Environmentally friendly methods for wastewater treatment

Abstract

A method for the environmentally friendly treatment of sulfate-containing wastewater is provided. Acidic sulfate-containing wastewater is treated in a sulfate reducing bioreactor, with the influent and effluent looped through the cathode compartment of an electrochemical cell. The electrochemical cell stabilizes the pH in the bioreactor by in situ generation of base in the cathode compartment. Additionally, hydrogen is produced and used as an electron donor for sulfate reduction in the bioreactor. The central compartment of the electrochemical cell contains a sulfide-rich aqueous solution, where extracted cations are replaced by protons from the anode compartment. This results in an acidification of the sulfide-rich solution, which is useful for extracting sulfides as H2S. This H2S can be used to precipitate metals at the beginning of the process, forming another loop.

Description

[Technical Field]

[0001] The present invention relates to a method for the treatment of wastewaters, in particular metallurgical and mining wastewaters. Such wastewaters are usually acidic and sulfate-rich, with a pH below 5, and contain a variety of metals and metalloids. [Background technology]

[0002] Traditionally, wastewater has been treated by adding CaO or Ca(OH)2 (lime) to the wastewater, resulting in alkalinity and removal of sulfates by precipitation as CaSO4.2H2O (gypsum), and precipitation of metals as metal hydroxides, or in the case of arsenic, as Ca3(AsO4)2. See Nazari et al., Hydrometallurgy, 174, pp. 258-281, 2017.

[0003] In addition to the high consumable costs associated with the use of lime, large amounts of metal-contaminated gypsum were not suitable for reuse and had to be disposed of. Furthermore, valuable metals and other potentially useful compounds are lost due to the difficulty of recovery from this precipitate. Looking more closely at the environmental aspects of this process, liming and precipitation of metal hydroxides are becoming increasingly insufficient to meet future stringent environmental standards. See Johnson et al., Sci. Total Environ, 338, 3-14, 2005.

[0004] For example, WO 2018 / 007297, EP 0880475, and EP 1578697 propose sulfate-reducing bioreactors as an alternative for treating sulfate- and metal-containing wastewater. In these bioreactors, bacteria reduce sulfate to sulfide, which can then be used to precipitate metals or metalloids. However, sulfate-reducing bioreactors rely on the addition of external electron donors, such as hydrogen gas or organic compounds, as well as the addition of acids and bases for pH control. When CO2 is used as the carbon source in sulfate-rich acidic wastewater, additional alkali is required to maintain a near-neutral pH in the bioreactor.

[0005] Not only does the use of consumables add to operational costs, but industrial setups also involve off-site production, purchasing, on-site storage, process controls, and safety measures associated with these chemicals. Looking again at the environmental aspects of such processes, the addition of alkali increases the salinity and conductivity of the wastewater, which can be problematic if intended for release into freshwater environments or on-site reuse.

[0006] WO 2010 / 002503 describes the use of electrobiochemical reactors to reduce target compounds such as arsenic, selenium, mercury, or nitrates to meet drinking water and emission standards. In this system, electrodes that act as electron donors to the microorganisms are inserted directly into the reactor.

[0007] Similar to WO 2010 / 002503, the electrobiochemical systems proposed by Blazquez et al., Water Research, 105, pp. 395-405, 2016, or Luo et al., Bioresource Technology, 167, pp. 462-468, 2014, are limited by slow electron transfer rates and limited microbial densities. Due to the low achievable current densities and low reduction rates (less than 5 g of sulfate per liter of bioreactor per day), these technologies require very large reactors and a large number of electrodes, resulting in high capital investments for the treatment of sulfate-rich metallurgical wastewater. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2018 / 007297 [Patent Document 2] European Patent No. 0880475 [Patent Document 3] European Patent No. 1578697 [Patent Document 4] International Publication No. 2010 / 002503 [Non-patent literature]

[0009] [Non-Patent Document 1] Nazari et al., Hydrometallurgy, 174, pp. 258-281, 2017 [Non-patent document 2] Johnson et al., Sci. Total Environ, 338, pp. 3-14, 2005 [Non-patent document 3] Blazquez et al., Water Research, 105, pp. 395-405, 2016 [Non-patent document 4] Luo et al., Bioresource Technology 167, pp. 462-468, 2014 Summary of the Invention [Problem to be solved by the invention]

[0010] Therefore, a common electrobiochemical system has yet to be demonstrated capable of treating sulfate-rich metallurgical wastewater at high flow rates while using a reactor of modest size. [Means for solving the problem]

[0011] The present disclosure provides a bioreactor-based method that allows for a high specific reactor reduction rate while avoiding the use of reagents such as alkali. This result is achieved by coupling the bioreactor with an external electrochemical cell. This combination is highly beneficial because the cathode compartment of the electrochemical cell produces H, the preferred electron donor for microorganisms, along with hydroxyl anions, which allow for the achievement of pH control in the bioreactor. No additional consumables are required to ensure optimal pH in the bioreactor. [Brief explanation of the drawings]

[0012] [Figure 1]1 is a block diagram of a flowsheet. - A: Bioreactor - B: Electrochemical cell, B1: Cathode, B2: Central compartment, B3: Anode - C: Stripping reactor (optional) - D: Water removal reactor (optional) - E: Sulfide precipitation reactor (optional). Stream 1 is the wastewater fed to the process. The wastewater can optionally be fed as stream 1' to precipitate sulfides beforehand. Streams 11 and 12 form a first recirculation loop L1. Bleeding stream 11' is the effluent stream, which is fed to a second recirculation loop L2, preferably formed by streams 21 and 22. Effluent stream 21' then becomes the preferred effluent stream. Stream 22' is essentially water from the water removal reactor. If the optional steps using the stripping reactor (C) or the water removal reactor (D) are not performed, the recirculation loop (L2) is closed, bypassing the missing reactor. Stream 31 is an acidic aqueous solution fed to the anode compartment (B3) of the electrochemical cell (B). Once activated, water only needs to be added to compensate for the electrolysis, which causes protons to migrate to the central compartment and oxygen to be released as a gas. The synergistic effect of the combination of a bioreactor and an electrochemical cell is demonstrated in the following embodiment. [Figure 2] 1 is a graph of the kinetics of the process shown in Example 1, showing the evolution of key process parameters: I: current in mA through the electrochemical cell (B); II: pH of the first recirculation loop (streams 11, 12, 11') of the bioreactor (A), respectively; III: pH of the wastewater or process feed stream (stream 1); IV: pH of the second recirculation loop or process effluent (streams 21, 22, 21'). DETAILED DESCRIPTION OF THE INVENTION

[0013] A first embodiment is a method for the treatment of wastewater (1) containing sulfate salts of cations capable of forming soluble sulfides, comprising: - providing a bioreactor (A) suitable for reducing sulfate to sulfide; - providing an electrochemical cell (B) through which an electric current flows, the electrochemical cell (B) having a central compartment (B2) connected by a cation exchange membrane to an adjacent cathode compartment (B1) and an adjacent anode compartment (B3), thereby producing H2 in the cathode compartment and O2 in the anode compartment; - flowing an aqueous solution containing cations (21) through the central compartment (B2); - supplying an acidic aqueous solution (31) to the anode compartment (B3); - circulating the sulfide solution (11, 12) in a first loop (L1) through the bioreactor (A) and the cathode compartment (B1), thereby maintaining the pH of the solution between 6 and 9 by adjusting the current through the electrochemical cell (B); - adding wastewater (1) to the first loop at a set flow rate; - draining a small portion (11') of the solution from the first loop (L1) at a flow rate corresponding to the flow rate at which the wastewater (1) is fed; The present invention relates to a method, comprising:

[0014] Cations capable of forming soluble sulfides include, for example, Na + , K. + , Ca 2+ , Mg 2+ , and NH4 + are cations from the list including

[0015] Use of a bioreactor in such a situation involves providing an electron donor and a carbon source, along with the selection of a suitable pH and temperature. Such conditions depend on the exact selection of the microorganism according to the general knowledge of those skilled in the art. A pH between 6 and 9 is generally suitable, with a pH between 7.5 and 8.5 being preferred, as is a temperature around normal room temperature.

[0016] The electrochemical cell (B) comprises a cathode compartment (B1), a central compartment (B2), and an anode compartment (B3), with adjacent compartments connected by a cation exchange membrane. The anode compartment (B3) contains an acidic aqueous solution. O2 and H +is produced at a rate that depends on the current passing through the cell. O2 may be recovered for other uses.

[0017] Due to the current flowing through the electrochemical cell (B), H + The soluble cations migrate from the anode compartment (B3) to the central compartment (B2) through the first cation exchange membrane, thereby acidifying the central compartment (B2). At the same time, the soluble cations migrate from the central compartment (B2) to the cathode compartment (B1) through the second cation exchange membrane, thereby basifying the cathode compartment (B1) containing the recycled sulfide solution of the bioreactor (A).

[0018] The basification serves to compensate for the general acidification reaction occurring in the bioreactor (A). This is an important feature of the present invention, namely, the pH of the bioreactor, which is prone to acidification, is controlled by adjusting the current flowing through the electrochemical cell (B). Therefore, there is no need to add reagents such as NaOH, as has been done conventionally. This feature therefore reduces the amount of salts discharged.

[0019] For this basification mechanism to be effective, the presence of cations capable of forming soluble sulfides is required. In the absence of such cations, H + Only the cations migrate from the central compartment (B2) to the cathode compartment (B1) through the second cation exchange membrane, which does not achieve the desired effect of basifying the recycled sulfide solution.

[0020] Therefore, the expression "cation-containing aqueous solution" refers to an aqueous solution containing at least the minimum concentrations of the above cations capable of forming soluble sulfides to ensure appropriate operating conditions. Pure water or pure acid without these cations would not have the same effect. The minimum concentration of cations capable of forming soluble sulfides depends on the pH and sulfate concentration of the wastewater, the reduction rate achieved in the bioreactor, and the amount of CO2 added to the system as a carbon source. To experimentally determine the amount of cations required, the wastewater can be tested in a laboratory-scale version of a treatment plant. A concentration of 20 mmol / L or higher is preferred.

[0021] One advantage of the first loop (L1) mentioned above is that, in principle, wastewater (1) can be added anywhere in the loop. Similarly, bleeding (11') can be extracted anywhere in the loop, making the system very flexible. This works because a relatively high recirculation rate, typically greater than 10, is used, which results in an essentially constant composition throughout the loop (L1). In a continuous process, the sulfide solution in the first loop (L1) also contains a limited concentration of sulfate, which originates from the wastewater (1). To avoid solution accumulation in the first loop, the effluent (11') should correspond to the inflow of wastewater (1).

[0022] A second embodiment relates to a method according to the previous embodiment, the method comprising: - circulating the cation-containing aqueous solution (21, 22) through a second loop (L2) passing through the central compartment (B2); - adding the effluent (11') from the first loop to a second loop; - draining a small portion (21') of the solution from the second loop (L2) at a flow rate corresponding to the flow rate at which the wastewater (1) is fed; Further includes:

[0023] For the same reasons as for the first loop (L1) described above, the inflow and outflow of the second loop (L2) can be anywhere within the loop. To avoid solution accumulation in the second loop (L2), the outflow (21') should correspond to the inflow of the outflow (11') and therefore the inflow of the wastewater (1). The first loop (L1), fed from the wastewater (1), is assumed to contain soluble sulfides with cations. The second loop (L2), fed via the outflow (11'), contains the same cations and can therefore be described as a "cation-containing aqueous solution."

[0024] In the absence of the optional stripping reactor (C) and water removal reactor (D), the recirculation loop (L2) is closed by connecting stream (22) directly to stream (21).

[0025] A third embodiment according to any of the previous embodiments relates to a process comprising inserting a stripping reactor (C) into the second loop (L2) for recovering H2S.

[0026] Since the aqueous solution of the second loop (L2) contains sulfides as H2S, it may be appropriate to strip this solution, thereby obtaining a sulfide-depleted effluent (21') and H2S. In this case, acidification in the central compartment (B2) of the electrochemical cell is beneficial to facilitate the stripping process. Apart from acidification, the stripping process may also benefit from high temperatures. A pH below 6 and a temperature above 40°C are preferred.

[0027] Another embodiment according to any of the preceding embodiments relates to a process comprising inserting a water removal reactor (D) into the second loop (L2).

[0028] The advantage of the water removal step is that it allows for an increase in the salinity of the second loop. By combining the proposed process with membrane processes such as ultrafiltration, nanofiltration, or reverse osmosis, the concentration of cations can be increased. The brine or concentrate from this process contains a high concentration of cations. This allows for the treatment of wastewater streams that are low in salts of cations that can form soluble sulfides.

[0029] With the inclusion of the water removal reactor (D), the low salinity stream (22') extracts most of the water entering the system along with a small portion of the salts, while the high salinity effluent (21') extracts most of the salts and a small portion of the water entering the system. Both streams (21') and (22') should correspond to the salts and water contained in the wastewater (1) inflow. By observing the water and salt mass balance, it is possible to avoid the accumulation of either water or salt in the second loop (L2).

[0030] Again, both the stripping reactor (C) and the water removal reactor (D) can be placed anywhere in the second loop (L2), which makes the system very flexible.

[0031] Alternatively, the water removal reactor (D) can be placed outside the second loop (L2) along the effluent (21'), in which case the effluent streams will contain separate solutions, one with a high salt content and one with a low salt content.

[0032] Another embodiment according to any of the previous embodiments relates to a method comprising injecting H2 produced in the cathode compartment (B1) into the first loop (L1), preferably by injecting H2 directly into the bioreactor (A).

[0033] Wastewater often contains elements that can form insoluble sulfides, such as elements from a list including Cu, Ni, Fe, As, and Hg, and it may be useful to pre-purify such wastewater by contacting it with sulfides, particularly H2S.

[0034] Therefore, another embodiment is a method for the treatment of wastewater (1') containing sulfate salts of cations capable of forming soluble and insoluble sulfides, comprising: - providing a sulfide precipitation reactor (E); - feeding the wastewater (1') into a sulfide precipitation reactor (E); - feeding H2S to a sulfide precipitation reactor, thereby precipitating insoluble sulfides; - separating the insoluble sulfides from the wastewater, thereby obtaining a wastewater (1); - treating said wastewater (1) according to any of the previous embodiments; The present invention relates to a method, comprising:

[0035] It is even more advantageous if all or part of this H2S comes from the stripping process in the stripping reactor (C) and is recycled and efficiently used in the precipitation reactor (E).

[0036] Therefore, another embodiment is a method for the treatment of wastewater (1') containing sulfates of cations capable of forming soluble and insoluble sulfides, provided that a stripping reactor (C) is provided: - providing a sulfide precipitation reactor (E); - feeding the wastewater (1') into a sulfide precipitation reactor (E); - feeding the H2S recovered from the stripping reactor (C) to a sulfide precipitation reactor, thereby precipitating insoluble sulfides; - separating the insoluble sulfides from the wastewater, thereby obtaining a wastewater (1); - treating said wastewater (1) according to any one of the previous embodiments; The present invention relates to a method, comprising:

[0037] In another embodiment, excess HS gas (i.e., HS that has not participated in the precipitation of insoluble sulfides) is sent back from the precipitation reactor (E) to the stripping reactor (C), optionally closing the gas loop. By closing the gas loop, the HS is optimally used to precipitate metals in the precipitation reactor (E). Advantageously, this also reduces the amount of HS that needs to be post-treated when it leaves the system at the end of the process.

[0038] Although it is possible to precipitate cations capable of forming insoluble sulfides in the bioreactor itself, the preferred option is to carry out this precipitation in a separate sulfide precipitation reactor (E), which avoids contamination of the bioreactor, which is likely to be beneficial for the bacteria, and reduces the need for cleaning and maintenance of the bioreactor.

[0039] Instead of stripping the sulfide as HS, the excess sulfide can be recovered by oxidizing it to elemental sulfur or precipitating it by adding iron chloride. The resulting elemental sulfur or sulfur-containing compounds, such as iron sulfide, can also be recovered and valorized.

[0040] Bioreactors require both a carbon source and an electron donor. CO2 and H2 are typical examples that fulfill these roles. These can be replaced with known alternative products.

[0041] The pH of a bioreactor (A) needs to be controlled. The pH is measured and compared to a setpoint, and this error is used to adjust the current passing through the electrochemical cell (B). A pH that is too low can be corrected by increasing the current, and vice versa.

[0042] pH control by adjusting the current through the electrochemical cell (B) is a key feature of the present invention. Influent wastewater is typically very acidic, and the bioreactor (A) operates at around pH 8. Therefore, continuous basification is necessary.

[0043] The wastewater releases protons during basification according to the following: HSO4 - →SO4 2- +H + (pKa=1.99) CO2 added to grow bacteria also releases protons according to the following: CO2+H2O→HCO3 - +H + (pKa=6.3) Sulfate reduction consumes protons according to the following: 4H2+SO4 2- +H + →HS - +4H2O

[0044] Typically, more protons are released than are consumed. Therefore, the solution must be vigorously basified to reach an operating pH of approximately 8 in the bioreactor (A). This is achieved by the electrochemical cell (B) according to the following reaction: 2H2O-4e - →O2+4H + (anode) H2O+2H + +2e - →H2+2OH - (cathode)

[0045] A current flowing through the electrochemical cell (B) causes protons to move across the membrane from the anode compartment (B3) to the central compartment (B2). + Soluble cations such as HCl migrate across the membrane from the central compartment (B2) to the cathode compartment (B1). The cathode becomes enriched in soluble hydroxides, which are available for basifying the recirculating bioreactor loop (L1). No external reagents are required.

[0046] The following examples further illustrate the present invention. [Example]

[0047] A synthetic wastewater medium is prepared by adding 85 mL of 98% H2SO4 (Sigma-Aldrich), 183 mg of KCl (>99.5%, Carl-Roth), 74 mg of NHCl (>99.7%, Carl-Roth), 33 mg of MgCl2.6H2O (>99%, Carl-Roth), and 37 mg of CaCl2.2H2O (>99%, Carl-Roth) to 10 L of demineralized water to reach the composition according to Table 1. Phosphate is added to the synthetic wastewater medium by diluting 0.21 mL of H3PO4 (85%, Sigma-Aldrich) in 10 L of wastewater. Trace elements and vitamins used in the methanogenic medium were prepared and added according to DSMZ 141 [DSMZ GmbH, DSMZ 141 Methanegenium Medium (H2 / CO2), (2017)]. Before use, the wastewater medium was stored at 4 °C and sparged with N2 for 30 min. A pH of 0.92 was reached by adding NaOH pellets (VWR).

[0048] The bioreactor is inoculated with a mixture of sulfate-reducing bacteria (Desulfovibrio spp., Desulfomicrobium spp.) and fermentative bacteria (Soehngenia spp., Lentimicrobium spp.). This inoculum can be obtained from microbial strain suppliers such as the American Type Culture Collection (ATCC), the Leibniz Institute DSMZ, or the Belgian coordinated collections of micro-organisms (BCCM).

[0049] The exact ratio of sulfate-reducing bacteria to fermenting bacteria is not critical and a wide range of tolerances is possible depending on the system. For example, if the electron donor is changed from H to molasses, a higher ratio of sulfate-reducing bacteria to fermenting bacteria may be preferred, e.g., 50:50. In this example, a ratio of 80:20 sulfate-reducing bacteria to fermenting bacteria is used.

[0050] This setup follows the scheme shown in Figure 1, but without the optional sulfide stripping reactor (C), dewatering unit (D), and precipitation reactor (E). Wastewater is pumped through a Watson-Marlow 300 pump and treated in the sulfate-reducing bioreactor (A) at a wastewater flow rate of 365 mL / day. A gas bag filled with N2 is connected to a 10 L wastewater bottle to maintain anaerobic conditions. An upflow expanded-bed bioreactor is used for hydrogenotrophic sulfate reduction. The bioreactor consists of a glass column with a diameter of 50 mm and a height of 60 cm. The bioreactor is packed with 53 g (dry weight) or 250 mL of granular activated carbon (CalgonCarbon Carbsorb™ 30) as a biocarrier. An upflow of 280 mL / h is provided, which expands the bed volume to 250 mL. The bioreactor has sampling ports for sampling the activated carbon, the gas phase, and the effluent.

[0051] Hydrogen gas is supplied to the bioreactor as the electron donor, generated via electrolysis using a separate electrochemical cell or directly from a gas cylinder. pH is measured using a sulfide-resistant probe (HA405-DXK-S8 / 120, Mettler-Toledo) and controlled using a PID loop acting on the current of the electrochemical cell.

[0052] Maintain a stable pH of 8 ± 0.15 in the reactor using a three-compartment electrochemical cell (B). 2A titanium-based iridium mixed metal oxide (Ti) electrode (Ir MMO) (Magneto Special Anodes) was used as the cathode and an iridium mixed metal oxide (Ti) electrode (Ir MMO) (Magneto Special Anodes) was used as the anode. The cathode and anode compartments were separated by a cation exchange membrane (CEM, Ultrex CMI-7000, Membrane International). A power supply (TDK-Lambda Z-20-30) was used to power the electrochemical cell (B), which was controlled by a PID loop. The pH of the bioreactor (A), the applied potential, and the current were recorded every 4 seconds. The bioreactor liquid, corresponding to the sulfide solution (12) from the first loop (L1), was recirculated to the cathode compartment (B1). If the pH of the bioreactor (A) fell below pH 7.85, the PID loop increased the applied current, and if the pH reached 8.15, the current decreased. The reactor was kept at room temperature between 15 and 25 °C.

[0053] The bioreactor effluent, corresponding to the effluent (11') from the first loop (L1), is sulfide-rich and sulfate-depleted. It is collected in a 1 L multi-port Schott bottle. The bioreactor effluent is recycled from the 1 L bottle to the central compartment (B2) of the electrochemical cell (B). Cations are extracted from this central compartment (B2) via the cathode compartment (B1) and returned to the bioreactor (A). These cations are replaced by protons produced in the anode compartment (B3). This acidified sulfate-reduced water optionally assists in the degassing of sulfides in the stripping reactor (C). The anolyte contains 0.25 M H2SO4 (B3). Oxygen produced at the anode is vented to the atmosphere. After acidification and cation extraction, the effluent is collected in a 10 L plastic container.

[0054] Table 1 shows the composition, pH, and conductivity of the treated water at various stages of treatment. Sulfate was 15.6 g / L SO4 in bioreactor (A). 2- to 0.2g / L SO4 2-The sulfate reduction rate is higher than that achieved with current electrobiochemical systems and conventional sulfate-reducing bioreactors already applied at industrial scale. The reduced sulfate is converted to approximately 3 g / L of dissolved sulfide (DS). The sulfur mass balance does not close completely, which can be explained by the instability of sulfides and the difficulty of measuring them, since these compounds are easily oxidized when in contact with air or other oxidizing agents. The extraction of cations reduces the conductivity from 9 mS / cm to 4 mS / cm. The conductivity of the feed stream (1) is lower due to the low pH and H + The conductivity is high (114 mS / cm).

[0055] [Table 1]

[0056] The operating principle of this technology can be seen in Table 1 and Figure 2. The concentrations of Na, NH4, K, and Mg (i.e., essentially the total cation concentration) are elevated in bioreactor (A) compared to the concentrations in the feed stream (1) or effluent (21'). This demonstrates that electrochemical cell (B) is effective in extracting cations from the electrochemical cell's central compartment (B2) to the cathode compartment (B1) and then to bioreactor (A) for in situ generation of alkali, e.g., NaOH, KOH, NH4(OH), Ca(OH)2, and Mg(OH)2, required for neutralization of acidic wastewater and CO2. As a result, the pH increases from 0.92 to 8.01 and remains stable throughout the experiment (see Figure 2). The cations extracted from the electrochemical cell's central compartment (B2) are replaced by protons from the anode compartment (B3), resulting in a decrease in pH from 8.01 to 2.23. In Figure 2, it can be seen that the pH in the central compartment (B2) and the applied current changed during the experiment. This can be explained by a slight change in sulfate reduction. As more sulfate is reduced, more protons are consumed and more sulfide is produced. As a result, less additional alkali is needed, and less current is required. Because less current is flowing, H + Furthermore, the higher the sulfide concentration in the bioreactor effluent (11'), the more cations are displaced by HS. - By protonation of H + is consumed, the decrease in pH in the central compartment (B2) is also offset. [Example]

[0057] In this example, the setup was expanded using optional sulfide stripping reactors (C) and precipitation reactors (E) according to Figure 1. A 1 L multi-port Schott bottle was converted into a sulfide stripping reactor by injecting N2 at a rate of 2 L / min using an ATEX KNF N922FTE 16 L gas pump. After stripping the H2S, the gas was injected into a 10 L evacuated glass vessel containing the wastewater medium from Example 1, which was contaminated with 468 mg NaAsO2, 70 mg PbCl2, 41 mg CdCl2, 24 mg CuCl2, and 2 mg TlCl2, resulting in a wastewater medium containing As (272 mg / L), Pb (5.2 mg / L), Cd (2.502 mg / L), Cu (1.14 mg / L), and Tl (0.152 mg / L). The wastewater medium is continuously mixed using a magnetic stirrer at 600 rpm.

[0058] After injection, the gas is recycled from the precipitation reactor (E) to the stripping reactor (C) to close the gas loop.

[0059] All metals and metalloids precipitate and are removed to concentrations less than 0.2 mg / L. Thus, the metals with the highest initial concentrations (metalloids) are removed from 272 mg / L to less than 0.05 mg / L. Cd is removed to a concentration of 0.172 mg / L. All other metals and metalloids are below the detection limit: Cu < 0.004 mg / L, Pb < 0.03 mg / L, and Tl < 0.02 mg / L. This demonstrates the effectiveness of using sulfides produced in bioreactor (A) and stripped in stripping reactor (C) to remove metals and metalloids as sulfide precipitates in precipitation reactor (E). This is most advantageous for elements such as Cu, which may be toxic to microorganisms and interfere with the function of bioreactor (A), and As, which is soluble at the pH of bioreactor (A) and may cause undesirable electrochemical reactions in electrochemical cell (B). [Explanation of symbols]

[0060] 1. Streams, wastewater containing sulfate salts of cations capable of forming soluble sulfides, feed streams 1' Stream, wastewater containing cationic sulfates capable of forming soluble and insoluble sulfides 11 Flow, sulfide solution 11' Bleeding stream, flow, effluent, small portion of solution 12 Flow, sulfide solution 21 Stream, process effluent, aqueous solution containing cations 22 Stream, process effluent, aqueous solution containing cations 21' effluent stream, process effluent stream, small portion of solution, effluent, high salinity effluent, stream 22' flow, low salinity flow 31 Flow, acidic aqueous solution A. Bioreactor B. Electrochemical cell B1 Cathode, Cathode Compartment B2 Central Section B3 Anode, anode compartment C. Stripping reactor D Water removal reactor, dehydration unit E. Sulfide Precipitation Reactor

Claims

1. 1. A process for the treatment of wastewater (1) containing sulfate salts of cations capable of forming soluble sulfides, comprising: providing a bioreactor (A) suitable for reducing sulfate to sulfide; - an electrochemical cell (B) through which current flows, having a central compartment (B2) connected by a cation exchange membrane to an adjacent cathode compartment (B1) and an adjacent anode compartment (B3), whereby H ２ and O in the anode compartment. ２ providing an electrochemical cell (B) that produces - flowing an aqueous solution containing cations (21) through the central compartment (B2); - supplying the anode compartment (B3) with an aqueous acidic solution (31); - circulating the sulfide solution (11, 12) in a first loop (L1) through the bioreactor (A) and the cathode compartment (B1), thereby maintaining the pH of the solution between 6 and 9 by adjusting the current through the electrochemical cell (B); - adding wastewater (1) to the first loop (L1) at a set flow rate; - draining a small portion (11') of the solution from the first loop (L1) at a flow rate corresponding to the flow rate at which the wastewater (1) is fed; A method comprising:

2. - circulating the aqueous cation-containing solution (21, 22) through a second loop (L2) passing through the central compartment (B2); - adding the effluent (11') from the first loop (L1) to the second loop (L2); - draining a small portion (21') of the solution from the second loop (L2) at a flow rate corresponding to the flow rate at which the wastewater (1) is fed; The method of claim 1 further comprising:

3. - H ２ inserting a stripping reactor (C) into the second loop (L2) to recover S; The method of claim 2 , comprising:

4. - inserting a water removal reactor (D) into the second loop (L2); The method of claim 2 , comprising:

5. - H produced in the cathode compartment (B1) ２ into the first loop (L1). The method of claim 1 , comprising:

6. 1. A method for the treatment of wastewater (1′) containing sulfates of cations capable of forming soluble and insoluble sulfides, comprising: - providing a sulfide precipitation reactor (E), - feeding the wastewater (1') into a sulfide precipitation reactor (E), - H ２ feeding S to a sulfide precipitation reactor (E) thereby precipitating insoluble sulfides; - separating the insoluble sulfides from the wastewater, thereby obtaining the wastewater (1); - treating said wastewater (1) according to claim 1; A method comprising:

7. 1. A method for treating wastewater (1') containing sulfates of cations capable of forming soluble and insoluble sulfides, provided that a stripping reactor (C) according to claim 3 is provided, - providing a sulfide precipitation reactor (E), - feeding the wastewater (1') into a sulfide precipitation reactor (E), - H recovered from the stripping reactor (C) ２ feeding S to a sulfide precipitation reactor, thereby precipitating insoluble sulfides; - separating the insoluble sulfides from the wastewater, thereby obtaining the wastewater (1); - treating said wastewater (1) according to claim 3; A method comprising:

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