Method and apparatus for electrochemical purification of wastewater

Abstract

The invention of the current application is directed to a wastewater treatment apparatus. The invention includes a divided membrane electrochemical cell including an anode contained within a anode compartment and cathode contained within a cathode compartment. The anode compartment and said cathode compartment are separated by a proton selective membrane. The invention also includes a voltage source, and a liquid-gas separator. The invention is an economically viable electrochemical advanced oxidation system that can cost-effectively treat recalcitrant COD with low energy, without the necessity for chemicals, and reduce or prevent sludge production in a single step.

Claims

1. A apparatus comprising: a divided membrane electrochemical cell comprising an anode contained within an anode compartment and cathode contained within a cathode compartment, wherein said anode compartment and said cathode compartment are separated by a proton selective membrane, and wherein said anode compartment additionally comprises at least one three dimensional electrode and at least one flow distributor is provided across the at least one three dimensional electrode wherein a first segment of the at least one three dimensional electrode comprises a stack of expanded metal substrates coated with hydroxyl radical catalyst layers, a second segment of the at least one three dimensional electrode comprises a stack of expanded metal substrates coated with hydroxyl radical catalyst layers with interspersed inert glass beads as turbulence promotors, and a third electrode segment comprises a stack of expanded metal substrates coated with hydroxyl radical catalyst layers interspersed with hydroxyl radical catalyst particles wherein at least one of said anode compartment or said cathode compartment comprises ion conducting materials in an amount from 5 to 95 volume % of said at least one of said anode compartment or said cathode compartment, a voltage source, and a liquid-gas separator, wherein said at least one three dimensional electrode increases, along its length, in at least one of, specific surface area per unit volume of electrode or mass transfer coefficient in the direction of wastewater flow.

2. The apparatus of claim 1 additionally comprising at least one pre-treatment unit suitable for removing anions, cations and dissolved CO.sub.2.

3. The apparatus of claim 1 wherein the energy consumption is less than or equal to 68 kWh/m.sup.3 of wastewater flow through the anode compartment.

4. The apparatus of claim 1 wherein the three dimensional electrode additionally comprises an ion conducting phase.

5. The apparatus of claim 1 wherein said at least one three dimensional electrode increases in specific surface area per unit volume of electrode in the direction of wastewater flow.

6. The apparatus of claim 1 wherein said at least one three dimensional electrode increases in mass transfer coefficient in the direction of wastewater flow.

7. A apparatus comprising: a divided membrane electrochemical cell comprising an anode contained within an anode compartment and cathode contained within a cathode compartment, wherein said anode compartment and said cathode compartment are separated by a proton selective membrane, and wherein said anode compartment additionally comprises at least one three dimensional electrode and at least one flow distributor is provided across the at least one three dimensional electrode, wherein the at least one three dimensional electrode comprises a pre-treatment segment comprising a porous substrate coated with a H.sub.2S, NH.sub.3 and/or metal oxidation catalyst and at least two voltage tabs wherein at least one of said anode compartment or said cathode compartment comprises ion conducting materials in an amount from 5 to 95 volume % of said at least one of said anode compartment or said cathode compartment, a voltage source, and a liquid-gas separator, wherein said at least one three dimensional electrode increases, along its length, in at least one of, specific surface area per unit volume of electrode or mass transfer coefficient in the direction of wastewater flow.

8. The apparatus of claim 7 wherein the at least one three dimensional electrode additionally comprises an insulating flow distributor positioned after the pre-treatment segment.

9. The apparatus of claim 8 wherein the at least one three dimensional electrode additionally comprises a subsequent segment positioned after the insulating flow distributor and comprising a porous substrate coated with an hydroxyl radical catalyst and a voltage tab.

10. The apparatus of claim 7 wherein the at least one pretreatment segment increases in at least one of specific surface area per unit volume of electrode or mass transfer coefficient in the direction of wastewater flow.

11. The apparatus of claim 7 wherein the at least one pretreatment segment increases in at least one of specific surface area per unit volume of electrode or mass transfer coefficient in the direction of wastewater flow.

12. The apparatus of claim 7 additionally comprising at least one pre-treatment unit suitable for removing anions, cations and dissolved CO.sub.2.

13. The apparatus of claim 7 wherein the energy consumption is less than or equal to 68 kWh/m.sup.3 of wastewater flow through the anode compartment.

14. The apparatus of claim 7 wherein the three dimensional electrode additionally comprises an ion conducting phase.

15. The apparatus of claim 7 wherein said at least one three dimensional electrode increases in specific surface area per unit volume of electrode in the direction of wastewater flow.

16. The apparatus of claim 7 wherein said at least one three dimensional electrode increases in mass transfer coefficient in the direction of wastewater flow.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view of a wastewater electrochemical treatment system.

(2) FIG. 2 is an exterior side-view of an exemplary anode-cathode cell.

(3) FIG. 3 is an interior side-view of an exemplary anode-cathode cell.

(4) FIG. 4 is a planar view of an exemplary divided anode compartment.

(5) FIG. 5. is a planar view of an exemplary 3D anode electrode configuration designed for high COD wastewater.

(6) FIG. 6. is a planar view of an exemplary 3D anode electrode configuration.

(7) FIG. 7. is a planar view of an exemplary 3D anode electrode configuration.

(8) FIG. 8. is a cross section view of a section of an exemplary 3D anode electrode configuration

(9) FIG. 9. is a planar view of an exemplary 3D electrode with a pre-treatment segment for inorganic electrochemical decomposition.

DETAILED DESCRIPTION OF DRAWINGS

(10) FIG. 1 is a schematic view of a wastewater electrochemical treatment system configured in accordance with the present invention. Wastewater from a source, such as an equalization tank 35, is pumped 37 to the electrochemical oxidation vessel 43. The wastewater optionally passes through a filter unit 36 when the TSS of the wastewater contains large particle larger than the smallest anode perforation or a high volume fraction of particles, greater than 5%, to prevent electrode clogging or shielding of activation sites. For monitoring and control purposes, COD 38 and pH 39 are monitored to detect any upset conditions and a flow meter 40 measures the flow rate. The flow rate may be increased or decreased automatically by adjusting the pump speed depending on the treatment objective and variations in measured COD concentration. The wastewater is fed into the cell vessel 43, passing through the anode compartment 2 or an array of anode compartments. A thermometer or thermocouple 42 measures the wastewater cell temperature for monitoring and control purposes. A power supply or potentiostat 44 applies a DC cell voltage across the anode-cathode pair(s) to provide an anode half-cell voltage large enough to produce OH*. The anode voltage at a point farthest from the bussbar feeder is measured by a reference electrode(s) 41 and this value is fed back to the power supply or potentiostat 44 for control of the applied cell voltage. The treated effluent and product gas (CO.sub.2, N.sub.2, O.sub.2) exits the anode compartment to a liquid-gas separator 53. The treated effluent is directed to discharge or a tank 46 where the COD 47 is measured to confirm the treatment objective. The anode product gas is passed through a scrubber 45, such as a carbon cartridge, to remove any fugitive volatile gases and then vented to atmosphere. A small side-stream of treated effluent is either continuously or intermittently pumped 48 through the cathode compartment 3 or array of cathode compartments. If a copious amount of scale forming ions are present in the treated effluent, the effluent may optionally pass through a pre-treatment unit 50, such as electro-deionization, capacitive deionization or ion exchange resin, to remove them. The cathode water and product H.sub.2 gas exit the cathode compartment to a liquid-gas separator 54. The H.sub.2 gas may be captured for use as an energy carrier, vented or flared to atmosphere and the cathode water is discharged back into the treated effluent. A sensor 49 measures the pH for monitoring and control purposes. Optional cleaning solutions stored in a small vessel 52 for cathode clean-in-place may be circulated to the cathode compartment in the cathode water. Optional cleaning solutions stored in a small vessel 55 for anode clean-in-place may be circulated to the anode compartment. If particulate deposits accumulate in the anode, the electrode may be back-flushed by reversing the flow of wastewater, by-passing the filter, to the wastewater feed.

(11) FIG. 2 is an exterior side-view of an exemplary anode-cathode cell 1. In FIG. 2, the anode compartment 2 is divided from the cathode compartment 3 by the membrane/separator 4. The membrane is composed of proton selective conducting ionomer, such as Nafion®.

(12) The anode and cathode compartments 2, 3 are enclosed in a cell housing 5a, 5b made of electrically insulating materials to prevent stray current loses. Suitable materials include polymers, glass, fiberglass reinforced plastic, ceramics, and polymer coated metals. The wastewater is fed into the anode compartment through the anode inlet 6 and the treated effluent and product gas (CO.sub.2, N.sub.2, O.sub.2) exit at the anode outlet 8. A side-stream of treated effluent is fed to the cathode compartment through the cathode inlet 7 and the cathode water and product H.sub.2 gas exit at the cathode outlet 9.

(13) FIG. 3 is an interior side-view of an exemplary anode-cathode cell. The exterior cell housing 5a, 5b keeps the cell liquid and gas tight, and is insulating to prevent any leakage current. The anode compartment is enclosed in the housing 5a and the cathode compartment is enclosed in an insulating housing 5b. 3 by the membrane/separator 4. The cell is sealed by gaskets 10 located between the membrane The cell is sealed by gaskets 10a 10b located between the membrane 4 and housing 5a, 5b. The membrane is composed of proton selective conducting ionomer. An anode functional coating 11 is located on the anode face of the membrane 4. The anode functional coating provides durability and enhances oxidation efficiency. The coatings may include one or more of particles to detach bubbles such as ZrO.sub.2 and TiO.sub.2, hard particles for erosion protection such as TiO.sub.2 and SnO.sub.2, mixed proton and electron conducting phases to distribute current such as ionomer, and doped SnP.sub.2O.sub.7, OH* scavenger such as MnO.sub.2 particles to protect membrane from OH* attack, and/or oxygen catalyst particles to oxidize metal ions and prevent membrane fouling. The phases may be mixed, and possess different composition along the length of the membrane or through the coating thickness. The wastewater is fed to the anode compartment 2 through the anode inlet 6 and flow distributor 20. The anode inlet and flow distributor are electrically insulating materials to prevent stray current loss. The flow distributor 20 is composed of a series of parallel or horizontal slots, and/or beads to provide uniform liquid distribution for high mass transfer efficiency. The wastewater flows through the 3D electrode which is located in the anode compartment 2. The organic and inorganic compounds in the wastewater are oxidized by OH* radicals generated on the surface of the 3D electrode as the wastewater flows through the anode. The anode voltage is applied to the electrode via the current collector 18. The anode half-cell voltage is measured and fed-back to the power supply by the reference electrode through the reference voltage port 21a. The treated effluent and product gas (CO.sub.2, N.sub.2, O.sub.2) exit the anode compartment 2 into the anode gas-liquid separator 16. The effluent exits the liquid-gas separator 16 through the anode liquid outlet 8 and the product gas exits through the anode gas vent 14. The anode liquid-gas separator 15, anode liquid outlet 9 and anode gas vent 17 are all composed of insulating materials to prevent stray current loss.

(14) The side-stream of treated effluent is fed to the cathode compartment 3 through the cathode inlet 7 and a flow distributor 20. Similarly, both the cathode inlet and flow distributor are electrically insulating materials to prevent stray current loss. The cathode electrode is a high surface area electrode located in the cathode compartment 3 and possesses enough porosity to prevent H.sub.2 gas accumulation and to enable cathode water/cleaning fluid flow through the cathode compartment 3. The cathode voltage is applied to the electrode via the current collector 19. The cathode half-cell voltage is measured by the reference electrode 51 through the reference voltage port 21b. A cathode functional coating is located on the cathode face of the membrane 12. The cathode functional coating provides erosion protection and enhances oxidation efficiency. The coating may contain one or more of hard particles for erosion protection such as carbides and metals, structural support for thin membrane such as fibers, cation scavengers to prevent their blocking catalyst sites such as activated carbon particles and mixed proton and electron conductive phases to distribute current such as ionomer and graphite. The particles may be mixed, possess different compositions along length of the cathode and/or through the coating thickness. The cathode water/cleaning fluid flows through the cathode compartment 3. The cathode water and product H.sub.2 gas exit the cathode compartment 3 into the cathode gas-liquid separator 15. The cathode water exits the liquid-gas separator 15 through the cathode liquid outlet 9 and the product H.sub.2 gas exits through the cathode gas vent 17. The cathode liquid-gas separator 15, cathode liquid outlet 9 and cathode gas vent 17 are all composed of insulating materials to prevent stray current loss.

(15) FIG. 4 is a planar view of an exemplary divided anode compartment. For large volumetric flows, scaling up the size of the anode-cathode cells is cost effective. The divided anode compartment has a common wastewater feed inlet, header 6. The flow distribution is accomplished either by a series of individual flow distributors 20, as shown, or one large distributor across the width. The compartment may be divided by machining grooves in the anode housing 5a to accommodate a set of 3D electrodes and provide mechanical support for the membrane. Alternatively, a large anode compartment is occupied by one 3D electrode that is divided by a series of current collectors 18 that run the length of the anode compartment and provide barriers across the width of the anode and anode compartment subdivisions 23. Mechanical support for the membrane is then provided by the current collectors 18. The cathode compartment is constructed in a similar manner so that the mechanical supports of the cathode and anode match. The electrode length is increased to provide for higher volumetric flows and the configuration of the 3D electrode is designed so that the product of mass transfer coefficient and electrode surface area (kA.sub.s) is maximized along the length of the electrode. One or more flow distributors 20 may be incorporated along the length of each anode compartment subdivisions 23.

(16) FIG. 5. is a planar view of an exemplary 3D anode electrode configuration designed for high COD wastewater. The embodiment shown is suitable for greater than 10,000 mg/L COD, with a TDS greater than 5000 ppm. The electrode is divided into three segments along its length by two flow distributors 20 in the direction of wastewater flow. The first segment of the 3D electrode is composed of a stack of expanded metal substrates coated with OH* catalyst layers 24. The stack thickness is specified based on COD concentration, wastewater flow rate, surface area A.sub.s and mass transfer coefficient k of the segment, maximum cell current and projected current density, volume of product gas produced, limiting current penetration depth and sum of iR losses at the operating current. The second segment of the 3D electrode is composed of a stack of the same expanded metal substrates coated with OH* catalyst layers 24 with interspersed inert glass beads 25 as turbulence promotors. The electrode surface area is the same as segment one, The glass beads 25 increase the mass transfer coefficient k of the segment to accommodate the reduction in the bulk COD leaving segment one and maintain a uniform limiting current and projected current density. The final electrode segment is composed of a stack of expanded metal substrates coated with OH* catalyst layers 24 interspersed with OH*catalyst particles 26. The presence of the catalyst particles 26 increase both the surface area A.sub.s and the mass transfer coefficient k of the segment to accommodate the further reduction in bulk COD leaving segment two and maintain a uniform limiting current and projected current density.

(17) FIG. 6. is a planar view of an exemplary 3D anode electrode configuration. The embodiment shows is suitable for low conductivity wastewater, less than 5000 ppm TDS. The 3D electrode is segmented into three regions with one flow distributor 20. The first electrode segment is composed of a stack of metal mesh substrates coated with OH* catalyst layer 27 interspersed with ion/proton conducting particles 28. The volume fraction of ion conducting particles is specified based on the value of the wastewater conductivity and the required limiting current penetration depth. For low penetration depths, a smaller stack of mesh substrates is used but a higher surface area mesh may be used to compensate for the loss in surface area A.sub.s. The second segment is composed of a higher surface area stack of mesh substrates coated with OH* catalyst layer 27. Since the limiting current penetration depth increases with decreasing COD concentration so the subsequent segment does not require an increase in conductivity to maintain the same penetration depth thus, ion conducting particles are not present. The higher surface area of the second segment is designed to match the limiting current of the first segment for the reduced COD. The final segment is composed of a bed of OH* catalyst particles 26 with a higher surface area than the second and first segments to maintain a constant limiting current.

(18) FIG. 7. is a planar view of an exemplary 3D anode electrode configuration. The embodiment shown includes a high electrode surface area provided by particles 29 consisting of mixed catalyst and proton conducting solid phases. The electrode is divided into three segments by two optional flow distributors 20. Expanded metal current collector(s) 13 are situated within the bed of agglomerate particles 29 to reduce its iR loss. The surface of the current collector 13 may be textured to increase surface roughness to improve electrical contact with the particles. To maintain a constant limiting current over the electrode, the size of the particles 29 are reduced down the length of the electrode in the direction of wastewater flow to increase surface area A.sub.s and mass transfer coefficient k as COD concentration is reduced.

(19) FIG. 8. is a cross section view of a section of an exemplary 3D anode electrode configuration. The embodiment shown includes a high electrode surface area provided by a mixture of discreet catalyst particles 26 and solid ionomer particles 28. Two expanded metal current collectors 13 are located within the bed of particles, aligned perpendicular to each other. The current collector closest to the membrane is flattened on the surface facing the membrane 4 to provide mechanical support and prevent perforation of the thin membrane 4. The anode and cathode functional coatings 11, 12 are shown on the membrane 4. The anode functional coating 11 prevents membrane degradation by preventing the direct contact of the membrane with the active catalyst particles 26 and current collector 13. The anode compartment wall 30 is shown with embossed dimples that improve mass transfer.

(20) FIG. 9. is a planar view of an exemplary 3D electrode with a pre-treatment segment 31 for inorganic electrochemical decomposition. The pre-treatment segment 31 consists of a porous substrate coated with a H.sub.2S, NH.sub.3 and/or metal oxidation catalyst and a voltage tab 33 which provides a lower anode half-cell voltage to the segment. An flow distributor 20 provides electrical separation and flow distribution into the subsequent segment 32. The subsequent segment 32 consists of a porous substrate with increasing surface area A.sub.s in the direction of flow that is coated with an OH* catalyst and a voltage tab 34 which provides a higher anode half-cell voltage to the segment.

(21) In some embodiments, there are at least 2 voltage tabs 34. In some embodiments there are 2-3 voltage tabs 34. In some embodiments the voltage tabs are separated by at least one insulating flow distributor 20.

Further Examples and Experimental Data

(22) A divided, flow-through reactor with anode and cathode compartments of 10 cm wide×40 cm high×6.4 mm thick was fabricated from PVDF. The inlet ports were at the bottom of the cell and flow distributors consisting of a series of parallel channels, 3 mm wide×4 mm long, were located at the entrance to both anode and cathode compartments. A proton selective membrane, Nafion 117™, divided the cell. The cell was sealed using silicone gaskets and a port for the reference electrode was located 1 cm above the anode flow distributor. In all experiments, the wastewater was fed to the anode compartment at room temperature and at a pH of 7. A constant cell voltage of 8 volts was applied, and only a single pass of the wastewater through the anode was used in all experiments. This translates into a treatment time of less than 4 minutes and 8 minutes for flow rates of 4 L/hr and 1.9 L/hr respectively. Samples of treated wastewater were taken after the oxidation process had run at steady state for at least 30-60 minutes. Wastewater analysis for COD and phenol concentrations were measured according to USEPA methods 410.4 and 420.1 respectively.

(23) Set 1

(24) Treatment of wastewater was performed in the divided reactor at a constant cell voltage of 8V. The wastewater was composed of 500 mg/L phenol (COD=1200 mg/L) in a solution of 8 g/L Na.sub.2SO.sub.4, that was fed to the anode compartment at either 4 or 1.9 liters/hour in a single pass. The catholyte was a solution of 8 g/L Na.sub.2SO.sub.4 that was recirculated through the cathode compartment during all experiments. The same 3D cathode with a constant mass transfer coefficient and constant surface area was used in all experiments. The cathode was a stack of two commercial mixed metal oxide (MMO) expanded metal cathodes (Magneto Special Anodes BV) with an effective surface area of 205 m/m.sup.3. The anodes in Examples 1, 2 and 4 consisted of 3D electrodes with constant mass transfer coefficient and constant surface area along their width and length. The anodes in Examples 1 and 4 are a stack of 3 commercial MMO expanded metal anodes (Magneto Special Anodes BV) occupying the full length of 40 cm of the anode compartment with an effective surface area of 410 m.sup.2/m.sup.3. Example 2 anode was a stack of 2 commercial MMO expanded metal anodes (Magneto Special Anodes BV) with an effective surface area of 205 m.sup.2/m.sup.3. Examples 3 and 5 anodes consist of 3D electrodes with variable mass transfer coefficient and variable surface area. The first 30 cm of anode length is a stack of 2 commercial MMO expanded metal anodes (Magneto Special Anodes BV) with an effective surface area of 205 m.sup.2/m.sup.3. The final 10 cm of the anode length consists of a loose random packing of sintered antimony doped tin oxide particles (Stannex ELR™), and current collection provided by 2 MMO expanded metal screens. The surface area of the bed of particles was estimated as 1272 m.sup.2/m.sup.3 from the particle size distribution assuming spherical shapes.

(25) Results

(26) The mass transfer coefficient was determined for each segment of the 3D electrodes using the well-known mass transfer correlations for a stack of expanded metal sheets and for a porous electrode with the kinematic viscosity of phenol=8×10.sup.−7 m.sup.2/s and the molecular diffusion coefficient of phenol in water=8.47×10.sup.−10 m.sup.2/s. The correlation for a stack of expanded metal sheets is defined as: Sh=0.71Re.sup.0.54Sc.sup.0.33(R.sub.h/α).sup.0.38 and the correlation for a porous electrode is defined as: eSt=0.45Re.sup.−0.41Sc.sup.−0.67 where: Sh=Sherwood number, Re=Reynolds number, Sc=Schmidt number, R.sub.h=hydraulic radius and α=aperture radius in direction of flow.

(27) Mass Transfer Coefficients for 3D Electrode Segments

(28) TABLE-US-00011 Wastewater Mass Transfer flow rate Coefficient Description (L/hr) (m/s) Stack of 3 MMO expanded 4 6.29 × 10.sup.−6 metal anode sheets 1.9 4.21 × 10.sup.−6 Stack of 2 MMO expanded 4 7.58 × 10.sup.−6 metal sheets 1.9 4.39 × 10.sup.−6 Loosely packed bed of 4 5.11 × 10.sup.−6 Stannex ELR ™ particles 1.9  3.3 × 10.sup.−6

(29) The limiting current at the front and at a point ¾ along the length of the 3D electrodes are shown below. Examples 3 and 5 with variable mass transfer coefficient and variable effective surface area have a uniform limiting current along the length of the electrode. Examples 1, 2 and 4 with a constant mass transfer coefficient and surface area have a drop in the limiting current of 3 to more than 4 times at ¾ point along the electrode length. This non-uniform current distribution leads to reduced oxidation efficiency at the lower limiting current regions and higher rates of anode degradation at the higher current regions shortening the lifetime of the electrode.

(30) Limiting Current for 3D Electrodes

(31) TABLE-US-00012 Limiting Current at Limiting Wastewater front end of Current at ¾ Example flow rate electrode point along # 3D Electrode Description (Lit/hr) (A) length (A) 1 Stack of 3 MMO expanded 4 5.7 1.4 metal anode sheets 2 Stack of 2 MMO expanded 4 3.8 1.0 metal sheets 3 Stack of 2 MMO expanded 4 3.8 3.8 metal sheets and loosely packed bed of Stannex ™ particles 4 Stack of 3 MMO expanded 1.9 3.5 0.8 metal sheets 5 Stack of 2 MMO expanded 1.9 2.1 2.1 metal sheets and loosely packed bed of Stannex ™ particles

(32) The predicted COD, measured COD, phenol removal percentages and energy consumption are shown below. Examples 3 and 5 with variable mass transfer coefficient and surface area are the most energy efficient having the lowest energy consumption per kg COD destroyed. Notably, Example 5 achieved a 25% higher COD removal than predicted. Comparing Examples 2 and 3, the addition of the higher surface area particle bed in the last ¼ segment of the electrode increased the destruction of COD by 40% and phenol by 60%, while reducing the energy consumption by 40%.

(33) COD Destruction and Energy Consumption

(34) TABLE-US-00013 Measured Measured Wastewater Predicted COD phenol Energy Example flow rate % COD destruction destruction Consumption # (Lit/hr) destruction (%) (%) (kWh/m.sup.3) 1 4 45 27 50 162 2 4 31 20 25 60 3 4 47 35 40 36 4 1.9 57 53 85 337 5 1.9 58 73 80 147
Set 2.

(35) Treatment of low conductivity wastewater was performed in the divided reactor at a constant cell voltage of 8V with 3D anodes and cathodes with and without ion conducting phases. The anode overpotential of 1 volt was measured using the reference electrode. The low conductivity wastewater was composed of 500 mg/L phenol (COD=1200 mg/L) in a solution of 1 g/L Na.sub.2SO.sub.4, pH=7 at room temperature, that was fed to the anode compartment at 4 liters/hour in a single pass. The resistivity of the wastewater and properties of the ion conducting phases are shown below.

(36) Properties of Ion Conducting Phases and Wastewater

(37) TABLE-US-00014 Conductivity (S/m) Particle size LiquGen ™ cation 2.78 Effective: 0.4-0.6 mm exchange resin beads Nafion ™ NR50 beads 10 3-4 mm 1 g/L Na.sub.2SO.sub.4 solution 0.16 n/a or wastewater 8 g/L Na.sub.2SO.sub.4 1.25 n/a wastewater

(38) Example 6 without an ion conducting phase consisted of a 3D anode with variable mass transfer coefficient and variable surface area. The first 30 cm of anode length is a stack of 2 commercial MMO expanded metal anodes (Magneto Special Anode BV) with an effective surface area of 205 m.sup.2/m.sup.3. The final 10 cm of the anode length consists of a loose random packing of sintered antimony doped tin oxide particles (Stannex ELR™), and current collection provided by 2 MMO expanded metal screens. The surface area of the bed of particles was estimated as 1272 m.sup.2/m.sup.3 from the particle size distribution assuming spherical shapes. The 3D cathode was a stack of three commercial MMO expanded metal cathodes (Magneto Special Anode BV) with an effective surface area of 410 m.sup.2/m.sup.3. The catholyte was a low conductivity solution of 1 g/L Na.sub.2SO.sub.4 recirculated through the cathode compartment.

(39) Example 7 with conducting ion phases consisted of a 3D anode with variable mass transfer coefficient and variable surface area. The first 30 cm of anode length is a stack of 2 commercial MMO expanded metal anodes (Magneto Special Anode BV) with an effective surface area of 205 m.sup.2/m.sup.3 and 50 volume % of cation exchange beads (LiquGen™) occupying the compartment volume. The final 10 cm of the anode length consisted of a loose random packing of sintered antimony doped tin oxide particles (Stannex ELR™) mixed with proton selective ionomer beads (Nafion™ NR50), and current collection provided by 2 MMO expanded metal screens. The bed volume of the ionomer particles was 50%. The 3D cathode was a stack of three commercial MMO expanded metal cathodes (Magneto Special Anode BV) with an effective surface area of 410 m.sup.2/m.sup.3 and 50 volume % cation exchange beads (LiquGen™). The catholyte was a low conductivity solution of 1 g/L Na.sub.2SO.sub.4 recirculated through the cathode compartment.

(40) Example 8 consisted of a 3D anode with variable mass transfer coefficient and variable surface area. The first 30 cm of anode length is a stack of 2 commercial MMO expanded metal anodes (Magneto Special Anode BV) with an effective surface area of 205 m.sup.2/m.sup.3 and 50 volume % of cation exchange beads (LiquGen™) occupying the compartment volume. The final 10 cm of the anode length consists of a loose random packing of 50 volume % sintered antimony doped tin oxide particles (Stannex ELR™) mixed with 50 volume % proton selective ionomer beads (Nafion™ NR50), and current collection provided by 2 MMO expanded metal screens. The 3D cathode was a stack of three commercial MMO expanded metal cathodes (Magneto Special Anode BV) with an effective surface area of 410 m.sup.2/m.sup.3 and 50 volume % cation exchange beads (LiquGen™) occupying the compartment volume. The catholyte consisted of effluent from the anode of low conductivity treated wastewater that was passed through the cathode compartment in a single pass without any pre-treatment.

(41) Results

(42) The ionic conductivity of the segments of the 3D anodes in Example 3 and 6 are equal to the wastewater conductivity. The ionic conductivity of segments of the 3D anode in Example 7 with ion conducting phases were calculated using the Maxwell theoretical model. The limiting current penetration depth for each segment of the 3D anodes, Examples 3, 6 and 7, were calculated for the applied overpotential of 1V and are shown below. None of the examples achieved full current efficiency throughout the full 3D electrode. Example 3 with the highest ion conductivity in both segments of the 3D anode had the lowest energy consumption. For low conductivity wastewater, comparing Examples 6 and 7, the addition of the ion conducting phase increased the limiting current penetration depth of both 3D anode segments by 50% or more, increased the 3D electrode ion conductivity by more than 50%, increased the phenol destruction by 50% and lowered the energy consumption by 25%. Comparing Examples 3 and 7, the 3D anode with ion conducting particles achieved equal or better phenol destruction of the low conductivity wastewater versus the 3D anode with a 3× higher conductivity wastewater.

(43) Example 3 (variable k and variable A.sub.e) is more advantageous than Examples 1, 2 (constant k and constant A.sub.e) because of its uniform limiting current, lower energy consumption and higher COD destruction.

(44) Example 7 (variable k, variable A.sub.e, ion conducting phases) is more advantageous than Example 6 (variable k, variable A.sub.e) because of increased electrode conductivity, increased limiting current penetration depth, increased phenol destruction, lowered energy consumption.

(45) Limiting Current Penetration Depth of 3D Electrodes

(46) TABLE-US-00015 Penetration Penetration depth of depth of Anode ionic Anode ionic limiting limiting Measured conductivity conductivity current current Phenol Energy Example segment 1 segment 2 segment segment Destruction Consumption # (S/m) (S/m) 1 (mm) 2 (mm) (%) (kWh/m.sup.3) 3 1.25 1.25 7.9 3.5 35 36 6 0.16 0.16 2.8 1.4 25 90 7 0.36 0.38 4.2 2.2 38 68

(47) Example 8 eliminated the catholyte recirculation loop by using a single pass of low conductivity anode effluent (0.16 S/m) directly, without pre-treatment. The resulting effluent from the cathode was then re-combined with the effluent from the anode according to FIG. 1. Example 8 had an ion conducting phase added to the 3D electrode to increase the cathode ion conductivity shown below. Comparing Example 6 and Example 8, the phenol destruction was equivalent and the energy consumption of Example 8 was 48% lower.

(48) Example 8 (variable k, variable A.sub.e, ion conducting phases, effluent from anode as catholyte) is more advantageous than Example 6 (variable k, variable A.sub.e, ion conducting phases, recirculating chemical catholyte) because Example 8 eliminates costly chemical recirculation loop, lower energy consumption for same COD destruction.

(49) Phenol Destruction and Energy Consumption

(50) TABLE-US-00016 Cathode Catholyte ionic Phenol Energy Example Conductivity conductivity Destruction Consumption # Catholyte (S/m) (S/m) (%) (kWh/m3) 6 1 g/L 0.16 0.16 25 90 Na.sub.2SO.sub.4 8 Effluent 0.16 0.36 25 44 from anode