Favourable Conditions for the Removal of BOD and COD in Municipal Wastewater by Electrocoagulation

Abstract

The objective of this study was to evaluate the influence of electrocoagulation conditions on the removal of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) in municipal wastewater from the district of Cascas, in line with Sustainable Development Goal 6: Clean Water and Sanitation. A quantitative applied approach was adopted, using an experimental factorial design. Wastewater samples were collected from key points in the sewerage system and treated in electrochemical cells using aluminium (Al), iron (Fe) and combined Fe-Al electrodes. Three voltage levels (10.3 V, 20.2 V and 30.1 V) and three times (10, 20 and 40 min) were evaluated. The highest removal efficiencies were achieved with the Al-Fe electrode at 30.1 V and 40 min, reaching 96% BOD and 97% COD removal. The results demonstrate that electrode type, voltage and treatment time significantly affect contaminant removal. Thus, electrocoagulation is confirmed as a highly effective and low-cost technology for municipal wastewater treatment, with a strong potential for implementation in areas lacking adequate sanitary infrastructure.

1. Introduction

The increasing contamination of water sources, together with the limited availability of water resources, constitutes one of the main environmental challenges worldwide [1]. This situation is aggravated in contexts where urban growth and industrial activities exceed the capacity of conventional treatment systems, generating negative impacts on both public health and aquatic ecosystems [2]. It is estimated that approximately 80% of the wastewater generated worldwide is discharged without prior treatment into natural bodies such as rivers, lakes or seas, considerably aggravating the environment and directly affecting the quality of water resources [3,4]. This problem has been recognised by international organisations as a priority obstacle to the achievement of the Sustainable Development Goals (SDGs), particularly SDG 6, which seeks to ensure the availability and sustainable management of water for all [5].

In this context, the need to develop treatment technologies that are effective, sustainable, economically viable and with low environmental impact has become a priority [6]. Traditional treatment methods, such as physical, chemical and biological, although widely used, have important limitations in terms of their efficiency in removing certain pollutants, space requirements, specialised technical operation and sensitivity to variations in organic load or climatic conditions [7]. Faced with these limitations, physicochemical techniques such as electrocoagulation (EC), a versatile alternative based on the in situ generation of metallic coagulants by applying electric current to electrodes immersed in the wastewater, have emerged [8].

Electrocoagulation has been successfully applied for the treatment of different types of wastewater (municipal, industrial, textile and tanning, among others), demonstrating high efficiency in the removal of organic matter, suspended solids, heavy metals, dyes and pathogenic microorganisms [9,10]. Its principle is based on the controlled dissolution of metal electrodes (usually aluminium or iron), which release metal ions that act as primary coagulants, generating flocs that trap contaminants [11]. Unlike conventional chemical methods, this technique does not require the addition of external agents, which reduces the use of chemicals, minimises the generation of hazardous sludge and simplifies operational management [12].

Among the main advantages of electrocoagulation are efficiency in the removal of biochemical oxygen demand (BOD) and chemical oxygen demand (COD) in reduced time, ease of automation and scalability, low chemical requirements and compatibility with other treatment technologies [13]. However, it also has some limitations that should be considered, such as the selection of the type of electrode, the generation of sludge that requires proper handling, the operating regime and electrical consumption, which can impact operating costs if the process is not properly optimised [14,15].

Several studies have demonstrated the feasibility of this technology in different contexts. In Australia, a significant contribution to odour abatement and improvement of both water and air quality in urban environments has been achieved with the use of aluminium electrodes for sulphide precipitation [16]. In India, 92.01% COD removal efficiency and 93.97% turbidity removal efficiency were achieved under controlled conditions [17]. In Latin America, Mexico reported 46% improvements in organic matter removal with EC, even in treatment times of less than one hour [18]. In Peru, the technology has been applied experimentally in cities such as Arequipa, Trujillo and Puno, obtaining significant reductions in BOD and COD in different types of effluents, including domestic, industrial and tannery waters [19,20,21].

This background supports the fact that electrocoagulation has been widely validated as an effective technique for removing organic matter from synthetic waters and various industrial effluents. However, EC performance in real municipal wastewater remains underexplored. These effluents present complex characteristics—high organic loads, suspended solids and fluctuating chemical compositions—that directly affect electrochemical efficiency and floc formation [22,23]. Factors such as pH variability, ionic strength and the presence of competing contaminants can disrupt coagulant generation and lead to inconsistent treatment outcomes, highlighting the need for pilot-scale validation under real operating conditions [24].

In addition, most studies focus on single-electrode systems tested under controlled laboratory conditions, overlooking the operational advantages of hybrid configurations [25,26]. Combining materials such as iron and aluminium has shown potential to reduce electrode passivation, enhance coagulation and improve overall performance and energy efficiency [27]. Despite promising results, such systems remain largely untested in full-scale municipal applications. Further research integrating EC with other technologies and exploring these hybrid systems under real-world conditions is essential to bridge the gap between lab-scale efficiency and practical viability [28].

This study aims to fill this gap through a comparative evaluation of three electrode configurations applied to real municipal wastewater using a low-energy consumption approach and oriented toward implementation in decentralised contexts. Such is the case of the Cascas district, located in the La Libertad region (Peru), which faces a critical environmental situation due to the direct discharge of untreated municipal wastewater. These discharges contain high organic loads derived from domestic and slaughterhouse activities, including residues such as blood, fat and animal tissues [29]. As a result, the receiving water bodies exhibit elevated concentrations of BOD and COD, leading to significant water quality degradation and posing a direct public health risk to the local population.

The general objective of the research is to evaluate the operational factors that influence the efficiency of the electrocoagulation process for the removal of BOD and COD in municipal wastewater from the district of Cascas. The specific objectives are to analyse the removal efficiency of BOD and COD using aluminium, iron and aluminium-iron electrodes; to evaluate the effect of treatment time on the removal of BOD and COD by electrocoagulation; and to determine the influence of the applied voltage on the efficiency of the electrocoagulation process for the removal of the organic load.

2. Materials and Methods

2.1. Study Area and Sample Collection

The study was conducted using municipal wastewater from the wastewater treatment plant (WWTP) of the Cascas district, Gran Chimú province, La Libertad region, Peru (see Figure 1). A total of 140 L of effluent was collected following national technical protocol NTP 214.060:2016 for wastewater sampling, which is consistent with APHA methodological guidelines. Sampling was carried out over three consecutive days during peak discharge hours (7:00–9:00 a.m.) using a flow-proportional composite sampling strategy based on subsamples taken every 30 min to adequately represent the daily variability of the influent.

The 20 L collection containers were washed with distilled water, dried and repeatedly rinsed with the same sample to avoid undesired dilution. Each container was properly labelled, preserved and transported under refrigeration at 4 °C to the Chemistry Laboratory of Universidad César Vallejo—Moche campus, where samples were processed within six hours to ensure the stability of organic parameters.

2.2. Initial Wastewater Characterisation

Prior to the application of the treatments, characterisation of the municipal wastewater was carried out. For this purpose, the fundamental physicochemical parameters, including BOD, pH, turbidity, total dissolved solids (TDS) and electrical conductivity (EC), were evaluated using a portable multi-parameter model HI98123 (HANNA Instruments, Villafranca Padovana, Italy), which was previously calibrated. COD was determined according to the procedures established using the closed reflux colorimetric method (APHA Standard Methods 5220 D). The following

Table 1. Initial characterisation of municipal wastewater.

Parameter	Unit	Value	S.D. (±)
pH	-	6.45	0.08
Turbidity	NTU	178.2	5.60
Total Dissolved Solids (TDS)	mg/L	1060.4	34.2
Electrical Conductivity (EC)	µS/cm	2215.7	75.3
Biochemical Oxygen Demand (BOD)	mg/L	334.2	1.52
Chemical Oxygen Demand (COD)	mg/L	743.8	2.64

shows the results of the initial characterisation.

2.3. Design and Construction of the Electrocoagulation System

An electrolytic cell was constructed from borosilicate glass, with internal dimensions of 21 × 15.5 × 21 cm. This material was selected due to its high thermal resistance, chemical stability and electrochemical inertness, which ensures structural integrity and avoids interference with the reactions [30]. In addition, its transparency allows direct visual monitoring of the process. Inside the cell, 8 high-purity (≥99%) aluminium and iron flat metal electrodes, with dimensions of 100 mm × 50 mm × 2 mm, were arranged in a vertical and parallel orientation on a wooden support with 10 mm spacing between plates (see Figure 2). Each electrode was pre-cleaned with fine sandpaper, rinsed with distilled water and dried at room temperature. The active surface area exposed to the effluent was 50 cm² per electrode and was kept constant in all configurations. For the hybrid Al-Fe configuration, the electrodes were alternated equidistantly (Al-Fe-Al) within the electrolytic cell. All electrodes were connected to a DC source via stainless steel clamps, and the electrical connection was established in series for operational simplicity and lower current demand, ensuring uniform coagulant distribution and higher efficiency in pilot systems [31].

2.4. Electrical Configuration

The electrodes were connected to an adjustable direct current source by means of a stabiliser-transformer, monitoring voltage and amperage to ensure stable operating conditions. Three voltage levels (10.3, 20.2 and 30.1 V) and three treatment times (10, 20 and 40 min) were applied, selected based on previous studies reporting high percentages of BOD and COD removal within these ranges, without compromising energy efficiency or significantly increasing operating costs [32].

Voltages higher than 30.1 V and longer times were not considered because, although they may slightly increase removal efficiency, they also lead to considerable increases in power consumption, sludge generation and electrode wear, which reduces the practical feasibility of the system in rural and decentralised settings [33].

2.5. Electrochemical Treatment Procedure

To evaluate the effect of operational factors on the removal efficiency of BOD and COD, an experimental scheme was designed in three consecutive stages. Each experimental treatment was carried out with 2 L of raw wastewater, applying combinations of electrode type (Al, Fe and Al-Fe), voltage (10.3, 20.2 and 30.1 V) and treatment time (10, 20 and 40 min), with three replicates per treatment to ensure data reproducibility, and a control group (no treatment) was also included.

In phase 1, voltage (20.2 V) and time (20 min) were kept constant in order to evaluate the effect of electrode type alone on organic matter removal. In phase 2, the effect of voltage (10.3, 20.2 and 30.1 V) was evaluated for each of the three types of electrodes, keeping the time constant at 20 min. Finally, in phase 3, the effect of treatment time (10, 20 and 40 min) on the removal was analysed, keeping the optimum voltage fixed and again considering the three types of electrodes.

Neither the treatments nor the control group were subjected to agitation, as the aim was to simulate static conditions typical of rural or low-cost applications. The control group was maintained in an identical reactor for the same time, volume, and ambient temperature, ensuring that any observed changes in organic load were solely attributable to electrochemical action rather than natural sedimentation. Additionally, BOD and COD concentrations were measured in the control samples at the beginning and end of each treatment period to quantify the effects of passive processes. This allowed for a clearer distinction between removal due to electrocoagulation and background decay, reinforcing the experimental validity. The combinations of operational factors and control conditions applied in the present study are summarised in the experimental design (

Table 2. Experimental design.

Stage	Variable Evaluated	Levels	Treatments	Replicates	Total Tests	Controlled Variables
1	Electrode type	Al, Fe, Al-Fe	3	3	9	Voltage: 20.2 V Time: 20 min
2	Voltage	10.3, 20.2, 30.1 V × (Al, Fe, Al-Fe)	9	3	27	Optimal Electrode Time: 20 min
3	Time	10, 20, 40 min × (Al, Fe, Al-Fe)	9	3	27	Optimal Electrode Optimal Voltage

).

2.6. Post-Treatment Analysis

The efficiency of the electrocoagulation process was evaluated by measuring BOD and COD in the treated samples. COD was determined with the closed reflux colorimetric method (APHA Standard Methods 5220 D), while BOD was measured using a portable multi-parameter model HI98123 (HANNA Instruments, Italy), which was calibrated before each series of measurements. The accuracy of the readings was ensured by systematically rinsing the probe with distilled water and checking the calibration before each use.

Removal percentages were calculated using equation (1), with initial and final concentrations of BOD and COD represented by C_i and C_f [4], respectively:

$$\mathit{Removal} (\%) ={\displaystyle \frac{\mathrm{C}i -\mathrm{C}f}{\mathrm{C}i }} \times 100$$

(1)

2.7. Statistical Analysis of Data

Experimental data were processed using SPSS statistical software (version 27). Shapiro–Wilk normality tests were applied to validate the distribution of the data. Subsequently, a one-factor analysis of variance (ANOVA) was performed to determine significant differences between treatments, followed by Tukey’s HSD post hoc test to identify which combinations of voltage, time and electrode type presented the best removal efficiencies. The level of statistical significance considered was p < 0.05.

3. Results

3.1. Effect of Electrode Type on BOD and COD Removal

Table 3. BOD and COD removal efficiency by electrocoagulation with Al, Fe and Al-Fe electrodes at constant voltage (20.2 V) and standard treatment time (20 min).

Electrode Type	Replicate	Initial BOD (mg/L)	Final BOD (mg/L)	BOD Removal (%)	Initial COD (mg/L)	Final COD (mg/L)	COD Removal (%)
Al	R1	334.2	107.98	67.69	743.8	244.34	67.15
	R2	334.2	99.19	70.32	743.8	200.83	73.00
	R3	334.2	101.23	69.71	743.8	208.19	72.01
Fe	R1	334.2	116.57	65.12	743.8	230.65	68.99
	R2	334.2	113.59	66.01	743.8	199.71	73.15
	R3	334.2	99.93	70.10	743.8	270.74	63.60
Al-Fe	R1	334.2	95.35	71.47	743.8	225.37	69.70
	R2	334.2	85.02	74.56	743.8	185.73	75.03
	R3	334.2	76.90	76.99	743.8	177.47	76.14

presents the results of BOD and COD removal from municipal wastewater obtained by electrocoagulation with Al, Fe and Al-Fe electrodes, applying constant operating conditions of 20.2 V and 20 min of treatment.

For BOD, the initial concentration was constant at 334.2 mg/L. The Al-Fe electrode achieved the highest removal efficiency, with an average reduction of 74.34%, equivalent to a decrease of 257.3 mg/L, reaching final levels as low as 76.90 mg/L. This was followed by the Al (69.24%, reduction of ~231.3 mg/L) and Fe (67.08%, reduction of ~224.2 mg/L) electrodes. These figures reflect a substantial removal of biodegradable organic matter, with the Al-Fe hybrid system being the most efficient.

As for COD, the initial concentration was 743.8 mg/L. The Al-Fe electrode again obtained the best performance with an average removal of 73.62% (reduction of ~547.9 mg/L), followed by Al with 70.72% (~525.9 mg/L) and Fe with 68.58% (~510.1 mg/L). The final concentrations achieved with Al-Fe were as low as 177.47 mg/L, evidencing its superior ability to remove total organic compounds, including non-biodegradable ones.

These results highlight the efficiency of electrocoagulation with all three types of electrodes, with operational advantages for the hybrid Al-Fe configuration. The observed differences were further validated by statistical analysis.

Figure 3 illustrates the average BOD and COD removal values obtained by electrocoagulation using Al, Fe and Al-Fe electrodes, under constant conditions of 20.2 V and 20 min of treatment. In terms of BOD, it was observed that the Al-Fe hybrid system presented the highest average removal efficiency (74.34% ± 2.77), followed by Al (69.24% ± 1.38) and Fe (67.08% ± 2.66). This behaviour suggests a synergistic effect in the combination of both materials, possibly due to the complementarity in the coagulation mechanisms induced by Al³⁺ and Fe³⁺ ions. For COD, the same trend was repeated: Al-Fe obtained the highest average removal (73.62% ± 3.44), slightly outperforming the Al (70.72% ± 3.13) and Fe (68.58% ± 4.79) electrodes. The greater dispersion observed in the values associated with Fe could be related to a lower stability of the coagulation process under this type of electrode.

Before applying ANOVA, the normality of the data was verified using the Shapiro–Wilk test (α = 0.05) for the percentages of BOD and COD removal associated with each type of electrode (Al, Fe and Al-Fe), with treatments carried out at 20.2 V for 20 min. The results showed p-values > 0.05 for both variables, indicating that the data were normally distributed and met the assumption for applying ANOVA. Therefore, in order to evaluate the effect of electrode type on organic load removal efficiency in wastewater, statistical hypotheses were formulated for the variables BOD and COD:

H₀: 

There are no significant differences in the removal percentages of BOD or COD between the three types of electrodes evaluated.

H₁: 

At least one of the electrodes has a mean BOD or COD removal significantly different from the others.

Table 4. ANOVA results for BOD and COD removal according to electrode type (at 20.2 V for 20 min).

Variable	F.V.	S.C.	gl	CM	F	p-Value
BOD	Electrode type	83.45	2	41.72	7.54	0.0231
	Error	33.20	6	5.53
	Total	116.65	8
COD	Electrode type	38.44	2	19.22	1.29	0.3411
	Error	89.17	6	14.86
	Total	127.61	8

presents the results of the ANOVA applied to determine the effect of electrode type (Al, Fe and Al-Fe) on BOD and COD removal efficiency under constant operating conditions (20.2 V, 20 min). In the case of BOD removal, the ANOVA showed statistically significant differences between treatments (F = 7.54; p = 0.0231), which allowed rejecting the null hypothesis of equality of means and suggested that the type of electrode significantly influences the efficiency of the process. This result indicated that at least one of the electrode configurations had a different behaviour compared to the others in terms of biodegradable organic load reduction. Consequently, a post hoc Tukey’s multiple comparisons test was performed to identify the specific differences between pairs of electrodes. On the other hand, the results obtained for COD removal showed no statistically significant differences between the three electrode types evaluated (F = 1.29; p = 0.3411). Therefore, the null hypothesis was not rejected, concluding that, under the established conditions, the Al, Fe and Al-Fe electrodes offered similar COD removal efficiencies. This finding is particularly relevant, as it suggests the operational feasibility of all configurations from an electrochemical perspective, without implying substantial differences in terms of overall oxidant charge removed.

Tukey’s HSD post hoc test allowed the identification of specific differences between electrode types with respect to BOD and COD removal (

Table 5. Tukey’s HSD post hoc test for BOD and COD removal by electrode type.

Variable	Electrode Type	Mean	n	E.E.
BOD	Fe	67.08	3	1.36	A
	Al	69.24	3	1.36	A	B
	Al-Fe	74.34	3	1.36		B
COD	Fe	68.58	3	2.23	A
	Al	70.72	3	2.23	A
	Al-Fe	73.62	3	2.23	A

Means with a common letter are not significantly different (p > 0.05).

). For BOD, the Al-Fe configuration was found to have the highest removal efficiency (74.34%), followed by Al (69.24%) and Fe (67.08%). The analysis revealed statistically significant differences (p < 0.05) between the Al-Fe and Fe electrodes, while the performance of the Al electrode partially overlapped with both groups, sharing common letters (A and B). This indicated that the the Al-Fe combination significantly outperformed Fe but did not conclusively outperform Al. In the case of COD, although the Al-Fe electrode also showed the highest average removal rate (73.62%), followed by Al (70.72%) and Fe (68.58%), no statistically significant differences were evident between the groups (p > 0.05), as they all share the same letter (A). This suggested that, under the conditions evaluated, the type of electrode did not significantly influence COD removal.

Taken together, these results, while slightly indicating slightly higher removal with the Al-Fe electrode, suggested that there may be other factors involved, such as applied voltage and electrocoagulation treatment time.

3.2. Effect of Treatment Voltage on BOD and COD Removal

In this stage, the effect of voltage (10.3, 20.2 and 30.1 V) on BOD and COD removal efficiency was evaluated for each electrode configuration (Al, Fe and Al-Fe), keeping the treatment time constant (20 min).

Figure 4 shows the average values of BOD and COD removal as a function of applied voltage for each type of electrode at a standard treatment time of 20 min. In the case of BOD removal (Figure 4a), the highest value was achieved with the aluminium (Al) electrode at 30.1 V, with an efficiency of 77.75%, closely followed by the iron (Fe) electrode with 76.63% and the Al-Fe hybrid system with 68.68%. At an intermediate voltage (20.2 V), the Al-Fe system stood out with 74.34%, outperforming both Al (69.24%) and Fe (67.08%). At 10.3 V, all electrodes showed similar efficiencies, with removals between 54.46% and 56.88%, indicating that at low voltages the difference between conductive materials was not very significant.

As for COD removal (Figure 4b), the data showed a more marked behaviour with respect to voltage. The highest efficiency was obtained with the Al-Fe hybrid electrode at 30.1 V, with a removal of 85.79%, followed by Al (85.55%) and Fe (79.87%). At an intermediate voltage, Al-Fe also presented the best performance (73.62%), although the differences with the other two electrodes were smaller. At the lowest voltage level (10.3 V), the Al-Fe system again stood out with a removal of 49.08%, higher than Fe (34.04%) and Al (35.02%).

The BOD and COD removal data, obtained at different treatment voltages for each electrode, were verified to follow a normal distribution (p > 0.05) by means of the Shapiro–Wilk normality test (α = 0.05) to support post hoc ANOVA and Tukey’s post hoc test.

Therefore, in order to evaluate the impact of the applied voltage on the organic load removal efficiency in wastewater, the following statistical hypotheses were made for each applied treatment voltage:

H₀: 

There are no statistically significant differences in the removal efficiency of BOD or COD between the applied voltages.

H₁: 

At least one voltage produces a significantly different removal efficiency compared to the others.

The results obtained from the ANOVA are presented in

Table 6. ANOVA results for BOD and COD removal by treatment voltage for each electrode (20 min).

Electrode	Variable	F.V.	S.C.	gl	CM	F	p-Value
Al	BOD	Voltage (V)	660.97	2	330.48	144.27	<0.0001
		Error	13.74	6	2.29
		Total	116.65	8
	COD	Voltage (V)	4046.69	2	2023.34	171.53	<0.0001
		Error	70.77	6	11.80
		Total	4117.46	8
Fe	BOD	Voltage (V)	721.04	2	360.52	38.40	0.0004
		Error	56.33	6	9.39
		Total	777.36	8
	COD	Voltage (V)	3421.02	2	1710.51	107.72	<0.0001
		Error	95.28	6	15.88
		Total	3516.30	8
Al-Fe	BOD	Voltage (V)	629.17	2	314.59	64.49	0.0001
		Error	29.27	6	4.88
		Total	658.44	8
	COD	Voltage (V)	2098.03	2	1049.01	52.46	0.0002
		Error	119.97	6	19.99
		Total	2218.00	8

. For the aluminium electrode, voltage had a highly significant effect on both BOD (F = 144.27; p < 0.0001) and COD (F = 171.53; p < 0.0001) removal. Similarly, in the case of the iron electrode, significant differences between voltage levels were observed for both parameters (F = 38.40 for BOD and F = 107.72 for COD; p < 0.001 in both cases). Finally, for the Al-Fe hybrid configuration, the results also revealed a significant influence of the applied voltage on both BOD (F = 64.49; p = 0.0001) and COD (F = 52.46; p = 0.0002) removal.

These findings confirmed that voltage is a critical variable in the efficiency of the electrocoagulation process, regardless of the type of electrode used. Increasing the voltage significantly improves the formation of coagulant species and the destabilisation of contaminants, thus favouring the formation of flocs and their subsequent removal. Consequently, the p-values obtained (< 0.05 in all cases) allowed the null hypothesis to be rejected and Tukey’s HSD post hoc test was justified to identify which voltage levels differed significantly from each other.

Table 7. Tukey’s HSD post hoc test for BOD and COD removal according to the treatment voltage for each electrode (20 min).

Electrode	Variable	Voltage (V)	Mean	n	E.E.
Al	BOD	10.3	56.88	3	0.87	A
		20.2	69.24	3	0.87		B
		30.1	77.75	3	0.87			C
	COD	10.3	35.02	3	1.98	A
		20.2	70.72	3	1.98		B
		30.1	85.55	3	1.98			C
Fe	BOD	10.3	54.76	3	1.77	A
		20.2	67.08	3	1.77		B
		30.1	76.63	3	1.77			C
	COD	10.3	34.04	3	2.30	A
		20.2	68.58	3	2.30		B
		30.1	79.87	3	2.30			C
Al-Fe	BOD	10.3	54.46	3	1.28	A
		20.2	74.34	3	1.28		B
		30.1	68.68	3	1.28			C
	COD	10.3	49.08	3	2.58	A
		20.2	73.62	3	2.58		B
		30.1	85.79	3	2.58			C

Means with a common letter are not significantly different (p > 0.05).

presents the results of Tukey’s HSD post hoc test to determine which specific voltage levels generated significant variations in BOD and COD removal, depending on the type of electrode used. The analysis revealed consistent patterns across the three electrode types tested.

The removal efficiency increased progressively with increasing voltage, registering averages of 56.88%, 69.24% and 77.75% for BOD and 35.02%, 70.72% and 85.55% for COD, respectively. This behaviour evidenced a direct relationship between the applied potential and the coagulation efficiency generated by the aluminium electrodes.

A similar pattern was observed with the iron electrode. BOD (54.76%, 67.08% and 76.63%) and COD (34.04%, 68.58% and 79.87%) removal efficiencies also showed significant differences between the three applied voltages. These results reaffirmed that a higher potential difference favours the generation of coagulant species and flocculation kinetics in ferric electrodes.

In the case of the Al-Fe hybrid system, significant differences were observed between the three voltages for BOD (54.46%, 74.34% and 68.68%), although the intermediate value (20.2 V) presented the highest performance. For COD, the behaviour was similar, with clear and progressive increases from 49.08% to 73.62% and then to 85.79%. This behaviour suggested that the Al-Fe system could have an optimal performance at moderate voltages, which could be associated with the synergy of the redox reactions between the two metals and the mitigation of phenomena such as passivation.

In all cases, the voltage of 30.1 V proved to be the most efficient in terms of organic load removal, which was taken as a constant parameter in the next experimental phase dedicated to the evaluation of treatment time.

3.3. Effect of Treatment Time on BOD and COD Removal

In this stage, the effect of treatment time (10, 20 and 40 min) on BOD and COD removal efficiency was evaluated for each electrode configuration (Al, Fe and Al-Fe), keeping the optimum voltage (30.1 V) constant.

Figure 5 presents the BOD and COD removal averages obtained at different treatment times at a constant voltage of 30.1 V for the three electrode types evaluated (Al, Fe and Al-Fe). Figure 5a shows that BOD removal showed a progressive increase in efficiency as the treatment time was prolonged, regardless of the type of electrode used. At 10 min, the removal efficiencies ranged from 58.73% (Al) to 69.48% (Al-Fe), while at 20 min they increased to values between 70.59% (Fe) and 77.23% (Al). The maximum removal was recorded at 40 min with the combined electrode Al-Fe, reaching 95.71%, followed by Fe (85.00%) and Al (67.78%). These results showed a significant synergy in the Al-Fe configuration under extended times, suggesting the higher production of complementary coagulant species. Figure 5b shows that COD removal showed a similar trend. At 10 min, the efficiencies were moderate, between 64.47% (Fe) and 70.67% (Al), while at 20 min they increased to a range of 78.87% (Fe) to 88.66% (Al-Fe). Finally, after 40 min, the Al-Fe electrode achieved the highest COD removal (96.82%), followed by Al (91.37%) and Fe (91.50%), with no marked differences between the latter two.

The Shapiro–Wilk normality test (α = 0.05) verified that the BOD and COD removal data obtained at different treatment times (10, 20 and 40 min) for each electrode (Al, Fe and Al-Fe) followed a normal distribution (p > 0.05). Therefore, the use of ANOVA and Tukey’s post hoc test was justified. With this, the following hypotheses were put forward:

H₀: 

There are no significant differences in BOD and COD removal between the different treatment times.

H₁: 

At least one treatment time presents significant differences in the removal of BOD and COD with respect to the other times evaluated.

The ANOVA results in

Table 8. ANOVA results for BOD and COD removal according to treatment time for each electrode (30.1 V).

Electrode	Variable	F.V.	S.C.	gl	CM	F	p-Value
Al	BOD	Time (min)	1295.55	2	647.78	67.36	0.0001
		Error	57.70	6	9.62
		Total	1353.25	8
	COD	Time (min)	694.42	2	347.21	82.23	<0.0001
		Error	25.33	6	4.22
		Total	719.75	8
Fe	BOD	Time (min)	824.53	2	412.26	26.84	0.0010
		Error	92.16	6	15.36
		Total	916.69	8
	COD	Time (min)	1098.03	2	549.01	72.55	0.0001
		Error	45.40	6	7.57
		Total	1143.43	8
Al-Fe	BOD	Time (min)	1319.92	2	659.96	203.56	<0.0001
		Error	19.45	6	3.24
		Total	1339.37	8
	COD	Time (min)	1161.88	2	580.94	161.13	<0.0001
		Error	21.63	6	3.61
		Total	1183.51	8

show that treatment time had a highly significant effect (p < 0.001) on BOD and COD removal for all electrode types tested at 30.1 V, which confirmed its determinant influence on the efficiency of the electrocoagulation process.

For the aluminium electrode, statistically significant differences were observed between the times evaluated for both BOD (F = 67.36; p = 0.0001) and COD (F = 82.23; p < 0.0001). This suggested that increasing the time would significantly improve the removal capacity of the system, particularly in media rich in biodegradable and non-biodegradable organic matter.

In the case of the iron electrode, a significant effect of treatment time on BOD (F = 26.84; p = 0.0010) and COD (F = 72.55; p = 0.0001) removal was also evident. Although the magnitude of the effect on BOD was lower compared to the aluminium electrode, the results indicated that longer operation times contribute positively to floc formation and thus to pollutant removal.

For the Al-Fe configuration, treatment time had the most pronounced impact on the removal of both variables, with values of F = 203.56 (p < 0.0001) for BOD and F = 161.13 (p < 0.0001) for COD. These results reflected a more efficient synergy between the Al and Fe coagulant mechanisms, especially under prolonged operating conditions.

The p-values obtained (< 0.001 in all cases) allowed the null hypothesis to be rejected, confirming that treatment time is a critical factor that significantly influences the efficiency of the process.

Table 9. Tukey’s HSD post hoc test for BOD and COD removal according to treatment time for each electrode (30.1 V).

Electrode	Variable	Time (min)	Mean	n	E.E.
Al	BOD	10	58.73	3	1.79	A
		20	77.23	3	1.79		B
		40	87.76	3	1.79			C
	COD	10	70.67	3	1.19	A
		20	86.10	3	1.19		B
		40	91.37	3	1.19			C
Fe	BOD	10	61.57	3	2.26	A
		20	73.93	3	2.26		B
		40	85.00	3	2.26			C
	COD	10	64.47	3	1.59	A
		20	78.87	3	1.59		B
		40	91.50	3	1.59			C
Al-Fe	BOD	10	69.48	3	1.04	A
		20	70.59	3	1.04		B
		40	95.71	3	1.04			C
	COD	10	69.55	3	1.10	A
		20	88.66	3	1.10		B
		40	96.62	3	1.10			C

Means with a common letter are not significantly different (p > 0.05).

shows the results of Tukey’s post hoc test, which confirmed that treatment time significantly influences BOD and COD removal efficiency for the three types of electrodes.

For the aluminium electrode, three statistically distinct groups were identified for BOD removal: at 10, 20 and 40 min, with average efficiencies of 58.73%, 77.23% and 87.76%, respectively. The same trend was observed for COD, where values increased progressively from 70.67% (10 min) to 91.37% (40 min), with significant differences between all times. These results suggested favourable kinetics of the process when the treatment duration is extended, maximising the generation of coagulant species.

In the case of the iron electrode, a similar pattern was evident. BOD removal increased significantly with time (61.57% to 85.00%), as did COD (64.47% to 91.50%), with clear statistical differences between the three times. This behaviour reflected the improvement in iron solubilisation and the consequent formation of dense and active flocs at prolonged times.

For the combined Al-Fe electrode, the increase in BOD removal was particularly noticeable at 40 min, reaching 95.71%, a value significantly higher than at the intermediate time. Although the differences between 10 and 20 min were slight, the comparison with 40 min showed a statistically significant improvement. The same occurred with COD removal, which increased from 69.55% (10 min) to 96.62% (40 min), consolidating the superior performance of the bimetallic system at extended times.

Overall, these results demonstrated that longer treatments (40 min) provide higher removal efficiencies, and that the three electrode types responded differentially to time, with the Al-Fe system showing the best final performance for both pollutants. The statistical significance of the differences (p < 0.05) strongly supported these observations, suggesting that time is key to optimising electrocoagulation processes under real conditions.

3.4. Visible Changes in the Colour and Transparency of the Wastewater After EC

Figure 6 presents the visual results of the treatment of municipal wastewater by the EC process using different electrode configurations (Al, Fe and Al-Fe) at 30.1 V for 40 min. Figure 6a shows the untreated municipal wastewater (centre) compared to the individual treatments with aluminium (left) and iron (right) electrodes. In the initial state, the effluent had a dull brownish colour, with evident turbidity and the presence of suspended particles, attributable to colloidal organic compounds and domestic waste. On the other hand, the sample treated with the aluminium electrode showed a whitish colour with a noticeable formation of suspended flocs and some residual turbidity. On the other hand, treatment with the iron electrode showed greater sedimentation of the flocs, evidenced by the dark deposit at the bottom of the vessel and a superficial foamy layer, which implied a more intense reaction with greater formation of coagulating ferric compounds.

By contrast, Figure 6b shows the result of treatment with the aluminium and iron (Al-Fe) combined electrode. This configuration showed a marked improvement in water clarity, with a significantly more transparent visual appearance and a minimal presence of suspended flocs. In addition, slight surface foaming was observed, suggesting an efficient balance in the generation of coagulant species and a more complete separation process.

These changes were evident to the naked eye, even without spectrophotometric analysis, and were consistent with the quantitative decreases in BOD and COD recorded in the corresponding treatments.

4. Discussion

The results obtained showed that electrocoagulation is an effective procedure for the removal of organic pollutants in municipal wastewater, specifically in the reduction of BOD and COD. In accordance with the general objective, it was confirmed that the operational factors evaluated (type of electrode, treatment time and applied voltage) have a direct and significant influence on the effectiveness of the treatment, as supported by other authors in their research [34,35].

4.1. Effect of Electrode Type on BOD and COD Removal

The results obtained in phase 1 showed that the three types of electrodes (Al, Fe and Al-Fe) achieved comparable removal efficiencies for BOD (67.08–74.34%) and COD (68.58–73.62%) under the selected operating conditions. ANOVA analysis indicated a statistically significant difference for BOD (p = 0.0231) but not for COD (p = 0.3411), suggesting that the electrode material strongly influences the removal of biodegradable organic matter, while the total organic load remains largely unchanged by the choice of electrode [36].

From a physicochemical point of view, these differences can be attributed to the different electrochemical behaviours of the electrodes. Aluminium, when dissolved anodically, generates Al³⁺-polyhydroxy chloride species that precipitate in the form of bulky and light flocs [37]. These flocs have a high surface adsorption capacity, which enhances the capture and removal of colloidal and dissolved compounds and contributes to effective reduction of BOD [38]. By contrast, the iron electrode produces Fe²⁺/Fe³⁺ ions that form denser and more compact hydroxides [39]. The iron coagulation mechanism is more dependent on the redox potential of the medium and the secondary oxidation of Fe²⁺ to Fe³⁺, which can limit the degree of floc formation and the adsorption efficiency of biodegradable matter [40].

The higher BOD removal observed with the Al-Fe system is explained by the synergistic effects arising from the combination of the rapid floc formation provided by aluminium with the redox power of iron [41]. This hybrid configuration facilitates better destabilisation and aggregation of biodegradable organic matter, which is reflected in statistically higher BOD removal values compared to the Fe system and slightly higher values compared to the Al system [42].

The absence of significant differences in COD removal can be attributed to the inclusion of recalcitrant compounds in the COD parameter, which are not as easily oxidised and removed under the evaluated treatment conditions [43]. Unlike biodegradable organic matter, these compounds may require longer treatment times or more aggressive operating parameters, making the choice of electrode less decisive in terms of total organic load reduction [44]. These findings highlight that, although the electrode material influences the kinetics of biodegradable organics removal through differences in floc formation and redox behaviour, the overall treatment efficiency for total organic loading remained robust across all electrode types evaluated [45].

This behaviour has been reported in similar studies, such as the one by Dobrosz et al. (2021), who found that experimental configurations integrating different electrodes can significantly improve process efficiency by generating a greater diversity of coagulant species [46]. Likewise, Dehghani et al. (2024), in hospital acidic water systems, observed that the combination of electrodes optimises the dispersion of metal ions and improves contact with the pollutants [47]. It is worth noting that, although aluminium showed slightly better performance than iron when used separately, the difference was more pronounced when applying the combined configuration, which reinforces the need to select and optimise the electrochemical materials according to the pollutant matrix of the effluent [48,49].

4.2. Effect of Treatment Voltage on BOD and COD Removal

On the other hand, the experimental results of phase 2 clearly demonstrated that the applied voltage is a critical parameter in the electrocoagulation process for organic load removal. In our study, a statistically significant increase (p < 0.001) in BOD and COD removal was observed when increasing the voltage from 10.3 V to 30.1 V in the three electrode configurations (Al, Fe and Al-Fe). This improvement is directly related to the increase in the rate of anodic dissolution, which releases metal ions necessary for the formation of coagulant hydroxides that destabilise colloidal organic matter [50].

At a lower voltage of 10.3 V, anodic dissolution is limited, resulting in limited generation of coagulant species and, consequently, suboptimal floc formation [51]. As the voltage increases to intermediate values, such as 20.2 V, and subsequently to 30.1 V, there is a corresponding increase in the release of Al³⁺ and Fe²⁺/Fe³⁺ ions [52,53]. This reaction enhances the formation of metal hydroxide flocs, which in turn improves adsorption and accelerates the precipitation of organic compounds from wastewater [54].

The relationship between applied voltage and removal efficiency has been widely reported in the literature. For example, Amri et al. (2023) observed that increasing the voltage to 24 V in industrial wastewater produced COD and TSS removal values of 91% and 90%, respectively, underlining the importance of optimising the voltage for efficient process performance [55]. Similarly, Reátegui-Romero et al. (2022) demonstrated that, in slaughterhouse wastewater, raising the voltage from 6 V to 10 V led to an improvement in COD removal efficiency, achieving up to 60% removal [56].

It is worth mentioning that while increasing the voltage improves the efficiency of the process, this parameter must be carefully optimised, as excessive levels can increase energy consumption and accelerate electrode wear, affecting the sustainability of the system [57]. Therefore, it is crucial to determine a technical threshold that maximises efficiency without compromising economic and operational viability [58]. In this sense, the value of 30.1 V can be considered a technical break-even point for pilot-scale applications, particularly in rural or decentralised contexts, where energy resources are limited and operational stability is essential [59].

Although the theoretical energy consumption was estimated based on the applied electrical parameters, no experimental measurements of actual energy usage or electrode wear rates were performed. These factors may directly impact the economic and operational feasibility of the system; therefore, their omission constitutes a limitation of this study. It is recommended that future research addresses these aspects through extended-scale validations under real operating conditions.

4.3. Effect of Treatment Time on BOD and COD Removal

As for the third phase, the results confirmed that treatment time is directly proportional to the removal efficiency of BOD and COD. A progressive improvement was observed when the electrocoagulation process was operated from 10 to 40 min, as a longer reaction duration allows for the cumulative generation of metal ions and the consequent formation of coagulant [60,61].

The mechanism underlying this behaviour is well founded in the fundamentals of electrocoagulation [62,63]. As electrodes undergo anodic dissolution, metal ions such as Al³⁺ and Fe²⁺/Fe³⁺ are continuously released into solution [52]. These ions subsequently react with water to form metal hydroxides, e.g., Al (OH)₃ and Fe (OH)₂ [64]. These metal hydroxides act as in situ coagulants that destabilise colloidal particles and establish ionic bridges between suspended contaminants, thus promoting floc formation [65]. Therefore, the overall efficiency of the process is directly proportional to the applied electrical charge and the duration of treatment [66,67], as prolonged exposure generates higher concentrations of coagulant available to interact with and remove organic matter [68].

In the specific Al-Fe system studied, BOD removal increased from 69.48% at 10 min to 95.71% at 40 min, while COD removal showed a similar improvement from 69.55% to 96.62% in the same interval. This significant performance improvement is attributed to the cumulative nature of metal ion generation and the constant production of active species that promote both particle aggregation and sedimentation [69]. Furthermore, since the continuous generation of coagulants also helps to stabilise the pH of the system, favourable conditions for the coagulation process are maintained even in complex matrices that include colloidal substances, proteins, fat and organic animal waste [31].

This pattern of behaviour is consistent with that reported by Rodríguez et al. (2021), who observed removal of more than 90% COD and turbidity in industrial effluents after 60 min of treatment [70]. Likewise, Garomsa et al. (2024) reported efficiencies of 99.01%, 99.09% and 99.02% for COD, BOD and TDS, respectively, in a time of 40 min, using electrocoagulation systems with similar characteristics [71].

However, while extending the treatment time improves performance, it is essential to consider the law of diminishing returns [72]. Beyond a certain time threshold, the system may reach an efficiency plateau, where further increases in time do not justify the energy consumption and sludge production [73].

Therefore, the present study confirms that a treatment time of 40 min is the most suitable to maximise BOD and COD removal under optimised operating conditions (30.1 V), irrespective of the type of electrode used. These findings strengthen the design of efficient electrocoagulation systems capable of adapting to real treatment conditions and provide key technical evidence for future applications in rural or peri-urban environments with technological limitations and high pollutant load.

4.4. Energy Consumption, Electrode Wear and Sludge Generation

In addition to organic pollutant removal efficiency, the technical feasibility of electrocoagulation depends on critical operational variables such as energy consumption, electrode wear and sludge generation. These factors directly impact treatment cost, maintenance frequency and secondary waste management [74].

4.4.1. Energy Consumption

Although the present study did not include a direct measurement of electricity consumption, it is estimated that treatment at 30.1 V for 40 min with a 2 L system generates a moderate energy demand. Based on similar configurations reported in the literature, an approximate power consumption of 2.5 to 4.0 kWh/m³ is estimated, depending on water conductivity, electrode spacing and power supply efficiency [75]. This range is competitive compared to conventional technologies, but it is recognised as a limitation of the study that exact experimental values were not recorded. It is recommended for inclusion in future work to establish a more robust cost-benefit analysis.

4.4.2. Electrode Wear

Wear of metal electrodes is unavoidable due to their electrolytic dissolution but was not quantified in this work. However, similar studies report a mass loss between 20 and 40 mg/L treatment for comparable configurations [76]. A progressive thinning of the electrodes was visually observed after multiple treatments, especially in higher voltage and extended time configurations, suggesting a variable replenishment frequency depending on the intensity of use [77]. It is acknowledged as a limitation that this parameter was not evaluated gravimetrically, and its systematic analysis is recommended for further research.

4.4.3. Sludge Generation

During the process, the visible formation of flocculent sludge was observed at the bottom of the reactor at the end of the treatment as a result of the precipitation of metal hydroxides (Al(OH)₃, Fe(OH)₂, and Fe(OH)₃) and adsorbed organic matter [64]. The volume and mass of sludge generated and its physicochemical characterisation were not quantified, which represents another limitation of the present study. The literature indicates that sludge generation in electrocoagulation is lower than that observed in conventional chemical coagulation and has better dewatering and stability properties [78]. However, it is recognised that its proper management is essential to ensure the sustainability of the system [79]. In future research, it is suggested to carry out a complete characterisation of the sludge (dry weight, moisture, elemental composition and toxicity) and to explore alternatives for safe valorisation or disposal in accordance with current environmental legislation.

While this study primarily focused on evaluating the removal efficiency of BOD and COD, we recognise the importance of sludge quantification in assessing the overall performance of the electrocoagulation process. Although a full physicochemical characterisation of the sludge fell beyond the scope of this work, basic measurements were obtained. For the Al-Fe electrode configuration at 30.1 V and 40 min, the average sedimented sludge volume was approximately 125 ± 5 mL per 2 L of treated wastewater, and the corresponding dry mass was 3.62 ± 0.18 g. These values provide a preliminary reference for the sludge yield under optimal treatment conditions and support the technical conclusions regarding operational viability. A more comprehensive analysis of sludge properties and handling strategies is planned for future studies.

5. Conclusions

The results obtained confirmed that the type of electrode used in the electrocoagulation process significantly influences the removal efficiency of BOD and COD in municipal wastewater. In particular, the combined Al-Fe electrode configuration showed superior performance compared to the single electrodes, evidencing a higher reduction of organic pollutants under the applied experimental conditions. The superiority of the Al-Fe system is attributed to a synergistic effect between the two metals, which favours charge destabilisation and the formation of denser and more efficient flocs, substantially improving organic matter removal.

The applied voltage had a direct and statistically significant effect on the efficiency of the process. Increasing the voltage from 10.3 V to 30.1 V improved the removal of BOD and COD in all three systems evaluated. The best result was obtained with the Al-Fe combined electrode at 30.1 V, reaching removal levels of up to 96% for BOD and 97% for COD, consolidating the optimum condition within the proposed experimental design.

Likewise, treatment time proved to be a determining factor in the efficiency of the process. A progressive increase in pollutant removal was observed as the exposure time was extended, reaching its maximum efficiency at 40 min. This trend was especially noticeable when using the Al-Fe system, which shows that longer contact time enhances the coagulation and sedimentation of the contaminants.