Thermal hydrolysis (TH) has been used to improve anaerobic digestion performance as well as the stability of heavy metals in sludge. Because the toxicity of heavy metals is closely related to both the concentration and the chemical speciation, more exhaustive studies on speciation distribution are urgently needed. This research aimed to investigate the effects of TH treatment (especially the time and temperature) on the concentration and stability of heavy metals in sludge, and to define the optimal TH conditions. The TH experiment indicated that the content of the stable form of Cu and Zn reached 83% and 47.4%, respectively, with TH at 210°C and 30 min. Compared with the raw sludge, the proportion of Cu and Zn increased by 11.88% and 7.3%, respectively. Results indicated that the heavy metals were combined with sludge in a more stable form with the pretreatment of TH, which improved the stability of heavy metals.

INTRODUCTION

The excess sludge from municipal wastewater treatment plants has become an outstanding environmental dilemma in recent years since it contains some harmful properties such as putrescibility, high content of pathogenic bacteria and presence of heavy metals or other chemical hazards (Romdhana et al. 2009). Among them, heavy metals have a negative effect. They can be easily concentrated in living organisms, and cause secondary contamination of groundwater or soil to some extent (Vymazal & Brezinova 2015). Regarding this, it is time to comprehensively study the principles of speciation distribution of the heavy metals in the excess sludge so as to take measures to get rid of them (Koncewicz-Baran et al. 2014). Researchers have gradually recognized that the severity of the problem depends not only on the total amount but also on the speciation distribution (Zhang et al. 2012). In the study of speciation of heavy metals, the BCR (Community Bureau of Reference) sequential extraction process is the most scientific process and has been extensively applied (Tokalioğlu et al. 2006). Among the four kinds of speciation identified by the BCR sequential extraction process, acid soluble and reduced state belong to the unstable state, whereas the oxidation and residual state refer to the stable state.

Thermal hydrolysis (TH) technology, as a way of improving anaerobic digestion performance of sludge, has been developed for a long time. Wang & Wang (2005) focused on improving gas production. Dewil et al. (2006) studied the effect of TH and Fenton oxidation on heavy metals of dewatered sludge. Obrador et al. (2001) carried out research on the speciation distribution rules of heavy metals through comparing the raw sludge with treated sludge under high temperature. Recent studies focused on the speciation distribution of heavy metals during anaerobic digestion. The majority of them indicated that the sludge stability was enhanced with TH treatment. Sun et al. (2010) investigated the principles of speciation distribution of Zn, Cu, Cd and Pb. They found that the fat and sugars as well as proteins were hydrolyzed into small molecular substances with the TH pretreatment. Therefore, the heavy metals would be released due to the loss of binding sites. From the above, it can be observed that those researchers mainly focused on the gas production or the speciation distribution of heavy metals during anaerobic digestion. However, there has been little research carried out on how TH treatment impacts the stability of heavy metals.

This research was aimed at evaluating the effect of TH treatment (hydrolysis time and temperature) on the total amount and stability of heavy metals in sludge, expecting to shed light on the optimal TH conditions in the sludge treatment.

MATERIALS AND METHODS

Materials and devices

The excess sludge of 84% moisture content was obtained from the sludge dewatering unit of the Yuliangzhou municipal wastewater treatment plant in Xiangyang, Hubei province, P.R. China. The typical quality parameters of excess sludge are shown in Table 1.

Table 1

The typical quality parameters of excess sludge

Parameters Value Parameters Value 
pH 6.5–7.0 SS/TS 98.4% 
TS 14.67–16.04% VSS/TS 46.0% 
VS/TS 55.4–63.4% VSS/SS 46.8% 
Alkalinity (CaCO3, mg·L−1200–560 Ammonia nitrogen/mg·L−1 67–180 
TCOD/mg·L−1 107,783–138,975 Fat/%VS 9.44 
SCOD/mg·L−1 1,516–1,690 Protein/%VS 59.16 
VFA/mg·L−1 246–500 Carbohydrate/%VS 31.4 
Parameters Value Parameters Value 
pH 6.5–7.0 SS/TS 98.4% 
TS 14.67–16.04% VSS/TS 46.0% 
VS/TS 55.4–63.4% VSS/SS 46.8% 
Alkalinity (CaCO3, mg·L−1200–560 Ammonia nitrogen/mg·L−1 67–180 
TCOD/mg·L−1 107,783–138,975 Fat/%VS 9.44 
SCOD/mg·L−1 1,516–1,690 Protein/%VS 59.16 
VFA/mg·L−1 246–500 Carbohydrate/%VS 31.4 

TS: total solids; VS: volatile solids; TCOD: total chemical oxygen demand; SCOD: soluble chemical oxygen demand; VFA: volatile fatty acids; SS: suspended solids; VSS: volatile suspended solids.

The experimental materials were: 0.11 mol L−1 CH3COOH, 0.5 mol L−1 NH2OH·HCl, H2O2, 1 mol L−1 CH3COONH4, 70% HNO3, 48% HF, and 70% HClO4.

The experimental devices and their models are shown in Table 2.

Table 2

The experimental devices and their models

Experimental devices Models 
Thermostatic oscillator China SHZ-82 
Digital display thermostatic water bath HH-S2s 
High-speed centrifuge TGL-185 
Constant temperature heating plate PERSDER946 
Constant temperature drying box DGG-9070A 
Atomic absorption spectrometer GBC AVANTA M 
Fourier transform infrared spectrometer Nicolet 6700 
Experimental devices Models 
Thermostatic oscillator China SHZ-82 
Digital display thermostatic water bath HH-S2s 
High-speed centrifuge TGL-185 
Constant temperature heating plate PERSDER946 
Constant temperature drying box DGG-9070A 
Atomic absorption spectrometer GBC AVANTA M 
Fourier transform infrared spectrometer Nicolet 6700 

Experimental methods and statistical analyses

Our previous studies suggested the optimal TH conditions, 170 °C and 30 min, at which the maximum gas production was obtained (Zhang et al. 2015). However, the stability of heavy metals which hydrolyzed at 170 °C was not improved markedly. Based on the theoretical analysis and practical operation, at high TH temperature (>210 °C) a large amount of polynitrogen that is difficult to degrade would be generated, which would impact the gas production during the anaerobic digestion process (Mavi et al. 2007; Jason et al. 2008). Therefore, in order to verify the reaction conditions that affect the stability of heavy metals in sludge, we determined that the reaction temperatures were 110, 170, 190 and 210 °C and the reaction times were 15, 30, 60, 90 and 120 min.

The experimental methods were as follows.

  1. A total of 30 g of dewatered sludge (moisture content was 840 mg·g−1) was dissolved in deionized water, and diluted to 250 mL.

  2. The mixed liquor was transferred into the TH reactor.

  3. Two groups of experiments were conducted. In group one, every sample of sludge was hydrolyzed at a different temperature (110, 170, 190, 210 °C) for 30 min. In group two, every sample of sludge was hydrolyzed for a different time (0, 15, 30, 60, 90, 120 min) at 210 °C.

  4. The hydrolyzed sludge was removed from the reactor after the reactions finished. The sludge was dried at 105°C for 24 h and ground to 100 mesh beads. The acquired samples were analyzed by atomic absorption spectrometry, while the stability of Cu and Zn was tested by the BCR sequential extraction process (Tokalioğlu et al. 2006).

RESULTS AND DISCUSSION

Effects of TH conditions on the concentration of Cu and Zn

According to Figure 1, the contents of Cu and Zn gradually increased with temperature. Also, the concentration of Cu ranged from 162.4 μg·g−1 in raw sludge to 193.3 μg·g−1 at the temperature of 210 °C, and Zn from 779.7 to 975.7 μg·g−1.
Figure 1

Effects of reaction temperature on the concentration of Cu and Zn (the reaction time was 30 min).

Figure 1

Effects of reaction temperature on the concentration of Cu and Zn (the reaction time was 30 min).

It could be deduced that during the TH process, the higher the temperature was, the more violent the reaction, which caused obvious sludge quantity loss with the unchanged quantity of heavy metals. As a result, the total percentage of Cu and Zn in unit mass sludge increased.

Compared with Figure 1, it can be observed from Figure 2 that the effect of hydrolysis time on the total amount of Cu was consistent with that of hydrolysis temperature. The concentration of Cu ranged from 162.36 μg·g−1 in raw sludge to 215.32 μg·g−1 with time and the increase was about 32.6 percentage points.
Figure 2

Effect of reaction time on the concentration of Cu and Zn (the reaction temperature was 210°C).

Figure 2

Effect of reaction time on the concentration of Cu and Zn (the reaction temperature was 210°C).

By contrast, the concentration of Zn reached the maximum in 30 min and then went down sharply to the level equal to that in the raw sludge after 90 min. The phenomenon could be explained as follows. During the TH process, the loss of some volatile organic substances in sludge with the unchanged amount of Zn contributed to the increase of Zn content in the initial 30 min. When the reaction lasted for more than 30 min, the concentration of Zn in the sludge reduced significantly. This was because parts of the organic complex that combined with Zn continued to volatilize, resulting in an obvious decline of the Zn ratio. In addition, some other studies (Conner 1990; Zorpas et al. 2001) presented a similar phenomenon. The explanation given by these studies was that the Zn in the treated sludge volatilized while the total solids reduced. This phenomenon gave the result that the concentration of Zn in the treated sludge did not change too much, or even went down.

Effects of TH conditions on the stability of Cu

Effects of reaction temperature on the stability of Cu

As shown in Figure 3, the ratio of Cu in acid soluble state remained low; especially at 170 °C and 190 °C, the Cu in acid soluble state was not detected. The content of residual Cu dropped to the minimum, 23.7% of the total amount, and accounted for less than half of its original portion in raw sludge at 170 °C. Unlike the residual state, the content of Cu in the oxidized state as well as the reduced state increased significantly after TH. This was especially true for the temperature of 170 °C, at which the maximum percentage of Cu in the oxidized state was obtained and the proportion in the reduced state accounted for 28.6% of the total amount. However, the proportion of the stable state (residual state and oxidized state) had no significant changes due to the increase of the oxidized state content, which could even offset the slump of the residual state.
Figure 3

Effects of reaction temperature on the stability of Cu (the reaction time was 30 min).

Figure 3

Effects of reaction temperature on the stability of Cu (the reaction time was 30 min).

With the increment of temperature, the percentage of residual Cu went up sharply to 44.5% at 210 °C, which was still less than that in raw sludge. Furthermore, the ratios of reduced and oxidized states declined while the increase of the content of the residual state made up for the decrease of the oxidized state. Hence, the ratio of Cu in the stable state continuously increased up to 83% at 210 °C. Furthermore, the stable form of Cu increased by 11.88 percentage points when compared with that in the raw sludge without TH.

The content of Cu in the residual state decreased obviously while the oxidation and reduced state grew gradually at 110 °C. It could be explained that parts of Cu absorbed in the crystal surface redissolved and the released free Cu was absorbed on the surface of oxides, which would inevitably cause the apparent growth of Cu in the reduced state. Meanwhile, a great number of intracellular organic matters that originated from the fracture of sludge cells would combine with Cu (Adam et al. 2005; Woo et al. 2006; Cretì et al. 2010).

With the consecutive growth of temperature, the oxidation of CuS at 170 °C was more complete when compared with 110 °C; so the percentage of residual Cu reduced further. The data showed that the small molecule organic matters had more undissociated carboxyl in comparison with macro-molecular organic matters. As a result, they had more metal binding sites. During this process, the macro-molecular organic compounds were degraded into small organic molecules that would combine with Cu, which made Cu in the oxidized state increase greatly.

The Maillard reaction, a kind of condensation reaction taking place between amino-group compounds and carboxides, occurred frequently in the TH when the hydrolysis temperature exceeded 170 °C. The reaction process is as follows:

And this reaction produced a kind of polynitrogen called melanoidin, which was difficult to degrade. Due to the polynitrogen generation, the smaller molecules were markedly consumed and the Cu which bound with these smaller molecules was released to the sludge again. Therefore, the proportion of Cu in the oxidized state would actually drop off. Furthermore, the reasons for the increase of the residual Cu were not only related to the conversion of the oxidized state but also to the decrease of the reduced state. In the process, due to the high temperature, H+ generated after hydrolysis accumulated on the surface of Fe and Mn oxides, which prevented Cu2+ from being adsorbed on the surface of the oxides (Laurent et al. 2009). Accordingly, the Cu in the oxidized state was reduced due to the effect of the Maillard reaction. Nevertheless, the content of Cu in the residual state rose with the process where Cu in the reduced state was released to the mixture liquids. And the increment of Cu in the residual state was more than the decrement in the oxidized state. Thus, the ratio of the stable form of Cu in the sludge increased finally.

According to the above analysis, the proportion of stable Cu reached the maximum at 210 °C.

Effects of reaction time on the stability of Cu

Figure 4 illustrates that the percentage of Cu in acid soluble state only had a slight rise while the percentage of stable Cu first increased and then decreased with time. Among them, the ratio of Cu in the oxidized state jumped to 18% after 15 min TH treatment, then steadily increased with the increase of time. Subsequently, its percentage reached 50.7% at the time of 90 min, and the ratio of oxidizable Cu did not change obviously in the next 30 min.
Figure 4

Effects of reaction time on the stability of Cu (the reaction temperature was 210°C).

Figure 4

Effects of reaction time on the stability of Cu (the reaction temperature was 210°C).

The assumption was that the disintegration of sludge happened quickly and then the internal organics within the broken cell wall dissolved out in the initial 15 min. However, the reaction between the sludge and dissolved organics was not fully complete due to the short duration of time, which would inevitably lead to a less intense Maillard reaction even when the temperature was 210 °C. Nevertheless, the polynitrogen was largely generated from 15 to 60 min, which impeded the formation of Cu in the oxidized state according to a similar analysis to that mentioned above in the section entitled ‘Effects of reaction temperature on the stability of Cu’. Additionally, when time continued to increase, the polynitrogen would hydrolyze, generating amino groups that could combine with Cu. As a consequence, Cu in the oxidized state increased.

In the initial 15 min, about 20% CuS was oxidized quickly. As a result, Cu2+ was released into the sludge and converted into other forms, which led to the decline of the residual state. Subsequently, the Cu bound with the silicate mineral crystal and transferred into the residual state. It is worth noting that parts of the crystal structure were damaged under high temperature and prolonged time after 60 min, which resulted in the release of Cu2+ originally wrapped up in the lattice of residual Cu. Ultimately, the percentage of residual Cu was only 22.2% when heating time reached 120 min.

To sum up, the optimal TH time was 30 min.

Effects of TH conditions on the stability of Zn

Effects of reaction temperature on the stability of Zn

From Figure 5, it was obvious that the stability of Zn had a dramatic change through thermal hydrolyzing at the temperature of 110 and 170 °C compared with that in raw sludge. The changes were as follows: at the temperature of 110 °C, the percentage of residual Zn decreased significantly to 13.5% and the ratio of Zn in the reduced state increased by 23.7 percentage points. It could be because the residual Zn on the crystal surface was released and Zn2+ was attracted by the metallic oxide, which contributed to a striking increase of Zn in the reduced state.
Figure 5

Effects of reaction temperature on the stability of Zn (the reaction time was 30 min).

Figure 5

Effects of reaction temperature on the stability of Zn (the reaction time was 30 min).

The percentage of residual Zn started to go up remarkably to 42.7%, which even exceeded that in untreated raw sludge, when the temperature rose up to 190 °C. When the temperature continued to rise, the percentage of residual Zn increased less sharply. The mechanism was that parts of Zn2+ might react with PO43− and the rest was compelled to enter the crystal lattice at high temperature and pressure. As a consequence, a series of hard-to-dissolve substances were formed and the content of residual Zn increased. But with the increase of temperature, the Zn that existed in the crystal reached saturation. Therefore, the content of residual Zn remained constant.

In the whole process, the content of Zn in acid soluble state decreased while Zn in the oxidized state had only a slight fluctuation in the whole process, which differed from the trend for Cu under the same hydrolysis conditions. The reasons were that the organic extracellular matter dissolved out from the inner cell and its decomposition products would combine with the Zn2+ to form the complex (Adam et al. 2005; Woo et al. 2006; Cretì et al. 2010). However, unlike Cu, the kind of complex that combined with Zn was not so stable because the dissociation process took place simultaneously during the hydrolysis. As a result, the content of Zn in the oxidized state was roughly unchanged.

In general, the ratio of stable Zn increased by 7.3 percentage points when compared with that in raw sludge after TH. Hence, the optimal TH temperature was 210 °C.

Effects of reaction time on the stability of Zn

Figure 6 indicates that the ratio of stable Zn reached a peak at 47.4% after 30 min, which is an increase of 7.3 percentage points compared with that in untreated sludge. The trends in the changes of Zn and Cu suggested that the metal sulfide was oxidized into other forms when heating. Differently, when the reaction time exceeded 90 min, the Zn in the residual state became stable.
Figure 6

Effects of reaction time on the stability of Zn (the reaction temperature was 210°C).

Figure 6

Effects of reaction time on the stability of Zn (the reaction temperature was 210°C).

The percentage of Zn in the residual state peaked at 30 min. The reason lay in the fact that the majority of Zn2+ was forced into the lattice and kept its structure undamaged. After that, the decomposition of the lattice would cause the reduction in the ratio of residual Zn. As for the oxidized state, it accumulated over time and the percentage was 12.1% at 120 min. The phenomenon differed to that of Cu. The reason was that the amino was more likely to combine with Cu than Zn. Thus, the Maillard reaction could affect the content of Cu in the oxidized state more significantly and cut off the increase of its content.

As for the unstable form, the percentage of Zn in the acid state dropped to 7.0% at the time of 30 min and the reduced state remained at approximately 50% for the whole process. The damage to the crystal structure in the residual Zn made Zn2+ in the lattice release and it was transformed into the other forms, which was propitious for the increase of Zn content in acid, oxidation and reduced states.

In summary, when the reaction time exceeded 30 min, the stability of Zn continued to decline; so the optimal time was 30 min.

The recovery rate

Because of the experimental errors (including improper operation, lower device accuracy etc.), the sum of ratios of all the states of Cu/Zn deviated slightly from 100%. To ensure that the results were effective, the recovery rate was adopted to test them. The recovery rate was used to check the ratio of the sum of all the four metal states to its total content in the same sludge.

As Table 3 shows, the recovery rates of Cu were 92.03 to 104.45% while Zn ratios were 95.7 to 104.82%; the deviations were within 5%. Therefore, we held the view that the results of this experiment were reliable.

Table 3

The recovery rate of heavy metals by modified BCR under different conditions of TH

Element Conditions of TH Recovery rate Element Conditions of TH Recovery rate 
Cu Raw sludge 92.03% Zn Raw sludge 102.60% 
30 min 110 °C 104.00% 30 min 110 °C 103.18% 
170 °C 101.10% 170 °C 101.20% 
190 °C 97.90% 190 °C 103.00% 
210 °C 104.45% 210 °C 97.90% 
210 °C 15 min 98.17% 210 °C 15 min 95.70% 
30 min 104.45% 30 min 97.90% 
1 h 103.70% 1 h 98.60% 
1.5 h 101.90% 1.5 h 104.82% 
2 h 102.20% 2 h 102.40% 
Element Conditions of TH Recovery rate Element Conditions of TH Recovery rate 
Cu Raw sludge 92.03% Zn Raw sludge 102.60% 
30 min 110 °C 104.00% 30 min 110 °C 103.18% 
170 °C 101.10% 170 °C 101.20% 
190 °C 97.90% 190 °C 103.00% 
210 °C 104.45% 210 °C 97.90% 
210 °C 15 min 98.17% 210 °C 15 min 95.70% 
30 min 104.45% 30 min 97.90% 
1 h 103.70% 1 h 98.60% 
1.5 h 101.90% 1.5 h 104.82% 
2 h 102.20% 2 h 102.40% 

The comparison of Fourier transform infrared spectra before and after TH treatment

Figure 7 indicates that the Fourier transform infrared (FT-IR) transmittance curve revealed a strong and broad absorption peak at wavenumber 3,800–2,500 cm−1, which represented the stretching vibration of —OH before and after TH. Simultaneously, the wavenumber 2,927 cm−1 and 2,855 cm−1 segments represented the methylene anti-symmetric and symmetric absorption peaks, respectively. The absorption peaks at wavenumber 1,445 and 1,449 cm−1 were formed by the deformation vibration of the O—H plane. The phenomena mentioned above could prove the existence of hydroxyl compounds. The manifestations of the existence of carbonyl in sludge were as follows: the strong absorption peak of C=O appeared at wavenumber 1,655 cm−1 and, at the same time, the bending vibration in the C—C=O plane appeared at wavenumber 529 cm−1; the wavenumber 3,400 cm−1 segment represented strong absorption peaks that belonged to the N—H stretching vibration of heteroaromatic compounds. The absorption eigen peak in the wavenumber 1,530–1,550 cm−1 segment represents the existence of N—H in-plane bending vibration while the wavenumber 778 and 96 cm−1 segment absorption peaks were two eigen vibration heteroaromatic compounds in the C—H out-of-plane absorption peak. In addition, the 1,032 cm−1 segment represented the absorption peak of P—O—C and the 694 cm−1 segment represented the absorption peak of C—Br vibration.
Figure 7

The FT-IR patterns. Gray (red in online version) line: untreated sludge; black line: sludge treated under optimal conditions (210 °C, 30 min). The full color version of this figure is available online.

Figure 7

The FT-IR patterns. Gray (red in online version) line: untreated sludge; black line: sludge treated under optimal conditions (210 °C, 30 min). The full color version of this figure is available online.

From above, the conclusion could be drawn that the change of groups was not obvious after TH. Nevertheless, differing from the smooth curve before TH, the 1,385 cm−1 segment represented the obvious absorption peak of carboxylate anion after TH (Choi & Yun 2006; Song et al. 2010). The mechanism was that the macro-molecules hydrolyzed and formed the carboxylate anion that easily to combined with heavy metals, which would be beneficial to the transformation of free heavy metals into the more stable organic combination forms. From the above analysis, it could be proved that the heavy metals would be more stable after 210 °C and 30 min TH.

CONCLUSIONS

  1. The concentration of Cu and Zn showed a gradual increase with the treatment temperature.

  2. The concentration of Cu was enhanced with reaction time while the concentration of Zn reached the maximum in 30 min and then sharply reduced after 90 min.

  3. For the sludge used in this experiment, the optimal conditions were 210 °C and 30 min. So a more stable fraction of heavy metals was obtained. The content of the stable form of Cu and Zn reached 83% and 47.4% respectively; when compared with the proportion of Cu and Zn in the raw sludge, they increased by 11.88% and 7.3%, respectively.

  4. In the FT-IR pattern, the 1,385 cm−1 segment represented the obvious absorption peak of the carboxylate anion after TH. Results indicated that the heavy metals were combined with sludge in a more stable form with the pretreatment of TH, which improved the stability of heavy metals.

ACKNOWLEDGEMENTS

This work was financially supported by the Independent Innovation Fund for Wuhan University of Technology (No. 155206009). The authors also deeply appreciate the help given by Hubei Guoxintianhui Energy Co., Ltd, in Xiangyang, P.R. China.

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