# Energy Plasma Graphite Wastewater Treatment System Based on Internet of Things Platform

Authors: Haobo Zhang; Chunlian Song; Jinmao Li
Journal: Wireless Communications and Mobile Computing
 (2022)

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## Abstract

If graphite plasma wastewater cannot be properly treated, it may cause serious pollution to the natural environment. In this paper, the conditions of wastewater degradation by high-voltage pulse discharge are optimized. On this basis, it is proposed to improve the discharge power supply and reactor to effectively strengthen the discharge and improve the degradation rate of organic matter. The wastewater is degraded by high-voltage pulse discharge and coagulation Fenton oxidation. The specific degradation effect is as follows: when the initial pH of aeration is about 10.00 mm and the initial pH of aeration is about 80 mm/L, the specific degradation effect of wastewater is about 1.00-200 mm. The degradation rate of chemical oxygen demand in 120 min can reach 49.84%. The graphite plasma wastewater treatment system designed based on the Internet of things platform has made a great contribution to the balance between industrial production and natural environment in China.


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## Body

## 1. Introduction

With the rapid development of China’s metallurgy, chemical industry, machinery, medical devices, nuclear energy, automobile, aerospace, and other industries, the demand for graphite and carbon products is growing. China’s graphite and carbon products industry will continue to grow rapidly in the future. According to statistics, the compound annual growth rate of sales revenue of China’s graphite and carbon products industry is 36.56%. Graphite will cause certain pollution to the environment in the production and manufacturing process, mainly including water pollution and atmospheric dust pollution. Among them, water pollution mainly includes alkali washing wastewater, acid leaching wastewater, acid washing wastewater, graphite sediment, and workshop cleaning wastewater produced by graphite workshop. The main characteristics of this graphite wastewater are the following: the wastewater contains acid-base substances and a small amount of metal ions, which is not only high in concentration and complex in composition but also relatively harmful. It belongs to a class of industrial pollutants strictly controlled by the state, which has a serious impact on the surrounding ecological environment [1]. Most of the existing graphite wastewater treatment processes are to precipitate, neutralize, oxidize, and filter the wastewater and then discharge it directly. Although this treatment method can reduce the pollution index, due to the large amount of chemical thunder, it will increase the output and load of sludge, and some high concentration brine contained in the wastewater will also cause secondary pollution [2].Advanced oxidation technology is a popular technology in recent years. It is a technology for the efficient treatment of organic pollutants in wastewater through strong oxidation active substances. The treatment of wastewater by high-voltage pulse discharge technology is one of the advanced oxidation technologies. This technology discharges the gas and liquid system through high-voltage pulse discharge power supply to produce low-temperature plasma, high-energy electrons, and ultraviolet light to degrade the wastewater. It is one of the effective treatment methods of wastewater [3, 4]. The Internet of things is an important part of the new generation of information technology. As the name suggests, the Internet of things is “the Internet connected with things.” This has two meanings: first, the core and foundation of the Internet of things is still the Internet, which is an extended and expanded network based on the Internet; second, its client extends and extends to any object to object for information exchange and communication. Therefore, the definition of the Internet of things is the following: through radio frequency identification (RFID), infrared sensor, global positioning system, laser scanner, and other information sensing equipment, according to the agreed protocol, connect any object with the Internet for information exchange and communication, so as to realize the intelligent identification, positioning, tracking, monitoring, and management of objects. With the continuous development of Internet of things technology, its technology is also widely used in various fields. The secondary utilization of domestic wastewater by using the sensor technology of the Internet of things can effectively protect the environment and alleviate the problems of water resources, which has played a role in promoting the implementation of sustainable development and building a harmonious society [5].


## 2. Literature Review

In today’s era of rapid development of “information and material,” according to the needs of industrial upgrading, the Internet of things products of sewage treatment systems will also be born. Therefore, the Internet of things is used to realize the remote control effect of equipment by using network technology, so as to realize man-machine separation and remote office. This feature also changes the traditional working mode in the factory during the current epidemic, reduces personnel contact, avoids the risk of cross-infection, and completes the epidemic prevention work. There is another highlight of the Internet of things function: operators can work anytime and anywhere, operate on computers, and operate on mobile phones. This function breaks the limitation of space and has great convenience. Using the function of the Internet of things can not only ensure that people do not contact each other but also improve work efficiency. It can also appropriately reduce the flow of people and the burden it brings to the factory, so as to bring more benefits to the factory and avoid some safety accidents [6]. Water is one of the necessary resources for human survival and development. With the continuous increase of population, the sustainable development of industrial and agricultural production, and the continuous improvement of residents’ living standards, the demand for water in human society will be greater and greater. With the development of human economic activities and the continuous progress of social civilization, the problem of water pollution has become increasingly prominent [7]. With the gradual increase of wastewater discharge, how to treat wastewater more effectively has become one of the hotspots of current research. There are many kinds of wastewater treatment methods, which are mainly classified as follows.


### 2.1. Physicochemical Method

The physicochemical method is the abbreviation of the physicochemical treatment method. There are mainly two kinds of physical methods to treat wastewater. One is aimed at removing insoluble suspended solids in water. The pure physical method is adopted. The commonly used treatment equipment and methods include sedimentation (sand setting), grid (screen), filtration, air flotation, and centrifugation (cyclone). Another treatment method is aimed at the removal of substances dissolved in wastewater, mainly including the adsorption method and extraction method.


### 2.2. Flocculation Method

Flocculation is the aggregation process of colloidal particles and microsuspended solids in liquid, which can play the role of flocculation and coagulation, so as to separate waste. The action mechanism of the flocculation method is the following: put the coagulant into the water and dissociate many positive ions. The negative charge on the surface of colloidal particles can be neutralized due to the existence of positive ions, which reduces the repulsion between particles and destroys the steady state of fine colloidal particles in water, so that the particles can get close to each other and condense into flocculent fine particles, so that larger flocs can be formed and precipitated through adsorption, winding, and bridging, so as to achieve the separation effect.


### 2.3. Membrane Separation

As a separation material, the membrane is characterized by selective separation. Membrane separation is a process that uses the function of membrane selective separation to realize the separation, purification, and concentration of different components of feed liquid 71. Compared with traditional filtration, the difference of membrane separation is that the membrane can be separated in the molecular range, and this process does not need phase transition or any additives. It is a physical process. The molecular weight (or pore diameter) of the membrane is generally micron. According to the different molecular weights of the membrane, the membrane can be divided into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.


### 2.4. Electrochemical Method

Electrochemistry is widely used in advanced wastewater treatment. These electrochemical technologies include electrocoagulation, capacitor desalination, and internal electrolysis 1241 [8, 9].


### 2.5. Biological Method

The biological method is to put microorganisms in sewage and use the decomposition characteristics of microorganisms to decompose organic matter into inorganic matter. The commonly used methods include the activated sludge process and biofilter process.


### 2.6. Advanced Oxidation Process

Advanced oxidation methods include Fenton oxidation, ozone oxidation, photooxidation, wet oxidation, sonochemical oxidation, and high-voltage pulse discharge. In this paper, the high-voltage pulse discharge method is used to treat wastewater. High-voltage pulse discharge technology is a kind of low-temperature plasma technology. Plasma is the fourth form of matter. It is a kind of conductive fluid. It is electrically neutral. It is composed of electrons, positive and negative ions, free radicals, neutral particles, and excited atoms. It is different from the solid state, liquid state, and gas state. They can be divided into two categories. According to the different characteristics of plasma, they are nonequilibrium plasma and equilibrium plasma.Equilibrium plasma is also known as thermal plasma. The thermal plasma system continuously introduces high electric energy, which can produce high-throughput heat flow. Through the thermal incineration process, it can deal with difficult water pollutants. Nonequilibrium plasma is also called low-temperature plasma. The whole system is in a low-temperature state and does not reach thermodynamic equilibrium, so it can provide high selectivity and energy efficiency [10]. At the same time, compared with equilibrium plasma, low-temperature plasma is easier to produce and has a wider prospect of industrial application. Based on these facts, plasma treatment of wastewater has attracted more and more attention [11]. High-voltage pulse discharge is a new type of water treatment technology, which combines a variety of advanced oxidation technologies, such as sonochemistry, high-energy electron radiation, and photochemistry, and can effectively decompose organic pollutants [12]. This technology has the advantages of good application effect, low equipment cost, no secondary pollution, and so on. At present, the treatment of wastewater by the high-voltage pulse discharge method is at the laboratory level, and its industrial application is less [13]. Several units have carried out high-voltage pulse discharge technology research. The research results show that the high-voltage pulse discharge method can improve the biodegradability of wastewater and high ammonia nitrogen removal rate, especially for PCBs, phthalates, heterocyclic compounds, and polycyclic aromatic hydrocarbons that are difficult to remove in wastewater. The application of the high-voltage pulse discharge method in wastewater treatment has attracted the attention of more and more scholars.


## 2.1. Physicochemical Method

The physicochemical method is the abbreviation of the physicochemical treatment method. There are mainly two kinds of physical methods to treat wastewater. One is aimed at removing insoluble suspended solids in water. The pure physical method is adopted. The commonly used treatment equipment and methods include sedimentation (sand setting), grid (screen), filtration, air flotation, and centrifugation (cyclone). Another treatment method is aimed at the removal of substances dissolved in wastewater, mainly including the adsorption method and extraction method.


## 2.2. Flocculation Method

Flocculation is the aggregation process of colloidal particles and microsuspended solids in liquid, which can play the role of flocculation and coagulation, so as to separate waste. The action mechanism of the flocculation method is the following: put the coagulant into the water and dissociate many positive ions. The negative charge on the surface of colloidal particles can be neutralized due to the existence of positive ions, which reduces the repulsion between particles and destroys the steady state of fine colloidal particles in water, so that the particles can get close to each other and condense into flocculent fine particles, so that larger flocs can be formed and precipitated through adsorption, winding, and bridging, so as to achieve the separation effect.


## 2.3. Membrane Separation

As a separation material, the membrane is characterized by selective separation. Membrane separation is a process that uses the function of membrane selective separation to realize the separation, purification, and concentration of different components of feed liquid 71. Compared with traditional filtration, the difference of membrane separation is that the membrane can be separated in the molecular range, and this process does not need phase transition or any additives. It is a physical process. The molecular weight (or pore diameter) of the membrane is generally micron. According to the different molecular weights of the membrane, the membrane can be divided into microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.


## 2.4. Electrochemical Method

Electrochemistry is widely used in advanced wastewater treatment. These electrochemical technologies include electrocoagulation, capacitor desalination, and internal electrolysis 1241 [8, 9].


## 2.5. Biological Method

The biological method is to put microorganisms in sewage and use the decomposition characteristics of microorganisms to decompose organic matter into inorganic matter. The commonly used methods include the activated sludge process and biofilter process.


## 2.6. Advanced Oxidation Process

Advanced oxidation methods include Fenton oxidation, ozone oxidation, photooxidation, wet oxidation, sonochemical oxidation, and high-voltage pulse discharge. In this paper, the high-voltage pulse discharge method is used to treat wastewater. High-voltage pulse discharge technology is a kind of low-temperature plasma technology. Plasma is the fourth form of matter. It is a kind of conductive fluid. It is electrically neutral. It is composed of electrons, positive and negative ions, free radicals, neutral particles, and excited atoms. It is different from the solid state, liquid state, and gas state. They can be divided into two categories. According to the different characteristics of plasma, they are nonequilibrium plasma and equilibrium plasma.Equilibrium plasma is also known as thermal plasma. The thermal plasma system continuously introduces high electric energy, which can produce high-throughput heat flow. Through the thermal incineration process, it can deal with difficult water pollutants. Nonequilibrium plasma is also called low-temperature plasma. The whole system is in a low-temperature state and does not reach thermodynamic equilibrium, so it can provide high selectivity and energy efficiency [10]. At the same time, compared with equilibrium plasma, low-temperature plasma is easier to produce and has a wider prospect of industrial application. Based on these facts, plasma treatment of wastewater has attracted more and more attention [11]. High-voltage pulse discharge is a new type of water treatment technology, which combines a variety of advanced oxidation technologies, such as sonochemistry, high-energy electron radiation, and photochemistry, and can effectively decompose organic pollutants [12]. This technology has the advantages of good application effect, low equipment cost, no secondary pollution, and so on. At present, the treatment of wastewater by the high-voltage pulse discharge method is at the laboratory level, and its industrial application is less [13]. Several units have carried out high-voltage pulse discharge technology research. The research results show that the high-voltage pulse discharge method can improve the biodegradability of wastewater and high ammonia nitrogen removal rate, especially for PCBs, phthalates, heterocyclic compounds, and polycyclic aromatic hydrocarbons that are difficult to remove in wastewater. The application of the high-voltage pulse discharge method in wastewater treatment has attracted the attention of more and more scholars.


## 3. Research Methods

### 3.1. High-Voltage Pulse Discharge Reactor

In order to put high-voltage pulsed discharge plasma technology into better use in industrial production, researchers are committed to the research and development of high-efficiency reactors. The improvement direction mainly includes the following aspects: (1) the effect of gas-liquid mass transfer needs to be further improved to increase the yield of free radicals, improve the spatial distribution of plasma, and optimize the mass transfer interface of the reactor; (2) a mixed gas-liquid reactor was developed, which can make full use of the synergistic effect of gas-liquid combined discharge to stimulate the degradation activity of the reactor; and (3) other new reactors were developed [14]. Based on the analysis of the existing technology, the types of reactors used for high-voltage pulse discharge plasma mainly include the following.New reactors include the wire simple reactor, needle plate reactor, ring simple reactor, wire plate (or plate) reactor, rod reactor, gas-liquid mixed reactor, three-phase filled reactor, and dielectric barrier discharge reactor [15].The discharge device used in this study uses the dielectric barrier discharge method. The dielectric barrier discharge (DBD) can discharge uniformly and stably under atmospheric pressure (normal temperature and pressure) to produce a large area of low-temperature plasma, as shown in Figure1. An insulating medium is inserted into the discharge space. When sufficient AC voltage is applied between the two electrodes, the huge voltage and current will cause the system temperature to rise sharply and produce a certain electric field intensity distribution [16]. The gas between the two electrodes will be broken down and ionized, and the surrounding air will also be ionized out of charged particles. The charged electrons will be accelerated to exchange momentum with the surrounding neutral gas molecules, producing dense plasma in the filament discharge channel, accompanied by acoustic, photoelectric, and thermal reactions [17]. Its plasma temperature and density are moderate, and it produces more active particles than ordinary chemical reactors. The discharge process has the advantage of easy regulation. The barrier medium can use quartz, ceramics, glass, and other materials. Due to the existence of the barrier medium, the general power supply system will use AC voltage. Discharge characteristics are related to voltage frequency, voltage amplitude, gas gap, electrode structure, and shape [18, 19].Figure 1
Dielectric barrier (DBD) discharge reactor.The treatment of wastewater by high-voltage pulse discharge technology is one of the advanced oxidation technologies. It can efficiently treat organic pollutants in wastewater through strong oxidation active substances. It is a multidimensional water treatment technology integrating light, electricity, and chemical oxidation [20]. This technology discharges the gas and liquid system through the high-voltage pulse discharge power supply to produce low-temperature plasma, high-energy electrons, and ultraviolet light to degrade the wastewater. It is one of the effective treatment methods of wastewater [21]. In this paper, firstly, the wastewater was treated by high-voltage pulse discharge, and the effects of acidity (initial pH value), electrode distance, Fe2+ addition, initial wastewater concentration, and aeration on its degradation were investigated. Acidity (initial pH value) and Fe2+ addition affect the hydroxyl radical concentration of active substances, electrode spacing affects the discharge mode, the initial concentration affects the treatment capacity, and the aeration rate affects the dissolved oxygen and discharge mode.


## 3.1. High-Voltage Pulse Discharge Reactor

In order to put high-voltage pulsed discharge plasma technology into better use in industrial production, researchers are committed to the research and development of high-efficiency reactors. The improvement direction mainly includes the following aspects: (1) the effect of gas-liquid mass transfer needs to be further improved to increase the yield of free radicals, improve the spatial distribution of plasma, and optimize the mass transfer interface of the reactor; (2) a mixed gas-liquid reactor was developed, which can make full use of the synergistic effect of gas-liquid combined discharge to stimulate the degradation activity of the reactor; and (3) other new reactors were developed [14]. Based on the analysis of the existing technology, the types of reactors used for high-voltage pulse discharge plasma mainly include the following.New reactors include the wire simple reactor, needle plate reactor, ring simple reactor, wire plate (or plate) reactor, rod reactor, gas-liquid mixed reactor, three-phase filled reactor, and dielectric barrier discharge reactor [15].The discharge device used in this study uses the dielectric barrier discharge method. The dielectric barrier discharge (DBD) can discharge uniformly and stably under atmospheric pressure (normal temperature and pressure) to produce a large area of low-temperature plasma, as shown in Figure1. An insulating medium is inserted into the discharge space. When sufficient AC voltage is applied between the two electrodes, the huge voltage and current will cause the system temperature to rise sharply and produce a certain electric field intensity distribution [16]. The gas between the two electrodes will be broken down and ionized, and the surrounding air will also be ionized out of charged particles. The charged electrons will be accelerated to exchange momentum with the surrounding neutral gas molecules, producing dense plasma in the filament discharge channel, accompanied by acoustic, photoelectric, and thermal reactions [17]. Its plasma temperature and density are moderate, and it produces more active particles than ordinary chemical reactors. The discharge process has the advantage of easy regulation. The barrier medium can use quartz, ceramics, glass, and other materials. Due to the existence of the barrier medium, the general power supply system will use AC voltage. Discharge characteristics are related to voltage frequency, voltage amplitude, gas gap, electrode structure, and shape [18, 19].Figure 1
Dielectric barrier (DBD) discharge reactor.The treatment of wastewater by high-voltage pulse discharge technology is one of the advanced oxidation technologies. It can efficiently treat organic pollutants in wastewater through strong oxidation active substances. It is a multidimensional water treatment technology integrating light, electricity, and chemical oxidation [20]. This technology discharges the gas and liquid system through the high-voltage pulse discharge power supply to produce low-temperature plasma, high-energy electrons, and ultraviolet light to degrade the wastewater. It is one of the effective treatment methods of wastewater [21]. In this paper, firstly, the wastewater was treated by high-voltage pulse discharge, and the effects of acidity (initial pH value), electrode distance, Fe2+ addition, initial wastewater concentration, and aeration on its degradation were investigated. Acidity (initial pH value) and Fe2+ addition affect the hydroxyl radical concentration of active substances, electrode spacing affects the discharge mode, the initial concentration affects the treatment capacity, and the aeration rate affects the dissolved oxygen and discharge mode.


## 4. Result Analysis

### 4.1. High-Voltage Pulse Discharge Power Supply and Reactor

In this experiment, the switch is the gas discharge tube power supply and the high-voltage pulse discharge power supply. The power supply is mainly composed of five parts: 1000 V DC charging power supply (400 A of the peak pulse current of the power supply, 3μF of the energy storage capacitance, and about 20 μs of the current pulse width), two gas discharge tubes, the trigger circuit of the discharge tube, the high-voltage pulse transformer, and the energy storage capacitor. The reactor is made of plexiglass, with an inner diameter of 100 mm, the diameter of the upper and lower plates of 95 mm, and the two pole plates are stainless steel plates. The high-voltage electrode is immersed in the solution, the earth electrode is placed on the liquid level, the distance between the electrodes is adjusted through three cylinders, and the gas is introduced from the aeration holes distributed at the bottom of the reactor [22, 23].The effect of the pulse method on wastewater treatment is to determine the chemical oxygen demand value of the water body to calculate the most degradation rate of chemical oxygen demand.


#### 4.1.1. COD Concentration Analysis Method

In the laboratory, the standard curve of COD is drawn according to the K2Cr2O7 oxidation method. The standard curve is y=0.0024x+0.0148,R2=0.999, There is a good linear relationship between 50 ppm and 250 ppm [24].COD removal rateη is used to express the effect of the pulse method on wastewater treatment. Its definition formula is as follows:
(1)η%=COD0−CODCOD0×100%.COD0 is the chemical oxygen demand value of simulated wastewater; CODc is the chemical oxygen demand value after discharge treatment. The unit is ppm.


### 4.2. Effect of Initial pH on Wastewater Degradation

Divalent iron ions are easy to form iron hydroxide precipitation under alkaline conditions. In order to avoid precipitation, the pH of simulated wastewater in the research process is less than 7.00. Adjust the initial pH of concentrated sulfuric acid to 2.00, 3.00, 4.01/3.99, 5.01, and 5.99, respectively. Take 50 mL of wastewater, add ferrous sulfate heptahydrate to make the content of divalent iron 1.80 mm, and adjust the distance between the upper and lower plates to 10 mm and 15 mm, respectively. The gas-liquid two-phase mixed discharge is adopted, and the aeration capacity is 200 L/h. The effect of discharge time on the chemical oxygen demand of wastewater is investigated, and the results are shown in Figure2.Figure 2
Effect of initial pH on COD degradation of wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes less with the extension of time. At the same time, when the initial pH of the solution is 3.00, the effect of high-voltage pulse discharge on the degradation of wastewater is the best. At 120 min, the COD of wastewater with initial pH of 3.00 decreased from 1383 ppm to 694 ppm, and the degradation rate reached 49.84%.


### 4.3. Effect of Electrode Spacing

When the electrode spacing is 3 mm and 5 mm, the upper plate is below the liquid level before discharge, the discharge mode is liquid phase discharge, and the chemical oxygen demand decreases slowly. When the electrode spacing is 10 mm, the upper electrode plate is just between the liquid level before discharge, and the discharge mode is gas-liquid mixed discharge. When the discharge spacing is 15 mm and 20 mm, the upper plate is above the liquid level before discharge, and the discharge mode is still gas-liquid mixed discharge.Adjust the initial pH of concentrated sulfuric acid to 3.00, take 50 mL of wastewater, add ferrous sulfate heptahydrate to make the content of divalent iron 1.80 mm, adjust the distance between the upper and lower plates to 3 mm, 5 mm, 10 mm, 15 mm, and 20 mm, respectively, and the aeration volume is 200 L/h. The effect of discharge time on chemical oxygen demand of wastewater is investigated, as shown in Figure3.Figure 3
Effect of electrode spacing on COD degradation in wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes less with the extension of time. At the same time, when the electrode spacing is 10 mm-15 mm, the effect of high-voltage pulse discharge degradation of wastewater is better. At 120 min, the COD of wastewater with electrode spacing of 10 mm-15 mm decreased from 1383 ppm to 694 ppm-711 ppm, and the degradation rate reached more than 48.63%.


### 4.4. Effect of Fe2+ Addition

Adjust the initial pH of concentrated sulfuric acid to 3.00; take 50 mL of wastewater, respectively; add ferrous sulfate heptahydrate to make the content of divalent iron 0, 1.08 mM, 1.80 mM, 2.52 mM, and 3.60 mM, respectively; adjust the distance between the upper and lower plates to 10 mm and 15 mm, respectively; adopt gas-liquid two-phase mixed discharge; and the aeration volume is 200 L/h. The effect of discharge time on chemical oxygen demand of wastewater is investigated. The experimental results are shown in Figure4.Figure 4
Effect of Fe2+ addition on COD degradation of wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes little with the extension of time. At the same time, when the amount of divalent iron is 1.80 mM, the effect of high-voltage pulse discharge on the degradation of wastewater is the best. At 120 min, the chemical oxygen demand with the addition of 1.80 mM of divalent iron decreased from 1383 ppm to 706 ppm, and the degradation rate reached 48.93%.


### 4.5. Effect of Initial Concentration

The pH of the two-phase mixed sulfuric acid is adjusted to be 1.80 mm/h, and the pH of the two-phase mixed sulfuric acid is adjusted to be 1.80 mm/h between the two-phase aeration plates. The pH of the two-phase mixed sulfuric acid is adjusted to be 1.80 mm/h between the two-phase aeration plates. Adjust the initial concentration of wastewater to 454 ppm/524 ppm, 916 ppm/1058 ppm, 1383 ppm/1587 ppm, and 1854 ppm/2125 ppm, respectively. The effect of discharge time on chemical oxygen demand of wastewater is investigated, and the experimental results are shown in Figure5.Figure 5
Effect of initial concentration on COD degradation of wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes less with the extension of time. At 120 min, the COD of wastewater with an initial concentration of 454 ppm decreased to 183 ppm, the degradation rate reached 59.65%, and the COD removal amount was 271 ppm. The COD of wastewater with initial concentrations of 916 ppm and 1383 ppm decreased to 420 ppm and 694 ppm, the degradation rate was 54.15% and 49.84%, and the COD removal was 496 ppm and 689 ppm. The COD of the wastewater with the initial concentration of 1854 ppm decreased to 1086 ppm, the degradation rate was only 41.45%, and the COD removal amount reached 769 ppm.


### 4.6. Effect of Aeration Rate

Adjust the initial pH of concentrated sulfuric acid to 3.00, take 50 mL of wastewater, add ferrous sulfate heptahydrate to make the content of divalent iron 1.80 mM, and adjust the distance between the upper and lower plates to 10 mm and 15 mm, respectively. The gas-liquid two-phase mixed discharge is adopted, the air is introduced into the aeration pump, and the aeration rate is adjusted to 0, 50 L/h, 100 L/h, 200 L/h, 400 l/h, 600 L/h, 800 L/h, and 1000 L/h, respectively. The effect of discharge time on the chemical oxygen demand of wastewater is investigated, and the experimental results are shown in Figure6.Figure 6
Effect of aeration rate on COD degradation of wastewater.The experimental results show that the aeration rate has a certain impact on the degradation of wastewater by high-voltage pulse discharge. When the electric pulse time is not more than 90 min, the chemical oxygen demand of wastewater decreases rapidly with the increase of pulse time. When the pulse time exceeds 90 min until the pulse time reaches 120 min, the decreasing trend of chemical oxygen demand of simulated wastewater with the increase of pulse time becomes gentle; that is, the pulse time has little impact on the degradation process. The experimental results also show that under the same conditions, the degradation effect is different with different aeration rates. The COD degradation rate of wastewater with pulse time of 120 min and aeration rate of 0-1000 L/h is 9.749%-49.69%. When the aeration rate is 200 L/h, the effect of wastewater degradation by high-voltage pulse discharge is the best, and the chemical oxygen demand decreases from 1387 ppm to 698 ppm. When there is no aeration, the effect of electric pulse degradation of wastewater is poor. Under a certain pulse time, the degradation rate of organic wastewater degraded by high-voltage pulse discharge increases with the increase of aeration. When the aeration rate reaches a certain value, the aeration rate is increased, the degradation rate becomes lower, and the effect of high-pressure pulse treatment also increases with the increase of the aeration rate until it is close to the optimal value. The optimum aeration rate is 200 L/h.


### 4.7. Actual Product Completion Diagram

The above experiments optimize and improve the graphite wastewater treatment system. The actual product drawings are shown in Figures7–9.Figure 7
Plasma reactor.Figure 8
Electrical control device.Figure 9
Overall internal diagram.


## 4.1. High-Voltage Pulse Discharge Power Supply and Reactor

In this experiment, the switch is the gas discharge tube power supply and the high-voltage pulse discharge power supply. The power supply is mainly composed of five parts: 1000 V DC charging power supply (400 A of the peak pulse current of the power supply, 3μF of the energy storage capacitance, and about 20 μs of the current pulse width), two gas discharge tubes, the trigger circuit of the discharge tube, the high-voltage pulse transformer, and the energy storage capacitor. The reactor is made of plexiglass, with an inner diameter of 100 mm, the diameter of the upper and lower plates of 95 mm, and the two pole plates are stainless steel plates. The high-voltage electrode is immersed in the solution, the earth electrode is placed on the liquid level, the distance between the electrodes is adjusted through three cylinders, and the gas is introduced from the aeration holes distributed at the bottom of the reactor [22, 23].The effect of the pulse method on wastewater treatment is to determine the chemical oxygen demand value of the water body to calculate the most degradation rate of chemical oxygen demand.


### 4.1.1. COD Concentration Analysis Method

In the laboratory, the standard curve of COD is drawn according to the K2Cr2O7 oxidation method. The standard curve is y=0.0024x+0.0148,R2=0.999, There is a good linear relationship between 50 ppm and 250 ppm [24].COD removal rateη is used to express the effect of the pulse method on wastewater treatment. Its definition formula is as follows:
(1)η%=COD0−CODCOD0×100%.COD0 is the chemical oxygen demand value of simulated wastewater; CODc is the chemical oxygen demand value after discharge treatment. The unit is ppm.


## 4.1.1. COD Concentration Analysis Method

In the laboratory, the standard curve of COD is drawn according to the K2Cr2O7 oxidation method. The standard curve is y=0.0024x+0.0148,R2=0.999, There is a good linear relationship between 50 ppm and 250 ppm [24].COD removal rateη is used to express the effect of the pulse method on wastewater treatment. Its definition formula is as follows:
(1)η%=COD0−CODCOD0×100%.COD0 is the chemical oxygen demand value of simulated wastewater; CODc is the chemical oxygen demand value after discharge treatment. The unit is ppm.


## 4.2. Effect of Initial pH on Wastewater Degradation

Divalent iron ions are easy to form iron hydroxide precipitation under alkaline conditions. In order to avoid precipitation, the pH of simulated wastewater in the research process is less than 7.00. Adjust the initial pH of concentrated sulfuric acid to 2.00, 3.00, 4.01/3.99, 5.01, and 5.99, respectively. Take 50 mL of wastewater, add ferrous sulfate heptahydrate to make the content of divalent iron 1.80 mm, and adjust the distance between the upper and lower plates to 10 mm and 15 mm, respectively. The gas-liquid two-phase mixed discharge is adopted, and the aeration capacity is 200 L/h. The effect of discharge time on the chemical oxygen demand of wastewater is investigated, and the results are shown in Figure2.Figure 2
Effect of initial pH on COD degradation of wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes less with the extension of time. At the same time, when the initial pH of the solution is 3.00, the effect of high-voltage pulse discharge on the degradation of wastewater is the best. At 120 min, the COD of wastewater with initial pH of 3.00 decreased from 1383 ppm to 694 ppm, and the degradation rate reached 49.84%.


## 4.3. Effect of Electrode Spacing

When the electrode spacing is 3 mm and 5 mm, the upper plate is below the liquid level before discharge, the discharge mode is liquid phase discharge, and the chemical oxygen demand decreases slowly. When the electrode spacing is 10 mm, the upper electrode plate is just between the liquid level before discharge, and the discharge mode is gas-liquid mixed discharge. When the discharge spacing is 15 mm and 20 mm, the upper plate is above the liquid level before discharge, and the discharge mode is still gas-liquid mixed discharge.Adjust the initial pH of concentrated sulfuric acid to 3.00, take 50 mL of wastewater, add ferrous sulfate heptahydrate to make the content of divalent iron 1.80 mm, adjust the distance between the upper and lower plates to 3 mm, 5 mm, 10 mm, 15 mm, and 20 mm, respectively, and the aeration volume is 200 L/h. The effect of discharge time on chemical oxygen demand of wastewater is investigated, as shown in Figure3.Figure 3
Effect of electrode spacing on COD degradation in wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes less with the extension of time. At the same time, when the electrode spacing is 10 mm-15 mm, the effect of high-voltage pulse discharge degradation of wastewater is better. At 120 min, the COD of wastewater with electrode spacing of 10 mm-15 mm decreased from 1383 ppm to 694 ppm-711 ppm, and the degradation rate reached more than 48.63%.


## 4.4. Effect of Fe2+ Addition

Adjust the initial pH of concentrated sulfuric acid to 3.00; take 50 mL of wastewater, respectively; add ferrous sulfate heptahydrate to make the content of divalent iron 0, 1.08 mM, 1.80 mM, 2.52 mM, and 3.60 mM, respectively; adjust the distance between the upper and lower plates to 10 mm and 15 mm, respectively; adopt gas-liquid two-phase mixed discharge; and the aeration volume is 200 L/h. The effect of discharge time on chemical oxygen demand of wastewater is investigated. The experimental results are shown in Figure4.Figure 4
Effect of Fe2+ addition on COD degradation of wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes little with the extension of time. At the same time, when the amount of divalent iron is 1.80 mM, the effect of high-voltage pulse discharge on the degradation of wastewater is the best. At 120 min, the chemical oxygen demand with the addition of 1.80 mM of divalent iron decreased from 1383 ppm to 706 ppm, and the degradation rate reached 48.93%.


## 4.5. Effect of Initial Concentration

The pH of the two-phase mixed sulfuric acid is adjusted to be 1.80 mm/h, and the pH of the two-phase mixed sulfuric acid is adjusted to be 1.80 mm/h between the two-phase aeration plates. The pH of the two-phase mixed sulfuric acid is adjusted to be 1.80 mm/h between the two-phase aeration plates. Adjust the initial concentration of wastewater to 454 ppm/524 ppm, 916 ppm/1058 ppm, 1383 ppm/1587 ppm, and 1854 ppm/2125 ppm, respectively. The effect of discharge time on chemical oxygen demand of wastewater is investigated, and the experimental results are shown in Figure5.Figure 5
Effect of initial concentration on COD degradation of wastewater.The experimental results show that the chemical oxygen demand decreases rapidly within 90 min of discharge time, and after 90 min, the chemical oxygen demand changes less with the extension of time. At 120 min, the COD of wastewater with an initial concentration of 454 ppm decreased to 183 ppm, the degradation rate reached 59.65%, and the COD removal amount was 271 ppm. The COD of wastewater with initial concentrations of 916 ppm and 1383 ppm decreased to 420 ppm and 694 ppm, the degradation rate was 54.15% and 49.84%, and the COD removal was 496 ppm and 689 ppm. The COD of the wastewater with the initial concentration of 1854 ppm decreased to 1086 ppm, the degradation rate was only 41.45%, and the COD removal amount reached 769 ppm.


## 4.6. Effect of Aeration Rate

Adjust the initial pH of concentrated sulfuric acid to 3.00, take 50 mL of wastewater, add ferrous sulfate heptahydrate to make the content of divalent iron 1.80 mM, and adjust the distance between the upper and lower plates to 10 mm and 15 mm, respectively. The gas-liquid two-phase mixed discharge is adopted, the air is introduced into the aeration pump, and the aeration rate is adjusted to 0, 50 L/h, 100 L/h, 200 L/h, 400 l/h, 600 L/h, 800 L/h, and 1000 L/h, respectively. The effect of discharge time on the chemical oxygen demand of wastewater is investigated, and the experimental results are shown in Figure6.Figure 6
Effect of aeration rate on COD degradation of wastewater.The experimental results show that the aeration rate has a certain impact on the degradation of wastewater by high-voltage pulse discharge. When the electric pulse time is not more than 90 min, the chemical oxygen demand of wastewater decreases rapidly with the increase of pulse time. When the pulse time exceeds 90 min until the pulse time reaches 120 min, the decreasing trend of chemical oxygen demand of simulated wastewater with the increase of pulse time becomes gentle; that is, the pulse time has little impact on the degradation process. The experimental results also show that under the same conditions, the degradation effect is different with different aeration rates. The COD degradation rate of wastewater with pulse time of 120 min and aeration rate of 0-1000 L/h is 9.749%-49.69%. When the aeration rate is 200 L/h, the effect of wastewater degradation by high-voltage pulse discharge is the best, and the chemical oxygen demand decreases from 1387 ppm to 698 ppm. When there is no aeration, the effect of electric pulse degradation of wastewater is poor. Under a certain pulse time, the degradation rate of organic wastewater degraded by high-voltage pulse discharge increases with the increase of aeration. When the aeration rate reaches a certain value, the aeration rate is increased, the degradation rate becomes lower, and the effect of high-pressure pulse treatment also increases with the increase of the aeration rate until it is close to the optimal value. The optimum aeration rate is 200 L/h.


## 4.7. Actual Product Completion Diagram

The above experiments optimize and improve the graphite wastewater treatment system. The actual product drawings are shown in Figures7–9.Figure 7
Plasma reactor.Figure 8
Electrical control device.Figure 9
Overall internal diagram.


## 5. Conclusion

In this paper, the optimal conditions of wastewater degradation by high-voltage pulse discharge are investigated. On this basis, it is proposed to improve the discharge power supply and reactor to effectively strengthen the discharge and improve the degradation rate of organic matter. The wastewater is degraded by high-voltage pulse discharge and coagulation Fenton oxidation. The specific conclusions are as follows: the initial pH is 3.00, and the electrode spacing is 10 mm-15 mm. When the ferrous iron addition amount is 1.80 mM and the aeration amount is 200 L/h, the degradation effect of wastewater with an initial concentration of about 1380 ppm by high-voltage pulse discharge is the best, and the degradation rate of chemical oxygen demand in 120 min can reach 49.84%. This graphite wastewater treatment device is mainly composed of the discharge device, high-frequency and high-voltage power supply, reactor, and water storage tank. Among them, the transformation and optimization of the power generation device is the focus of the whole equipment design. The quality of the discharge device will directly affect the effect of wastewater treatment, which is mainly affected by discharge mode, discharge material, and device tightness. The high-frequency and high-voltage power supply is the source of the energy of the whole system, and the stability of the power supply voltage will also have an immeasurable impact on the experiment. Therefore, the power supply voltage of the graphite wastewater treatment system adopts the formed product as the guarantee of the stable output of the experimental energy. There are two main ideas in the design of the reactor. One is to put the electrode directly into the graphite waste liquid for discharge, but this will produce microbubbles around the discharge electrode, which will short circuit the equipment. In this case, the wasted energy of generating microbubbles will account for about 75% of the initial injected energy, and the utilization rate of electric energy will also be reduced. Therefore, another scheme was selected when optimizing the reactor, which abandoned the design and research of direct discharge in water. Instead, the discharge electrode is discharged in the gas, and then, the plasma gas generated by the discharge is flushed into the graphite wastewater through the conduit, which can not only avoid the risk of energy loss and short circuit but also fully integrate the discharge gas and graphite wastewater, so as to achieve the expected effect of degrading graphite wastewater. It has far-reaching significance for the degradation of graphite wastewater.First of all, there is still a lot of work to be done in the basic research of high-voltage pulse discharge technology in wastewater treatment, and the understanding of the reaction mechanism needs to be deepened. The high-voltage pulse reaction is very complex, and it is necessary to further explore the generation and transformation process of active substances and their physical effects on the degradation process, rationally optimize the process, and systematically explain the mechanism of high-voltage pulse oxidation. In addition, a deeper investigation of how the high-voltage pulse interacts with various contaminant molecules in the mother liquor is required. In addition, studies on the effects on the solubility, volatility, hydrophobic/hydrophilic properties, thermal stability, and diffusivity of the system should be carried out.



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*Source: 5746553-2022-05-19.xml*