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Simultaneous removal of phosphorus and potassium from synthetic urine through the precipitation of magnesium potassium phosphate hexahydrate
Chemosphere 84 (2011) 207–212
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Simultaneous removal of phosphorus and potassium from synthetic urine through the precipitation of magnesium potassium phosphate hexahydrate Kangning Xu a, Chengwen Wang a,⇑, Haiyan Liu b, Yi Qian a a b
School of Environment, Tsinghua University, Beijing 100084, PR China School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 4 February 2011 Received in revised form 20 April 2011 Accepted 20 April 2011 Available online 18 May 2011 Keywords: Urine Phosphorus Potassium Struvite Magnesium potassium phosphate hexahydrate
a b s t r a c t This study investigated the simultaneous removal of P and K from synthetic urine through the precipitation of magnesium potassium phosphate hexahydrate (MPP, MgKPO46H2O) in bench-scale experiments. Results show that the removal efficiencies of P and K are mainly determined by the solution pH and the molar ratio of Mg:K:P. Co-precipitation of struvite-type compounds, i.e., magnesium ammonium phosphate hexahydrate (MAP, MgNH4PO46H2O), magnesium sodium phosphate heptahydrate (MSP, MgNaPO47H2O), and MPP, was confirmed by analysis of the solid precipitates using a Scanning Electron Microscope/Energy Dispersive X-ray Apparatus and an X-ray Diffractometer. The co-precipitation significantly influenced the removal of K. As much ammonium as possible should be removed prior to MPP precipitation because MAP had higher tendency to form than MPP. The inevitable co-precipitation of MPP and MSP resulted in the addition of more MgCl26H2O and Na2HPO412H2O to obtain the high removal of K. In total, the removal efficiencies of P and K were 77% and 98%, respectively, in the absence of ammonium when pH was 10 and the molar ratio of Mg:K:P was 2:1:2. The results indicate that the MPP precipitation is an efficient method for the simultaneous removal of P and K to yield multi-nutrient products. Ó 2011 Published by Elsevier Ltd.
1. Introduction Urine source-separation has been considered to be a sustainable alternative for the traditional sewerage system (Maurer et al., 2006). According to previous studies (Larsen and Gujer, 1996; Fittschen and Hahn, 1998; Larsen et al., 2001; Otterpohl, 2002; Rauch et al., 2003; Ronteltap et al., 2007), human urine contributes about 75–87% of the total N, 40–50% of the P, and 54–90% of the K to municipal wastewater with only 1% of raw sewerage volume (Larsen et al., 2001; Vinneras and Jonsson, 2002). Thus, urine has high concentrations of nutrients, including 8028 ± 1298 mg N L1, 631 ± 214 mg P L1 and 2196 ± 343 mg K L1 (Fittschen and Hahn, 1998; Udert et al., 2006). Much attention has been paid to yielding phosphorus products from wastewater in recent studies (Maurer et al., 2006; Le Corre et al., 2009; Uysal et al., 2010). New sources of K are also urgently needed, especially in agricultural countries, such as China, that rely heavily on imported K fertilizer (Sun et al., 2009; Ma et al., 2010). Urine source-separation is, therefore, an attractive method for both pollutant removal and multi-nutrient products yielding (Maurer et al., 2006; Wilsenach et al., 2007; Zhao et al., 2007; Le Corre et al., 2009). ⇑ Corresponding author at: Room 735, School of Environment, Tsinghua University, Beijing 100084, PR China Tel.: +86 10 62771551; fax: +86 10 62788148. E-mail address: [email protected] (C. Wang). 0045-6535/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.chemosphere.2011.04.057
Urea is the main form of N in fresh urine. Under the catalytic effects of urease, urea can be hydrolyzed to ammonium and bicarbonate (Hotta and Funamizu, 2008). Several techniques, including precipitation of magnesium ammonium phosphate hexahydrate (MAP, a struvite-type compound) (Maurer et al., 2006; Ronteltap et al., 2007; Wilsenach et al., 2007; Tilley et al., 2008), nitritation–Anammox (Udert et al., 2003; Wilsenach, 2006; Udert et al., 2008), complete autotrophic nitrogen removal (Wilsenach, 2006) and stripping–absorption (Basakcilardan-Kabakci et al., 2007), have been developed to remove N from urine. Nitrification (Feng et al., 2008) has also been used to oxidize ammonium to nitrate for urine stabilization. After the removal or oxidation of ammonium, high P and K concentrations remain in the urine. Precipitation of magnesium potassium phosphate hexahydrate (MPP, MgKPO46H2O), another struvite-type compound, could occur and simultaneously remove P and K (Wilsenach et al., 2007). This precipitation process has been used to yield multi-nutrient products from seawater bittern, and these products have been considered to be efficient fertilizers (Lozano et al., 1999; Lozano and Sanvicente, 2002). Wilsenach et al. (2007) studied the conditions necessary to form MPP in synthetic urine, and demonstrated that high P removal could be achieved through MPP precipitation. To date, however, the removal efficiency of K under identical parameter has not been reported.
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The objective of this study is to optimize the parameters for the simultaneous removal of P and K from synthetic urine through MPP precipitation. By conducting bench-scale experiments, the factors that influence the removal efficiencies of P and K, including pH, molar ratio of Mg:K:P, and ammonium concentration were investigated. The effect of magnesium sodium phosphate heptahydrate (MSP, MgNaPO47H2O) on MPP precipitation was also examined. The results obtained could provide a basis for selecting optimal conditions with which to simultaneously remove K and P to yield multi-nutrient products from nutrient-abundant solutions such as urine.
Table 2 Experimental conditions of MPP precipitation for the simultaneous removal of P and K.
2. Material and methods 2.1. Synthetic urine
Serial
pH
Molar ratio of Mg:K:Pa
Ammonium (mg N L1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
7 8 9 10 11 12 10 10 10 10 10 10 10 10 10 10
1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.1:1:1.1 1.6:1:2 2:1:1.6 2:1:2 2:1:2.5 2.5:1:2 2:1:2 2:1:2 2:1:2 2:1:2
0 0 0 0 0 0 0 0 0 0 0 0 40 100 300 500
A typical recipe for synthetic urine (Griffith et al., 1976) has been used in previous studies (Wilsenach et al., 2007; Tilley et al., 2008). In this recipe, the following assumptions are made: urea is hydrolyzed, and ammonium and carbonate are completely removed in advance without losing other substrates. The characteristics of synthetic urine used in this study are shown in Table 1.
Ammonium concentrations were analyzed before and after the experiments.
2.2. Experimental procedures
2.3. Analytical methods
Three factors that affect P and K removal were studied in a batch reactor: pH, molar ratio of Mg:K:P, and ammonium concentration. The detailed precipitation parameters are listed in Table 2. The molar ratio of Mg:K:P in synthetic urine was calculated as 0.06:1:0.59. Based on the stoichiometric molar ratio (1:1:1) of MPP precipitation, Mg and P sources should be added to yield high removal efficiencies of nutrients, especially K. The experiments were conducted in 500 mL beakers in duplicates. An impeller was submerged into the middle of the reaction zone for stirring and controlled at 200 rpm by a variable speed motor. A Mg source (MgCl26H2O) and a phosphate source (Na2HPO412H2O) in solid phase were added to the beaker to adjust the molar ratio of Mg:K:P. Then, 300 mL synthetic urine was added to the beaker with stirring. The solution pH was adjusted with 1 M NaOH. After reacting for 20 min, stirring was ceased to allow the precipitates to settle. After a sedimentation time of 20 min, the supernatant was collected for analysis. To test the effect of ammonium concentration on the removal of P and K, synthetic urine was prepared by adding NH4Cl. In this way, the experiment simulated the technique where urine is pretreated for ammonium removal as described earlier. In all cases, small amounts of ammonium remained in solution, and these affect the P and K removal. To test the volatilization of free ammonia during the experiments, a similar sample of synthetic urine was prepared without any MgCl2. The solutions were then stirred at 200 rpm for 20 min and allowed to settle for 20 min at pH 10.
A handheld pH meter (Hach Sension 1, USA) was used to measure the solution pH. All solution samples were filtered through 0.45 lm membranes before analysis. Ammonium and phosphate concentrations were analyzed colorimetrically with a spectrophotometer (Hach DR 5000, USA) according to standard methods (Ministry of Environmental Protection, 2002). The concentrations of K, Na, Ca, and Mg were measured with an Inductively Coupled Plasma- Atomic Emission Spectroscope (Thermo, USA). Precipitates were first collected through filtration using 0.45 lm filters, and then dried in an oven at 50 °C for about 48 h. The crystalline forms were assayed with an X-ray Diffractometer (XRD, Rigaku TTR-III, Japan). The morphology and chemical composition of the precipitates were obtained on a Scanning Electron Microscopy–Energy Dispersive X-ray Apparatus (SEM-EDX, Cambridge S-360, UK).The removal efficiency of nutrient elements (RE, E = P or K) was determined as the quotient of the element in the precipitates divided by the total element in synthetic urine:
Table 1 Characteristics of synthetic urine.a
a
Composition
g L1
mM
CaCl22H2O MgCl26H2O NaCl Na2SO4 Na3C6H8O72H2O Na2C2O4 KH2PO4 KCl C4H7N3O
0.65 0.65 4.60 2.30 0.65 0.02 4.20 1.60 1.10
4.4 3.2 78.7 16.2 2.6 0.15 30.9 21.5 9.7
pH = 4.2.
a
Initial molar ratio at neutral pH.
RE ¼ 1
Es ; Ei þ Ea
ð1Þ
1
where Es (mg L ) is the concentration of P or K in the supernatant, Ei (mg L1) is the initial concentration in synthetic urine, and Ea (mg L1) is the concentration added to synthetic urine. The volatilized ammonium was taken into account for N removal. Thus, the removal efficiency of N (RN) was calculated as:
RN ¼ 1
Ns þ Nv ; Ni
ð2Þ
where Ns (mg L1) is the ammonium concentration in the supernatant, Nv (mg L1) is the ammonium concentration volatilized during the experiments, and Ni (mg L1) is the initial ammonium concentration in the synthetic urine. 3. Results and discussion 3.1. Effect of pH Consistent with most previous studies, pH in a range of 5.5–11 played an important role in struvite precipitation (Gunay et al., 2008; Le Corre et al., 2009; Di Iaconi et al., 2010). The optimum pH was determined when the molar ratio of Mg:K:P was
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209
3.2. Effect of molar ratio of Mg:K:P
Fig. 1. Effect of pH on the removal of K (j) and P (h) from synthetic urine through the precipitation of MPP. The error bars represent the standard deviation of parallel samples.
1.6:1:1.6. Higher pH value significantly improved the removal of P and K, although the removal efficiency of K decreased at pH 12 (Fig. 1). This was mainly due to the dissociation of H2 PO 4 and 3 3 HPO2 4 to PO4 at higher pH. Higher PO4 concentrations enhanced the precipitation of MPP. The optimum pH for the removal of P and K was 11, but the removal efficiency of P and K was only slightly higher at pH 11 than at pH 10 by 0.8% and 0.6%, respectively. Furthermore, less NaOH was necessary to adjust the pH to 10 instead of 11. Thus, the pH was set to 10 for the succeeding experiments detailed in this study. Increasing the pH above 11 did not show further improvements in the removal of P, whereas it decreased the removal of K. This may be caused by the formation of other precipitates. For example, the precipitation of Mg(OH)2 at pH 11–12 may lead to stronger competition for OH with PO3 4 (Warmadewanthi and Liu, 2009). With increasing pH, Mg3(PO4)2, rather than struvite, also precipitates out (Lee et al., 2003; Zhang et al., 2009).
Experiments were conducted at pH 10 to determine the effects of Mg:K and P:K ratios on the removal of K and P. The removal of K was found to improve significantly with increasing molar ratios of Mg:K and P:K when the Mg:P ratio was fixed at 1:1 (Fig. 2a). Theoretically, 100% K should be removed when the molar ratio of Mg:K:P in the solution is equal to the stoichiometric value. However, the removal efficiency of K was only 48% when the Mg:K:P ratio was 1.1:1:1.1. While the removal efficiency of K increased to 77% when the Mg:K:P ratio was 2:1:2, it remained lower than the theoretical value. When the molar ratio of Mg:P was kept at 1:1, the removal efficiency of P was about 98%; this value did not change significantly with increasing Mg:K ratio. The effect of Mg:K ratio was determined at a fixed P:K ratio of 2:1 (Fig. 2b). Higher Mg:K ratios improved the removal of P because most of the phosphate could be precipitated under such a
Fig. 3. Effect of ammonium concentration on the removal of K (j), N (N), and P (h). The error bars represent the standard deviation of parallel samples.
(a)
(b)
(c)
(d)
Fig. 2. Effect of the Mg:K:P molar ratio on the removal of K (j) and P (h), and the concentration of residual phosphate from synthetic urine through the precipitation of MPP: (a) Mg:P ratio = 1:1, (b) P:K ratio = 2:1, (c) Mg:K ratio = 2:1, and (d) residual P concentration. The error bars represent the standard deviation of parallel samples.
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condition (Song et al., 2007). The removal efficiency of K first increased, and then decreased with increasing Mg:K ratio. This indicates the existence of an optimum Mg:K ratio for the removal of K. The decrease in K removal may be caused by the precipitation of Mg3(PO4)2 because the precipitation potential of this compound would also be enhanced by the addition of more Mg source. The effect of P:K ratio was determined at a fixed Mg:K ratio of 2:1 (Fig. 2c). Higher P:K ratios increased the removal of K but decreased the removal of P. The residual P was 62 mg L1 when the Mg:K:P ratio was 2:1:2; it increased to 797 mg L1 at a Mg:K:P ratio of 2:1:2.5 (Fig. 2d). More residual phosphate was detected in the synthetic urine solution at a Mg:K:P ratio of 1.6:1:2 than at 1.6:1:1.6. Thus, overdosing with the P source is unfavorable from the perspective of yielding nutrient products from urine. 3.3. Effect of ammonium
Fig. 4. Morphology and chemical composition of solid precipitates produced at pH 10 and a Mg:K:P molar ratio of 2:1:2 as analyzed via SEM-EDX.
During determination of the optimum values of pH and molar ratio of Mg:K:P for the removal of P and K, all the ammonium in synthetic urine was supposed to be removed in advance. However, a certain amount of ammonium always remains in urine. The removal efficiency of ammonium was reported to be 99% by stripping (Basakcilardan-Kabakci et al., 2007) and 95% by nitrification (Feng et al., 2008). The residual ammonium may interfere with the removal of K and P due its promotion of MAP precipitation. Thus, batch tests were conducted with the ammonium concentration ranging from 40 to 500 mg N L1 at a Mg:K:P molar ratio of 2:1:2 and pH 10. Ammonium in synthetic urine significantly reduced the removal of K (Fig. 3). The removal efficiency of K decreased from 77% to 65% as the ammonium concentration increased from 0 to 500 mg N L1. In contrary, the removal efficiency of P remained at about 97% with slight variations. Ammonium was also removed by MAP precipitation; 97% ammonium was removed when the
(a)
(d)
(b)
(e)
(c)
Fig. 5. XRD patterns of the standard crystals and precipitates from synthetic urine: (a) MgKPO46H2O [Powder Diffraction File (PDF) No. 35-0812], (b) MgNaPO47H2O (PDF No. 75-2269), (c) MgNH4PO46H2O (PDF No. 15-0762), (d) the precipitate formed at Mg:K:P = 2:1:2 and pH = 10, and (e) the precipitate formed at Mg:K:P = 2:1:2, pH = 10, and 500 mg NH4–N L1.
K. Xu et al. / Chemosphere 84 (2011) 207–212
ammonium concentration was 40 mg N L1. However, the removal efficiency of N decreased to 73% when ammonium concentration increased to 500 mg N L1. The removal of both K and ammonium indicates that MAP and MPP co-precipitated in the synthetic urine. 3.4. Co-precipitation of struvite-type compounds MAP and MSP are both isomorphous analogues of MPP (Stefov et al., 2004). These struvite-type compounds have been reported to have a general formula of M2+N+XO4nH2O, where M@Mg or Ca, N@K, Rb, Cs, Tl, Na or NH4, X@P or As, and n = 6–8 (Dickens and Brown, 1972; Banks et al., 1975; Mathew et al., 1982). Besides the principal elements of MPP, i.e. Mg, K, P and O, abundant Na was detected in the solid precipitates (Fig. 4). This indicates the co-precipitation of MPP and MSP. The simultaneous removal of both ammonium and K in the presence of ammonium in synthetic urine also indicates that MPP and MAP co-precipitated. In addition, needle-like crystals were observed, similar in shape to the MAP crystal reported (Song et al., 2007; Zhao et al., 2007). The crystal forms of the precipitates were confirmed by XRDanalysis (Fig. 5). The precipitates shown in Fig. 5d and e were respectively produced in the absence and presence of ammonium in synthetic urine. Both precipitates showed similar XRD patterns with nearly all of the characteristic peaks of the standard crystals (Fig. 5a–c). Thus, the co-precipitation of MPP and MSP in the absence of ammonium, and the co-precipitation of MAP, MPP, and MSP in the presence of ammonium in synthetic urine were confirmed. The pKsp (minus denary logarithm of solubility product constant) of MAP is reported to range from 12.60 to 13.36 (Ronteltap et al., 2007) while the pKsp of MPP is 10.62 (Taylor et al., 1963). MAP has a higher tendency to form than MPP because the minimal solubility of struvite crystal produces the maximal potential for its formation (Gunay et al., 2008). This hypothesis could also be speculated from reports showing that K and Na are scarcely detected in most studies on MAP precipitation (Zhao et al., 2007; Tilley et al., 2008). Therefore, in order to simultaneously remove P and K, the ammonium in urine should be removed prior to the precipitation of MPP. In the absence of ammonium in synthetic urine, co-precipitation of MPP and MSP reduced the theoretical removal efficiency of K. The tendencies of MPP and MSP to precipitate could not be directly compared due to the absence of the solubility product constant of MSP. In this study, Na originated mainly from the sodium salts used in synthetic urine, Na2HPO412H2O as the P source, and NaOH employed for pH adjustments. The molar ratio of Na:K was calculated as 7.5 when the solution pH was 10 and the molar ratio of Mg:K:P was 2:1:2 in the batch tests. MPP precipitation removed about 39% of the total Mg2+ and 39% of the total phosphate in solution, while MSP precipitation removed no more than 61% of the total Mg2+ and 61% of the total phosphate. This indicates that MPP has a higher tendency to form than MSP in synthetic urine. The co-precipitation of MPP and MSP resulted in the addition of more sources of Mg and P to obtain high K removal. This finding may be a bottleneck for future applications of MPP precipitation to remove of K from urine. 4. Conclusions The results of this study showed that the removal efficiencies of P and K in the absence of ammonium when pH was 10 and the molar ratio of Mg:K:P was 2:1:2 were 77% and 98%, respectively. The pH and the molar ratio of Mg:K:P are the key factors for the simultaneous removal of P and K. MPP and MSP co-precipitated in synthetic urine in the absence of ammonium. Thus, addition of MgCl26H2O and Na2HPO412H2O
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was necessary to adjust the molar ratio of Mg:K:P to obtain high K removal. MPP, MAP, and MSP co-precipitated in synthetic urine in the presence of ammonium. MAP has a higher tendency to form than MPP. A small quantity of residual ammonium in synthetic urine significantly reduced the removal of K. Thus, as much ammonium as possible should be removed from solution prior to MPP precipitation. Acknowledgements This study was financially supported by the Ministry of Science and Technology of the People´s Republic of China (Nos. 2007BAK12B11 and 2008BAJ08B13). We would like to acknowledge Dr. Hongxiang Fu, Dr. Jian Wang and Dr. Yujue Wang for their assistance in writing this paper. References Banks, E., Chianelli, R., Korenstein, R., 1975. Crystal chemistry of struvite analogs of type MgMPO4.6H2O (M+ = K+, Rb+, Cs+, Ti+, NHþ 4 ). Inorg. Chem. 14, 1634–1639. Basakcilardan-Kabakci, S., Ipekoglu, A.N., Talini, I., 2007. Recovery of ammonia from human urine by stripping and absorption. Environ. Eng. Sci. 24, 615– 624. Di Iaconi, C., Pagano, M., Ramadori, R., Lopez, A., 2010. Nitrogen recovery from a stabilized municipal landfill leachate. Bioresour. Technol. 101, 1732–1736. Dickens, B., Brown, W.E., 1972. The crystal structure of CaKAsO48H2O. Acta Crystallogr. B-Struct. 28, 3056–3065. Feng, D.L., Wu, Z.C., Xu, S.H., 2008. Nitrification of human urine for its stabilization and nutrient recycling. Bioresour. Technol. 99, 6299–6304. Fittschen, I., Hahn, H.H., 1998. Characterization of the municipal wastewater part human urine and a preliminary comparison with liquid cattle excretion. Water Sci. Technol. 38 (6), 9–16. Griffith, D.P., Musher, D.M., Itin, C., 1976. The primary cause of infection induced urinary stones. Invest. Urol. 13, 346–350. Gunay, A., Karadag, D., Tosun, I., Ozturk, M., 2008. Use of magnesit as a magnesium source for ammonium removal from leachate. J. Hazard. Mater. 156, 619– 623. Hotta, S., Funamizu, N., 2008. Evolution of ammonification potential in storage process of urine with fecal contamination. Bioresour. Technol. 99, 13–17. Larsen, T.A., Gujer, W., 1996. Separate management of anthropogenic nutrient solutions (human urine). Water Sci. Technol. 34 (3–4), 87–94. Larsen, T.A., Peters, I., Alder, A., Eggen, R., Maurer, M., Muncke, J., 2001. Reengineering the toilet for sustainable wastewater management. Environ. Sci. Technol. 35, 192A–197A. Le Corre, K.S., Valsami-Jones, E., Hobbs, P., Parsons, S.A., 2009. Phosphorus recovery from wastewater by struvite crystallization: a review. Crit. Rev. Environ. Sci. Technol. 39, 433–477. Lee, S.I., Weon, S.Y., Lee, C.W., Koopman, B., 2003. Removal of nitrogen and phosphate from wastewater by addition of bittern. Chemosphere 51, 265–271. Lozano, J.A.F., Colmenares, A.R., Rosas, D., 1999. A novel process for the production of multinutrient phosphatic base fertilizers from seawater bittern and phosphoric acid. Interciencia 24, 317–320. Lozano, J.A.F., Sanvicente, L., 2002. Multinutrient phosphate-based fertilizers from seawater bitterns. Interciencia 27, 496–499. Ma, H., Su, S., Liu, H., Yang, X., Peng, H., Yu, Z., 2010. Potassium resource and sustainable development of potash salt industry in China. Earth Sci. Front. 17, 294–310 (in Chinese). Mathew, M., Kingsbury, P., Takagi, S., Brown, W.E., 1982. A new struvite-type compound, magnesium sodium phosphate heptahydrate. Acta Crystallogr. BStruct. 38, 40–44. Maurer, M., Pronk, W., Larsen, T.A., 2006. Treatment processes for source-separated urine. Water Res. 40, 3151–3166. Ministry of Environmental Protection, P.R.C., 2002. Monitoring and Analytical Methods of Water and Wastewater. China Environmental Science Press, Beijing (in Chinese). Otterpohl, R., 2002. Options for alternative types of sewerage and treatment systems directed to improvement of the overall performance. Water Sci. Technol. 45 (3), 149–158. Rauch, W., Brockmann, D., Peters, I., Larsen, T.A., Gujer, W., 2003. Combining urine separation with waste design: an analysis using a stochastic model for urine production. Water Res. 37, 681–689. Ronteltap, M., Maurer, M., Gujer, W., 2007. Struvite precipitation thermodynamics in source-separated urine. Water Res. 41, 977–984. Song, Y.H., Yuan, P., Zheng, B.H., Peng, H.F., Yuan, F., Gao, Y., 2007. Nutrients removal and recovery by crystallization of magnesium ammonium phosphate from synthetic swine wastewater. Chemosphere 69, 319–324. Stefov, V., Soptrajanov, B., Spirovski, F., Kuzmanovski, I., Lutz, H.D., Engelen, B., 2004. Infrared and Raman spectra of magnesium ammonium phosphate hexahydrate
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Simultaneous removal of phosphorus and potassium from synthetic urine through the precipitation of magnesium potassium phosphate hexahydrate Kangning Xu a, Chengwen Wang a,⇑, Haiyan Liu b, Yi Qian a a b
School of Environment, Tsinghua University, Beijing 100084, PR China School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 4 February 2011 Received in revised form 20 April 2011 Accepted 20 April 2011 Available online 18 May 2011 Keywords: Urine Phosphorus Potassium Struvite Magnesium potassium phosphate hexahydrate
a b s t r a c t This study investigated the simultaneous removal of P and K from synthetic urine through the precipitation of magnesium potassium phosphate hexahydrate (MPP, MgKPO46H2O) in bench-scale experiments. Results show that the removal efficiencies of P and K are mainly determined by the solution pH and the molar ratio of Mg:K:P. Co-precipitation of struvite-type compounds, i.e., magnesium ammonium phosphate hexahydrate (MAP, MgNH4PO46H2O), magnesium sodium phosphate heptahydrate (MSP, MgNaPO47H2O), and MPP, was confirmed by analysis of the solid precipitates using a Scanning Electron Microscope/Energy Dispersive X-ray Apparatus and an X-ray Diffractometer. The co-precipitation significantly influenced the removal of K. As much ammonium as possible should be removed prior to MPP precipitation because MAP had higher tendency to form than MPP. The inevitable co-precipitation of MPP and MSP resulted in the addition of more MgCl26H2O and Na2HPO412H2O to obtain the high removal of K. In total, the removal efficiencies of P and K were 77% and 98%, respectively, in the absence of ammonium when pH was 10 and the molar ratio of Mg:K:P was 2:1:2. The results indicate that the MPP precipitation is an efficient method for the simultaneous removal of P and K to yield multi-nutrient products. Ó 2011 Published by Elsevier Ltd.
1. Introduction Urine source-separation has been considered to be a sustainable alternative for the traditional sewerage system (Maurer et al., 2006). According to previous studies (Larsen and Gujer, 1996; Fittschen and Hahn, 1998; Larsen et al., 2001; Otterpohl, 2002; Rauch et al., 2003; Ronteltap et al., 2007), human urine contributes about 75–87% of the total N, 40–50% of the P, and 54–90% of the K to municipal wastewater with only 1% of raw sewerage volume (Larsen et al., 2001; Vinneras and Jonsson, 2002). Thus, urine has high concentrations of nutrients, including 8028 ± 1298 mg N L1, 631 ± 214 mg P L1 and 2196 ± 343 mg K L1 (Fittschen and Hahn, 1998; Udert et al., 2006). Much attention has been paid to yielding phosphorus products from wastewater in recent studies (Maurer et al., 2006; Le Corre et al., 2009; Uysal et al., 2010). New sources of K are also urgently needed, especially in agricultural countries, such as China, that rely heavily on imported K fertilizer (Sun et al., 2009; Ma et al., 2010). Urine source-separation is, therefore, an attractive method for both pollutant removal and multi-nutrient products yielding (Maurer et al., 2006; Wilsenach et al., 2007; Zhao et al., 2007; Le Corre et al., 2009). ⇑ Corresponding author at: Room 735, School of Environment, Tsinghua University, Beijing 100084, PR China Tel.: +86 10 62771551; fax: +86 10 62788148. E-mail address: [email protected] (C. Wang). 0045-6535/$ - see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.chemosphere.2011.04.057
Urea is the main form of N in fresh urine. Under the catalytic effects of urease, urea can be hydrolyzed to ammonium and bicarbonate (Hotta and Funamizu, 2008). Several techniques, including precipitation of magnesium ammonium phosphate hexahydrate (MAP, a struvite-type compound) (Maurer et al., 2006; Ronteltap et al., 2007; Wilsenach et al., 2007; Tilley et al., 2008), nitritation–Anammox (Udert et al., 2003; Wilsenach, 2006; Udert et al., 2008), complete autotrophic nitrogen removal (Wilsenach, 2006) and stripping–absorption (Basakcilardan-Kabakci et al., 2007), have been developed to remove N from urine. Nitrification (Feng et al., 2008) has also been used to oxidize ammonium to nitrate for urine stabilization. After the removal or oxidation of ammonium, high P and K concentrations remain in the urine. Precipitation of magnesium potassium phosphate hexahydrate (MPP, MgKPO46H2O), another struvite-type compound, could occur and simultaneously remove P and K (Wilsenach et al., 2007). This precipitation process has been used to yield multi-nutrient products from seawater bittern, and these products have been considered to be efficient fertilizers (Lozano et al., 1999; Lozano and Sanvicente, 2002). Wilsenach et al. (2007) studied the conditions necessary to form MPP in synthetic urine, and demonstrated that high P removal could be achieved through MPP precipitation. To date, however, the removal efficiency of K under identical parameter has not been reported.
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The objective of this study is to optimize the parameters for the simultaneous removal of P and K from synthetic urine through MPP precipitation. By conducting bench-scale experiments, the factors that influence the removal efficiencies of P and K, including pH, molar ratio of Mg:K:P, and ammonium concentration were investigated. The effect of magnesium sodium phosphate heptahydrate (MSP, MgNaPO47H2O) on MPP precipitation was also examined. The results obtained could provide a basis for selecting optimal conditions with which to simultaneously remove K and P to yield multi-nutrient products from nutrient-abundant solutions such as urine.
Table 2 Experimental conditions of MPP precipitation for the simultaneous removal of P and K.
2. Material and methods 2.1. Synthetic urine
Serial
pH
Molar ratio of Mg:K:Pa
Ammonium (mg N L1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
7 8 9 10 11 12 10 10 10 10 10 10 10 10 10 10
1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.6:1:1.6 1.1:1:1.1 1.6:1:2 2:1:1.6 2:1:2 2:1:2.5 2.5:1:2 2:1:2 2:1:2 2:1:2 2:1:2
0 0 0 0 0 0 0 0 0 0 0 0 40 100 300 500
A typical recipe for synthetic urine (Griffith et al., 1976) has been used in previous studies (Wilsenach et al., 2007; Tilley et al., 2008). In this recipe, the following assumptions are made: urea is hydrolyzed, and ammonium and carbonate are completely removed in advance without losing other substrates. The characteristics of synthetic urine used in this study are shown in Table 1.
Ammonium concentrations were analyzed before and after the experiments.
2.2. Experimental procedures
2.3. Analytical methods
Three factors that affect P and K removal were studied in a batch reactor: pH, molar ratio of Mg:K:P, and ammonium concentration. The detailed precipitation parameters are listed in Table 2. The molar ratio of Mg:K:P in synthetic urine was calculated as 0.06:1:0.59. Based on the stoichiometric molar ratio (1:1:1) of MPP precipitation, Mg and P sources should be added to yield high removal efficiencies of nutrients, especially K. The experiments were conducted in 500 mL beakers in duplicates. An impeller was submerged into the middle of the reaction zone for stirring and controlled at 200 rpm by a variable speed motor. A Mg source (MgCl26H2O) and a phosphate source (Na2HPO412H2O) in solid phase were added to the beaker to adjust the molar ratio of Mg:K:P. Then, 300 mL synthetic urine was added to the beaker with stirring. The solution pH was adjusted with 1 M NaOH. After reacting for 20 min, stirring was ceased to allow the precipitates to settle. After a sedimentation time of 20 min, the supernatant was collected for analysis. To test the effect of ammonium concentration on the removal of P and K, synthetic urine was prepared by adding NH4Cl. In this way, the experiment simulated the technique where urine is pretreated for ammonium removal as described earlier. In all cases, small amounts of ammonium remained in solution, and these affect the P and K removal. To test the volatilization of free ammonia during the experiments, a similar sample of synthetic urine was prepared without any MgCl2. The solutions were then stirred at 200 rpm for 20 min and allowed to settle for 20 min at pH 10.
A handheld pH meter (Hach Sension 1, USA) was used to measure the solution pH. All solution samples were filtered through 0.45 lm membranes before analysis. Ammonium and phosphate concentrations were analyzed colorimetrically with a spectrophotometer (Hach DR 5000, USA) according to standard methods (Ministry of Environmental Protection, 2002). The concentrations of K, Na, Ca, and Mg were measured with an Inductively Coupled Plasma- Atomic Emission Spectroscope (Thermo, USA). Precipitates were first collected through filtration using 0.45 lm filters, and then dried in an oven at 50 °C for about 48 h. The crystalline forms were assayed with an X-ray Diffractometer (XRD, Rigaku TTR-III, Japan). The morphology and chemical composition of the precipitates were obtained on a Scanning Electron Microscopy–Energy Dispersive X-ray Apparatus (SEM-EDX, Cambridge S-360, UK).The removal efficiency of nutrient elements (RE, E = P or K) was determined as the quotient of the element in the precipitates divided by the total element in synthetic urine:
Table 1 Characteristics of synthetic urine.a
a
Composition
g L1
mM
CaCl22H2O MgCl26H2O NaCl Na2SO4 Na3C6H8O72H2O Na2C2O4 KH2PO4 KCl C4H7N3O
0.65 0.65 4.60 2.30 0.65 0.02 4.20 1.60 1.10
4.4 3.2 78.7 16.2 2.6 0.15 30.9 21.5 9.7
pH = 4.2.
a
Initial molar ratio at neutral pH.
RE ¼ 1
Es ; Ei þ Ea
ð1Þ
1
where Es (mg L ) is the concentration of P or K in the supernatant, Ei (mg L1) is the initial concentration in synthetic urine, and Ea (mg L1) is the concentration added to synthetic urine. The volatilized ammonium was taken into account for N removal. Thus, the removal efficiency of N (RN) was calculated as:
RN ¼ 1
Ns þ Nv ; Ni
ð2Þ
where Ns (mg L1) is the ammonium concentration in the supernatant, Nv (mg L1) is the ammonium concentration volatilized during the experiments, and Ni (mg L1) is the initial ammonium concentration in the synthetic urine. 3. Results and discussion 3.1. Effect of pH Consistent with most previous studies, pH in a range of 5.5–11 played an important role in struvite precipitation (Gunay et al., 2008; Le Corre et al., 2009; Di Iaconi et al., 2010). The optimum pH was determined when the molar ratio of Mg:K:P was
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3.2. Effect of molar ratio of Mg:K:P
Fig. 1. Effect of pH on the removal of K (j) and P (h) from synthetic urine through the precipitation of MPP. The error bars represent the standard deviation of parallel samples.
1.6:1:1.6. Higher pH value significantly improved the removal of P and K, although the removal efficiency of K decreased at pH 12 (Fig. 1). This was mainly due to the dissociation of H2 PO 4 and 3 3 HPO2 4 to PO4 at higher pH. Higher PO4 concentrations enhanced the precipitation of MPP. The optimum pH for the removal of P and K was 11, but the removal efficiency of P and K was only slightly higher at pH 11 than at pH 10 by 0.8% and 0.6%, respectively. Furthermore, less NaOH was necessary to adjust the pH to 10 instead of 11. Thus, the pH was set to 10 for the succeeding experiments detailed in this study. Increasing the pH above 11 did not show further improvements in the removal of P, whereas it decreased the removal of K. This may be caused by the formation of other precipitates. For example, the precipitation of Mg(OH)2 at pH 11–12 may lead to stronger competition for OH with PO3 4 (Warmadewanthi and Liu, 2009). With increasing pH, Mg3(PO4)2, rather than struvite, also precipitates out (Lee et al., 2003; Zhang et al., 2009).
Experiments were conducted at pH 10 to determine the effects of Mg:K and P:K ratios on the removal of K and P. The removal of K was found to improve significantly with increasing molar ratios of Mg:K and P:K when the Mg:P ratio was fixed at 1:1 (Fig. 2a). Theoretically, 100% K should be removed when the molar ratio of Mg:K:P in the solution is equal to the stoichiometric value. However, the removal efficiency of K was only 48% when the Mg:K:P ratio was 1.1:1:1.1. While the removal efficiency of K increased to 77% when the Mg:K:P ratio was 2:1:2, it remained lower than the theoretical value. When the molar ratio of Mg:P was kept at 1:1, the removal efficiency of P was about 98%; this value did not change significantly with increasing Mg:K ratio. The effect of Mg:K ratio was determined at a fixed P:K ratio of 2:1 (Fig. 2b). Higher Mg:K ratios improved the removal of P because most of the phosphate could be precipitated under such a
Fig. 3. Effect of ammonium concentration on the removal of K (j), N (N), and P (h). The error bars represent the standard deviation of parallel samples.
(a)
(b)
(c)
(d)
Fig. 2. Effect of the Mg:K:P molar ratio on the removal of K (j) and P (h), and the concentration of residual phosphate from synthetic urine through the precipitation of MPP: (a) Mg:P ratio = 1:1, (b) P:K ratio = 2:1, (c) Mg:K ratio = 2:1, and (d) residual P concentration. The error bars represent the standard deviation of parallel samples.
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condition (Song et al., 2007). The removal efficiency of K first increased, and then decreased with increasing Mg:K ratio. This indicates the existence of an optimum Mg:K ratio for the removal of K. The decrease in K removal may be caused by the precipitation of Mg3(PO4)2 because the precipitation potential of this compound would also be enhanced by the addition of more Mg source. The effect of P:K ratio was determined at a fixed Mg:K ratio of 2:1 (Fig. 2c). Higher P:K ratios increased the removal of K but decreased the removal of P. The residual P was 62 mg L1 when the Mg:K:P ratio was 2:1:2; it increased to 797 mg L1 at a Mg:K:P ratio of 2:1:2.5 (Fig. 2d). More residual phosphate was detected in the synthetic urine solution at a Mg:K:P ratio of 1.6:1:2 than at 1.6:1:1.6. Thus, overdosing with the P source is unfavorable from the perspective of yielding nutrient products from urine. 3.3. Effect of ammonium
Fig. 4. Morphology and chemical composition of solid precipitates produced at pH 10 and a Mg:K:P molar ratio of 2:1:2 as analyzed via SEM-EDX.
During determination of the optimum values of pH and molar ratio of Mg:K:P for the removal of P and K, all the ammonium in synthetic urine was supposed to be removed in advance. However, a certain amount of ammonium always remains in urine. The removal efficiency of ammonium was reported to be 99% by stripping (Basakcilardan-Kabakci et al., 2007) and 95% by nitrification (Feng et al., 2008). The residual ammonium may interfere with the removal of K and P due its promotion of MAP precipitation. Thus, batch tests were conducted with the ammonium concentration ranging from 40 to 500 mg N L1 at a Mg:K:P molar ratio of 2:1:2 and pH 10. Ammonium in synthetic urine significantly reduced the removal of K (Fig. 3). The removal efficiency of K decreased from 77% to 65% as the ammonium concentration increased from 0 to 500 mg N L1. In contrary, the removal efficiency of P remained at about 97% with slight variations. Ammonium was also removed by MAP precipitation; 97% ammonium was removed when the
(a)
(d)
(b)
(e)
(c)
Fig. 5. XRD patterns of the standard crystals and precipitates from synthetic urine: (a) MgKPO46H2O [Powder Diffraction File (PDF) No. 35-0812], (b) MgNaPO47H2O (PDF No. 75-2269), (c) MgNH4PO46H2O (PDF No. 15-0762), (d) the precipitate formed at Mg:K:P = 2:1:2 and pH = 10, and (e) the precipitate formed at Mg:K:P = 2:1:2, pH = 10, and 500 mg NH4–N L1.
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ammonium concentration was 40 mg N L1. However, the removal efficiency of N decreased to 73% when ammonium concentration increased to 500 mg N L1. The removal of both K and ammonium indicates that MAP and MPP co-precipitated in the synthetic urine. 3.4. Co-precipitation of struvite-type compounds MAP and MSP are both isomorphous analogues of MPP (Stefov et al., 2004). These struvite-type compounds have been reported to have a general formula of M2+N+XO4nH2O, where M@Mg or Ca, N@K, Rb, Cs, Tl, Na or NH4, X@P or As, and n = 6–8 (Dickens and Brown, 1972; Banks et al., 1975; Mathew et al., 1982). Besides the principal elements of MPP, i.e. Mg, K, P and O, abundant Na was detected in the solid precipitates (Fig. 4). This indicates the co-precipitation of MPP and MSP. The simultaneous removal of both ammonium and K in the presence of ammonium in synthetic urine also indicates that MPP and MAP co-precipitated. In addition, needle-like crystals were observed, similar in shape to the MAP crystal reported (Song et al., 2007; Zhao et al., 2007). The crystal forms of the precipitates were confirmed by XRDanalysis (Fig. 5). The precipitates shown in Fig. 5d and e were respectively produced in the absence and presence of ammonium in synthetic urine. Both precipitates showed similar XRD patterns with nearly all of the characteristic peaks of the standard crystals (Fig. 5a–c). Thus, the co-precipitation of MPP and MSP in the absence of ammonium, and the co-precipitation of MAP, MPP, and MSP in the presence of ammonium in synthetic urine were confirmed. The pKsp (minus denary logarithm of solubility product constant) of MAP is reported to range from 12.60 to 13.36 (Ronteltap et al., 2007) while the pKsp of MPP is 10.62 (Taylor et al., 1963). MAP has a higher tendency to form than MPP because the minimal solubility of struvite crystal produces the maximal potential for its formation (Gunay et al., 2008). This hypothesis could also be speculated from reports showing that K and Na are scarcely detected in most studies on MAP precipitation (Zhao et al., 2007; Tilley et al., 2008). Therefore, in order to simultaneously remove P and K, the ammonium in urine should be removed prior to the precipitation of MPP. In the absence of ammonium in synthetic urine, co-precipitation of MPP and MSP reduced the theoretical removal efficiency of K. The tendencies of MPP and MSP to precipitate could not be directly compared due to the absence of the solubility product constant of MSP. In this study, Na originated mainly from the sodium salts used in synthetic urine, Na2HPO412H2O as the P source, and NaOH employed for pH adjustments. The molar ratio of Na:K was calculated as 7.5 when the solution pH was 10 and the molar ratio of Mg:K:P was 2:1:2 in the batch tests. MPP precipitation removed about 39% of the total Mg2+ and 39% of the total phosphate in solution, while MSP precipitation removed no more than 61% of the total Mg2+ and 61% of the total phosphate. This indicates that MPP has a higher tendency to form than MSP in synthetic urine. The co-precipitation of MPP and MSP resulted in the addition of more sources of Mg and P to obtain high K removal. This finding may be a bottleneck for future applications of MPP precipitation to remove of K from urine. 4. Conclusions The results of this study showed that the removal efficiencies of P and K in the absence of ammonium when pH was 10 and the molar ratio of Mg:K:P was 2:1:2 were 77% and 98%, respectively. The pH and the molar ratio of Mg:K:P are the key factors for the simultaneous removal of P and K. MPP and MSP co-precipitated in synthetic urine in the absence of ammonium. Thus, addition of MgCl26H2O and Na2HPO412H2O
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