HK40016107B - Stack of electrochemical cells for wastewater treatment with isolated electrodes - Google Patents

Description

Stack of electrochemical cells with isolated electrodes for wastewater treatment

Technical Field

The present invention relates to a stack of electrochemical cells for wastewater treatment, and in particular to a stack of electrochemical cells for removing organic and inorganic pollutants, the stack comprising at least one electrochemical cell immersed in a reactor tank, wherein one electrode of the electrochemical cell is protected by a cover which isolates the electrode from the wastewater or from any other solution in the reactor tank.

background

For new wastewater treatment scheme, particularly for cost-effective, sustainable, do not produce secondary pollution, meet water quality standards and have minimum operation requirement and the demand of the water treatment system of maintenance requirement there is obvious growth.The preferred method of processing wastewater is by non-chemical oxidation technology, for example, by electrochemical oxidation.Electrochemical oxidation is effective in eliminating a wide range of pollutants such as persistent organic pollutants, dioxins, nitrogen substances (for example ammonia), medicine, pathogen, microorganism and most of priority pollutants and pesticides.

A variety of cell configurations including flow-through parallel plates, divided chambers, packed bed electrodes, stacked disks, concentric cylinders, moving bed electrodes, and filter presses have been developed for direct and indirect electrochemical oxidation in wastewater treatment.

A configuration of an electrolytic cell for wastewater treatment uses a solid polymer electrolyte (SPE), as described, for example, in the applicant's patent publication WO2012167375. The system includes an electrolytic cell comprising a cathode having a cathode gas diffusion layer and a cathode catalyst layer, an anode having an anode diffusion layer and an anode catalyst layer, and a solid polymer membrane electrolyte separating the anode layer and the cathode layer. By guiding the wastewater through the flow field channels arranged in the anode flow field plate placed adjacent to the anode fluid delivery layer, the wastewater is uniformly delivered to the anode fluid delivery layer and delivered from the anode fluid delivery layer. The hydrogen generated during the electrochemical treatment of the wastewater is collected from the cathode and guided out of the electrolytic cell by means of the flow field channels arranged in the flow field plate placed adjacent to the cathode fluid delivery layer. The system can include a plurality of electrolytic cells that are stacked and arranged in series and/or in parallel, and can operate without a cathode electrolyte or other supporting electrolytes.

The applicant has also developed a system as disclosed in the applicant's patent application WO2017123969, in which a stack of electrochemical cells is immersed in a reactor tank containing wastewater to be treated, wherein each electrochemical cell includes a solid polymer electrolyte (SPE) membrane and an anode catalyst layer and a cathode catalyst layer and an open poremesh, each catalyst layer is adjacent to one side of the solid polymer electrolyte membrane, and each open poremesh is adjacent to the catalyst layer. The system also includes a compression frame, each frame adjacent to the open poremesh and having a compression arm that is deployed in an area defined by the periphery of the frame, and the compression arms are connected to each other at a connection portion. The system also includes a fastener that passes through a hole at a connection portion in the arm of the compression frame, through a hole in the open poremesh, and through a catalyst coated membrane protruding to compress the solid polymer electrolyte membrane, the catalyst layer, and the open poremesh between the two compression frames. This system has been shown to achieve a higher pollutant removal rate at a lower operating cost due to the removal of flow field plates and random, heterogeneous porous media (gas diffusion layers). The system offers low voltage operation and energy consumption and can operate at variable effluent flow rates.

In a system in which a stack of electrochemical cells is immersed in a reactor tank having both a cathode catalyst layer and an anode catalyst layer exposed to wastewater to be treated, such as in the system described in the applicant's co-pending patent application, an intermediate reagent formed at the anode during the oxidation of the wastewater can poison the cathode catalyst. In addition, wastewater that is oxidized on the anode side and is substantially free of primary pollutants but may potentially contain an intermediate reagent of the oxidation process can be contacted with the cathode catalyst, and the intermediate reagent can be reduced, thereby reversing the cleaning oxidation process that occurs on the anode side. In addition, in some cases, such as in the case of treatment of wastewater containing ammonia, sodium chloride (NaCl) is added to the reactor tank to induce the formation of in situ hypochlorite and complete the indirect oxidation of the contaminated water. In these cases, some of the sodium chloride can be retained in the treated water in the reactor tank and can be discarded from the tank by transporting the treated water to the outlet pipe of the treated water collection tank. Sodium chloride has a high electrical conductivity and therefore can cause the risk of corrosion of pipes downstream from the treatment system in some cases.

Therefore, the performance of a system in which a stack of electrochemical cells is immersed in a reactor tank can be further improved by isolating the anode or cathode from the bulk solution in the reactor tank.The present invention addresses this need, providing several benefits as disclosed herein.

SUMMARY OF THE INVENTION

The present invention describes a stack of electrochemical cells for wastewater treatment, the stack comprising at least one electrochemical cell, the electrochemical cell comprising a solid polymer electrolyte membrane, an anode catalyst layer adjacent to a first side of the solid polymer electrolyte membrane, and a cathode catalyst layer adjacent to a second side of the solid polymer electrolyte membrane opposite the first side. The electrochemical cell further comprises a first open-pore grid adjacent to the anode catalyst layer and a second open-pore grid adjacent to the cathode catalyst layer, a first compression frame adjacent to the first open-pore grid adjacent to the anode catalyst layer, and a second compression frame adjacent to the second open-pore grid adjacent to the cathode catalyst layer, each of the compression frames having compression arms extending within an area defined by the perimeter of the frame, the compression arms connected to each other at a connection location. Fasteners protrude through holes provided in the compression arms of the first and second compression frames at the connection location, through holes provided in the first and second open-pore grids, and through the solid polymer electrolyte membrane and the anode and cathode catalyst layers. The fasteners, together with the compression frames, provide a force that compresses the solid polymer electrolyte membrane, the catalyst layer, and the open-pore grid between the two compression frames. The cover is attached to a first compression frame placed on the anode catalyst layer side of the cell or to a second compression frame placed on the cathode catalyst layer side of the cell to form an enclosure for protecting the anode catalyst layer or cathode catalyst layer by isolating the anode catalyst layer or cathode catalyst layer from the solution in the reactor tank when the stack is immersed in the solution in the reactor tank.

In this first embodiment of the invention, the cover has a side surface provided with an opening for allowing wastewater, anode solution or cathode solution to enter the anode catalyst layer or, respectively, the cathode catalyst layer of the electrochemical cells in the stack. The cover also includes an inlet duct for feeding wastewater, anode solution or cathode solution into the housing formed by the cover, an outlet duct for removing reaction products formed at the anode catalyst or at the cathode catalyst from the housing formed by the cover, and a ventilation duct for removing gases from the housing formed by the cover.

The stack also comprises a seal between the side of the cover where the opening is provided and the compression frame immediately adjacent to this side.

The cover is made of a non-conductive material.

The stack includes a plurality of electrochemical cells connected by at least one rod that holds the electrochemical cells in the stack at a distance from adjacent electrochemical cells.

In another embodiment of the present invention, a stack of electrochemical cells comprises at least one repeating unit, the repeating unit comprising two electrochemical cells, each electrochemical cell comprising a solid polymer electrolyte membrane, an anode catalyst layer adjacent to a first side of the solid polymer electrolyte membrane, and a cathode catalyst layer adjacent to a second side of the solid polymer electrolyte membrane opposite the first side. Each electrochemical cell further comprises a first open grid and a second open grid, a first compression frame and a second compression frame, the first open grid adjacent to the anode catalyst layer, the second open grid adjacent to the cathode catalyst layer, the first compression frame adjacent to the first open grid, the second compression frame adjacent to the second open grid, each of the compression frames having compression arms extending within an area defined by the perimeter of the frame, the compression arms connected to each other at a connection location. Fasteners protrude through holes provided in the compression arms of the first and second compression frames at the connection location, through holes provided in the first and second open grids, and through the solid polymer electrolyte membrane and the anode and cathode catalyst layers, and together with the compression frames provide the force necessary to compress the components of the electrochemical cell. The stack also includes at least one rod for connecting the electrochemical cells in the stack and holding the first electrochemical cell in the repeating unit at a certain distance from the second electrochemical cell in the repeating unit, such that the anode side of the first electrochemical cell faces the anode side of the second electrochemical cell, or such that the cathode side of the first electrochemical cell faces the cathode side of the second electrochemical cell.

In this embodiment, the cover is placed between the compression frames of two adjacent electrochemical cells of the repeating unit and forms a shell that spans the distance between the two adjacent cells, thereby isolating the anode catalyst layer or cathode catalyst layer of the two adjacent electrochemical cells of the repeating unit from the solution in the reactor tank.

In one embodiment, the electrochemical cells in a stacked repeating unit are positioned such that a compression frame adjacent to the open grid of the cathode catalyst layer of a first electrochemical cell adjacent to the repeating unit faces a compression frame adjacent to the open grid of the cathode catalyst layer of a second electrochemical cell adjacent to the repeating unit, the second electrochemical cell being adjacent to the first electrochemical cell in the repeating unit in the stack.

In this embodiment, the cover is placed between the compression frames of adjacent electrochemical cells of a repeating unit, and the cover has an inlet duct, an outlet duct, a ventilation duct, a first side and a second side, wherein the inlet duct is used to feed the cathode solution into the outer shell formed by the cover, the outlet duct is used to remove the reaction product formed at the cathode catalyst from the outer shell formed by the cover, and the ventilation duct is used to remove the gas from the outer shell formed by the cover, the first side and the second side are opposite to each other, and each side is provided with an opening to allow the cathode solution to enter the cathode catalyst layer of two adjacent cells.

In another embodiment, the electrochemical cells in the stack are positioned such that a compression frame adjacent to the open grid of the anode catalyst layer of a first electrochemical cell adjacent to the repeating unit faces a compression frame adjacent to the open grid of the anode catalyst layer of a second electrochemical cell adjacent to the repeating unit, the second electrochemical cell being adjacent to the first electrochemical cell in the repeating unit.

In this embodiment, the cover is placed between the compression frames of the first electrochemical cell and the second electrochemical cell in the repeating unit, and the cover has an inlet pipe, an outlet pipe and a ventilation pipe, a first side and a second side, the inlet pipe is used to feed wastewater or anode solution into the shell formed by the cover, the outlet pipe is used to remove the reaction products formed at the anode catalyst layer from the shell formed by the cover, and the ventilation pipe is used to remove gas from the shell formed by the cover, the first side and the second side are opposite to each other, and each side is provided with an opening to allow wastewater or anode solution to enter the anode catalyst layer of two adjacent cells in the repeating unit.

In embodiments where the cover is provided with two opposing sides, each side having an opening, a seal is placed between each side of the cover and the compression frame immediately adjacent that side.

In all presented embodiments, the solid polymer electrolyte membrane may be an anionic solid polymer electrolyte or a cationic solid polymer electrolyte, and the cover is made of a non-conductive material.

The invention also relates to a system for treating wastewater, comprising at least one stack of electrochemical cells as described herein, the stack being immersed in a reactor tank containing wastewater or a cathode solution. The stacks can be connected in series or in parallel.

Also described is a method for wastewater treatment comprising the steps of:

a. providing at least one stack of electrochemical cells, the stack comprising a cover attached to a compression frame on the anode side of at least one electrochemical cell in the stack, the cover having one side with an opening facing the anode catalyst layer side of the electrochemical cell, the stack being immersed in a reactor tank containing wastewater to be treated,

b. supplying the anode solution to the housing formed by the cover and the compression frame,

c. providing a voltage across the electrochemical cell, and

d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater.

Also described is a method for wastewater treatment comprising the steps of:

a. providing at least one stack of electrochemical cells, the stack comprising a cover attached to a compression frame on the anode side of at least one electrochemical cell in the stack, the cover having one side with an opening facing the anode catalyst layer side of the electrochemical cell, the stack being immersed in a reactor tank containing a cathode solution,

b. supplying the wastewater to be treated to the housing formed by the hood and the compression frame,

c. providing a voltage across the electrochemical cell, and

d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater.

Another method for wastewater treatment according to another embodiment of the present invention comprises the following steps:

a. providing at least one stack of electrochemical cells, the stack comprising a cover attached to a compression frame on the cathode side of at least one electrochemical cell in the stack, the cover having one side with an opening facing the cathode catalyst layer side of the electrochemical cell, the stack being immersed in a reactor tank containing wastewater to be treated,

b. supplying the cathode solution to the housing formed by the cover, which is attached to the compression frame,

c. providing a voltage across the electrochemical cell, and

d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater.

Another method for wastewater treatment is disclosed, comprising the steps of:

a. providing at least one repeating unit comprising two electrochemical cells and a cover placed between compression frames of two adjacent electrochemical cells of the repeating unit, the cover having an opening on each of its two opposite sides, each opening facing the cathode catalyst layer side of one of the two adjacent cells, the stack being immersed in a reactor tank containing wastewater to be treated,

b. supplying the cathode solution to the housing formed by the cover and the compression frame of two adjacent cells,

c. providing a voltage across the electrochemical cell of the repeating unit, and

d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater.

A method for wastewater treatment is disclosed, comprising the steps of:

a. providing at least one repeating unit comprising two electrochemical cells and a cover placed between compression frames of two adjacent electrochemical cells of the repeating unit, the cover having an opening on each of its two opposite sides, each opening facing the anode catalyst layer side of one of the two adjacent cells, the stack being immersed in a reactor tank containing wastewater to be treated,

b. supplying the anode solution to the housing formed by the cover and the compression frame of two adjacent cells,

c. providing a voltage across the electrochemical cell of the repeating unit, and

d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater.

A method for wastewater treatment is disclosed, comprising the steps of:

a. providing at least one repeating unit comprising two electrochemical cells and a cover placed between compression frames of two adjacent electrochemical cells, the cover having an opening on each of its two opposite sides, each opening facing the anode catalyst layer side of one of the two adjacent cells, the stack being immersed in a reactor tank containing a cathode solution,

b. supplying wastewater to a housing formed by the cover and two compression frames of two adjacent cells of a repeating unit,

c. providing a voltage across the electrochemical cell, and

d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate specific preferred embodiments of the invention, but should not be considered as limiting the spirit or scope of the invention in any way.

FIG1 illustrates an exploded view of an electrochemical cell used in the present system for wastewater treatment.

2 illustrates a schematic diagram of a stack of electrochemical cells including a cover positioned between the cathodes of two adjacent cells in the stack in accordance with the present system.

FIG. 3 illustrates an exploded view of a module for wastewater treatment comprising a reactor tank and the stack of electrochemical cells illustrated in FIG. 2 .

FIG4 illustrates a schematic diagram of a stack of electrochemical cells according to a first embodiment of the present invention.

FIG. 5 illustrates an exploded view of a cover according to the present invention that may be placed between two adjacent batteries.

6 illustrates a schematic diagram of a stack of electrochemical cells according to the present invention, the electrochemical cells comprising a cover placed between the anodes of two adjacent cells in the stack, the stack being immersed in wastewater in a reactor tank.

7 illustrates a schematic diagram of a stack of electrochemical cells comprising a cover placed between the anodes of two adjacent cells in the stack, the stack being immersed in a cathode solution in a reactor tank, with wastewater circulating through the enclosure formed by the cover and the two adjacent cells.

8 illustrates a schematic diagram of a stack of electrochemical cells having a cover placed on the cathode side of each of the cells in the stack.

FIG. 9 illustrates an exploded view of a cover that may be attached to one of the electrochemical cells in the stack as illustrated in FIG. 8 .

Details

Certain terms are used in this description and are intended to be interpreted according to the definitions provided below. In addition, terms such as "a" and "comprise" should be considered open-ended. In addition, all U.S. patent publications and other references cited herein are intended to be incorporated by reference in their entirety.

SPE herein stands for Solid Polymer Electrolyte and may be any suitable ion-conducting ionomer (anionic or cationic, organic or inorganic form), such as Thus, an SPE electrochemical cell is a cell comprising an SPE as an electrolyte to which electrical energy is supplied to achieve the desired electrochemical reaction (wherein a positive voltage is applied to the anode of the cell).

An exemplary electrochemical cell for wastewater treatment used in the present system is illustrated in an exploded view of FIG1 . The electrochemical cell 100 includes a catalyst coated membrane 102 (CCM) consisting of a solid polymer electrolyte membrane 104 coated with a catalyst layer 106 on each of its two sides. Only one catalyst layer 106 on a first side of the membrane is shown in FIG1 , for example, this can be an anode catalyst layer, but those skilled in the art will readily appreciate that the opposite side of the membrane is also coated with a catalyst layer, which in this embodiment will be a cathode catalyst layer and can have substantially the same area as the anode catalyst layer. In this context, in the present disclosure, the anode active area of an electrochemical cell is defined as the area of the membrane (or of the open mesh as described further below in an alternative embodiment) coated with a catalyst layer on the anode side, and the cathode active area is defined as the area of the membrane (or of the open mesh) coated with a catalyst layer on the cathode side. In the illustrated embodiment, the solid polymer electrolyte membrane 104 is provided with holes 105 that allow fasteners 122 to penetrate the membrane during assembly of the electrochemical cell, as described further below. In an alternative embodiment, the solid polymer electrolyte membrane 104 is not pre-formed with holes, and in this case, the fasteners penetrate the membrane during the assembly process of the electrochemical cell. The electrochemical cell also includes an open mesh 108 and an open mesh 110 placed adjacent to the catalyst coated membrane 102 on each side of the CCM, and a compression frame 112 and a compression frame 114 placed adjacent to the open mesh 108 and the open mesh 110, respectively. The open mesh 108 and the open mesh 110 are meshes provided with open holes to allow for a relatively large porosity of the mesh, and they are also provided with holes 116 that allow fasteners 122 to penetrate during assembly of the electrochemical cell. The area of each of the open mesh 108 and the open mesh 110 is substantially the same as the anode active area of the electrochemical cell and the cathode active area, which is the catalyst coated area of the membrane. An area 128 at the periphery of the CCM (102) is not coated with catalyst along its periphery and has an electrical isolation function.

In the illustrated embodiment, compression frames 112 and 114, each having a rectangular shape with four sides, are each provided with compression arms 118 that are connected to each other at connection points 120 and extend within the area between the four sides of the compression frame. Holes 119 are provided in the compression frames at the connection points 120 to allow fasteners 122 to penetrate during assembly of the electrochemical cell. The connection points are distributed within the area between the four sides of each compression frame. Compression frames 112 and 114 are provided with leads 130 for electrical connection to a power source, typically a DC power source. Those skilled in the art will appreciate that compression frames 112 and 114 can have shapes other than the rectangular shape illustrated in the accompanying drawings of the present invention, and that the compression arms 118 and connection points 120 for each compression frame are distributed within the area defined by the perimeter of the compression frame. In the case of rectangular compression frames, the perimeter of the frame is defined by the sides of the frame.

In Figures 1 to 3, the fastener 122 is illustrated as a threaded bolt that cooperates with the nut 126 to ensure the required compressive force, but those skilled in the art will readily understand that any other fastener, such as a rivet, may be used to provide the compressive force exerted by the compression frame on the open-cell grid and on the CCM, and such a fastener may not require any additional elements such as the nut 126 for ensuring the required compressive force.

The SPE membrane 104 provides a reduced gap between the electrodes (catalyst layers on the anode and cathode sides of the membrane). In the present invention, there is no gas diffusion layer to support the catalyst layer, and the electrode only includes an anode catalyst layer and a cathode catalyst layer 106. In this embodiment, each catalyst layer is deposited on one side of the membrane, which also contributes to lower operating costs. The open grid 108 and the open grid 110 provide local current collection, and due to their relatively high porosity, allow contaminated water and treated water to easily enter and leave the reaction site on the catalyst layer, and are easy to remove the gas formed next to the catalyst layer. The compression frame 112 and the compression frame 114 allow the peripheral current collection of the open grid 108 and the open grid 110, and mainly due to the distribution of the compression arms and the distribution of the corresponding connection parts, the compression arms 118 of the open grid span the entire anode active area and the corresponding cathode active area to achieve a substantially uniform compression of the open grid, membrane and catalyst layer. For example, the compression frame 112 and the compression frame 114 are made of a conductive metal or ceramic with a thickness of 0.5mm to 5mm. Those skilled in the art will appreciate that the number of connection sites and aspect ratio of the compression frame can be varied and can be configured to allow for substantially uniform compression of the open cell grid and CCM, and to accommodate different sizes of commercially available solid polymer membranes.

For the above purposes, open mesh 108 and open mesh 110 have a relatively high porosity. In the context of the present invention, porosity is defined as the ratio between the open area and the volume of the mesh. Types of meshes that can be used include, but are not limited to, sintered titanium fiber mesh supplied by Bekaert having a mesh thickness between 250 microns and 550 microns, a fiber diameter between 22 microns and 50 microns, and a porosity of 50% to 85%, and expanded metal mesh supplied by Dexmet having a mesh thickness between 10 microns and 5,000 microns, a line width between 0.04 inches and 0.055 inches, and a porosity between 30% and 95%, with approximately 33 to 493 openings per square inch and wherein the diamond-shaped openings have a size for LWD (long side of the diamond) between 0.075 inches and 0.289 inches and a size for SWD (short side of the diamond), where LWD and SWD are the dimensions of the diagonals of the diamond-shaped openings, as explained, for example, on the supplier's website. Preferably, the open-cell mesh is made of a conductive metal or ceramic and has a thickness between 10 microns and 5,000 microns and a porosity between about 30% and 95%.

The electrochemical cell is assembled by compressing the CCM 102 between the open-cell grid 108 and the open-cell grid 110 and between the compression frame 112 and the compression frame 114 using fasteners 122, which pass through holes 119 provided in the compression arms 118 at the connection 120, through holes 116 provided in the open-cell grid 108 and the open-cell grid 110, through the catalyst layer 106, and through holes 105 provided in the solid polymer electrolyte membrane 104. When the solid polymer electrolyte membrane 104 does not include any holes, the fasteners 122 can penetrate the membrane directly when the electrochemical cell is assembled. The fasteners 122 can be provided with washers 124 that spread the compressive force from the fastener to the compression arms 118, or alternatively can have a shape that allows for the spread of the compressive force.

Fasteners 122, washers 124, and nuts 126 are made of non-conductive materials. In the electrochemical cell of the present invention, fasteners 122 penetrate the connection site, the open-pore grid, and the CCM to ensure that the compressive force is substantially evenly distributed across the entire active area of the electrochemical cell and to maintain a reduced gap between the electrodes. This is different from the compression systems described in the prior art, in which compression of the electrochemical cell is achieved solely by peripheral compression of the frame via spring-loaded bolts to prevent any compression device from penetrating the SPE, more specifically the SPE membrane.

The stacking of the electrochemical cells used in this system is illustrated in Figure 2. Stacking 200 includes a plurality of electrochemical cells 100 having the same configuration as that illustrated in Figure 1 described above. The cells are connected to each other by at least one rod 202, which provides the required interval between the individual electrochemical cells 100. The illustrated stacking includes 6 electrochemical cells, but it will be readily understood by those skilled in the art that stacking according to the present invention can include more electrochemical cells, or for some very small-scale applications, less than 6 electrochemical cells. In a preferred embodiment, a stacking includes 50 cells, but the stacking can include up to about 500 individual electrochemical cells. A cover 260 is placed between adjacent cells 100 to isolate the cathodes of two adjacent cells from the solution in the reactor tank, as further explained below and illustrated in Figure 4.

As further illustrated in an embodiment of the present invention, when assembled into a stack, the electrochemical cells can be arranged so that the anode side of one electrochemical cell faces the cathode side of an adjacent cell, or so that the cathode side of one electrochemical cell faces the cathode side of an adjacent cell and the anode side of one electrochemical cell faces the anode side of an adjacent electrochemical cell.

The module 300 for wastewater treatment includes a stack 200 of electrochemical cells immersed in a reactor tank, as illustrated in the exploded view in Figure 3. Stack 200 is housed in a reactor tank 302 so that one electrode (anode or cathode, as described below) of each of the electrochemical cells in the stack is directly exposed to wastewater and pollutants or exposed to the solution contained in the reactor tank, while the other electrodes are isolated from the wastewater or solution in the reactor tank by a cover. Module 300 also includes an outer cover 304 provided with a feed port (feed port) (not shown) and an exhaust port 314 and an inner cover 306 also provided with a feed port (not shown) and an exhaust port 316, both inner and outer covers covering the reactor tank 302 on the upper portion thereof to accommodate wastewater and stack 200 and control emissions from the module. Module 300 is also provided with a liquid level sensor 308 to ensure that stacking operation is stopped when the water level is below a desired threshold value, which provides protection for membrane and electrode systems from resistance burnout and uneven hydration. Within the reactor tank, a level sensor 308 for monitoring the water level within the tank is housed within a pipe 310. The module 300 is also provided with a level switch 312 for stopping the flow of wastewater into the reactor tank when the liquid level in the tank reaches a predetermined level.

The covers placed between two adjacent batteries are only schematically illustrated in Figures 2 and 3, and a person skilled in the art will readily understand that each cover may include additional elements, such as inlet and outlet ducts and ventilation ducts, as further described below and illustrated in Figures 5 and 9.

In a schematic review of the reactions occurring at individual electrochemical cell levels in a stack of electrochemical cells from the prior art, the electrochemical cell has both an anode and a cathode that are directly exposed to wastewater, more specifically, wastewater containing ammonia, the electrochemical oxidation processes on the anode fall into the categories of direct, indirect surface-mediated, and indirect secondary oxidant-mediated oxidation, where the specific reaction depends on the type of solid polymer electrolyte (SPE) used, the choice of catalyst, and the composition of the wastewater solution. Positive charge carriers are transferred using cationic SPEs, while negative charge carriers are transferred using anionic SPEs. On the anode side, when the contaminated wastewater is exposed to the anode catalyst layer, a stepwise oxidation process occurs, involving direct, indirect surface-mediated, or indirect secondary oxidant-mediated oxidation, as shown in Equations 1 to 3 for cationic SPEs and in Equations 6 and 7 for anionic SPEs, respectively.

For cells based on cationic SPEs, when wastewater (e.g., wastewater with ammonia contaminants) is exposed to the anode catalyst layer, a stepwise oxidation process occurs at the anode, involving direct oxidation as shown in Equation 1, or indirect oxidation as shown in Equations 2(a) and 2(b), or as shown in Equations 3(a) and 3(b):

Formula 1: Direct oxidation of ammonia (anode half reaction):

Equation 2: Indirect oxidation of ammonia via (a) generation of hydroxyl surface species from water and (b) oxidation of ammonia via the surface hydroxyl species (anodic half-reaction):

Equation 3: Indirect secondary oxidant-mediated oxidation of ammonia via (a) production of hypochlorite species from NaCl and (b) indirect oxidation of ammonia via hypochlorite (anodic half-reaction):

For a cationic SPE-based electrochemical cell in which the anodic half-reaction is shown in Equation 1 or Equation 2, the cathode reaction involves the direct production of hydrogen gas from protons transported across the SPE, as shown in Equation 4:

For a cationic SPE-based electrochemical cell in which the anode half-reaction is shown in Equation 3, the cathode reaction involves the direct production of sodium hydroxide via the transport of sodium ions across the SPE, as shown in Equation 5(a). The sodium hydroxide then undergoes a subsequent reaction in solution with the products of the anode reaction to reform salt and water, as shown in Equation 5(b).

Formula 5:

Alternatively, for anionic SPE-based electrochemical cells, when wastewater (in this case ammonia contaminants) is exposed to the anode catalyst layer, a stepwise indirect oxidation process occurs at the anode involving hydroxyl surface species or hypochlorite, as shown in Equations 6 and 7, respectively:

Equation 6: Indirect oxidation of ammonia via surface hydroxyl species (anodic half-reaction):

Equation 7: Indirect oxidation of ammonia (anodic half-reaction) via (a) production of hypochlorite species from Cl ions transported across the SPE and (b) indirect oxidation of ammonia via hypochlorite:

For an anionic SPE-based electrochemical cell in which the anodic half-reaction is shown in Equation 6, the cathodic reaction involves the production of hydroxyl charge carriers and hydrogen gas from water, as shown in Equation 8:

For an anionic SPE-based electrochemical cell in which the anodic half-reaction is shown in Equation 7, the cathodic reaction involves the production of chloride ion charge carriers and hydrogen gas from NaCl and water, as shown in Equation 9:

The reactions shown in Equations 1 to 3, and correspondingly in Equations 6 and 7, are anode half-reactions, and as those skilled in the art will appreciate, in many cases, there may be many intermediate steps in the reaction, and therefore, many intermediate substances. However, such intermediate substances are generally oxidized into final products, which typically include carbon-containing pollutants such as CO ₂ , nitrogen-containing pollutants such as N ₂ , and sulfur-containing pollutants such as SO _x .

At the cathode, pollutants may also be reduced when in contact with the cathode catalyst layer, and such reduction reactions may also contribute to the gradual removal of wastewater pollutants and the oxidation of intermediate compounds formed at the anode.

As described in the prior art, in a system of a stack of electrochemical cells in which both the anode and cathode are directly exposed to wastewater in a reactor tank, some of the intermediate substances produced at the anode or present as background species in the wastewater are not fully oxidized and can be carried by the wastewater and can reach the cathode, poisoning it. For example, background organic substances can polymerize on the cathode, and/or trace metals in the solution can be electrodeposited on the cathode, which prevents the cathode from performing the desired electrochemical reactions shown in Equations 5(a), 5(b), 8, and 9.

In addition, in the case of an indirect ammonia oxidation process, the wastewater reduction process occurring at the cathode can electrochemically reduce intermediate ammonia oxidation products from the overall ammonia oxidation reaction, which can mix with the wastewater in the tank, thereby reaching the cathode and reducing the efficiency of the system in removing ammonia from the wastewater.

During the indirect oxidation of ammonia, salt (NaCl) is added to the contaminated wastewater to be oxidized at the anode and used to generate hypochlorite species in situ that can oxidize the ammonia contaminants in the wastewater. The salt from the wastewater can mix with the decontaminated water in the tank and reach the pipes that transport the clean water from the tank to a selected discharge location, such as a municipal wastewater treatment plant, which can increase pipe corrosion.

The present invention solves all the drawbacks presented above and thereby increases the removal rate of the system, whereby the present invention describes a system in which the anode or respectively the cathode of at least one of the electrochemical cells in the stack is isolated from the solution in the tank.

A first embodiment of the present system is schematically illustrated in Figure 4. The system is illustrated as having a stack comprising a repeating unit 400 comprising two electrochemical cells 410 and 420 with the cathode sides "C" of the electrochemical cells facing each other, and one skilled in the art will understand that a stack according to the present invention may comprise more repeating units 400 assembled together by at least one rod 402 such that each anode of the electrochemical cells in one repeating unit 400 faces the anode of one of the electrochemical cells in the repeating unit positioned immediately adjacent to the repeating unit.

Each of the two electrochemical cells 410 and 420, respectively, includes a catalyst coated membrane (CCM) consisting of a solid polymer electrolyte membrane 411 and a solid polymer electrolyte membrane 421 coated with an anode catalyst layer 412 and a corresponding anode catalyst layer 422 on one side and a cathode catalyst layer 413 and a corresponding cathode catalyst layer 423 on the other side. Each electrochemical cell also includes an open mesh 414 and an open mesh 415, and an open mesh 424 and an open mesh 425, respectively, which are placed on each side of the catalyst coated membrane adjacent to the anode catalyst layer and the corresponding cathode catalyst layer; and a compression frame 416 and a compression frame 417, and a compression frame 426 and a compression frame 427, respectively, which are each placed adjacent to the open mesh. As described with respect to 2 and further illustrated in FIG4, the cells are connected to each other by rods 402, which provide a certain distance between the individual cells 410 and 420. Even though the assembly is illustrated in FIG. 4 as having two rods 402 , one skilled in the art will understand that at least one rod 402 is required for positioning the electrochemical cells in the stack with the desired spacing therebetween.

The cover 460 is placed between the two electrochemical cells 410 and 420, connecting the compression frame 417 and the compression frame 427, each of which is placed adjacent to the open mesh on the cathode side of each of the two adjacent electrochemical cells 410 and 420. The cover forms an impermeable shell 490 for wastewater between the cathodes of these electrochemical cells and thereby isolates the cathodes from the wastewater in the tank. The schematic diagram of the assembly in Figure 4 shows gaps between the two compression frames 416 and 417 and the solid polymer membrane 411, and between the compression frames 426 and 427 and the solid polymer membrane 421, respectively, and those skilled in the art will understand that because the size of the catalyst layer and the size of the open mesh along the X axis are very small (e.g., less than 5 mm), and because the compression system compresses these elements together, there are very small gaps between the compression frames 416 and 417 and the membrane 411, and between the compression frames 426 and 427 and the membrane 421, respectively. Portions of membrane 411 and membrane 421 extend beyond the active area (catalyst coated area of the membrane) and are placed between compression frames to electrically isolate them from each other.

The entire stack formed by repeating units 400 is submerged in a tank 430 containing contaminated wastewater 450 .

The cathode solution 470 is fed to the housing 490, and the product 480 of the reaction occurring on the cathode side is removed from the housing. During the ammonia removal process, a solution 470 such as a salt (NaCl) and water is fed to the housing 490. Chloride ions ( ^Cl- ) are transferred to the anode side by the anion exchange membrane to form hypochlorous acid (HClO), which is also used for ammonia oxidation and removed from wastewater. The product 480 of the reaction occurring on the cathode side (including, for example, NaOH) is flushed out of the housing. A venting hole 475 is provided to allow the elimination of gases (such as hydrogen) produced during the reaction occurring on the cathode side.

A three-dimensional view of the cover 460 is illustrated in FIG5 . The cover includes a body 466 having two opposing sides 461 and 462 that, in the assembled stack of the embodiment illustrated in FIG4 , are positioned adjacent to the compression frame facing the cathode side of each of two adjacent electrochemical cells. Each of the sides 461 and, respectively, the sides 462 is provided with an opening 467 to allow cathode solution introduced through an inlet conduit 464 to enter the cathode side of the electrochemical cells in the assembled stack, and is provided with a seal 463 that prevents wastewater from leaking from the reactor tank into the housing 490 formed by the cover and the compression frames of the adjacent cells in the assembled stack, and prevents cathode solution from leaking from the housing 490 into the reactor tank. The cover is provided with an inlet conduit 464 through which cathode solution 470 can flow into the housing, and an outlet conduit 465 through which products 480 of the reaction occurring at the cathode catalyst can flow out of the housing. A ventilation duct 476 is provided to allow gas to be vented from the housing 490 to the atmosphere or to a gas containment tank through the vent hole 475. The inlet duct 464, the outlet duct 465, and the ventilation duct 476 can be threaded into the body of the hood, and they can be molded with the body of the hood or welded to the body of the hood. It should be understood that the location of the inlet duct and the outlet duct on the hood can vary according to the stacking design of different embodiments of the present invention, but generally the hood is provided with an inlet duct, an outlet duct, and a ventilation duct.

The electrochemical cell illustrated in Figure 4 has a similar construction to that described in Applicant's co-pending application 62/279,631 and illustrated in Figure 1. This applies to all embodiments of the present invention.

The electrochemical cells in the stack illustrated in FIG4 are arranged so that the cathodes of two adjacent electrochemical cells face each other and are exposed to the housing 490 formed by the cover 460, thereby isolating them from the wastewater solution in the tank. In yet another embodiment of the present invention, schematically illustrated in FIG6, the electrochemical cells are assembled in a stack with the anode sides "A" of two adjacent electrochemical cells 510 and 520 facing each other, so that the catalyst layers on the anode sides of electrochemical cells 510 and 520 are exposed to the housing 590 formed by the cover 560 and the compression frame of the two adjacent cells, thereby isolating them from the wastewater solution 550 in the tank. FIG6 does not show further details of the construction of each electrochemical cell or the construction of the cover, but it should be understood that the electrochemical cell in this embodiment has the same construction as the electrochemical cell illustrated in FIG1 to FIG5, more specifically, the electrochemical cell includes a catalyst coated membrane placed between two open-cell meshes and two compression frames, and fasteners protruding through the assembly formed by the CCM, the meshes, and the compression frame to ensure stack compression. The cover 560 has a similar construction to the cover illustrated in FIG. 5 .

In the embodiment illustrated in Figure 6, a stack comprising at least one repeating unit 500 is immersed in wastewater 550 within a reactor tank 530. A cover 560 is placed between the compression frames of two adjacent electrochemical cells 510 and 520 located on the anode ("A") side of each electrochemical cell. The cover 560 forms a housing 590 that isolates the anodes (anode catalyst layers) of the two electrochemical cells from the wastewater in the reactor tank. In this embodiment, pollutants in the wastewater are reduced on the cathode ("C") side of the electrochemical cell, and an anode solution 570 comprising, for example, _H2O and, in some cases, an electrolyte such as NaOH or _H2SO4 is introduced into the housing 590 to reach the _anodes of the electrochemical cells 510 and 520. On the anode side, water is electrolyzed to form protons that cross to the cathode side. On the cathode side, the protons participate in the electroreduction of wastewater compounds (such as nitrates, nitrites, or urea). Reaction products 580 flow out of the housing 590. Reactive gases (O ₂ , N ₂ , etc.) are exhausted out of the enclosure through vents 575 .

In the embodiment illustrated in FIG7 , a stack comprising at least one repeating unit 600 is immersed in a cathode solution 650 within a reactor tank 630. A cover 660 is placed between the compression frames (not shown) of two adjacent cells 610 and 620 on the anode (“A”) side of the cells to create an enclosure 690 that isolates the anodes (anode catalyst layers) of the two adjacent electrochemical cells from the solution in the reactor tank. A stream of wastewater 670 is introduced into the enclosure 690, and the wastewater is oxidized at the anodes of the electrochemical cells so that contaminants are removed, and a stream of clean water 680 flows out of the enclosure 690. Vents 675 are provided to allow for the elimination of gases produced during the reaction occurring on the anode side. The construction of the embodiment illustrated in FIG7 is the same as that of the embodiment illustrated in FIG6 , with the only difference being that the solution in the reactor tank in FIG7 is a cathode solution rather than wastewater, and that wastewater, rather than an anode solution, circulates through the enclosure 690.

The reaction that occurs at the anode and cathode of electrochemical cell 610 and electrochemical cell 620 is similar to the reaction that occurs in the embodiment illustrated in Fig. 4.For ammonia removal process, for example, the cathode solution 650 in the tank comprises salt (NaCl) and water. Chloride ion ^Cl- is transferred to the anode side by anion exchange membrane, forms hypochlorous acid (HClO), and on the anode side, hypochlorous acid is also used for ammonia oxidation and is removed from wastewater. The product (comprising such as NaOH) of the reaction that occurs on the cathode side remains in the tank. The hydrogen formed during the reaction that occurs on the cathode side is discharged or captured from the top of reactor tank.

Another embodiment of the present invention is illustrated in FIG8 , which shows a stack 700 including an electrochemical cell 710 and an electrochemical cell 720 having a configuration similar to the electrochemical cell illustrated in FIG1 . It will be understood by those skilled in the art that a stack can include only one electrochemical cell or more batteries as needed. If the stack includes more than one electrochemical cell, the electrochemical cell is preferably arranged so that the anode side A of one electrochemical cell faces the cathode side C of the adjacent electrochemical cell in the stack, but other arrangements are also possible, whereby the anode of one electrochemical cell faces the anode of the adjacent cell, and the cathode of one electrochemical cell faces the cathode of the adjacent cell. Stack 700 is immersed in wastewater 750 within reactor tank 730. Cover 760A and corresponding cover 760B are attached to the compression frame (not shown) on the cathode side of electrochemical cell 710 and corresponding electrochemical cell 720. The cathode solution 770 is fed into the housing 790A and housing 790B formed between the cover 760A and the cover 760B respectively and the compression frame of the electrochemical cell placed on the cathode side, and the reaction product stream 780A and reaction product stream 780B formed by the reaction occurring at the cathode are removed from the housing.

Pollutants in the wastewater are removed by the oxidation reaction occurring on the anode catalyst, and intermediate products formed on the cathode side that cross the membrane to the anode side also contribute to the removal of pollutants, similar to the process described, for example, with respect to the ammonia oxidation occurring at the anode in the case of the embodiment illustrated in FIG4. Although not shown in the schematic diagram of FIG8, the covers 760A, 760B are provided with vents to allow gas to be discharged from the housing 790A and, respectively, the housing 790B, similar to the vents 775 illustrated in FIG9.

In another embodiment of the present invention, not shown, a stack of electrochemical cells in which the anode of one electrochemical cell faces the cathode of an adjacent cell comprises at least one electrochemical cell having a cover attached to its compression frame on the anode side, or preferably comprises more or all of the electrochemical cells in the stack, each having a cover attached to its respective compression frame on the anode side. Anode solution or wastewater can be supplied to the housing thus formed in a similar manner to that described with respect to the embodiments shown in Figures 6 and 7.

In the embodiment illustrated in Figure 8, the cover 760A and cover 760B for isolating the cathode of the electrochemical cell have a different construction from the cover 460 illustrated in Figure 5. Such a cover is illustrated as the cover 760 in Figure 9. The cover 760 has a main body 766, and the main body 766 has only one open side (open side) 761, and the open side 761 is provided with an opening 767. In the assembled stack of the embodiment illustrated in Figure 8, the opening 767 is placed adjacent to the compression frame of the cathode side facing the electrochemical cell. The open side 761 is provided with a seal 763, which prevents wastewater from leaking from the reactor tank into the shell 790 formed by the compression frame placed on the cathode side of the cover and the electrochemical cell, and prevents cathode solution from leaking from the shell 790 into the reactor tank. The side 762 opposite to the side 761 is a continuous panel without any openings and prevents any wastewater from the reactor tank from reaching the shell 790. The housing is provided with an inlet conduit 764 through which the cathode solution can flow into the housing, and an outlet conduit 765 through which the products of the reaction occurring at the cathode can flow out of the housing. A vent conduit 776 is provided to allow gas to be exhausted from the housing 790 through the vent hole 775 to the atmosphere or to a gas holding tank.

The wastewater treatment system may include more than one module 300 illustrated in FIG3 . Contaminated wastewater is stored in a holding tank from which it is pumped to a reactor tank of module 300 where it is treated for removal of pollutants. In some embodiments, the reactor tank may include a recirculation pump or a stirring mechanism or may use product gas to help mix the wastewater within the tank. If the system includes more than one module 300, the stacks in the modules 300 may be connected in series or in parallel, as illustrated, for example, in applicant's commonly owned U.S. patent application Ser. No. 14/648,414.

In the embodiments provided herein, each electrochemical cell can include a catalyst coated membrane (CCM) as illustrated in Figure 1. In alternative embodiments, the anode catalyst and the cathode catalyst can be deposited, for example, on the side of the open mesh that faces the membrane when the electrochemical cells are assembled together. Additionally, in other embodiments, when the electrochemical cells are assembled together, the anode catalyst can be deposited on one side of the membrane and the cathode catalyst can be deposited on the side of the open mesh that faces the other side of the membrane; or when the electrochemical cells are assembled together, the cathode catalyst can be deposited on one side of the membrane and the anode catalyst can be deposited on the side of the open mesh that faces the other side of the membrane. In all embodiments, there is a reduced gap between the electrodes (the catalyst layer is placed adjacent to or deposited on the anode side and corresponding cathode side of the membrane), which gap is fixed by the uniform compression system of the system.

In all embodiments of the present invention, "electrode" is understood to be a catalyst layer, because the electrode does not include a gas diffusion layer, so that the anode is in fact an anode catalyst layer, and the cathode is a cathode catalyst layer. Anode catalyst and cathode catalyst can include a variety of catalyst materials, including but not limited to platinum, platinum-derived alloys comprising iridium, ruthenium, rhodium, palladium, cobalt, nickel, iron and iron alloys, copper and copper alloys, mixed metal oxides, diamond, and ceramic-derived catalysts. As known in the art, the use of supported catalysts can improve the dispersion of catalytic materials and therefore improve utilization, and the interaction between certain catalysts and supports can improve catalytic activity and durability. The example of a catalyst support that can be used in combination with the list of catalyst materials in the present invention includes titanium, niobium, nickel, iron, graphite, mixed metal oxides and ceramics. Anode catalyst and cathode catalyst can also include stainless steel or graphite.

The covers described in this invention are made of a non-conductive material, such as, for example, polytetrafluoroethylene (PTFE) or PVDF, or other such plastic materials.

The advantages of the electrochemical cell of the present invention for wastewater treatment and the method of operating the electrochemical cell of the present invention are numerous compared to solutions from the prior art. These advantages include (1) isolation of the cathode electrode from the wastewater being treated to prevent scaling and poisoning of the cathode during operation, (2) combination with anionic SPE materials, which allows the use of a closed loop of high-concentration brine at the cathode to generate hypochlorous acid on demand at the anode, and (3) isolation of the anode to allow the cathode to pre-treat the wastewater by reducing, for example, urea to ammonia, where the product ammonia then flows through the anode housing for oxidation.

The disclosure of U.S. Provisional Patent Application Serial No. 62/465,448, filed March 1, 2017, is incorporated herein in its entirety.

While particular elements, embodiments, and applications of the present invention have been shown and described, it will, of course, be understood that the invention is not limited thereto, as modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.

Claims (21)

1. A stack of electrochemical cells for wastewater treatment, the stack comprising at least one electrochemical cell, the electrochemical cell comprising: a. Solid polymer electrolyte membrane; b. An anode catalyst layer and a cathode catalyst layer, wherein the anode catalyst layer is adjacent to a first side of the solid polymer electrolyte membrane, and the cathode catalyst layer is adjacent to a second side of the solid polymer electrolyte membrane opposite to the first side; c. A first open-cell grid and a second open-cell grid, wherein the first open-cell grid is adjacent to the anode catalyst layer and the second open-cell grid is adjacent to the cathode catalyst layer; d. A first compression frame and a second compression frame, the first compression frame being adjacent to the first perforated grid and the second compression frame being adjacent to the second perforated grid, each of the compression frames having a compression arm extending in an area defined by the periphery of the frame, the compression arms being connected to each other at a connection point. e. A fastener that passes through holes in the compression arms of the first and second compression frames at the connection point, through holes in the first and second open grids, and protrudes through the solid polymer electrolyte membrane and the anode catalyst layer and the cathode catalyst layer. The cover is attached to the first compression frame or the second compression frame to form a housing for isolating the anode catalyst layer or the cathode catalyst layer. 2. The stack of claim 1, wherein the shroud has an inlet pipe, a side, an outlet pipe, and a ventilation pipe, the inlet pipe for feeding wastewater, a cathode solution, or an anolyte into the housing formed by the shroud, the side being provided with an opening for allowing the wastewater, the anolyte, or the cathode solution to enter the anode catalyst layer or the cathode catalyst layer of the electrochemical cell in the stack, the outlet pipe for removing reaction products formed at the anode catalyst or the cathode catalyst from the housing formed by the shroud, and the ventilation pipe for removing gases from the housing formed by the shroud. 3. The stack of claim 2, further comprising a seal between the side of the cover having the opening and the compression frame immediately adjacent to the side. 4. The stack as claimed in claim 1, wherein the cover is made of a non-conductive material. 5. The stack of claim 1, comprising a plurality of electrochemical cells connected by a rod, the rod holding one of the electrochemical cells in the stack at a distance from an adjacent electrochemical cell. 6. A stack of electrochemical cells for wastewater treatment, the stack comprising at least one repeating unit, the repeating unit comprising two electrochemical cells, each electrochemical cell comprising: a. Solid polymer electrolyte membrane; b. An anode catalyst layer and a cathode catalyst layer, wherein the anode catalyst layer is adjacent to a first side of the solid polymer electrolyte membrane, and the cathode catalyst layer is adjacent to a second side of the solid polymer electrolyte membrane opposite to the first side; c. A first open-cell grid and a second open-cell grid, wherein the first open-cell grid is adjacent to the anode catalyst layer and the second open-cell grid is adjacent to the cathode catalyst layer; d. A first compression frame and a second compression frame, the first compression frame being adjacent to the first perforated grid and the second compression frame being adjacent to the second perforated grid, each of the compression frames having a compression arm extending in an area defined by the periphery of the frame, the compression arms being connected to each other at a connection point. e. A fastener that passes through holes in the compression arms of the first and second compression frames at the connection point, through holes in the first and second open grids, and protrudes through the solid polymer electrolyte membrane and the anode catalyst layer and the cathode catalyst layer. The stack also includes: - At least one rod for connecting the electrochemical cells in the stack and holding a first electrochemical cell in the repeating unit at a distance from a second electrochemical cell in the repeating unit, such that the anode side of the first electrochemical cell faces the anode side of the second electrochemical cell, or such that the cathode side of the first electrochemical cell faces the cathode side of the second electrochemical cell; and A cover is placed between the compressed frames of the two electrochemical cells of the repeating unit, forming a shell spanning the distance between the two electrochemical cells of the repeating unit, thereby isolating the anode catalyst layer or the cathode catalyst layer of the two electrochemical cells of the repeating unit from the solution in the reactor tank. 7. The stack of claim 6, wherein the shroud has an inlet pipe, an outlet pipe, a ventilation pipe, a first side and a second side, the inlet pipe for feeding an anolyte, wastewater or a cathode solution into the housing formed by the shroud, the outlet pipe for removing reaction products formed at the anolyte catalyst or the cathode catalyst from the housing formed by the shroud, the ventilation pipe for removing gases from the housing formed by the shroud, the first side and the second side being opposite to each other, each side having an opening to allow the anolyte solution or the wastewater or the cathode solution to enter the anolyte catalyst layer or correspondingly the cathode catalyst layer of each of the two electrochemical cells of the repeating unit. 8. The stack of claim 7 further includes a seal between each side of the cover and the compression frame immediately adjacent to that side. 9. The stack of claim 6, wherein the solid polymer electrolyte membrane is an anionic solid polymer electrolyte. 10. The stack of claim 6, wherein the solid polymer electrolyte membrane is a cationic solid polymer electrolyte. 11. The stack of claim 6, wherein the cover is made of a non-conductive material. 12. A system for treating wastewater, comprising at least one stack of electrochemical cells as claimed in claim 1, the stack being immersed in a reactor tank containing wastewater or a cathode solution. 13. The system of claim 12, wherein the stacks are connected in series or in parallel. 14. A system for treating wastewater, comprising at least one stack of electrochemical cells as claimed in claim 6, the stack being immersed in a reactor tank containing wastewater or a cathode solution. 15. The system of claim 14, wherein the stacks are connected in series or in parallel. 16. A method for wastewater treatment, comprising the following steps: a. Providing a stack of at least one electrochemical cell as claimed in claim 1, the stack comprising at least one electrochemical cell as claimed in claim 1, the electrochemical cell being immersed in a reactor tank containing the wastewater to be treated. b. Supplying the anolyte solution to the housing formed by the shroud, which is attached to the first compression frame on the anode side. c. Providing a voltage across the electrochemical cell, and d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater. 17. A method for wastewater treatment, comprising the following steps: a. Providing a stack of at least one electrochemical cell according to claim 1, the stack comprising at least one electrochemical cell according to claim 1, the electrochemical cell being immersed in a reactor tank containing a cathode solution. b. The wastewater to be treated is supplied to the housing formed by the shroud, which is attached to the first compression frame on the anode side. c. Providing a voltage across the electrochemical cell, and d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater. 18. A method for wastewater treatment, comprising the following steps: a. Providing a stack of at least one electrochemical cell as claimed in claim 1, the stack comprising at least one electrochemical cell as claimed in claim 1, the electrochemical cell being immersed in a reactor tank containing the wastewater to be treated. b. Supplying cathode solution to the housing formed by the shroud, which is attached to the second compression frame on the anode side. c. Providing a voltage across the electrochemical cell, and d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater. 19. A method for wastewater treatment, comprising the following steps: a. Providing a stack of at least one electrochemical cell as claimed in claim 6, the stack comprising at least one repeating unit as claimed in claim 6, the repeating unit being immersed in a reactor tank containing the wastewater to be treated. b. The cathode solution to be processed is supplied to the housing formed by the cover, the cover being attached to the second compression frame of the first electrochemical cell of the repeating unit and the second compression frame of the second electrochemical cell of the repeating unit. c. Providing a voltage across the first electrochemical cell and the second electrochemical cell, and d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater. 20. A method for wastewater treatment, comprising the following steps: a. Providing a stack of at least one electrochemical cell as claimed in claim 6, the stack comprising at least one repeating unit as claimed in claim 6, the repeating unit being immersed in a reactor tank containing the wastewater to be treated. b. Supplying the anolyte solution to the housing formed by the shroud, the shroud being attached to the first compression frame of the first electrochemical cell of the repeating unit and the first compression frame of the second electrochemical cell of the repeating unit. c. Providing a voltage across the electrochemical cell, and d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater. 21. A method for wastewater treatment, comprising the following steps: a. Providing at least one stack of the electrochemical cell of claim 6, the stack comprising at least one repeating unit of claim 6, the repeating unit being immersed in a reactor tank containing a cathode solution. b. Supplying wastewater to the outer casing formed by the shroud, the shroud being attached between the first compression frame of the first electrochemical cell of the repeating unit and the first compression frame of the second electrochemical cell of the repeating unit. c. Providing a voltage across the electrochemical cell, and d. Operate the electrochemical cell at a certain current density to degrade pollutants in the wastewater.