Abstract

Bioleaching has been demonstrated to be an effective technology for the removal of heavy metals and sludge dewaterability. Since bacteria gain nutrients by diffusion of soluble compounds, the insolubility of elemental sulfur may slow the growth of bacteria. Thus, it is very important to find an energy substance quickly become available to Acidithiobacillus thiooxidans. This paper studies the improvement of sludge dewaterability and heavy metal removal with sodium thiosulfate as the source of energy for Acidithiobacillus thiooxidans. Through orthogonal experiments with specific resistance to filtration (SRF) as the target index, four factors (FeSO4 dosage, Na2S2O3 dosage, sludge reflux ratio and sludge moisture content) were identified to be the important influencing parameters. The optimal conditions were: FeSO4 dosage, 8 g/L; Na2S2O3 dosage, 1.5 g/L; sludge reflux ratio, 30%; sludge moisture content, 97%. Results indicated that the SRF of the sludge decreased from 9.89 × 1012 to 1.03 × 1011 m/kg. The removal efficiencies of heavy metals Cu, Zn, Pb and Cr could reach 83%, 78%, 31% and 38% within 3 days, respectively. These results confirm the potential of sodium thiosulfate as an alternative energy substance in bioleaching to improve sludge dewaterability as well as removal of metals.

INTRODUCTION

In the year 2012, a large amount of wastewater was treated in China, yielding approximately 9.1 million dry tons of excess sludge, containing complex components (Zhen et al. 2014). This yield of sludge is predicted to continue to increase in the foreseeable future because of the enforcement of newly enacted environmental regulations dealing with wastewater treatment. As a result of this treatment, a large quantity of major byproducts will be generated that are of serious environmental concern (Pathak et al. 2009).

Because the water content of the activated sludge is very high, dewatering is essential to reduce the sludge volume before further treatment to reduce the costs of its transportation, disposal, and storage (Chen & Lin 2000; Xiao et al. 2016). Sewage sludge consists primarily of biological flocs formed by growth of microorganisms and by adsorption of particles from the influent, such as single cells, filamentous bacteria, organic fibers, inorganic particles (salt and sand), and extracellular polymeric substances (EPS) (Christensen et al. 2015). Sludge dewatering has been deemed one of the most expensive and the least understood methods (Chen et al. 2010). Therefore, sound, sustainable and environmentally friendly methods should be chosen to enhance sludge dewatering.

Heavy metals, contained in municipal wastewater and industrial discharges, may accumulate in waste sludge in wastewater treatment plants, which can cause a sludge disposal problem (Zhang et al. 2008). The presence of heavy metals in sludge restricts its use as a fertilizer. Essential microelements in the environment, such as Cr, Sn, Zn, F, Mn, and Cu, present in appropriate concentrations are not harmful to living organisms (Ignatowicz 2017). However, repeated application of contaminated sludge may result in the release of heavy metals in the soil as a result of decomposition of the sludge organic matter, changing its fertility and lowering plant productivity and quality (Ignatowicz 2017). Consumption of plant matter contaminated with toxic heavy metals results in the entry of the metals into the food chain, and may ultimately cause metabolic disorder and chronic disease in human beings (Ozoreshampton et al. 2005). The presence of high concentrations of toxic heavy metals such as Cu, Zn, Pb, and Cr in sludge is always one of the major obstacles to the recycling of organic matter (Zhou et al. 2008; Islam et al. 2015).

Bioleaching is a powerful and versatile alternative to chemical and physical treatment of excess sludge, owing to low-cost, environmentally friendly processes and low generation of waste byproducts and waste solution (Chen & Lin 2004). Sludge dewatering may be improved by use of microwave, hydrothermal and ultrasound treatments, as well as by chemical conditioning. However, the application of these methods is constrained due to their high cost and the environmentally harmful wastewater produced (Liu et al. 2012a; Guo et al. 2015). In contrast, sludge dewatering may be 4- to 10-fold improved in bioleaching. The moisture content of the dewatered sludge may be as low as 60% after compression by a diaphragm filter (Liu et al. 2012a, 2012b). The Acidithiobacillus thiooxidans (A. thiooxidans) and Acidothiobacillus ferrooxidans (A. ferrooxidans) can obtain energy from the oxidation of substances and cause bioacidification and solubilization of heavy metals (Chen & Lin 2004). After bioleaching, heavy metals could be separated from the sludge digest, and pathogenic bacteria and the sludge odor eliminated (Tyagi et al. 1998; Wang et al. 2010; Li et al. 2015).

Since the cells of A. thiooxidans obtain nutrients from the external environment by diffusion, they must, in the case of extracellular elemental sulfur, first promote its solubilization (Knickerbocker et al. 2000). However, elemental sulfur is hard to dissolve in water. The insolubility of elemental sulfur may slow the growth of bacteria. The energy of most thiobacillus derives from the oxidation of reduced or partially reduced sulfur compounds, including sulfides, elemental sulfur and thiosulfate, the final oxidation product being sulfate (Trudinger 1967). Compared with sulfur, sodium thiosulfate has excellent solubility and can serve as the source of energy for Acidithiobacillus thiooxidans (Kelly 1989; Chan & Suzuki 1994).

However, because the effect of sodium thiosulfate on sludge dewaterability and removal of metals from sludge by bioleaching bacteria has not been reported, it is the subject of the present study. The optimal conditions were determined by orthogonal tests.

MATERIALS AND METHODS

Municipal sewage sludge sampling

The municipal sludge used in this work was obtained from a local wastewater treatment plant in Zhengzhou, China. After 2 h of settling, the supernatant was removed. The sludge sample was stored at 4 °C to avoid a change in physical and chemical properties. The physiochemical characteristics of the tested municipal sewage sludge are listed in Table 1.

Table 1

Physicochemical characteristics of the tested municipal sewage sludge

pH ORP (mv) Organic matter (%) Solid content (%) SRF × 1013 (m/kg) Cu (mg/kg) Zn (mg/kg) Pb (mg/kg) Cr (mg/kg) 
6.86 14 50.81 2.69 1.33 513.5 986.3 103.9 206.6 
pH ORP (mv) Organic matter (%) Solid content (%) SRF × 1013 (m/kg) Cu (mg/kg) Zn (mg/kg) Pb (mg/kg) Cr (mg/kg) 
6.86 14 50.81 2.69 1.33 513.5 986.3 103.9 206.6 

Note: Oxidation-reduction potential (ORP), specific resistance to filtration (SRF).

Microbial acclimation

Methods have been developed for the selection of microorganisms capable of performing a desired process under conditions of stress. Preliminary tests showed a need for acclimatizing microbial inocula to the use of Na2S2O3. A. ferrooxidans (ATCC 23270) and A. thiooxidans (ATCC 53990) uesd in experiments were grown in modified 9 K and Waksman liquid medium, respectively. The Waksman medium without elemental sulfur was autoclaved at 121 °C for 15 min before being supplemented with 10 g/L Na2S2O3·5H2O as the energy source. Erlenmeyer flasks (250-mL size) containing 90 mL above Waksman medium were inoculated with 5 mL each of A. ferrooxidans and A. thiooxidans and shaken at 180 rpm and 28 °C until the pH of the medium dropped below 2.0. Then 10 mL bacteria solution was withdrawn from the Erlenmeyer flasks for the next acclimation step. The acclimation procedure described above was repeated three times.

Preparation of inoculum

Inoculum was prepared by introducing 15 mL of acclimated culture into 135 mL of fresh sludge containing 1.0 g/L Na2S2O3·5H2O and 6.0 g/L FeSO4·7H2O in a 250 mL Erlenmeyer flask, which was then incubated on a rotary shaker at 180 rpm at 28 °C. When the pH had dropped to approximately 2.0, 15 mL of bioacidified sludge were transferred to 135 mL of medium. This step was repeated three more times. The final subculture was then used as inoculum in the bioleaching experiment.

Sludge bioleaching experiments

The sludge bioleaching experiments were optimized by orthogonal experiments. FeSO4 dosage, Na2S2O3 dosage, sludge return reflux and sludge moisture content with four levels were chosen in the sludge bioleaching experiments (see Tables 2 and 3). Several nutrients and bioleaching inoculum were added to fresh sludge in 250 mL Erlenmeyer flasks (for each process and bioleaching experiments, the additions were performed as shown in Tables 2 and 3). The total amount of sludge medium in each Erlenmeyer flask was 100 mL. Each Erlenmeyer flask was plugged with 6 layers of gauze and all were shaken at 180 rpm at 28 °C. Water lost by evaporation was replaced daily with ultra-pure water. During the bioleaching treatment, pH and oxidation-reduction potential (ORP) were monitored regularly at 12-h intervals. Specific resistance to filtration (SRF) and heavy metal concentrations were determined after 3 days. The leaching rate of heavy metals was calculated by comparing the concentrations of heavy metals before and after treatment. Each experiment and all determinations were run in triplicate.

Table 2

Levels and factors affecting the process

Level Factors
 
FeSO4 A (g/L) Na2S2O3 B (g/L) Sludge reflux Ratio C (%) Sludge moisture content D (%) 
0.5 10 95 
20 96 
1.5 30 97 
10 40 98 
Level Factors
 
FeSO4 A (g/L) Na2S2O3 B (g/L) Sludge reflux Ratio C (%) Sludge moisture content D (%) 
0.5 10 95 
20 96 
1.5 30 97 
10 40 98 
Table 3

Results of orthogonal experiment

Experiment no. Factor A (g/L) Factor B (g/L) Factor C (%) Factor D (%) SRF (1012 m/kg)
 
0.5 10 95 6.926 8.424 9.890 
20 96 5.967 7.027 8.808 
1.5 30 97 2.737 1.037 1.665 
40 98 0.464 0.375 0.560 
0.5 20 97 0.862 1.858 1.143 
10 98 0.966 1.042 1.320 
1.5 40 95 0.935 1.260 1.963 
30 96 0.881 1.132 1.054 
0.5 30 98 0.242 0.295 0.394 
10 40 97 0.275 0.339 0.384 
11 1.5 10 96 0.769 0.806 0.958 
12 20 95 0.844 0.977 1.397 
13 10 0.5 40 96 0.235 0.413 0.373 
14 10 30 95 1.607 1.024 1.968 
15 10 1.5 20 98 0.107 0.321 0.103 
16 10 10 97 0.319 0.563 0.453 
K1(10124.490 2.588 2.706 3.101    
K2(10121.204 2.563 2.451 2.369    
K3(10120.640 1.055 1.169 0.969    
K4(10120.624 0.752 0.631 0.518    
R(10123.866 1.836 2.075 2.583    
Experiment no. Factor A (g/L) Factor B (g/L) Factor C (%) Factor D (%) SRF (1012 m/kg)
 
0.5 10 95 6.926 8.424 9.890 
20 96 5.967 7.027 8.808 
1.5 30 97 2.737 1.037 1.665 
40 98 0.464 0.375 0.560 
0.5 20 97 0.862 1.858 1.143 
10 98 0.966 1.042 1.320 
1.5 40 95 0.935 1.260 1.963 
30 96 0.881 1.132 1.054 
0.5 30 98 0.242 0.295 0.394 
10 40 97 0.275 0.339 0.384 
11 1.5 10 96 0.769 0.806 0.958 
12 20 95 0.844 0.977 1.397 
13 10 0.5 40 96 0.235 0.413 0.373 
14 10 30 95 1.607 1.024 1.968 
15 10 1.5 20 98 0.107 0.321 0.103 
16 10 10 97 0.319 0.563 0.453 
K1(10124.490 2.588 2.706 3.101    
K2(10121.204 2.563 2.451 2.369    
K3(10120.640 1.055 1.169 0.969    
K4(10120.624 0.752 0.631 0.518    
R(10123.866 1.836 2.075 2.583    

Note: Specific resistance to filtration (SRF).

Orthogonal array experimental design

In this study, the experimental design was based on a standard orthogonal array matrix. Four important factors affecting the bioleaching performances were: FeSO4 dosage (factor A), Na2S2O3 dosage (factor B), sludge return reflux (factor C) and sludge moisture content (factor D) at four levels. An orthogonal experiment L16 (44) with the value of SRF as the target index was employed to assign the considered factors as shown in Table 2. All the trials were repeated three times.

Analysis of range and variance of orthogonal experiment was employed to reveal the optimal conditions of bioleaching. Direct observation analysis was employed to calculate the mean values (K1, K2, K3 and K4) for different levels of each factor to assess the effects of the different factors on the SRF. The order of importance of the influence of the factors was estimated using the mean value difference (R), which is the difference between the maximum and minimum of SRF response for every level. Furthermore, the analysis of range and variance was used to estimate the values of the sum of squares and the deviation factor F to determine the contribution degree of each factor and the experimental error.

Analytical methods

pH and ORP were determined with pHS-3C digital pH meter (pHS-3C, Shanghai INESA Scientific Instrument, China). SRF was determined by the Buchner funnel vacuum suction method (Wang et al. 2013). The sludge cake produced by the Buchner funnel filtration process was dried at 105 °C for 2 h to determine the moisture content of filter cake. The Pb, Zn, Cr and Cu contents were measured by inductively coupled plasma atomic emission spectrometry using a HNO3-HClO4 method.

Statistical analysis

All treatments were performed in triplicate. Each data point in the graphs represents the mean. The analysis of range (ANORA) and the analysis of variance (ANOVA) of the test results were performed via IBM SPSS Statistics 19. Differences were considered statistically significant when p < 0.05. Graphs were prepared using Origin 7.5.

RESULTS AND DISCUSSION

Microbial acclimation

The changes in pH and ORP during bioleaching of the sludge in separate batch reactors are illustrated in Figure 1. The pH in all reactors rose slightly in the first day, followed by a considerable decrease over time (Figure 1(a)). However, the pH of the significant reduction began to occur after 3 days in the first batch, which gradually decreased from an initial pH of 4.0 to about 1.75 after 6 days. In the third batch, to which sodium thiosulfate was added, the pH dropped more rapidly in that it declined to 1.2 in 3 days.

Figure 1

Variation in sludge pH (a) and ORP (b) with time during acclimation.

Figure 1

Variation in sludge pH (a) and ORP (b) with time during acclimation.

Changes in ORP during the acclimation process are shown in Figure 1(b). Unlike the pH, which decreased, the ORP rose to highest values (300–340 mV) after 6 days. The extent of ORP changes was related to corresponding pH changes. The final ORP of the third batch was the highest and that of the first batch the lowest.

Sodium thiosulfate disproportionates into elemental sulfur and sulfite in acidic solution. Both sulfur and sulfite are able to serve as energy sources for members of Acidithiobacillus and some members of Thiobacillu (Pryor 1960). The hydrolysis of sodium sulfite and sodium thiosulfate will release OH-, raising the pH of the solution slowly (Aylmore & Muir 2001). Therefore, the pH will rise gradually initially. As a result of bio-oxidation of sodium sulfite and elemental sulfur, sulfuric acid is formed, leading to a sharp decline in pH (Zhou et al. 2008). The changes in pH are a reflection of the activity of sulfur-oxidizing bacteria and their growth (Zhang et al. 2008). The results shown in Figure 1(a) reflect the adaptation of Thiobacillus to the use of sodium thiosulfate as its energy source.

A high ORP coupled with a low pH in the growth medium has been considered as indicative of a substantial population of Thiobacillus (Chartier & Couillard 1997; Xiang et al. 2000). The rise in ORP and drop in pH in the present experiment are consistent with this.

Range analysis

An orthogonal array is a useful design for optimizing a bioleaching process. All of the 16 experimental results of the four levels of each factor are given in Table 3. The mean values as well as the values of R for SRF were also calculated. The SRF values ranged from 1.03 × 1011 m/kg to 9.890 × 1012 m/kg. The mean value for the different factors at different levels is also shown. The R-values of the FeSO4 and the Na2S2O3 dosage, the sludge reflux ratio, and the sludge moisture content were 3.866, 1.836, 2.075, and 2.583, respectively.

Table 3 gives the results of the range analysis for the orthogonal experiments, where Ki imply the mean values of the evaluation indexes of four levels in every factor. Moreover, the value of R which can be calculated by R = max(Ki) − min(Ki), is a standard to estimate the impact level of each factor on the experimental results. A higher mean value of difference factors at different levels, as mentioned above, demonstrates that the corresponding level affects SRF more deeply. Hence, the optimal level for the four factors are as follows: FeSO4 dosage was 10 g/L, Na2S2O3 dosage was 2 g/L, sludge reflux ratio was 40% and sludge moisture content was 98%; since the K-value was the highest at these combination (A4B4C4D4). Meanwhile, the R-value demonstrates the significance of the influence exerted by such factors and a larger R means that such factor may affect the SRF in a deeper way. The descending order RA > RD > RC > RB indicates that the degree of significance of different factors is as follows: FeSO4 dosage > sludge moisture content > sludge reflux ratio > Na2S2O3 dosage. The mean values of SRF as a function of four levels of four factors are discussed in detail to represent how the experimental conditions can influence the results of bioleaching as flow.

FeSO4 dosage

Figure 2(a) shows the infuence of FeSO4 concentrations on the SRF of sludge. The sludge SRF decreased dramatically with an increase in the FeSO4 dosage. With an FeSO4 dosage of 10.0 g/L, the sludge SRF reached 6.24 × 1011 m/kg−1, indicating that the dewaterability of the sludge was significantly improved. As shown in Figure 2(b), the pH of sludge with FeSO4 dosage of 4.0 g/L and 6.0 g/L decreased to around 3.0 on the first day, then began to increase to around 3.2. The pH of the sludge to which 10.0 g/L of FeSO4 had been added dropped from an initial value of 6.4 to about 2.3 in 3 days and then stayed at that level until the end of the bioleaching.

Figure 2

Effect of FeSO4 dosage on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching with FeSO4 at different dosages.

Figure 2

Effect of FeSO4 dosage on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching with FeSO4 at different dosages.

During bioleaching, Fe3+ generated from the bio-oxidation of Fe2+ by A. ferrooxidans may act as an effective oxidant in the bioleaching system (Zhou et al. 2008). It is widely known that Fe3+ oxidizes metal sulfides, elemental sulfur and thiosulfate into dissolved sulfate which leads to metal dissolution (Chen et al. 2003). The probable explanation for the increase in pH of sludge with FeSO4 dosage of 4.0 g/L and 6.0 g/L after the first day may be the lack of release of H+ resulting from the hydrolysis of Fe3+ from the bio-oxidation of Fe2+ by A. ferrooxidans (Daoud & Karamanev 2006; Liu et al. 2012b). The suitable FeSO4 addition limit for improving the dewaterability was 8 g/L. Excess addition of FeSO4 would not improve sludge dewaterability further.

Sludge moisture content

The changes in sludge SRF during the bioleaching of sludge with different moisture contents are shown in Figure 3(a). The sludge SRF decreased with an increase in sludge moisture content, i.e. the higher the moisture content of the sludge, the greater was the sludge dewatering efficiency. After 3 days of reaction, the pH decreased to 3.3, 2.7, 2.4, and 2.3 at sludge moisture contents of 95, 96, 97, and 98%, respectively (Figure 3(b)). The pH of the sludges having moisture contents of 97 and 98%, respectively, declined faster than the pH of the sludges with moisture contents of 95 and 96%, respectively.

Figure 3

Effect of sludge moisture content on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching at different moisture contents.

Figure 3

Effect of sludge moisture content on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching at different moisture contents.

The sludge moisture content remarkably influenced the rate of decrease of the pH. Generally, the acidification rates of sludge increased as the sludge moisture content increased (Chen & Lin 2004). This was because sludge of lower the sludge moisture content had higher buffering capacity, it required more time and acid to attain the pH at which the heavy metals could be leached from the sludge (Chen & Lin 2000). Hence, the sludge moisture content acted as a critical factor that greatly affected the change in pH during the bioleaching process. With sludge moisture content of 97% and 98%, the sludge SRF reached behind 1 × 1012 m/kg, indicating that the dewaterability of the sludge was significantly improved. Considering the cost, the sludge moisture content of 97% was more appropriate.

Sludge reflux ratio

The influence of the sludge reflux ratio on dewatering by sludge filtration after bioleaching is shown in Figure 4(a). The sludge SRF decreased as the sludge reflux ratio increased. As shown in Figure 4(b), the sludge reflux ratio affected the initial pH. At reflux ratios of 30 and 40%, the sludge medium pH decreased sharply on the first day and then remained at pH 3.3 and 2.4, respectively. At reflux ratios of 10 and 20%, the medium pH increased on the first day and then declined slightly over time.

Figure 4

Effect of sludge reflux ratio on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching at different reflux ratios.

Figure 4

Effect of sludge reflux ratio on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching at different reflux ratios.

Because acid pH favors the development of Thiobacillus cultures, the amount of initial acid addition has a strong influence on the growth of Thiobacillus bacteria at the beginning of the leaching reaction (Chartier & Couillard 1997). In addition, recycling acidified bioleached sludge into the original sludge in subsequent batch bioleaching was needed for the sake of lowering the buffering capacity of the sludge (Zhou et al. 2005). The changes in pH values at different rates of reflux are probably affected by the different buffering capacities of the medium, which are the result of the addition of different amounts of acidified bioleached sludge. The dewatering efficiency of sludge decreased as the reflux ratio increased. For cost-effective operation, the optimal sludge reflux ratio is 30% in bioleaching process.

Na2S2O3 dosage

The SRF after bioleaching at different dosages of Na2S2O3 is shown in Figure 5(a). The SRF of the sludge decreased sharply with an excess in the dosage of Na2S2O3. With an Na2S2O3 dosage of 2.0 g/L, the sludge SRF reached 7.52 × 1011 m/kg, indicating that the dewaterability of the sludge was significantly improved. Figure 5(b) shows that at Na2S2O3 dosages of 1.5 and 2.0 g/L, the pH dropped rapidly to around 3.0 on the first day and then remained stable until the end of the bioleaching. In contrast, at Na2S2O3 dosages of 0.5 and 1 g/L, the pH decreased slightly over 3 days.

Figure 5

Effect of Na2S2O3 dosage on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching with Na2S2O3 at different dosages.

Figure 5

Effect of Na2S2O3 dosage on sludge SRF (a) and variation in sludge pH (b) with time during bioleaching with Na2S2O3 at different dosages.

As mentioned above, sodium thiosulfate disproportionated into sulfite and elemental sulfur in acidic solution, which could be used as the energy source for Thiobacillus (Pryor 1960). A. thiooxidans is able to utilize thiosulfate as an energy source and forms sulfate as a byproduct of metabolism (Lengke & Southam 2005). The possible main pathways (Suzuki 2011) of the oxidation of thiosulfate by A. thiooxidans to sulfate are shown in following reaction:

  • 1.
    Mechanism with thiosulfate cleavage 
    formula
    (1)
     
    formula
    (2)
     
    formula
    (3)
  • 2.
    Mechanism forming tetrathionate 
    formula
    (4)
     
    formula
    (5)
Reaction 1 + reaction 2 + reaction 3 + reaction 4 + reaction 5: 
formula

From this experiment, it can be seen that when treated with 2 g/L and 1.5 g/L Na2S2O3, pH of sludge changes rapidly with time and dropped to a lower level, while that with 1 g/L and 0.5 g/L changed little (Figure 5(b)). This indicates that the growth and proliferation of A. thiooxidans requires an adequate energy source. Addition of enough Na2S2O3 as an energy source is helpful in enhancing the activities of A. thiooxidans. The more an adequate energy source exists, the more quickly the A. thiooxidans entered the logarithmic growth phase, propagating rapidly, and led to a constant rate of decrease in pH value.

Analysis of variance

From the analysis of variance for the orthogonal experiments in Table 4, the significance of FeSO4 dosage, sludge return ratio, sludge moisture content and Na2S2O3 dosage was all 0, indicating that they affected the SRF of sludge extremely significantly (p < 0.001). The values of the sum of squares (SS) and the deviation of factor F for each factor are also presented in Table 4.

Table 4

Analysis of variance for the orthogonal experiments

Parameters Sum of the deviation square (1025Degree of freedom Mean square (1025F ratio Significance 
Factor A 12.370 4.122 42.140 0.000 
Factor B 3.412 1.137 11.628 0.000 
Factor C 3.592 1.197 12.241 0.000 
Factor D 5.203 1.734 17.731 0.000 
Error 3.423 35 0.098   
Total Error 42.520 48    
Parameters Sum of the deviation square (1025Degree of freedom Mean square (1025F ratio Significance 
Factor A 12.370 4.122 42.140 0.000 
Factor B 3.412 1.137 11.628 0.000 
Factor C 3.592 1.197 12.241 0.000 
Factor D 5.203 1.734 17.731 0.000 
Error 3.423 35 0.098   
Total Error 42.520 48    

ANOVA was employed to estimate the values of the SS and the deviation factor F to determine the magnitude of the contribution of factors to SRF. It was found that the order of sum of the deviation square was the same as that of R-value: FeSO4 dosage > sludge moisture content > sludge return ratio > Na2S2O3 dosage, indicating that the level of significance of factors are as follows: FeSO4 dosage > sludge moisture content > sludge return ratio > Na2S2O3 dosage. This is consistent with discussions presented in the previous sections.

Removal of heavy metals

Table 5 shows the effect of pH and ORP on the mobilization of the heavy metals (Zn, Cr, Cu, and Pb) during the bioleaching. The removal efficiency of Cu and Zn increases from an initial 1.37% to 83.55% and 1.09% to 78.92% at termination of bioleaching while the pH decreased from 6.53 to 2.06, respectively. However, Cr began to be mobilized rapidly only at a pH of 2–3 (Liu et al. 2012b). Yet, the removal efficiency of Cr remained low until the pH of the sludge decreased to 2.06. Then the Cr removal efficiency increased significantly as the pH dropped below 2.0 (Zhou et al. 2005). By contrast, the removal efficiency of Pb was lower even when the pH dropped to 2.06. The maximum removal efficiencies of heavy metals Cu, Zn, Pb and Cr were 83%, 78%, 31% and 38%, respectively.

Table 5

Contents of heavy metals in sludge after bioleaching

Number pH ORP (mV) Cu (%) Zn (%) Pb (%) Cr (%) 
6.53 15 1.37 1.09 1.12 1.66 
5.7 62 3.92 2.51 2.49 3.30 
3.39 194 51.46 47.58 4.33 5.14 
2.43 249 65.88 57.32 18.13 23.97 
3.2 205 56.80 50.89 9.03 17.56 
3.18 206 59.57 51.22 10.21 19.07 
3.23 203 53.47 48.60 8.80 10.97 
3.15 208 62.36 52.91 15.01 20.50 
2.22 261 83.42 78.36 30.47 35.82 
10 2.3 296 82.18 74.33 28.14 32.60 
11 2.56 242 83.02 76.70 26.86 29.45 
12 3.08 211 63.27 54.08 16.48 20.96 
13 2.36 253 70.93 59.59 22.51 29.31 
14 3.45 191 48.78 40.99 3.61 4.37 
15 2.06 270 83.55 78.92 31.68 37.95 
16 2.42 250 67.10 58.43 19.76 24.36 
Number pH ORP (mV) Cu (%) Zn (%) Pb (%) Cr (%) 
6.53 15 1.37 1.09 1.12 1.66 
5.7 62 3.92 2.51 2.49 3.30 
3.39 194 51.46 47.58 4.33 5.14 
2.43 249 65.88 57.32 18.13 23.97 
3.2 205 56.80 50.89 9.03 17.56 
3.18 206 59.57 51.22 10.21 19.07 
3.23 203 53.47 48.60 8.80 10.97 
3.15 208 62.36 52.91 15.01 20.50 
2.22 261 83.42 78.36 30.47 35.82 
10 2.3 296 82.18 74.33 28.14 32.60 
11 2.56 242 83.02 76.70 26.86 29.45 
12 3.08 211 63.27 54.08 16.48 20.96 
13 2.36 253 70.93 59.59 22.51 29.31 
14 3.45 191 48.78 40.99 3.61 4.37 
15 2.06 270 83.55 78.92 31.68 37.95 
16 2.42 250 67.10 58.43 19.76 24.36 

Note: Oxidation-reduction potential (ORP).

It is widely known that pH is the most important parameter influencing sludge-borne heavy metals solubilization during bioleaching. The results obtained in the study indicated that the removal efficiency of Cu and Zn increased to above 50% when pH decreased to 3.39 and 3.2, respectively. Data presented here demonstrated that pH taken as a threshold value for extensive Zn and Cu solubilization during bioleaching was nearly 5.0 and 4.0, respectively. The bioleaching reached a maximum removal efficiency of approximately 80% at the pH of 2.0. The solubilization of Cr took place very rapidly at pH of 3.0 or below. It means that Cr requires a threshold pH of around 2.0 to initiate its sufficient solubilization (Zhou et al. 2008). The removal of Pb was not as effective as that of Cr even if at pH around 2.0. This was probably because dissolved Pb2+ reacts with SO42− to form PbSO4, which does not dissolve readily in mineral acid (Xiang et al. 2000).

CONCLUSIONS

This study first proposed adopting sodium thiosulfate to replace the insoluble elemental sulfur as an energy source for Thiobacillus in order to improve the sewage sludge dewaterability. The optimization of process parameters was performed by an orthogonal experiment with SRF as the target index. The results were examined by range analysis and showed that FeSO4 dosage, Na2S2O3 dosage, sludge reflux ratio and sludge moisture content played important roles in bioleaching. According to the range analysis, the ranking of the significance of factors for bioleaching were as follows: FeSO4 dosage > sludge moisture content > sludge return ratio > Na2S2O3 dosage. The optimal process parameters are as follows: FeSO4 dosage was 8 g/L, Na2S2O3 dosage was 1.5 g/L, sludge return ratio was 30% and sludge moisture content was 97%. The removal efficiencies of heavy metals increased gradually while the pH decreased. The removal efficiencies of heavy metals Cu, Zn, Pb and Cr were able to reach 83%, 78%, 31% and 38% within 3 days, respectively.

ACKNOWLEDGEMENTS

The present study was supported jointly by the Cutting-edge Technologies Project of Henan Province Scientific and Technological Department of China (152300410036) and the Key Research Projects of Henan Province Education Department of China (15A610020).

CONFLICT OF INTEREST

The authors declare that they have no conflict of interest.

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