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Published online 20 April 2005
Published in J Environ Qual 34:816-824 (2005)
DOI: 10.2134/jeq2004.0307
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Heavy Metals in the Environment

Bioproduction of Ferric Sulfate Used during Heavy Metals Removal from Sewage Sludge

Patrick Drogui, Guy Mercier and Jean-François Blais*

Institut National de la recherche scientifique (INRS-Eau, Terre et Environnement), 490, rue de la Couronne, Québec, QC, Canada G1K 9A9

* Corresponding author (blaisjf{at}ete.inrs.ca)

Received for publication August 9, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Toxic metals removal from wastewater sewage sludge can be achieved through microbial processes involving Acidithiobacillus ferrooxidans. The oxidation of ferrous ions by A. ferrooxidans, cultured in sewage sludge filtrate, was studied in both batch and continuous flow stirred tank reactors. Sewage sludge filtrate containing natural nutrients (phosphorus and nitrogen) was recovered as effluent following the dehydration of a primary and secondary sludge mixture. Batch and continuous flow stirred tank reactor tests demonstrated that A. ferrooxidans were able to grow and completely oxidize ferrous iron in a culture medium containing more than 80% (v v–1) sewage sludge filtrate with 10 g Fe(II) L–1 added. Toxic levels were reached when total organic carbon in the sewage sludge filtrate exceeded 250 mg L–1. The ferric iron solution produced in the sludge filtrate by A. ferrooxidans was used to solubilize heavy metals in primary and secondary sludge. The solubilization of Cu, Cr, and Zn yielded 71, 49, and 80%, respectively. This is comparable with the yield percentages obtained using a FeCl3 solution. The cost of treating wastewater sewage sludge by bioproducing a ferric ion solution from sewage sludge is three times less expensive than the conventional method requiring a commercial ferric chloride solution.

Abbreviations: BFSS, bioproduced ferric iron solution • ORP, oxidation–reduction potential • SSF, sewage sludge filtrate • TOC, total organic carbon


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ENVIRONMENTAL CONTAMINATION caused by toxic metals has created a societal health risk. Numerous studies have focused on the removal of toxic metals from sewage sludge (Ito et al., 2000; Mercier et al., 2002; Naoum et al., 1998; Wong and Henry, 1988; Wong et al., 2004; Yoshizaki and Tomida, 2000). Researchers at L'Institut National de la Recherche Scientifique (Quebec, Canada) have developed innovative bioleaching processes (METIX-BS [also known as SSDML process], METIX-BF) to decontaminate wastewater sludge (Blais et al., 1992, 2004; Couillard and Mercier, 1991; Tyagi et al., 1993). The principal microorganism involved in these bioleaching processes is A. ferrooxidans, a chemoautotrophic bacterium that can grow on ferrous ion (Jensen and Webb, 1995; Kelly and Wood, 2000). The oxidation of ferrous ion by A. ferrooxidans increases the oxidizing potential of the sludge, thus increasing the solubilization of toxic metals (Blais et al., 1993; Couillard et al., 1994). The biooxidation of ferrous ion is also used in hydrometallurgy for the leaching of metals (Torma, 1983) and for the treatment of acid mine effluent (Nakamura et al., 1986). The bioleaching process is effective in removing heavy metals from sludge but most often, it requires long residence time compared with a chemical process. Indeed, the longer retention time constitutes a major obstacle to its practical application at full scale.

To increase the efficiency and reduce the operating costs substantially, a hybrid process (biological and chemical processes combined) called indirect bioleaching process (IBP) was developed. The indirect bioleaching takes advantage of coupling a biological process (reduction of operating cost) to a chemical process (shorter retention time). The indirect bioleaching process involves the production of a concentrated ferric sulfate solution from the biological oxidation of ferrous sulfate, shown in Fig. 1 . The ferric ion solution produced is used to maintain a high oxidation–reduction potential (ORP) in the wastewater sludge that is required for the succeeding 2- to 6-h metal solubilization step. Leached sludge is then conditioned by adding an organic polymer. Subsequently, the sludge is dehydrated and 5 to 10% (v v–1) of the acidic filtrate (pH between 2.0 and 2.5) is transferred into a bioreactor tank. This results in a decrease in acid consumption. The residual volume of acid leachate (90–95% of the total volume) is neutralized at a pH between 7 and 8 to recover the remaining metals. Dehydrated and decontaminated sludge is neutralized at pH 7 or 12 by adding calcium hydroxide (lime). The neutralized sludge is used as organic fertilizer on agricultural lands. During leaching, pH adjustment, dewatering, and metal precipitation are routine operations. The production of a ferric ion solution via biological oxidation of ferrous iron requires further examination. Indeed, a concentrated ferric ion solution is produced in a separate tank (compared with direct bioleaching process) in view of reducing the volume of the tank used for sludge decontamination and minimizing the energy cost required for oxygen supply. The objective of this study is to refine bioproduction of ferric sulfate techniques to optimize (reduce costs and increase effectiveness) the indirect bioleaching process.



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  		Fig. 1. Hybrid (chemical and biological) leaching process for heavy metals removal from sewage sludge.

 
Biological production of ferric iron via ferrous iron usually requires the addition of nutrients, mainly nitrogen and phosphorus, to support the growth and activity of indigenous iron-oxidizing bacteria (Jensen and Webb, 1995; Silverman and Lundgren, 1959). The challenge is to produce a microbial ferric ion solution via ferrous iron oxidation without adding nutrient salts. The sewage sludge filtrate containing natural nutrients (phosphorus and nitrogen) can then be used as culture medium for A. ferrooxidans growth. This practice should reduce the operating costs involved in metal extraction. Numerous parameters, such as the ferrous ion concentration, nutrient concentrations, and organic matter content (such as organic acids), can considerably influence microbial ferrous oxidation (Gu and Wong, 2004; Tuovinen et al., 1971; Tuttle and Dugan, 1976).

This study evaluates a technique involving the oxidation of ferrous ions into ferric ions during which sludge filtrate is used as culture medium for A. ferrooxidans growth. Additionally, the leaching of metals from sewage sludge through the use of iron is evaluated and compared with conventional chemical leaching using ferric chloride solution.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sludge Filtrate Sampling
Sewage sludge filtrate (SSF) was used as a nutrient supplement for the bacterial growth of A. ferrooxidans. The SSF, produced during primary and secondary sludge dehydration by belt filter, was supplied by the Quebec Urban Community wastewater treatment plant (Québec, Canada). Samples were collected in polypropylene bottles, shipped cold, and kept at 4°C before use. The total organic carbon (TOC) of the sludge filtrate ranged from 200 to 600 mg L–1 and the inorganic carbon content ranged from 90 to 100 mg L–1.

Microorganisms
The microorganisms used throughout this study were a consortium of bacteria, the majority of which were indigenous iron-oxidizing bacteria. These bacteria were obtained directly from sewage sludge (Blais et al., 1993). The bacteria were originally cultivated on ferrous sulfate in a culture medium containing SSF and a synthetic mineral (9K medium) solution developed by Silverman and Lundgren (1959).

Oxidant Agent Production
The biooxidation experiments were conducted in both batch and continuous flow stirred tank reactors. A working volume of 5.15 L was used for all experiments. Mixing was achieved by a variable speed mixer operated at 500 rpm and using a three blade axial impeller with a 7.62-cm diameter. The reactors were operated at room temperature (25 ± 2°C). Diffused air to each reactor was monitored through a regulator and flowmeter. The concentration of dissolved oxygen stayed at approximately 8 mg L–1. Reactions were not limited by the available oxygen supply. Ferrous iron was added as FeSO4·7H2O. Bacteria were considered to be adapted when the ORP in the run tests stabilized at 500 to 600 mV.

Batch Reactor Run Experiments
The ferric ion solution was produced in four batch reactors (A, B, C, and D) at different concentrations of SSF with one reactor maintained as a control. Bioreactor A, containing 100% (v v–1) 9K solution, was used as a control. The control consisted of 3.65 L of the 9K synthetic solution and 1.5 L of the inoculum. The three bioreactors (B, C, and D) each contained a mixture of 3.65 L comprised of SSF and 9K synthetic medium, in which 1.5 L of inoculum was added. Bioreactors B, C, and D, respectively, contained a mixture prepared as follows: 34% (v v–1), 67% (v v–1), and 100% (v v–1) of SSF were respectively added to 66% (v v–1), 33% (v v–1), and 0% (v v–1) of 9K synthetic medium. The inoculum was a culture of A. ferrooxidans previously acclimated to grow on ferrous sulfate in the presence of sludge filtrate. Each biological reactor contained 10 g Fe(II) L–1 at pH = 2.0. The biological batch reactors were monitored daily for ORP, pH, and ferrous and ferric ions concentration. Once the oxidation of Fe(II) was successfully completed in the batch tests running in Bioreactor B, the liquid volume was divided between Bioreactors B, C, and D. The working volume of Bioreactors C and D reached 5.15 L after addition of a liquid portion from Bioreactor B, whereas the liquid volume in Bioreactor B needed to be adjusted to 5.15 L. Thus, Bioreactor B was filled with 5.15 L of a feed solution made up of 34% (v v–1) SSF and 66% (v v–1) 9K solution. The solution in Bioreactor A experienced evaporation; thus, the volume was increased to 5.15 L with tap water. It is important to underscore that, before continuous flow-through reactors tests were performed, a second set of batch reactor experiments were conducted to achieve a high concentration of bacteria (A. ferrooxidans) within each bioreactor at the beginning of continuous run experiments (i.e., ferric iron concentration and ORP maximal). Four to six days of incubation were required to completely oxidize ferrous iron in Bioreactors B, C, and D. Subsequently, the bioreactors were fed continuously by a mixture containing various concentrations of SSF.

Continuous Flow-Through Reactor Experiments
The continuous flow-through reactor tests consisted of gradually supplying A. ferrooxidans with a feed solution containing 10 g Fe(II) L–1 and increasing concentrations of sludge filtrate. During the first set of continuous run tests, Bioreactors B, C, and D were fed by an identical solution comprised of 34% (v v–1) of SSF and 66% (v v–1) of 9K medium, while Bioreactor A was only supplemented with 100% (v v–1) 9K synthetic solution (control). After 10 d of continuous run experiments, a second set of experiments was conducted and Bioreactors B, C, and D were fed by solutions prepared as follows: 60% (v v–1), 80% (v v–1), and 90% (v v–1) of SSF were respectively added to 40% (v v–1), 20% (v v–1), and 10% (v v–1) of 9K synthetic medium, respectively. Bioreactor A was always used as the control. Bioreactors C and D had been intermediately inoculated (1 L of inoculum in each reactor) from the output solution of Bioreactor A on the fourth and fifth day, respectively, of incubation during the first set of continuous flow-through reactors tests. The hydraulic retention time in all bioreactors was 48 h. Figure 2 illustrates one of the four microbial ferrous sulfate oxidation reactors used for the continuous tests. Continuous test were conducted in all four bioreactors (A, B, C and D) operated in parallel with an initial pH = 2.0. Influent was metered to each reactor by a peristaltic pump. Feed solutions were maintained at pH = 2.0. Samples were taken daily from each bioreactor and monitored for ORP, ferrous ion content, ferric ion content, and pH. Nitrogen and phosphorus contents were measured to ascertain that nutrients were present in sufficient quantity for biological growth and to obtain a nutrient requirement estimate for A. ferrooxidans. It worth noting that intermediate inoculations were performed to accelerate microbial ferrous ion oxidation.



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  		Fig. 2. Setup of the ferrous sulfate oxidation bioreactor.

 
Similar continuous experiments were conducted with feed solution but without further addition of the 9K synthetic solution. The feed solutions were made up of either 25% (v v–1), 50% (v v–1), 75% (v v–1), or 100% (v v–1) SSF. The remainder of the solution consisted of tap water and 10 g Fe(II) L–1. Bacteria previously acclimated to grow on ferrous sulfate in a mixture containing low concentration of 9K medium (10% 9K and 90% of SSF) were used as inoculum. During this set of experiments, several tests were performed using SSF sampled at different times. The initial TOC concentration in the SSF samples was not the same. The initial sludge filtrate (SSF) diluted by 25, 50, or 75% (v v–1) produced different total organic concentrations. The objective of these tests was to determine at what TOC concentrations bacterial activity would be inhibited.

Metal Leaching System
Bioproduced ferric iron solution (BFSS) was then used to solubilize metals in a primary and secondary sludge mixture resulting from a biofiltration process and supplied by the Quebec Urban Community wastewater treatment plant. The total solids content in the sludge was 19 g L–1. Experiments were performed in a 2-L tank containing 1 L of well-mixed sludge. The experiments were conducted at room temperature. The sludge had been previously acidified to pH = 2.2 ± 0.2 with H2SO4 (5 M). Subsequently, ferric iron solution was added to increase ORP between 400 and 450 mV. The total leaching time was 6 h. The ORP and pH were measured every 2 h. Fifty-milliliter samples were also drawn at 2-h intervals and 4.5 mL of the polymer solution (1 g Percol L–1) were added to the sample. The sample was then vacuum-filtered on a Whatman (Maidstone, UK) 934-AH membrane. The addition of Percol solution enables the solid and liquid phase to separate and enhances filtration. After the addition of 0.2% hydrochloric acid, 20 mL of the filtrate was stored at 4°C awaiting heavy metal analysis.

Analytical Techniques
An Orion (Beverly, MA) 205A model pH meter was used to obtain ORP and pH values. The pH and ORP electrodes were in Ag/AgCl and in platinum, respectively. The ORP measurements were checked with quinhydrone at pH = 4 and 7. Oxygen concentrations were determined with a dissolved oxygen meter (Model 50B; Yellow Spring Instruments, Yellow Springs, OH). Total organic carbon (TOC) and inorganic C were measured with a Shimadzu (Kyoto, Japan) TOC 5000. The concentrations of nitrogen and phosphorus compounds were measured using a Technicon (Tarrytown, NY) autoanalyzer. Others nutrients (Ca, Mg, and K) were determined by plasma emission spectrophotometry (ICP–AES) using a Varian (Palo Alto, CA) apparatus (Vista AX). The concentrations of the ferrous and ferric ions were measured by a colorimetric method with phenanthroline (American Public Health Association, 1999). The absorbance at 510 nm was measured after 5 to 10 min. To assess the concentration of total iron, ferric ion was reduced to ferrous ion using hydroxylammonium chloride as the reducing agent. The concentration of ferric iron in solution was calculated by subtracting the ferrous ion concentration from the total iron concentration. A calibration curve of known FeSO4·7H2O concentrations was used to calculate the iron concentrations. The absorbance measurements (510 nm) were performed using Spectronic (Garforth, UK) 501 and 601 spectrophotometers.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Batch Reactor Run Results
The ORP and residual ferrous ion concentration were measured following biological oxidation of ferrous iron:

[1]

Figures 3 and 4 show the changes in ORP and Fe(II) concentrations in Bioreactors A, B, C and D. After 3 and 8 d of incubation, the ORP level in Bioreactors A and B increased from 420 mV to stabilize at 625 mV. Ferrous ion concentrations in Bioreactors A and B decreased from approximately 7.5 to 0 g L–1. The increase in ORP is primarily attributed to the increased Fe(III) to Fe(II) ratio that frequently occurs during bacterial growth (Pesic et al., 1989). Previous studies (Lundgren et al., 1964) have suggested that the growth of A. ferrooxidans is directly proportional to the number of ferrous ions oxidized. It is common in bacterial oxidation of ferrous ion studies to evaluate bacterial growth by measuring substrate concentrations of bacteria rather than determining bacterial biomass (Harvey and Crundwell, 1997; Lizama and Suzuki, 1989).



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  		Fig. 3. Effect of sludge filtrate concentration on oxidation–reduction potential during batch reactors run tests. SSF, sewage sludge filtrate; 9K, synthetic mineral solution.

 


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  		Fig. 4. Effect of sludge filtrate concentration on ferrous ion concentration during batch reactors run tests. SSF, sewage sludge filtrate; 9K, synthetic mineral solution.

 
In contrast, Bioreactors C and D did not show a significant decrease in ferrous iron concentration indicating a lack of bacterial growth. Furthermore, the ORP remained constant (400 mV) in Bioreactors C and D throughout the experiment. Ferrous iron was not oxidized in these bioreactors, owing to possible inhibitory effects caused by high concentrations of organic matter or insufficient nutrients.

Continuous Flow-Through Reactor Results
Bacterial activity in the continuous flow-through reactors is presented in Fig. 5 . Ferric ion concentrations are plotted against time of incubation. Interestingly, the oxidation of ferrous ions into ferric ions occurred in all culture media. In Bioreactor A, ferric ion concentration increased from 7 to 9 g L–1 after 3 d of incubation and remained stable. In Bioreactor B, ferric ion concentration increased from 3 to 8.5 g L–1 after 5 d of incubation. In Bioreactors C and D, ferric iron concentration decreased from 5 to about 1 g L–1 after 4 d of incubation, then increased rapidly reaching 8 g L–1 after 7 and 9 d, respectively. Indeed, the lag times required to increase ferric ion concentration indicated the period of time needed for the bacteria to adapt to the new medium. It is very important to notice that Bioreactors C and D had been intermediately inoculated (1 L of inoculum in each reactor) from the output solution of Bioreactor A, respectively, on the fourth and fifth day of incubation. Intermediate inoculations were performed to accelerate microbial ferrous ion oxidation.



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  		Fig. 5. Variation in ferric ion concentration during first continuous reactor run tests. Hydraulic retention time = 48 h. SSF, sewage sludge filtrate; 9K, synthetic mineral solution.

 
In continuous bioreactor systems (Bioreactors B, C, and D), A. ferrooxidans previously acclimated to grow on a feed solution containing 34% (v v–1) sludge filtrate and 66% (v v–1) 9K synthetic solution were supplied with feed solutions containing 60% (40% 9K), 80% (20% 9K), and 90% (10% 9K) SSF, respectively. Bioreactor A was fed with 100% 9K synthetic medium and used as a control assay. Figure 6 shows that microbial ferrous oxidation occurred in Bioreactors B, C, and D. The ferric iron concentration varied between 8 and 9 g L–1 in each bioreactor. The steady state operation was considered to be established when the ferric ion concentration varied by less than 5% during a period of time equal to three hydraulic retention times. A lag time of only 1 d was recorded in the three bioreactors despite high sludge filtrate concentrations used in the feed solutions (60, 80, and 90%) as compared with previous continuous flow-through tests. Bioreactor performance was superior during the second set of continuous experiments than during the first set of continuous experiments. Performance improved as a result of the gradual acclimation of bacteria contributing to a biomass accumulation within the bioreactors.



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  		Fig. 6. Variation in ferric ion concentration during second continuous reactor run tests. Hydraulic retention time = 48 h. SSF, sewage sludge filtrate; 9K, synthetic mineral solution.

 
A third set of continuous experiments were conducted to determine the limit of TOC for which A. ferrooxidans were inhibited. A feed solution without the addition of 9K synthetic medium but containing 10 g Fe(II) L–1 was prepared. In Fig. 7 , ORP and ferric iron concentration are plotted against TOC. The ORP and Fe(III) concentration values correspond to the steady state values obtained during the incubation period. The ORP (550 mV) and the ferric ion concentration (8 g L–1) remained constant up to 250 mg TOC L–1 then decreased simultaneously to 400 mV and 1 g Fe(III) L–1. Total organic carbon concentrations greater than 250 mg L–1 inhibited bacterial growth. Chemical interactions between ferrous iron and certain compounds outside the cell may inhibit iron oxidation depending on the organic compound used (Tuttle and Dugan, 1976). Several organic compounds, such as carboxylic acids, anionic detergents, cationic surfactants, amino acids, and carbohydrates, have been reported to inhibit iron oxidation and bacterial growth in iron media (Tuttle and Dugan, 1976). Tuttle and Dugan (1976) also demonstrated that carboxylic acids were the most toxic chemicals tested.



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  		Fig. 7. Effect of total organic carbon in sludge filtrate on oxidation–reduction potential and Fe(III) measured during continuous reactor run tests.

 
Humic substances, proteins, carbohydrates, tannins, and anionic detergents can be found in sewage sludge filtrate samples (Rebhun and Manka, 1971). Gu and Wong (2004) have recently shown that organic acids, especially acetic and propionic acids, were found in anaerobically digested sewage sludge submitted to a bioleaching treatment using Acidithiobacillus ferrooxidans, at concentrations higher than their inhibitory levels.

The microbial Fe(II) oxidation rate of the process studied was 0.2 g Fe(II) L–1 h–1. This value can be compared with values obtained in different experimental conditions. The microbial oxidation of ferrous iron of concentration below 10 g L–1 has been investigated by several authors. The maximum oxidation rate obtained by freely suspended bacteria is below 0.4 g Fe(II) L–1 h–1 (Braddock et al., 1984; Torma, 1977). Armentia and Webb (1992) studied ferrous ion oxidation using A. ferrooxidans cells immobilized in polyurethane foam support particles and fed by 9K synthetic solution. The resulting ferrous ion oxidation rate was 0.23 g Fe(II) L–1 h–1 with an hydraulic retention time of 20 h. The microbial Fe(II) oxidation rates obtained during the present study are consistent with those obtained by Armentia and Webb (1992). The process proposed by Armentia and Webb (1992) requires the addition of nutrients for bacterial growth. In the present study, bacterial growth was achieved in sludge filtrate without addition of nutrients. Table 1 compares the nutrient content of the 9K synthetic medium and other mineral media frequently used for thiobacilli cultures to those of the SSF. Essential nutrients such as phosphorus, magnesium, and nitrogen were present in higher concentrations in the 9K medium compared with the SSF. Complete Fe(II) oxidation was obtained using SSF regardless of low nutrient concentrations. This was primarily due to prior acclimation of the bacteria. Bacterial growth without the addition of any chemical products reduces the operating costs required for biooxidation processes applied to metal recovery in mining, municipal sewage sludge, or industrial waste products.


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  		Table 1. Composition of different synthetic media and sewage sludge filtrate (SSF) medium.

 
Comparison of Bioproduced Ferric Iron Solution and Ferric Chloride Used as Solubilizing Agents for Metal Leaching
The ferric ion solution produced by A. ferrooxidans in the sewage sludge filtrate was then used to solubilize the heavy metals contained in untreated sewage sludge. The results were then compared with chemical leaching results using FeCl3. The main characteristics of the ferric ion solutions used are given in Table 2. The biological ferric sulfate solution (BFSS) was obtained from the biological oxidation of Fe(II) [10 g Fe(II) L–1]. The concentration of ferric iron in FeCl3 was 15 times greater than that measured in SSF. Chromium (122 mg Cr kg–1), copper (200 mg Cu kg–1), and zinc (390 mg Zn kg–1) had the highest concentrations in the sludge mixture. However, the maximum concentrations of Cr, Cu, and Zn permitted in decontaminated sludge used for agricultural purposes in Québec are 210, 100, and 500 mg kg–1, respectively (criteria C1) (Gouvernement du Québec, 2001). Accordingly, copper was the only metal in the sludge exceeding the maximum value permitted.


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  		Table 2. Comparison between bioproduced ferric iron solution (BFSS) and FeCl3 solution used as solubilizing agent during metal extraction from sludge after 6 h.

 
The total percentages of metal solubilization obtained after 6 h of leaching are indicated in Table 2. The metal extraction yield was obtained by dividing the final concentration of metal by the corresponding metal content in the sewage sludge as determined after digestion. Comparisons between metal leaching yields using BFSS or FeCl3 as solubilization agents show significant differences in Cr and Zn extraction. However, for Cu solubilization a yield of 96% was obtained using FeCl3 compared with 71% obtained with BFSS. This difference can be explained by the quantity of Fe(III) used during the leaching process. Using SSF, the amount of ferric iron required to increase the ORP of the sludge to between 400 and 450 mV was 32.1 kg Fe(III) Mg–1 dry sludge compared with 44.6 kg Fe(III) Mg–1 dry sludge consumed using the FeCl3 process. It is likely that copper was strongly bound within the sludge and, consequently, a high concentration of ferric ions was required for Cu removal. Indeed, an increase of ferric ions results in an increase in the available energy that can be used for the breakdown of Cu in the sludge.

Treatment using BFSS, sludge pre-acidified to pH = 2.0, requires 108 kg H2SO4 Mg–1 dry sludge. Treatment using FeCl3 requires 82 kg H2SO4 Mg–1 dry sludge; thus, the FeCl3 treatment reduces H2SO4 consumption by 23%. In fact, the pH of the FeCl3 solution was initially very low (pH = 0.24) and consequently it contributed to sludge acidification. By comparison, the initial pH of the BFSS was 2.36, which did not significantly affect the pH of sludge previously acidified to pH = 2.0.

Given that the experimental objective was to produce sludge suitable for agriculture use, it was important to verify the nutrient content during the leaching process. The fertilizing value of the sludge was evaluated by measuring key nutrient concentrations (Ca, K, Mg, and P) present in the sludge dewatering filtrate (untreated and treated sludge). The nutrient concentrations in the untreated-sludge filtrate were as follows: Ca (96.0 mg L–1), K (24.0 mg L–1), Mg (15 mg L–1), and P (2.4 mg L–1). By comparison, the concentrations recorded in the treated-sludge filtrate using SSF and FeCl3 processes are summarized in Table 2. An increase in Ca, K, and Mg concentrations (estimated to 70, 36, and 52%, respectively) was recorded regardless if BFSS or FeCl3 was used as the solubilizing agent. In fact, the nutrients initially linked to the insoluble fraction of the sludge were leached during the metal leaching process, although a non-negligible concentration of nutrients remained in the solid fraction of sludge. In particular, the concentration of phosphorus (3.0 mg L–1) measured in the treated-sludge filtrate using FeCl3 was quite similar to that measured in the untreated-sludge filtrate (2.4 mg L–1). By comparison, treatment using BFSS induced a high solubilization of phosphorus (38 mg L–1). It is worth noting that according to the analysis of samples withdrawn every two hours (2, 4, and 6 h), the concentration of phosphorus increased in the solution at the beginning of the leaching process followed by a gradual decrease in phosphorus content. For example, after 2 h of leaching using BFSS, the concentration of phosphorus recorded in solution was 48 mg L–1 and, at the end of the treatment (after 6 h of leaching), a concentration of 38 mg L–1 was measured. The subsequent decrease in phosphorus concentration is attributed to coprecipitation of phosphate ions with ferric ions:

[2]

Thus, FePO4 remained in the insoluble fraction of sludge and induced a reduction of phosphorus concentration. The formation of FePO4 increases with ferric ion concentration. The highest concentration of phosphorus (38 mg L–1) recorded in the BFSS treated-sludge filtrate was likely due to a relatively low concentration of ferric ions [32 kg Fe(III) Mg–1 dry sludge] used for metal leaching. This can be compared with 45 kg Fe(III) Mg–1 dry sludge consumed using FeCl3. Correspondingly, the BFSS produced from the SSF initially contained phosphorus, resulting in a higher phosphorus concentration in the BFSS treated-sludge filtrate.

It is also interesting to compare the costs of chemical products required for metal extraction using BFSS versus commercial ferric chloride solution. At a cost of US$208 Mg–1 of H2SO4 and US$100 Mg–1 FeSO4·7H2O, the decrease in pH and increase in ORP using BFSS involved a total cost of US$26 Mg–1 dry sludge. By comparison, at a cost of US$1300 Mg–1 of FeCl3, metal leaching using FeCl3 required a total cost of US$75 Mg–1 dry sludge, including sludge pre-acidification. Consequently, metal extraction from sewage sludge using BFSS process was three times less expensive than the FeCl3 process.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study has demonstrated that sewage sludge filtrate can be used as a nutrient source for the growth of Acidithiobacillus ferrooxidans during the biological oxidation of ferrous iron. Sewage sludge filtrate must be diluted to reduce the concentration of dissolved organic carbon to below the toxicity level for A. ferrooxidans. Total organic carbon (TOC) in excess of 250 mg L–1 was inhibitory to bacterial growth. The ferric ion solution produced in the sewage sludge filtrate by A. ferrooxidans was used to remove heavy metals (71% Cu, 49% Cr, and 80% Zn of total concentration) from a primary and secondary sludge mixture. With the exception of Cu, the percentage yields were similar to those obtained using a FeCl3 solution. The cost of treatment using the bioproduction of a ferric ion solution in sewage sludge filtrate was US$26 Mg–1 dry sludge (chemical use only). This is three times less expensive than the treatment process using a ferric chloride solution. The biological production of ferric iron in sewage sludge filtrate eliminates the need for additional nutrients, creating a more economical process for the removal of toxic metals from sewage sludge. Parameters such as hydraulic retention time, temperature, and substrate concentration must be studied further to optimize the bioproduction of ferric sulfate and increase the efficiency of heavy metals removal from sewage sludge. In addition, a statistical study should be performed to critically demonstrate the performances of the indirect bioleaching process. Finally, an economic study should be also performed to sharply demonstrate the economic advantage of indirect bioleaching application (including chemical cost, energy cost required for oxygen supply, and cost required to built the bioreactor) compared with conventional direct bioleaching.


   ACKNOWLEDGMENTS
 
Sincere thanks are extended to the Canada Research Chairs and to the National Sciences and Engineering Research Council of Canada for their financial contribution to this study.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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