Accepted Manuscript Short Communication A New Method for Nutrients Removal and Recovery from Wastewater Using a Bioelectrochemical System Fei Zhang, Jian Li, Zhen He PII: DOI: Reference:
S0960-8524(14)00823-2 http://dx.doi.org/10.1016/j.biortech.2014.05.105 BITE 13513
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
12 April 2014 24 May 2014 27 May 2014
Please cite this article as: Zhang, F., Li, J., He, Z., A New Method for Nutrients Removal and Recovery from Wastewater Using a Bioelectrochemical System, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/ j.biortech.2014.05.105
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A New Method for Nutrients Removal and Recovery from Wastewater Using a Bioelectrochemical System
Fei Zhang, Jian Li and Zhen He * Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
Intended for Bioresource Technology Type of contribution: Short Communication
*
Corresponding author. Phone: (540) 231-1346; fax: (540) 231-7916; e-mail: [email protected] (Z.
He)
Abstract Nutrients management is a key task of wastewater treatment and removal of nutrients is usually associated with significant energy/economic cost. A new bioelectrochemical system, named “R2BES”, was developed for removing and possibly recovering nutrients from wastewater. This R2BES takes advantage of bioelectricity generation from oxidation of organic compounds to drive ammonium migration out of wastewater, and uses hydroxide ions produced from the cathode reaction as a medium to exchange phosphate ions from wastewater at the same time. Under an applied voltage of 0.8 V, the R2-BES removed 83.4 ± 1.3% of ammonium nitrogen and 52.4±9.8% of phosphate, significantly higher than those (3.6±3.7% and 21.1±2.6%) under an open circuit condition. Applying an external voltage can increase current generation, COD removal, and nutrient removal. Those results demonstrate a proof of concept that the R2-BES may be potentially applied to remove and recover nutrients through appropriate integration into the existing treatment facilities.
Keywords: Nutrients Removal and Recovery; Nitrogen and Phosphorus; Bioelectrochemical system; Microbial fuel cells; Wastewater Treatment
1
1. Introduction It is increasingly recognized that wastewater should be treated in a sustainable way for minimizing contamination and maximizing the recovery of valuable resources such as energy, water and nutrients (Verstraete et al., 2009). Nutrient removal and recovery is of particular interest, because of potential threat of nutrient compounds to natural water bodies, the limited mining resource and high cost associated with nutrient production (Rittmann et al., 2011). In addition to nutrient removal by conventional biological treatment (nitrification, denitrification and ANAMMOX) and ammonia stripping, various biochemical and chemical methods were investigated and applied for nutrient recovery. For example, microalgae can uptake both nitrogen and phosphorus with generation of biomass that can be further utilized for production of energy or value-added compounds; however, separation of algae from the treated wastewater represents a significant challenge (Cai et al., 2013). Chemical precipitation such as forming struvite can recover both nitrogen and phosphorus (Corre et al., 2009), but the cost of struvite production is high and struvite is not widely used in agriculture (Hao et al., 2013). Membrane-based technologies exhibit great promise for wastewater treatment. The studies show that phosphorus can be recovered using electrodialysis or forward osmosis/membrane distillation (Xie et al., 2014; Zhang et al., 2013b). In general, nutrient removal and recovery from wastewater requires costeffective treatment technologies (Lee et al., 2013), and it will be advantageous to address multiple factors (e.g., organics, nitrogen, and phosphorus) in a single system.
Bioelectrochemical systems (BES) are an emerging treatment technology based on microbial interaction with solid electron acceptors/donors, and the representative BES include microbial fuel cells (MFCs), microbial electrolysis cells (MECs), and microbial desalination cells (MDCs). 2
While energy recovery from organic compounds is a focus in BES research (Ge et al., 2014), nutrient removal and recovery has also gained much attention (Kelly and He, 2014). Nitrogen is removed in a BES through bioelectrochemical denitrification (sometimes coupled with nitrification), or recovered via ammonium migration driven by electricity generation (Clauwaert et al., 2007; Kuntke et al., 2012; Virdis et al., 2008). Phosphorus removal and recovery in a BES usually relies on the high-pH condition as a result of cathode reduction reaction to precipitate phosphorus compounds (Ichihashi and Hirooka, 2012). BES may provide an energy-efficient approach for nutrient removal and recovery by using bioenergy produced from organic compounds in wastewater, but there are also some challenges such as lack of systems for simultaneously removing/recovering both nitrogen and phosphorus, and systematic demonstration of nutrient removal and recovery (more discussion can refer to a recent review article (Kelly and He, 2014)).
Intrigued by the findings that ammonium ions can be separated from wastewater driven by electron flow in a BES (Cord-Ruwisch et al., 2011), a new method was developed and investigated in this study for removal of both ammonium and phosphate ions from synthetic wastewater, named “R2-BES”. The key factor is the use of both cation and anion exchange membranes in a BES, which allow the migration of both cations and anions at the same time, though in different mechanisms (Fig. 1). This method is different from nutrient removal by ion exchange methods that require additional ion input into wastewater.
2. Materials and Methods 2.1 BES Setup
3
A liter-scale R2-BES was built based on a rectangular MFC in a glass tank, which functioned as an anode compartment with a liquid volume of ~ 8.5 L. Two 1-m long carbon brushes were folded to fit into the anode compartment as the anode electrode. A membrane “pocket” was made of a piece of cation exchange membrane (CEM) and a piece of anion exchange membrane (AEM, Membranes International, Inc., Glen Rock, NJ, USA), each of which had a size of 20 cm × 30 cm, with three sides sealed and the top side open to the air. A piece of carbon cloth (20 cm × 50 cm) was folded to fit into the membrane pocket as the cathode electrode, and coated with 0.5 mg cm-2 of activated carbon powder as a cathode catalyst as previously described (Zhang et al., 2013a). This membrane pocket was inserted into the glass tank and acted as a cathode compartment with a liquid volume of about 300 mL.
2.2 BES Operation The R2-BES was operated in a continuously-fed mode at room temperature (~ 20 ºC). The anode compartment was inoculated with the anaerobic sludge from a municipal wastewater treatment plant (Peppers Ferry Regional Wastewater Treatment Plant, Radford, VA). The synthetic anode solution was prepared as (per L of tap water): glucose, 0.2 g; NH4Cl, 0.11 g; NaCl, 0.1 g; MgSO4, 0.003 g; CaCl2, 0.004 g; NaHCO3, 0.02 g; NaOH, 0.1 g; K2HPO4, 0.028 g; and trace elements, 0.4 mL (Angenent and Sung, 2001). The conductivity of the synthetic solution was about 1.13 mS cm-1. The anode solution was fed at 4.51 mL min-1, resulting in a hydraulic retention time (HRT) of 31.4 h. The anolyte was internally mixed by a submerging pump at 9.5 L min-1. The initial catholyte was tap water, and 10-mL tap water was added to compensate for evaporation on daily basis. No active aeration was provided to the cathode compartment. When the external voltage was applied (0.4 or 0.8 V), a power supply was connected to the BES circuit according to
4
a previous study (Yossan et al., 2013).
2.3 Measurement and Analysis The voltage across an external resistor (1 Ω) was recorded every 5 min by a digital multimeter (2700, Keithley Instruments Inc., Cleveland, OH, USA). The pH was measured using a benchtop pH meter (Oakton Instruments, Vernon Hills, IL, USA). The conductivity was measured by a benchtop conductivity meter (Mettler-Toledo, Columbus, OH, USA). The concentrations of chemical oxygen demand (COD), nitrogen (ammonium, nitrate and nitrite), and phosphate were measured using a colorimeter according to the instructions of the manufacturer (Hach Company, Loveland, CO, USA). Coulombic efficiency (CE) was calculated according to the previous work (Logan et al., 2006). The energy consumption by nitrogen removal (kWh kg-1) under an applied voltage was estimated by calculating total energy input (integrating power with time, kWh) within time t, which was then divided by the amount of removed nitrogen within time t (kg). An abiotic test (containing no electrodes) was conducted for examining ion exchange in an H-type MFC similar to that in a previous study (Yossan et al., 2013), containing AEM and with controlled electrolyte pH.
3. Results and Discussion The effectiveness of nutrient removal in the R2-BES was demonstrated through comparing its performance under a closed-circuit condition and an open circuit condition, as shown in Fig.2. With current generation of 68.7 ± 4.6 mA under 0.8 V applied (Fig. 2A), the R2-BES decreased the NH4+-N concentration in the anolyte from 28.5 ± 1.3 (influent) to 4.7 ± 0.5 mg L-1 (effluent) (Fig. 2B), representing 83.4 ± 1.3% reduction. The concentration of phosphate (PO43--P) was
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reduced by 52.4 ± 9.8%, decreasing from 5.3 ± 0.2 to 2.5 ± 0.4 mg L-1. For comparison, the R2BES under the open-circuit condition (~ 0.63 V, Fig. 2A) removed 3.6 ± 3.7% of ammonium nitrogen and 21.1 ± 2.6% of phosphate. As a result of ion migration from the anode compartment into the cathode compartment, the catholyte was expected to concentrate those ions. Under the closed-circuit condition, the concentrations of ammonium nitrogen and phosphate in the catholyte reached 350.0 ± 57.7 mg L-1 and 302.5 ± 29.0 mg L-1, respectively, which could be possibly recovered as either ammonia gas and phosphate precipitates by taking advantage of the high pH of the catholyte (13.1 ± 0.2), or struvite with addition of magnesium salts. The present study focused on the nutrient removal, and the recovery will be investigated in future work to reveal the appropriate recovering method and the recovered products. Nitrate and nitrite was detected at the concentration of 20-30 mg L-1 in the catholyte, indicating the occurrence of nitrification, likely due to the contamination of autotrophic microorganisms with an open cathode compartment.
The comparison between the closed- and the open-circuit conditions shows the key role of electrical current in nutrient removal. There could be two mechanisms of ion movement in the R2-BES, both of which are affected by current generation: (1) unidirectional movement is driven by current generation and ions move from the anolyte to the catholyte, which should be the major mechanism for ammonium removal under the closed-circuit condition; and (2) bidirectional movement is via ion exchange, and the exchange between hydroxide ions (a product of current generation in the catholyte) and phosphate ions (anolyte) could be the mechanism for phosphorus removal. The ion exchange membranes used in this study are not selective for ammonium nitrogen or phosphate ions; therefore, other ions would also migrate and
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compete with nitrogen and phosphate ions. It was found that the conductivity of the anolyte decreased from 1.13 ± 0.05 to 0.51 ± 0.06 mS cm-1 under the closed-circuit condition, while there was almost no change in the anolyte conductivity under the open-circuit condition. The R2BES achieved a Coulombic efficiency of 38.2 ± 3.8%. Assuming one electron moves one ammonium ion and according to the calculation of charge transfer efficiency in a previous study (Jacobson et al., 2011), migration of ammonium ions could contribute to 18.0 ± 1.7% of current generation, indicating that movement of other cations was largely involved in current generation and part of current generation was related to hydroxyl movement from the cathode into the anode through AEM.
Likewise, competition among anions during ion exchange with hydroxide ions was also expected, and phosphate ions would be outcompeted by other dominant anions such as chloride ions due to a smaller charge affinity and a larger hydration diameter. The abiotic tests of ion exchange between phosphate and hydroxide ions revealed that more phosphate ions could be exchanged at a higher catholyte pH (32.7% removal at pH 13.0 vs. 23.4% at pH 6.0), and the presence of chloride ions significant decreased phosphate removal (18.8% removal at pH 13.0). The ion exchange brought hydroxide ions into the anolyte (wastewater), which could provide alkalinity and buffer pH decrease during the anaerobic oxidation of organic compounds. In the R2-BES under a 0.8-V applied voltage, the exchange of phosphate ions could add alkalinity of 4.5 ± 0.9 mg L-1 as CaCO3. The actual alkalinity addition should be more than that value, because phosphate ions were at a lower concentration than other anions in the anode feeding solution, and electric current would also promote the unidirectional movement of hydroxide ions into the anolyte through AEM. The alkalinity addition and its buffering effects are of great interest and
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will be investigated in more details in future studies.
To further understand the effects of externally applied voltage on the R2-BES, the system was operated under three voltages, 0, 0.4 and 0.8 V. There was significant difference in current generation: the 0.8-V condition produced a current of 68.7 ± 4.6 mA, three times that of the system without any external voltage (22.4 ± 0.5 mA at 0 V). As a result of the high current generation at 0.8 V, the R2-BES removed 94.4 ± 0.8% of COD, while the low current generation at 0 V led to the lowest COD removal of 79.0 ± 2.6% (Fig. 3A). Nutrient removal was also improved with the applied voltage, but nitrogen and phosphorus exhibited different trends likely due to the different removal mechanisms (Fig. 3B). Ammonium migration depends on electron flow, and thus its removal shows an almost linear relationship with current generation, varying from 20.3 ± 0.5% at 0 V to 83.4 ± 1.3% at 0.8 V. Phosphate movement relies on the gradient of anions across the AEM and migration of competitive anions. The 0.8 V condition still had the highest phosphate removal of 52.4 ± 9.8%, while the other two conditions had insignificant difference in their removal (34.7 ± 4.9% at 0.4 V and 30.8 ± 4.4% at 0 V). At this moment, it is not clearly understood how phosphate movement is affected by the competition and anion gradient; the pH gradient is clearly important but its effect may be weakened by the presence of other major factors (e.g., anion competition and unidirectional anion movement from the cathode into the anode): the anolyte pHs under those three applied voltages were similar at ~ 6.4, while the catholyte pHs of the 0.4- and 0.8-V conditions were similar at 13.1 and the one at 0 V was lower at 11.8.
Although applying an external voltage generally improves the performance of the R2-BES and
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further increasing the external voltage (>0.8 V) may further increase the efficiency because of higher current generation, one must also note the additional energy input with this approach (beyond energy consumption by regular operation such as pumping systems). For example, at 0.8 and 0.4 V, the additional energy input for nitrogen removal is 8.58 ± 1.02 and 3.41 ± 0.39 kWh kg N-1, respectively. Assuming a wholesale electricity price of $0.07 kWh-1, the cost of this energy input is about $0.23-0.60 kg N-1. In addition, a high external voltage (>1.2 V) may cause water electrolysis: despite high current generation under that condition, the use of BES method for nutrient removal and recovery may lose its significance (which lies in the use of electrons from organic wastes, instead of water oxidation). The improved performance with additional energy input may compensate for the increased economic cost, but this needs further analysis. The detailed economic analysis is not feasible at the current scale of the system, and comparison with the existing technologies will require further development of the system to a reasonable scale (to avoid any overpromises from bench-scale systems).
The present R2-BES has accomplished nutrient removal to a certain degree, and the nutrient concentrations need to be further reduced to meet the discharge limit. The performance may be improved through adding more cathode units/membrane areas. In the present R2-BES, the size of the anode volume is nearly 30 times that of the cathode volume, and thus there is room for more cathode units. The unidirectional movement of hydroxide ions from the catholyte into the anolyte driven by electric current could slow down phosphate migration (in the opposite direction); future R2-BES design may consider an additional AEM compartment for ion exchange only, with supply of the high-pH catholyte from the CEM compartment that has electrochemical activities. Identifying appropriate application niche can accelerate the R2-BES development. For example,
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installing the R2-BES in the anoxic zone will take advantage of the existing facilities and have it as pretreatment of the aerobic process, thereby reducing the need for nutrient removal after aerobic treatment. The R2-BES can also be linked to anaerobic digestion for side stream treatment to polish anaerobic digestion effluent by reducing organic contents and decreasing the concentrations of both ammonia and phosphorus (and recovering the concentrated nutrients).
Future work will investigate the R2-BES from several aspects: (1) detailed balances of major cations and anions will be established to understand the competition of ion movement; (2) recovery of the concentrated ions in the catholyte will be studied; (3) the influence of alkalinity addition with hydroxyl on wastewater treatment will be further examined; (4) the function of the R2-BES may be expanded to removal of heavy metals; and (5) the stability of the system will be investigated through long-term operation with examining main influence factors such as HRT, loading rates, and membrane performance.
4. Conclusions This study has demonstrated a proof-of-concept bioelectrochemical system for simultaneous removal of nitrogen and phosphorus from synthetic wastewater. It was found that current generation played a central role in the removal process, and higher current generation with externally applied voltage would improve the removal efficiency. Nitrogen was removed by direct current-driven ammonium migration, and phosphorus was removed via ion exchange with hydroxyl that was generated as a result of the cathode reaction. With further understanding of ion transport/competition and improvement of system configuration/operation, this system may provide a new approach for energy-efficient removal and recovery of nutrients from wastewater.
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Acknowledgements This work was financially supported by a grant from Gannett Fleming, Inc. The authors would like to thank Peppers Ferry Regional Wastewater Treatment Authority for providing wastewater samples.
References 1. Angenent, L.T., Sung, S. 2001. Development of anaerobic migrating blanket reactor (AMBR), a novel anaerobic treatment system. Water Res., 35, 1739-1747. 2. Cai, T., Park, S.Y., Li, Y. 2013. Nutrient recovery from wastewater streams by microalgae: Status and prospects Renew. Sust. Energ. Rev., 19, 360-369. 3. Clauwaert, P., Rabaey, K., Aelterman, P., Schamphelaire, L.D., Pham, T.H., Boeckx, P., Boon, N., Verstraete, W. 2007. Biological denitrification in microbial fuel cells. Environ. Sci. Technol., 41, 3354-3360. 4. Cord-Ruwisch, R., Law, Y., Cheng, K.Y. 2011. Ammonium as a sustainable proton shuttle in bioelectrochemical systems. Bioresour. Technol., 102, 9691-9696 5. Corre, K.S.L., Valsami-Jones, E., Hobbs, P., Parsons, S.A. 2009. Phosphorus Recovery from Wastewater by Struvite Crystallization: A Review. Crit. Rev. Env. Sci. Technol., 39, 433-477. 6. Ge, Z., Li, J., Xiao, L., Tong, Y., He, Z. 2014. Recovery of Electrical Energy in Microbial Fuel Cells. Environ. Sci. Technol. Lett., 1, 137-141. 7. Hao, X., Wang, C., Loosdrecht, M.C.M.v., Hu, Y. 2013. Looking Beyond Struvite for PRecovery. Environ. Sci. Technol., 47, 4965–4966. 8. Ichihashi, O., Hirooka, K. 2012. Removal and recovery of phosphorus as struvite from swine wastewater using microbial fuel cell. Bioresour. Technol., 114, 303–307. 9. Jacobson, K.S., Drew, D., He, Z. 2011. Efficient salt removal in a continuously operated upflow microbial desalination cell with an air cathode. Bioresour. Technol., 102, 376-380. 10. Kelly, P., He, Z. 2014. Nutrients removal and recovery in bioelectrochemical systems: a review. Bioresour. Technol., 153, 351-360. 11. Kuntke, P., Smiech, K.M., Bruning, H., Zeeman, G., Saakes, M., Sleutels, T.H., Hamelers, H.V., Buisman, C.J. 2012. Ammonium recovery and energy production from urine by a microbial fuel cell. Water Res, 46, 2627-36. 12. Lee, E.J., Criddle, C.S., Bobel, P., Freyberg, D.L. 2013. Assessing the scale of resource recovery for centralized and satellite wastewater treatment. Environ Sci Technol, 47, 10762-70. 13. Logan, B.E., Hamelers, B., Rozendal, R.A., Schroder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., Rabaey, K. 2006. Microbial fuel cells: methodology and technology. Environ. Sci. Technol., 40, 5181-5192. 14. Rittmann, B.E., Mayer, B., Westerhoff, P., Edwards, M. 2011. Capturing the lost 11
phosphorus. Chemosphere, 84, 846-53. 15. Verstraete, W., Caveye, P.V.d., Diamantis, V. 2009. Maximum use of resources present in domestic “used water”. Bioresour. Technol., 100, 5537–5545. 16. Virdis, B., Rabaey, K., Yuan, Z., Keller, J. 2008. Microbial fuel cells for simultaneous carbon and nitrogen removal. Water Res., 42, 3013-3024. 17. Xie, M., Nghiem, L.D., Price, W.E., Elimelech, M. 2014. Toward Resource Recovery from Wastewater: Extraction of Phosphorus from Digested Sludge Using a Hybrid Forward Osmosis−Membrane Distillation Process. Environ. Sci. Technol. Lett., 1, 191195. 18. Yossan, S., Xiao, L., Prasertsan, P., He, Z. 2013. Hydrogen production in microbial electrolysis cells: choice of catholyte. Int. J. Hydrogen Energy, 38, 9619-9624. 19. Zhang, F., Ge, Z., Grimaud, J., Hurst, J., He, Z. 2013a. Long-term performance of literscale microbial fuel cells installed in a municipal wastewater treatment facility. Environ. Sci. Technol., 47, 4941-4948. 20. Zhang, Y., Desmidt, E., Van Looveren, A., Pinoy, L., Meesschaert, B., Van der Bruggen, B. 2013b. Phosphate separation and recovery from wastewater by novel electrodialysis. Environ Sci Technol, 47, 5888-95.
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Figure captions Figure 1. The Schematic of the R2-BES. Figure 2. Comparison between the closed- and the open-circuit conditions: (A) current generation and voltage; and (B) the concentrations of ammonium nitrogen and phosphate in the anode influent and effluent under those two conditions. Figure 3. The effects of the applied voltage on the R2-BES: (A) current generation and COD removal; and (B) the removal of ammonium nitrogen and phosphate.
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Highlights • A new bioelectrochemical system (BES) is developed for nutrient removal/recovery; • The key factor for this BES is the use of both cation and anion exchange membranes; • Nitrogen is removed driven by current generation and may be recovered as ammonia; • Phosphorus is removed through ion exchange with hydroxyl.
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S0960-8524(14)00823-2 http://dx.doi.org/10.1016/j.biortech.2014.05.105 BITE 13513
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
12 April 2014 24 May 2014 27 May 2014
Please cite this article as: Zhang, F., Li, J., He, Z., A New Method for Nutrients Removal and Recovery from Wastewater Using a Bioelectrochemical System, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/ j.biortech.2014.05.105
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A New Method for Nutrients Removal and Recovery from Wastewater Using a Bioelectrochemical System
Fei Zhang, Jian Li and Zhen He * Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
Intended for Bioresource Technology Type of contribution: Short Communication
*
Corresponding author. Phone: (540) 231-1346; fax: (540) 231-7916; e-mail: [email protected] (Z.
He)
Abstract Nutrients management is a key task of wastewater treatment and removal of nutrients is usually associated with significant energy/economic cost. A new bioelectrochemical system, named “R2BES”, was developed for removing and possibly recovering nutrients from wastewater. This R2BES takes advantage of bioelectricity generation from oxidation of organic compounds to drive ammonium migration out of wastewater, and uses hydroxide ions produced from the cathode reaction as a medium to exchange phosphate ions from wastewater at the same time. Under an applied voltage of 0.8 V, the R2-BES removed 83.4 ± 1.3% of ammonium nitrogen and 52.4±9.8% of phosphate, significantly higher than those (3.6±3.7% and 21.1±2.6%) under an open circuit condition. Applying an external voltage can increase current generation, COD removal, and nutrient removal. Those results demonstrate a proof of concept that the R2-BES may be potentially applied to remove and recover nutrients through appropriate integration into the existing treatment facilities.
Keywords: Nutrients Removal and Recovery; Nitrogen and Phosphorus; Bioelectrochemical system; Microbial fuel cells; Wastewater Treatment
1
1. Introduction It is increasingly recognized that wastewater should be treated in a sustainable way for minimizing contamination and maximizing the recovery of valuable resources such as energy, water and nutrients (Verstraete et al., 2009). Nutrient removal and recovery is of particular interest, because of potential threat of nutrient compounds to natural water bodies, the limited mining resource and high cost associated with nutrient production (Rittmann et al., 2011). In addition to nutrient removal by conventional biological treatment (nitrification, denitrification and ANAMMOX) and ammonia stripping, various biochemical and chemical methods were investigated and applied for nutrient recovery. For example, microalgae can uptake both nitrogen and phosphorus with generation of biomass that can be further utilized for production of energy or value-added compounds; however, separation of algae from the treated wastewater represents a significant challenge (Cai et al., 2013). Chemical precipitation such as forming struvite can recover both nitrogen and phosphorus (Corre et al., 2009), but the cost of struvite production is high and struvite is not widely used in agriculture (Hao et al., 2013). Membrane-based technologies exhibit great promise for wastewater treatment. The studies show that phosphorus can be recovered using electrodialysis or forward osmosis/membrane distillation (Xie et al., 2014; Zhang et al., 2013b). In general, nutrient removal and recovery from wastewater requires costeffective treatment technologies (Lee et al., 2013), and it will be advantageous to address multiple factors (e.g., organics, nitrogen, and phosphorus) in a single system.
Bioelectrochemical systems (BES) are an emerging treatment technology based on microbial interaction with solid electron acceptors/donors, and the representative BES include microbial fuel cells (MFCs), microbial electrolysis cells (MECs), and microbial desalination cells (MDCs). 2
While energy recovery from organic compounds is a focus in BES research (Ge et al., 2014), nutrient removal and recovery has also gained much attention (Kelly and He, 2014). Nitrogen is removed in a BES through bioelectrochemical denitrification (sometimes coupled with nitrification), or recovered via ammonium migration driven by electricity generation (Clauwaert et al., 2007; Kuntke et al., 2012; Virdis et al., 2008). Phosphorus removal and recovery in a BES usually relies on the high-pH condition as a result of cathode reduction reaction to precipitate phosphorus compounds (Ichihashi and Hirooka, 2012). BES may provide an energy-efficient approach for nutrient removal and recovery by using bioenergy produced from organic compounds in wastewater, but there are also some challenges such as lack of systems for simultaneously removing/recovering both nitrogen and phosphorus, and systematic demonstration of nutrient removal and recovery (more discussion can refer to a recent review article (Kelly and He, 2014)).
Intrigued by the findings that ammonium ions can be separated from wastewater driven by electron flow in a BES (Cord-Ruwisch et al., 2011), a new method was developed and investigated in this study for removal of both ammonium and phosphate ions from synthetic wastewater, named “R2-BES”. The key factor is the use of both cation and anion exchange membranes in a BES, which allow the migration of both cations and anions at the same time, though in different mechanisms (Fig. 1). This method is different from nutrient removal by ion exchange methods that require additional ion input into wastewater.
2. Materials and Methods 2.1 BES Setup
3
A liter-scale R2-BES was built based on a rectangular MFC in a glass tank, which functioned as an anode compartment with a liquid volume of ~ 8.5 L. Two 1-m long carbon brushes were folded to fit into the anode compartment as the anode electrode. A membrane “pocket” was made of a piece of cation exchange membrane (CEM) and a piece of anion exchange membrane (AEM, Membranes International, Inc., Glen Rock, NJ, USA), each of which had a size of 20 cm × 30 cm, with three sides sealed and the top side open to the air. A piece of carbon cloth (20 cm × 50 cm) was folded to fit into the membrane pocket as the cathode electrode, and coated with 0.5 mg cm-2 of activated carbon powder as a cathode catalyst as previously described (Zhang et al., 2013a). This membrane pocket was inserted into the glass tank and acted as a cathode compartment with a liquid volume of about 300 mL.
2.2 BES Operation The R2-BES was operated in a continuously-fed mode at room temperature (~ 20 ºC). The anode compartment was inoculated with the anaerobic sludge from a municipal wastewater treatment plant (Peppers Ferry Regional Wastewater Treatment Plant, Radford, VA). The synthetic anode solution was prepared as (per L of tap water): glucose, 0.2 g; NH4Cl, 0.11 g; NaCl, 0.1 g; MgSO4, 0.003 g; CaCl2, 0.004 g; NaHCO3, 0.02 g; NaOH, 0.1 g; K2HPO4, 0.028 g; and trace elements, 0.4 mL (Angenent and Sung, 2001). The conductivity of the synthetic solution was about 1.13 mS cm-1. The anode solution was fed at 4.51 mL min-1, resulting in a hydraulic retention time (HRT) of 31.4 h. The anolyte was internally mixed by a submerging pump at 9.5 L min-1. The initial catholyte was tap water, and 10-mL tap water was added to compensate for evaporation on daily basis. No active aeration was provided to the cathode compartment. When the external voltage was applied (0.4 or 0.8 V), a power supply was connected to the BES circuit according to
4
a previous study (Yossan et al., 2013).
2.3 Measurement and Analysis The voltage across an external resistor (1 Ω) was recorded every 5 min by a digital multimeter (2700, Keithley Instruments Inc., Cleveland, OH, USA). The pH was measured using a benchtop pH meter (Oakton Instruments, Vernon Hills, IL, USA). The conductivity was measured by a benchtop conductivity meter (Mettler-Toledo, Columbus, OH, USA). The concentrations of chemical oxygen demand (COD), nitrogen (ammonium, nitrate and nitrite), and phosphate were measured using a colorimeter according to the instructions of the manufacturer (Hach Company, Loveland, CO, USA). Coulombic efficiency (CE) was calculated according to the previous work (Logan et al., 2006). The energy consumption by nitrogen removal (kWh kg-1) under an applied voltage was estimated by calculating total energy input (integrating power with time, kWh) within time t, which was then divided by the amount of removed nitrogen within time t (kg). An abiotic test (containing no electrodes) was conducted for examining ion exchange in an H-type MFC similar to that in a previous study (Yossan et al., 2013), containing AEM and with controlled electrolyte pH.
3. Results and Discussion The effectiveness of nutrient removal in the R2-BES was demonstrated through comparing its performance under a closed-circuit condition and an open circuit condition, as shown in Fig.2. With current generation of 68.7 ± 4.6 mA under 0.8 V applied (Fig. 2A), the R2-BES decreased the NH4+-N concentration in the anolyte from 28.5 ± 1.3 (influent) to 4.7 ± 0.5 mg L-1 (effluent) (Fig. 2B), representing 83.4 ± 1.3% reduction. The concentration of phosphate (PO43--P) was
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reduced by 52.4 ± 9.8%, decreasing from 5.3 ± 0.2 to 2.5 ± 0.4 mg L-1. For comparison, the R2BES under the open-circuit condition (~ 0.63 V, Fig. 2A) removed 3.6 ± 3.7% of ammonium nitrogen and 21.1 ± 2.6% of phosphate. As a result of ion migration from the anode compartment into the cathode compartment, the catholyte was expected to concentrate those ions. Under the closed-circuit condition, the concentrations of ammonium nitrogen and phosphate in the catholyte reached 350.0 ± 57.7 mg L-1 and 302.5 ± 29.0 mg L-1, respectively, which could be possibly recovered as either ammonia gas and phosphate precipitates by taking advantage of the high pH of the catholyte (13.1 ± 0.2), or struvite with addition of magnesium salts. The present study focused on the nutrient removal, and the recovery will be investigated in future work to reveal the appropriate recovering method and the recovered products. Nitrate and nitrite was detected at the concentration of 20-30 mg L-1 in the catholyte, indicating the occurrence of nitrification, likely due to the contamination of autotrophic microorganisms with an open cathode compartment.
The comparison between the closed- and the open-circuit conditions shows the key role of electrical current in nutrient removal. There could be two mechanisms of ion movement in the R2-BES, both of which are affected by current generation: (1) unidirectional movement is driven by current generation and ions move from the anolyte to the catholyte, which should be the major mechanism for ammonium removal under the closed-circuit condition; and (2) bidirectional movement is via ion exchange, and the exchange between hydroxide ions (a product of current generation in the catholyte) and phosphate ions (anolyte) could be the mechanism for phosphorus removal. The ion exchange membranes used in this study are not selective for ammonium nitrogen or phosphate ions; therefore, other ions would also migrate and
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compete with nitrogen and phosphate ions. It was found that the conductivity of the anolyte decreased from 1.13 ± 0.05 to 0.51 ± 0.06 mS cm-1 under the closed-circuit condition, while there was almost no change in the anolyte conductivity under the open-circuit condition. The R2BES achieved a Coulombic efficiency of 38.2 ± 3.8%. Assuming one electron moves one ammonium ion and according to the calculation of charge transfer efficiency in a previous study (Jacobson et al., 2011), migration of ammonium ions could contribute to 18.0 ± 1.7% of current generation, indicating that movement of other cations was largely involved in current generation and part of current generation was related to hydroxyl movement from the cathode into the anode through AEM.
Likewise, competition among anions during ion exchange with hydroxide ions was also expected, and phosphate ions would be outcompeted by other dominant anions such as chloride ions due to a smaller charge affinity and a larger hydration diameter. The abiotic tests of ion exchange between phosphate and hydroxide ions revealed that more phosphate ions could be exchanged at a higher catholyte pH (32.7% removal at pH 13.0 vs. 23.4% at pH 6.0), and the presence of chloride ions significant decreased phosphate removal (18.8% removal at pH 13.0). The ion exchange brought hydroxide ions into the anolyte (wastewater), which could provide alkalinity and buffer pH decrease during the anaerobic oxidation of organic compounds. In the R2-BES under a 0.8-V applied voltage, the exchange of phosphate ions could add alkalinity of 4.5 ± 0.9 mg L-1 as CaCO3. The actual alkalinity addition should be more than that value, because phosphate ions were at a lower concentration than other anions in the anode feeding solution, and electric current would also promote the unidirectional movement of hydroxide ions into the anolyte through AEM. The alkalinity addition and its buffering effects are of great interest and
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will be investigated in more details in future studies.
To further understand the effects of externally applied voltage on the R2-BES, the system was operated under three voltages, 0, 0.4 and 0.8 V. There was significant difference in current generation: the 0.8-V condition produced a current of 68.7 ± 4.6 mA, three times that of the system without any external voltage (22.4 ± 0.5 mA at 0 V). As a result of the high current generation at 0.8 V, the R2-BES removed 94.4 ± 0.8% of COD, while the low current generation at 0 V led to the lowest COD removal of 79.0 ± 2.6% (Fig. 3A). Nutrient removal was also improved with the applied voltage, but nitrogen and phosphorus exhibited different trends likely due to the different removal mechanisms (Fig. 3B). Ammonium migration depends on electron flow, and thus its removal shows an almost linear relationship with current generation, varying from 20.3 ± 0.5% at 0 V to 83.4 ± 1.3% at 0.8 V. Phosphate movement relies on the gradient of anions across the AEM and migration of competitive anions. The 0.8 V condition still had the highest phosphate removal of 52.4 ± 9.8%, while the other two conditions had insignificant difference in their removal (34.7 ± 4.9% at 0.4 V and 30.8 ± 4.4% at 0 V). At this moment, it is not clearly understood how phosphate movement is affected by the competition and anion gradient; the pH gradient is clearly important but its effect may be weakened by the presence of other major factors (e.g., anion competition and unidirectional anion movement from the cathode into the anode): the anolyte pHs under those three applied voltages were similar at ~ 6.4, while the catholyte pHs of the 0.4- and 0.8-V conditions were similar at 13.1 and the one at 0 V was lower at 11.8.
Although applying an external voltage generally improves the performance of the R2-BES and
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further increasing the external voltage (>0.8 V) may further increase the efficiency because of higher current generation, one must also note the additional energy input with this approach (beyond energy consumption by regular operation such as pumping systems). For example, at 0.8 and 0.4 V, the additional energy input for nitrogen removal is 8.58 ± 1.02 and 3.41 ± 0.39 kWh kg N-1, respectively. Assuming a wholesale electricity price of $0.07 kWh-1, the cost of this energy input is about $0.23-0.60 kg N-1. In addition, a high external voltage (>1.2 V) may cause water electrolysis: despite high current generation under that condition, the use of BES method for nutrient removal and recovery may lose its significance (which lies in the use of electrons from organic wastes, instead of water oxidation). The improved performance with additional energy input may compensate for the increased economic cost, but this needs further analysis. The detailed economic analysis is not feasible at the current scale of the system, and comparison with the existing technologies will require further development of the system to a reasonable scale (to avoid any overpromises from bench-scale systems).
The present R2-BES has accomplished nutrient removal to a certain degree, and the nutrient concentrations need to be further reduced to meet the discharge limit. The performance may be improved through adding more cathode units/membrane areas. In the present R2-BES, the size of the anode volume is nearly 30 times that of the cathode volume, and thus there is room for more cathode units. The unidirectional movement of hydroxide ions from the catholyte into the anolyte driven by electric current could slow down phosphate migration (in the opposite direction); future R2-BES design may consider an additional AEM compartment for ion exchange only, with supply of the high-pH catholyte from the CEM compartment that has electrochemical activities. Identifying appropriate application niche can accelerate the R2-BES development. For example,
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installing the R2-BES in the anoxic zone will take advantage of the existing facilities and have it as pretreatment of the aerobic process, thereby reducing the need for nutrient removal after aerobic treatment. The R2-BES can also be linked to anaerobic digestion for side stream treatment to polish anaerobic digestion effluent by reducing organic contents and decreasing the concentrations of both ammonia and phosphorus (and recovering the concentrated nutrients).
Future work will investigate the R2-BES from several aspects: (1) detailed balances of major cations and anions will be established to understand the competition of ion movement; (2) recovery of the concentrated ions in the catholyte will be studied; (3) the influence of alkalinity addition with hydroxyl on wastewater treatment will be further examined; (4) the function of the R2-BES may be expanded to removal of heavy metals; and (5) the stability of the system will be investigated through long-term operation with examining main influence factors such as HRT, loading rates, and membrane performance.
4. Conclusions This study has demonstrated a proof-of-concept bioelectrochemical system for simultaneous removal of nitrogen and phosphorus from synthetic wastewater. It was found that current generation played a central role in the removal process, and higher current generation with externally applied voltage would improve the removal efficiency. Nitrogen was removed by direct current-driven ammonium migration, and phosphorus was removed via ion exchange with hydroxyl that was generated as a result of the cathode reaction. With further understanding of ion transport/competition and improvement of system configuration/operation, this system may provide a new approach for energy-efficient removal and recovery of nutrients from wastewater.
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Acknowledgements This work was financially supported by a grant from Gannett Fleming, Inc. The authors would like to thank Peppers Ferry Regional Wastewater Treatment Authority for providing wastewater samples.
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Figure captions Figure 1. The Schematic of the R2-BES. Figure 2. Comparison between the closed- and the open-circuit conditions: (A) current generation and voltage; and (B) the concentrations of ammonium nitrogen and phosphate in the anode influent and effluent under those two conditions. Figure 3. The effects of the applied voltage on the R2-BES: (A) current generation and COD removal; and (B) the removal of ammonium nitrogen and phosphate.
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Highlights • A new bioelectrochemical system (BES) is developed for nutrient removal/recovery; • The key factor for this BES is the use of both cation and anion exchange membranes; • Nitrogen is removed driven by current generation and may be recovered as ammonia; • Phosphorus is removed through ion exchange with hydroxyl.
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