Fraction Distributions of Phosphorus in Sewage Sludge and Sludge ...

Waste Biomass Valor (2012) 3:355–361 DOI 10.1007/s12649-011-9103-5

ORIGINAL PAPER

Fraction Distributions of Phosphorus in Sewage Sludge and Sludge Ash Huacheng Xu • Hua Zhang • Liming Shao Pinjng He

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Received: 15 September 2011 / Accepted: 15 December 2011 / Published online: 24 December 2011 Ó Springer Science+Business Media B.V. 2011

Abstract Previous publications on phosphorus (P) in sewage sludge and sludge ash have mostly focused on the total contents, while information on the fraction distribution of P is rarely examined. In this study, P in sewage sludge was, for the first time, fractioned into NH4Cl-P, BDP, NaOH-P, HCl-P and Res-P via increasing chemical reagents. Its speciation evolution during sludge incineration was also investigated. Results showed that the total P content in sewage sludge had a wide range (0.97–1.74%), and NaOH-P and Res-P were the dominant fractions, which contributed with 67.1–81.2% of sludge P. Sludge incineration process increased the relative proportion of HCl-P from the initial 9.1–11.7% (sewage sludge) to 63.0–78.0% (from ash when sludge was incinerated at 900°C). Furthermore, P in sludge ash was respectively extracted by acid (HCl) and base (NaOH). It was found that the extraction percentage by HCl reached more than 90%, while that by NaOH was less than 30%. The different extraction efficiencies could be explained by the P sequential extraction procedure applied in this study. Keywords Fraction distributions Phosphorus Sewage sludge Sludge ash Extraction efficiency

Introduction The accumulation of sewage sludge produced from wastewater treatment plants (WWTPs) is a growing

H. Xu H. Zhang L. Shao P. He (&) State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China e-mail: [email protected]

environmental problem. It is expected that the annual dried sludge output in China is about 6 million tonnes. Land application and incineration are two widely applied methods for sludge disposal in many parts of the world [1]. Land application could utilize the valuable components in sludge, making it an attractive and feasible option for sludge disposal. Among the valuable components of sewage sludge, phosphorus (P) is an important nutrient that stimulates the growth of crops and other photosynthetic microorganisms [2]. Total P content of land application sludge has been widely reported in literature [3–6], whereas information about the speciation fractions would be more useful in evaluating its contribution, since not all P in sludge is readily available to crops. Commonly, P in sludge consists of organic P and inorganic P, and the latter could be further divided into various fractions based on the different binding abilities. Attempts have been made to explore the fractions of P in sludge [2, 7], but most of these studies only divided sludge P into non-apatite inorganic P, apatite P, and organic P, which was insufficient to evaluate the inorganic P. Thus, a more detailed P extraction procedure, commonly used in soils and lake sediments [8, 9], was applied in this study for the first time to fractionate the sludge P. Using this procedure, NH4Cl-P and BD-P are considered as labile P, NaOH-P as the P bound to metal oxides (aluminum and iron), and HCl-P represents to P associated with calcium and magnesium. Sludge incineration has the benefits of volume reduction with hazardous compound destruction and has become an increasingly adopted sludge treatment process [10]. Previous publications on sludge incineration focused mainly on the emissions and speciation evolution of heavy metals [11, 12], whereas information about the speciation evolution of P during sludge incineration was rarely provided.

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As already known, P in sewage sludge is non-volatile, causing incineration sludge ash to be rich in P (4–9%), which indicates a potential P resource [13–15]. P in sludge ash can be extracted by chemical extraction solvents [16]. Extensive experimental work has been conducted to explore the influencing factors on P extraction. For example, Hong et al. [16] studied the effect of acid concentration on P extraction from sludge ash, and Cohen [17] further studied the effects of acid amounts and concentrations on P extraction. The exact acid requirement for complete P extraction (0.013 and 0.016 g H?/g ash, respectively) was obtained, but the reason to the different results was not given due to the non-performance of the P fraction for sewage sludge ashes. This study attempted to explain the different release efficiencies reported by previous researchers using the above mentioned P extraction procedure. In this study, sewage sludge samples were collected from four full-scale WWTPs in Anhui province (the middle-east of China). The amounts and fraction distributions of P in sludge with respect to land application were firstly investigated, then the sludge samples were incinerated at different temperatures and the P speciation evolutions were examined. Finally, P extraction from sludge ash was studied by acid and base, and the different extraction efficiencies were explained. Increasing our knowledge on this could help us to better understand P in sewage sludge and sludge ash, and even to evaluate the maximum P release potential for some environmental samples. Table 1 Physical–chemical characteristics for the four sludge samples

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Methods and Materials Samples Sludge samples obtained from WWTPs were dried at 48°C for 72 h to a constant weight, and then ground by a ballmill (Retsch MM 400, Germany) to obtain fine powders. The powdered samples were sieved by passing them through a nylon fiber sieve. The resulting samples with particle sizes smaller than 70 lm were separated and stored in a desiccator at room temperature. The agronomic parameters are shown in Table 1. Dried sludge samples were then, respectively, incinerated at 500, 600, 700, 800 and 900°C for 120 min in a laboratory furnace reactor (SX2-12-10, Shanghai, China). As can be seen, the four sludge samples both had relatively low organic matter contents (\45%), and this would be due to their long sludge retention time ([17 days). Total P contents displayed a wide range, with ZZJ sludge being smallest (0.97%) and WXY sludge producing the largest (1.74%). The P contents reported herein were comparable with those in animal manure [18], but higher than that in lake sediments [8]. N/P ratios in the four samples ranged from 1.75 to 2.73, which were lower than the typical N/P uptake ratio of 7.5 for crops, indicating that P applied with sludge would far exceed the amount of P required by crops [19]. Thus, long-term sludge application to agricultural land would lead to soil P accumulation and an acceleration of P transfer in run-off to water bodies. High contents of

Name of WWTPs

Chaohu (CH)

Wangtang (WT)

Wangxiaoying (WXY)

Zhuzhuanjing (ZZJ)

Process

Oxidation ditch

Oxidation ditch

Oxidation ditch

SBR

Organic matter (%)

44.35 ± 0.08

38.16 ± 0.05

39.96 ± 0.09

33.89 ± 0.02

Chemical precipitation

No

Ferrous sulfate

Calcium chloride

No

Wastewater composition

90% of domestic and 10% of industrial wastewater



C70% of domestic wastewater

TN (%)

3.22 ± 0.04

2.60 ± 0.00

3.05 ± 0.03

2.64 ± 0.07

TP (%)

1.48 ± 0.01

1.19 ± 0.04

1.74 ± 0.03

0.97 ± 0.02

K (mg/kg)

13,550 ± 30

12,010 ± 23

14,207 ± 36

10,579 ± 45

Na (mg/kg)

2,934 ± 12

3,056 ± 14

3,550 ± 16

3,223 ± 23

Mg (mg/kg)

14,170 ± 35

12,409 ± 33

15,840 ± 17

10,975 ± 46

Ca (mg/kg)

13,845 ± 22

13,785 ± 37

16,266 ± 44

12,361 ± 27

Al (mg/kg)

22,672 ± 19

35,160 ± 58

28,798 ± 44

35,273 ± 35

Fe (mg/kg)

21,035 ± 56

25,912 ± 29

27,128 ± 17

31,010 ± 54

N/P

2.18

2.18

1.75

2.73

Ca/P

0.94

1.16

0.94

1.28

Al/P

1.53

2.95

1.66

3.65

Fe/P

1.42

2.17

1.56

3.21


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iron and aluminum in sludge samples would be due to the chemical precipitation and/or high industrial wastewater proportion (Table 1). With regard to other macro-elements, calcium was the most abundant (12,361–16,266 mg/kg), followed by magnesium (10,975–15,840 mg/kg), potassium (10,579–14,207 mg/kg) and sodium (2,934–3,550 mg/kg), taken over all samples. The Sequential Extraction of P The fractionation of P was carried out using the sequential extraction procedure of Psenner et al. [20] with slight modifications by Hupfer et al. [21]. Sludge was subjected to sequential chemical extraction with 1 mol/L NH4Cl, 0.11 mol/L Na2S2O4/NaHCO3, 1 mol/L NaOH and 0.5 mol/L HCl. The solvent extracts were centrifuged at 6,000 rpm for 20 min, and the supernatants were filtered through a 0.45 lm P-free membrane for P determination. The total P content was achieved by acid digestion with nitric and sulfuric acids at 250°C. Residual P (Res-P) was defined as the difference between the total P and the extracted P in four extracts. Based on this extraction procedure, P in sludge was fractionated into NH4Cl-P, BD-P, NaOH-P, HCl-P and Res-P. NH4Cl-P and BD-P are considered as labile P, and NaOH-P and HCl-P are, respectively denoted as Al/Fe-P and Ca/Mg-P, while Res-P represents the stabilized P in sludge matrix. P Release from Sludge Ashes The extraction experiments were carried out at room temperature. Before analysis, all glassware was treated in a 5% nitric acid solution for 24 h and washed with de-ionized water. The ash samples were extracted using different types of extraction solvents such as HCl and NaOH at varying concentrations (0.01–0.8 mol/L) and liquid/solid (L/S) ratios (25–200 ml/g). A measured amount of ash sample (0.5 g) was mixed with the extraction solvents in a conical flask and shaken at 120 rpm for 2 h, then the slurry was immediately filtered and the filtrate was analyzed for P concentration. Table 2 Content and fraction distribution of P in sewage sludge

Values in parentheses are the proportion (%) of each fraction relative to total P

Fractions

Other Analytical Methods Total nitrogen (TN) was measured with an element analyzer (vario EL III, Elementar, German). P was determined by the molybdenum blue/stannous chloride method [22]. Metals in sludge were analyzed by inductively coupled plasma-optical emission spectroscopy (ICP-OES). All experiments were carried out in duplicate and the reagents used in this work were of analytical reagent grade.

Results and Discussion Contents and Fraction Distribution of Phosphorus in Sewage Sludge Table 2 summarizes the contents and fraction distribution of P in sludge and the relative contributions of each fraction to total P. Different P distribution patterns were observed among the four sludge samples. NH4Cl-P is termed as the immediately available P and is easily released into water [9]. In this study, CH sludge and WXY sludge released 13.4 and 15.8% of total P into NH4Cl extraction solvent, respectively, while WT sludge and ZZJ sludge only released 5.2 and 2.6% of total P, respectively. The lower proportions of NH4Cl-P in WT and ZZJ sludge samples would be due to their higher (Al ? Fe)/P ratios (Table 1). Pastene [23] suggested that higher (Al ? Fe)/P ratios would lead to very low effectiveness of sludge as a P fertilizer. Hedley and McLaughlin [24] also found that sewage sludge stabilized with Fe or Al was generally found to be less effective source of P for plant growth than Ca-stabilized or non-chemically treated sludge. The BD-P, which represents the redox-sensitive P fraction, were no more than 1 mg/g in four samples, and the relative contributions to total P were both less than 7%. In addition, the sum of NH4Cl-P and BD-P accounted for 7.1–21.5% of total P, which was lower than those of Turner and Leytem [18], who reported that most of P in swine manure was determined as NH4Cl-P (55%) and BD-P (23%). NaOH-P represents the P bound to metal oxides (Al and Fe) which is exchangeable against OH- [25, 26].

Sludge samples CH

WXY

WT

ZZJ

NH4Cl-P

1.98 ± 0.03 (13.4)

2.76 ± 0.00 (15.8)

0.62 ± 0.02 (5.2)

0.25 ± 0.01 (2.6)

BD-P NaOH-P

0.65 ± 0.03 (4.4) 4.25 ± 0.08 (28.8)

1.00 ± 0.03 (5.7) 5.32 ± 0.12 (30.5)

0.75 ± 0.02 (6.3) 4.97 ± 0.03 (41.7)

0.43 ± 0.00 (4.5) 4.85 ± 0.05 (50.2)

HCl-P

1.53 ± 0.02 (10.3)

1.98 ± 0.05 (11.4)

1.09 ± 0.02 (9.1)

1.14 ± 0.06 (11.7)

Res-P

6.37 ± 0.12 (43.1)

6.36 ± 0.08 (36.6)

4.49 ± 0.04 (37.7)

2.99 ± 0.03 (31.0)

14.78 ± 0.12 (100)

17.40 ± 0.03 (100)

11.92 ± 0.20 (100)

9.67 ± 0.13 (100)

Total P

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NaOH-P contents in this study exhibited slight fluctuations as follows: 4.25 mg/g in CH sludge; 5.32 mg/g in WXY sludge; 4.97 mg/g in WT sludge; and 4.85 mg/g in ZZJ sludge, while their corresponding proportions to total P were 28.8, 30.5, 41.7 and 50.2%, respectively. The proportions of NaOH-P in sludge samples were correlated with the molar ratios of Fe/P (R2 = 0.98, p = 0.019) and Al/P (R2 = 0.99, p = 0.003) (Table 1). HCl-P is defined as Ca– Mg-associated P, and the proportions of HCl-P in four samples stabilized between 9.1 and 11.7%, which were comparable with results from other sludge samples [2], but lower than those in lake sediments [9]. The relatively low HCl-P proportions in sewage sludge were attributed to the low Ca/P ratios (\1.3, Table 1). The relative contribution of Res-P ranged from 31.0 to 43.1%, suggesting that inorganic P was the main P fraction in sewage sludge. Similar results were also reported in other environmental samples [2, 26].

of NH4Cl-P, BD-P and NaOH-P, and their amounts only accounted for 15–20% of total P for ashes produced at 900°C, whilst the HCl-P sharply increased from the initial 9.1–11.7% (sewage sludge) to 63.0–78.0% (ashes produced at 900°C). As a result, Res-P decreased drastically from 31.0–43.1% (sewage sludge) to 7.0–11.7% (ashes produced at 900°C). Organic matter was burned during sludge incineration, causing the proportion of inorganic P increasing from 56.9–69.0% (sewage sludge) to 88.3–93.0% (ashes produced at 900°C). Furthermore, inorganic P mainly consisted of NaOH-P for the sewage sludge, whilst HCl-P became the main inorganic P species for the sludge ashes. In general, the proportion of NaOH-P and HCl-P accounted for about 90% with elevated incineration temperature (e.g. 900°C). The results were consistent with Peplinski et al. [27] who showed that P-bearing mineral phases in sewage sludge ashes were aluminium phosphates (AlPO4) and whitlockite (Ca3(PO4)2).

Speciation Evolutions of P During Sludge Incineration

P Recovery from Sludge Ash by Extraction

Mass reduction and P content in sludge ash during the incineration process is summarized in Table 3. Sludge reduction was observed during incineration process, and higher incineration temperatures caused a greater sludge reduction. As shown in Table 3, the remaining parts for CH sludge decreased from 66.65 ± 0.08% to 53.13 ± 0.05% when increasing the incineration temperature from 500 to 900°C. Consequently, P contents in sludge ashes increased from 22.18 ± 0.05 to 27.75 ± 0.04 mg/g. In addition, the sludge reduction and P contents in sludge ashes among other sludge samples were similar to those of the CH sludge. By comparing the P contents in sewage sludge and sludge ashes, it was found that P was not volatilized during the whole process of incineration. Figure 1 depicts the speciation evolution of P under different incineration temperatures. The application of the incineration process significantly decreased the proportions

Sludge ash obtained from incineration at 800°C was recommended for P extraction [14]. Figure 2 shows the P extraction from sludge ash by acid and base at varying extraction solvent concentrations and L/S ratios. The extraction percentage increased with increasing solvent concentrations and L/S ratios. As shown in Fig. 2a, P extraction at an L/S ratio of 25 ml/g, and HCl concentration of 0.2 mol/L was about 60%. Increasing the L/S ratio to 100 ml/g resulted in 92.5% of P extraction, and further increasing the L/S ratio did not significantly improve the P extraction. Thus, the most cost-effective acid consumption, 0.2 mol/L HCl with a L/S ratio of 100 ml/g were considered as the most suitable conditions. It was noticeable that the extraction percentage of P by HCl extraction was more than 90%, while those by NaOH extraction was less than 30% (Fig. 2b), clearly showing that base extraction was not an effective method. In addition, P extraction by water was

Table 3 Mass reduction and P content in sludge ash during the incineration process Sludge samples

Parameters

Incineration temperatures (°C) Dried sludge

CH sludge WT sludge WXY sludge

Remainder (%)

ZZJ sludge

700

800

900

66.65 ± 0.08

54.80 ± 0.09

53.78 ± 0.02

53.64 ± 0.03

53.13 ± 0.05

14.78 ± 0.12

22.18 ± 0.05

27.03 ± 0.02

27.47 ± 0.06

27.59 ± 0.07

27.75 ± 0.04

Remainder (%) P content (mg/g)

100 11.92 ± 0.20

61.84 ± 0.05 19.21 ± 0.06

61.59 ± 0.02 19.40 ± 0.01

60.46 ± 0.08 19.73 ± 0.06

60.01 ± 0.08 19.89 ± 0.03

59.61 ± 0.08 19.96 ± 0.09

Remainder (%) Remainder (%) P content (mg/g)

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600

P content (mg/g)

P content (mg/g)

100

500

100 17.40 ± 0.03 100 9.67 ± 0.13

60.04 ± 0.09

59.65 ± 0.01

59.06 ± 0.09

58.73 ± 0.03

58.36 ± 0.12

28.90 ± 0.07

29.16 ± 0.03

29.51 ± 0.02

29.63 ± 0.05

29.84 ± 0.03

66.11 ± 0.02

65.41 ± 0.05

65.13 ± 0.06

64.73 ± 0.04

64.33 ± 0.02

14.66 ± 0.08

14.79 ± 0.10

14.85 ± 0.03

14.98 ± 0.02

15.02 ± 0.04


359

Fig. 1 The speciation evolutions of P under different incineration temperatures

minimal (\0.2%), regardless of L/S ratios, indicating that P in ash is present in the form of insoluble salts [15]. Previous researchers had investigated the extraction efficiencies of P from sludge ash and also found that NaOH was not an efficient extraction solvent. For example, Stark et al. [4] showed that the extraction efficiency increased from 75 to 85% when the HCl concentration increased from 0.5 to 1.0 mol/L. Kuligowski and Poulsen [28] further reported a 94% extraction efficiency using 0.6 mol/L H2SO4. And Biswas et al. [13] found that the extraction efficiency was less than 30% by NaOH. However, the reasons for the different extraction efficiencies by acid and base were not explained in the previous studies. In this study, the P sequential procedure applied could explain the above-mentioned reasons. According to the assignment of P fractions, the sum of NH4Cl-P, BD-P and NaOH-P represented the parts that can be dissolved by NaOH, and those of NH4Cl-P, BD-P, NaOH-P and HCl-P denoted the parts that can be extracted by HCl. The total proportions of NH4Cl-P, BD-P and NaOH-P in the four ashes (Fig. 1) ranged from 14.4 to 24.7%, which theoretically explained the results of Biswas et al. [13] and in Fig. 2b that NaOH only extracted less than 30% of the total P. The total

proportions of NH4Cl-P, BD-P, NaOH-P and HCl-P in the four ashes accounted for about 90% of total P (Fig. 1), which is consistent with the results of Kuligowski and Poulsen [28] and in Fig. 2a that more than 90% of the extraction was obtained by HCl. In addition, the minimal release of P in water (Fig. 2) was also consistent with the results of Fig. 1, that NH4Cl-P only accounted for \1% of the total P. These consistent results showed that the P sequential procedure applied in this study could theoretically calculate the maximum P release potential for sewage sludge and sludge ashes, and even for other environmental samples, such as animal manure and lake sediments, which require further study.

Conclusions Sewage sludge has a high P content, ranging from 0.97 to 1.74%, and NaOH-P and Res-P were the main P fractions. Sludge incineration decreased the proportions of NH4Cl-P, BD-P and NaOH-P, while increased the HCl-P proportion. Sludge ashes are rich in P content and acid was a more effective extraction solvent than base for P extraction. The

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6.

7.

8.

9.

10.

11.

12.

13.

14. 15. Fig. 2 P extraction from sludge ash by a HCl and b NaOH

different extraction efficiencies can be successfully explained by the P sequential procedure applied in this study.

16.

17. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (20977066), the Program of Shanghai Subject Chief Scientist (10XD1404200) and the Specialized Research Fund for Doctoral Program of Higher Education of China (200802470029).

18.

19.

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