Determination of critical operational conditions favoring sulfide production from domestic wastewater treated by a sulfur-utilizing denitrification process.

The aim of this study was to determine the critical operational conditions leading to the generation of sulfide in a domestic wastewater treated by a sulfur-utilizing denitrification process. The influence of various important parameters on the reduction of the sulfates present in denitrified domestic wastewaters to sulfide was studied. Experiments were carried out in batch mode with denitrified domestic wastewaters containing various amounts of both organic matter and sulfates. Preliminary results showed that aqueous sulfide was generated for DOC and sulfate contents higher than 56 mg/L and 371 mg/L, respectively, while DOC and sulfate contents of 77 mg/L and 412 mg/L, respectively, were required to allow the release of gaseous H2S. Good correlations were also observed between gaseous sulfide production and the values of ORP and DOC, while the amounts of dissolved sulfide produced seemed to be correlated with the ORP values and the concentration of sulfates. Additional experiments were conducted using a Box-Behnken methodology to determine if the production of aqueous or gaseous sulfide can be predicted depending on the DOC (from 50 to 90 mg/L) and sulfate contents (from 160 to 380 mg/L) at various temperatures ranging from 5 to 25 °C. The highest sulfide generation (H2S(g) = 84.8 ppm and H2S(aq) = 2.42 mg/L) occurred at 25 °C with DOC and sulfate concentrations starting from 90 mg/L and 270 mg/L, respectively, indicating that the production of sulfides from denitrified domestic wastewaters required conditions not likely to occur at the effluent of a sulfur-based denitrification unit following secondary treatment.

Assessment of sulfide production risk in soil during the infiltration of domestic wastewater treated by a sulfur-utilizing denitrification process.

This study aimed to determine the potential of sulfide generation during infiltration through soil of domestic wastewater treated by a sulfur-utilizing denitrification process. Three types of soil with different permeability rates (K s = 0.028, 0.0013, and 0.00015 cm/s) were investigated to evaluate the potential risk of sulfur generation during the infiltration of domestic wastewater treated by a sulfur-utilizing denitrification system. These soils were thoroughly characterized and tested to assess their capacity to be used as drainages for wastewaters. Experiments were conducted under two operating modes (saturated and unsaturated). Sulfate, sulfide, and chemical oxygen demand (COD) levels were determined over a period of 100 days. Despite the high concentration of sulfates (200 mg/L) under anaerobic conditions (ORP = -297 mV), no significant amount of sulfide was generated in the aqueous (<0.2 mg/L) or gaseous (<0.15 ppm) phases. Furthermore, the soil permeability did not have a noticeable effect on the infiltration of domestic wastewater treated by a sulfur-utilizing denitrification system due to low contents of organic matter (i.e., dissolved organic carbon, DOC). The autotrophic denitrification process used to treat the domestic wastewater allowed the reduction of the concentration of biochemical oxygen demand (BOD5) below 5 mg/L, of DOC below 7 mg/L, and of COD below 100 mg/L.

Factors influencing the Zn and Mn extraction from pyrometallurgical sludge in the steel manufacturing industry.

In this laboratory study, a process has been developed for selectively leaching zinc and manganese from pyrometallurgical sludge produced in the steel manufacturing industry. In the first part, the yield of Zn extraction was studied using four factors and four levels of the Box-Behnken response surface design. The optimum conditions for the step of Zn leaching were determined to be a sulfuric acid concentration of 0.25 mol/L, a pulp density of 10%, an extraction temperature of 20 °C, and three stages of leaching. Under such conditions, 75% of the Zn should be leached. For Mn leaching, the optimum conditions were determined to be a sulfuric acid concentration of 0.25 mol/L, a Na2S2O5/Mn stoichiometry of 1, a leaching time of 120 min and two leaching steps. In this case, 100% of the Mn should be leached.

A new process for nickel ammonium disulfate production from ash of the hyperaccumulating plant Alyssum murale.

The extraction of nickel (Ni) from ultramafic soils by phytomining can be achieved using Alyssum murale cultures. This study presents a new process for the valorization of Ni accumulated by this plant through the production of a Ni ammonium disulfate salt (Ni(NH(4))(2)(SO(4))(2).6H(2)O). The process comprises an initial leaching of the ashes of A. murale with a sulphuric acid solution (1.9 M H(2)SO(4), T=95 °C, t=240 min, TS=150 g ash L(-1)), producing a leachate rich in Ni (10.2 g Ni L(-1); 96% Ni solubilisation), Mg, P, K, Fe, Ca and Al. The pH of the acid leachate is increased to 5.0 with NaOH (5M), followed by an evaporation step which produced a purified solution rich in Ni (21.3 g NiL(-1)) and an iron hydroxide precipitate. The cold crystallization (T=2 °C, t=6h) of this solution by the stoichiometric addition (× 1.2) of ammonium sulfate generates a Ni ammonium disulfate salt, containing 13.2% Ni, that is potentially valuable to industry.

[Hydrological and edaphic structure of an oyster-farming site: Duna Blanca (Bay of Dakhla, south Morocco)]. / Structure hydrologique et edaphique d'un site ostreicole: Duna Blanca (la baie de Dakhla sud du Maroc).

Morocco shelters lagoons, estuaries and bays along its paralic coasts which are among the most productive in the world. The Bay of Dakhla is the longest and the most important site in Morocco due to its halieutic richness. In fact, this bay is an ecosystem with great potential in terms of aquaculture, mainly shellfish farming, and shelters favorable zones for tapiculture, mytiliculture and ostreiculture, in accordance with ecological planning. A hydrological study (temperature, salinity, chlorophyll "a", suspended matter and organic matter) was conducted on a breeding project of the cupped oyster (Crassostrea gigas) installed on the Duna Blanca site since April 2003. A seasonal follow-up of sediment structure was carried out involving granulometric and metallic studies. The hydrological sampling was performed monthly on the surface and at the bottom, during spring tides, low tide and high tide. The results confirmed that the site encompasses a wealth of nutritive elements and a significant chlorophyll-rich biomass. A gradient of hypersalinity is well correlated with seasonal variation of the temperature. The sedimentary structure ranges from muddy-sandy to sandy type. The metal concentrations in the sediments never exceeded the toxicity thresholds. However, the site's production potential cannot be limitless and could be affected by the extension and installation of new conchylaceous farms.

[Treatment of PAH-contaminated aluminium wastes by flotation using amphoteric surfactants]. / Traitement de dechets d'aluminerie contamines en hap par flottation en presence de surfactants amphoteres.

The aluminium industry produces wastes polluted with polycyclic aromatic hydrocarbons (PAH). The most important PAH found in these wastes is benzo(b,j,k)fluoranthene (BJK) at concentrations exceeding the permitted levels (>1000 mg kg(-1)). The objective of this research was to compare the performances of amphoteric (BW and CAS) and non-ionic surfactants (Triton X-100 and Tween 80), at a concentration of 0.5% (w w(-1)), for PAH removal (and particularly for BJK) during washing treatment of aluminium industry wastes. The best removal yield of BJK (35%) has been measured during treatment with CAS. The efficiency of this surfactant has been further improved by using a flotation process. Flotation tests have also been realized at different CAS concentrations (0.1, 0.2, 0.25 and 0.5% w w(-1)) and using different total solids (7, 10, 15 and 20% w v(-1)). The highest BJK removal yield (68%) has been obtained using 0.5% CAS and a total solids concentration of 15%. The rate of hazardous wastes produced in these conditions represents 10% of the initial weight of aluminium wastes treated.

[PAH removal from black sludge from aluminium industry by flotation using non-ionic surfactants]. / Enlèvement des hap des boues noires d'aluminerie par flottation et en présence de surfactants non ioniques.

The valorization of black sludge is an environmental problem of the aluminium industry because of Polycyclic Aromatic Hydrocarbon (PAH), fluoride and alumina contents. A soil washing operated in a cell flotation with non-ionic surfactants (Brij 35, Igepal CA-720, Triton X-100, Tween 80) at different concentrations has been tested in order to remove the PAH (benzo(a)anthracene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(j)fluoranthene, benzo(k)fluoranthene, fluoranthene) detected in black sludge. The Tween 80 at 0.5% gave the best results with PAH removal rates between 35 and 50%. These percentages remain low due to the presence in large quantity of fine solid particles (45%) in the PAH concentrate produced by the flotation process. The substitution of the cell flotation by a column flotation allowed the reduction of this problem. Thus, the PAH removal rates reached 60%. This removal yield has been obtained with the lowest wash water flow-rate (150 ml min(-1)) and pulp total solids (10% p p(-1)) tested.

[Study of the metal precipitation from decontamination leachates of municipal wastes fly ash incinerators]. / Etude de la précipitation des métaux de lixiviats de décontamination des cendres volantes d'incinérateurs de déchets municipaux.

This research work focuses on the development of a new process for the decontamination of municipal wastes incinerators fly ashes. The objective of this study was to evaluate different total and selective precipitation methods for metals removal from ash decontamination leachates. The tested options include 1) use of hydrated lime and caustic soda for selective (pH 5.0) and total (pH 8.5) metal precipitation; 2) addition of different chemicals (H3PO4, Na2S and FeCl3) in a pH range from 6.0 to 9.0. Fly ash decontamination assays using alkaline and acid washing steps were initially performed using optimal conditions previously established. Treated fly ashes respected the standards based on the TCLP leaching test for all studied metals and SPLP. Total metal precipitation tests carried out at pH 8.5 achieve removal yields for all metals > or = 90% using hydrated lime and > or = 83% using caustic soda. Selective precipitation tests alone at pH 5.0 show removal yields > or = 97% for Cr and between 75 and 87% for Al and Pb. Moreover, assays carried out using a stoechiometric addition of Na2S have allowed the separation of Cd (> or = 99%) and Zn (> or = 71%) as metal sulphides (CdS and ZnS). From an economical point of view, the most interesting option seems to be the leachates neutralization at pH 7.0 using Ca(OH)2 combined with the reuse of the treated leachates in the fly ash leaching steps. Metal precipitation cost at pH 7.0 has been estimated to be 22.7 CAN dollars tct-1 using Ca(OH)2, and 26.7 CAN dollars tct-1 using NaOH.

Removal of metals in leachate from sewage sludge using electrochemical technology.

Heavy metals in acidic leachates from sewage sludge are usually removed by chemical precipitation, which often requires high concentration of chemicals and induces high metallic sludge production. Electrochemical technique has been explored as an alternative method in a laboratory pilot scale reactor for heavy metals (Cu and Zn) removal from sludge leachate. Three electrolytic cell arrangements using different electrodes materials were tested: mild steel or aluminium bipolar electrode (EC cell), Graphite/stainless steel monopolar electrodes (ER cell) and iron-monopolar electrodes (EC-ER cell). Results showed that the best performances of metal removal were obtained with EC and EC-ER cells using mild steel electrodes operated respectively at current intensities of 0.8 and 2.0 A through 30 and 60 min of treatment. The yields of Cu and Zn removal from leachate varied respectively from 92.4 to 98.9% and from 69.8 to 76.6%. The amounts of 55 and 44 kg tds(-1) of metallic sludge were respectively produced using EC and EC-ER cells. EC and EC-ER systems involved respectively a total cost of 21.2 and 13.1 CAN dollars per ton of dry sludge treated including only energy consumption and metallic sludge disposal. The treatment using EC-ER system was found to be effective and more economical than the traditional metal precipitation using either Ca(OH)2 and/or NaOH.

Cocoa shells for heavy metal removal from acidic solutions.

The development of economic and efficient processes for the removal of heavy metals present in acidic effluents from industrial sources or decontamination technologies has become a priority. The purpose of this work was to study the efficiency with which cocoa shells remove heavy metals from acidic solutions (pH 2) and to investigate how the composition of these solutions influences heavy metal uptake efficiency. Adsorption tests were conducted in agitated flasks with single-metal solutions (0.25 mM Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn), multi-metal solution (comprised of 0.25 mM of each of the cations above) and an effluent obtained from chemical leaching of metal-contaminated soil, in the presence of different cocoa shell concentrations (5-40 g/l). Results from the single-metal solution assays indicated that the fixation capacity of heavy metals by cocoa shells followed a specific order: Pb>Cr>Cd=Cu=Fe>Zn=Co>Mn=Ni=Al. Cocoa shells are particularly efficient in the removal of lead from very acidic solutions (q(max)=6.2 mg Pb/g, pH(i)=2.0 and T=22 degrees C). The presence of other metals and cations in solution did not seem to affect the recovery of lead. It was also observed that the maximum metal uptake was reached in less than 2 h. This research has also demonstrated that the removal of metals caused a decline in solution proton concentration (pH increase) and release of calcium, magnesium, potassium and sodium from the cocoa shells.

Comparison of natural adsorbents for metal removal from acidic effluent.

Adsorption tests were carried out in acidic synthetic solutions (pH 2.0) using 20 g l(-1) of various natural adsorbents and 0.25 mM of 11 different metals. In decreasing order, the most efficient adsorbents tested were: oyster shells, cedar bark, vermiculite, cocoa shells and peanut shells. In contrast, weak metal adsorption was demonstrated by: red cedar wood, peat moss, pine wood, corn cobs and perlite. Metal adsorption capacities in acidic synthetic solution followed the order: Pb2+> Cr3+> Cu2+> Fe2+> Al3+> Ni2+> Cd2+ > Mn2+ > Zn2+ >> Ca2+, Mg2+. Alkaline treatment (0.75 M NaOH) increased the effectiveness of metal removal for the majority of adsorbents. In contrast, acid treatment (0.75 M H2SO4) either reduced or did not affect the adsorption capacity of the materials tested. Finally, oyster shells, red cedar wood, vermiculite, cocoa shells and peanut shells, were effective natural adsorbents for the selective recovery of lead and trivalent chromium from acidic effluent.

[Bioleaching kinetic of a pyrite mining residue using organic wastes as culture media for Acidithiobacillus ferrooxidans]. / Cinétique de biolixiviation d'un résidu minierde pyrite en présence d'effluents organiquesutilisés comme milieu de culture pour Acidithiobacillus ferrooxidans.

In this study, the results of the leaching of metal sulphide concentrate using organic wastes as culture media for Acidithiobacillus ferrooxidans are summarized. These results indicate that the liquid fraction of municipal sewage sludge, paper mill sludge and pig manure, containing 10% (w v(-1)) pulp density of a pyritic mine waste concentrate can support the growth of the leaching bacteria and allow metal solubilization. The inhibition by dissolved organic carbon (DOC) appeared when the concentration in pig manure liquid fraction and sewage sludge filtrate is higher than 180 mg l(-1) and 500 mg l(-1), respectively. However, increase in organic concentration up to 650 mg l(-1) using paper mill sludge supernatant had no inhibitory effect on the bacterial growth. An important decrease of the DOC has been measured during all bioleaching tests. The organic matter was probably consumed by heterotrophic microorganisms activity. The growth rate of the iron-oxidizing bacteria varied from 0.05 to 0.07 h(-1). The dissolution of pyrite (FeS2) in organic waste media led to a yield of Fe solubilization of about 35%. Copper and zinc were also solubilized during the bioleaching tests. The yields of Cu and Zn solubilization ranged from 12 to 24%.

Different options for metal recovery after sludge decontamination at the Montreal Urban Community wastewater treatment plant.

The MUG (Montreal Urban Community) treatment plant produces approximately 270 tons of dry sludge daily (270 tds/day) during the physico-chemical treatment of wastewater. Recently, this treatment plant endowed a system of drying and granulation of sludge for valorization as an agricultural fertilizer having a capacity of 70 tds/day (25% of the daily sludge production). However, the metal content (mainly Cu and Cd) of the sludge surpasses the norms for biosolids valorization. In order to solve this problem, a demonstration project, from the lab scale to the industrial pilot plant, was carried out to test the Metix-AC technology for the removal of metals. A strongly metal-loaded filtrate was generated during the sludge decontamination. Tests concerned the study of the metal recovery by total precipitation and selective precipitation, as well as the use of alternative products for the metal precipitation. Other works consisted to simulate the acid filtrate recirculation from the decontaminated sludge (25% of the total volume) in the untreated sludge (75% of the total volume) intended for the incineration. The total precipitation with hydrated limeappearedeffectivefortherecoveryof metals (87% Cd, 96% Cr, 97% Cu, 98% Fe, 71% Ni, 100% Pb, 98% Zn). However, this option entails the production of an important quantity of metallic residue, which should be disposed of expensively as dangerous material. The selective iron precipitation does not appear to bean interesting option because the iron in solution within the leached sludge was principally present in the form of ferrous iron, which cannot be precipitated at pH lower than five. On the other hand, the use of commercial precipitating agents (TMT-15, CP-33Z, CP-NB and CPX) without pH adjustment of filtrate gave good results for the recovery of Cu and, to a lesser degree for the recovery of Pb. However, the efficiency for the other metals' (Cd, Cr, Fe, Ni and Zn) recovery was weaker (< 25%). Finally, the acid filtrate recirculation containing solubilised metals in untreated sludge destined for incineration appears to be the most interesting option. Metals in solution in the acid filtrate, precipitate or adsorb effectively (97% Cd, 97% Cr, 99% Cu, 82% Ni, 100% Pb and 87% Zn) on the solids of the untreated sludge. Moreover, TCLP tests were done on ashes produced during the incineration of sludge mixed with the acid filtrate produced during sludge decontamination. These tests showed that there were no significant differences, as regards the extractability of metals, between such ashes and those produced during the untreated sludge incineration without addition of filtrate. Therefore, it was predictable that this method can respect the current environmental standards required by the different governmental authorities.

[Lead and zinc recovery by adsorption on peat moss during municipal incinerator used lime decontamination]. / Récupération du plomb et du zinc par adsorption sur tourbe lors de la décontamination de chaux usées d'incinérateur de déchets municipaux.

The objective of this research was to evaluate the efficiency of peat moss for lead and Zn recovery from alkaline leachates (pH 11.5) produced during decontamination of municipal incinerator fly ash. Tests carried out with peat moss columns (density of 0.13 g m l(-1)) gave very high removal yields for lead (98.9 to 100%) and Zn (98.4 to 99.8 %). The initial metal concentrations in the leachates were 126 to 138 mg Pb l(-1) and 14.4 to 23.5 mg Zn l(-1). The columns were fed using two rates (20 and 40 ml min(-1)), which correspond respectively to 17.6 and 8.8 min as contact time on peat moss. Adsorption efficiencies of 16 to 18 mg Pb g(-1) and 1.7 to 3.2 mg Zn g(-1) have been measured during this fly ash leachate treatment study. The adsorbed toxic metals can be desorbed using a chlorhydric acid solution. The peat moss can be regenerated and reused for several adsorption cycles without loss of the lead and Zn adsorption efficiencies.

A decontamination process to remove metals and stabilise Montreal sewage sludge.

The Montreal Urban Community (MUC) treatment plant produces approximately 270 tons of dry sludge daily (tds/day) during physicochemical wastewater treatment. The sludges are burned and contribute to the greenhouse effect by producing atmospheric CO2. Moreover, the sludge emanates a nauseating odour during its thermal stabilisation and retains unpleasant odours for the part (25%) that is dried and granulated. To solve this particular problem, the treatment plant authorities are currently evaluating an acidic chemical leaching (sulfuric or hydrochloric acid) process at a pH between 2 and 3, using an oxidizing agent such as ferric chloride or hydrogen peroxide (METIX-AC technology, patent pending; [20]). They could integrate it to a 70 tds/day granulated sludge production process. Verification of the application of METIX-AC technology was carried out in a pilot plant set up near the sludge production plant of the MUC. The tests showed that METIX-AC technology can be advantageously integrated to the process used at the MUC. The residual copper (274 +/- 58 mg/kg) and cadmium (5.6 +/- 2.9 mg/kg) concentrations in the treated sludge meet legislation standards. The results have also shown that odours have been significantly eliminated for the dewatered, decontaminated, and stabilized biosolids (> 97%) compared to the non-decontaminated biosolids. A high rate of odour elimination also was obtained for the liquid leached biosolids (> 93%), compared to the untreated liquid biosolids. The fertilising value (N and P) is well preserved by the METIX-AC process. Dissolved organic carbon measurements have showed that little organic matter is brought in solution during the treatment. In fact, the average concentration of dissolved organic carbon measured in the treated liquid phase is 966 +/- 352 mg/l, whereas it is 1190 +/- 325 mg/l in untreated sludge. The treated sludge was first conditioned with an organic polymer and a coagulant aid. It was successfully dewatered with various dehydration equipments (filter press, rotary press, centrifuge).

Prediction of metal precipitates in tannery sludge leachate based on thermodynamic calculations.

Thermodynamic calculations were performed in order to predict the formation of metal precipitates during the tannery sludge leaching. Gibbs free energy deltaG=RTln(Qc/Kc) of precipitation reaction was used to examine the formation of precipitates during the leaching process. The values of activity equilibrium constant (Ka) of various precipitation reactions were adopted from the literature. The Ka values were corrected to obtain the corresponding values of concentration product (Qc) and concentration equilibrium constant (Kc) using the activity coefficients (gamma) of the ions. The activity coefficients (gamma) of the ions was calculated using Davies equation Lngammai=-1.172Zi2((I0.5/(1+I0.5))-0.3I) (aqueous solution, 25 degrees C, 1 atm, I>0.3 mol x l(-1)). The values of ionic strength (I) at different sludge solids concentration and leaching pH were obtained by measuring the concentration of all ionic species in the leachate. The thermodynamic calculations indicated that the possible metal precipitates formed during the leaching process were (am)Cr(OH)3, (am)CrPO4, (am)Fe(OH)3, alpha-FeOOH(goethite), FePO4.2H2O, (am)AIPO4-2H2O, CaSO4.2H2O(gypsum). The solubility of these precipitates was found to decrease with the increase in the ionic strength (or sludge solids concentration). The results of computations were supported by the experimental observations.

Partitioning of trace metals before and after biological removal of metals from sediments.

Metal removal by biological solubilization in three strongly contaminated sediments was carried out in a two-liter stirred bioreactor. Biological treatment yielded metal removal efficiencies in the range of 11-30%, 43-57%, 60-79%, 61-90%, 18-21%, 0-10% for Pb, Cu, Zn, Cd, Ni and Cr, respectively. The treated sediments were then rinsed with a NaCl solution (0.5 M), resulting in an increase by nearly 47% in Pb removal for the three sediments, while for other metals (Cu, Zn, Cd, Ni, Cr), the NaCl rinse did not seem to allow any significant increase in metal solubilization. A standard procedure of sequential selective extraction (SSE) was applied to the sediments before and after each treatment. With regard to Pb, Zn and Cd, the carbonate bound fractions (2/3 sediments) represented 18-42% of metals prior to treatment, while the iron and manganese oxides bound fraction constituted 39-60% of metals for the three sediments. Between 90 and 100% of Pb, Zn and Cd removed by the process came from the fractions bound to carbonates and from those bound to Fe and Mn oxides. The organic matter and sulfide bound fractions contained 65-72% of total Cu present before treatment and the process removed, on average, 63% Cu present in this fraction. In contrast, Ni and Cr were found mainly in the residual fractions (50-80%). Finally, this biological treatment did not solubilize Cr appreciably, while removal of Ni mostly originated from the carbonate and Fe/Mn oxides fractions (70-80%).

Extraction of Cr(III) and other metals from tannery sludge by mineral acids.

A comprehensive investigation on the extraction conditions of Cr(III) and other metals from tannery sludge using mineral acids was performed. The effect of various factors (the extraction time, the type of mineral acid, the consumption of acid, pH, sludge solids concentration, temperature, and the type of sludge) on the leaching yield of metals was studied. The results indicate that the metal extraction time for most of the metals was 2h at 25 degrees C. The most suitable acid for Cr(III) extraction was sulphuric acid. A relationship for the acid requirement to adjust different sludge pH at varying sludge solids concentration was established. The leaching yield of chromium varied with sludge pH and the suitable pH for Cr(III) extraction was between 2.0 to 3.0. The optimum sludge solids concentration for Cr(III) extraction was 78.5 g x l(-1). The leaching yield of Cr(III) decreased with the increased temperature. The type of sludge (wet or dry) has no effect on the leaching yield of Cr(III) The metal adsorption and bonding on the insoluble organic colloid matter as well as the formation of metal precipitates may be two possible reasons for the decreased extraction yield of metals with increased solids concentration.

Comparison of Microbial Sulfuric Acid Production in Sewage Sludge from Added Sulfur and Thiosulfate.

Microbial leaching is one of the most attractive methods of removing toxic metals from sewage sludge. Sulfuric acid produced by indigenous microflora by the oxidation of elemental sulfur and sulfur compounds solubilizes toxic metals. The oxidation of sulfur compounds can be achieved by the direct oxidation to sulfates or by indirect oxidation, through the production and accumulation of soluble intermediate (S2 O2- 3 , S3 O2- 6 , S4 O2- 6 ) compounds. The production of these intermediates may create a potential danger of acidification of the receiving waters or the agricultural soil where the leached sludge is ultimately destined, via slow oxidation of the intermediates with subsequent sulfuric acid production. The objective of this research was to investigate the formation of S2 O2- 3 , S3 O2- 6 , and S4 2- 6 during metal bioleaching using elemental sulfur and thiosulfate as energy substrates for growth of indigenous thiobacilli (sulfur-oxidizing microorganisms) in sludge. The results obtained showed that intermediates were not formed when elemental sulfur was used as a substrate, whereas trithionate and tetrathionate accumulated in the sludges when thiosulfate was used as substrate. Moreover, the metabolism of thiosulfate was much slower than that of elemental sulfur in sludge medium. Therefore, the utilization of thiosulfate for the growth of indigenous thiobacilli in sewage sludge is a less attractive alternative for the metal bioleaching.

Bacterial leaching of metals from sewage sludge by indigenous iron-oxidizing bacteria.

Bioleaching of metals can be achieved in sewage sludge using Thiobacillus ferrooxidans, which obtains its energy requirements from the oxidation of added ferrous iron. The purpose of this study was to verify the presence of indigenous T. ferroxidans and to evaluate their adaptive capacity and leaching potential. Nineteen sludges (primary, secondary, aerobically and anaerobically digested, oxidation pond) were tested and all of them contained indigenous iron-oxidizing bacteria. The acclimation of these organisms by successive transfers allowed a rise of sludge redox potential over 450 mV and a decrease of sludge pH between 3.8 and 2.2 over a 10-day incubation period. The metal solubilization efficiencies were Cd: 55-98%, Cr: 0-32%, Cu: 39-94%, Mn: 71-98%, Ni: 37-98%, Pb: 0-31% and Zn: 66-98%, were reached with these indigenous strains. The results obtained show that the metal bioleaching may be easily realized by direct acclimation of sludge microflora.