Removal of nutrients from piggery wastewater using struvite precipitation and pyrogenation technology

Bioresource Technology 102 (2011) 2523–2528

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Removal of nutrients from piggery wastewater using struvite precipitation and pyrogenation technology Haiming Huang a,⇑, Chunlian Xu a,b, Wei Zhang a a b

Center for Environmental Engineering Design, Chinese Academy of Environmental Sciences, Beijing, 100012, China Graduate School of Life and Environmental Sciences, University of Tsukuba, Tennodai, Tsukuba, Ibaraki, 305-8572, Japan

a r t i c l e

i n f o

Article history: Received 27 July 2010 Received in revised form 11 November 2010 Accepted 12 November 2010 Available online 19 November 2010 Keywords: Magnesite Piggery wastewater Nutrients Recycling struvite Struvite precipitation

a b s t r a c t In this paper, removal of nutrients from piggery wastewater by struvite crystallization was conducted using a combined technology of low-cost magnesium source in struvite precipitation and recycling of the struvite pyrolysate in the process. In the present research, it was found that high concentrations of K+ and Ca2+ present in the solution significantly affected the removal of nutrients. When the struvite crystallization formed at the condition of dosing the magnesite pyrolysate at a Mg:N:P molar ratio of 2.5:1:1, and having a reaction time of 6 h, a majority of nutrients in piggery wastewater can be removed. Surface characterization analysis demonstrated that the main components of the pyrolysate of the obtained struvite were amorphous magnesium sodium phosphate (MgNaPO4) and MgO. When the struvite pyrolysate was recycled in the process at the pH range of 8.0–8.5, the precipitation effect was optimum. When the struvite pyrolysate was recycled repeatedly at pH 8.5 or without any adjustment of pH, the outcome of the removal of the nutrients in both cases was similar. With the increase in the number of recycle times, the performance of struvite precipitation progressively decreased. An economic evaluation showed that the combination of using low-cost material and recycling of struvite was feasible. Recycling struvite for three process cycles could save the chemical costs by 81% compared to the use of pure chemicals. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Nitrogen is an essential nutrient for living organisms; however, it can provoke water eutrophication when it is present in excess. Therefore, ammonia–nitrogen removal from wastewaters is of importance to prevent environmental pollution. Piggery wastes are considered to be a type of waste with the greatest environmental impact because of the presence of high concentrations of organic matters, nitrogen, and phosphorous in it. Usually, anaerobic digestion is accepted as a principal method of clearing up a majority of organic matters in piggery wastes, but the problem of nutrient enrichment of the digestion liquor still remains (Obaja et al., 2003). The most common and economical method to remove nutrients from wastewater is through the biological process (Cooper et al., 1994). However, high content of ammoniacal nitrogen has a toxic effect on microorganisms (Kim et al., 2008), which may lead to a decrease in the treatment effectiveness of the biological process. Struvite precipitation is a valid alternative for the removal of high ammonia concentrations from the anaerobically digested liquor of piggery wastes due to its high removal effectiveness, reaction rate, and solid–liquid separation capability. Struvite (MAP,

⇑ Corresponding author. Tel.: +86 10 84935398; fax: +86 10 84935653. E-mail address: [email protected] (H. Huang). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.11.054

MgNH4PO46H2O) is a white crystalline compound composed of magnesium, ammonium, and phosphate in an equal molar ratio (Di Iaconi et al., 2010). It also has a very low solubility. Removal of ammonia as struvite has been widely investigated on the treatment of wastewaters that is rich in ammonia, such as rare-earth wastewater (Huang et al., 2009), landfill leachate (Di Iaconi et al., 2010), coking wastewater (Chen et al., 2009), semiconductor wastewater (Kim et al., 2009; Warmadewanthi and Liu, 2009), and human urine (Ganrot et al., 2007). The removal rate of ammonia could be reached to 85% within 30 min under a molar ratio of Mg:N:P of 1:1:1 for treating landfill leachate (Ozturk et al., 2003). Since the amount of PO43 and Mg2+ in wastewaters are usually inadequate, a great amount of phosphate and magnesium salts are required for the effective removal of ammonia. This leads to a high operating cost, which hampers the widespread application of the struvite process. To solve this problem, many researchers have used low-cost materials containing magnesium as magnesium source of struvite precipitation, such as the byproducts generated in the production of magnesium oxide (Chimenos et al., 2003; Quintana et al., 2005; Quintana et al., 2008), the pyrolysate of magnesite (Huang et al., 2010), magnesite mineral (Gunay et al., 2008), bittern (Lee et al., 2003), and seawater (Kumashiro et al., 2001). In addition, process recycling of the struvite pyrolysate for the reduction of operation cost was proposed by some researchers (He et al., 2007; Türker and Çelen, 2007; Huang

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et al., 2009; Zhang et al., 2009). However, the simultaneous use of low-cost materials containing magnesium and the pyrolysate of struvite for ammonia removal has not yet been evaluated. In earlier published literatures, there are some papers focusing on the pyrogenation and process recycling of struvite. For example, Kenichi et al. (1975) found that an ammonia removal of 86.3% was achieved by recycling the pyrolysate of struvite, which was produced in alkali solution and at 70–80 °C. Huang et al. (2009) introduced a technology of recycling struvite, which could achieve an ammonia removal of 99% and remaining phosphorus of less than 1 mg/L. He et al. (2007) reported that an ammonium-release ratio greater than 96% could be reached when struvite was pyrolyzed in the following conditions: OH:NH4+ = 1:1; decomposition temperature, 90 °C; heating time, 2 h. In addition, Zhang et al. (2009) considered that the optimal condition for struvite pyrolysate production was controlled at an OH:NH4+ molar ratio of 2:1, a heating temperature of 110 °C, and a heating time of 3 h. The pyrogenation of struvite in NaOH solution could be depicted by the following reaction equation: MgNH4 PO4  6H2 OðsÞ þ NaOHðsÞ ! MgNaPO4 ðsÞ þ NH3 ðgÞ þ 7H2 OðgÞ ð4Þ +

2+

+

Zhang et al. (2009) reported that a NH4 –OH –Mg –Na –PO43 solution system could be formed when struvite was mixed with NaOH solution. As the NH4+–OH structure was unstable, ammonia could be released according to the Equation NH4+ + OH M NH3 + H2O when the NH4+–OH–Mg2+–Na+–PO43 solution system was heated at a given temperature. As a result, the pyrogenation of struvite takes place, leading to the formation of MgNaPO4. Struvite is an orthorhombic structure consisting of PO43 tetrahedral, Mg(6H2O)2+ octahedral and NH4+ groups held together by hydrogen bands (Whitaker and Jeffery, 1970). MgNaPO47H2O is an isomorphous analogue with struvite. Besides, a large number of other struvite analogues such as MgKPO46H2O, MgRbPO46H2O, MgTlPO46H2O, and MgCsPO46H2O have been reported (Mathew and Schroeder, 1979; Banks et al., 1975). As the stability of the struvite analogues generally declines with the decrease in the size of the univalent ion (Banks et al., 1975), when MgNaPO4 was added to a solution containing NH4+, the Na+ in the MgNaPO4 could be replaced by NH4+ forming more stable MgNH4PO46H2O because the size of NH4+ is larger than that of Na+. Consequently, the reuse of MgNaPO4 for ammonia removal was achieved. The objective of this study is to investigate the struvite-precipitation recycle technology with the use of low-cost magnesium sources for ammonia removal, for the purpose of reducing the chemical cost of struvite precipitation. First, the effect of K+ and Ca2+ in solution on struvite precipitation was evaluated by using synthetic swine wastewater. Second, the investigations focused on the conditions of using the pyrolysate of magnesite as the magnesium source and the efficiency of ammonia removal from piggery wastewater by internally recycling struvite pyrolysate. In addition, the solid before and after the struvite pyrogenation were characterized by a scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Finally, an economic evaluation of the recycle process was performed. 

2. Methods 2.1. Raw wastewater The raw wastewater used in the experiments was the anaerobically digested liquor of piggery wastewater, which was taken from a pig farm located in a city in Guangdong province. Before being

used, solid–liquid separation was performed. The composition of the supernatant is shown in Table 1. 2.2. Chemicals for struvite formation Piggery wastewaters generally contain negligible magnesium and a low concentration of phosphate in comparison to ammonia (NH3-N) (see Table 1). For the effective removal of ammonia, some phosphate and magnesium salts are required to be added. In this study, the pyrolysate of magnesite that calcined at 700 °C for 1.5 h and with a 53% Mg content was used as a source of magnesium in struvite precipitation (Huang et al., 2010). The original magnesite mineral was purchased from Xinxing Magnesium Powder Plant (Liaoning Province, China). As for the phosphate source, H3PO4 (85%) was used. In addition, KCl, CaCl2, NH4H2PO4, MgCl26H2O, and Na2HPO412H2O of analytical grade were used in this study. 2.3. Experiments for the influence of K+ and Ca2+ To study the effect of K+ and Ca2+ on the struvite precipitation of piggery wastewater, synthetic swine wastewater with a NH3-N concentration of 985 mg/L prepared by dissolving NH4H2PO4 into deionized water was used. The experimental procedures are shown in the following. First, 200 mL synthetic wastewater was fed into a jar with an airtight lid. Second, MgCl26H2O was added to the wastewater at the Mg2+:NH4+:PO43 stoichiometric molar ratio of 1:1:1, and then KCl (or CaCl2) was fed into the wastewater at K+:Mg2+ (or Ca2+:Mg2+) molar ratio range of 0–0.75. Third, the reaction solution was stirred by a magnetic stirrer for 15 min at pH 9. Last, the solution supernatant after a precipitation of 10 min was filtered through a 0.45-lm membrane filter for component analysis. 2.4. Experiments for the use of the magnesite pyrolysate The use of the pyrolysate of magnesite (its main composition was MgO) as the magnesium source of struvite precipitation was performed at different magnesium:ammonia molar ratios (1.5– 3.5). Experimental procedures are depicted as follows. 500 mL piggery wastewater and a given dose of magnesite pyrolysate were added to a jar with an airtight lid, and H3PO4 was supplemented to the stoichiometric ratio of struvite formation. The pH value of the supernatant was measured after the agitation was intermitted for 1 min, at different time intervals (0.5–8 h). Thereafter, the supernatant of 5 mL was removed and filtered through a 0.45lm membrane filter for the component analysis. In addition, control experiments using magnesite pyrolysate and H3PO4 as magnesium and phosphate sources was conducted by using pure MgCl26H2O and Na2HPO412H2O. The experimental procedures Table 1 Composition of piggery wastewater supernatant after solid–liquid separation. Parameters

Average values

Standard deviation

pH COD (mg/L) BOD5 (mg/L) TN (mg/L) NH3-N (mg/L) TP (mg/L) PO43–-P (mg/L) K (mg/L) Ca (mg/L) Mg (mg/L) Al (mg/L) Fe (mg/L)

7.8 2388 1035 1212 985 182 161 797 135 6.7 5.8 2.1

0.2 238 164 55 31 19 11 27 23 3.6 2.5 0.8

H. Huang et al. / Bioresource Technology 102 (2011) 2523–2528

were similar to that described in Section 2.3 (reaction time, 15 min; precipitation time, 10 min; Mg2+:NH4+:PO43, 1:1:1). 2.5. Experiments for the recycle of the struvite pyrolysate The struvite formed in 500 mL piggery wastewater under optimum conditions was collected using a 0.45-lm membrane filter and washed for three times with pure water, and then decomposed under the following conditions: pyrogenation temperature, 110 °C; pyrogenation time, 3 h; NaOH:NH4+; 1:1. All the resulting pyrolysate was added to 500 mL wastewater. The mixed solution was agitated for 1 h at given experimental pH, having a precipitation time of 10 min. This process was repeated four more times. After each experiment, the supernatant of the effluent was filtered through a 0.45-lm membrane filter for component analysis. All the experiments were performed in triplicate, at a wastewater temperature range of 23–25 °C. To prevent the volatilization of ammonia, the jars were airproofed during the reaction and the samples taken were diluted with deionized water to the desired concentration. 2.6. Analytical methods The composition of the piggery wastewater, ammonia, and phosphate of the effluent were determined according to the Standard Methods (SEPA, 2002). The struvite before pyrogenation and the pyrolysate after pyrogenation were washed with deionized water three times, dried in an oven at 35 °C for 48 h, and then characterized by SEM (FEI Quanta 400, America), FTIR (Bruker VENTEX70, Germany), and XRD (Philips Model PW1830). 3. Results and discussion 3.1. The effect of K+ and Ca2+

Ammonia removal ratio (%) Rem. PO4-P (mg/L)

Fig. 1 reveals the removal efficiency of ammonia in the individual presence of potassium and calcium ions as well as the remaining PO4-P as a function of the n(Cation):n(Mg2+) molar ratio. It can be seen that there is an obvious decrease in ammonia removal and remaining PO4-P in the presence of Ca2+ and K+. At identical molar concentration of metal ions, the ammonia-removal efficiency and remaining PO4-P concentration were lowest for Ca2+ present in the solution, followed by K+. As shown in Fig. 1, when the molar ratios of Ca2+:Mg2+and K+:Mg2+ increased from 0 to 0.75, the removal efficiency of ammonia decreased from 87.7% to 58% and 79%,

+

+

+

2+

K + NH4

60

Ca + NH4

40 20 0 90 80 70 60 50 0.00

0.15

0.30 0.45 n(Cation):n(Mg 2+)

0.60

0.75

Fig. 1. Ammonia-removal ratio and remaining PO4-P concentration during struvite precipitation at different n(Cation):n(Mg2+) molar ratios.

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respectively, and there was a decrease in the remaining PO4-P concentration from 58.7 mg/L to 0.3 mg/L and 48 mg/L, respectively. The behavior of K+ in the solution could be explained by the following reaction (Wilsenach et al., 2007): þ þ Mg2þ þ HPO3 4 þ K ! MgKPO4 þ H

ð1Þ

The K+ present in the solution could compete with NH4+ to form the potassium struvite (Schuiling and Anrade, 1999; Di Iaconi et al., 2010). Nevertheless, the K-struvite is less stable than struvite and decomposes with time when removed from the mother liquor (Banks et al., 1975). This may be a reason why the ammoniaremoval ratio and remaining PO4-P concentration slowly decreased with the increase in the K+:Mg2+ ratio. As for Ca2+, its presence could consume the PO43 available for struvite precipitation through the formation of calcium phosphate precipitation, inhibiting the generation of struvite crystal.

3.2. The use of the magnesite pyrolysate The experimental results of the use of the magnesite pyrolysate as magnesium source are shown in Fig. 2(a–c). It was found that with increases in the Mg:N molar ratio and the reaction time, the solution pH, ammonia-removal ratio, and PO4-P removal ratio also increased. When the Mg:N molar ratio increased from 1.5 to 2.5 at a given reaction time, the ammonia and PO4-P removal ratio rapidly increased, whereas further increases in the Mg:N range of 2.5–3.5 caused a slow increase in the removal ratio of ammonia and PO4-P. When the Mg:N molar ratio was 2.5 and the reaction time was 6 h, the solution pH, the ammonia, and PO4-P removal ratio were more than 8.6, 80%, and 96%, respectively. In order to obtain high removal efficiencies of nutrients, the struvite obtained at the Mg:N molar ratio of 2.5 and reaction time of 6 h was adopted in the subsequent recycle experiments. The struvite formation with the use of magnesite pyrolysate proceeds by the following reactions:

MgO þ 2Hþ ! Mg2þ þ H2 O

ð2Þ

þ þ Mg2þ þ HPO2 4 þ NH4 þ 6H2 O ! MgNH4 PO4  6H2 O þ H

ð3Þ

MgO has a very low solubility in the neutral solution. Nevertheless, in this study, due to the presence of large amounts of H+ caused by the addition of phosphoric acid, the MgO present in the magnesite pyrolysate can play a dual function on the struvite precipitation perfectly, that is, neutralizing the H+ in solution and releasing Mg2+ as magnesium source of struvite precipitation. In the previous literatures, there have been some papers reporting ammonia removal by using low-grade MgO and the mechanism for struvite formation involving the use of low-grade MgO has been discussed. Chimenos et al. (2003) proposed that the struvite formation takes place on the surface of MgO particle instead of the bulk solution, which thereby stops further reaction of MgO. The mechanism was also confirmed by Chen et al. (2009). In this work, in order to compare the ammonia-removal effect of the magnesite pyrolysate as magnesium source, pure MgCl26H2O was used and the experimental results are shown in Fig. 3. From the figure, we can observe that at the pH range of 8–8.5, the ammonia-removal ratio reached the maximum of about 83%, with a residual concentration of PO4-P of 10 mg/L. Although the amount of active magnesium in the experiments of using magnesite pyrolysate and MgCl26H2O as magnesium sources may be different, we can consider that using magnesite pyrolysate as the magnesium source could achieve ammonia-removal efficiency, which may be close to that of MgCl26H2O.

H. Huang et al. / Bioresource Technology 102 (2011) 2523–2528

Magnesium:Ammonia

a

pH 3.5

3

2.5

2

1.5 0.5

2

3.5

5

6.5

8

Ammonia removal ratio (%)

b

3.5

Magnesium:Ammonia

2526

3

2.5

2

1.5 0.5

2

3.5

5

6.5

8

Time (h)

Time (h) 3.5

Magnesium:Ammonia

Phosphorus removal ratio (%)

c

3

2.5

2

1.5 0.5

2

3.5

5

6.5

8

Time (h) Fig. 2. Struvite precipitation with magnesite pyrolysate as magnesium source at different Mg:N molar ratios and reaction time (a) the variation of solution pH, (b) the variation of ammonia-removal ratio, and (c) the variation of PO4-P removal ratio.

3.3. Process recycling of the collected struvite 3.3.1. Struvite pyrogenation and surface characterization analysis In this work, SEM, FITR, and XRD analysis before and after pyrogenation were performed to identify the transformation of the struvite collected at a Mg:N molar ratio of 2.5:1 and a reaction time of 6 h. SEM images showed that the pyrolysate was coarse in comparison with the nonpyrolyzed struvite and their sizes were irregular (3–30 lm). The FITR pattern indicated that the ammonium characteristic bands (1435 cm1) of the pyrolysate almost completely disappeared compared to the struvite. This suggests that ammonia releases from struvite (Stefov et al., 2004). Further, in the XRD analysis, the comparison before and after the pyrogenation of the struvite precipitate indicated that the characteristic peaks of struvite disappeared after heating, and only the peaks of MgO remained. The main composition of the pyrolysate could be considered to be the nonreacted MgO and amorphous magnesium sodium phosphate (MgNaPO4). 3.3.2. The optimum pH for the struvite pyrolysate reuse The solution pH is one of the most important controlling factors for struvite crystallization reaction (Song et al., 2007). H3PO4, H2PO4, HPO42, MgOH+, MgNH4PO4, MgPO4, MgH2PO4+ and MgHPO4 can be formed in the system involving Mg2+, PO43 and

NH4+ aqueous solutions when the pH varied (Mijangos et al., 2004). The optimum pH of struvite precipitation has been widely investigated. In the previous literatures concerning struvite precipitation of piggery wastewater, the optimum pH of 8.5 (Suzuki et al., 2002; Çelen et al., 2007), 9 (Jaffer et al., 2002), 8.9–9.25 (Nelson et al., 2003), 9.5–10.5 (Song et al., 2007) was reported. In this study, to determine the optimum pH for the reuse of the struvite pyrolysate, experiments were performed at a pH range of 6.5–10, with a reaction time of 1 h. The experimental results are shown in Fig. 4. It is observed that the remaining PO4-P concentration decreased rapidly with the increase of the pH and reached 1 mg/L at pH 10. The maximum ammonia-removal ratio appeared in the range of 8–8.5, which was in agreement with that of pure MgCl26H2O. When the pH was over the optimum range, Mg3(PO4)2 and Ca3(PO4)2 were formed instead of struvite with an increase in the pH, which lead to the decrease of phosphate available for struvite crystallization. Finally, this brought a decrease in the ammonia-removal ratio. When the pH was below the optimum range, the increase of H+ in the solution inhibited the struvite crystallization, resulting in the decrease of the ammonia-removal ratio. 3.3.3. Multirecycles of the struvite pyrolysate For determining the effect of multirecycle precipitation, two modes of reuse of the struvite pyrolysate, with a reaction time of

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100 120

Remaining PO4-P (mg/L)

60 30 0 80 70

60 40 20

60

0 50 6.5

7.0

7.5

8.0

8.5 pH

9.0

9.5

10.0 10.5

Fig. 3. The ammonia and PO4-P removal ratios with MgCl2 and Na2HPO4 as magnesium and phosphate sources at different pHs: the Mg2+:NH4+:PO43 molar ratio for struvite precipitation was 1:1:1.

Ammonia removal ratio (%) Rem. PO4-P (mg/L)

At pH 8.5 Without any adjustment of pH

80

90

160 120

Ammonia removal ratio (%)

Ammonia removal ratio (%) Rem. PO4-P (mg/L)

H. Huang et al. / Bioresource Technology 102 (2011) 2523–2528

80 70 60 50 40

80

1

40 0 80

2

3 Recycle time

4

5

Fig. 5. The performance of the struvite pyrolysate recycles at different recycle times.

70 60 50

6.5

7.0

7.5

8.0

8.5

9.0

9.5

10.0

pH

Fig. 4. The effect of pH on the process recycling of struvite pyrolysate, with reaction time of 1 h.

1 h, were investigated. In the first mode, the pyrolysate was reused at pH 8.5. In the second mode, the pyrolysate was added to the wastewater without any adjustment of pH. It was found from the experiments that in the second mode, the supernatant pH at the end of reaction decreased from the initial 9.38 to 8.45 in the fifth cycle. In addition, it was observed that the ammonia-removal ratio and the remaining PO4-P concentration in both modes were basically close per cycle (Fig. 5). With the increase in recycle times, the ammonia-removal ratio decreased gradually, and the remaining PO4-P concentration increased. The increases of inactive Mg3(PO4)2 and Mg2P2O7 in recycled pyrolysate (Schulze-Rettmer et al., 2001) and the losses of Mg2+ and PO43 in the supernatant per recycle time might be responsible for this present result. Besides, the accumulation of some components from wastewater (e.g. potassium, calcium, Section 3.1) might also be a factor inhibiting the formation of struvite in later cycles. 3.4. Economic evaluation of recycling struvite Economic analysis of process recycling of struvite obtained with the magnesite pyrolysate was carried out and compared to the struvite precipitation using pure chemicals (without internal recycling of struvite). In this assessment, the manpower costs, as well as the market values of the recovered struvite and ammonia were

not taken into account, and only the costs of the chemicals used and energy consumed were considered. The market prices of chemicals used and energy consumed in the calculations are given in Table 2. It can be calculated that the total cost of chemicals and energy is $10.3/m3 of piggery wastewater when pure MgCl26H2O and Na2HPO412H2O were used for the formation of struvite in piggery wastewater, whereas it is $4.9/m3 of piggery wastewater when the magnesite pyrolysate and H3PO4 were used without internal recycling of struvite (see Table 3). The results of the economic analysis for internal recycling of struvite (Table 3) indicate that the average costs decrease progressively with the increase of cycles of struvite recycle, and about 59% of the average cost could be reduced by recycling struvite for three cycles. Further, in comparison to the costs of using pure chemicals, the cost of recycling struvite obtained with the magnesite pyrolysate and H3PO4 for three process cycles could be reduced by 81%. Thus, it can be seen that this process can greatly lower the costs of struvite precipitation. In addition, the use of MgO and H3PO4 in struvite precipitation does not increase the salinity of wastewater, which is beneficial to the biological treatment of the effluents. In previous literatures, there are some papers reporting the reduction of struvite-precipitation cost. He et al. (2007) reduced about 44% of chemical costs by recycling struvite for three cycles. Table 2 The market prices of the chemicals used and energy consumed in the experiments. Chemicals/energy

Price

Magnesite H3PO4 (85%) MgCl26H2O Na2HPO412H2O NaOH Energy consumption

0.013 $/kg 0.61 $/kg 0.079 $/kg 0.36 $/kg 0.32 $/kg 0.1 $/kWh

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H. Huang et al. / Bioresource Technology 102 (2011) 2523–2528

Table 3 Economic analysis of the nutrients removal process by struvite precipitation compared to struvite pyrogenation and recycle technology. Cost for struvite precipitation with MgCl2 and Na2HPO4 ($/m3 of wastewater) Cost for struvite precipitation with magnesite pyrolysate and H3PO4 ($/m3 of wastewater) Average cost for recycling the pyrolysate of struvite obtained with magnesite pyrolysate and H3PO4 The average cost for 1 recycle cycle ($/m3 of wastewater) The average cost for 2 recycle cycles ($/m3 of wastewater) The average cost for 3 recycle cycles ($/m3 of wastewater) The average cost for 4 recycle cycles ($/m3 of wastewater) The average cost for 5 recycle cycles ($/m3 of wastewater)

10.3 4.9

2.97 2.32 2.0 1.81 1.68

Huang et al. (2009) decreased 48.7% of struvite-precipitation cost by using a struvite-recycle technology, and Gunay et al. (2008) reduced the operation cost by about 18% by using magnesite as a magnesium source. 4. Conclusions The presence of high contents of K+ and Ca2+ in solution could inhibit the formation of pure struvite. When the pyrolysate of magnesite was used as magnesium sources, a Mg:N molar ratio of 2.5:1 and reaction time of 6 h is required for a satisfactory nutrients removal from piggery wastewater (i.e. 80% of NH3-N and 96% of PO4-P). The multi-recycling of struvite pyrolysate in the recovery process without any pH adjustment may provide a sustainable method to remove nutrient from piggery wastewater. Using low-cost material in combination with the process recycling of the struvite pyrolysate is as effective as and less costly than using pure chemicals. Acknowledgements The authors are indebted the anonymous reviewers for their insightful comments and suggestions that significantly improved the manuscript. This work was financially supported by Major Specific Projects of National Science and Technology of China, ‘‘Control and Treatment of Water Pollution’’ (Grant Nos. 2009ZX07529-007). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biortech.2010.11.054. References Banks, E., Chianelli, R., Korenstein, R., 1975. Crystal chemistry of struvite analogs of the type MgMPO4 6H2O (M+ = potassium (1+), rubidium (1+), cesium (1+), thallium (1+), ammonium (1+). Inorg. Chem. 14 (7), 1634–1639. Chen, T., Huang, X., Pan, M., Jin, S., Peng, S., Fallgren, P.H., 2009. Treatment of coking wastewater by using manganese and magnesium ores. J. Hazard. Mater. 168, 843–847. Chimenos, J.M., Fernández, A.I., Villalba, G., Segarra, M., Urruticoechea, A., Artaza, B., Espiell, F., 2003. Removal of ammonium and phosphates from wastewater resulting from the process of cochineal extraction using MgO-containing byproduct. Water Res. 37, 1601–1607. Cooper, P., Day, M., Thomas, V., 1994. Process options for phosphorus and nitrogen removal from wastewater. J. Inst. Water Environ. Manage. 8, 84–92. Çelen, I., Buchanan, J.R., Burns, R.T., Robinson, R.B., Raman, D.R., 2007. Using a chemical equilibrium model to predict amendments required to precipitate phosphorus as struvite in liquid swine manure. Water Res. 41, 1689–1696. Di Iaconi, C., Pagano, M., Ramadori, R., Lopez, A., 2010. Nitrogen recovery from a stabilized municipal landfill leachate. Bioresour. Technol. 101, 1732–1736. Ganrot, Z., Dave, G., Nilsson, E., 2007. Recovery of N and P from human urine by freezing, struvite precipitation and adsorption to zeolite and active carbon. Bioresour. Technol. 98, 3112–3121.

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