WO2005038091A2

WO2005038091A2 - Use of electrochemical cell to produce hydrogen peroxide and dissolved oxygen

Info

Publication number: WO2005038091A2
Application number: PCT/US2004/033146
Authority: WO
Inventors: Marilyn J. Niksa; Andrew J. Niksa
Original assignee: Niksa Marilyn J; Niksa Andrew J
Priority date: 2003-10-11
Filing date: 2004-10-07
Publication date: 2005-04-28
Also published as: WO2005038091A3

Abstract

An apparatus and process for making relatively low concentrations of hydrogen peroxide and increased dissolved oxygen in aqueous solutions uses direct oxygen reduction at a gas diffusion interface which is spaced from an electrochemically stable anode, wherein the aqueous electrolyte has a minimal amount of metal ions or hardness which will compromise cathode performance. The process can operate on a side stream from the main receiving water and then inject the treated solution back into said stream. Alternatively, feed water from one source can be fed into the electrolytic generator and injected into a separate receiving water. If peroxide residual is not desirable, an activated carbon canister can decompose the peroxide, thusly increasing the dissolved oxygen concentration. The process generates peroxide from water and electricity and oxygen (from air) and does not affect the pH or chemical composition of the receiving water.

Description

USE OF ELECTROCHEMICAL CELL TO PRODUCE HYDROGEN PEROXIDE AND DISSOLVED OXYGEN

TECHNICAL FIELD This invention relates to the use of direct oxygen reduction in an electrochemical cell for making relatively low concentrations of hydrogen peroxide and increased dissolved oxygen in aqueous solutions. The reaction occurs at a gas diffusion interface which is spaced from an electrochemically stable anode using an aqueous electrolyte having a minimal amount of hardness and/or metal ions which will compromise cathode performance. BACKGROUND ART Hydrogen peroxide is generally effective at low concentrations for most applications. However, all commercial procedures for making hydrogen peroxide aim at large scale production which produce highly concentrated solutions for ease of shipment. These solution concentrations are generally much too high for the end use and are diluted by as much as 1000X at point of use. Also, additives are required to minimize decomposition and stabilize the peroxide, and many of these are undesirable for human, animal or plant consumption. A simple, safe, inexpensive process for making hydrogen peroxide at the point of use from water without chemical additives would offer great benefit to consumers. Prior art for on-site production of peroxide suffers from complexity of design and high cost, and/or the probability that the oxygen reducing cathode will foul quickly and cease to operate adequately. In addition, many approaches require addition of chemicals to the aqueous streams to be treated which would negatively affect water quality. Typically, alkaline peroxide is the product which would unacceptably affect water pH. Finally, most prior art utilizes a separator, usually a perfluorinated ion exchange membrane, to allow high peroxide concentrations, typically 3% - 5% by weight, without compromising galvanic current efficiency. Galvanic efficiency using these systems can average 60% - 80%. However, the separators require handling acidic and basic recirculating streams, and the membranes are extremely expensive and highly sensitive to hardness or metal impurities. The electrolytic production of hydrogen peroxide has an extensive history. For many years, hydrogen peroxide was manufactured by electrolysis where persulfate is formed at a high over- voltage anode and then subsequently hydrolyzed (Kirk-Othmer Encyclopedia of Chemical Technologies, 3^rd Edition. Volume 13, 1981). This process suffered from high energy costs and has been displaced by reduction of oxygen by hydrogen with anthraquinone as the catalyst (^"Kirk-Othmer Encyclopedia of Chemical Technologies. 3^rd Edition. Volume 13, 1981). This process requires large capital expenditures in equipment and is unsuitable for small scale or on-site production. High strength hydrogen peroxide (up to 50%) is generated, thus minimizing transportation costs. Significant work has been done on electrolytic production of peroxide via reduction of oxygen to provide a reasonable process for on-site production. Three basic types of cathodes have been developed: a flow-through or three dimensional cathode wherein the oxygen needed to support the electrochemical reaction is transported through the electrolyte; a "gas diffusion" electrode wherein the cathode is typically a carbon cloth or fiber structure with one surface highly hydrophobic and generally conscribes the cell wall. Oxygen from the air or other source then passes through this layer while liquid does not. The other face is compressed carbon particles where the electrochemical reaction transpires. The advantage of this approach is that oxygen supply is not limited by oxygen solubility in the aqueous solution (which is a maximum of about 38 ppm at atmospheric conditions). The disadvantage is that the electrolytic cell cannot tolerate high hydraulic pressure differential so that it must operate at atmospheric conditions and cannot be placed directly into a pressurized water line. Finally, the most recent innovation is the fuel cell or SPE (Solid Polymer Electrolyte) wherein the cathode is actually printed onto a polymer film that acts as the electrolyte. The catalysts used are expensive, precious metals and the polymer film is typically a fluoropolymer film that is also expensive and prone to fouling by hardness and/or metal ions. The high unit area costs drive the system to operate at high current density exacerbating problems with polarization at anode and cathode which can drive the cathode into hydrogen evolution and/or result in high operating cell voltages. Regardless of the cathode used, the electrochemistry is equivalent:

Cathode: 2e^" + O₂ + H₂O ► HO₂ ^" + OH^"

Anode: H₂O ► l/2O₂ + 2H^"1" + 2e^'

V2 O₂ + 2H₂O ► HO₂ ^" + OH^" + 2H^"1" followed by: HO₂ ^" + H₂O ► H₂O₂ + OH^"

It should be noted that when the cathode is catalyzed with an appropriate material, such as silver, MnO₂, or cobalt macrocycle, peroxide is not the predominant product of oxygen reduction: Reactions with catalyzed cathode:

O₂ + 2H₂O + 4e^" ► 4 OH^" Flow through or three dimensional carbon cathodes were utilized by Huron Tech Co., Dow Chemical Company and the Canadian government to develop a process to make alkaline peroxide useful in the bleaching of pulp for paper making. Oxygen gas is reduced within a electrode bed of carbon particles or chips. There is a diaphragm or ion exchange separator to allow the production of the alkaline peroxide (D. Dong at The Ninth International Forum of Electrolysis in the Chemical Industry, Clearwater Beach, November, 1995, U.S. Patents 3,969,201, 6,200,440 & 4,431,494). Operating and capital costs were not competitive with peroxide produced in large facilities which make peroxide via the anthraquinone route, and the process was not commercially accepted. In addition, the technology suffered from continuous degradation of the carbon chips that made up the cathode and the expense of providing gaseous oxygen. Another three dimensional cathode is described in Patent 4,430,176 which teaches a process for producing an alkaline solution of hydrogen peroxide. A fluid permeable, conductive cathode comprising vitreous carbon foam is separated from the anode via a diaphragm or ion exchange membrane and oxygenated alkaline electrolyte is circulated within the permeable cathode. This process is limited in operating current density due to oxygen solubility restrictions in the alkaline environment. A similar patent, 4,350,575, describes use of a reticulated vitreous carbon cathode in lieu of a gas diffusion electrode and has equivalent limitations. Another example of the use of reticulated vitreous carbon as a peroxide generating cathode for treatment of municipal sewage is described in "Hydrogen peroxide production by water electrolysis: Application to disinfection" P. Drougui, S. Elmaleh, M. Rumeau, C. Bernard, A. Rambaud, UMR 5569 UM

II/CNRS/TRD Hydrosciences Montpelier, CC 24, University Montpelier, 34095 Montpelier Cedex 5, France. The maximum peroxide concentration obtained in this work without active aeration or oxygenation of the electrolyte was only 0.13 mg/liter/min. The cathode activity is rate controlled by diffusion of dissolved oxygen to the cathode surface so that a very large surface area is needed to manufacture commercial amounts of peroxide. In addition, the electrode is submersed in a very contaminated aqueous environment which results in rapid fouling, dramatically limiting lifetime. Patent 6,224,744 describes a process for destroying organic pollutants using a gas diffusion electrode and an anode, either dimensionally stable or sacrificial, and circulating polluted water within a cell body. The peroxide generated cathodically acts in concert with either metal ions discharged from the anode or by direct oxidation of the polluting organic at the anode surface if the anode is anodically stable (such as precious metal coated titanium). However, commercial utility is limited due to the rapid contamination of the gas diffusion cathode in water containing excessive metal ions. Some ions, such as iron and copper, will plate onto the cathode since the potential for this electrochemical reaction is lower than that producing peroxide. More damaging is the deposition of divalent metal hydroxides, such as magnesium and calcium, that precipitate within the carbon structure of the gas diffusion cathode. This blocks access of oxygen to electro-active sites and causes the cathode to flood with inevitable generation of hydrogen in lieu of oxygen reduction. Traditionally, electrochemical reactors are subjected to current reversal to "clean" the scale from the cathode. During this step, oxygen and protons are generated at the cathode which effectively dissolve the scale. Gas diffusion cathodes, however, cannot be subjected to current reversal since such action will result in the degeneration of the fragile carbon layer due to generation of oxygen gas formed during the reverse cycle. Indeed, much work has been done to allow gas evolution on gas diffusion cathodes, with little technical success. Hence, operating a cell as described would have a very short lifetime. Patent 6,254,762 describes an electrolytic cell capable of operation in extremely low conductivity water. A dimensionally stable anode is separated from the gas depolarized cathode by an ion exchange membrane and the cathode is separated from the ion exchange membrane by ion exchange resin particles. The ion exchange membrane is in direct contact with the anode. Again, the process and equipment are complex and expensive. Patent 6,159,349 describes a system that doses hydrogen peroxide into seawater solutions comprising a water electrolysis cell that provides hydrogen and oxygen gases to another electrolytic cell. This cell contains a hydrogen depolarized anode and oxygen depolarized cathode which generate hydrogen peroxide in the seawater solutions without co-production of chlorine or chlorine species. There is complexity and cost for such a scheme. In addition, fouling of the gas depolarized cathode will occur due to the extreme level of hardness in the seawater electrolyte. Patent 4,758,317 describes a process for producing hydrogen peroxide in an alkaline electrolyte with a flow through cell and a separator that is either microporous or an ion exchange membrane. The solutions must be "urged" or pumped through the chamber. Again, this approach is inappropriate for dosing peroxide into waters which cannot tolerate introduction of chemical species (i.e. alkalinity) and requires a separator. Patent 5,645,700 describes an electrolytic cell which generates peroxide comprising an "SPE" or fuel cell electrode that is able to operate at ambient temperature and pressure and requires no chemical additives, thus allowing introduction of the peroxide directly into a stream to be treated. The cell comprises a polymer membrane, semi-permeable to either protons or hydroxide ions sandwiched between an anode and cathode with catalysts that allow a stream of dissolved oxygen or oxygen bubbles passing over the cathode to be reduced to hydrogen peroxide. Again, this cell utilizes precious metal catalysts, and expensive membranes which are very sensitive to operating conditions and water quality. In addition, the cathode must discharge oxygen that is dissolved in the aqueous stream with the same limitations as three dimensional cathodes. Patent 4,455,203 describes a system using a solid electrolyte made of perfluorinated polymer and gas permeable coatings as electrodes and supplying water to the anode side and oxygen to the cathode side and withdrawing peroxide on the cathode side. Both the catalysts used and the polymer membrane are expensive and sensitive to operating conditions and impurities. DISCLOSURE OF THE INVENTION The present invention relates to an electrochemical cell and a process for producing hydrogen peroxide and dissolved oxygen in water. The electrochemical cell has at least one gas diffusion cathode and at least one electrochemically stable anode spaced from the cathode at a distance between about 0.O20" and about 0.125". The water that is used as an electrolyte should not {P contain metal ions greater than about 70 parts per million (ppm) and a hardness (as calcium carbonate) of no greater than about 35 ppm or 2 grains per gallon of water). The electrochemical cell is operated at a current density from about 0.001 amp per square inch to about 0.20 amp per square inch. The differential hydraulic pressure across the cathode is less than about 36", preferably from about 0" to about 3O" of water pressure. The gas diffusion cathode preferably contains carbon comprising acetylene black. The electrochemically stable anode typically is a precious metal coated titanium. The feed water to the cell is pretreated by reverse osmosis and/or ion exchange if necessary to reduce metal contamination to less than about 70 ppm and hardness to below about 35 ppm. The concentration of generated hydrogen peroxide in the discharge water is from about 2 ppm to about 2000 ppm. If desired, the peroxide may be catalytically converted to water and dissolved oxygen using a decomposer element comprising an activated carbon bed or carbon block. The carbon bed or block may be further catalyzed with MnO to improve peroxide decomposition activity. An additional post treatment prior to use of the decomposer unit may be employed to generate hydroxyl free radicals. The additional post treatment is ultra violet light, ozone, or ultrasonic waves. The water discharged from the electrochemical cell may be diluted 1.5 — 200 times by water that has bypassed the electrochemical cell(s). This bypass can be achieved either by taking the feed water to the cell(s) as a slipstream from an incoming water source or, alternatively, the source of said feed water can be totally separate from the bypass water stream. The hydrogen peroxide is typically dosed into the bypass water using a pump or eductor. The invention also relates to a process for the use of water that has been treated so as to contain an enhanced level of hydrogen peroxide and dissolved oxygen. The enhanced level of peroxide and dissolved oxygen is generated by passing a feed water stream having a metal contamination less than about 70 ppm and a hardness less than about 35 ppm through an electrochemical cell having a gas diffusion cathode and an electrochemically stable anode. The electrodes are spaced from one another a distance between about 0.020" and about 0.125". The cell is operated at a current density of between about 0.001 amp per square inch and about 0.20 amp per square inch and a differential hydraulic pressure across the cathode less than about 36", preferably between 0" and 30" water pressure. The treated water is discharged from the electrochemical cell and is applied in a sufficient quantity for the designated purpose. Among these uses for water having enhanced levels of peroxide and dissolved oxygen are hydroponic or soil-less agricultural irrigation; disinfection of irrigation lines; destruction of H₂ S, oxidation of soluble iron, treatment of holding tanks for potable or industrial water; and the removal or prevention of biofilms. The invention also includes an electrolytic cell useful for enhancing the peroxide and dissolved oxygen concentration in water. The cell includes a gas diffusion cathode and a stable anode spaced from the cathode a uniform distance of between about 0.020" and about 0.125". The cell has a liquid inlet, a liquid discharge, and a passage between the electrodes for the water to flow. The cell is operated at an electrical current density of between about 0.001 amp and about 0.20 amp per square inch. The cell uses a gas diffusion cathode containing carbon, such as acetylene black, which has not been treated to render it catalytic. Said gas diffusion cathode typically has a hydrophobic face which avoids egress of electrolyte and conscribes at least one cell wall. The anode is a valve metal with an electrocatalytic coating comprising a platinum group metal or metal oxide and can be a solid sheet or, alternatively, foraminous. The electrodes are planar or curvilinear and are directly opposite to one another. They comprise one anode and one cathode, or one anode and two cathodes spaced on either side of said anode. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a process stream wherein a total mcoming stream is treated; Figure 2 shows a process for splitting and recombining the incoming stream after treating a portion of the stream; and Figure 3 shows the treatment of the incoming stream and combining it with a second stream. MODES FOR CARRYING OUT THE INVENTION Turning now to the drawings, and with particular reference to Figure 1, a feed water stream 110 to the electrochemical cell(s) 112 flows through or around anode 116 and cathode 114 and peroxide and dissolved oxygen treated water exits 122. The feed water 110 is fed through hardness / metal ion removal means 118 if the water does not meet purity criteria. Discharge water is fed through decomposer element 120 if it is desirous to decompose peroxide to water and dissolved oxygen. As an option, the discharge from the cell(s) 112 flows through a disinfection unit 132, such as a source of ultraviolet light, ozone or ultrasonic generators, before being fed through the decomposer element. These disinfection units are well known in the art and are commonly used for water disinfection. Figure 2 shows feed water 210 separated via a splitter 224 and a portion is fed to the electrochemical cell(s) 212 flowing through or around cathode 214 and anode 216. Peroxide and dissolved oxygen treated water exits at 222 and is re- introduced into the bypass portion portion 228 of the feed water by injection means 226. Feed water is fed through hardness / metal ion removal means 218 if the water does not meet purity criteria. Discharge water is optionally fed through the disinfection unit 232, and thence the decomposer element 220 if it is desirous to decompose peroxide to water and dissolved oxygen. The final treated water exits at discharge 230 for use. Turning next to Figure 3, feed water 310 is fed into the electrochemical cell(s) 312 flowing through or around cathode 314 and anode 316. Peroxide and dissolved oxygen treated water exits 322 and is fed into a separate stream 328 of receiving water via injection means 326. Feed water is fed through hardness / metal ion removal means 318 if the water does not meet the purity criteria. As shown in Figures 1 and 2, discharge water is optionally fed through a disinfection unit 332, and thence is fed through decomposer element 320 if it is desirous to decompose peroxide to water and dissolved oxygen. The present invention provides a simple, inexpensive device and method for manufacturing low strength hydrogen peroxide accompanied by increased dissolved oxygen in a receiving stream. The electrolytic cell includes a gas diffusion electrode which has a hydrophobic surface to avoid egress of electrolyte while allowing oxygen from air to pass through the structure to the electro-active surface which, is wetted by the electrolyte. These types of cathodes are readily available from, i.e. Fuel Cell Technologies, or Yardney Electric. The carbon utilized for the active face should preferably not be catalyzed by any of the commonly used catalysts, such as platinum or cobalt macrocycles, since this will favor a four electron oxygen reduction to hydroxide instead of the two electron oxygen reduction to hydrogen peroxide. Peroxide generation can be further enhanced by using low surface area carbon, such as acetylene black. Gas diffusion electrodes are superior to flow through carbon cathodes because the oxygen is provided from air, where the oxygen partial pressure changes very little, and is also superior to the SPE which has the drawbacks of very high cost and the susceptibility of the membrane to foul. The anode is comprised of a metal that is electrochemically stable in the aqueous electrolyte and can be comprised of a high quality stainless steel, such as AL6XN, available from Allegheny Ludlum, or precious metal coated titanium available from, i.e. DeNora SPA. The anode and cathode are spaced from each other by a gap of about 0.020" to 0.125" which is readily obtainable by present commercial manufacturing methods. Water fed to the electrochemical cell is controlled to low hardness, typically less than 2 grains per gallon or about 35 ppm as calcium carbonate and a metal concentration less than about 70 ppm. If the feed water to be treated does not meet this criteria, the water is treated by, i.e. ion exchange or reverse osmosis. This avoids the fouling of the gas diffusion cathode surface with scale which compromises cell performance and will ultimately result in the cathode reactions shifting from oxygen reduction which forms hydrogen peroxide to hydrogen evolution. The preferred operating current density is maintained low to avoid large changes in cell voltage which arise from inevitable variations in water conductivity for multiple applications. In this fashion, the power supply equipment is dramatically reduced in cost and complexity. In addition, low operating current density maintains a low electrode potential and again lessens the propensity of the gas diffusion cathode to shift from oxygen reduction to hydrogen evolution as it ages. Typically, gas diffusion electrodes can tolerate very little difference in hydraulic pressure between the air side of the cathode and the electrolyte side. Hence, the system described is preferably operated under atmospheric conditions or very slight pressure differential. If it is desirous to dose the electrochemically generated peroxide into a pressurized line, then injection means, such as eductors, chemical metering pumps, or flow proportional devices similar to the apparatus sold under the Dosamatic trademark, should be employed. If the receiving water stream does not meet the hardness criteria, it is preferred that a side stream be diverted through a treatment device and then onto the cell compartment(s). This side stream should preferably be from about 2% to

20% of the total flow to be treated. A unique advantage of this scheme is that only a very small amount of the total flow need be treated to remove hardness and/or metal ions. This dramatically reduces the initial and on-going cost of the treatment device. Some applications either require high dissolved oxygen concentrations

(such as soil-less growing feed solutions) where a peroxide residual is undesirable (such as potable water disinfection). The process is easily modified to accomplish this by putting a peroxide decomposer on the discharge side of the cell. Preferably, this device is a simple cartridge filled with activated carbon or a carbon block filter. Further peroxide decomposition efficiency is obtained by catalyzing the carbon with manganese dioxide prepared by soaking in an aqueous solution of permanganate. It is significantly advantageous to generate peroxide as a precursor to dissolved oxygen since it can store large amounts of oxygen in a small volume. For example, one liter of 2,000 ppm peroxide can be diluted to provide 12.5 gallons of water containing 20 ppm dissolved oxygen, a concentration typically desired for agricultural use. The disinfection capability of peroxide can be dramatically increased by adding a source of ultra violet light, ozone, or ultrasonic generators. One of these devices can easily be placed either on the discharge of the cell or directly into the receiving water, after the point at which the peroxide is dosed. It is well known that adding these types of devices generates hydroxyl free radicals which are a far more effective disinfectant than any of these means alone. EXAMPLE 1 An electrolytic cell having an anode and cathode area of 20 square inches was operated in water with less than 2 grains per gallon total hardness and a conductivity of 450 micro Siemens at a current of 2 amps. The air cathode had an uncatalyzed carbon made from Teflon bound BP2000 carbon (available from Fuel Cell Technologies, or Yardney Electric). The anode was an EC-600 iridium oxide coated titanium expanded mesh available from Eltech Systems Corporation. Average cunent efficiency over the 1100 minute test time was 50%. The results are shown in the following graph.

peroxide vs time, 7 quarts, 2 amps

200 400 600 800 1000 1200 minutes

EXAMPLE 2 Three electrolytic cells with an active area of three square inches were operated at various current densities relating to production rates of peroxide. The feed water was made of distilled water with reagent grade sodium sulfate added to make a conductivity of 500 micro Siemens and then autoclaved to exterminate any indigenous microbes. This was then inoculated with non-coliform heterotrophic bacteria which were dosed at a level of about 3 times that expected in a typical potable water source. Three-fourths gallon of inoculated water were placed into three vessels; one vessel had no cell inserted and acted as a control. Two other vessels had a cell as described. HPC (heteropbilic plate count, colonies per inch with 25 being the sensitivity of the procedure) analyses were done as shown below.

After 63 hours the cells were removed from the vessels. The HPC data is as shown below:

As can be seen, the electrolytically generated peroxide acted to disinfect and provide bacteriostatic conditions for a potable or industrial water supply. EXAMPLE 3 A flow through type electrolytic cell with an active area of about 56 square inches was operated on water of hardness less than 2 grains per gallon and was softened using a standard commercial water softener. The cathode utilized uncatalyzed BP2000 as the electrochemically active carbon face, and the anode was an iridium oxide coated mesh. The cell gap was approximately 0.035". Operating pressure was atmospheric. Analyses of peroxide were done directly on the discharge of the cell. The effect of current on peroxide generation rate and galvanic efficiency are shown below:

Typically, the flow rate of aqueous electrolyte through the type of cell described in Example 3 is in the range of between about 25 ml/minute to about 300 ml/minute. However, the electrochemical reactions in Examples 1 and 2 are described as being conducted under static conditions. Generally, the water temperature during electrolysis and the subsequent treatments and reactions preferably are maintained below about 25° C to prevent decomposition of the peroxide. INDUSTRIAL APPLICABILITY The dissolution of oxygen into aqueous solutions is essential in a number of industries. Hydroponic culture of plants requires continuous aeration of the liquid growing medium for optimum growth rate and yield. Analogously, aquaculture of various marine food sources requires maintenance of dissolved oxygen levels of greater than about 5 or 6 ppm. Ponds must be continually aerated, or areas of depleted oxygen will result in the death of indigenous species. The invention is useful for making hydrogen peroxide or increasing dissolved oxygen on-site or on-demand for these and other purposes, such as of water purification, groundwater remediation, activated carbon regeneration, or to maintain disinfection or bacteriostatic conditions in potable or industrial water reservoirs. It is understood that the entire relevant content of the priority Provisional United States patent application is incorporated herein by reference. While the invention has been described in combination with specific embodiments thereof, there are other alternatives, modifications, and variations that are likewise deemed to be within the scope thereof. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.

Claims

WHAT WE CLAIM IS:

1. A process for producing hydrogen peroxide and dissolved oxygen in water comprising employing an electrochemical cell with a gas diffusion cathode and an electrochemically stable anode spaced from the cathode at a distance between about 0.020" and about 0.125" and an aqueous electrolyte feed x- stream wherein the contamination from metals ions is less than about 70 parts per million and hardness of less than about 35 ppm as calcium carbonate, and the operating current density is from about 0.001 amp per square inch to 0.20 amp per square inch and a differential hydraulic pressure across the gas diffusion cathode is less than about 36" water pressure.

2. The process of claim 1 wherein the gas diffusion cathode contains carbon comprising acetylene black and is free of added catalytic materials.

3. The process of claim 1 wherein the electrochemically stable anode is precious metal coated titanium.

4. The process of claim 1 wherein the concentration of generated hydrogen peroxide in the discharge water is from about 2 ppm and about 2000 ppm.

5. The process of claim 1 wherein the peroxide is catalytically converted to water and dissolved oxygen using a decomposer element.

6. The process of claim 4 where the decomposer element is an activated carbon bed or carbon block.

7. The process of claim 4 where the decomposer element is an activated carbon bed further catalyzed with MnO₂.

8. The process of claim 1 wherein the pretreatment is carried out by ion exchange or reverse osmosis.

9. The process of claim 1 where an additional treatment is employed to generate hydroxyl free radicals.

10. The process of claim 9 where said additional treatment is ultra violet light, ozone, or ultrasonic waves.

11. The process of claim 1 wherein the water discharge from the electrochemical cell is diluted 1.5 - 200 times by water that has bypassed the cell.

12. The process of claim 11 wherein the hydrogen peroxide is dosed into said bypass water via use of a pump or eductor.

13. The process of claim 11 wherein the feed water to the electrochemical cell is a slipstream from the bypass water stream.

14. The process of claim 13 wherein the source of said feed water is separate from the bypass water stream.

15. A process for the use of water containing an enhanced level of hydrogen peroxide and dissolved oxygen comprising the step of passing at least a portion of a feed water having a metal ion contamination less than about 70 ppm and a hardness level of less than 35 ppm as calcium carbonate through an electrochemical cell having a gas diffusion cathode and an electrochemically stable anode spaced from the cathode at a distance between about 0.020" and A" about 0.125", operating the cell at a current density of between about 0.001 amp per square inch and about 0.20 amp per square inch and a differential hydraulic pressure across the cathode less than about 36" water pressure, removing the treated water from the electrochemical cell and applying a sufficient quantity of the water for a purpose selected from the following: hydroponic or soil-less agricultural irrigation; disinfection of irrigation lines; destruction of H₂S, oxidation of soluble iron, treatment of holding tanks for potable or industrial water; and the removal and/or prevention of biofilms.

16. The process of claim 15 wherein the treatment of holding tanks for potable or industrial water provides bacteriostatic conditions within the holding tanks or provides bacterial disinfection of holding tanks for potable or industrial water.

17. An electrolytic cell with a gas diffusion cathode and an anodically stable anode spaced from the cathode a uniform distance of between about 0.020" and about 0.125", the cell having a liquid inlet, and a liquid discharge, said cell capable of increasing the dissolved oxygen and hydrogen peroxide content of an aqueous electrolyte flowing between said cathode and said anode when an electrical current density of between about 0.001 amp and about 0.20 amp per square inch is passed through the electrolyte in the cell.

18. The electrolytic cell according to claim 17 wherein the gas diffusion cathode is uncatalyzed.

19. The electrolytic cell according to claim 17 wherein the anode is a valve metal with an electrocatalytic coating comprising a platinum group metal or metal oxide.

20. The electrolytic cell according to claim 17 wherein the anode is foraminous.

21. The electrolytic cell according to claim 17 wherein the electrodes are planar or curvilinear and directly opposite to one another.

22. The electrolytic cell according to claim 17 wherein the electrodes comprise one anode and two cathodes spaced on either side of said anode.