Performance of Continuous Electrocoagulation Processes (CEPs) as an Efficient Approach for the Treatment of Industrial Organic Pollutants: A Comprehensive Review

Abstract

Electrocoagulation (EC) processes have emerged as an efficient solution for different inorganic and organic effluents. The main characteristics of this versatile process are its ease of operation and low sludge production. The literature indicates that EC can be successfully used as a single process or a step within a combined treatment system. If used in a combined system, this process could be employed as a pre-, a post-, or middle treatment step. Additionally, the EC process has been used in both continuous and batch modes. In most studies, EC has achieved significant improvements in the treated water quality and relatively low total energy consumption. This review presents a comprehensive evaluation and analysis of standalone and combined continuous EC processes. The influence of key operational parameters on continuous EC performance is thoroughly discussed. Furthermore, recent advancements in reactor design, modeling, and process optimization are addressed. The benefits of integrating other treatment processes with the EC process, such as advanced oxidation, membranes, chemical coagulation, and adsorption, are also evaluated. The performance of most standalone and combined EC processes used for organic pollutant treatment and published in the last 25 years is critically analyzed. This review is expected to give researchers many insights to improve their treatment scenario with recent and efficient environmental experiences, sustainability, and circular economy. The clearly presented information is expected to guide researchers in selecting efficient, cost-effective, and time-saving treatment alternatives. The findings ensure the considerable potential of continuous EC treatment processes for organic pollutants. However, more research is warranted to enhance process design, operational efficiency, scale-up, and economic viability.

1. Introduction

Currently, various types of organic and inorganic waste are produced from human activities. Most of these pollutants, which are mainly found in industrial wastewater effluents, are toxic and may represent a problem for public health and could cause a threat to ecosystems if not successfully managed [1,2,3]. For this reason, research is being carried out to remove harmful pollutants from wastewater. These harmful chemicals include both natural and synthetic organic and inorganic compounds and ions, such as pharmaceuticals, dyes, fats, fluoride, arsenic, heavy metal ions, and many others [4,5,6,7]. Several conventional treatment technologies have been utilized to manage both wastewater and drinking water. These treatment technologies include adsorption [8,9,10], Nano filtration [4,9,11], chemical coagulation [12,13,14], advanced oxidation [15], biodegradation [16,17], electrocoagulation (EC) [18,19], and magnetic separation [20]. These treatment processes have both advantages and disadvantages. Accordingly, the efficient use and feasibility of these different processes depend mainly on the special application. The main drawbacks include the high maintenance and operating costs, the production of large volumes of secondary pollutants that need management, such as activated sludge, and the requirement for the use of some chemicals. Moreover, most of the treatment processes are still in the lab scale, and their scale-up faces many challenges [18,19,20].

Accordingly, improvement of the performance of the present processes and development of more efficient techniques is of primary interest to obtain improved treatment technologies of high removal efficiency, effectiveness, and sustainable performance [21]. In the last decades, the EC process has emerged as an efficient and fast treatment process compared to the previous traditional processes. This treatment process provides important merits as an environmentally friendly approach due to its versatility, simple setup, and simple operation. In addition, the EC process usually produces minimum sludge quantities with no need for new chemicals [22,23,24]. Moreover, the solar-powered EC process provides a low operational cost alternative since it allows for efficient removal of pollutants with a minimal energy cost [25,26,27]. However, most of the research applications of EC processes are still in the batch mode and on the lab scale. It is noticed that single and combined continuous EC processes have been applied for two decades to treat both organic and inorganic pollutants [28,29,30]. Moreover, promising and significant findings concerning continuous EC treatment processes have been reported in dozens of published research studies in the last decades.

According to the intensive literature survey, about 60 papers have been published in the last 25 years concerning continuous EC processes for organic pollutant removal. In this review, the term organic pollutants encompass a broad spectrum of anthropogenic and naturally occurring compounds found in wastewater, including dyes, phenols, pharmaceuticals, petrochemical derivatives, humic substances, and organic acids. While the focus is primarily on conventional organic pollutants commonly found in industrial effluents, this review also references treatment efforts related to emerging contaminants, such as endocrine-disrupting compounds (EDCs), pharmaceuticals, and, to a limited extent, per- and polyfluoroalkyl substances (PFASs). However, due to the limited number of CEP studies specifically targeting PFASs, their treatment remains outside the core scope of this review. Instead, emphasis is placed on pollutants that have been more extensively studied in continuous EC systems over the past 25 years. In addition, in all these papers, the conclusions have confirmed that the performance of these continuous EC processes for organic pollutant removal is superior to batch EC processes. The distribution of the published papers between standalone and combined EC processes, the type of treated pollutant, and the years of publication are shown in Figure 1. Data in Figure 1 were collected from Scopus and Web of Science databases using the search terms “continuous electrocoagulation” AND “organic pollutant” OR “industrial wastewater” in titles, abstracts, and keywords. The search included peer-reviewed journal articles published between 2000 and 2025 since EC is a recent technology, and it is rarely cited before the year 2000. Duplicates were removed, and only English-language articles focusing on continuous (non-batch) EC applications were included.

Figure 1a shows that publications on continuous standalone and combined EC processes have increased significantly over the past ten years. In the periods 2000–2004, 2005–2009, 2010–2014, 2015–2019, and 2020–2025, there are 1, 1, 5, 12, and 15 publications for standalone [3,28,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62] and 0, 0, 4, 5 and 10 publications for combined EC [25,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80], respectively. On the other hand, the number of publications for the four major types of wastewaters: dyes, industrial, agricultural, and pretreated, are 14, 11, 7, and 2 publications for standalone and 8, 9, 1, and 3 publications for combined EC processes, respectively. These results indicate that more attention is being given to continuous, standalone, combined EC systems due to their significant percentage removal, low cost, and ease of operation.

Despite the proven effectiveness of EC processes, a comprehensive review of continuous EC systems remains largely absent in the literature. A thorough literature survey reveals that most existing reviews have focused primarily on batch-mode EC processes [6,18,21,30], with limited attention given to continuous systems. EC systems can be operated in batch or continuous modes. In batch EC systems, a fixed volume of wastewater is treated in a closed reactor for a defined time, after which the treated water is removed. This approach is simple but less efficient for large-scale applications due to downtime between cycles, lower throughput, and limited automation. In contrast, continuous EC systems allow wastewater to flow continuously through the reactor, enabling consistent treatment, greater scalability, and better integration into industrial processes. Al-Qodah et al. [25] emphasized the growing need for continuous EC designs, particularly for high-flow industrial effluents, due to their superior operational stability and potential for energy optimization. Additionally, EC processes can be categorized as standalone systems, where electrocoagulation is used independently, or combined systems, where EC is integrated with other treatment technologies such as advanced oxidation, membrane filtration, chemical coagulation, or adsorption. Defining these terms is essential because their configuration significantly impacts removal efficiency, cost, and feasibility. Despite the promising advantages of continuous EC, most published research has focused on batch-mode systems. Therefore, this review aims to address this gap by critically analyzing the performance, design innovations, and optimization strategies of continuous EC processes for organic pollutant removal.

Although López-Guzmán et al. [2] provided a useful overview of continuous EC systems, their review was restricted to single and combined processes targeting pharmaceutical pollutants. To the best of our knowledge, no study to date has comprehensively examined continuous EC applications for a broader range of organic pollutants across various wastewater types. This review aims to fill that gap by evaluating the current state and potential advancements in continuous EC treatment technologies for wastewater contaminated with organic compounds. It systematically summarizes, compares, and analyzes the influence of key EC operational parameters on pollutant removal efficiency. Moreover, it highlights innovative reactor configurations, modeling approaches, and optimization tools that have been developed for continuous EC systems. Economic feasibility, environmental impact, and future research directions are also discussed.

This review covers studies published over the past 25 years and offers critical insights into the performance and practicality of continuous EC processes. By providing a broad analysis that extends beyond chemical oxygen demand (COD) removal, it compiles data on various indicators, including color, turbidity, total organic carbon (TOC), biochemical oxygen demand (BOD₅), humic substances, phenols, and microbial contaminants such as E. coli and fecal coliforms. This work is intended to guide researchers and practitioners toward more efficient, sustainable, and cost-effective wastewater treatment solutions.

2. Continuous Electrocoagulation Processes for Organic Pollutants

2.1. Standalone EC Treatment Processes

Kim et al. [31] reported the first research on standalone continuous electrocoagulation processes (CEPs) for organic effluents. They studied the role of current density (CD), which is the electric current applied per unit area of the electrode, electrode number (n), representing the total number of electrodes used in the system, dyes and electrolyte concentration, electrode interdistance, pH, and flow rate on color removal from synthetic wastewater by a continuous EC unit. They found that the dye percentage removal and reaction rate constant, k, which quantifies the rate of pollutant degradation, values were directly proportional to the electrode number, current density, and electrolyte concentration but inversely proportional to electrode interdistance, dye concentration, and flow rate. In addition, dye removal efficiency was higher when using aluminum compared to SUS and Fe electrodes, and pH had a small effect on it. In addition, power consumption increased with increasing CD, electrode interdistance, and electrolyte concentration but decreased as the electrode number increased. On the other hand, the flow rate, dye concentration, and solution pH did not significantly affect power consumption.

The second study using a continuous EC cell was performed by Mollah et al. [32], who studied the removal of orange II dye with NaCl electrolyte solution. Their cell consisted of five parallel Fe electrodes. They examined several parameters, including dye concentration, flow rate (F), CD and pH, finding that dye removal approached 98.5% under optimal conditions. The analysis of the residue by XRD indicated the formation of γ-Fe₂O₃ and Fe₃O₄ after the oxidation of the Fe electrodes. Subsequently, Merzouk et al. [3] studied CEP performance for COD and dye removal from synthetic wastewater, assessing the impact of conductivity, influent pH, inlet dye concentration, CD, and pollutant residence time to optimize process performance. They reported that COD and color percentage removal reached 85% each, with an initial COD of 2.5 g·L⁻¹ and dye concentration below 0.20 g·L⁻¹. Meas et al. [28] applied a continuous EC process to remediate fluorescent penetrant liquid-contaminated rinse water from aviation parts. The contaminated rinse water was clarified, allowing reuse four times. COD, color, and turbidity removal efficiencies were 95%, 99%, and 99%, respectively. They also estimated the operation cost with an investment-return period of approximately 17 weeks. Moussavi et al. [33] analyzed an EC process performance for the removal of total petroleum hydrocarbon (TPH) from petroleum-contaminated groundwater. In their experiments, the operational conditions varied as follows: pH from 4 to 11; CD from 2 to 18 mAcm⁻²; reaction time from 2 to 60 min; and electrode materials were Al, Fe, and steel. They concluded that a pseudo-second order reaction best described the TPH elimination rate, and the reaction constant decreased with aeration from 477 to 78 mL g⁻¹ min⁻¹. In addition, the percentage removal of TPH improved from 67.2 to 93.4% by increasing the hydraulic retention time (HRT) from 10 to 60 min.

Zodi et al. [34] utilized a continuous electrochemical reactor to remove direct red 81 dye by electrocoagulation/flotation. The reactor included sludge separation and electrochemical cells. They studied the effects of inlet flow rate and CD on dye removal efficiency, finding optimal dye removal at a CD up to 200 Am⁻² and a low flow rate of 10 L/h, although electrical energy consumption increased with flow rate. A significant relationship between current density, flow rates, and current efficiency (Faradic yield) was observed. Specific electrical energy consumption remained at 52–58 kWh·kg⁻¹ Al at 0.15 mA·cm⁻².

During the last 25 years, about 34 papers concerning the standalone continuous EC treatment process for organic pollutants have been published [2,3,28,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,76,77,78,79]. The most recent research was that of Abbasi et al., Abdul Rahman et al., Mehralian et al., and Purkait et al. [76,77,78,79].

Abbasi et al. [76] investigated the treatment of real licorice processing wastewater using a continuous lab-scale EC reactor equipped with Fe rods serving as both anodes and cathodes. They applied response surface methodology (RSM), a statistical and mathematical technique used to develop, improve, and optimize processes, to model the influence of key operational parameters, including electrolysis time, CD, NaCl concentration, and mixing speed, on the removal efficiencies of color, soluble COD, and turbidity. RSM helps in understanding the interactions between multiple variables and in determining the optimal conditions for achieving the highest treatment efficiency. Additionally, they compared the performance of this rod-based EC cell design with a previous setup that used Fe plate electrodes. The results demonstrated superior performance with Fe rod electrodes, achieving removal efficiencies of 94.6% for color, 90.1% for COD, and 59.35% for turbidity. Furthermore, the optimal CD and electrolysis time required for effective treatment were lower with rod electrodes, indicating their potential for more energy-efficient operation. These findings suggest that Fe rod electrodes offer a promising and cost-effective alternative for the removal of high organic pollutant loads from industrial wastewater.

Abdul Rahman et al. [77] applied a continuous EC process to treat Borneo tropical brackish peat wastewater under varying residence times and initial salinity conditions. Their results showed that the treated effluent complied with recommended environmental regulations. Color, total suspended solids (TSS), COD, and turbidity percentage removal were 94.09, 89.91, 92.71, and 86.04%, respectively, with 5 min as the optimal residence time. The optimal energy consumption cost was only 0.03 $·m⁻³ of brackish peat water treated. These findings demonstrate that the continuous EC treatment process is a cost-effective one to treat natural organic pollutants.

Additionally, Mehralian et al. [78] studied a continuous EC reactor to treat aged landfill leachate using iron and Al electrodes in a novel configuration to enhance the process performance. They modelled the impact of CD, pH, and HRT on TOC, COD, BOD₅, and turbidity, color, and heavy metal ion removal using Box–Behnken design (BBD), a response surface methodology-based statistical tool used to evaluate the relationships between multiple process variables and optimize operational conditions with a reduced number of experimental trials. The findings showed that the applied models are accurate with an R² of 0.92 for Al and 0.97 for Fe electrodes. They reported that the TOC, COD, BOD₅, and NH₃-N percentage removal were maximal at an HRT of 50 min, CD of 1.1 mA·cm⁻², and a pH of 11, reaching 59, 64, 55, and 27%, respectively, in the EC with Fe electrodes. The EC with Fe electrodes showed a higher turbidity and color removal by 86 and 59% than the EC with Al electrodes. Furthermore, the total landfill leachate treatment cost was 0.21 $·m⁻³. Finally, Purkait et al. [79] investigated the CEP for real textile effluent treatment. At the optimal conditions of a CD of 6.5 mA·cm⁻², influent flow of 10 mL·min⁻¹, and contact time of 80 min, the percentage removal of TOC, COD, and turbidity was 75, 86, and 95% for Fe electrodes and 73, 77, and 96% for Al electrodes, respectively. In addition, the optimal operational prices were determined to be 1.851 $·m⁻³ for Al and 1.562 $·m⁻³ for Fe electrodes. A summary of the main parameters and results from standalone EC treatments over the last 25 years is presented in

Table 1. Summary of key operational parameters and outcomes from the treatment of industrial organic pollutants using standalone continuous electrocoagulation processes (CEPs) over the past 25 years.

Wastewater Type	Electrodes Information			Operational Parameters Value				RE (%)	Ref.
	Type	No.	Arrangement, Distance (cm)	CD (mA·cm−2)	pH T (°C)	Flow Rate (mL·min−1)	Co (mg·L−1)
Reactive and disperse dyes	Al, Fe, SS	8–14	MPP, 0.5–3.0	1.01–4.50	4–10	50–200	870	98	[31]
Orange II dye solution	Fe	5	BPS, 0.6	15.95–39.87	5–11 25	350–600	50	99	[32]
Synthetic textile wastewater	Al	2	Parallel	31.25	6–9	Residence time 14 min	2500	80 for color 85 for COD	[3]
Fluorescent penetrant liquid	Al	2	Parallel	0.333	6.5 23 ± 3	1.0 to 3.25	1500	COD 95 99 NTU 99 Color	[28]
Petroleum-contaminated groundwater	Al, Fe Steel	2	Parallel, Al–Fe, St–Fe, Fe-St, St-Al Al-St, Fe-Al, Fe-Fe, Al-Al, St-St	2–18	4–11 23.5 ± 3	Residence time 10–60 min	NR	93.4 TPH	[33]
Direct red 81 (DR 81) dye	Al	6	BP Al-Al	20	7.5 20	166.7	50	90.2%	[34]
Rice mill	SS	2	SS-SS	5−25	4.98	50 to 100	2200	89 for COD and TSS	[35]
Cheese whey	Fe	2	Screw type	40–60	3–7	Retention time 20–60 min	15.500	86.4 COD	[36]
Synthetic HA solution	Fe	2	Parallel	0.5–15	3–7	1500	21.59	Dissolved organic carbon (DOC)	[37]
Compost leachate	Al, Fe	2	Al–Al, Al–Fe Fe–Fe, Fe–Al	19 V	NR	Hydraulic residence time 75 min	13,600	96% COD 99% TSS	[38]
Textile r	Al	2	Cylindrical cathode Central anode	2–12	7 25	Retention time 10–30 min	300	91.5 COD, 95.5 color	[39]
Reactive textile dye	Al	2	Parallel	10–30	2.3–8.8	15–60 L·h−1	300	90 turbidity, 97 color	[40]
Pulp and paper mill	Fe	2–6	MPS	Optimum CD 5.556	7	0.5–4.0 L·h−1	2000	82.15 COD, 90 color	[41]
Dairy	Al	4	MPS	0.2–1.2	7.8–9.2 25	Residence time 30–90	2200	94 color, 93 NTU 65 TOC, 69 COD	[42]
Dye house	Al	4	MPS	2–8.5	6.5–7.1 18–22	0.01–0.20	200	85, 77 COD, 76, 72 TOC 95, 95 turbidity for Fe and Al	[43]
Textile	Al	12	MPS	2–8	7.31 30 ±3.2	Retention time 5–40 min	2240	98 turbidity, 88 color 93 COD, 94 TSS, 52 TDS	[44]
Tannery	Al, Fe	10	MPP	7–14	5.5	Retention time 25–100 min	2500–3000	73, 67 COD, 94, 93 color 100, 100 Cr, 51, 46 NH3-N for AL and Fe	[45]
Textile dye Acid Red 336	Al	2, 4	MPS, BPS	10–40	NR	15 L·h−1	1000	97.5 color, 98.5 turbidity	[46]
Indigo dyeing wastewater	Fe	28	MPS	0–220 V	3–10	1–3 L/min	2080	93.8 color, −92.07 COD	[47]
Paint	Al, Fe	10	MPS	0–5 A	6.6–7.0	8 L·h−1	1000	69,7 COD, 62.1 Al	[48]
Backwash	SS, Fe	13	MPS	6–30 V	7.3–7.7	Retention time 10 min	178.2	98.4 COD	[49]
Licorice processing	Fe	12	BPS	10–50	6.5 18–22	11–16 mL·h−1	1600	90.1 color, 89.4 COD 82 turbidity, 73.3 alkalinity	[50]
Sarawak peat	Cu	10–20	MPP, MPS, BPS	0.12–0.599	NR	Retention time 10–37.778	8	100 turbidity 90.91 TSS	[51]
Metalworking	Al, Fe	4	MPS	9	7 25	10–200 Retention time 350–17.5 min For Al	17,312	92.6–71.3 COD, 83.3–64.9 TOC 99.9–88.9 turbidity	[52]
Synthetic colored effluent	SS, Al	Packed bed	Packed bed, particulate, and planar	3–6 A	3–8	9.4–24 L·h−1	140	98 Color	[53]
Peat water	Cu	10–20	MPP, MPS, BPS	0.12–0.599	6–7	7.1 L·h−1	NR	100 turbidity, 90 r TSS 78 COD, 97 TOC	[54]
Azo dyes methyl orange	Fe	2	Fold plate electrode	10–15	6 Room temp.	15–105 L·h−1	100	92.35 color	[55]
Hospital	Fe, Al	2	Parallel	20–40 V	4–8	Retention time 10–60 min	502.8	75.5 COD 59.2 BOD5 80.7 phenols 85.3 Phosph, 75.6 TSS	[56]
Indigo dye	Fe	30	BPS	20–65 V	2.5–7.5	1.5–3	10–60	93.18	[57]
Licorice processing	Fe	12	Rod electrodes	28	6.3	Retention time 71.8 min	NR	94.6 color, 90.1 COD 72 turbidity	[76]
Borneo tropical brackish peat	Al	10	MPP	0.625	4.74	Retention time 5 min	NR	94.01 color 91.43 COD	[77]
leachate	Fe, Al	2 anodes 12 cathodes	MPS	1.1	11	40.0	75	59 COD, 64 TOC 55 BOD5, 27 NH3-N	[78]
Textile	Fe, Al		NR	1–6.5	NR	10–200	NR	95 turbidity 75 TOC, 86 COD	[79]

.

The table includes critical details such as wastewater type, electrode material, configuration (e.g., monopolar/parallel, bipolar/serial, and distance between electrodes), current density, pH, temperature, flow rate or HRT, and the achieved removal efficiencies for parameters such as COD, BOD₅, TOC, color, turbidity, and TSS. These parameters are essential for comparing treatment performance across various wastewater types, including textile effluents, landfill leachate, dairy wastewater, peat water, petroleum-contaminated groundwater, and food-processing wastewater such as cheese whey or licorice processing discharge. Electrode materials ranged from aluminum (Al), iron (Fe), stainless steel (SS), to copper (Cu), with current densities spanning from as low as 0.333 mA·cm⁻² up to 500 A·m⁻² depending on the application. Flow rates varied from 1 mL·min⁻¹ to over 1.5 L·min⁻¹ or residence times from 5 to 100 min, and the pH of influents ranged between 2.3 and 11 in most cases. Removal efficiencies varied widely depending on the application and operating conditions, reaching up to 99% for color and turbidity, 96% for COD, and over 90% for TSS and TOC in many cases. The table provides a comprehensive comparison that supports the process feasibility for various wastewater streams.

In addition to summarizing operating conditions and removal efficiencies, the data in Table 1 also reveal key performance trends that help interpret the EC process behavior. For example, aluminum electrodes often achieved higher removal of turbidity and color, while iron electrodes showed stronger performance in COD and organic load removal. Operating at acidic to neutral pH favored the formation of effective coagulant species, particularly in dye-laden or high-organic-content wastewater. Similarly, moderate current densities (10–50 mA·cm⁻²) were generally optimal, providing a balance between pollutant removal and energy use, whereas extreme current densities sometimes led to diminishing returns or excessive sludge production. These patterns underscore the importance of tailoring EC conditions to specific wastewater characteristics to ensure both technical and economic efficiency.

It is evident from Table 1 that about 34 studies have utilized standalone EC treatment processes during the last 25 years. The main parameters influencing the performance of such processes and their removal efficiencies are the type of wastewater, flow rate, type, electrode number, their arrangement, and current density. In most research, the removal efficiencies are relatively high, especially those of turbidity and color removal. However, COD or TOC removal efficiencies are usually lower than those of turbidity removal because the COD load is usually high in most wastewater resources. This can be attributed to the differing chemical characteristics and reactivity of the targeted pollutants. Color and turbidity are primarily associated with colloidal particles, suspended solids, and dye molecules, which are readily destabilized and removed via charge neutralization, electro-flotation, and sweep coagulation mechanisms facilitated by EC-generated metal hydroxides. In contrast, COD and TOC represent the total organic load, including stable and soluble low-molecular-mass compounds such as organic acids, alcohols, and surfactants, many of which are non-ionized and less reactive toward coagulation. These molecules may remain in the aqueous phase due to their solubility and chemical stability, making them less susceptible to removal through electrocoagulation alone. Thus, while EC is highly effective for removing particulate and chromophore pollutants, its efficiency for complete organic carbon removal is inherently limited by molecular structure and solubility. In this case, the use of combined continuous EC processes with suitable pretreatment is more efficient and can reduce the sludge that could precipitate on the electrodes, causing a serious electrode passivation drawback. When the percentage removal of an EC process is relatively low, a post-treatment process is needed to remove part of the pollutant load prior to the EC step. This will increase the treatment efficiency of wastewater to fulfill the environmental regulations in most countries.

2.2. Combined Continuous Electrocoagulation Processes

As mentioned above, combined EC treatment processes are essential alternatives to the standalone EC process to efficiently treat high organic load industrial wastewater [75]. Such systems utilize the EC process in one of the three different configurations:

• A first treatment process, followed by another second treatment process such as biological treatment or adsorption, followed it [72,80,81,82].
• A second or third treatment process after another treatment process, such as chemical coagulation or advanced oxidation processes [61,69,83,84].
• An intermediate treatment process between other pre- and post-treatment processes is applied [25].

The main reasons for using the EC process as a first or pretreatment process are to achieve one of these two goals:

➢

To detoxify the wastewater by degrading some toxic materials that act as inhibitors in the biological treatment step [58].

➢

To electrocoagulate a high percentage of the organic chemicals found in the wastewater and cause fouling to membranes or inhibition in bioreactors [61].

On the other hand, the use of a post-treatment step is important to further remove the pollutant concentrations in the effluents to achieve environmental standards and to become appropriate for reuse or for release into rivers [82,83,84]. The reasons for using EC as an alternative post-treatment step are due to its compact technology, which needs only a relatively small space; its ease of continuous operation; low capital operational and maintenance cost; and the possible use of cost-effective solar-powered treatment systems [61,62,66,69,84].

The utilization of the EC process as an intermediate step between other pre- and post-treatment processes is limited. The literature indicates that only one recent published research used three subsequent treatment steps of chemical coagulation (CC) as a pretreatment step, EC, and adsorption as a third and post-polishing treatment step to treat pharmaceutical wastewater [25]. In this study, Al-Qodah et al. used a continuous solar-powered EC process with CC as a pretreatment step and continuous packed bed adsorption as a final third step to improve the overall removal efficiency to about 97.12% and a relatively low cost of 0.3 $·m⁻³ treated effluents. A conceptual integration of continuous electrocoagulation (CEP) within hybrid wastewater treatment systems is shown in Figure 2. The diagram of Figure 2 illustrates CEP’s flexibility to serve as a primary, intermediate, or polishing treatment step in combination with physical, chemical, and biological processes.

The literature survey concerning combined EC continuous processes for organic pollutant treatment shows that there are about 18 successful applications with significant findings during the last 25 years. A total of 13 of them used EC as a pretreatment step, and 4 of them used EC as a post-treatment, and only 1 study used the EC as a middle treatment step between CC and adsorption. Moisés et al. [58] and Phalakornkule et al. [60] are research groups that treated organic pollutants using combined EC treatment systems. Moisés et al. [58] combined a treatment system for process wastewater that comprises an EC cell containing aluminum electrodes, followed by a clarifier and finally a bioreactor. The feed flow rate varied between 50 and 200 mL/mi. On the other hand, the bioreactor was operated by using several active sludge cultures. They reported that the optimal flow rate was 50 mL·min⁻¹, and the most active cultures in the bioreactor were flagellate protozoa and ciliates. At the optimal conditions, the overall percentage removal of COD, turbidity, and color was 80, 92, and 94%, respectively. Subsequently, Phalakornkule et al. [60] treated HA found in raw waters obtained from two dams by an electromagnetic double EC treatment system. They reported that the combined system had a significant percentage removal of HA. The cost of the EC process for treating direct red 23 and Reactive Blue 14 was 1.40 and 0.69 kWh·m⁻³, respectively. The final percentage removal of the COD, color, and TS was 93, 99, and 89%, respectively.

Subsequently, Bani-Melhem et al. [61], Makwanaa and Ahammed [62], Jiménez et al. [63], GilPavas et al. [64], and Gunawan et al. [66] treated greywater, anaerobically treated municipal wastewater, dye-containing grey water, colored organic solution, and oil, textile, and yarn-dyed wastewater, respectively. Additional research results are summarized in

Table 2. Summary of studies on the treatment of industrial organic pollutants using combined CEP published over the past 20 years.

Wastewater Type	Combined Process Type		EC Process								Removal (%)	References
			Electrodes Information				Operational Parameters
			Type	No.	Arrangement	CD mA·cm−2	pH	T °C	Flow Rate mL·min−1	Co (mg·L−1)
EC as a pretreatment step
Industrial wastewater	ES and post-activated sludge		Al	12	MPS	3.4 A	8	RT	50–200	2000	94 color 92 turbidity 80 COD 80	[58]
HA	Electromagnetic treatment-doubled EC		Fe, Al, SS	2	Parallel	9.1–14.5	4–12	30–120	10–50	20	52 HA at pH 3	[59]
Dye-containing wastewater	ES and post gas separation tank and two sediments		Fe	50	MPP	3–4	9.6	RT	0.07–1140	100	99 color 93 COD 89 TS	[60]
Grey water	ES and post-submerged membrane bioreactor (SMBR)		Al	2	Parallel	0.7–1.4	5–10	22.24	- HRT 2–40 h	463	Nearly (100%)	[61]
Kaolin suspension, colored organic solution, and oil-in-water emulsion	ES and post-electroflotation		Al and Fe	3–5	-	2.5	4–4.5	25	Residence time 10–55 min-	200	60 turbidity 80 COD ≥80 color	[63]
Textile effluents	EC and electrochemical oxidation (EO)		Fe, Al, Ge, Ti	6	MPS-	1–5	4–10 -	--	Retention time 29.63 min	200–3000	70 DCOD 100 color	[64]
Landfill leachate	ES and anaerobic treatment technique		Iron	2	Parallel	20–50	3–9		13–24	6400	92 COD	[65]
Yarn dyed wastewater	EC and Fenton oxidation		Al	2	Parallel	-	6.8	25	30–90	--	80 COD 97.8 color	[66]
Algae cultivation	EC and flotation		Fe and AL	2	Parallel	1.2 (Al)/ 3.2 (Fe)	7.15–7.3	RT	10 L/h	200	Microalgae removal 88 Fe electrodes 73 Al electrodes	[68]
River water	EC and ultrafiltration (UF)		Al	6	MPS	3.7–7.4	6	17	500	-	TDS 72.20, turbidity 99.11, BOD5 94.35, COD 81.55	[71]
Dyes	EC-adsorption		Fe	10	BPS	1.3–21.	5–11	-	0.5–1 L/min	-	96.87 color 96.87 COD 84.46 TSS	[72]
Hydraulic fracturing effluents	EC and membrane distillation		Fe, Al	5	BPS	5 A	7.2	20	200	100–1000	42 TOC	[73]
Acid Brown 14 diazo dye	EC+ (photo) electro-Fenton recirculation		Fe	2–9	MPS	50	4–10		10 L/h	50	TOC reduction 90 in chloride media 97 in sulfate media	[74]
Greywater	EC, bed filter, and adsorption		Al Fe	4	MPS-	0.45–1.4	7–8		0.05–0.1 L/min	245	85–90 COD	[75]
		EC as a post-treatment step
Municipal wastewater	ES as post-treatment method for UASB reactor effluents		Al	4	MPP	1–5	6–7	25–27	4.32–12.96 L/h	274	>99.8 reduction in total and fecal coliforms	[62]
Real urban treated effluents	Electro-disinfection and EC		Iron, diamond	-	BPS	0.5–1	7–7.6	-	50 L/h	6.1	100 turbidity 100 E. coli	[67]
Distillery spent Wash	EC and ozone assisted EC		Al	6	MPS	5.25–10.75	3.1–6.9	24–62	3 gm/h	3875	72 COD 92 color	[69]
Bio-digested landfill leachate	Bed biofilm reactor (CF-SBBR) and EC		Fe, Al	6	Parallel	7–42	2–10		1.6–3.3 L/min	1900	46.5, 54.4 COD 95.8, 98.5 color 83.5, 78.6 TOC 40.9, 57.9 TN, for Al, Fe, respectively	[70]
EC between pre- and post-treatment processes
Dairy	Chemical coagulation, EC, and adsorption		Fe	2–6	MPS, MPP, BPS	1–4	5.6	25	19–45	100	97.1 TOC	[25]

below.

Ghernaout et al., 2010 [59], were the first to apply EC as a post-polishing process integrated with electromagnetic separation of humic acid (HA). Subsequently, Makwana et al. [62], Cotillas et al. [67], Parmentier et al. [68], and Dan et al. [70] successfully treated anaerobically pretreated municipal wastewater, secondary clarifier effluents, and landfill leachate using the EC step as a post-treatment step. Table 2 contains the main conditions and main results obtained by the combined continuous EC systems used for organic pollutants.

Table 2 shows recent parameters in published research on CEP treatment systems used to treat wastewater containing mainly organic pollutants. The table shows pre-, post-, and middle-EC systems, the values of main operational variables, and final values of the percentage removal of the pollutant. In most applications, the CEP combined systems achieved a high percentage of removal. Types of treated effluents include dyed, dairy, grey, landfill leachate, and others. On the other hand, the EC integrated processes include adsorption, chemical coagulation, a submerged membrane bioreactor (SMBR), electro-flotation, biological treatment, oxidation, and tight ultrafiltration (UF).

The only research that utilized three subsequent combined processes, including chemical coagulation (CC), solar-powered EC, and a packed column adsorber for dairy effluents, was conducted by Al-Qodah et al. [25]. In the first CC step, several coagulants were evaluated to remove TOC. The second SPEC step examined the effect of many variables, such as electrode number and arrangement, flow rate, initial TOC, and CD, on TOC percentage removal. Finally, the third adsorption step used some cost-effective adsorbents such as sea sand (SS). The influence of wastewater flow rate (F), adsorbent height (H), and initial pollutant concentration on TOC removal was studied. The combined three-step system showed high removal performance and achieved percentage removal of 50.4, 76.5, and 80.2% for CC, SPEC, and adsorption, respectively. The overall treatment three-step system achieved a percentage removal of 97.1%. In addition, the application of solar power decreases the cost by 83% compared to conventional powered processes. These results confirmed the potential of the three-step solar-powered treatment system as an efficient, sustainable, and cost-effective approach to produce high-quality remediated wastewater for reuse.

3. Mathematical Modeling of Continuous Electrocoagulation Processes

Batch or continuous EC processes usually involve complex electrochemical reactions on the electrodes that lead to anode dissolution and the evolution of some gases on the cathodes. In addition, several physical and chemical processes take place in the electrolyte containing pollutants that promote the formation of coagulants [80,81,82]. The mechanism of these processes is familiar, and it is affected by the type of electrodes, pollutant, and operation parameters applied to the system [83,84,85]. Accordingly, mathematically modeling the EC processes as any other chemical process is crucial, as it allows a better understanding of the impact of different operational or design parameters on its overall performance, especially achieving high pollutant removal efficiencies in a cost-effective process of environmental compliance [86,87,88]. However, modeling the EC processes usually faces difficulties due to the complex nature of the chemical, physical, and physiochemical processes involved. In addition, the impact of EC parameters is not stable and varies from one process application to another and the consequent conditions [89]. Accordingly, a suitable mathematical model that successfully describes the performance of the EC treatment processes is still difficult to achieve [90].

After an intensive literature survey on continuous EC processes, several suitable references were found to find what types and objectives of any mathematical model used in them. The two main types of mathematical models found in the available literature are: models developed to understand the mechanism and kinetics of the EC process, and models applied to optimize experimental parameters and to achieve their efficient ranges and to reduce the operation in addition to the capital cost.

Accordingly, the following subsections will discuss types of mathematical models used in continuous EC processes and the main obtained results.

3.1. Kinetic and Isotherm Modeling of Continuous Electrocoagulation Processes

3.1.1. Standalone Processes

Several standalone EC processes applied certain kinetic models to fit the experimental data and describe the mechanisms of these processes. Kim et al. [31] published the first research to describe the flow model in the EC cell. They reported that the electrodes in their study were arranged in a zigzag configuration, with a small gap between them and with a high length-to-width ratio. They noticed that this arrangement retards fluid mixing in the forward direction. They concluded that the flow in the EC cell reactor is similar to that which occurs in a plug flow tubular reactor (PFTR) rather than a continuous stirred tank reactor (CSTR). The decolorization of dyes in their study followed a first-order reaction kinetic model, where the rate constant (k) is obtained from the slope of the linear plot of −ln(C/Co) versus time (t).

Abdul Rahman et al. [51] used the same model to calculate the reaction rate constant for turbidity and TSS found in Sarawak peat water. Wu et al. [55] found that the second-order kinetic model describes well the experimental results of the adsorption of methyl orange (MO) on Fe(OH)₃ formed coagulant with a correlation coefficient between experimental and theoretical data of R² = 0.9954. Moreover, Wu et al. [55] uniquely examined the adsorption isotherm behavior of MO onto the coagulants produced during EC. They compared the Langmuir and Freundlich isotherm models, which are traditionally used to describe adsorption phenomena rather than the overall EC process. Their results showed that the Langmuir model provided a better fit, suggesting monolayer adsorption of MO onto the Fe(OH)₃ surface. These findings were validated using R² values as a measure of fit.

Mahesh al. [41] used continuous EC treatment for pulp and paper mill wastewater. They used the instantaneous charge efficiency (ICE) to have a quantitative estimation of the percentage removal of the pollutants. The ICE was estimated from the following Equation (1):

$$ICE={\displaystyle \frac{4FV[{\left(COD\right)}_t-{\left(COD\right)}_{t+∆t}]}{I∆t}}{\displaystyle \frac{1}{32}}$$

(1)

where (COD)_t and (COD)_t+Δt are the COD values at time t and t + Δt (gO₂·L⁻¹), respectively, V is the volume of the wastewater (L), 4 is the number of electrons exchanged per mole of oxygen consumed, and 32 is the molecular mass of oxygen (gO₂·mol⁻¹).

The ICE decreases as the charge increases and exhibits lower values at higher volumetric flow rates. Subsequently, the concept of the Electrochemical Oxidation Index (EDI) for the organic compound was introduced to provide insight into the current efficiency of the process. The EDI was determined using Equation (2):

$$EDI={\displaystyle \frac{{\int }_{t_1}^{t_2}ICE\left(t\right)dt}{∆t}}$$

(2)

where Δt is EC time. The estimated EDI values at the flow rates of 2, 3, and 4 L·h⁻¹ were 1.16, 0.72, and 0.21%, respectively. Similarly, at flow rates of 0.5 and 1 L·h⁻¹, the EDI values were 1.73 and 1.51%, respectively. A larger EDI value means more easily the compounds are oxidized.

3.1.2. Combined Processes

Combined continuous EC processes have rarely applied kinetic or other models to describe pollutant removal rate. The main reason is the limited number of such combined processes. Gunawan et al. [66] applied an integrated electrocoagulation–Fenton treatment system to remove yarn from industrial wastewater. They reported that the pseudo-first-order rate kinetic model was the best to describe the COD removal in the EC process with and without the presence of Fenton reagent and assuming that the hydroxyl radical is in excess. The values of the model are constant at molar ratios of Fe(II)/H₂O₂ of 1:4.5 and 1:10 are 0.0199 and 0.0705 min⁻¹, respectively. These values were derived from the linear fit of ln(C/Co) versus time, and the goodness-of-fit was evaluated using correlation coefficients.

Recently, Al-Qodah et al. [25] studied a three-step continuous treatment system for dairy wastewater to remove TOC and turbidity. The three steps are CC, EC, and packed bed adsorber. Al-Qodah et al. [25] used the breakthrough curves to describe the changes of the (TOC/TOC_o) of the packed bed effluent as a function of time and applied the Bohart–Adams model (Equation (3)). They considered C/C_o = 0.2 as a design value above which the bed becomes exhausted, and the sand bed requires regeneration. The main parameters that influence the bed performance are the initial TOC concentration, the packed bed height, and the flow rate. Al-Qodah et al. [25] applied the Bohart–Adams model shown in Equation (3) to describe the performance of a sand bed with time and TOC content in the column effluents [90]:

$$\mathrm{l}\mathrm{n}\left(C_o/(C_t-1\right))={K_{AB}N}_o\left(Z/U\right){-K}_{AB}{C}_{o}t$$

(3)

C_t, and C_o are the exit and inlet TOC concentration in mg·L⁻¹; t is time (min); Z is the adsorption bed height (cm); U is the superficial wastewater velocity (cm·min⁻¹). N_o is the adsorption capacity (mg·L⁻¹), and K_AB is the mass transfer coefficient (L·mg⁻¹·min⁻¹). A plot of ln(C/C_o) vs. time (t) can give the values of the column parameters. Some results of their investigations are shown in Figure S1.

Table S1 presents the variations of N_o and K_AB under different operating conditions, confirming that higher TOC_o and flow rates lead to increased N_o, indicating more TOC passes untreated through the column. These modeling efforts help quantify and optimize the adsorptive capacity of the final treatment stage.

3.2. Operational Parameters of Continuous Electrocoagulation Processes Optimization by Statistical and AI Methods

As mentioned above, optimizing the experimental variables of the EC process is crucial to improve the pollutant percentage removal and reduce the cost of the environmentally safe treatment process, in addition to the possible process scaling up from lab to an industrial level [87]. The complex relations between the different input and output parameters need a suitable optimization methodology [91]. Among these optimization methodologies, the statistical ones, such as RSM and artificial intelligence (AI), represent suitable tools for modeling the response of EC systems variables [92,93].

In RSM, the regression model’s implementation hinges on the chosen experimental design. Parameter evaluation and optimization are visualized through surface and contour plots. In RSM, the implementation of the regression model depends on the chosen experimental design, which dictates the data points used to build the model. Parameter evaluation depends on surface and contour plots, allowing for the identification of optimal conditions [87]. The main objectives of the RSM approach are to optimize a process by investigating the best combination of input variables to achieve the desired output and to determine the correlation between manual input parameters and the response variable [94,95,96,97]. In other words, RSM is applied to determine the importance of many experimental parameters [98]. There are three major steps in the optimization procedure: 1. Realizing experimental design. 2. Make coefficient approximations using a suitable model to predict the response. 3. Check the model validity [99].

According to Phu et al. [88], the most applied design processes are central composite design (CCD) and BBD. The development of these two was to reduce the handy experimental runs compared to the full factorial design (FFD) method. Equations (4) and (5) show how to calculate the required number of experiments, N, for the development of both (BBD) and (CCD), respectively [94]:

$$N=2k\left(k-1\right)+C_o$$

(4)

$$N=2^k+2k+C_o$$

(5)

where C_o, k are the number of center points and the number of factors, respectively.

On the other hand, using AI processes to simulate the rate of removal in EC cells can enhance both experimental speed and accuracy. However, AI faces several challenges in EC systems. These challenges include the data quality, difficulty understanding how models make decisions, the need for high computations, and finally, overfitting risk, which affects the accuracy and scalability. In addition, the integration of AI into existing systems requires experts since it is difficult to obtain consistent results in the absence of standard guidelines [87]. For this reason, AI has not been used so far in EC processes.

A summary of published studies dealing with parameter optimization for studies of standalone and combined continuous EC systems is given in the following sections.

3.2.1. Standalone Continuous Electrocoagulation Processes Optimization Using Statistical and AI Methods

Karichappan et al. [35] applied a continuous EC treatment process to treat TSS and COD from rice industry effluents. This research was the first to apply a four-factor, three-level BBD and combine with RSM to help the achievement of an optimum solution to maximize pollutant removal with the minimum energy consumption. They selected initial pH, CD, flow rate, and electrode interdistance as BBD independent variables. They considered TSS and COD removal and electrical energy consumption EEC as the response functions. They reported that the obtained response values were in high agreement with manual experimental results, as shown in Figure 3.

Figure 3 contains diagnostic plots that show predicted versus actual plots taken from references [35,47], respectively. It is clear that the data points are very close to the diagonal in the two figures, which reflects a high agreement between predicted and experimental data. Accordingly, the quadratic model was successfully applied to describe the EC continuous process for the treatment of rice mill effluents [35].

The adequacy of the applied mathematical models was tested by the Pareto analysis of variance (ANOVA). In addition, mathematical models were applied to plot the response surface contour graphs to investigate the interactive effect of independent variables on responses [95,96]. Numerical optimization has been applied to BBD results to investigate the optimum operating parameters to achieve efficient treatment in terms of economic viability and percentage removal.

A similar approach was applied by Un et al. [36] who developed an empirical model through RSM, using some effective experimental parameters, including CD, pH, and residence time. They reported that the best obtained fit was a nonlinear model R² of 85%. Subsequently, Amani et al. [38] investigated the treatment performance of high pollutant load leachate in a continuous electrocoagulation/flotation (ECF) system. The interactive effects of the operational parameters on COD and TSS percentage removal in continuous runs using Al electrodes were analyzed and then correlated by RSM. They obtained predicted responses by fitting adequate cubic correlations to the results. The obtained 95% confidence interval value indicates a high agreement between predicted and experimental results.

Recently, Hendaoui et al. [57] investigated the decolorization of indigo dye using a continuous EC process with Fe electrodes. They optimized the process parameters by using RSM. The high 0.978 value of R² and ANOVA analyses indicate a good agreement between the predicted and experimental results. They reported that the optimal conditions were a pH of 7.5, a concentration of dye of 60 mg·L⁻¹, a voltage of 47 V, and an inlet flow rate of 2 L·min⁻¹. The predicted and experimental color percentage removals were 94.083% and 93.972%, respectively. The total cost was 0.1 $·m⁻³ of remediated effluent.

Abbasi et al. [76] selected three model analyses, including statistical software (Stat-Ease, Version 10.0.7.0), CCD, and ANOVA under RSM. They evaluated three operating parameters, namely electrolysis time, CD, and mixing intensity at three different levels of −1, 0, and 1 for the EC process containing iron rod electrodes. According to their simulation results, Abbasi et al. [76] reported that CD and electrolysis time are the most influential variables on the different responses, such as COD, color, and turbidity percentage removal.

In the last contribution of using a standalone continuous EC treatment system, Mehralian et al. [78] employed BBD to investigate a regression model that describes the impact of some operational parameters on the removal of COD as a response and using both electrodes. Based on the ANOVA results, they concluded that the selected model has a high R² value. They reported that all used parameters are effective in COD removal, with pH as the most effective variable.

3.2.2. Combined Continuous Electrocoagulation Processes Optimization Using Statistical and AI Methods

Ghernaout et al. [59] used electromagnetic treatment-doubled EC processes to remove HA found in dams’ water. They applied RSM with a second-degree model and conducted minimal tests to simulate the optimum parameters and to interpret the results. They reported that the obtained results indicated that RSM is an efficient optimization method for the EC parameters for HA removal. The analysis of variance indicated a high coefficient value (R² = 0.99). This presents a satisfactory agreement between the experimental results and the second order model predictions. The successful removal of pollutants suggests that this combined process could be scaled up to a larger scale. The same RSM model was applied by Jiménez et al. [63] to optimize the influence of some operating conditions, such as CD, residence time, and pollutant concentration, on the performance of a combined continuous ECEF reactor.

Other potential results were investigated by GilPavas et al. [64], who applied a continuous combined EC and electro-oxidation system to treat industrial textile effluents and used RSM to optimize the operation parameters to achieve maximum chemical oxygen demand degradation (DCOD) efficiency removal.

Finally, Hendaoui et al. [72] optimized a combined continuous EC adsorption treatment system for textile industrial effluent. They reported that the additional lab-scale results under optimal conditions agree with RSM-predicted results. This confirms the ability of the RSM model to successfully predict suitable results in the defined system. In addition, the applied combined process represents an eco-friendly system for industrial effluent treatment and reuse at a relatively low cost.

Table 3. Summary of experimental design models used in standalone and combined CEP for the treatment of organic pollutants since 2013.

Wastewater Type	Model Used	Optimum Values of the Operating Parameters				Predicted Responses at Optimum Conditions	Ref.
1. Standalone EC Processes
		pH	CD mA·cm−2	Flow rate mL·min−1	Electrode distance, cm
Rice mill	RSM using BBD	7	15	70	5	97.41% COD 89.09% TSS 7.24 KWh EEC	[35]
Cheese whey	RSM	Initial 4.54	60	Retention time 20 min	-	2.112 mg·L−1 COD	[36]
Compost leachate	RSM	Inlet COD 13,600 mg·L−1	Voltage 19 V	HRT 75 min	3	96% COD 99% TSS	[38]
Dairy	Factorial design methodology ANOVA	Voltage 10 V	1.33	1000	1	94 color, 93 NTU 65 TOC, 69 COD	[42]
Indigo dyeing wastewater	RSM CCD	7.85	Voltage 101 V	1300	-	89.2 color, 76.1 COD 29.76 conductivity	[47]
Licorice processing	RSM CCD	Mixing intensity 45 rpm	35.0	Electrolysis time 81.8 min	NaCl concentration 300 mg·L−1	90.1 color, 89.4 COD 82 turbidity 73.3 alkalinity	[50]
Sarawak peat	RSM	-	0.3861	Electrolysis time 37.778 min	-	2.0247 NTU turbidity 2.8629 mg·L−1 TSS	[51]
Azo dyes methyl orange	RSM using BBD	MO Co 134 mg·L−1	10.1	Electrolysis time 30 min	-	92.35 color	[55]
Hospital	RSM using BBD	7.92	Voltage 40 V	HRT 15 min	-	75.5 COD, 59.2 BOD5 80.7 phenols 85.3 phosphates, 75.6 TSS	[56]
Indigo dye	RSM ANOVA	7.5	Voltage 47 V	2000	Solution concentration 60 mg·L−1	Predicted color removal 94.083%	[57]
Licorice processing	RSM CCD ANOVA analysis	Mixing intensity 45 rpm	CD 2.8 mA·cm−2	Electrolysis time 71.8 min	-	94.6 color 90.1 COD 72 turbidity	[76]
Leachate	RSM using BBD	11	1.1	HRT 50 min	-	59 COD, 64 TOC 55 BOD5, 27 NH3-N	[78]
2. Combined EC processes
Anaerobically treated municipal wastewater	RSM using BBD	-	CD 2 mA·cm−2	Residence time 5 min	Influent COD 274 mg·L−1	90 mg·L−1 effluent COD 0.57 mg·L−1 phosphate 15.2 NTU turbidity	[62]
Kaolin organic solution and oil-in-water emulsion	RSM	-	CD 2 mA·cm−2	Residence time 20 min	Influent COD 250 mg·L−1	60 turbidity 80 COD ≥80 color	[63]
Textile wastewater	RSM using BBD	4	CD 4.1 mA·cm−2	Retention time 29.63 min	Conductivity of 3.7 mS·cm−1	70 DCOD 100 color	[64]
Textile effluent	RSM using BBD	8.24	Voltage 70 V	Effluent Flow rate 0.5 L·min−1	Clay flow rate 100 mL·min−1	96.87 color, 96.87 COD 84.46 TSS 0.75 $·m−3 as total cost	[72]

presents a summary of experimental design models used in standalone and combined continuous EC processes applied to organic pollutants.

It is evident from Table 3 that most of the research used RSM with BBD models to optimize the operational parameters in both standalone and combined continuous EC processes. These results confirmed good relationships between predicted and experimental data. In addition, in most cases, the optimized parameters include the current density or applied voltage, flow rate or residence time or HRT, initial pollutant concentration, pH, and mixing speed. In all cases, the main result of optimizing these parameters is a significant increase in the pollutant removal efficiencies. Accordingly, the use of experimental design models is necessary in EC experiments. In addition, the inclusion of flow rate in the optimized parameters is very important since it is the main parameter that differentiates batch from continuous processes.

4. Design Innovations in Electrocoagulation Processes

The efficient design of the EC cell in standalone EC processes and other processes in combined continuous EC processes is a critical step in achieving high pollutant removal efficiencies at the lowest cost. For this reason, this section will consider the design of the EC reactors, including the electrode shapes and layout inside the cell, as well as the reactor shape and pollutant flow pattern within it. More attention is given to significant innovations in these parameters compared to traditional plate electrodes arranged in a rectangular reaction EC cell.

4.1. Standalone Continuous EC Reactors

As previously stated, the design of the EC cell is crucial, as it has direct effects on the process performance, and it is decisive in the process scaling up. There are numerous parameters to consider during the design process. Some authors concentrated on cell geometry and its influence on the flow regimes of contaminants within the cell. Others have concentrated on the electrode design to reduce metal consumption and electrode passivation [25]. The success in potential and current distribution on all the electrodes enhances the mass transfer and mixing conditions. Many studies considered sludge production and the potential for electrode passivation, sludge sedimentation, or flotation to achieve efficient sludge separation. Finally, another component that determines process efficiency is the flow rate and residence time of the pollutant, which have an impact on the mass transfer/mixing conditions [2]. Cell geometry and electrode arrangement, as well as hydrodynamics, are two other significant factors that influence the reactor’s physical design characteristics. Therefore, it is possible to infer that the best reactor physical design and operational parameters significantly affect pollutant removal efficiency in the EC process. Even though EC methods have been widely investigated in recent years, there is no one standard reactor design since they are tailored to each process and pollutant, making it impossible to evaluate their performance.

In this part, we sought to explain the most original design of continuous EC cells in terms of shape, as well as electrode shape and arrangement. A novel continuous EC reactor has been designed by Un et al. [36]. Figure 4a shows the experimental setup of this process used to treat cheese whey wastewater.

As shown in Figure 4a, the cell contains a horizontally spinning screw-type Fe anode equipped with impellers and a U-shaped Fe cathode. The horizontal screw anode was positioned 1 cm above the base of the cathode and fully immersed in water. This design will generate turbulent flow inside the cell from one side, potentially lowering sludge deposition on the electrodes and hence electrode passivation from the other. A similar novel electrode design characterizes that of Naje et al. [39].

The developed EC cell consists of a 10 L volume cylindrical reactor with Al electrodes. The rotating anode is made up of ten impellers, while the cathode is formed by ten rings. Each impeller comprises four primary rods along with rings measuring 172 mm in outer diameter, 134 mm in inner diameter, and 12 mm in thickness, spaced 30 mm apart. The reactor is also equipped with three baffles placed at equal intervals to inhibit both rotation and tangential fluid movement, thereby maintaining the structure of the cathode rings.

Tiaiba et al. [46] design a reactor that comprises a 3 L rectangular tank connected to a solid settling flotation tank of 3.5 L volume. The treated liquid exits the continuous EC chamber as an overflow and gently moves into the settling portion. The lighter, floating sludge was consistently removed from the second chamber via overflow, enabling the heavier substances to settle at the bottom.

Abbasi et al. [50] employed a treatment system that was composed of two components: the flotation/sedimentation chamber and the EC chamber. The anode and cathode layouts are vertical and horizontal, respectively, as shown in Figure S2. This novel has the ability to provide good mixing and turbulent flow. Furthermore, the sedimentation/flotation chamber houses the EC chamber.

Rodrigues et al. [53] employed a treatment system that included a fixed bed of metallic particles with a diameter of 2.4 mm that functioned as an anode and were filled with carbon steel or aluminum. In contrast, the cathode, which was made out of an identically sized Al plate, was affixed to the rear face and shielded from the bed particles by a polyamide mesh and a polyethylene screen. The scientists found that employing steel particles in the packing resulted in the lowest energy consumption (EC·g⁻¹) values. When continuously operated, the factors considered included the current density, initial pH, and flow rate. The use of particle anode was equivalent to that of planar anodes.

The electrode design employed in the reactor design was the main focus of Wu et al.’s study [55]. They used folded iron sheets as the anode and cathode to control the flow of fluids. However, Abbasi et al. [58] created another novel EC reactor using Fe rod electrodes. This continuous EC system has Fe rod electrodes measuring 10 mm in diameter and 10 mm in height. When the scientists compared the present design’s performance to that of a previous system that employed Fe plate electrodes, they noticed that the technique removed more turbidity, 59.35% against 47.88% with Fe plate electrodes.

Finally, Mehralian et al. [78] utilized a new reactor configuration that included two cylindrical EC reactors with an internal capacity of 2 L and 12 parallel tubes as anodes and cathodes. A smaller EC vessel was put into the larger anode to enhance the surface area. Electrodes composed of iron or aluminum might be used. They concluded that treating high COD-loaded leachate with a CFR-EC reactor is both cost-effective and efficient.

In conclusion, most of the research discussed in this section has focused on two main aspects: electrode shape and configuration and the settling or flotation of the generated sludge. The majority of innovative designs have resulted in improved pollutant removal efficiency. A summary of the key innovations in standalone ECP is presented in

Table 4. Key variable of standalone continuous EC reactors.

The Design Innovation	Impact of the Innovation	Ref.
A single reactor for both electrolysis and coagulate settling.	High percentage removal of textile effluents.	[34]
A U-shaped cathode and a horizontally rotating screw anode to treat cheese whey effluents.	Removal efficiency of 86.4%.	[36]
Rotating anode with 10 impellers and 10 rings as cathodes.	Passivation reduction on the anode and increased adsorption onto the rotating anode.	[39]
Rectangular tank EC cell for both metal dissolution and solid settling.	Color and turbidity removal of 97.5 and 98.5%.	[46]
Vertical iron plates as anodes and cathodes, with horizontal bipolar electrodes located between the main electrodes (anode and cathode).	The use of CCD with RSM to find the optimal operating conditions leads to high removal efficiencies of 90.15 for COD, 91 for color, and 82% for turbidity.	[50]
Fixed bed of metallic particles as anode. The Al plate cathode was of the same dimensions, attached to the back face.	Effective electrochemical reactor with particulate anode and 90% removal efficiency of dyes.	[53]
Continuous-flow electrocoagulation (CFR-EC) reactor consists of two cylinders as anodes equipped with 12 parallel tubes utilized as cathode.	An effective and affordable EC cell for landfill leachate removal with relatively low cost.	[78]

.

Table 4 indicates that there are few innovative designs for the EC cell parts, especially for the electrodes. The traditional rectangular plate shape of cathodes and anodes could negatively affect the flow inside the cell, and their activity can easily be reduced by the deposited sludge, which causes electrode passivation. The new designs mentioned in Table 4 indicate that the percentage removal in the EC process increases with relatively low cost.

4.2. Combined Electrocoagulation Process

Combined continuous EC process EC shows successful employment with promising and significant findings in the last two decades when used to treat both organic and inorganic chemical pollutants. Accordingly, the design of the combined EC continuous systems is critical and important to give high efficiency of removal and treatment. For this reason, this section will discuss the most innovative designs of combined EC continuous systems according to their design and arrangement of the treatment processes and according to the type of system combined with the EC system.

Moisés et al. [58] published the first continuous EC and biological treatment system for treating organic contaminants. They used a continuous system that featured an aerobic biological activated sludge reactor following an EC pretreatment step. With ciliate and flagellate protozoa and optimum conditions, the COD, color, turbidity, and removal efficiency were 80, 94, and 92%, respectively. The system’s performance in treating industrial wastewater was assessed.

However, Phalakornkule et al. [60] employed two sedimenters, a gas separation tank, and continuous-mode EC, as shown in Figure S3. The first unit is an EC reactor with an 8 L acrylic column and 25 pairs of round iron electrodes.

Similarly, Jiménez et al. [63] used a continuous operation setup consisting of a methacrylate-based electrocoagulation–electroflotation reactor, as shown in Figure S4. The reactor is divided into two distinct zones. The first zone consists of three to five aluminum plates functioning as anodes, responsible for generating coagulant species essential to the coagulation process. The second zone features a horizontally positioned polished stainless-steel cathode located at the reactor’s base, which generates hydrogen bubbles that lift the coagulated flocs to the surface.

Parmentier et al. [68] performed algae harvesting experiments in a tubular electrocoagulation–flotation reactor shown in Figure 5.

Figure 5 shows that the EC cell is made up of two vertical concentric tubular electrodes that are separated by 0.6 cm. This electrochemical cell consists of two vertical concentric tubular electrodes separated by 0.6 cm. The inner SS cathode has a height of 36 cm and an outside diameter of 1.5 cm. The outer anode, on the other hand, is made of aluminum or iron and is 36 cm in height and 2.7 cm in diameter. The flocculation tower is mounted above the EC unit to enable direct flotation-based separation of coagulated particles, thereby minimizing any required energy typically needed for aeration. The optimal height of the flotation tower is 25 cm, which was essential to maintain consistent removal of algae. A tubular mixing device connects the EC cell to the flotation column, ensuring effective integration. Ultimately, the treated wastewater is discharged.

Gökkuş et al. [74] used a continuous flow method to treat Acid Brown 14 (AB14) solutions with EC, as shown in Figure S5. Figure S5 shows a magnetic pump supplying wastewater into a 3 L cylindrical open reactor with Fe rod electrodes scattered in a concentric pattern by inserting them into holes on the reactor’s top surface. This reactor uses compressed air to

Accelerate floc production by increasing the oxidation of Fe²⁺ ions created during the reaction.

Increase convection of contaminants and electrolytes towards anodes for quicker oxidation.

The arrangement of the EC-EF and EC-PEF systems was carried out by integrating the EC reactor with a pre-pilot flow system, as illustrated in Figure S5. This setup consisted of a single electrochemical filter-press compartment equipped with a Pt anode and a carbon-PTFE air diffusion cathode. A 2.5 L solution treated by the EC process was recirculated from the reservoir through the EC cell at a constant flow rate. The pH was regulated to 3.0, the optimal condition for both EF and PEF treatments targeting organic contaminants. The EC’s output was linked to a 640 mL annular glass photoreactor, which was then connected to the reservoir’s outflow. In PEF testing, the photoreactor irradiated the solution with an Omnilux E27 125-W UVA lamp (320–400 nm, λ_max = 360 nm). This light was irradiated at 105 W·m⁻².

Recently, Zivari-Moshfegh et al. [100] employed an undivided tubular flow EC cell, comprising an iron tube functioning as the cathode with a 12 mm internal diameter and 11.2 m overall length. This tube was segmented into 14 equal parts connected as illustrated in Figure 6.

It is evident from Figure 6 that the aluminum rods act as anodes within the iron tubes, covering a total surface area of 2750 cm². The cathode surrounds the anode rods, while the pump transfers effluent from the bottom to the top.

Finally, Z. Al-Qodah et al. [25] designed a three-step combined EC system to treat dairy effluents. As shown in Figure S6, the first step was CC, the second step was solar-powered electrocoagulation (SAEC), and the third step was adsorption. This reactor was loaded with effluents from the preceding coagulation (CC) operation. This method employs Fe sacrificial electrodes. To prevent electrode passivation and sludge buildup to improve liquid flow within the tank, perforated electrodes were used. Furthermore, the electrodes contain several holes placed all around them to improve their surface area. Another advantage of this system is that it uses solar panels (PSM36S-90, Philadelphia Solar, Amman, Jordan) to significantly reduce the treatment cost. A sealed lead–acid battery was utilized to store solar power. A DC-DC inverter converted DC into alternating AC, and a digital potentiometer voltmeter for current intensity control. The advantages of using solar energy stored in solar cells as a source of electricity in the design increase uniqueness and shed light on focusing on green sources of energy in any project.

In conclusion, most of the research discussed in this section focused on designing the combined process by using the most efficient post- or pretreatment process in which the treatment process gives its highest efficiency.

Table 5. Key variables of designing combined continuous EC reactors.

The Combined System	The Design Innovation	Impact of the Innovation	Ref.
Electrocoagulation—Activated sludge	EC reactor + clarifier + aerobic biological reactor 12 Al electrodes, 6 anodes, and 6 cathodes.	Percentage of removal of color 94%, turbidity 92%, COD 80%	[58]
ES	ES with gas separation tank and two sedimenters.	Recovery of hydrogen High removal efficiencies	[60]
Combined EC-EF reactor	Two-zone reactor: The first has 3 to 5 sheets like Al anodes. The second has polished SS cathode at the reactor bottom in a horizontal position to produce H2 bubbles. Settled and floated solids are collected separately.	High removal of pollutants that form low-density solids during the EC stage	[63]
New tubular EC-EF reactor design	The EC cell consists of a pair of concentric tubular electrodes: The inner cathode is made of SS Outer anode is made of Al or Fe	Effective tubular EC-EF reactor for industrial-scale microalgae harvesting.	[68]
EC reactor and a (photo) electro-Fenton recirculation system	The electrodes were Fe rods distributed in a concentric configuration. Air diffusion cathode.	High TOC reduction, close to 90% and 97% in chloride and sulfate media, respectively	[74]
A coagulation-based continuous tubular electrochemical reactor	Iron tube cathode with a commercial aluminum rod anode	Short treatment time, energy efficiency, high mass transfer rate, and low ohmic drop, no need to adjust the pH	[70]
Chemical coagulation (CC), solar powered electrocoagulation (SAEC) and post-adsorption for dairy wastewater treatment	Three-step continuous treatment system with a packed bed adsorber	1- 97.1% removal efficiency 2- 83% reduction in operational costs.	[25]

summarizes key variables of designing combined continuous EC reactors.

Table 5 shows that several combined EC treatment processes have few innovative designs for the EC cell or the other treatment step in the combined system. These design innovations lead to improved performance of the treatment system, increase pollutant removal efficiencies, and reduce waste production. However, it is clear that the use of combined EC processes is still in the first steps and needs intensive research to optimize all the parameters and to advance performance after removing all the problems that hinder this process.

5. Scale-Up of Continuous Electrocoagulation Processes

It is noted that most processes applied for wastewater treatment, including EC, are still at the lab or bench scale level. For this reason, it is crucial to conduct industrial-scale processes to discover the most efficient and cost-effective operation at high volumetric wastewater flow rates [67]. In addition, it is apparent from the available research that the EC process is a potential application for industrial effluent treatment due to its ease of continuous operation and the possible employment of renewable resources to power the process. These facts could motivate intensive research to develop larger-scale EC processes into pilot and industrial-scale applications and their coupling with other processes in a systematic approach [2,101]. However, EC technology could face some scale-up difficulties because the presently used designs are mainly empirical and depend on some experimental parameters that are used to investigate the kinetics and percentage removal of any contaminant. These parameters are treatment time, temperature, CD, and electrode characteristics, in addition to flow rates in continuous processes. In addition, the analysis of the electrical potential and current density are important parameter in continuous large-scale reactor design [2].

As mentioned above, it is apparent from the literature survey on continuous EC processes for organic pollutant removal that most of the published continuous EC processes are of small lab or bench-scale installations. However, these small-scale studies have made it possible to visualize the most important behavior of a continuous process, such as flow rate or residence time, in addition to the flow modes inside the EC cell. This flow mode is directly affected by the electrode type, shape, number, and arrangement. Accordingly, the design of a continuous EC cell must consider the effects of these parameters.

Among the literature on continuous EC treatment processes for organic pollutants, only six research studies applied pilot- or industrial-scale processes, according to their reports. These are Meas et al. [28], Zodi et al. [34], Amour et al. [40], and Yánes et al. [56] who used EC reactors of 3.75 and 23, 2, 3.1, and 3 L volume. The studies of the previous research of Meas et al. [28], McBeath et al. [37], Amour et al. [40], and Cotillas et al. [67] used reactors with flow rates of 1.2 and 6 m³·day⁻¹, 15 L·h⁻¹, 10 L·min⁻¹, and 50 L·h⁻¹. Based on the reactor volumes and daily treated volume of the wastewater in the previous studies, only the pilot- and large-scale studies of Meas et al. [28], McBeath et al. [37], and Cotillas et al. [67] are considered and discussed in this section.

Meas et al. [28] published the first attempt to scale up a continuous process comprising an EC reactor for the treatment of fluorescent penetrant-contaminated liquid using sacrificial Al electrodes. They used a pilot- and industrial-scale system of 3.75, and 23 L volume with a treatment capacity of 1.2, and 6 m³·day⁻¹, respectively. Figure 7 shows a scheme of the industrial-scale system used to reduce the color, COD, and turbidity found in the specific wastewater.

It is evident from Figure 7 that the industrial unit comprised an EC cell, followed by a clarification chamber, a sludge-conditioning tank, and s and or activated carbon filter beds. Figure S7 depicts the effect of flow rate on the percentage removal of COD and color.

As shown in Figure S7, the removal efficiencies of color and COD ranged from 94 to 97% for color and 95 to 98% for COD when the flow rate was about 1.0 o 3.25 L·min⁻¹, or a flow rate of 1.44 and 4.68 m³·day⁻¹, respectively.

One important improvement in the present design is the partial sludge supernatant recirculation to enhance coagulation and consequently increase the percentage removal at constant energy consumption. If these design improvements are installed in the large-scale design, a low-cost and suitable quality of the remediated water will be obtained.

McBeath et al. [37] built a pilot-scale, continuous EC cell with Fe electrodes to treat drinking water. They investigated the impact of several operating parameters, including current density, metal loading (ML), and the inter-electrode distance (d), on natural organic matter (NOM) removal at a flow rate of 1.35 L·min⁻¹. DOC removal performance was evaluated and found to increase as d increases. In addition, they built a pilot-scale continuous reactor to accommodate a flow rate of 10 L·min⁻¹. They incorporated suitable baffles at the reactor inlet to enhance flow distribution inside the whole electrochemical cells. The main result obtained in this research is that as d increases, DOC decreases at the cost of being the most energy-intensive due to the greater resistivity associated with the lower conductivity of the water. Current density effects on DOC were the greatest at the lowest operating conditions, a phenomenon in agreement with the literature. Using the same methodology employed in this outlined research paper, a scaled-up flow rate of 10 L·min⁻¹ should be tested.

The most recent investigation in this section was conducted by Cotillas et al. [67], who examined the scaling up of the integrated electro-disinfection–electrocoagulation (ED-EC) system for reclaiming actual urban secondary wastewater. The EC unit utilized boron-doped diamond (BDD) anodes along with Fe bipolar electrodes. The prototype system expanded the anode surface area threefold and the bipolar electrode area fifteenfold compared to the laboratory-scale configuration. Their findings confirmed the feasibility of achieving complete disinfection and turbidity reduction using current densities between 0.5 and 2 mA·cm⁻². Chlorine-based disinfectants, both free and combined, were generated in situ from the effluents’ chloride content (without the addition of external chemicals) and were effective in eliminating microbial contaminants.

Simultaneously, turbidity was mitigated by the iron coagulant species produced via anodic dissolution of the bipolar electrodes. The prototype demonstrated improved turbidity removal, primarily attributed to the increased electrode area. Additionally, it was shown that the treated wastewater could be safely reclaimed with electric charges under 0.07 kAh·m⁻³, while avoiding the generation of harmful chlorate by-products, even at current densities exceeding 7 A·m⁻². The integrated ED-EC process was implemented in a pilot-scale setup consisting of three electrochemical cells, as depicted in Figure 8.

It is evident from Figure 8 that each cell has a filter press design equipped with a BDD anode, an SS cathode, and five perforated bipolar Fe electrodes placed between them. The observed performance differences between bench-scale and pilot-scale systems can be partially attributed to the difference in their operational modes. The bench-scale usually operates in a batch mode, while the pilot-scale operates in continuous mode. In the continuous setup, the effluent flows through the cell a single time, whereas in batch mode, the same volume is recirculated multiple times through the electrochemical reactor. Although, the contact duration between the fluid and electrodes is equivalent in both modes due to consistent current and total electric charge. The continuous operation achieves this exposure in one pass, while the batch process does so over multiple passes. During the intervals between these passes in batch mode, the oxidizing agents produced remain active in the effluent, potentially enhancing disinfection before being lost to parasitic side reactions. However, this operational difference alone does not fully account for the superior disinfection results achieved in the pilot-scale system. Therefore, a more detailed investigation into the behavior and contribution of chlorine species at pilot scale is necessary, particularly in relation to the electric charge applied under varying current densities.

Given these findings, the scale-up of continuous EC systems for the removal of organic contaminants remains an underdeveloped area, with limited systematic research. Consequently, further investigation into scale-up methodologies represents a promising and innovative direction, with the potential to significantly advance the feasibility of applying continuous EC processes on a large scale.

6. Circular Economy in Electrocoagulation-Based Wastewater Treatment

The circular economy offers significant advantages over the traditional linear take-make-dispose model by addressing inefficiencies in resource use and mitigating environmental, social, and economic risks. Unlike the linear model, which depletes resources and generates excessive waste, the circular economy emphasizes continual reuse, recycling, and materials regeneration, preserving their value throughout their lifecycle. This closed-loop system reduces dependency on finite natural resources, minimizes environmental degradation, and fosters resilience in supply chains. It supports a sustainable society by promoting environmental preservation, economic security, and social equity, ensuring long-term viability for both human and ecological systems [102].

EC offers a versatile platform for advancing circular economy goals in wastewater treatment by integrating pollutant removal with opportunities for resource recovery and reuse [103,104,105]. Beyond its core advantages, minimal chemical input, low sludge toxicity, and energy-efficient contaminant removal, EC supports the reclamation of valuable compounds such as nutrients, metals, and energy carriers [106,107,108]. EC aligns with circular economy principles through several ways. It minimizes chemical usage, reduces secondary pollution, and enables recovery of metals, nutrients, and energy. Table 6 below categorizes circular economy opportunities across the different stages of EC processes, illustrating how each component of the treatment cycle can contribute to sustainability.

Circular economy principles can be effectively embedded within the continuous EC process for wastewater treatment, extending beyond product recovery to cover the entire treatment chain. The pre-recovery stage is increasingly used as a crucial opportunity for extracting beneficial and expensive compounds from wastewater streams even before full treatment is initiated. For instance, phenolic compounds from agro-industrial effluents like olive mill wastewater can be selectively recovered through EC-assisted processes, contributing to value generation upstream and reducing the pollutant load entering the main treatment system [106]. At the input stage, renewable or cheap energy resources such as solar or biogas can be employed to power EC units, which significantly reduces the environmental footprint and operating costs of continuous EC plants [107]. During the process stage, the electrochemical reaction generates hydrogen gas at the cathode, offering a clean and renewable energy vector that can be recovered and reused internally to offset energy requirements [108]. In the coagulation stage, advancements in electrode design allow the use of low-cost and recycled electrode materials, which support resource efficiency and cost reduction without compromising treatment performance [109]. The by-product stage involves sludge generation, but unlike conventional systems where sludge is often a disposal burden, EC-generated sludge can be chemically inert and suitable for valorization in the production of cement, bricks, or other construction materials [110]. The water output stage provides opportunities for the reuse of treated effluent in non-potable applications, such as agricultural irrigation or industrial processes, thereby conserving freshwater resources and closing water loops [111]. Finally, in the nutrient and material recovery stage, essential nutrients like phosphorus and calcium, along with valuable metals such as chromium and copper, can be effectively recovered and reintroduced into industrial or agricultural supply chains, reinforcing circular economy goals through resource recycling and potential revenue generation [112]. Table 6 summarizes the contributions of the EC process to circular economy dimensions.

Table 6. Contributions of electrocoagulation to circular economy dimensions.

Stage	CE Opportunities	Example	Ref.
Pre-recovery stage	Recovery of organic value-added compounds from wastewater before full treatment	Phenolic compound recovery from agro-industrial effluents (e.g., olive mill wastewater)	[106]
Input stage	Use renewable energy sources (solar, biogas) to power EC units	Solar-powered CEP systems	[107]
Process stage	Recover hydrogen gas during electrolysis	Hydrogen fuel for internal use	[108]
Coagulation stage	Optimize electrode materials	Low-cost recycled electrodes	[109]
By-product stage	Utilize sludge instead of disposal	Utilize sludge for construction (cement, bricks)	[110]
Water output stage	Water reuse for non-potable applications (irrigation, industrial use)	Treated water used for irrigation	[111]
Nutrient and material recovery	Recovery of nutrients (P, Ca) and valuable metals (Cr, Cu)	Resource recycling and sales	[112]

Table 6. Contributions of electrocoagulation to circular economy dimensions.

Stage	CE Opportunities	Example	Ref.
Pre-recovery stage	Recovery of organic value-added compounds from wastewater before full treatment	Phenolic compound recovery from agro-industrial effluents (e.g., olive mill wastewater)	[106]
Input stage	Use renewable energy sources (solar, biogas) to power EC units	Solar-powered CEP systems	[107]
Process stage	Recover hydrogen gas during electrolysis	Hydrogen fuel for internal use	[108]
Coagulation stage	Optimize electrode materials	Low-cost recycled electrodes	[109]
By-product stage	Utilize sludge instead of disposal	Utilize sludge for construction (cement, bricks)	[110]
Water output stage	Water reuse for non-potable applications (irrigation, industrial use)	Treated water used for irrigation	[111]
Nutrient and material recovery	Recovery of nutrients (P, Ca) and valuable metals (Cr, Cu)	Resource recycling and sales	[112]

In contrast to batch processes, CEP allows for uninterrupted treatment, real-time monitoring, and better control over sludge and by-product handling. Figure 9 presents a schematic of how CEP can be integrated into a circular economy model for wastewater treatment. Figure 9 illustrates a closed-loop model where every output from the CEP system is reintegrated into the production cycle. Before entering the main EC stream, a pre-recovery stage can be incorporated to selectively extract high-value compounds, such as phenolics, volatile organics, or nutrients, from wastewater, reducing pollutant load and creating early opportunities for valorization [106]. Solar or biogas energy drives the EC reactors [107]. Hydrogen gas, produced at the cathode, can be harvested for internal energy use or fuel cells [108]. Coagulated pollutant forms sludge that is repurposed for construction applications [110]. Treated effluent is directed to reuse systems (irrigation and industrial processes) [111], and valuable materials are extracted and sold or reused in production [112]. This schematic underscores how CEP can contribute to environmental sustainability and economic resilience.

EC systems are increasingly designed not only for pollutant removal but also for resource recovery, supporting the transition toward sustainable and circular wastewater treatment technologies. Continuous-mode EC has attracted growing interest due to its operational stability, better scalability, and ability to integrate with renewable energy or hybrid treatment systems [106,108,109]. Despite these advantages, Table 7 shows that only a few studies have implemented continuous EC configurations for nutrient, energy, or material recovery. For example, a continuous EC system achieved 70% hydrogen recovery alongside >97% dye removal [107], while a continuous-flow reactor using Fe/Al electrodes enabled significant metal recovery (e.g., 99% Mg²⁺, 80% As³⁺, and 70% Pb²⁺) along with ammonia–nitrogen (NH₃-N) removal [78]. Another integrated system combining EC with constructed wetlands and filtration achieved recovery rates of 82.49% for copper, 95% for cyanide, and 71% for grease from industrial wastewater [112]. However, nutrient recovery under continuous modes remains limited in the literature, and energy metrics are often unreported or generalized, highlighting an area needing further exploration and standardization.

In contrast, batch-mode EC systems dominate current research, offering a controlled platform for investigating recovery mechanisms and optimizing operating conditions. As summarized in Table 7, batch EC setups have demonstrated high recovery efficiencies for phosphorus and nitrogen, such as 97.3% total phosphorus via EC–struvite precipitation [113], 98.9% phosphorus and 85% nitrogen in another study [114], and up to 70.9% ammonium ion (NH₄⁺) removal with energy consumption ranging from 5.87 to 7.93 kWh·kg⁻¹ NH₃-N [115]. Material recovery from batch EC is also well documented, including 93.14% sulfate and 94.86% calcium [116] and complete (100%) oil recovery from oily wastewater [117]. While these findings validate the strong potential of batch EC in circular economy applications, their operational limitations and discontinuous nature pose challenges for real-world, large-scale deployment.

Overall, Table 7 illustrates that while batch EC systems remain prevalent in academic studies, continuous systems represent the future direction for industrial applications aligned with circular economy objectives. However, extensive research is encouraged to optimize nutrient and energy recovery in continuous setups, integrate renewable energy sources, and establish performance benchmarks across treatment scales. Critically, no existing study has yet demonstrated an EC system that integrates circular economy principles across all treatment stages, from energy input and electrode sourcing to product recovery and by-product valorization. Most current research isolates one or two recovery aspects (e.g., energy or nutrient) but fails to design EC systems as fully sustainable loops. This represents a significant gap in the literature, underscoring the need for future studies to develop integrated EC frameworks that align operational efficiency with circular economy principles at each step of the process. Bridging this gap is essential for advancing EC from a pollutant removal technique to a regenerative, resource-conserving treatment solution.

Table 7. Electrocoagulation systems for resource recovery: mode, nutrient, energy, and material recovery.

ES System	Nutrient Recovery		Energy Recovery	Material Recovery	Ref.
Continuous EC
Reverse electrodialysis + EC	--		--	99% Cr(VI)	[118]
EC	--		70% H2	>97% Blue dye	[107]
EC—filtration and constructed wetlands	--		--	Cu 82.49% (R); CN 95% (R); 71% Grease	[112]
Continuous-flow reactor + Fe/Al electrodes (CFR-EC)	NH3-N: 27%		--	Heavy metals: Cr6+ (50%), Pb2+ (70%), As3+ (80%), Mg2+ (99%), B3+ (81%), Mn3+ (99%), Ni2+ (20%), Ba2+ (65%)	[78]
EC	--		14.3–16.3 kWh·kg−1 COD removed	TS: 65% removal	[41]
Batch EC
EC—struvite precipitation	97.3% TP		2.35 kWh·m−3	--	[113]
EC—struvite precipitation	24.6% N; 88.4% P		--	--	[119]
EC	90% PO43−		--	--	[109]
EC	74% P; 76% TP		--	--	[120]
EC	98.9 % P, 85 % N;		--	--	[114]
EC	1.13–1.50 kg NH3-N·m−2·d−1; 5.87–7.93 kWh·kg−1 NH3-N; 56.3–70.9% NH4+		--	--	[115]
EC		--	--	93.14% SO42−; 94.86% Ca2+	[116]
EC		--	--	100% Oil	[117]

Table 7. Electrocoagulation systems for resource recovery: mode, nutrient, energy, and material recovery.

ES System	Nutrient Recovery		Energy Recovery	Material Recovery	Ref.
Continuous EC
Reverse electrodialysis + EC	--		--	99% Cr(VI)	[118]
EC	--		70% H2	>97% Blue dye	[107]
EC—filtration and constructed wetlands	--		--	Cu 82.49% (R); CN 95% (R); 71% Grease	[112]
Continuous-flow reactor + Fe/Al electrodes (CFR-EC)	NH3-N: 27%		--	Heavy metals: Cr6+ (50%), Pb2+ (70%), As3+ (80%), Mg2+ (99%), B3+ (81%), Mn3+ (99%), Ni2+ (20%), Ba2+ (65%)	[78]
EC	--		14.3–16.3 kWh·kg−1 COD removed	TS: 65% removal	[41]
Batch EC
EC—struvite precipitation	97.3% TP		2.35 kWh·m−3	--	[113]
EC—struvite precipitation	24.6% N; 88.4% P		--	--	[119]
EC	90% PO43−		--	--	[109]
EC	74% P; 76% TP		--	--	[120]
EC	98.9 % P, 85 % N;		--	--	[114]
EC	1.13–1.50 kg NH3-N·m−2·d−1; 5.87–7.93 kWh·kg−1 NH3-N; 56.3–70.9% NH4+		--	--	[115]
EC		--	--	93.14% SO42−; 94.86% Ca2+	[116]
EC		--	--	100% Oil	[117]

Critically, no existing study has yet demonstrated an EC system that integrates circular economy principles across all treatment stages, from energy input and electrode sourcing to product recovery and by-product valorization. Most current research isolates one or two recovery aspects (e.g., energy or nutrient) but fails to design EC systems as fully sustainable loops. This represents a significant gap in the literature, underscoring the need for future studies to develop integrated EC frameworks that align operational efficiency with circular economy principles at each step of the process. Bridging this gap is essential for advancing EC from a pollutant removal technique to a regenerative, resource-conserving treatment solution.

7. Life Cycle Assessment in Electrocoagulation Systems

Life cycle assessment (LCA) has emerged as a valuable tool used to evaluate the environmental sustainability of EC processes used in wastewater treatment. Despite the growing interest in EC due to its efficiency and simplicity, its environmental implications throughout the treatment life cycle remain underexplored. In this context, the LCA is applied to quantify key environmental impacts, including carbon footprint (global warming potential) and energy consumption, across all stages of the EC process, from material production and operation to waste management. A literature survey in the LCA in EC reveals that a limited number of 40 studies have been published so far. Figure 10 presents the distribution of these studies over the years, and it includes both batch and continuous EC systems.

As shown in Figure 10, 18% originated from China, 17% from India, 9.4% from the USA, and the remaining were distributed across countries like Iran, the United Kingdom, Italy, and others. This limited geographical spread and publication volume highlight the early stage of integrating LCA into EC research. In addition, several studies have applied continuous EC systems for wastewater treatment, with some incorporating LCA to evaluate environmental impacts. For instance, Al-Qodah et al. [25] and Çetinkaya et al. [121] demonstrated the integration of EC with solar and biogas energy sources, achieving effective treatment and significantly reduced greenhouse gas emissions; biogas in particular showed superior sustainability compared to grid electricity [121]. In another study, Maćerak et al. [122] evaluated continuous EC for municipal wastewater, finding that higher flow rates permitted increased current densities and reduced energy consumption, while COD removal remained stable, thus making the system efficient for high-flow treatment [122]. Additionally, a study by Mahes et al. [41] on continuous EC of pulp and paper mill effluents reported effective removal of COD and color, with specific energy consumption decreasing as residence time was reduced; sludge handling and operating cost (0.9 $·m⁻³) were also evaluated, indicating industrial feasibility [41].

Several recent studies have applied LCA to investigate the environmental impact of batch-mode EC processes for treating various wastewaters. In treating textile wastewater, Sedaghat et al. [123] reported high COD removal (96.6%) under optimized conditions, with electricity consumption emerging as the major contributor to environmental impacts such as global warming, and human health burden. Goyal and Mondal [124] compared EC and adsorption for removing arsenic and fluoride from groundwater, finding EC significantly more environmentally friendly (GWP of 4.5 vs. 35.2 kg CO₂-eq) and cost-effective. Li et al. [125] assessed phosphate removal using different anode materials and found aluminum electrodes delivered the highest efficiency (90%) and lowest environmental burden. Similarly, Safwat et al. [126] demonstrated effective manganese removal (up to 96.5%) using titanium electrodes, with relatively low associated environmental impacts. Across these studies, electricity usage and electrode material were identified as key impact drivers, and EC consistently showed strong performance in both treatment efficiency and environmental sustainability, particularly when aluminum or titanium electrodes were used under optimized conditions. Overall, these findings from LCA studies provide critical insights into the environmental performance of EC systems by identifying major impact contributors and guiding the optimization of operational parameters and electrode selection. This facilitates the development of more sustainable, efficient, and cost-effective EC wastewater treatment technologies.

8. Concluding Remarks and Future Research Perspectives

Continuous electrocoagulation processes (CEPs) represent a promising and evolving technology for the treatment of wastewater contaminated with organic pollutants. This review has demonstrated that both standalone and combined CEPs have achieved high removal efficiencies for a wide range of pollutants, including color, turbidity, COD, TOC, and TSS [3,25,28,58,59,60,61]. Furthermore, the integration of CEPs with complementary processes—such as adsorption, biological treatment, and advanced oxidation—enhances overall system performance and broadens the scope of applications [25,65,68,76]. Innovations in reactor design, electrode configurations, and statistical modeling have further advanced the process efficiency and cost-effectiveness of CEPs [35,36,50,57].

Despite these advancements, several technical challenges continue to hinder the scalability and long-term sustainability of CEPs:

• Electrode Passivation: One of the most significant operational barriers is the passivation of electrodes, which occurs due to the deposition of inorganic scales or precipitated coagulants on the electrode surface [36,39,50]. This phenomenon reduces the effective electrochemical surface area, increases energy consumption, and compromises pollutant removal efficiency. Addressing this issue requires a better understanding of the chemical and electrochemical mechanisms of passivation under different wastewater conditions. Future research should focus on the following:

  ✓

  Developing anti-fouling electrode materials and coatings;

  ✓

  Investigating the use of alternating current (AC) or pulse current modes to minimize passive layer buildup;

  ✓

  Designing self-cleaning or rotating electrode systems that promote turbulence and reduce scale accumulation [36,39];

  ✓

  Applying online monitoring and control strategies to detect and mitigate passivation in real-time.

• Sludge Management Constraints: Although EC generally produces less sludge than chemical coagulation, the nature of the sludge—rich in metal hydroxides and adsorbed organics—makes it difficult to handle and dispose of [34,50,60]. Moreover, in continuous systems, the accumulation of sludge can interfere with flow dynamics and electrode accessibility. To improve the sustainability of CEPs, future research should explore the following:

  ✓

  Efficient sludge separation techniques, such as electro-flotation and gravity-assisted settling [46,50];

  ✓

  Valorization of EC sludge through resource recovery (e.g., phosphorus and metals) or conversion into construction materials;

  ✓

  Real-time monitoring of sludge characteristics to optimize operating parameters dynamically;

  ✓

  Hybrid reactor designs that integrate sludge management modules directly into the EC process.

• System Scaling and Standardization: Most CEP studies remain at the lab or pilot scale. Scaling these systems to industrial applications requires addressing issues related to uniform current distribution, flow optimization, heat management, and long-term system stability [25,60,66]. Standardizing reactor design and establishing scale-up protocols will be essential for commercialization.
• Process Optimization and Automation: The application of statistical tools like RSM and emerging artificial intelligence (AI) techniques for predictive modeling and control optimization remains limited [35,38,57,60]. Expanding these tools will allow operators to adapt EC systems to variable wastewater compositions and operational conditions, thereby improving robustness and reliability.
• Energy Efficiency and Sustainability: Although solar-powered EC systems have shown promise, more work is needed to improve the energy efficiency of both standalone and hybrid CEPs [25,28,50]. Lifecycle assessment (LCA) and cost–benefit analyses are also essential to demonstrate the environmental and economic viability of CEPs relative to conventional treatment technologies.

Based on the challenges identified, the following directions are recommended for future research:

• Develop and validate novel electrode materials and geometries that resist passivation and reduce maintenance frequency [36,39].
• Investigate integrated process configurations that combine EC with sludge minimization and reuse strategies [25,50].
• Advanced reactor automation and digitalization, including sensor-based monitoring and AI-driven control systems [60,66,76].
• Conduct comprehensive scale-up studies under real-world operating conditions to evaluate reliability, cost, and environmental impact [25,60].
• Establish international guidelines and performance standards for continuous EC reactor design, operation, and maintenance.
• Promote interdisciplinary research combining electrochemistry, fluid dynamics, materials science, and environmental engineering to holistically enhance CEPs.

In conclusion, while CEPs are a highly promising treatment technology for a broad spectrum of organic pollutants, overcoming the challenges of electrode passivation, sludge handling, and scalability is crucial for their full-scale implementation. Future research, guided by a systems-based and innovation-driven approach, will be essential to unlock the full potential of this environmentally sustainable technology.