Environmental-friendly Process for the Treatment of Wastewater

Abstract

Provided is a process for the environmental-friendly treatment of sulfate-containing wastewater. The acidic, sulfate-containing wastewater is treated in a sulfate reducing bioreactor with influent and effluent looped through to the cathode compartment of an electrochemical cell. The electrochemical cell stabilizes the pH in the bioreactor by the in-situ production of base in the cathode compartment. Additionally, hydrogen is produced which is used in the bioreactor as electron donor for the sulfate reduction. The middle compartment of the electrochemical cell contains a sulfide rich aqueous solution in which the extracted cations are displaced by protons from the anode compartment. This results in the acidification of the sulfide rich solution, which is beneficial for the extraction of sulfides as H.sub.2S. This H.sub.2S can be used for the precipitation of metals in the beginning of the process, forming another loop.

Claims

1-7. (canceled)

8. Process for the treatment of wastewater containing sulfate salts of cations susceptible of forming soluble sulfides, comprising the steps of: providing a bioreactor suitable for the reduction of sulfates to sulfides; providing an electrochemical cell through which a current is passed, having a central compartment connected by cation exchange membranes to an adjacent cathode compartment, and to an adjacent anode compartment, thereby generating H.sub.2 in the cathode compartment, and O.sub.2 in the anode compartment; passing a cation-bearing aqueous solution through the central compartment; providing an acidic aqueous solution in the anode compartment; circulating a sulfidic solution in a first loop over a path passing through the bioreactor and the cathode compartment, whereby the pH of the solution is maintained between 6 and 9 by adjusting the current through the electrochemical cell; adding the wastewater to the first loop at a set flow rate; and, bleeding a minor part of the solution from the first loop at a rate corresponding to the flow rate of feeding the wastewater.

9. Process according to claim 8, comprising the additional steps of: circulating a cation-bearing aqueous solution in a second loop over a path passing through the central compartment; adding the bleed from the first loop to the second loop; and, bleeding a minor part of the solution from the second loop at a rate corresponding to the flow rate of feeding the wastewater.

10. Process according to claim 8, comprising the step of inserting a stripping reactor in the second loop, for the recovery of H.sub.2S.

11. Process according to claim 8, comprising the step of inserting a water removal reactor in the second loop.

12. Process according to claim 8, comprising the step of sparging the H.sub.2 generated in the cathode compartment into the first loop.

13. Process for the treatment of wastewater containing sulfate salts of cations susceptible of forming soluble and insoluble sulfides, comprising the steps of: providing a sulfide precipitation reactor; feeding the wastewater to the sulfide precipitation reactor; feeding H.sub.2S to the sulfide precipitation reactor, thereby precipitating the insoluble sulfides; separating the insoluble sulfides from the wastewater, thereby obtaining wastewater; and, processing said wastewater according to claim 8.

14. Process for the treatment of wastewater containing sulfate salts of cations susceptible of forming soluble and insoluble sulfides, with the proviso that stripping reactor is provided according to claim 10, comprising the steps of: providing a sulfide precipitation reactor; feeding the wastewater to the sulfide precipitation reactor; feeding H.sub.2S recovered from stripping reactor to the sulfide precipitation reactor, thereby precipitating the insoluble sulfides; separating the insoluble sulfides from the wastewater, thereby obtaining wastewater; and, processing said wastewater according to claim 10.

Description

EXAMPLE 1

[0080] A synthetic wastewater medium is prepared by adding 85 mL of 98% H.sub.2SO.sub.4 (Sigma-Aldrich), 183 mg KCl (>99.5%, Carl-Roth), 74 mg NH.sub.4Cl (>99.7%, Carl-Roth), 33 mg MgCl.sub.2.Math.6H.sub.2O (>99%, Carl-Roth) and 37 mg CaCl.sub.2.Math.2H.sub.2O (>99%, Carl-Roth) to 10 L demineralized water, so as to reach the composition according to Table 1. Phosphate is added to the synthetic wastewater medium diluting 0.21 mL of H.sub.3PO.sub.4 (85%, Sigma-Aldrich) in 10 L of wastewater. Trace elements and vitamins are prepared and added according to DSMZ 141, as used for methanogenic media [DSMZ GmbH, DSMZ 141 Methanogenium medium (H.sub.2/CO.sub.2), (2017)]. Prior to use, the wastewater medium was stored at 4? C. and sparged 30 minutes with N.sub.2. The pH of 0.92 is reached by addition of NaOH pellets (VWR).

[0081] The bioreactor is inoculated with a mixture of sulfate reducers (Desulfovibrio spp., Desulfomicrobium spp.), and fermenters (Soehngenia spp., Lentimicrobium spp.). This inoculum can be obtained from suppliers of microbial strains such as the American Type Culture Collection (ATCC), the Leibniz Institute DSMZ or the Belgian coordinated collections of micro-organisms (BCCM).

[0082] The exact ratio of sulfate reducers to fermenters is not critical, and allows for a wide margin, depending on the system. For example, if the electron donor is changed from H.sub.2 to molasses, a higher sulfate reducers to fermenters ratio may be preferred, such as 50:50. In the present example, a sulfate reducers to fermenters ratio of 80:20 is used.

[0083] The set-up is construed according to the scheme shown in FIG. 1, without the optional sulfide stripping reactor (C), dewatering unit (D) and precipitation reactor (E). The wastewater is pumped and treated in a sulfate reducing bioreactor (A) at a wastewater flow rate of 365 mL/day via a Watson-Marlow 300 pump. Gas bags filled with N.sub.2 are connected to 10 L wastewater bottles to maintain anaerobic conditions. An up-flow expanded bed bioreactor is used for hydrogenotrophic sulfate reduction. The bioreactor consists of a glass column of 50 mm diameter and 60 cm height. The bioreactor is filled with 53 g (dry weight) or 250 mL of granular activated carbon (CalgonCarbon Carbsorb? 30) as biocarrier. An up-flow of 280 mL/h is provided, resulting in an expanded bed volume of 250 mL. Sampling ports are present in the bioreactor for sampling of the activated carbon, gas phase and effluent.

[0084] Hydrogen gas is provided to the bioreactor as electron donor, generated via electrolysis using a separate electrochemical cell or directly from a gas cylinder. The pH is measured with a sulfide resistant probe (HA405-DXK-S8/120, Mettler-Toledo) and is controlled using a PID loop acting on the current of the electrochemical cell.

[0085] A three-compartment electrochemical cell (B) is used to maintain a stable reactor pH of 8?0.15. A stainless-steel mesh (100 cm.sup.2) is used as a cathode while an iridium mixed-metal oxide titanium-based (Ti) electrode (Ir MMO) (Magneto Special Anodes) is used as an anode. Cathode and anode compartment are separated by cation exchange membranes (CEM, Membrane International Ultrex CMI-7000). A power source (TDK-Lambda Z-20-30) is used to power the electrochemical cell (B), controlled by the PID loop. The pH of the bioreactor (A), applied potential and current are registered every 4 s. Bioreactor liquid, corresponding to sulfidic solution (12) from the first loop (L1), is recirculated over the cathode compartment (B1). When the pH of the bioreactor (A) drops below pH 7.85, the PID loop increases the applied current, while the current is lowered when a pH of 8.15 was reached. The reactor is kept at room temperature, between 15 and 25? C.

[0086] The bioreactor effluent, corresponding to bleed (11) from the first loop (L1), is rich in sulfides and depleted in sulfate. It is collected in a 1 L multi-neck Schott bottle. The bioreactor effluent is recirculated from the 1 L bottle over the central compartment (B2) of the electrochemical cell (B). Cations are extracted from this central compartment (B2) via the cathode compartment (B1) back into the bioreactor (A). These cations are replaced by protons produced in the anode compartment (B3). This acidified sulfate-depleted water, supports degassing of sulfides, optionally in a stripping reactor (C). The anolyte contains 0.25 M H.sub.2SO.sub.4 (B3). Oxygen produced at the anode is vented into the atmosphere. After acidification and cation extraction, the effluent is collected in a plastic 10 L vessel.

[0087] Table 1 shows the composition, pH and conductivity of the treated water at different stages of the treatment. Sulfate is removed in the bioreactor (A) from 15.6 g/L SO.sub.4.sup.2? to 0.2 g/L SO.sub.4.sup.2?, corresponding to a sulfate reduction rate of 22.5 g sulfate per liter per day. This reduction rate is higher than those achieved by current electro-biochemical systems and conventional sulfate reducing bioreactors already applied at industrial scale. The reduced sulfate is converted into ca. 3 g/L of dissolved sulfides (DS). The mass balance of sulfur does not fully close, which may be explained by the instability of sulfides and difficulty to measure sulfides, as these compounds are easily oxidized when in contact with air or other oxidants. Due to the extraction of cations, the conductivity decreases from 9 to 4 mS/cm. The conductivity of the feed (1) is high (114 mS/cm) due to the low pH and conductivity of H.sup.+

TABLE-US-00001 TABLE 1 Process stream characteristics Conductivity Cl.sup.? PO.sub.4.sup.3? SO.sub.4.sup.2? Dissolved sulfides pH (mS/cm) (mg/L) (mg/L) (mg/L) (mg/L) Stream (1) 0.92 114 526 481 15518 0 Wastewater Loop 1 Bioreactor 8.01 9 405 305 244 3040 Stream (21) Output 2.23 4 422 335 194 1200 Na.sup.+ NH.sub.4.sup.+ K.sup.+ Ca.sup.2+ Mg.sup.2+ Total cations* (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mmol/L) Stream (1) 164 25 96 10 4 12 Wastewater Loop 1 Bioreactor 1623 251 909 66 36 114 Stream (21) Output 288 46 139 6 4 19 *Total cations = [Na.sup.+] + [K.sup.+] + [NH.sub.4.sup.+] + 2 ? [Mg.sup.2+] + 2 ? [Ca.sup.2+]

[0088] The working principle of the technology can be observed in Table 1 and FIG. 2. Na, NH.sub.4, K, and Mg concentrations (i.e. essentially the total cation concentration) are elevated in the bioreactor (A) compared to the concentrations in the feed (1) or in the output (21). This shows that the electrochemical cell (B) is effective in extracting cations from the central compartment (B2) of the electrochemical cell to the cathode compartment (B1) and further to the bioreactor (A) for the in-situ production of alkali such as NaOH, KOH, NH.sub.4(OH), Ca(OH).sub.2 and Mg(OH).sub.2, which are required for neutralization of acidic wastewater and CO.sub.2. This results in an increase of the pH from 0.92 to 8.01, which is maintained in a stable manner throughout the experiment (see FIG. 2). The cations, extracted from the central compartment (B2) of the electrochemical cell (B), are replaced by protons from the anode compartment (B3), resulting in a decrease in pH from 8.01 to 2.23. It can be seen in FIG. 2 that the pH of central compartment (B2) and applied current varied over the course of the experiment. This can be explained by slight variations in sulfate reduction. When more sulfate is reduced, more protons are consumed, and more sulfides are produced. As a result, the need for additional alkali decreases and less current is required. Due to the lower current, less cations are replaced by H.sup.+ in the central compartment (B2). Additionally, higher sulfide concentrations in the bioreactor bleed (11) also counteracts the pH drop in the central compartment (B2) as H.sup.+ are consumed by the protonation of HS.sup.?.

EXAMPLE 2

[0089] In this example, the set-up is expanded with the optional sulfide stripping reactor (C) and precipitation reactor (E), according to FIG. 1. The 1 L multi-neck Schott bottle is converted into a sulfide stripping reactor by sparging N.sub.2 at a rate of 2 L/min using an ATEX KNF N922FTE 16L gas pump. After H.sub.2S stripping, the gas is sparged into a de-aerated glass 10 L vessel containing the wastewater medium according to Example 1, spiked with 468 mg NaAsO.sub.2, 70 mg PbCl.sub.2, 41 mg CdCl.sub.2, 24 mg CuCl.sub.2, 2 mg TlCl.sub.2, resulting in wastewater medium with As (272 mg/L), Pb (5.2 mg/L), Cd (2.502 mg/L), Cu (1.14 mg/L) and TI (0.152 mg/L). This wastewater medium is continuously mixed with a magnetic stirrer at 600 rpm.

[0090] After sparging, the gas is recirculated from the precipitation reactor (E) back to the stripping reactor (C), closing the gas loop.

[0091] All metals and metalloids are precipitated and removed down to concentrations below 0.2 mg/L. As, the metal(loid) with the highest initial concentration, is removed from 272 mg/L down to less than 0.05 mg/L. Cd is removed down to a concentration of 0.172 mg/L. All other metals and metalloids are below the limit of detection: Cu<0.004 mg/L, Pb<0.03 mg/L and Tl<0.02 mg/L. This shows the efficacy of using sulfides produced in the bioreactor (A) and stripped in the striping reactor (C) for the removal of metals and metalloids as sulfide precipitates in the precipitation reactor (E). This is most advantageous for elements such as Cu, which are toxic for the microorganisms and may hamper the functioning of the bioreactor (A), and elements such as As, which are soluble at the pH of the bioreactor (A) and may result in undesirable electrochemical reactions in the electrochemical cell (B).