Three-Phase Three-Dimensional Electrochemical Process for Efficient Treatment of Greywater

Water shortages around the world have intensified the search for substitute sources. Greywater can serve as a solution for water requirements. Compared to two-dimensional electrochemical processes for water treatment, the addition of particle activated carbon enhances the conductivity and mass transfer or the adsorption of pollutants in a three-dimensional (3D) electrochemical process. The large specific surface areas of these particles can provide more reactive sites, resulting in a higher removal efficiency. In this study, the treatment of greywater by the electro-Fenton (E-Fenton) method was carried out in a 3D electrolytic reactor. The effects of the operating conditions, such as electrode spacing, applied voltage, treatment time, and activated carbon loading, on the efficacy of the E-Fenton process were investigated, and the corresponding optimum conditions were found to be 7 cm, 9 V, 2 h, and 10 g. The results showed that CODCr removal of greywater treated using the 3D electrochemical process was 85%. With the help of the Box–Behnken experiment design and the response surface methodology, the parameters were optimized to determine the optimal conditions. The results of the response surface analysis were consistent with the experimental results. The above findings illustrate that the proposed three-phase 3D electrochemical process is feasible for the efficient treatment of greywater.


Introduction
With the ongoing global progress in social and economic development, the problem of water shortages is becoming increasingly alarming, especially due to the unwise and inefficient use of water resources [1][2][3]. The World Water Council projects that global water consumption will increase by approximately 50% by 2034 [4]. In China, greywater accounts for approximately 30% of urban domestic wastewater [5]. Since it is moderately polluted, it can be recycled and reused. From an environmental perspective, it is wiser to recycle greywater than further pollute urban wastewaters [6]. Draining greywater directly (i.e., without treatment) into a drainage system will cause pollution of natural water system [7][8][9]. Moreover, it will produce destructive and cumulative biological diseases and have a greater impact on human health [10,11]. For example, most cases of enteric virus infections originate from contaminated drinking water resources, recreational waters, and foods contaminated by sewage and sewage effluent waters [12,13].
Within this context, conventional biological treatment does not always achieve satisfactory results, and traditional physicochemical methods are relatively expensive, ineffective, or may lead to secondary contamination. For example, the dissolved air flotation method [14][15][16] involves injection of a large number of dense bubbles into treated wastewater, whereupon impurities adhere to the bubbles, effectively forming a liquid with a density less than that of water. The primary disadvantage of this treatment method is that it is difficult to directly contact the suspended sludge, which results in secondary sludge formation.
In coagulation-flocculation treatment methods [17,18], colloidal particles in contaminated water collide and agglomerate, thus forming larger particles or flocs. However, these methods are expensive and ineffective at removing anionic detergents and pathogenic pollutants from greywater [19].
Advancements in water treatment technologies enable efficient treatment of wastewater [20][21][22]. Electrochemical technologies are a huge improvement in the field of wastewater treatment because of their high efficiency, environmental protection, and versatility. Despite these advantages, conventional two-dimensional (2D) electrochemical electrodes have a mass transfer limitation, small space-time yield, and low area-volume ratio. The development of three-dimensional (3D) electrochemical electrodes provides an outstanding solution to the above shortcomings that limit the application of 2D electrodes. Compared with conventional electrochemical technologies, 3D electrochemical processes can overcome the shortcomings of plane electrode design due to the increased electrode surface area per reactor unit volume and higher throughput. This enables high current efficiency, improved productivity, compact design, decolorization, and efficient removal of heavy metals. Moreover, the biochemical characteristics of processed wastewater can also be improved. High treatment capacity, lack of secondary pollution, and mild reaction conditions are among the other advantages of this technology [23,24]. Table 1 presents a comparison of the performances of a 3D electrochemical process and other physiochemical treatment processes for different target pollutants. It clearly illustrates the high efficiency of the 3D electrochemical process in COD Cr removal. However, the particle electrode may lose its adsorption capacity and catalytic activity due to the accumulation of pollutants on particle surfaces over continuous runs [25]. In general, 3D electrochemical technology stimulates the further development of electrocatalysis technology with the aim of applying it to treatment of highly concentrated wastewaters [26][27][28]. This can also help solve the problem of the treatment and reuse of greywater [29][30][31]. 1 Chemical Oxygen Demand. 2 Coagulation and intermittent sand filtration. 3 Photocatalytic electrolysis membrane reactor. 4 External loop airlift membrane bioreactor. 5 Divided electrolysis cell. 6 Two-dimensional electrochemical technology. 7 Three-dimensional electrochemical technology. 8 Fish sauce manufacturing. 9 Coarse sand. 10 Membrane area. 11 Chloride concentration. 12 Current density. 13 Reaction time.
Herein, the experiments were carried out in a homemade 3D electrode reactor. The effects of the interelectrode spacing, voltage, treatment time, and activated carbon loading on the performance of greywater treatment were investigated. The feasibility and efficiency of the 3D electrochemical process for the treatment of greywater were also verified. Additional analysis aimed at the optimization of the process parameters was performed using the response surface method and the Box-Behnken design [37][38][39]. These findings are expected to encourage the application of 3D electrochemical technology in greywater Membranes 2022, 12, 514 3 of 13 treatment. A three-dimensional process can serve as a pretreatment process to increase the biodegradability of effluent. This will be a trend in future development.  ). The shower gel and activated carbon were purchased from a local market. Ultrapure water was produced in laboratory.

Preparation of Simulated Greywater
According to a certain proportion (see Table 2), the reagents were weighed and dissolved in pure water and then mixed well under ultrasound. The simulated greywater was characterized by a high concentration and complex composition. The characteristics of the simulated greywater water are listed in Table 3, which was provided by Jiangsu Longmem Environmental Technology Co., Ltd. (Changzhou, China).

The Electrolysis System
The mechanism of the electro-Fenton (E-Fenton) method [40] involves the reduction of O 2 to H 2 O 2 at the cathode, which produces •OH radicals via the subsequent Fenton reaction involving Fe 2+ . These radicals then oxidize organic matter to CO 2 and H 2 O or small organic molecules [41,42].
The dioxygen required for Reaction (1) can be supplied to the cathode of the electrolysis reactor by means of external aeration or produced on the anode according to Reactions (3) or (4). The constructed E-Fenton system with 3D electrodes is capable of degrading pollutants in different ways [43,44]. In addition to the direct oxidation at the anode, the cathode has strong adsorption and catalytic properties, which can reduce the dissolved oxygen present in the system to H 2 O 2 . In the presence of H 2 O 2 and added Fe 2+ ions, •OH radicals are generated during the Fenton reaction and oxidize the organic matter. In addition, the electric field between the main electrodes can also cause the activated carbon particles to be charged with positive and negative charges due to the fact of electrostatic induction, forming an independent miniature electrolytic cell. As a result, electrochemical redox reactions can proceed simultaneously on the surface of each particle. The mechanism of the electrolysis reaction is presented in Figure 1.
The dioxygen required for Reaction (1) can be supplied to the cathode of the elect ysis reactor by means of external aeration or produced on the anode according to R tions (3) or (4).
The constructed E-Fenton system with 3D electrodes is capable of degrading po tants in different ways [43,44]. In addition to the direct oxidation at the anode, the cath has strong adsorption and catalytic properties, which can reduce the dissolved oxy present in the system to H2O2. In the presence of H2O2 and added Fe 2+ ions, •OH radi are generated during the Fenton reaction and oxidize the organic matter. In addition, electric field between the main electrodes can also cause the activated carbon particle be charged with positive and negative charges due to the fact of electrostatic induct forming an independent miniature electrolytic cell. As a result, electrochemical redox actions can proceed simultaneously on the surface of each particle. The mechanism of electrolysis reaction is presented in Figure 1.

Pretreatment of the Particle Electrodes
In this experiment, the activated carbon particles were repeatedly washed sev times beforehand in order to avoid the adsorption effect of the activated carbon on effectiveness of the 3D electrodes in treating greywater. The cleaned activated carbon ultrasonically treated in the greywater. Each ultrasonic treatment step was carried out 3 h. After three repeated ultrasonic treatments, the adsorption of activated carbon w considered to have reached saturation.

Three-Dimensional Electrodes
In this experiment, a homemade 3D electrode reactor was used. The reactor was b from transparent organic glass and had a usable volume of 1.5 L. A stainless-steel p was used as the anode, and a graphite plate with a thickness of 2 mm and an effec treatment area of 70 cm 2 was used as the cathode.
The prepared greywater was added to the catalytic reactor, followed by the addi of the weighed quantity of granulated activated carbon. The experimental device is sho in Figure 1.

Electro-Fenton Process for Greywater Using Three-Dimensional Electrodes
The analysis methods of water quality correlation are shown in Table 3. The CO of the greywater was determined by the potassium dichromate method [45] using a CO

Pretreatment of the Particle Electrodes
In this experiment, the activated carbon particles were repeatedly washed several times beforehand in order to avoid the adsorption effect of the activated carbon on the effectiveness of the 3D electrodes in treating greywater. The cleaned activated carbon was ultrasonically treated in the greywater. Each ultrasonic treatment step was carried out for 3 h. After three repeated ultrasonic treatments, the adsorption of activated carbon was considered to have reached saturation.

Three-Dimensional Electrodes
In this experiment, a homemade 3D electrode reactor was used. The reactor was built from transparent organic glass and had a usable volume of 1.5 L. A stainless-steel plate was used as the anode, and a graphite plate with a thickness of 2 mm and an effective treatment area of 70 cm 2 was used as the cathode.
The prepared greywater was added to the catalytic reactor, followed by the addition of the weighed quantity of granulated activated carbon. The experimental device is shown in Figure 1.

Electro-Fenton Process for Greywater Using Three-Dimensional Electrodes
The analysis methods of water quality correlation are shown in Table 3. The COD Cr of the greywater was determined by the potassium dichromate method [45] using a COD Cr detector (HACH DR3900, Loveland, CO, USA). The conductivity of the greywater was analyzed by a conductivity meter (HACH HQ40d, Loveland, CO, USA). The voltage in the 3D electrode system was provided by a DC regulated power supply (GWINSTEK GPS-3030DD).

The Effect of Electrode Spacing on the Degree of Greywater Treatment
The effectiveness of the proposed treatment in decreasing chemical oxygen demand (COD Cr ) and other characteristics of greywater was studied by varying the process parame-

The Effect of Electrode Spacing on the Degree of Greywater Treatment
The effectiveness of the proposed treatment in decreasing chemical oxygen dema (CODCr) and other characteristics of greywater was studied by varying the process para eters within reasonable limits: voltage, 5-11 V; treatment time, 0-5 h; interelectrode sp ing of 3, 5, and 7 cm; activated carbon loading of 10 g. The results are presented in  As can be seen in Figure 2, with the increase in interelectrode spacing, the CO removal of greywater increased [46]. When the other variables were kept constant, t was mainly due to the small distance between electrodes and the low energy of the el trolytic system, which affected the mass transfer efficiency. With the increase in the d tance between electrodes, the mass transfer process became more intensive due to concentration gradient between organic matter and solution. This improved the efficien of the degradation of the organic pollutants.

The Treatment Time
At early stages of the treatment process, CODCr removal increased rapidly with increase in processing time. After a period of time, the CODCr removal basically remain As can be seen in Figure 2, with the increase in interelectrode spacing, the COD Cr removal of greywater increased [46]. When the other variables were kept constant, this was mainly due to the small distance between electrodes and the low energy of the electrolytic system, which affected the mass transfer efficiency. With the increase in the distance between electrodes, the mass transfer process became more intensive due to the concentration gradient between organic matter and solution. This improved the efficiency of the degradation of the organic pollutants.

The Treatment Time
At early stages of the treatment process, COD Cr removal increased rapidly with the increase in processing time. After a period of time, the COD Cr removal basically remained unchanged. This was mainly because the concentration of organic matter in the system gradually decreased during electrolysis and the catalytic effect diminished. The results are shown in Figure 3a. unchanged. This was mainly because the concentration of organic matter in the system gradually decreased during electrolysis and the catalytic effect diminished. The results are shown in Figure 3a.

Applied Voltage
With the increase in voltage, the CODCr removal efficiency initially tended to increase but then decreased. The voltage affected the amount and rate of •OH production. If the voltage was too small, the voltage on the particle electrode was insufficient, resulting in less •OH and a weaker catalytic effect. Thus, the voltage at the particle electrode could not reach the anode or cathode. Contact between the particle electrode causes short circuiting, which reduces the efficiency of the electrolytic process, and a high voltage. The electrodes were subject to side reactions, such as hydrogen evolution reactions, which affected the current efficiency and reduced the effectiveness of the CODCr removal.

Activated Carbon Loading
With the increase in activated carbon loading, the CODCr removal from the greywater by the 3D electrodes showed a trend of first increasing and then decreasing but basically remained above 60%. The highest CODCr removal of 85.7% was achieved at 10 g of activated carbon. This was mainly because the amount of particle electrodes added affects the electrolysis efficiency of the system. The lower the activated carbon loading, the fewer reaction sites are involved in the reaction, resulting in a lower electrolysis efficiency of the system. When the activated carbon loading increased, there were more reaction sites in the system, shortening the mass transfer distance between pollutants. However, when the activated carbon loading was excessive, the increased resistance caused the system to have side effects, resulting in a higher temperature of the system, thus reducing the electrolysis rate.
From Table 4, it can be seen that the 3D electrochemical process had a higher CODCr removal efficiency than the conventional 2D electrochemical process in the treatment of wastewater.

Applied Voltage
With the increase in voltage, the COD Cr removal efficiency initially tended to increase but then decreased. The voltage affected the amount and rate of •OH production. If the voltage was too small, the voltage on the particle electrode was insufficient, resulting in less •OH and a weaker catalytic effect. Thus, the voltage at the particle electrode could not reach the anode or cathode. Contact between the particle electrode causes short circuiting, which reduces the efficiency of the electrolytic process, and a high voltage. The electrodes were subject to side reactions, such as hydrogen evolution reactions, which affected the current efficiency and reduced the effectiveness of the COD Cr removal.

Activated Carbon Loading
With the increase in activated carbon loading, the COD Cr removal from the greywater by the 3D electrodes showed a trend of first increasing and then decreasing but basically remained above 60%. The highest COD Cr removal of 85.7% was achieved at 10 g of activated carbon. This was mainly because the amount of particle electrodes added affects the electrolysis efficiency of the system. The lower the activated carbon loading, the fewer reaction sites are involved in the reaction, resulting in a lower electrolysis efficiency of the system. When the activated carbon loading increased, there were more reaction sites in the system, shortening the mass transfer distance between pollutants. However, when the activated carbon loading was excessive, the increased resistance caused the system to have side effects, resulting in a higher temperature of the system, thus reducing the electrolysis rate.
From Table 4, it can be seen that the 3D electrochemical process had a higher COD Cr removal efficiency than the conventional 2D electrochemical process in the treatment of wastewater. Greywater U = 9 V, GAC 7 = 10 g, ES 8 = 7 cm, HRT = 120 min 85 This work 1 Two-dimensional electrochemical reactor. 2 Three-dimensional electrochemical reactor. 3 Heavy oil refinery. 4 Current density. 5 Reaction time. 6 NaCl concentration. 7 Granular activated carbon. 8 Electrode spacing.

Changes in the Electrical Conductivity of Greywater during Treatm
The conductivity of greywater changes depending on its sal catalytic process proceeded, more of the solute in solution was i tivity increased. However, in general, the change in conductivity (Figure 4).

Changes in the Turbidity of Greywater during Treatment
As can be seen in Figure 5, the turbidity of the treated water d the first 1-2 h of treatment. However, it started to decrease mo subsequent 3 h of treatment (i.e., 2-5 h since the beginning of the turbidity was significantly reduced during the electrocatalytic p and porous structure of activated carbon [51] (Figure 1). The atom face was not saturated with surface energy and, thus, the surface adsorption of molecules. As the treatment time increased, the tu decreased, and the adsorption capacity of activated carbon gradu

Changes in the Turbidity of Greywater during Treatment
As can be seen in Figure 5, the turbidity of the treated water decreased rapidly during the first 1-2 h of treatment. However, it started to decrease more gradually during the subsequent 3 h of treatment (i.e., 2-5 h since the beginning of the process). The greywater turbidity was significantly reduced during the electrocatalytic process due to the loose and porous structure of activated carbon [51] (Figure 1). The atomic force field on its surface was not saturated with surface energy and, thus, the surface energy was reduced via adsorption of molecules. As the treatment time increased, the turbidity of treated water decreased, and the adsorption capacity of activated carbon gradually reached saturation. Membranes 2022, 12, x FOR PEER REVIEW Figure 5. Effect of treatment time on the turbidity of greywater (the activated carbon load 10 g, the interelectrode spacing was 7 cm, and the voltage was 9 V).

Box-Behnken Design and Response Surface Methodology
Through previous experiments, it is found that when the interelectrode spac 7 cm, the voltage was 9 V, the activated carbon loading was 10 g, and the proces was 2 h, the CODCr of greywater treated using the 3D electrode decreased by 85%. optimization of the process parameters-voltage, treatment time, and activated loading-was performed using the Box-Behnken experimental design and the r surface methodology (RSM). The values for these three factors in run 3, as obtain the steepest ascent path (Table 5), were taken as the central points. The respective high levels for each factor were coded as shown in Table 6. Fitting the experimen using regression analysis gave the following second-order polynomial equation: where Y is the predicted CODCr removal; A, B, and C are the code variables for time, and activated carbon loading, respectively. The obtained F-value of 1671.80 implies that the model was significant. Th only a 0.01% chance that such a large F-value was due to the fact of noise. Based o values for A, B, and C, the relative influence of the three factors on CODCr remo lowed the order: time > voltage > activated carbon loading. The "predicted R-s value of 0.9938 was in reasonable agreement with the "adjusted R-squared" v 0.9989", i.e., the difference was less than 0.2. The p-value is usually used to test th Figure 5. Effect of treatment time on the turbidity of greywater (the activated carbon loading was 10 g, the interelectrode spacing was 7 cm, and the voltage was 9 V).

Box-Behnken Design and Response Surface Methodology
Through previous experiments, it is found that when the interelectrode spacing was 7 cm, the voltage was 9 V, the activated carbon loading was 10 g, and the process period was 2 h, the COD Cr of greywater treated using the 3D electrode decreased by 85%. Further optimization of the process parameters-voltage, treatment time, and activated carbon loading-was performed using the Box-Behnken experimental design and the response surface methodology (RSM). The values for these three factors in run 3, as obtained from the steepest ascent path (Table 5), were taken as the central points. The respective low and high levels for each factor were coded as shown in Table 6. Fitting the experimental data using regression analysis gave the following second-order polynomial equation: where Y is the predicted COD Cr removal; A, B, and C are the code variables for voltage, time, and activated carbon loading, respectively.  The obtained F-value of 1671.80 implies that the model was significant. There was only a 0.01% chance that such a large F-value was due to the fact of noise. Based on the F-values for A, B, and C, the relative influence of the three factors on COD Cr removal followed the order: time > voltage > activated carbon loading. The "predicted R-squared" value of 0.9938 was in reasonable agreement with the "adjusted R-squared" value of 0.9989", i.e., the difference was less than 0.2. The p-value is usually used to test the significance of a variable. The smaller the p-value, the more significant the corresponding variable. As shown in Table 7, the p-values for A, B, and C were much less than 0.0001, indicating that the voltage, time, and activated carbon loading are important process parameters influencing the removal of COD Cr .

Results of the Response Surface Optimization of the Proposed Greywater Treatment Method
The response surfaces are presented in Figures 6-8 in the form of 3D surfaces and contour plots. As can be seen from the figures, the response surfaces were convex with each plot representing an optimal condition, and the variables had maxima. In addition, Figure 8 shows a better ellipse, indicating better interaction between the variables representing time and activated carbon loading. However, the interaction between voltage, time, and activated carbon loading was not significant, which is consistent with the results of the response surface analysis. The response surface analysis showed that the greywater COD Cr decreased by 88.51% at a voltage of 8.68 V, treatment duration of 2.50 h, and an activated carbon loading of 10.28 g.

Conclusions
A method for greywater treatment using 3D electrodes was developed and applied with good results. Single-factor experiments show that the treatment duration, voltage,

Conclusions
A method for greywater treatment using 3D electrodes was developed and applied with good results. Single-factor experiments show that the treatment duration, voltage,

Conclusions
A method for greywater treatment using 3D electrodes was developed and applied with good results. Single-factor experiments show that the treatment duration, voltage,

Conclusions
A method for greywater treatment using 3D electrodes was developed and applied with good results. Single-factor experiments show that the treatment duration, voltage, and activated carbon loading are three key factors influencing the COD Cr level of greywater. The Box-Behnken design and the response surface method were used for more advanced optimization of the three factors listed above and to determine the optimal reaction conditions. Specifically, it was found that for a voltage of 8.7 V, a treatment duration of 2.5 h, and an activated carbon loading of 10.3 g, the COD Cr decreased by 88.5%. When the interelectrode spacing, voltage, treatment duration, and activated carbon loading were 7 cm, 9 V, 2 h, and 10 g, respectively, the COD Cr of treated greywater decreased by 85.6%. The experimental values and the predicted values coincided well.
Author Contributions: Conceptualization, methodology, W.L. and P.Z.; software P.Z.; validation, formal analysis, investigation and resources W.L. and P.Z.; data curation, writing-original draft preparation and visualization P.Z.; supervision, project administration and funding acquisition W.W.; writing-review and editing, W.L. and W.W. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.