Hybrid electrochemical-fenton-ozonation system for urban and industrial wastewater treatment

Abstract

The present invention provides a comprehensive hybrid system for the treatment of urban and industrial wastewater, integrating electrochemical processes, Fenton oxidation, and ozonation The system is designed to efficiently remove a wide range of pollutants, including suspended solids, organic compounds, and refractory substances that are resistant to conventional biological treatment. The hybrid process begins with electrochemical treatment, which generates coagulant ions and promotes the removal of both dissolved and suspended contaminants. This is followed by Fenton oxidation, where hydroxyl radicals produced from hydrogen peroxide and iron ions degrade persistent organic compounds. The final stage involves ozonation, which further oxidizes remaining pollutants, increases dissolved oxygen levels, and disinfects the effluent.

Description

[0001] Hybrid Electrochemical-Fenton-Ozonation System for Urban and Industrial Wastewater T reatment

[0002] Field of the Invention

[0003] The present invention relates to the field of wastewater treatment technologies. More specifically, it concerns a hybrid system for the treatment of urban and industrial wastewater that combines electrochemical processes with Fenton oxidation and ozonation. The invention targets the removal of a wide range of organic and inorganic pollutants, including recalcitrant compounds that are resistant to conventional biological treatment. This system is designed to improve treatment efficiency, reduce residual chemical oxygen demand (COD) and biochemical oxygen demand (BOD), and produce treated effluent suitable for reuse in various applications. The hybrid approach provides a scalable, integrated solution for both municipal and industrial wastewater treatment facilities.

[0004] Background Art

[0005] Wastewater production is one of the common problems of industrial development [1 , 2], Water becomes polluted during production processes or from employees’ activities [3], Treating or reusing this water requires proper purification. There are many methods to treat industrial wastewater, and the most important and common ones are biological methods [4], One of the main groups of pollutants in industrial wastewater is organic compounds [5], These compounds can exist in water as suspended solids or dissolved substances, increasing the chemical oxygen demand (COD) of the water. To fully oxidize organic compounds, according to Equation 1 , each molecule of organic matter needs v molecules of oxygen (O2). For example, this means that 0.12 grams of oxygen are needed to completely convert one gram of glucose into carbon dioxide and water. It should be noted that Equations 1 to 3 show the biochemical reactions of organic matter consumption by bacteria, without considering biomass production and the nitrification process. This means that nitrogen finally turns into ammonia and does not undergo further transformation [6], CaHbOcNd+ vO2-► aCO2+ dNH3+ wH2O Eq. 1) b—3d w = - 2 Eq. 2)

[0006] (Eq. 3)

[0007] A wide range of organic compounds are not toxic, but some microorganisms can use them as a carbon source or an energy source [7], So, when organic compounds are present in water, bacteria use them for carbon or energy, and at the same time, they consume the oxygen in the water [8], This reduces the dissolved oxygen level in the water and makes it harder for aquatic organisms to survive. Therefore, releasing organic compounds into water is a serious environmental risk. There are different methods to remove organic compounds from water, including physical, chemical, electrochemical, and biological methods [9], The most important physical method is water distillation

[0010] , In this method, water is heated to its boiling point. Pure water evaporates, while pollutants remain in the boiler. The water vapor can then be condensed back into liquid water using a cooler. Although this method is very effective and produces high-quality water, it is energy-intensive and expensive. It is also difficult to separate organic compounds that have a boiling point close to water, which requires complex and costly equipment. Biological methods are among the most common ways to remove organic pollutants

[0011] , As mentioned earlier, many organic compounds can be easily used by bacteria. By providing wastewater that contains organic compounds to a high concentration of bacteria and enough oxygen, organic compounds can be converted into carbon dioxide and water over a certain period (Equation 1)

[0012] . Various biological methods, such as trickling filters and activated sludge systems, can be used to remove organic compounds from water

[0013] , Biological treatment works well only when the difference between chemical oxygen demand (COD) and biological oxygen demand (BOD) is not large. Normally, if the COD / BOD ratio is less than 3, microorganisms can decompose most of the organic compounds

[0014] , However, when COD is much higher than BOD (more than three times), it indicates the presence of compounds that are either hard to degrade or toxic to microorganisms. These compounds reduce microbial activity and limit the efficiency of biological processes. As a result, a significant portion of organic pollutants remains in the wastewater. To solve this problem, chemical oxidation processes are used

[0015] , In these methods, compounds such as hydrogen peroxide (H2O2), potassium bicarbonate (KHCO3), hexavalent iron (Fe(VI)), and potassium permanganate (KMnO4) act as oxidants. These substances can destroy the molecular structure of complex pollutants by accepting electrons from the pollutants or producing active radicals. For example, hydrogen peroxide breaks down under ultraviolet (UV) light and produces highly reactive hydroxyl radicals ( OH) according to Equation 4:

[0008] H2O2+ hv ^ 20H. (Eq. 4)

[0009] These radicals have a high oxidation potential (E° » 2.8 V) and can almost completely oxidize all organic compounds into simple products like carbon dioxide (CO2) and water (H2O).

[0010] OH. +RH ~^ R. +H2O (Eq. 5)

[0011] In Equation 5, RH is an organic compound. The hydroxyl radical (-OH) attacks the organic molecule and removes one hydrogen atom. This produces an organic radical (R-) and water.

[0012] R+(?2 - * ROO. (Eq. 6)

[0013] R- is very unstable and quickly reacts with dissolved oxygen. The product of this reaction is a peroxy radical (ROO-) (Eq. 6).

[0014] The peroxy radical decomposes through a chain of further reactions

[0016] , Eventually, the organic compound is converted into carbon dioxide and water. In the oxidation process with potassium permanganate, the Mn04“ ion also acts as a strong oxidant and converts organic compounds into CO2and H2O, while it itself is reduced to MnO2or Mn2+. Equation 8 shows the reaction of potassium permanganate (KMnO4) with ethanol (C2H5OH). In this reaction, permanganate acts as a strong oxidizer and oxidizes ethanol to acetic acid (CH3COOH).

[0015] 4KMnO4+ 3C2H5OH -► 4H2O + KOH + 4MnO2+ 3CH3COOH (Eq. 8)

[0016] If the reaction stops at this stage, part of the organic pollutant (ethanol) is only transformed into another compound (acetic acid) and is not fully mineralized. Acetic acid still contains carbon and contributes to chemical oxygen demand (COD) and biochemical oxygen demand (BOD). The presence of acetic acid is usually not a major problem because it is highly biodegradable and can be easily consumed by microorganisms in the next biological stage. If the concentration of KMnO4is higher and the environment is sufficiently alkaline or acidic, acetic acid can also be further oxidized. In this case, the C- C bonds break, and the final mineral products, such as carbon dioxide and water, are formed. The overall reaction in an alkaline environment can be written as follows:

[0017] 2KMnO4+ 3CH3COOH -► 4H2O + 2K0H + 6CO2(Eq. 9)

[0018] If the environment is acidic, meaning that Mn04is reduced to Mn2+, Reaction 10 occurs.

[0019] 12H+8KMnO, + 5CH3COOH -► 4K++ 4H2O + 8Mn2++ 10CO2(Eq. 10)

[0020] Hexavalent iron (Fe(VI)), which exists as the ferrate ion (Fe042“), is also known as a green oxidant. It can break down toxic and persistent organic compounds. Its general reaction is as follows: 4H2O + Fe3+-► 3e~ + 8H++ Fe04“2Eq. 11)

[0021] In these reactions, Fe(VI) accepts electrons from pollutants and oxidizes them. Generally, oxidants either react directly with organic compounds to break their molecular structure or produce active oxygen species such as OH and O2“, starting chain reactions of radicals. These reactions not only mineralize organic compounds but also reduce wastewater toxicity and make it more suitable for later biological treatment. Using a combination of chemical oxidation and biological methods (called hybrid or combined processes) is one of the most efficient approaches for treating wastewater containing resistant pollutants. In this approach, part of the toxic and stable compounds is first broken down by chemical oxidation, and then the remaining more biodegradable organic matter is removed by microorganisms. Another chemical treatment method uses photocatalysts such as TiO2, ZnO, or MgO. Light exposure on these photocatalysts can produce free hydroxyl radicals in water, which break down organic compounds. Oxidizing glucose in water using oxidants or photocatalysts is usually costly and complex. Electrochemical methods have also been effectively used in many studies to remove pollutants from wastewater

[0017] , In these methods, several electrodes with specific materials and surface areas are placed in wastewater and connected to electricity at a certain voltage and current. This allows removal of organic pollutants. Electrochemical treatment can also remove suspended particles, color, and harmful bacteria from wastewater

[0018] , Typically, three types of electrodes — graphite, iron, and aluminum — are used. Using iron or aluminum electrodes not only oxidizes organic compounds directly but also releases Fe3+or Al3+ions into the water. These ions act as coagulants and neutralize the surface charges of non-settling (colloidal) suspended particles, allowing them to clump together into larger flocs. Some flocs settle, and some float on the water surface, making separation possible. Therefore, electrochemical methods can easily remove both dissolved organic compounds and suspended solids from wastewater.

[0022] These methods are usually simple and inexpensive but are generally combined with other treatment methods for maximum effectiveness. Sometimes, a combination of the above methods is used in industries for complete wastewater treatment. Several electrochemical oxidation pathways have been proposed for organic matter. The most important include: (a) direct oxidation at the anode; (b) indirect oxidation with hydroxyl radicals ( OH) at the anode (inactive); (c) electro-Fenton and cathodic Fenton; (d) indirect oxidation with active chlorine in the presence of chloride; (e) sulfate electrolysis and sulfate radical pathway; (f) other indirect oxidation pathways (anodic ozonation, peroxynitrate, etc.); and (g) cathodic reduction (cathode catalysts and reductive decomposition). The sacrificial electrode can be either iron or aluminum. If aluminum is used, Al3+ions are released into the water according to Equation 11. If iron is used, Fe2+and Fe3+ions are released. Using the following equations, it is possible to calculate how many moles of metal ions are released per mole of electrode in water.

[0023] Al -^ Al3++ 3e“ Eq. 11)

[0024] Al3+and Fe3+ions hydrolyze in water according to Equations 13 and 14. The solid metal hydroxides (AI(OH)3or Fe(OH)3) form flocs that trap colloids and suspended solids. In designing the process, pH and alkalinity must be considered because hydrolysis depends on pH.

[0025] Al3++ 3H2O -► Al(OH)3(S) + 3H+(Eq. 13)

[0026] Fe3++ 3H2O -► Fe(OH)3(s) + 3H+(Eq. 14)

[0027] One important reaction in electrochemical treatment is the cathodic reduction of oxygen to hydrogen peroxide, which acts as a precursor for the Electro-Fenton process (see Equation 15).

[0028] O2+ 2H++ 2e“ H2O2(Eq. 15) This reaction is used to produce hydrogen peroxide in situ. When it combines with Fe2+ions, hydroxyl radicals are generated (according to the next reaction).

[0029] Fe2++ H2O2^ Fe3+. OH + OH~ (Eq. 16)

[0030] The hydroxyl radical ( OH) is a very strong oxidant that attacks resistant organic compounds and mineralizes them. In the Electro-Fenton process, Fe2+ions are electrochemically regenerated at the cathode. The direct production of OH radicals on inactive anodes is another phenomenon that can occur during electrochemical wastewater treatment. Water oxidation on the surface of an “inactive” anode occurs according to the following equation.

[0031] H2O OH + H++ e~ (Eq- 17)

[0032] RH+. OH ^. R + H2O -► CO2+ H2O (Eq. 18)

[0033] Boron-doped diamond (BDD) and some metal oxide anodes enhance this indirect oxidation pathway. This method is suitable for deep mineralization. Using Faraday’s law, the mass of dissolved metal or the number of moles of generated species can be calculated. To calculate the number of moles of metal released into the water during electrolysis (n), Equation 19 is used, and to determine the mass of metal released (m), Equation 20 is used. Eq. 19)

[0034] _ MIt

[0035] (Eq. 20) zf In these equations, I is the current (amperes), t is the time (seconds), z is the number of electrons transferred per metal atom (3 for Al3+, 2 for Fe2+), F is Faraday’s constant (about 96485 C mol-1), and M is the molar mass (grams per mole). These equations are used to determine electrode sizes and estimate the electric charge needed to produce the target dose of coagulant. To calculate the electric current applied to the electrodes, Equation 21 can be used.

[0036] In this equation, j is the current density (amperes per square meter), I is the current (amperes), and A is the anode surface area (square meters). The treatment rate and electrode dissolution increase proportionally with current density. Therefore, j should be chosen to balance the production rate and electrode lifespan. When chloride ions (Cl") are present, they are first converted to molecular chlorine (Cl2) during electrochemical reactions (Equation 22), and then water is hydrolyzed to produce HOCI / OCI" (Equation 23). This process is called the indirect chlorination pathway.

[0037] 2Cl~ -► Cl2+ 2e“ Eq. 22)

[0038] Cl2+ H2O HOCl + H++ c Eq. 23)

[0039] HOCI / OCI" ions are oxidizers that remove organic compounds, but they can also form chlorinated by-products. If the feed water contains chloride, this must be considered. The production of sulfate radicals through anodic persulfate is another oxidation pathway that occurs during electrochemical wastewater treatment (see Equations 24 and 25).

[0040] 2SO4 -> S2Og + 2e (Eq. 24) The sulfate radical (S04“) is a strong oxidant used to break down resistant organic compounds. This pathway can be applied in advanced electrochemical oxidation. Studying various oxidation methods, especially their combination with electrochemical processes, shows that an efficient system can be achieved for removing resistant pollutants from industrial wastewater. This approach not only improves effluent quality but also allows the water to be reused.

[0041] Summary of the Invention.

[0042] The present invention provides a comprehensive hybrid system for the treatment of urban and industrial wastewater, integrating electrochemical processes, Fenton oxidation, and ozonation. The system is designed to efficiently remove a wide range of pollutants, including suspended solids, organic compounds, and refractory substances that are resistant to conventional biological treatment. The hybrid process begins with electrochemical treatment, which generates coagulant ions and promotes the removal of both dissolved and suspended contaminants. This is followed by Fenton oxidation, where hydroxyl radicals produced from hydrogen peroxide and iron ions degrade persistent organic compounds. The final stage involves ozonation, which further oxidizes remaining pollutants, increases dissolved oxygen levels, and disinfects the effluent.

[0043] The combined approach allows for high treatment efficiency, minimizes chemical consumption, and produces effluent with reduced chemical oxygen demand (COD) and biochemical oxygen demand (BOD). The system is scalable for municipal and industrial applications and provides the potential for water reuse in various sectors, including irrigation and industrial processes. By integrating these three treatment methods, the invention offers a robust, automated, and cost-effective solution for wastewater treatment, ensuring both environmental protection and operational efficiency.

[0044] Description

[0045] Electrochemical System Combined with Fenton Oxidation and Ozonation

[0046] In this study, a complete treatment system called the electrochemical system combined with Fenton oxidation and ozonation is proposed to remove pollutants from wastewater. This system can remove a large portion of organic pollutants present in domestic and industrial wastewater. Although the system has high efficiency, its design and construction are very costly and require a large area. Skilled personnel are also needed to operate the system. All parts of the system are described in detail in the following sections. To achieve high efficiency in removing wastewater pollutants, a combination of three methods — electrochemical treatment, Fenton oxidation, and ozonation — is used. The proposed system includes nine separate units, each of which will be examined in detail in the following sections. Figure 1 shows a diagram of the proposed system.

[0047] Electrochemical Treatment and Sedimentation Tank (Unit 1)

[0048] Electrochemical treatment is an effective, simple, and low-cost method for removing a wide range of organic and inorganic compounds from industrial and even municipal wastewater. In this system, electrodes — usually made of aluminum, iron, or graphite — are placed in the reactor (Figure 2). Half of the electrodes act as anodes, and the other half as cathodes. A power supply, typically with an output voltage below 40 volts for worker safety, provides the required electricity. When electricity is applied to the electrodes, water begins to split into hydrogen and oxygen. Oxygen and hydrogen molecules form thousands of very small bubbles that are released into the tank. These bubbles slowly rise to the water surface and attach to suspended particles in the wastewater, causing them to float. The oxygen gas also dissolves in the water, increasing the dissolved oxygen (DO) concentration. This helps reduce potential odors in the following treatment units. Passing electric current through the water also triggers extensive electrochemical reactions among the organic and inorganic substances in the wastewater. As a result, some organic compounds are mineralized or converted into simpler organic compounds, which can be more easily removed in subsequent treatment stages.

[0049] As mentioned, aluminum, iron, and graphite electrodes can be used in the electrochemical treatment tank. When graphite electrodes (inactive electrodes) are used, only the previously mentioned electrochemical reactions occur. However, when iron or aluminum electrodes are used, in addition to these reactions, Fe3+or Al3+ions are released into the wastewater. These ions are strong coagulants and can aggregate many suspended particles in the wastewater into large flocs. In many cases, these flocs are lighter than water and float on the surface. Therefore, equipment to collect floating particles should be installed for this tank. To prevent anaerobic conditions and control odors, the electrolysis process should be repeated at least once every hour. Each pair of electrodes is connected to a power supply with a voltage between 24 and 40 volts and a current appropriate for the reactor volume. It is recommended that transformer voltages be adjustable between 24 and 40 volts so that, by adjusting the voltage under different conditions, energy consumption can be reduced while achieving more effective electrochemical treatment. Although voltages above 40 volts may improve treatment efficiency, it is recommended that they be used only under special conditions with all necessary safety measures, due to the risk of electric shock. Aluminum electrodes are recommended because they provide the highest efficiency for removing pollutants and suspended solids. Iron electrodes can also be used, and in special operational conditions, replacing aluminum electrodes with iron ones can help adjust system efficiency. Electrodes need to be periodically removed from the tank, and the inactive layer formed on their surface should be removed by sanding. After cleaning, the electrodes are reinstalled. Over time, the electrodes must also be replaced to maintain system performance. In the sedimentation section, inclined plates are installed at an angle between 45 and 60 degrees. These plates ensure sufficient efficiency in removing suspended particles in the sedimentation tank. If the angle is less than 45 degrees or greater than 60 degrees, the plates will not improve system performance. It is expected that a significant portion of the pollutants in the wastewater will be removed in this section.

[0050] Fenton Process (Units 2, 3, and 5)

[0051] The Fenton process is an effective method for treating wastewater containing recalcitrant compounds. During the Fenton process, hydroxyl radicals ( OH) are produced from the reaction between iron ions and hydrogen peroxide. These radicals are strong oxidants with a standard potential of 1.8 volts versus the standard hydrogen electrode and can degrade a wide range of organic compounds. Many of the resistant organics are mineralized, ultimately converting into carbon dioxide, water, and mineral salts. The iron used in this process can also act as a coagulant, helping to clarify the water. The Fenton unit consists of three parts: an oxidation tank, chemical storage tanks, and a pH adjustment tank (Figure 3). Since the Fenton process works best under acidic conditions, one of the tanks shown in Figure 3 contains sulfuric acid. Another tank is dedicated to a basic substance, such as sodium hydroxide for pH adjustment after the process. The tanks must be made of corrosion-resistant materials. The pH of the Fenton oxidation tank is lowered to around 3 by adding sulfuric or hydrochloric acid. A suitable amount of ferrous sulfate solution (Fe2+) and hydrogen peroxide is then dosed into the oxidation tank using dosing pumps, with quantities determined from pilot-scale laboratory tests. After completing the Fenton process, the wastewater is transferred to the pH adjustment tank, where the pH is raised back to 7 using a sodium hydroxide solution. All of these steps are controlled automatically by computer software. The software monitors pH in real-time through sensors in both the oxidation tank and the pH tank. After this stage, the wastewater moves to the next unit, ozonation.

[0052] Anaerobic Sludge Digestion Tank (Unit 4)

[0053] In most treatment units, settled or floating residual materials form, which are called sludge. Sludge is a mixture of organic and inorganic compounds that must have its organic portion decomposed and then be dried. The dried sludge is disposed of as hazardous waste following special procedures. Figure 4 shows a diagram of the sludge digester. In this digester, sludge enters and remains for a long period, often several months. During this time, the sludge is thoroughly mixed by a mixer. During anaerobic digestion, methane gas is produced. This gas must be collected by a suction system and transferred to a flare for burning. After complete digestion, the sludge is gradually transferred to drying beds, where it dries. The dried sludge is finally collected and disposed of as hazardous waste.

[0054] Sedimentation Tank (Unit 6)

[0055] The sedimentation tank in this system is used to separate suspended particles formed in different treatment sections. By reducing the water flow velocity, the tank allows suspended particles to settle. The settled particles are eventually transferred to the sludge digestion tank using a sludge pump.

[0056] Ozonation System and Ozone Contact Tank (Units 7 and 8)

[0057] Ozone is one of the strongest oxidants and can decompose any remaining organic compounds that have survived previous treatment stages. In this section, very resistant compounds, even at low concentrations, are eliminated. Ozone decomposes quickly and converts into molecular oxygen, which increases the dissolved oxygen (DO) in the wastewater and helps prevent odor formation. Ozonation also fully disinfects the wastewater, making it suitable for subsequent uses. After ozonation, the wastewater enters the ozone contact tank and remains there for about half an hour to allow the ozone to achieve maximum effect.

[0058] Chlorine Contact Tank (Unit 9)

[0059] Since ozone does not leave a residual in the water, the use of treated wastewater — especially for irrigation — can be limited. Therefore, chlorine is added to the wastewater to maintain a residual concentration of 0.3 to 0.8 mg / L, making the water suitable for broader applications. Typically, chlorine gas is dissolved in water to produce a high- concentration stock solution. This stock chlorine solution is injected into the wastewater using a dosing pump. The amount of chlorine injected should be determined during operation by performing a breakpoint test and measuring the residual chlorine in the treated wastewater.

[0060] Brief Description of Drawings

[0061] Figure 1 : Diagram of the proposed units for maximum removal of organic compounds from wastewater

[0062] Figure 2: Diagram of the electrochemical treatment and sedimentation tank (Unit 1)

[0063] Figure 3: Diagram of the Fenton process

[0064] Figure 4: Sludge digester diagram

[0065] Figure 5: Sedimentation tank diagram

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Claims

Claims1. A hybrid system for treating urban and industrial wastewater, comprising: (a) an electrochemical treatment unit equipped with aluminum, iron, or graphite electrodes for the removal of suspended and dissolved contaminants, wherein coagulant ions are generated during electrolysis; (b) a Fenton oxidation unit configured to mix hydrogen peroxide (H2O2) and iron ions (Fe2+) under acidic conditions to generate hydroxyl radicals ( OH) for degrading refractory organic compounds; and (c) an ozonation unit for further oxidation of remaining pollutants, increasing dissolved oxygen and disinfecting the treated effluent.

2. The system of claim 1 , wherein the electrochemical treatment unit further comprises a sedimentation tank for settling or floating coagulated particles, with sloped plates installed at an angle of 45°-60° to enhance removal efficiency.

3. The system of claim 1 , wherein the Fenton oxidation unit includes: (a) an oxidation tank, (b) a chemical storage tank for hydrogen peroxide and iron sulfate solution, and (c) a pH adjustment tank to maintain the optimal pH for the Fenton reaction and neutralize effluent post- treatment.

4. The system of claim 1 , wherein the ozonation unit includes a contact tank configured to retain the wastewater for a period of 30 minutes to ensure complete reaction with ozone.

5. The system of claim 1 , further comprising a chlorine contact tank to add residual chlorine (0.3-0.8 mg / L) for extended disinfection and to prepare the effluent for reuse in irrigation or industrial applications.

6. The system of claim 1 , wherein the hybrid combination of electrochemical, Fenton, and ozonation processes allows for high removal efficiency of chemical oxygen demand and biochemical oxygen demand and the mineralization of persistent organic compounds.

7. The system of claim 1 , wherein operation parameters including voltage, current density, chemical dosage, and retention time are controlled automatically via a computer software system to optimize treatment efficiency.

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