Bioresource Technology 166 (2014) 1–8

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Recovery of phosphorus and nitrogen from alkaline hydrolysis supernatant of excess sludge by magnesium ammonium phosphate Wei Bi a,1, Yiyong Li a,2, Yongyou Hu a,b,⇑ a Ministry of Education Key Laboratory of Pollution Control and Ecological Remediation for Industrial Agglomeration Area, College of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China b State Key Lab of Pulp and Paper Engineering, College of Light Industry and Food Science, South China University of Technology, Guangzhou 510640, PR China

h i g h l i g h t s  Two-stage alkaline hydrolysis process was first applied to treat excess sludge.  The running conditions of two-stage alkaline hydrolysis process were determined.  P and N were recovered from supernatant of two-stage alkaline hydrolysis sludge.  The supernatant pH of the two-stage alkaline hydrolysis sludge was below 10.5. 3

 Optimum conditions for PO4

a r t i c l e

-P recovery were determined.

i n f o

Article history: Received 15 February 2014 Received in revised form 20 April 2014 Accepted 25 April 2014 Available online 10 May 2014 Keywords: Magnesium ammonium phosphate (MAP) Excess sludge Alkaline hydrolysis Phosphorus recovery Nitrogen recovery

a b s t r a c t Magnesium ammonium phosphate (MAP) method was used to recover orthophosphate (PO3 4 -P) and ammonium nitrogen (NH+4-N) from the alkaline hydrolysis supernatant of excess sludge. To reduce alkali consumption and decrease the pH of the supernatant, two-stage alkaline hydrolysis process (TSAHP) was + designed. The results showed that the release efficiencies of PO3 4 -P and NH4-N were 41.96% and 7.78%, respectively, and the pH of the supernatant was below 10.5 under the running conditions with initial pH of 13, volume ratio (sludge dosage/water dosage) of 1.75 in second-stage alkaline hydrolysis reactor, 20 g/L of sludge concentration in first-stage alkaline hydrolysis reactor. The order of parameters influencing MAP reaction was analyzed and the optimized conditions of MAP reaction were predicted + through the response surface methodology. The recovery rates of PO3 4 -P and NH4-N were 46.88% and 16.54%, respectively under the optimized conditions of Mg/P of 1.8, pH 9.7 and reaction time of 15 min. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction In a second level biochemical sewage treatment system, more than 90% phosphorus and parts of nitrogen in influent will be transferred into the sludge (Balmer, 2004), and thus phosphorus and nitrogen usually accounted for up to 4% and 9% of the dry sludge. If the sludge was directly disposed in sea or landfills, the leaching of phosphorus and nitrogen would lead to serious secondary pollution problems (Korboulewsky et al., 2002). Besides, in the sewage treatment system, 20–50% of phosphorus and part of ⇑ Corresponding author at: College of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China. Tel.: +86 20 39380506; fax: +86 20 39380508. E-mail addresses: [email protected] (W. Bi), [email protected] (Y. Li), [email protected] (Y. Hu). 1 Tel.: +86 13763306840. 2 Tel.: + 86 15820278056. http://dx.doi.org/10.1016/j.biortech.2014.04.092 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

nitrogen in effluent sludge would be returned back to the wastewater treatment system. Thus, if phosphorus and nitrogen of the excess sludge can be recycled through magnesium ammonium phosphate (MAP) method, it can not only recover the limited phosphorus resource, but also reduce the phosphorus and nitrogen loading of the sewage treatment system. Besides, the MAP hexahydrate (MgNH4PO46H2O), commonly known as struvite, can also be utilized as an effective slow-release fertilizer in agriculture. To find an efficient way to release phosphorus and nitrogen from excess sludge is the first step for phosphorus and nitrogen recovery. Recently, various physical and chemical methods have been used to disintegrate sludge, such as thermal hydrolysis (Appels et al., 2010; Xue and Huang, 2007), ozone (Yan et al., 2009), chlorine dioxide (Wang et al., 2011), acid and alkaline hydrolysis (Chen et al., 2007; Stark et al., 2006), ultrasonic (Wang et al., 2010; Yan et al., 2010), microwave (MW) irradiation (Park et al., 2004), chlorine dioxide (ClO2)-ultrasonic disruption

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W. Bi et al. / Bioresource Technology 166 (2014) 1–8

(Lin et al., 2012) and microwave (MV)-hydrogen peroxide(H2O2) method (Eskicioglu et al., 2008). In comparison with other methods, alkaline hydrolysis has the advantages of simple devices, easy operation and high efficiency. The preferred alkali, in most cases, was sodium hydroxide (NaOH) which was reported to yield greater solubilization efficiency than calcium hydroxide (Ca(OH)2) (Torres and Llorens, 2008). Kim et al. (2003) observed that the accessorial solubilized COD were 39.8%, 36.6%, 10.8% and 15.3% respectively, more than the control treated by NaOH, KOH, Mg(OH)2 and Ca(OH)2 at pH 12. Besides, pH was the key parameter of sludge alkaline hydrolysis. pH value and alkaline sludge disintegration increased with the increase of alkaline dosage (Li et al., 2008), and it was only at pH higher than 11 that the sludge cells were disrupted (Becerra et al., 2010). However, most of the investigations were focused on the release of organic carbon, there is only a few publications investigated the release of phosphorus and nitrogen. In addition, it has been published the pH for alkaline hydrolysis ranged from 12 to 13, the pH of the alkaline hydrolysis supernatant was still close to 12 after 24 h (Li et al., 2012) due to the addition of large dose of NaOH, which was not beneficial to the subsequent phosphorus and nitrogen recovery by MAP reaction. So far, MAP method was applied to recover phosphorus and nitrogen from the supernatant of anaerobic digestion sludge and alkaline fermentation liquid of excess sludge. Uludag-Demirer and Othman (2009) observed that around 63% PO3 4 -P and 64% NH+4-N could be recovered from the supernatant of anaerobic digestion sludge within 10 min at pH 9.0 and a Mg/N/P molar ratio of 1:1:1. Tong and Chen (2007) found that under the conditions of a Mg/P molar ratio of 1.8, pH 10 and reaction time of 2 min, 92.8% of the orthophosphate (SOP) could be recovered from the alkaline fermentation liquid of waste activated sludge. They (Tong and Chen, 2009) also observed that around 83% soluble orthophosphate (SOP) and 75% ammonium nitrogen (NH+4-N) could be recovered from the alkaline fermentation liquid under the conditions of a Mg/N molar ratio of 1.8, pH 10.41 and a P/N molar ratio of 1.16. But there have been few investigations which have focused on the recovery of phosphorus and nitrogen from the alkaline hydrolysis supernatant of excess sludge. In this study magnesium dichloride (MgCl26H20), as the additional source of Mg2+, was added to the alkaline hydrolysis supernatant of excess sludge to recover MAP. In order to reduce alkali consumption and decrease the pH of the supernatant, twostage alkaline hydrolysis process (TSAHP) was designed. The effects of pH, sludge concentration and reaction time on the release of phosphorus and nitrogen were firstly explored through the single factor experiments. The influences of Mg/P (molar ratio of Mg2+/NH+4-N), pH and reaction time on the recovery efficiencies + of PO3 4 -P, TP and NH4-N for MAP reaction of the effluent from the TSAHP were investigated. Finally, the response surface methodology (RSM) was used to analyze the order of parameters influencing MAP reaction, and predict the optimum reaction conditions for conversion.

2. Methods 2.1. Excess sludge The excess sludge was taken from the secondary sedimentation tank of A/O process in Guangzhou, china. The sludge was concentrated by settling at 4 °C for 24 h. Then the supernatant was abandoned. The main characteristics of the sludge were as follows: pH 6.45–6.73, total solid (TS) 16.44–37.35 g/L, total phosphorus (TP) 225–275 mg/L, total nitrogen (TN) 780–932 mg/L, and water content 98.0–98.5%.

2.2. Single factor experiment for alkaline hydrolysis sludge Certain amount of excess sludge with an average concentration of TS of 16.44 g/L were placed in a 250 mL beaker, NaOH of 10 mol/ L was then added to adjust pH to 8.0, 9.0, 10.0, 11.0, 12.0, 12.5, 13.0 and 13.5 when the sludge was stirred at 150 rpm. After 0.5 h, 1 h, 2 h, 5 h, 8 h and 12 h, the mixture was centrifuged at 4000 rpm for 10 min, and the supernatant was directly used for measurement of TP, TN and ammonia nitrogen (NH+4-N). The supernatant was filtered through a 0.45 lm-pore-size syringe filter unit and then the filtrate was used for PO3 4 -P determination. 2.3. Two-stage alkaline hydrolysis of excess sludge One aim of developing the TSAHP was to decrease the pH of the supernatant. While the supernatant pH was determined by two factors: alkaline hydrolysis pH (a) and volume ratio (sludge dosage/water dosage (b)). Here, twelve groups of experiment were carried out to optimize the running conditions. For each group of experiment, the pH and PO3 4 -P concentration of the supernatant were determined. The running conditions of the twelve groups of experiment were given in Table 1. The flow chart of the TSAHP was shown in Fig. 1. The operating procedures were as follows: (1) Starting up: for the first-stage alkaline hydrolysis, X L of the fresh excess sludge with a concentration of 20 g/L along with 10 mol/L NaOH were injected into reactor A to obtain final pH of 10 under continuous stirring at 150 rpm. After 1 h, the mixture was centrifuged, and the first-stage alkaline hydrolysis supernatant was discharged. The residual solid from reactor A was injected into reactor B, in which X/b L of water was injected. Then 10 mol/L NaOH was added to reactor B under constant stirring till pH to a. After 1 h, the mixture was centrifuged, and the second-stage alkaline hydrolysis supernatant was injected into reactor A. The residual solid from reactor B was discharged. (2) Running: after starting, no any NaOH was needed adding to reactor A to adjust pH but other operating procedures unchanged. During the whole operating procedures, the residual solid was discharged (solid line), and the first-stage alkaline hydrolysis supernatant (dotted line) was collected + for the recovery of PO3 4 -P and NH4-N. 2.4. Recovering phosphorus and nitrogen by MAP It was observed that the pH values of the alkaline hydrolysis supernatant ranged from 9.0 to 10.5, which met the requirements of pH for MAP reaction. Therefore, magnesium dichloride

Table 1 Running conditions of the twelve groups of experiment for the TSAHP. Run

a

b

Supernatant pH

PO3 4 -P concentration (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12

11.5 11.5 11.5 11.5 12.25 12.25 12.25 12.25 13.0 13.0 13.0 13.0

0.75 1 1.75 2.5 0.75 1 1.75 2.5 0.75 1 1.75 2.5

7.99 ± 0.14 7.83 ± 0.15 7.67 ± 0.17 7.72 ± 0.16 8.95 ± 0.10 8.81 ± 0.24 8.69 ± 0.20 8.35 ± 0.23 12.44 ± 0.18 11.68 ± 0.17 9.72 ± 0.15 9.09 ± 0.14

38 ± 1.15 42 ± 2.00 50 ± 2.31 34 ± 2.00 40 ± 2.31 52 ± 3.05 52 ± 2.00 48 ± 2.00 50 ± 2.31 54 ± 2.00 62 ± 2.31 64 ± 3.05

a is alkaline hydrolysis pH, b is volume ratio (sludge dosage/water dosage).

W. Bi et al. / Bioresource Technology 166 (2014) 1–8

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Fig. 1. The flow chart of the two-stage alkaline hydrolysis process (TSAHP).

(MgCl26H2O), as the additional source of Mg2+, could be directly added to the alkaline hydrolysis supernatant to recover phosphorus and nitrogen through MAP reaction. Firstly, placing 100 mL of the alkaline hydrolysis supernatant in a 250 mL beaker, calculating the amount of magnesium salt needed through the PO3 4 -P concentration and Mg/P. Then magnesium salt was added to the supernatant when the mixture was stirred at 200 rpm. And the pH was adjusted with 2 mol/L NaOH or 2 mol/L HCl as required. After the reaction procedure, the mixture was centrifuged at 4000 rpm for 10 min, and TP and NH+4-N were determined for the supernatant. Then the supernatant was filtered through a 0.45 lm-pore-size syringe filter unit for PO3 4 -P determination. The dried precipitate was examined by energy dispersive spectroscopy (EDS). 2.5. Main indicators and analysis methods Total phosphorus (TP) and orthophosphate (PO3 4 -P) were measured with Ammonium molybdate spectrophotometric method (MEP, 2002). Ammonia nitrogen (NH+4-N) was determined by the method of Nessler’s reagent photometry (MEP, 2002). Total nitrogen (TN) was measured with Alkaline potassium persulfate digestion UV spectrophotometric method (MEP, 2002). pH was measured with a pH meter (PHS-3C). And EDS analysis for the dried precipitate was carried out on EVO LS10 (Carl ZEISS) system. 3. Results and discussion 3.1. Effects of single factor on release of phosphorus and nitrogen from alkaline hydrolysis sludge 3.1.1. Effect of pH The pH was adjusted with NaOH of 10 mol/L. After continuous stirring for 2 h, the changes of the concentrations of TP, PO3 4 -P, TN and NH+4-N in the alkaline hydrolysis supernatant with the alkaline hydrolysis pH were shown in Fig. 2a. As was shown in Fig. 2a, when the alkaline hydrolysis pH was ranged from 8 to 10, there was only a slight increase in TN, TP, NH+4-N and PO3 4 -P. As the alkaline hydrolysis pH was higher than 10, the concentrations of TP, PO3 4 -P and TN were significantly increased, among which the increase of TP was much greater than that of PO3 4 -P. In addition, when the alkaline hydrolysis pH was ranged from 11 to 12.5, a platform about TN concentration appeared, but the concentrations of TP and PO3 4 -P were still significantly increased. Visibly, higher alkaline hydrolysis pH was beneficial to the release of phosphorus. Similar results were also reported by Xiao and Liu (2006). At low alkaline hydrolysis pH, only the sludge flocs broke. It was only at high alkaline hydrolysis pH that the sludge cells broke and then the proteins and nucleic

acids were hydrolyzed, and the polysaccharides from the bacteria were also decomposed, resulting in the release of phosphorus and nitrogen. Because the sludge cell membrane was mainly composed of phospholipids bilayer. And at pH 11 and above, there was still a release of part of phosphorus after the sludge cell membrane was disrupted. Therefore with the further disruption of the sludge cells, the concentrations of TP and PO3 4 -P would be still increased. In addition, when the alkaline hydrolysis pH was higher than 11, the disruption of the sludge cell membrane had started, which resulted in most of the intracellular material leaching into the aqueous phase. And part of TN released by the hydrolysis of nitrogen-containing organic matter were transformed into NH+4-N. Simultaneously, part of ammonia nitrogen could be converted to ammonia gas to escape from sludge under high alkaline hydrolysis pH (Ma et al., 2012). While NH+4-N produced was slightly more than that escaped, thus the concentration of NH+4-N in the supernatant increased slightly. In the range of pH of 11–12.5, the amount of TN released by the hydrolysis of nitrogen-containing organic matter was close to that of NH+4-N escaped. Therefore, in the range of pH of 11–12.5, there was a platform of TN concentration. The study also found that the pH of the alkaline hydrolysis sludge would drop with time, but the decrease of the sludge pH was significantly limited (only 0.2–1.3 decreases). Especially when the alkaline hydrolysis pH was higher than 12.5, the decrease of the sludge pH was almost negligible. It may be that high alkaline dosage broke through the buffer capacity of sludge (Li et al., 2012). Hence, the alkaline hydrolysis pH should be controlled at 12.5. 3.1.2. Effect of sludge concentration Different concentrations of sludge were prepared and then 10 mol/L NaOH was added to adjust pH to 12.5 under continuous stirring for 2 h. The changes of the concentrations of TP, PO3 4 -P, TN and NH+4-N in the alkaline hydrolysis supernatant with sludge concentration were shown in Fig. 2b. As was shown in Fig. 2b, the concentrations of TN, TP, NH+4-N and PO3 4 -P increased with the increase of sludge concentration, among which the increase of TN was the fastest, the increase of TP was faster than that of PO3 4 -P, while the increase of NH+4-N was unapparent. The reason may be that, at high alkaline hydrolysis pH, the sludge flocs were firstly broken and then the sludge cells were disrupted (Li et al., 2008). In addition, extracellular polymeric substances (EPS) was one of the three components of the sludge flocs structure, proteins and polysaccharides accounted for 70–80% of the entire EPS (Higgins and Novak, 1997). The release of TN was mainly from the hydrolysis of nitrogenous organic matter, such as protein. Therefore, the TN increased fastest among the substances above. While, part of NH+4-N produced by the TN conversion could be converted to

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+ Fig. 2. Changes of TP, PO3 4 -P, TN and NH4-N in the alkaline hydrolysis supernatant with (a) alkaline hydrolysis pH, (b) sludge concentration, and (c) reaction time.

ammonia gas to escape from sludge under the alkaline hydrolysis pH of 12.5, thus the increase of NH+4-N was unapparent. It also can be observed that the higher the sludge concentration, the more cellular substances from the alkaline hydrolysis sludge leaching into aqueous phase. However, the concentrations of PO3 4 -P and TN decreased after the sludge concentration reached + 50 g/L. Besides, the release efficiencies of PO3 4 -P and NH4-N decreased with the increase of sludge concentration, according

to the calculation of Fig. 2b. The most probable explanation was that at high sludge concentration, the water content of the sludge was low, resulting in the decrease of intracellular material leaching into the aqueous phase. Visibly, a low sludge concentration of 20–30 g/L was beneficial to the release of PO3 4 -P and NH+4-N. Therefore, the sludge concentration should be controlled at 20–30 g/L and the sludge concentration was maintained at 20 g/L in the subsequent experiments.

W. Bi et al. / Bioresource Technology 166 (2014) 1–8

3.1.3. Effect of reaction time 10 mol/L NaOH was added to the sludge with a concentration of 20 g/L to adjust pH to 12.5. The changes of the concentrations of TP, + PO3 4 -P, TN and NH4-N in the alkaline hydrolysis supernatant with the reaction time were shown in Fig. 2c. As was shown in Fig. 2c, the release of TP, NH+4-N and PO3 4 -P mainly took place in the first 30 min. Then the concentrations of TP, NH+4-N and PO3 4 -P increased slowly after 30 min, especially after 1 h. Similar results were also found by other researchers (Li et al., 2008, 2013; Xiao and Liu, 2006). Li et al. (2008) observed that the alkaline hydrolysis process included two stages: an initial rapid phase of 0.5 h and a subsequent slow stage. However, the concentration of TN still increased from 480 to 525 mg/L after 1 h. In addition, among the substances above, the concentration of TN was the highest, the concentration of TP was higher than that + of PO3 4 -P, while the concentration of NH4-N was the lowest. The reason might be that protein was a major component of the sludge and the dissolution ratio of particle proteins increased after the sludge was alkaline hydrolyzed. Therefore, the concentration of TN was the highest among all the released substances above. While part of NH+4-N might be converted to ammonia gas to escape from sludge under the alkaline hydrolysis pH of 12.5, so the concentration of NH+4-N was the lowest. According to the calculation of Fig. 2c, it can be seen that in the first 1 h of alkaline hydrolysis, + the release efficiencies of PO3 4 -P and NH4-N were 26.05% and 3.72%, respectively. And after 1 h, the release efficiencies of PO3 4 P and NH+4-N stayed constant around 31% and 5%, respectively. In addition, during the process of alkaline hydrolysis sludge, solid particles containing phosphorus could be hydrolyzed to soluble organic phosphorus in a relatively short period of time, and the rate of organic phosphorus further converted into PO3 4 -P might be faster than that of TN further converted into NH+4-N. Visibly, a good PO3 4 -P release effect could be achieved during the 0.5–1 h of alkaline hydrolysis sludge.

3.2. Two-stage alkaline hydrolysis 3.2.1. The optimal running conditions As shown in Table 1, the PO3 4 -P concentrations in group 11 and 12 were much higher than other groups. In addition, the supernatant pH in group 11 and 12 were 9.72 and 9.09 respectively, both meeting the aim of developing the TSAHP. Since the soluble chemical oxygen demand (SCOD) in group 11 (2177 mg/L) was much higher than that of group 12 (1214 mg/L), and a higher SCOD indicated a better sludge stabilization after alkaline hydrolysis as well as a bigger potential for methane production by anaerobic digestion of the supernatant after phosphorus and nitrogen recovery, the running conditions of group 11 (a = 13 and b = 1.75) was utilized as the optimal running conditions for the TSAHP.

3.2.2. Release effect of phosphorus and nitrogen through two-stage alkaline hydrolysis Under the optimal running conditions, the data of 16 batches of experiments proved that it could not only maintain the pH of the alkaline hydrolysis supernatant at 9.0–10.5, but also save more than 15% of the alkali consumption than that of the single-stage alkaline hydrolysis process (SSAHP). 2.8 L fresh sludge with a concentration of 20 g/L was treated in 16 batches, the experimental statistics showed that the concentra+ tions of TP, PO3 4 -P and NH4-N in the alkaline hydrolysis supernatant were 72.5–75 mg/L, 62–64 mg/L and 62.5–64.5 mg/L, + respectively. The release efficiencies of PO3 4 -P and NH4-N were 41.96% and 7.78%, respectively. In addition, pH of the alkaline hydrolysis supernatant was 9.50–9.88.

5

3.3. Effect of influence factors on recovery of phosphorus and nitrogen by MAP method 3.3.1. Effect of Mg/P ratio on recovery of phosphorus and nitrogen The initial molar ratio of N/P was calculated by the concentra+ tions of PO3 4 -P and NH4-N from the supernatant of the alkaline hydrolysis sludge. It was found that the molar concentration of NH+4-N was more than two times higher than that of PO3 4 -P in the supernatant. So the concentration of NH+4-N was surplus during MAP reaction, and the concentration of nitrogen was not taken into consideration. While the experimental statistics showed that the pH of the supernatant was 9.26–9.51, which met the pH condition of MAP reaction. Therefore, the molar ratio of Mg/P (Mg2+/PO3 4 -P) was the critical factor for the recovery of phosphorus and nitrogen from the alkaline hydrolysis supernatant by MAP. The formation of MAP requires the presence of Mg, N and P with a theoretical molar ratio of 1:1:1, it was observed that the molar concentration of NH+4N was greater than that of PO3 4 -P, thus Mg/P could be rate-limiting step of MAP reaction. After 10 min of MAP reaction at different Mg/ P, the mixture was centrifuged. The changes of the recovery efficiency of NH+4-N, PO3 4 -P and TP with Mg/P were shown in Fig. 3a. Fig. 3a showed that the recovery efficiencies of TP, PO3 4 -P and NH+4-N increased in turn, with the increase of Mg/P. But when Mg/P was greater than 1.4, the recovery efficiency of PO3 4 -P stayed constant around 23.53%, while the recovery efficiency of TP still increased. And the recovery efficiency of TP also stayed constant around 23.33% when further increasing Mg/P from 1.8 to 2.0. Because MAP reaction mainly occurred with PO3 4 -P, so when Mg/P increased, the peak of the recovery efficiency of PO3 4 -P was firstly achieved. In addition, when Mg/P increased from 1.4 to 1.8, except that PO3 4 -P reacted to form MAP, some other species of phosphorus (such as organic phosphorus) also precipitated from the supernatant, or even combined with magnesium. Therefore, the recovery efficiency of TP still increased after the recovery efficiency of PO3 4 -P stayed constant. Besides, the recovery efficiency of NH+4-N increased slowly when Mg/P was higher than 1.4. It could be seen that Mg/P of 1.4:1–1.8:1 was greater than the theoretical value of 1:1 in the form of MAP. The possible reason was that there was high concentration of SCOD, such as proteins, lipids in the alkaline hydrolysis supernatant, and some of the magnesium would combine with SCOD or even co-precipitate with SCOD (Tong and Chen, 2007). So the dosage of magnesium was greater than the theoretical value during the recovery of phosphorus and nitrogen in the form of MAP. 3.3.2. Effect of pH on recovery of phosphorus and nitrogen pH is an important parameter in MAP reaction. Although H+ does not directly affect ion equilibrium in the solution, the concentrations of free NH+4 and PO3 ions are highly dependent on pH 4 (Nelson et al., 2003). Therefore, pH not only affects the yield of struvite, but also influences the purity of struvite. Lee et al. (2003) used a chemistry equilibrium model (MINTEQ) to indicate the mechanism of struvite formation, and it was observed that struvite precipitation occurred across a pH range of 7.5–10, when pH was higher than 10, the mainly components of the precipitate was magnesium phosphate (KSP = 9.8  1025), which was less soluble than struvite, but when pH was higher than 11, magnesium hydroxide would be the dominant species. In this study, in order to determine the optimum pH of MAP reaction, pH was adjusted from 8.0 to 10.5. Magnesium source was added at a Mg/P molar ratio of 1.4:1. The following step was the same as described in Section 3.3.1. The changes of the recovery efficiency of NH+4-N, PO3 4 -P and TP with pH were shown in Fig. 3b. Fig. 3b showed that there was a platform about TP recovery efficiency when pH ranged from 8.0 to 8.5. After the pH of 8.5, the recovery efficiency of TP increased with the increase of pH. And

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W. Bi et al. / Bioresource Technology 166 (2014) 1–8

tion. Therefore, the recovery efficiency of TP was low. However, when pH increased from 9.5 to 10.0, the recovery efficiency of PO3 4 -P remained constant, while TP recovery efficiency still increased slowly. At this point, the reason why the recovery efficiency of TP increased might be that part of the organic phosphorus precipitated from the supernatant by flocculation precipitation. In addition, the recovery efficiency of NH+4-N increased slowly when pH was lower than 9.0, then it increased significantly after pH increased to 9.0, and the peak of NH+4-N recovery efficiency was attained around 41.98% at pH 10.5. This might be due to the ammonium volatilization by conversion to free ammonia as pH increased. Therefore, the suitable pH should be controlled at about 9.5.

3.3.3. Recovery efficiency of phosphorus and nitrogen with reaction time At pH of 9.5, magnesium source was added at a Mg/P molar ratio of 1.4:1. The changes of the recovery efficiency of TP, PO3 4 P and NH+4-N with reaction time were shown in Fig. 3c. It could be seen that MAP reaction was completed within 10 min. Then + the recovery efficiencies of TP, PO3 4 -P and NH4-N increased slowly 3 after 10 min, among which, PO4 -P recovery efficiency was the highest, the recovery efficiency of TP was the lowest. The reason might be that MAP reaction, which was based on PO3 4 -P precipitation product, occurred quickly, and could be completed in short time. In addition, the molar concentration of NH+4-N was more than two times higher than that of PO3 4 -P in the supernatant from the TSAHP. Therefore, PO3 4 -P recovery efficiency was higher than that of NH+4-N. Similar results were also found by other researchers (Lee et al., 2003; Uludag-Demirer and Othman, 2009). It could be seen that 10 min was enough for efficient phosphorus and nitrogen recovery.

3.4. Optimum conditions of recovery of phosphorus and nitrogen from alkaline hydrolysis supernatant of excess sludge by MAP Based on the results of Section 3.3, the experiments were conducted following a three-level-three-variable Box–Behnken design (BBD), which was designed by Design Expert software to evaluate the effects of molar ratio of (Mg/P), pH, and reaction time on the recovery of PO3 4 -P. The coded levels of the variables were shown in Table 2. In addition, seventeen batch experiments designed by BBD together with the response values obtained from the experiments were given in Table 3.

3.4.1. Establishment of prediction model According to the Design Expert software, PO3 4 -P recovery efficiency could be fitted by a quadratic model, which was shown in Eq. (1) (in terms of coded factors).

PO3 4 -P recovery efficiency ð%Þ ¼ 34:19 þ 7:66A þ 3:63B þ 1:61C + Fig. 3. The changes of recovery efficiency of TP, PO3 4 -P and NH4-N with (a) Mg/P molar ratio, (b) pH, and (c) reaction time.

the peak of TP recovery efficiency was achieved around 25.81%, when pH increased to 10.0. As pH increased, the recovery efficiency 3 of PO3 4 -P also increased, but a platform about PO4 -P recovery efficiency occurred and stayed constant after pH 9.5. The possible reason was that the solubility of struvite decreased with the increase of pH, when pH ranged from 8.0 to 9.5 (Doyle and Parsons, 2002). Especially, in pH range of 8–8.5, the struvite produced by PO3 4 -P reaction had certain solubility, resulting in less struvite precipita-

 1:61AB þ 0:81AC þ 0:81BC  3:39A2  3:39B2 þ 3:87C2

ð1Þ

where A is Mg/P, B is pH, C is reaction time/min.

Table 2 RSM design for factors and levels. Factor

Mg/P pH Reaction time (min)

Code

A B C

Level 1

0

1

1.0 9.0 5

1.4 9.5 10

1.8 10.0 15

7

W. Bi et al. / Bioresource Technology 166 (2014) 1–8 Table 3 Box–Behnken experimental design and results. Run

A

B

C

PO3 4 -P recovery efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

1.4 1.4 1.4 1.8 1.8 1.4 1.0 1.0 1.4 1.4 1.4 1.8 1.0 1.0 1.4 1.8 1.4

9.0 9.5 9.5 9.5 10.0 9.5 9.0 10.0 9.5 9.5 10.0 9.5 9.5 9.5 9.0 9.0 10.0

5 10 10 5 10 10 10 10 10 10 15 15 15 5 15 10 5

29.03 35.48 32.26 38.71 38.71 32.26 12.90 22.58 35.48 35.48 41.94 41.94 29.03 29.03 32.26 35.48 35.48

A: Mg/P, B: pH, C: Reaction time (min).

3.4.2. Analysis of variance for PO3 4 -P recovery model The analysis of variance (ANOVA) for PO3 4 -P recovery efficiency was shown in Table 4. The Model F-value of 11.55 implied that the model was significant. Values of ‘‘Prob > F’’ less than 0.05 indicated that the model terms were significant. As seen in Table 4, A (Mg/P), B (pH), A2, B2 and C2 were the significant model terms, among which Mg/P was more significant than pH, whereas C (reaction time) was insignificant. The ‘‘Lack of fit F-value’’ of 4.17 implied that the Lack of fit was not significant relative to the pure error. In addition, the ‘‘R-squared’’ of 0.9369 indicated that the model could explain 93.69% of the variability. And the ‘‘Adep Precision’’ of 14.536 indicated an adequate signal. Therefore, this model can be used to navigate the design space. 3.4.3. Determination of optimum process parameters The optimum conditions of Mg/P of 1.8, pH 9.7 and reaction time of 15 min were determined through predicting optimum value of PO3 4 -P recovery efficiency by RSM. Under such conditions, the optimum recovery efficiency of PO3 4 -P, which was predicted by RSM, was 45.35%. In addition, three experimental batches were conducted to test the model of recovery of PO3 4 -P, and the corresponding experimental PO3 4 -P recovery efficiency was 46.88%. It can be seen that the experimental value was quite close to the model value, besides the recovery efficiency of NH+4-N was 16.54%. The dried precipitate was analyzed by EDS, the results showed that the main elemental components of the precipitate were magnesium and phosphorus, and the molar ratio of Mg/P was 0.66:1, which was less than the theoretical value of 1:1 for MAP. The rea-

son might be that some extra of PO3 4 -P bound to the precipitate by adsorption and/or coprecipitation with MAP. Actually, it was observed that SCOD clearly decreased from 2213.6 mg/L to 2059.2 mg/L after phosphorus and nitrogen recovery in the form of MAP, which suggested a coprecipitation of biopolymers such as protein and polysaccharide with MAP. This phenomenon was also observed by Tong and Chen (2007). The EDS suggested that Al, Ca and Fe existed in the precipitate, especially with a high content for Al. Here, it should be pointed out that aluminum sulfate was added before the secondary sedimentation tank of A/O process for strengthening the biological phosphorus removal. And it is well known that aluminum salt could be dissolved both with acid and base (Petzet et al., 2011). For the high pH (above 9.5) in TSAHP, the aluminum salt from the excess sludge would be dissolved and transferred into aluminate, which would enter into the supernatant. Guibaud et al. (2009) found that Al, Ca and Fe can combine with proteins and polysaccharides. Therefore, Al, Ca and Fe may also enter into the precipitate with the biopolymers (proteins and polysaccharides) by adsorption and/or co-precipitation. Moreover, abundant Al playing a role of coagulant may strength the adsorption and/or coprecipitation. This could partly explain the high decrease of SCOD as talked above. Since the precipitate was complicated, an analysis of the precipitate by X-ray diffraction (XRD) and scanning electron microscopy (SEM) that commonly used for struvite analysis failed (data not shown). In order to further confirm the existence of MAP in the precipitate, the precipitate was dissolved with dilute hydrochloric acid and the molar ratio of Mg2+, NH+4-N and PO3 4 -P in the solution was determined. The result showed that the molar ratio of Mg2+/NH+4-N/PO3 4 -P was 1.15:1:1.66. It indicated a molar ratio of Mg2+/PO3 4 -P being 0.69:1, which was approximated to the molar ratio of Mg/P being 0.66:1 as observed by EDS. While, the possible reason for the molar ratio of Mg2+/NH+4-N in the precipitate being greater than 1:1 was that some Mg2+ combined with organic substances (Tong and Chen, 2007). Therefore, though other compounds (including phosphate compounds not in the form of MAP) would coprecipitate during the recovery of phosphorus and nitrogen in the form of MAP, MAP should exist in the precipitate. The above analysis suggested that the existence of rich aluminum in excess sludge would affect MAP formation as well as its purity. Therefore, in order to recover phosphorus and nitrogen from excess sludge by MAP, an addition of aluminum sulfate to strengthen the biological phosphorus removal should not be advocated. 4. Conclusions Volume ratio (sludge dosage/water dosage) of 1.75 and pH 13 in reactor B, and 20 g/L of fresh sludge concentration in reactor A

Table 4 Analysis of variance for RSM quadratic model. Source

Sum of squares

Degree of freedom

Mean square

F value

P-value (Prob > F)

Model A-Mg/P B-pH C-reaction time AB AC BC A2 B2 C2 Residual Lack of fit Pure error Cor total

765.04 469.56 105.36 20.81 10.41 2.60 2.60 48.30 48.30 63.09 51.51 39.02 12.49 816.55

9 1 1 1 1 1 1 1 1 1 7 3 4 16

85.00 469.56 105.36 20.81 10.41 2.60 2.60 48.30 48.30 63.09 7.36 13.01 3.12

11.55 63.81 14.32 2.83 1.41 0.35 0.35 6.56 6.56 8.57

0.0020 pH > reaction time, and Mg/P of 1.8, pH 9.7 and reaction time of 15 min were the optimized conditions for PO3 4 -P recovery. Acknowledgement This research was supported by the Guangdong Scientific and Technological Project of China (2007A032302001). References Appels, L., Degreve, J., Van der Bruggen, B., Van Impe, J., Dewil, R., 2010. Influence of low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion. Bioresour. Technol. 101 (15), 5743–5748. Balmer, P., 2004. Phosphorus recovery – an overview of potentials and possibilities. Water Sci. Technol. 49 (10), 185–190. Becerra, F.Y.G., Acosta, E.J., Allen, D.G., 2010. Alkaline extraction of wastewater activated sludge biosolids. Bioresour. Technol. 101 (18), 6972–6980. Chen, Y.G., Jiang, S., Yuan, H.Y., Zhou, Q., Gu, G.W., 2007. Hydrolysis and acidification of waste activated sludge at different pHs. Water Res. 41 (3), 683–689. Doyle, J.D., Parsons, S.A., 2002. Struvite formation, control and recovery. Water Res. 36 (16), 3925–3940. Eskicioglu, C., Prorot, A., Marin, J., Droste, R.L., Kennedy, K.J., 2008. Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H(2)O(2) for enhanced anaerobic digestion. Water Res. 42 (18), 4674–4682. Guibaud, G., van Hullebusch, E., Bordas, F., d’Abzac, P., Joussein, E., 2009. Sorption of Cd(II) and Pb(II) by exopolymeric substances (EPS) extracted from activated sludges and pure bacterial strains: modeling of the metal/ligand ratio effect and role of the mineral fraction. Bioresour. Technol. 100 (12), 2959–2968. Higgins, M.J., Novak, J.T., 1997. Characterization of exocellular protein and its role in bioflocculation. J. Environ. Eng. 123 (5), 479–485. Kim, J., Park, C., Kim, T.H., Lee, M., Kim, S., Kim, S.W., Lee, J., 2003. Effects of various pretreatments for enhanced anaerobic digestion with waste activated sludge. J. Biosci. Bioeng. 95 (3), 271–275. Korboulewsky, N., Dupouyet, S., Bonin, G., 2002. Environmental risks of applying sewage sludge compost to vineyards: carbon, heavy metals, nitrogen, and phosphorus accumulation. J. Environ. Qual. 31 (5), 1522–1527. 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 (4), 265– 271. Li, H., Jin, Y.Y., Mahar, R., Wang, Z.Y., Nie, Y.F., 2008. Effects and model of alkaline waste activated sludge treatment. Bioresour. Technol. 99 (11), 5140–5144. Li, H., Li, C.C., Liu, W.J., Zou, S.X., 2012. Optimized alkaline pretreatment of sludge before anaerobic digestion. Bioresour. Technol. 123, 189–194.

Li, H., Zou, S.X., Li, C.C., Jin, Y.Y., 2013. Alkaline post-treatment for improved sludge anaerobic digestion. Bioresour. Technol. 140, 187–191. Lin, J.T., Hu, Y.Y., Wang, G.H., Lan, W.C., 2012. Sludge reduction in an activated sludge sewage treatment process by lysis-cryptic growth using ClO2ultrasonication disruption. Biochem. Eng. J. 68, 54–60. Ma, H.J., Zhang, S.T., Lu, X.B., Xi, B., Guo, X.L., Wang, H., Duan, J.X., 2012. Excess sludge reduction using pilot-scale lysis-cryptic growth system integrated ultrasonic/alkaline disintegration and hydrolysis/acidogenesis pretreatment. Bioresour. Technol. 116, 441–447. Ministry of Environmental Protection, China (MEP), 2002. Standard Methods for the Examination of Water and Wastewater, fourth ed. Ministry of Environmental Protection, Beijing, China. Nelson, N.O., Mikkelsen, R.L., Hesterberg, D.L., 2003. Struvite precipitation in anaerobic swine lagoon liquid: effect of pH and Mg:P ratio and determination of rate constant. Bioresour. Technol. 89 (3), 229–236. Park, B., Ahn, J.H., Kim, J., Hwang, S., 2004. Use of microwave pretreatment for enhanced anaerobiosis of secondary sludge. Water Sci. Technol. 50 (9), 17–23. Petzet, S., Peplinski, B., Bodkhe, S.Y., Cornel, P., 2011. Recovery of phosphorus and aluminium from sewage sludge ash by a new wet chemical elution process (SESAL-Phos-recovery process). Water Sci. Technol. 64 (3), 693–699. Stark, K., Plaza, E., Hultman, B., 2006. Phosphorus release from ash, dried sludge and sludge residue from supercritical water oxidation by acid or base. Chemosphere 62 (5), 827–832. Tong, J., Chen, Y.G., 2009. Recovery of nitrogen and phosphorus from alkaline fermentation liquid of waste activated sludge and application of the fermentation liquid to promote biological municipal wastewater treatment. Water Res. 43 (12), 2969–2976. Tong, J.A., Chen, Y.G., 2007. Enhanced biological phosphorus removal driven by short-chain fatty acids produced from waste activated sludge alkaline fermentation. Environ. Sci. Technol. 41 (20), 7126–7130. Torres, M.L., Llorens, M.D.E., 2008. Effect of alkaline pretreatment on anaerobic digestion of solid wastes. Waste Manage. 28 (11), 2229–2234. Uludag-Demirer, S., Othman, M., 2009. Removal of ammonium and phosphate from the supernatant of anaerobically digested waste activated sludge by chemical precipitation. Bioresour. Technol. 100 (13), 3236–3244. Wang, G.H., Sui, J., Shen, H.S., Liang, S.K., He, X.M., Zhang, M.J., Xie, Y.Z., Li, L.Y., Hu, Y.Y., 2011. Reduction of excess sludge production in sequencing batch reactor through incorporation of chlorine dioxide oxidation. J. Hazard. Mater. 192 (1), 93–98. Wang, X.X., Qiu, Z.F., Lu, S.G., Ying, W.C., 2010. Characteristics of organic, nitrogen and phosphorus species released from ultrasonic treatment of waste activated sludge. J. Hazard. Mater. 176 (1–3), 35–40. Xiao, B.-Y., Liu, J.-X., 2006. Study on treatment of excess sludge under alkaline condition. Huan Jing Ke Xue. 27 (2), 319–323. Xue, T., Huang, X., 2007. Releasing characteristics of phosphorus and other substances during thermal treatment of excess sludge. J. Environ. Sci. 19 (10), 1153–1158. Yan, S.T., Chu, L.B., Xing, X.H., Yu, A.F., Sun, X.L., Jurcik, B., 2009. Analysis of the mechanism of sludge ozonation by a combination of biological and chemical approaches. Water Res. 43 (1), 195–203. Yan, Y.Y., Feng, L.Y., Zhang, C.J., Wisniewski, C., Zhou, Q., 2010. Ultrasonic enhancement of waste activated sludge hydrolysis and volatile fatty acids accumulation at pH 10.0. Water Res. 44 (11), 3329–3336.