Wastewater treatment by way of electrocoagulation

Abstract

The present invention relates to devices (10) and methods for wastewater treatment by way of electrocoagulation. The devices (10) comprise spirally wound electrode sheets (15) for the cathode and the anode and at least one stirrer (16) below the electrodes (15).

Description

[0001] Wastewater treatment by electrocoagulation

[0002] Technical area

[0003] The present invention relates to devices and methods for wastewater treatment by electrocoagulation.

[0004] Description of the state of the art

[0005] Electrocoagulation (EC) is an electrochemical technology for treating water and wastewater by in-situ generation of metal coagulants through the electrolytic dissolution of anode materials, triggered by an electric current applied across the electrodes. The metal ions formed from the anode, preferably Fe or Al ions, form insoluble compounds in the electrolyte with the OH ions formed at the cathode in aqueous solutions, which then act as coagulants. In contrast to chemical coagulation through the addition of soluble Fe or Al salts, electrocoagulation does not produce wastewater with an elevated salt content.

[0006] EC is successfully used to remove contaminants from aqueous solutions. Typical contaminants include organic substances, sulfur and phenols, dyes, other suspended solids, emulsified oil, and heavy metals.

[0007] A conventional electrocoagulation apparatus is described in US Pat. No. 8,133,382 B2. The device comprises a plurality of spaced-apart rectangular reaction plates arranged within a reaction chamber. A voltage is applied to selected reaction plates to generate an electric field within the electrocoagulation chamber. The plates are arranged vertically, thereby inducing a vertical fluid flow through a device.

[0008] WO 2009 / 009465 A1 discloses an electrocoagulation reactor and water purification systems and methods using the reactor. The electrocoagulation reactor has a spirally wound assembly in which the electrocoagulation treatment takes place. The spirally wound assembly comprises spaced-apart electrode sheets spirally wound, with a region for fluid flow in the space between the electrode sheets.

[0009] WO 2021 / 034580 A2 relates to iron electrocoagulation reactors (Fe-EC) for removing contaminants from water, consisting of an array of folded or spirally wound iron-containing anode and cathode plates separated by perforated insulating spacers. An oxidizing agent can be added to the contaminated water to accelerate the oxidation of Fe(II) ions released from the anode and / or to oxidize the contaminant.

[0010] In both reactors, the fluid to be treated flows axially through the spirals formed by the electrodes, making it difficult to ensure uniform flow through the channels. Both reactors are only suitable for the treatment of lightly contaminated fluids that are not prone to foaming and exhibit only very low solids precipitation.

[0011] WO 2004 / 089832 A1 discloses an electrocoagulation reactor in which two essentially parallel plates are arranged in a spiral coaxial manner, so that fluid entering the innermost cylindrical region follows a spiral path outward and then, upon reaching an outermost cylindrical region, is guided along a spiral path between the two plates back to the innermost cylindrical region. The spiral flow channels are sealed at the top and bottom by the electrodes, so this reactor is also unsuitable for the treatment of heavily contaminated wastewater, which tends to foam and form large amounts of solid particles.

[0012] Object of the invention

[0013] Against this background, the object of the invention is to provide a device and a method for electrocoagulation even for heavily contaminated fluids / wastewater, which have a high degree of separation and low energy consumption and ensure good and rapid mixing of the produced coagulants with the contaminants.

[0014] Short description of the figures

[0015] Figure 1 shows two perspective views of an embodiment of the device according to the invention;

[0016] Figure 2 shows the voltage curve over time in a device according to the invention and an electrocoagulation reactor of the prior art;

[0017] Figure 3 shows a comparison of foam development in a device according to the invention and an electrocoagulation reactor of the prior art;

[0018] Figure 4 shows the voltage curve over time in a device according to the invention at different stirring speeds;

[0019] Figure 5 shows a comparison of the sedimentation of flocs produced in a device according to the invention and in an electrocoagulation reactor of the prior art after 15 minutes of standing time;

[0020] Figure 6 shows a plan view of another embodiment of the device according to the invention;

[0021] Figure 7 shows a side view of the device of Figure 6;

[0022] Figure 8 shows a plan view of another embodiment of the device according to the invention; Figure 9 shows a plan view of an embodiment of the device according to the invention with multiple electrodes.

[0023] Summary

[0024] An electrocoagulation device is provided with spirally wound electrodes arranged in a preferably cylindrical central chamber, with the longitudinal axis of the central chamber forming the center axis of the spirals. The electrodes comprise spirally wound electrode sheets for the cathode and anode, which are arranged at a distance from one another. The fluid to be treated flows radially through the spiral channels, ensuring uniform flow even with multiple electrodes. The channels are open in the axial direction to reliably prevent blockage of the channels by foam and solid flakes, even with heavily contaminated fluids.

[0025] Furthermore, a method for electrocoagulation using the device according to the invention is provided. The spaces between the electrode sheets are filled with the liquid to be treated. By applying a direct voltage to the electrodes, coagulants are electrochemically generated while the liquid flows radially in the channels of the device according to the invention. The generated coagulants cause the impurities contained in the liquid to flocculate, which can then be easily separated.

[0026] Detailed description

[0027] The invention relates to an electrocoagulation device comprising spirally wound electrode sheets for the cathode(s) and anode(s), which are arranged at a distance from one another in a preferably cylindrical central space, wherein the longitudinal axis of the central space forms the center axis of the spirals. According to the invention, the channels formed between the electrode sheets are open in the axial direction to ensure safe operation even with wastewater. Blockage of the channels by foam and the formation of solid flakes is thus reliably prevented, even with heavily contaminated fluids, since the density differences cause these to escape upwards or downwards from the spiral channels. The fluid to be treated flows radially through the spiral channels, ensuring uniform flow even when multiple electrodes are used.

[0028] The cylindrical central chamber is bounded at the bottom by a base spaced from the spirals to ensure the downward removal of solids formed from the spiral channels. In one embodiment of the device, the cylindrical central chamber has at least one liquid inlet. In another embodiment, the liquid inlet is arranged near the lower end of the cylinder. In one embodiment, the cylindrical central chamber has at least one liquid outlet. In another embodiment, the liquid outlet is arranged near the upper end of the cylinder.

[0029] In one embodiment, the upper end of the cylindrical central chamber is open, allowing liquid to escape over the upper edge of the cylindrical central chamber. In another embodiment, the cylindrical central chamber is limited at the top by a lid, which is then also spaced apart from the spirals to ensure the removal of foams formed from the spiral channels.

[0030] In one embodiment, the wall of the cylindrical central chamber is made of plastic. Since the liquid to be treated typically contains corrosive chemicals and gases, the container material should be corrosion-resistant. Examples of suitable plastics are polypropylene (PP), polyethylene (PE), or polyvinyl chloride (PVC). In another embodiment, the plastic is polycarbonate or polymethyl methacrylate ("Plexiglas").

[0031] In the device according to the invention, at least two electrodes, alternating anode and cathode, are wound in the form of at least two combined Archimedean spirals in such a way that spiral flow channels are formed between the anodes and cathodes, through which liquid flows radially during operation.

[0032] In one embodiment of the device, the radial flow is generated by at least one agitator arranged in the cylindrical central space. In one embodiment of the device, one agitator is arranged above the electrodes. In another embodiment of the device, one agitator is arranged below the electrodes. In yet another embodiment of the device, a cylindrical area is kept free in the core of the spirals, in which at least one agitator is located. A combination of agitators is also possible. If the agitator is arranged below the electrodes, the flocs formed are not damaged by the agitator. Particularly good flow through the spiral channels and good separation performance are achieved with a central agitator below the spiral body.

[0033] The presence of at least one agitator has a significant impact on the effectiveness of contaminant removal and energy consumption in the device. In a device according to the invention with an active agitator, the voltage increase during operation is significantly lower than in a device without agitator, the flocs formed are much larger and can be separated more easily, and a significantly higher proportion of contaminants is removed.

[0034] The device according to the invention is operated in batch mode. In a further development of the device according to the invention, this is intended for continuous operation and has at least one liquid inlet and at least one liquid outlet, which are arranged at a distance from one another, for example opposite one another on the jacket of the cylindrical central space or near the upper and lower ends of the cylindrical central space. In this embodiment, an agitator is optional. In one embodiment of the device, the anode contains metallic iron, aluminum, or titanium. In a further embodiment, the anode consists of metallic iron, steel, aluminum, or titanium. In a further embodiment, the anode and cathode consist of the same material. In another embodiment, the anode and cathode consist of different materials.In a further embodiment, the cathode contains graphite or metallic copper.

[0035] When selecting electrode materials, two factors should be considered: first, the availability of the metals and second, the pH of the feed. Since the anode is a consumable part that must be replaced from time to time, iron and aluminum are the most important materials due to their availability. The material of the sacrificial anode also depends on the pH and the type of contaminants. For example, if the pH is above 9, an iron electrode is preferred. If the pH is between 5 and 8, an aluminum electrode is suitable.

[0036] In one embodiment, the device has a cylindrical outer space arranged coaxially to the cylindrical central space, which has a larger diameter than the cylindrical central space and is designed to collect liquid escaping at the top of the cylindrical central space. In a further embodiment, the cylindrical outer space forms a collar around the cylindrical central space. This collar serves to collect any foam that may form. In a further embodiment, the length of the cylindrical outer space is shorter than the length of the cylindrical central space. In a further embodiment, the cylindrical outer space is arranged around the cylindrical central space such that the upper edge of the cylindrical outer space is at the same height as the upper edge of the cylindrical central space.In one embodiment, the wall of the cylindrical outer chamber is made of corrosion-resistant plastic, which allows for the treatment of acidic or alkaline solutions. In another embodiment, the plastic is polypropylene or PVC.

[0037] The invention also relates to a process for treating contaminant-containing water (feed) by electrocoagulation. In the process according to the invention, the contaminant-containing water is filled into a device according to the invention. The contaminant-containing water is continuously circulated in the device by at least one agitator. A direct voltage is applied to the electrodes, and an electric current is passed through the device, electrochemically generating metal hydroxides that cause the contaminants to flocculate. The flocculated contaminants are then separated.

[0038] In one embodiment of the process, the contaminants are fed into a settling tank after treatment with EC. Since flocs settle due to their high density, they can be disposed of without filtration. The settling time depends on the concentration of contaminants in the feed, but is typically between 10 and 30 minutes. The remainder flows to a filtration process.

[0039] Some parameters of the process according to the invention (e.g. operating time or current density) depend on the quality of the feed and are adjusted according to the concentration of impurities in the feed.

[0040] In one embodiment of the process, the pH of the feed is from pH 3 to pH 12. In one embodiment of the process, the ratio (S / V) of the electrode surface area (S) to the volume of the feed (V) is 0.5 to 0.7. In one embodiment of the process, the electrode surface area (S) is 700 to 900 cm 2In one embodiment of the process, the speed of the at least one agitator is from 100 to 800 rpm, for example from 200 to 600 rpm, in particular from 400 to 500 rpm.

[0041] In one embodiment of the process, the impurities contained in the water comprise organic substances. In another embodiment, the impurities contained in the water comprise phenols. In another embodiment, the impurities contained in the water comprise sulfur. In another embodiment, the impurities contained in the water comprise dye particles. In one embodiment of the process, the impurities contained in the water comprise suspended solids. In another embodiment of the process, the impurities contained in the water comprise emulsified oil. In another embodiment of the process, the impurities contained in the water comprise (heavy) metals or semi-metals, for example copper, zinc, chromium, cadmium or silicon.

[0042] The device and the method according to the invention are particularly suitable for use in oil refineries and the petrochemical industry, in the food industry, the metal industry and the textile industry.

[0043] When evaluating the use of electrolysis, energy consumption is the decisive operating cost factor. For energy reasons, both the efficiency of the electrolysis and the required power of the agitator are very important. These variables are significantly influenced by the geometry of the reactor. In the device according to the invention, the configurations of the electrodes and the reactor are designed so that the electrochemically generated coagulants mix very well and very quickly with the contaminants, allowing flocs to form.

[0044] Electrolysis takes place at the cathode electrode, leading to the formation of gas bubbles that, if the flow is poor, can adhere to the electrode surface and cause considerable additional resistance and voltage increase. Due to the round cross-section of the cylindrical central chamber, at least one agitator operates very effectively, achieving a uniform flow over the electrode surfaces in the spiral channels, which improves electrolysis efficiency. Due to the shape of the electrodes, the flow velocity at the surface of the electrodes is high, and the liquid is thoroughly mixed, resulting in less foam formation. For solids, emulsified oil, and (semi-)metals such as copper, chromium, lead, mercury, or silicon, a removal efficiency of more than 90 percent is achieved.

[0045] The inventive solution offers numerous advantages compared to conventional EC systems with plate electrodes: lower energy consumption due to the removal of passivation by the flow pattern; smaller space requirements due to the integration of three processes (electrocoagulation, electroflotation, and sedimentation) into one system; shorter treatment time due to the thorough mixing of coagulants and contaminants; lower filtration effort due to the large flocs formed; and lower foam formation due to the higher velocity of the flow surface. Further advantages and embodiments of the invention will become apparent from the description and the accompanying illustrations.

[0046] Examples

[0047] Figure 1 shows two perspective views of an embodiment of the device 10 according to the invention. The left figure shows a side view, the right figure a view obliquely from above.

[0048] A cylindrical central chamber 11 of the device 10 is made of a transparent plastic and is open at the upper end. Arranged around the cylindrical central chamber 11 is a cylindrical outer chamber 12, which forms a collar of the cylindrical central chamber 11 and is also made of transparent plastic. The cylindrical central chamber 11 has a liquid inlet 13 near its lower end, and the cylindrical outer chamber 12 has a liquid outlet 14 near its upper edge. Inside the cylindrical central chamber 11, spirally curved electrode blades 15 in the form of two combined Archimedean spirals are arranged such that spiral flow channels are formed between the anode and cathode. The spirals 15 are arranged at a distance from the bottom 17 of the cylindrical central chamber 11.

[0049] Using the device according to the invention and a prior art electrocoagulation device with rectangular flat plate electrodes arranged parallel in a rectangular housing, beneath which a stirrer is arranged, several operating tests of different durations were carried out, and the voltage curve over time was measured and the foam formation was observed.

[0050] The test conditions are summarized in Tables 1-3. The feed used was a halogen-free machining emulsion with an oil content of 8 wt.% and a solids content of <5 wt.%, pH 9.0, which contained 136 mg / l ammonium, <2 mg / l cadmium, 2 mg / l lead, 7 mg / l copper, 13 mg / l zinc, 6 mq / l chromium, 0.076 wt.% sulfur, 921 mq / l phosphorus, 2,182 mq / l aluminum, and 1,800 mq / l silicon. Figure 2 shows the voltage curve over time in the device according to the invention (solid line) and the prior art electrocoagulation reactor (dashed line). For the three tests conducted, the respective operating times were selected according to the impurity concentration in the feed. Higher concentrations require a longer operating time. The different output voltages are also due to the concentration differences.

[0051] Table 1

[0052] Table 2

[0053] Table 3 The two electrocoagulation devices, whose electrodes are each made of aluminum sheets with an area of ​​836 cm 2 In all experiments, the test tubes were filled with 1300 ml of liquid, and a current of 5 A was passed through the device for the respective duration. The feed concentration was adjusted by mixing appropriate volumes of water with the feed. The agitator was operated at a stirrer speed of 400 rpm to continuously circulate the liquid in the device. The conductivity of the liquid, its pH value, the liquid temperature, and the applied voltage were measured at the beginning of the experiment (start) and at the end of the experiment.

[0054] As can be seen from Figure 2, the device according to the invention shows a significantly lower voltage requirement and thus a reduced energy requirement at the same current intensity than the device of the prior art.

[0055] Figure 3 shows the foam development in the device according to the invention (left row) and the prior art electrocoagulation reactor (right row) compared after 5, 10, 15, and 20 minutes of operation. As can be seen in the images in Figure 3, a large volume of foam was generated on the liquid surface in the prior art device. This indicates that the plate electrodes act as a barrier to the proper mixing of Al ions and OH ions for the production of coagulants, as well as to the mixing of the produced coagulants with the impurities, resulting in lower separation efficiency and thus foam formation during stirring. In the device according to the invention, however, the velocity at the surface is high due to the spiral configuration, which leads to proper mixing and negligible foam formation, and thus also higher efficiency.

[0056] Experiments were conducted to determine the effect of stirring on electrocoagulation in a device according to the invention. The electrocoagulation device according to the invention, whose electrodes each consist of aluminum sheets with an area of ​​836 cm 2 passed, in all experiments, 1300 ml of liquid was filled, and a current of 5 A was passed through the device for 10 minutes. The conductivity of the liquid, its pH value, the liquid temperature, and the applied voltage were measured at the beginning of the experiment (start) and at the end of the experiment (end). The results are summarized in Table 4.

[0057] Table 4

[0058] Figure 4 shows the voltage curve over time in the device according to the invention at the different stirring speeds.

[0059] The experimental results show that stirring during electrocoagulation (EC) has a significant impact on contaminant removal and energy consumption. The experiments show that the voltage increase during EC without stirring is approximately three times greater than with stirring. The efficiency of contaminant removal (measured by liquid turbidity) for EC with stirring speeds of 400 to 500 rpm is 82-90%, while only 56% is achieved without stirring.

[0060] The flocs formed during operation with stirring are very large and can be easily separated. Without stirring, the flocs formed are very small and require extensive filtration to separate. Figure 5 shows a comparison of the sedimentation of the flocs formed by EC 15 minutes after the end of the EC treatment: on the left (a) the liquid treated by EC with stirring and on the right (b) the liquid treated by EC without stirring. Figure 6 shows a top view of another embodiment of a device 20 according to the invention; Figure 7 shows a side view of the device 20.

[0061] The batch reactor 20 is mounted on a frame 29 and has an inlet 21 with an inlet valve 22 for the supply of liquid to be treated into the batch reactor 20. An agitator 24 is attached to a stirrer holder 23 and is driven by an electric motor 25. Spiral-shaped aluminum electrodes 26 are located inside the batch reactor 20. At the lowest point of the batch reactor 20, an outlet 28 with an outlet valve 27 is arranged, through which the batch reactor 20 can be emptied. shows a plan view of another embodiment of the device 30 according to the invention. Inside a cylindrical container, a cathode 31 and an anode 32 made of spirally rolled metal sheets are arranged such that they do not touch each other. In the center of the electrodes 31, 32 runs the

[0062] Axis of a stirrer 33, the blades of which are arranged below the electrodes 31, 32.

[0063] Figure 9 shows a top view of an embodiment of the device 40 according to the invention with multiple electrodes 41, 42. Inside a cylindrical container, multiple cathodes 41 and anodes 42 are arranged alternately so that they do not touch each other. The individual sheets of the cathodes 41 and anodes 42 are each bent so that all the electrodes 41, 42 together form a vortex. The axis of an agitator 43 runs along the central axis of the device 40, the blades of which are arranged below the electrodes 41, 42. List of Reference Symbols

[0064] 10 Electrocoagulation device

[0065] 11 cylindrical reactor chamber

[0066] 12 cylindrical outer space / overflow

[0067] 13 Fluid intake

[0068] 14 Fluid drainage

[0069] 15 spiral electrodes

[0070] 16 agitator

[0071] 17 Floor

[0072] 20 batch reactor

[0073] 21 Inlet

[0074] 22 Inlet valve

[0075] 23 Stirrer holder

[0076] 24 agitator

[0077] 25 electric motor

[0078] 26 aluminum electrode

[0079] 27 Exhaust valve

[0080] 28 Procedure

[0081] 29 frame

[0082] 30 Electrocoagulation device

[0083] 31 Cathode

[0084] 32 Anode

[0085] 33 agitator

[0086] 40 Electrocoagulation device

[0087] 41 Cathode

[0088] 42 Anode

[0089] 43 agitator

Claims

Patent claims 1. Device (10) for electrocoagulation, comprising spirally wound electrode sheets (15) for cathode and anode, which are spaced apart from each other in a cylindrical central space (11 ), wherein the longitudinal axis of the central space (11 ) forms the central axis of the spirals (15), characterized in that the cylindrical central space is bounded downwards by a base (17) which is spaced apart from the spirals (15), 2. Device according to claim 1, wherein at least one agitator (16) is arranged in the cylindrical central space (11).

3. Device (10) according to claim 2, wherein at least one agitator (16) is arranged below the electrodes (15).

4. Device (10) according to claim 2 or 3, wherein at least one agitator (16) is arranged above the electrodes (15).

5. Device (10) according to one of claims 2 to 4, wherein a cylindrical area is kept free in the core of the spirals (15) in which at least one agitator (16) is arranged.

6. Device (10) according to one of claims 1 to 4, wherein the cylindrical central space (11) has at least one liquid inlet (13) arranged near the lower end of the cylinder.

7. Device (10) according to one of claims 1 to 6, which has a cylindrical outer chamber arranged coaxially to the cylindrical central space (11).

8. Device according to any one of claims 1 to 7, wherein the anode contains metallic iron, aluminum, or titanium.

9. A method for treating water containing impurities by electrocoagulation, in which the water containing impurities is filled into a device (10) according to one of the preceding claims, the water containing impurities is continuously guided radially in the device (10) through the spiral channels formed between the spirally wound electrode sheets (15), a direct voltage is applied to the electrodes (15), whereby metal hydroxides are electrochemically generated, causing the impurities to flocculate, and the flocculated impurities are subsequently separated.

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