the selectivity of the ammonia electrooxidation toward nitrogen is 100% at the applied voltage region studied. Hence, no parallel reactions such as the formation of nitrogen oxidize species are expected, and the low experimental efficiencies obtained could be attributed meanly to ohmic losses in the system (e.g., current collectors, tabs, wires and cables, etc.). The faraday efficiency (η) was calculated through:
Figure 3. Cross validation test for the quadratic prediction of ammonia fractional conversion at 0.8 V. Test set points are presented with a confidence interval of 95% of three sample repetitions.
from the surface response analysis can predict the conversion obtained in the AEC with a normalized error MNE = 0.069. Hydrogen Production. ANOVA results indicated that the cell voltage and solution pH are the only factors that presented a statistically significant correlation with a confidence level >95% with the H2 production (see Table S2, Supporting Information), where the solution pH was identified as the most significant factor (p = 0.000). A surface response analysis (Figure S2, Supporting Information) provided a quadratic regression model that takes into account binary interactions among the three experimental factors studied with a model regression coefficient R2 = 97.55%. Although the statistical significance between the flow rate of wastewater and the hydrogen production was found to be below the desired confidence level, the flow rate is still taken into account in the model presented in eq 6.
η=
mnFt sMQ
(7) −1
where m is the ammonia removal rate in kg·h calculated from the experimental fractional conversion, s is the stoichiometric coefficient of ammonia in eq 3, M is the molecular weight of ammonia, Q is the measured charge capacity in A·h, η is the equivalent mole (6 eqiv for the ammonia electro-oxidation reaction in eq 3), F is the Faraday constant (26.8 A·h·eq−1), and t is the test time, which was 1 h for all the experiments.
Y2 = 0.121 − 0.1095X1 − 0.0165X 2 − 0.0053X3 + 0.045X12 + (7.125 × 10−4)X 22 + (1.92 × 10−4)X32 + 0.004X1X 2 + 0.0096X1X3 − (1.8 × 10−4)X 2X3 (6) 12170
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A composite optimization, including the specific energy consumed along with the response variables presented earlier, was performed using Minitab 15. The optimization parameters and the optimum solution, which consider the same weight and importance for all the response variables, are presented in Table 4.
goal
lower
*Tel.: +1 740 593 9670. Fax: +1 740 593 0873. E-mail: botte@ ohio.edu. Notes
The authors declare no competing financial interest.
target
upper
weight
solution
min
4
4
5
1
4.7129
max max
0 0.5
0.01 0.9
0.01 0.9
1 1
0.0096 0.5629
■
ACKNOWLEDGMENTS The authors would like to thank the financial support of the Center for Electrochemical Engineering Research at Ohio University and the Department of Defense through the U.S. Army Construction Engineering Research Laboratory (W9132T-09-1-0001). The content of the information does not reflect the position or the policy of the U.S. government.
The optimization plot (Figure S3, Supporting Information) shows a global solution at X1 = 0.7657 V, X2 = 13 and X3 = 2.39 mL·min−1. At these conditions, the composite desirability indicates that the targeted values presented in Table4 are satisfied in a 33.22%, reaching a conversion of 56.29%. The specific energy consumption value of 4.77 kWh·kg−1 of NH3 implies a reduction of at least 12% over the specific energy consumption of 5.4 kWh·kg−1 of NH3 reported by Wett33 for the biological nitrification/denitrification process. Moreover, the net energy required for ammonia electrolysis will be much lower after the quantification of the energy provided by the hydrogen produced in the process.
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REFERENCES
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4. CONCLUSIONS The feasibility of using the ammonia electrolysis technology for the deammonification of synthetic swine wastewater was evaluated. An accurate prediction of an optimized ammonia conversion value could be obtained from the quadratic statistical model acquired in the surface response analysis, where the pH of wastewater was found to be the most significant factor toward ammonia conversion and hydrogen production. The hydrogen production in the electrolyzer is not affected by the flow rate of the synthetic wastewater through the electrolyzer. The presence of different ions (Mg2+, Ca2+, PO43−, and SO42−) does not affect drastically the performance of the ammonia electrolyzer. Therefore, the ammonia electrolyzer has the potential to be used for wastewater remediation. If hydrogen is not recovered, energy consumption as low as 3.00 kWh·kg−1 of NH3 can be achieved with the ammonia electrolysis process. This value can be significantly reduced with the integration of the ammonia electrolysis technology with a combined heat and power generation system to take advantage of the hydrogen produced in the process. The results of this study show the ammonia electrolysis process as a feasible and energy efficient alternative for the ammonia removal from wastewater. The results presented in this study can be used as a starting point for the evaluation of the process with real swine wastewater and for the scale up of the electrochemical deammonification process toward a pilot test station.
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AUTHOR INFORMATION
Corresponding Author
Table 4. Composite Optimization Parameters energy (kWh·kgNH3−1) H2 (SLPM) NH3 fractional conversion
Article
ASSOCIATED CONTENT
S Supporting Information *
Table S1: Analysis of variance for the ammonia fractional conversion. Table S2: Analysis of variance for the hydrogen production in SLPM. Figure S1: Contour plots for ammonia fractional conversion. Figure S2: Contour plots for hydrogen production in SLPM. Figure S3: Composite optimization plot. This information is available free of charge via the Internet at http://pubs.acs.org/. 12171
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