Processes Coupled to Electrocoagulation for the Treatment of Distillery Wastewaters

Abstract

Vinasse is acidic, dark brown wastewater obtained as a residue from the alcohol distillation process, the main component of which is water, in addition to mineral nutrients and a high organic load. Electrocoagulation (EC) is a technology that generates coagulating substances in situ by oxidizing sacrificial anodes through an electric current applied to the electrodes. During the last decade, the electrocoagulation process has been intensively investigated in several reviews, due to its ease of operation, versatility, sustainability and low environmental impact. The objective of the present work has been to make a general review of the EC process, its principle, reaction mechanism and operating parameters involved in the electrocoagulation process. In this research, the PRISMA method was used for the analysis of articles from different databases such as Scopus, ScienceDirect and Google Scholar. This review collects numerous studies of the EC process in stillage wastewater treatment and makes a comparison between these experimental results mainly in terms of chemical oxygen demand removal. In addition, this review makes a comprehensive analysis of EC coupled to other processes, taking into account their operating parameters and stillage contaminant removal efficiency. The conclusion of this research points out that electrocoagulation coupled with other treatment processes is very necessary because it reduces energy consumption and increases the rate of pollutant removal from wastewater.

1. Introduction

Distilleries are currently ranked as one of the 17 most polluting industries because they produce large quantities of highly concentrated wastewater, which constitutes a serious environmental challenge [1]. Generally, each liter of ethanol produced generates 10 to 13 L of vinasse, which is characterized as a foul-smelling waste with a high organic load [2], with extremely high chemical oxygen demand (COD) content (100,000 mg/L), acidic pH (3.75–4.5), strong odor [3], high biochemical oxygen demand (BOD) (45,000–60,000 mg/L) [4] and also nutrients in the form of nitrogen, phosphorus and potassium. For this reason, advanced technologies for wastewater treatment have been developed. Among these technologies, the most widely used are physicochemical processes, which have been known for centuries for water treatment [5]. Several methods are available for stillage treatment, such as anaerobic digestion [6], Fenton [7], ozone [8], electrodialysis [9], nanofiltration [10] and electrocoagulation [3,11]. As for physical treatments, modified materials such as adsorbents have been found to be effective for the adsorption of specific pollutants [3]. Carbohydrate adsorbents based on natural polymers with specific functional groups can enhance their potential applications towards the noticeable removal of dyes from wastewater [12].

One of them is electrocoagulation (EC) technology [13], which stands out as one of the most viable alternatives because it does not use chemical reagents [14] and removes various contaminants from metals to persistent organic pollutants [13].

Some researchers have studied the treatment of distillery effluents for the removal of color and chemical oxygen demand (COD) by different methods, such as biological and electrochemical methods [15]. Savita Dubey et al. have employed a combined sono-electrocoagulation process in the treatment of distillery wastewater, and the factors studied were ultrasonic power, current density, pH and electrolysis time, obtaining a COD removal of 93% and a color removal of 88% with iron electrodes [16]. Badur Karmankar et al. applied photovoltaic solar cell-driven electrocoagulation for the treatment of distillery effluents, with various combinations of Fe/Fe, Fe/Al, Al/Fe and Al/Al electrodes and taking into account the operating parameters such as pH, current density, separation and electrode combination. Obtaining the results of 94.9% COD reduction and 81.3% color removal helps in concluding that the photovoltaic electrocoagulation system is better than conventional EC [17]. However, most studies focus only on electrocoagulation as a treatment method without considering the benefits that can be obtained by integrating it with new advanced technologies, giving rise to hybrid methods that increase the removal efficiency of the organic pollutant load. This review study evaluates the efficiency of hybrid electrocoagulation processes with conventional electrocoagulation, taking into account the effect of various operating parameters such as current density, electrolysis time, pH, voltage and electrode specifications. In addition, it will emphasize the contributions of these studied methods to obtain a clearer picture and select the best treatment technology for vinasse.

2. Vinasse and Its Characterization

The production and characteristics of distillery residue are highly variable and depend on the feedstock used and various aspects of the ethanol production process [18]. Vinasse is characterized by a deep brown color, high turbidity, high content of organic compounds (acids, alcohols, aldehydes, ketones, esters and sugars) and a high concentration of ions (K⁺, Ca²⁺, Mg²⁺, Fe³⁺, Cl⁻ y SO₄²⁻) [19]. The COD and BOD values of these wastes are due to the presence of a number of organic compounds, such as polysaccharides, proteins, polyphenols, waxes and melanoidins, which cause environmental contamination [20].

Table 1. Physical and chemical characteristics of distillery vinasse.

Characteristics	Units	Values
		[9,20,22]	[22,23,24]	[22,25,26]
pH	-	4.0–4.5	3.8–5	3.5–4
TS	mg/L	59,000–82,000	48,000–62,000	142,150 ± 4600
VS	mg/L	38,000–66,000	42,800–50,300	102,430 ± 1900
TSS	mg/L	2400–5000	23,000–31,500	-
COD	mg/L	100,000–150,000	60,000–72,000	155,833 ± 8065
BOD	mg/L	35,000–50,000	25,000–50,500	18,160 ± 1378
Conductivity	mS/cm	8.3 ± 0.7	12.1 ± 2.3	9.49 ± 0.02
Nitrogen—NH3	mg/L	1660–4200	<1.00	12,209.52–2018.27
Total phosphorus	mg/L	225–308	-	260.80–16.76
Potassium	mg/L	9600–15,475	-	-
Iron	mg/L	1550–1800	6.7–9.0	190–204
Sulfates	mg/L	2100–2300	-	-
Calcium	mg/L	2300–2500	-	329–371
Magnesium	mg/L	220–250	<1.00	598–609

TS: total solids; VS: volatile solid; TSS: total suspended solids; N-NH₃: total ammonia nitrogen. [9,20,21,22,23,24]: references.

shows the physical and chemical characteristics reported by different researchers. These conductivity values are relatively high, mainly due to the presence of sodium, potassium, chloride and sulfate ions in the stillage. Most of the total solids correspond to volatile total solids, which are organic in nature. Total nitrogen includes all natural products, such as proteins, peptides and amino acids [20]. Its acidic character is mainly due to the addition of sulfuric acid to the solution during ethanol fermentation. In addition, its low pH is related to the presence of organic acids in the stillage, such as acetic acid [21].

3. Main Fundamentals of Electrocoagulation

The electrocoagulation process has attracted great attention in industrial wastewater treatment due to its versatility and environmental compatibility. This technique has several advantages over alternative methods such as simple equipment, ease of operation, shorter retention times, reduction in or even the elimination of the need to add chemicals, the rapid settling of the flocs generated by the process and less sludge generation [27]. This process combines the basic principles of anodic oxidation and cathodic reduction [28], coagulation, flocculation and electrochemical process [29]. EC is an electrochemical process that destabilizes contaminant charges through an applied electric current that causes the dissolution of the electrode and traps contaminants in flocs that can be separated from the electrolyte mixture [30]. Metal ion compounds were created by dissolving a “sacrificial anode” with an electric current. These ions bind to form flocs, which absorb dissolved substances and decrease the stability of colloidal contaminants in wastewater [31]. Hydroxyl ions produced from water molecules will react with metal ions that subsequently bind with each other to form metal hydroxide coagulants in situ [32]. Metal hydroxyl ions, due to their polar moment and oxidizing capacity, form complexes with contaminants, leading to the formation of flocs [30]. The eradication mechanism may be through charge neutralization, sweep coagulation or adsorption [33]. The adsorption process occurs when molecules of an aqueous phase bind to the surface of the solid coagulant, which subsequently forms insoluble precipitates [32]. Current density (CD), initial pH, concentration, operating time, conductivity and agitation speed are the main factors that have a significant influence on the EC treatment process [34]. Due to its wide field of application, the EC can be used in the treatment of industrial, domestic and municipal wastewater, among others [29].

3.1. Mechanisms of Electrocoagulation

When electric current flows, oxidation and reduction reactions occur at the anode and cathode, respectively [35]. At a suitable pH, metal ions can form a wide range of metal hydroxides and coagulated species that destabilize and collect suspended particles or adsorb and precipitate dissolved contaminants. This process as we can see in Figure 1 produces metal ions from the anode, forming hydroxides that adsorb and aggregate particles that are suspended, precipitate and adsorb dissolved contaminants [36]. O₂ and H₂ gases are simultaneously emitted near the anode and cathode, respectively. These gases promote flotation, which causes flocs to rise to the surface of the liquid and lighter objects to sink [31]. Therefore, the main electrochemical and chemical reactions in an electrocoagulation system are as follows [37]: In summary, in a CE process, three consecutive steps of the coagulated ions occur in situ. These are (i) the electrolytic oxidation of sacrificial electrodes to detach coagulants, (ii) the destabilization of contaminants, suspension of particles and breaking of emulsions and (iii) the accumulation of the destabilized phase to produce flocs [6].

Anodic reaction (coagulation):

$$\mathrm{M}\to {\mathrm{M}}^{\mathrm{n}+}+\mathrm{n}{\mathrm{e}}^{-}$$

(1)

In addition to the oxidation of metals, oxidation of water can also occur at the anode, depending on the anode potential.

$${2\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to 4{\mathrm{H}}^{+}+{\mathrm{O}}_2+4\mathrm{e}$$

(2)

Cathodic reaction (floating):

$$2{\mathrm{e}}^{-}+{2\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{H}}_2$$

(3)

In acid solution:

$${\mathrm{M}}^{\mathrm{n}+}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{M}{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{n}}+\mathrm{n}{\mathrm{H}}^{+}$$

(4)

In alkaline solution:

$${\mathrm{M}}^{\mathrm{n}+}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{M}{\left(\mathrm{O}\mathrm{H}\right)}_{\mathrm{n}}$$

(5)

3.2. Types of Electrodes

In the electrocoagulation process, there are different types of electrode metals, and mostly aluminum and iron electrodes are used. The anodic process involves the oxidative dissolution of aluminum in aqueous solution, as well as the reductive dissociation of water [38], as shown in the following reaction:

For aluminum electrodes [39]:

$$\mathrm{Anode}: \mathrm{A}\mathrm{l}\to {\mathrm{A}\mathrm{l}}^{3+}+3{\mathrm{e}}^{-}$$

(6)

$$\mathrm{Cathode}: 4{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2+4{\mathrm{O}\mathrm{H}}^{-}$$

(7)

$$\mathrm{F}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{o}\mathrm{f} {Al\left(OH\right)}_3 : {\mathrm{A}\mathrm{l}}^{3+}+3{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_3$$

(8)

In the case of iron, when an iron anode is used in electrocoagulation, Fe dissolves in the wastewater from the oxidation of Fe²⁺ at the anode as follows [40]:

Anode [39]:

$${\mathrm{F}\mathrm{e}}_{\left(\mathrm{s}\right)}\to {\mathrm{F}\mathrm{e}}^{2+}+2{\mathrm{e}}^{-}$$

(9)

$${\mathrm{F}\mathrm{e}}^{2+}+2{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

(10)

while the hydroxide ion and H₂ gas are generated at the cathode from the following reaction [40]:

Cathode [39]:

$$\mathrm{F}\mathrm{e}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{H}}_2$$

(11a)

$${\mathrm{F}\mathrm{e}}^{2+}+1.5{\mathrm{H}}_2\mathrm{O}+0.25{\mathrm{O}}_2\to \mathsf{\gamma }-\mathrm{F}\mathrm{e}\mathrm{O}\mathrm{O}{\mathrm{H}}_{\left(\mathrm{s}\right)}+{2\mathrm{H}}^{+}$$

(11b)

Fe was released into the solution as Fe (II) (9) and then immediately oxidized to Fe (III) due to water and an oxygenated environment or by dissolved oxygen. Then, Fe (III) precipitated in the formation of iron oxides, particularly nanoscale lepidocrocite (γ-FeOOH), which is responsible for the removal of organic and inorganic COD and other metal ions, and ultimately removes color and COD from wastewater [39]. In addition to Al and Fe electrodes, the use of copper (Cu) as the electrode material also helps in the removal of various wastewaters [14]. Among these metals, due to the higher conductivity of Cu, it can be used as an alternative electrode in EC operation [41]. The equations of reactions carried out in EC operation at the anode and cathode are as follows:

Reaction at the anode [14]:

$$2Cu\to 2C{u}^{2+}+4e^{-}$$

(12)

Reaction at the cathode:

$$4H_{2}O+4e^{-}\to 2H_2+4{OH}^{-}$$

(13)

$$2Cu+{4H}^{+}\to {2Cu}^{2+}+{2H}_2$$

(14)

Zn is an attractive metal to be used as an electrode for electrocoagulation treatment due to its low cost, spontaneous reactivity and ability to rapidly remove contaminants; these advantages translate into high removal efficiency and lower energy consumption [42]. Electrochemical reactions involving Zn electrodes are as follows for the Zinc electrode [42]:

$$\mathrm{Anode} \left(\mathrm{coagulation}\right): Zn\to {Zn}^{2+}+{2e}^{-}$$

(15)

$$\mathrm{Cathode} \left(\mathrm{flotation}\right): 2H_2{O}_{\left(l\right)}+2e^{-}\to H_{2\left(g\right)}+2{OH}^{-}$$

(16)

$$\mathrm{General} \mathrm{reaction}: {Zn}_{\left(aq\right)}^{2+}+{2H}_2{O}_{\left(l\right)}\to {Zn\left(OH\right)}_2+{2H}_{\left(aq\right)}^{+}$$

(17)

The electrochemical reactions with titanium metal acting as electrodes are described as follows for the Titanium electrode [43].

$$\mathrm{Anode}: Ti\to {Ti}^{4+}+{4e}^{-}$$

(18)

$$\mathrm{Cathode}: 4H_{2}O+4e^{-}\to 2H_2+4{OH}^{-}$$

(19)

$$\mathrm{General} \mathrm{reaction}: {Ti}_{\left(aq\right)}^{4+}+{4H}_2{O}_{\left(l\right)}\to {Ti\left(OH\right)}_4+{4H}_{\left(aq\right)}^{+}$$

(20)

Electrochemical reactions of magnesium electrodes [40]:

For the Magnesium electrode, the anode is

$${Mg}_{\left(s\right)}\to {{Mg}^{2+}}_{\left(aq\right)}+{2e}^{-}$$

(21)

$$\mathrm{Cathode}:2H_2{O}_{\left(l\right)}+2e^{-}\to 2H_{2\left(g\right)}+2{OH}^{-}$$

(22)

$$\mathrm{General} \mathrm{reaction}: {Mg}_{\left(aq\right)}^{2+}+{2H}_2{O}_{\left(l\right)}\to {Mg\left(OH\right)}_2\downarrow +{2H}^{+}$$

(23)

3.3. Electrocoagulators of Reactors

In wastewater treatment by electrocoagulation, electrocoagulators are devices used to conduct electric current through water. This action induces electrochemical reactions that contribute to the removal of pollutants and suspended particles in the water. Each type of electrocoagulator has advantages and disadvantages in terms of efficiency and treatment capacity, as well as operating and maintenance costs, among other aspects. The selection of the most suitable electrocoagulation will depend on the particular characteristics of the water to be treated and the specific requirements of the wastewater treatment process. Currently, there are no researchers who have applied these types of electrocoagulators to the treatment of wastewater specifically from distilleries such as vinasse. However, there is research where wastewater from other types of industry has been treated with different types of electrocoagulators, showing a significant percentage of contamination removal.

Table 2. Types of electrocoagulators.

Types	References
Electrocoagulator with spiral electrode configuration.	[44]
Electrocoagulator with centrifugal electrodes.	[45]
Electrocoagulator with cylindrical electrode.	[46]
Electrocoagulator with cylindrical anode with multiple fins.	[47]
Electrocoagulator with integrated advanced oxidation system.	[48]
Electrocoagulator equipment with mobile electrodes.	[49]
Continuous flow electrocoagulator.	[50]
Discontinuous flow electrocoagulator.	[51]
Hybrid electrocoagulation–adsorption system.	[52,53]

shows different types of electrocoagulators with their respective references from previous studies carried out.

3.4. Data Analysis

• Faraday’s Law

The consumption of an electrode (C) in reference to the unit of treated wastewater (Kg electrode/m³) is calculated with Faraday’s law [54].

$$\mathrm{C}={\displaystyle \frac{\mathrm{I}\times \mathrm{t}\times \mathrm{M}}{z\times F\times V\times 1000}}$$

(24)

• I: current intensity (A).
• t: electrolysis time (s).
• V: volume of treated wastewater (m³).
• F: Faraday constant (96,487 C/mol).
• M: molar mass of electrode consumed (g/mol).
• z: electron transfer number.

• Energy consumption

Cost evaluation in large-scale electrochemical treatment is strongly influenced by energy consumption. A lower energy demand leads to a reduction in the costs associated with the treatment. Specific energy consumption is defined as the amount of energy required per unit mass of organic matter removed, which can be COD, ammonia nitrogen or colorant, among others [55]. The evaluation of this specific energy consumption can be performed as follows:

$$\mathrm{S}\mathrm{E}\mathrm{C}={\displaystyle \frac{U\times {\int }_0^{t}Idt}{\left({COD}_x-{COD}_y\right)\times V}}$$

(25)

• SEC: the specific energy consumption (kWh/kg COD removed).
• U: applied voltage (V).
• I: current intensity (A).
• t: electrolysis time (h).
• COD_x: chemical oxygen demand before treatment (g/L) and COD_y: chemical oxygen demand after treatment (g/L).

• Operating Cost

The determining factor for the implementation of the treatment is the operating cost, which includes both the cost of the materials (electrodes) and the cost of electricity [41]:

$$\mathrm{O}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g} \mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}\left({\displaystyle \frac{\mathrm{U}\mathrm{S}\mathrm{\$}}{{\mathrm{m}}^3}}\right)=\mathrm{a}\times \mathrm{e}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{e} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}+\mathrm{b}\times \mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y} \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{m}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}$$

(26)

where a and b are the current market price of the electrode material and the current price of electricity in USD/kWh, respectively. As the COD concentration increases, so does the energy consumption.

Previous studies mentioned that the electrode material can affect the operating cost and energy consumption during the CE process, and it has been reported that the brass electrode showed better performance with respect to Cu, Fe and Al electrodes with lower energy consumption (21.58 kWh/m³) and lower operating costs (1797 USD/m³) [56]. According to another study, an electrode spacing of 1 cm and a current density of 2.2 mA/cm² resulted in the lowest electrical energy consumption (11.0 kWh/m³) and the lowest increase in electrolyte temperature (9.5 °C) [11].

4. Main Operating Parameters

Several parameters influence the efficiency of electrocoagulation and its ability to remove contaminants from wastewater. Adjusting and optimizing these parameters improves the efficiency and effectiveness of the treatment process. The parameters that affect the efficiency of EC are related to the operating conditions, such as electric current, electric potential and treatment time, and to the characteristics of the wastewater, such as pH, alkalinity, suspended solids and conductivity, electrode arrangement and EC reactor geometry (electrode surface area, electrode spacing) [48]. Figure 2 illustrates some of the key parameters that researchers and operators take into account when applying electrocoagulation to treat industrial effluents.

4.1. Influence of pH

In an electrocoagulation (EC) process, the influence of pH as an operating factor plays a crucial role in the removal efficiency because it is closely linked to the formation of coagulants in the form of hydroxide complexes. These complexes are produced at the cathode of the electrocoagulator due to the release of hydrogen gas and the concentration of hydroxyl ions $\left({\mathrm{O}\mathrm{H}}^{-}\right)$ [57]. In highly alkaline conditions using Fe- and Al-type electrodes, they form $\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}$ and $\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_4^{-}$ and obtain a low adsorption performance, while in slightly alkaline or basic conditions, their adsorption capacity increases due to the formation of ions $\mathrm{F}\mathrm{e}{\left(\mathrm{O}\mathrm{H}\right)}_3^{-}$ and $\mathrm{A}\mathrm{l}{\left(\mathrm{O}\mathrm{H}\right)}_3^{-}$ [58]. Therefore, the pollutant removal efficiency is reduced as the pH of the solution increases or decreases from the optimum pH [59], the value of which depends on the type of vinasse to be treated, since they have different physicochemical natures. A recent study using sugar cane vinasse observed an increase in pH during electrolysis in its different experimental conditions, suggesting that this electrocoagulation process is a suitable method for neutralizing the pH of vinasse since it does not require the addition of salts [11].

4.2. Influence of Current Density

It is a parameter that allows for regulating the rate of electron release due to the dissociation of metal ions from the electrodes. It also influences the amount of coagulant at the anode and the release of hydrogen gas $\left({\mathrm{H}}_2\right)$ at the cathode [60]. Likewise, the amount of bubbles plays an important role because it modifies the mass transfer between contaminants and coagulants, regulating the collision speed of coagulated particles and contributing to the formation of flocs [60,61]. On the other hand, an excess of current may allow secondary reactions which would reduce the efficiency of the process by forming a colloidal charge [57]. In that sense, current density is a crucial variable in the CE process that is optimized to achieve desirable treatment performance [15,30].

When comparing the sedimentation curves in the vinasse electrocoagulation process by [11], it was shown that the sludge generated at the end of the process under optimal conditions presented a shorter coalescence phase and a higher sedimentation rate compared to the sludge generated under other experimental conditions. This could be attributed to the lower load applied, although it would imply a longer electrolysis time.

4.3. Influence of Electrolysis Time

The electrolysis time is directly related to the removal of contaminants. As time passes, the efficiency also increases; but beyond the optimum time, the amount of removal does not increase because only a certain amount of available flocs are formed to adsorb a certain amount of contaminants [59]. After completion of the EC, the time provided for the coagulated species to settle is called the retention time [62]. The solution must be kept at rest to allow the coagulated species to settle, resulting in a clear supernatant liquid and sludge (adsorbed contaminant).

4.4. Influence of Electrode Spacing

When the separation between the anode and cathode increases, so does the resistance offered by the electrocoagulation cell. As the distance increases, the space between electrodes is partially filled with gases during electrolysis, which increases its electrical resistance [63]. In addition, the amount of oxidized metal is reduced [64], and thus the removal of contaminants decreases due to the lower formation of metal hydroxides [65]. Some researchers state that the optimum range of electrode spacing varies from 0.5 to 1.5 cm for different wastewaters and another to no less than 10 mm for low cell voltages [64,66,67,68].

4.5. Influence of Electrode Type

The type of electrode material has a major influence on the efficiency of the treatment because it influences the nature of the reaction, pollutant removal mechanisms and the cost-effectiveness of the process [69]. The most common electrodes used are those of Al, Fe and stainless steel; but As, Ag, Ca, Ba, Cr, Cd, Cs, Mg, Fe, Na, Sr, Si, Zn and some others such as SnO₂, PbO₂, graphite, Ni and boron-doped diamond (BDD) are used [70]. On the other hand, the effectiveness of these electrodes varies according to the type of pollutant load to be removed. Moreover, researchers proposed the idea that Fe electrodes could offer higher efficiency compared to Al electrodes, due to their durability, lower cost and higher adsorption capacity for ferrous and hydrated iron hydroxides than aluminum hydroxides [71].

4.6. Influence of Mode of Electrode Connection

The arrangement of the electrodes in the electrocoagulation reactor also influences the pollutant removal capacity and energy consumption [72]. The utmost distinctive connection modes of anodes and cathodes are parallel monopolar electrode connections (MP-P), serial monopolar electrode connections (MP-S) and serial bipolar electrode connections (BP) [66]. Figure 3 depicts the mode of electrode connections.

• Monopolar electrodes with parallel connection (MP-P)

The anodes and cathodes are arranged alternately and in parallel at the same potential; furthermore, the current is divided between all electrodes for the resistance of the individual cells [63,65]. Each unit has equal voltage, and the total current adds up to all the subcurrents, which, so far, is the most cost-effective way with aluminum and iron electrodes in wastewater treatment [72], because it allows for efficient current distribution and is advantageous from a cost perspective. In the MP-P configuration (Figure 3A), the anodes and cathodes are connected in parallel to the power supply.

• Monopolar electrodes with series connection (MP-S)

Internally, the sacrificial electrodes are connected exclusively to each other and are not connected in any way to the two electrodes located at the ends. In this configuration, the current flowing through all the electrodes is constant, but the overall voltage is the sum of all the voltages present in each individual electrolytic cell [60]. The outer electrodes of the MP-S configuration (Figure 3B) are connected to the power supply, but the inner electrodes are interconnected with each other.

• Bipolar electrode with connection (BP)

It consists of two external electrodes that are connected to the energy source. The outer electrodes are a monopolar type, while the inner electrodes are a bipolar type. The bipolar electrodes do not maintain any connection with each other, and each of their sides performs, at the same time, the function of the anode and cathode. This means that the opposite sides of each bipolar electrode have opposite charges. The anodic dissolution occurs on the positive side, while the negative side is prone to cathodic reactions [60]. For the most part, electrode corrosion and sediment accumulation are more pronounced in the case of bipolar connections, which in turn increases the operating costs of the process, while the monopolar connection is the most economical [73]. With the BP electrode configuration (Figure 3C), only the two outermost electrodes are connected to the power supply, and all inner electrodes are not connected at all.

It is not clear which electrical configuration is best because, in addition to the electrical configuration, the removal efficiency is highly dependent on other operating parameters, as well as the nature of the solution and the pollutants [7]. Previous studies have shown that when parallel arrangements are used, significant advantages in terms of energy consumption are achieved [8].

In addition to the electrode connection mode, the electrode orientation also has an influence on the EC efficiency. Although, vertical orientation is used in most applications [12].

4.7. Influence of Temperature

Generally, the electrocoagulation method is carried out at room temperature [67]. The influence of this parameter is directly related to molecular mobility, increasing the number of collisions between particles, favoring the formation of flocs, and a greater elimination of organic load [74]. At high values, the formation of metallic hydroxides increases; however, at even higher temperatures, these coagulants become more soluble, and consequently, the amount of flocs is potentially reduced [74,75].

4.8. Influence of Stirring Speed

The agitation speed contributes to maintaining homogeneous conditions and preventing the formation of concentration gradients in a CD [76]. By increasing this parameter, an increase in contaminant removal efficiency is experienced due to the mobility of the ions generated and the rapid formation of flocs [77]. It also influences the reduction in solution turbidity and COD values [78]. However, it was shown that rapid agitation, in addition to saturating the solution with auxiliary electrolytes, disturbs the connection between the colloids, destabilizing them. The agitation speed favors the efficient transport of the coagulant generated in the electrolytic cell through the entire volume of the reactor. It also homogenizes other device variables, such as pH and temperature [79]. A more intense agitation speed could break the flocs in the reactor and produce smaller flocs, which are difficult to remove from the water [80]. Studies have examined the effect of agitation speed on the EC process. They reported that at a DC of 0.5 mA/cm², the contaminant removal efficiencies were close to 74%, 85% and 69% for 100 rpm, 150 rpm and 250 rpm, respectively [81]. Recent EC studies have reported that agitation significantly affected the turbidity removal rate of sugarcane stillage [11]. Similar results were obtained in previous studies on the EC treatment of other effluents [82].

4.9. Influence of Initial Concentration

The concentration of contaminants varies according to the type of vinasse. If there is a higher number of contaminants, they can react with more ions generated, and a higher efficiency can be achieved. At the same time, it involves higher energy consumption due to the organic load in the solution and proper management of the waste generated. On the other hand, other researchers mention that removal efficiency decreases as the initial concentration of contaminants increases [83].

4.10. Influence of Conductivity

The fluid with low conductivity requires a high potential to overcome the resistance offered by the solution present between two electrodes, as there are fewer dissolved ions available to carry the electrical charges [84]. A vinasse with high conductivity favors electrical conduction to flow more efficiently through the solution facilitating the coagulation and precipitation of contaminants, which means that less current is required to achieve the same contaminant removal efficiency [83]. The current density increases with increasing electrolytic conductivity due to the decrease in ohmic resistance of the water/wastewater. High conductivity also reduces the treatment time required to achieve a certain degree of elimination [85]. Sodium chloride (NaCl) is frequently used to increase the electrolytic conductivity of wastewater in the electrocoagulation process, which allows for a decrease in energy consumption during the process [86]. For very high current densities, chloride anions can also oxidize to active chlorine, such as hypochlorite anions, which can oxidize organic compounds. In one of the studies, it was found that the amount of aluminum generated increases rapidly with increasing NaCl dosage. It was observed that the amount of aluminum generated increases rapidly as the NaCl dose varies from 0 to 15 ppm. However, the amount of aluminum ions (coagulant) produced beyond this concentration becomes almost constant [87].

4.11. Influence of Voltage

Voltage is a necessary factor that determines the amount of electrical energy supplied to the system. An increase in voltage results in a greater availability of energy to stimulate the reaction, which in turn accelerates the coagulation process of the contaminant load [88]. However, an increase in voltage leads to higher energy consumption. It is important to note that, if the voltage is raised too high, it will not only increase the temperature of the system but may also affect the quality of the treated effluent due to the possible occurrence of side reactions. Photovoltaics, which converts solar radiation into electricity, represent a clean and sustainable energy technology. Use can help reduce the carbon footprint, improve sustainability and reduce the operating cost of the electrocoagulation process. Karmankar et al. evaluated the cost of distillery wastewater treatment using solar photovoltaic electrocoagulation and compared it with the conventional electrocoagulation process, concluding that electrical energy consumption can be reduced by using photovoltaics [17]. They also mention that the conventional system required more time and a slightly higher voltage than the photovoltaic system to obtain the same pollutant removal efficiency.

Table 3. Treatment of distillery wastewater by conventional electrocoagulation process.

Type of Wastewater	Initial Parameters	Electrode Specifications	Operating Conditions	Treatment Efficiency	R
Vinasse from distillery wastewater	COD: 100.16 g/L	n = 2; type anode: Fe Dimensions: 20 cm, 3 cm, 3 mm; area ≈ 28.5 cm2	pH: 4.3; 5; 6; current: 2.38; 2.66; 2.98 A:10 V; time: 1 h; 200 RPM	COD: 4.83; 8.59; 13.96% pH: 4.90; 5.85; 7.50	[3]
Distillery vinasse residues	TS: 91.89 g/L TSS: 11.78 g/L; TDS: 80.11 g/L;	n = 2; type: Fe Dimensions: 2, 0.3, 0.03 dm; area ≈ 0.636 dm2; distance: 0.5 dm	Current density: 3.9; 4.72; 5.5 A/dm2; volume: 1 L; pH: 4.4; time: 8 h; voltage: 10.4; 11.8; 15.1 V	TS: 97.5; 90.5; 79.44 g/L TSS: 10.3; 9.11; 2.29 g/L TDS: 87.29; 81.40; 77.15 pH: 7.3; 7.3; 7.5	[89]
Vinasse waste from the bioethanol industry	COD:113.70 g/L	n = 2; type: Fe Dimensions: 2 × 0.3 × 0.03 dm Active area: 0.95 × 0.3 × 0.06 dm; distance: 0.55 dm	Agitation speed: 0, 250, 500 rpm; volume: 1 L; T: 301.65°K; pH: 4.1 Voltage: 12.6; 12.3; 12.4 V	pH: 6.2; 6.7; 7 COD: 67.62%	[35]
Distillery spent wash	COD: 120 g/L	n = 2; type: Al-Al, Fe-Fe, Al-Fe; dimensions: 150 × 32 × 1 in mm; active area: 50 mm × 32 mm; distance: 3 cm	Volume: 300 mL; time: 2 h; pH: 3 Current density: 0.187 A cm2; Agitation speed: 500 rpm	COD: with Al-Al: 73.3% with Al-Fe: 60% whit Fe-Fe: 46.6%	[90]
Distillery effluent	COD: 5.15 g/L BOD: 0.8 g/L Conductivity: 5.9 mS/cm	Type: Al	Time: 93 min pH: 3.5–9.5 Current density: 44.5–225.5 A/m2	COD: 85.1%; color: 79.4% pH: 8; current density: 135 A/m2	[91]
Vinasse residues	COD: 100.16 g/L	Type: Fe Dimensions: 0.2 m × 0.03 m × 0.003 m; distance: 0.055 m	Voltage: 7.5–12.5 V 200 rpm; pH: 6 Temperature: 105–110 °C	COD: 83.17 kg/m3	[88]
Distillery effluent	pH: 4.8 COD: 140 g/L Conductivity: 32 mS/cm	Type: Fe Dimensions: 10 cm × 2.5 cm × 0.5 cm Distance: 2 cm	Agitation speed: 100 RPM; Time 2 h; pH: 3–9 Intensity: 0.5; 1; 1.5; 1.9 A	pH: 4.8; COD: 19.87%, 51.67%; color: 83.75%	[92]
Distillery wastewater effluent	COD: 13.8 g/L	n = 4; type: Al Dimensions: 8 cm × 7 cm; area: 56 cm2 Distance: 2.0 cm	Voltage: 0–30 V; volume: 1.5 dm3 Time: 2 h; I: 0–5 A; D.C: 89.3 A/m2	COD: 93%; pH: 8; color: 87%, light yellow	[93]
Treatment of sugarcane vinasse	Turbidity, 2440 ± 400 NTU; total dissolved solids, 6810 ± 840 mg/L; total suspended solids, 5200 ± 300 mg/L	Aluminum electrodes (125 × 80 × 2 mm) connected in parallel monopolar mode	Electrode spacing (1 cm), current density 6.1 mA cm−2; 430 rpm; time 3 h	98.8% NTU; % total dissolved solids; total suspended solids, 20% and 96%	[11]
Vinasse effluents	COD 46,550 mg/L; turbidity 697 NTU	Anode of aluminum, steel cathode (60 mm × 50 mm × 2 mm)	1 A, pH 7 and time 45 min; constant circulation flow rate of 10 mL s−1	COD removal 80.8%; turbidity removal 73.10%	[94]
Distillery spent wash	COD 112,948.5; 27.2 mS cm−1; pH 4.4; total solid 92,624.25 mg L−1	Electrodes (iron plates) with dimensions of length, width and thickness of 20.3 and 3 mm	Currents (2.5, 3 and 3.5 A) and initial pH (4.4, 5.0 and 7.0); 500 rpm; time 6 h	COD removal (74.9%)	[95]

TS: total solids; TSS: total suspended solids; TDS: total suspended solids; COD: chemical oxygen demand; R: references.

shows previous studies on the treatment of vinasse by conventional electrocoagulation. Previous studies have reported the effect of different electric current densities on the concentration (g/L) of TS, TSS and TDS in stillage. Table 3 shows that, for lower densities, TS increases slightly and decreases at higher densities. However, TSS decreases for all densities and is more pronounced at higher densities, and TDS shows the same trend as TS.

5. Hybrid Methods in Electrocoagulation

Hybrid electrocoagulation methods represent a technology that combines electrocoagulation with other wastewater treatment processes to improve treatment efficiency and effectiveness. These combined approaches can be more efficient at removing a wider range of contaminants and can be better adapted to different types of wastewater and specific treatment conditions. Several researchers have worked on the development of these hybrid electrocoagulation processes as effective distillery wastewater treatment techniques, as illustrated in Figure 4.

• Electrocoagulation–Adsorption

Some researchers indicate that this process has been shown to achieve significant reductions in contamination levels. Color, COD and TOC removals of 96%, 72% and 61%, respectively, have been achieved. In addition, it is important to note that activated carbon has proven to be highly effective in the removal of a wide range of contaminants and dyes in this industry [96]. With this hybrid method, electrocoagulation and adsorption, to reduce pollutants from a refinery effluent containing crucial pollutants such as COD, turbidity and TSS, it was treated with aluminum electrodes and activated carbon, and the result was 82% COD, 96.5% turbidity and 98.64% TSS eliminated in the effluent [97]. On the other hand, activated carbon is important because it helps to improve the recovery efficiency of highly dispersed carbon materials [98]. Activated carbon has a porous structure that allows it to retain organic molecules on its surface. This is due to the presence of chemical groups on its surface that interact with contaminants and retain them [99].

b.

Electrocoagulation–Electroflotation

The synergistic effect produced by electrocoagulation and electro-Fenton creates applied stress, and the operation of the membrane facilitates the demulsification of the emulsified oily wastewater along with the creation of substantial turbulence on the membrane surface through the formation of hydrogen bubbles [100]. The efficacy of this process was investigated under different conditions, and it was found that it could effectively remove fluoride in a hydraulic retention time of only 30 min [101].

c.

Electrocoagulation–Ozone

The ozone-coupled electrocoagulation system involves a direct attack of ${Fe}^{+2}$ with $O_3$ to generate the intermediate ${\left(FeO\right)}^{+2}$. The intermediate species produce ${\left(OH\right)}^{-}$ radicals. Therefore, the ozone-assisted electrocoagulation system can accelerate the removal efficiency of color and COD [102]. Ozone is important because it is a very powerful oxidizing agent that can degrade a wide variety of organic pollutants in water and air and also degrade dyes in an aqueous solution [103]. When ozone is combined with electrocoagulation, there is a synergy between the two processes. In addition, ozone can improve the efficiency of electrocoagulation by enhancing floc formation, improve the flocculation of pollutants, and can also generate hydroxyl radicals ${\left(OH\right)}^{-}$ which are highly reactive and can attack and degrade pollutants [104]. A hybrid electrocoagulation–ozonation design was found to reduce high contaminants such as COD, BOD, chlorides and cyanides. This process had an optimum removal efficiency of 99.8% cyanide ions, 94.7% COD and 95% chlorides [105].

d.

Electrocoagulation–Hydrogen Peroxide

The addition of hydrogen peroxide ${(H}_2{O}_2)$ in the electrocoagulation (EC) treatment enhances the removal of toxic compounds through a mechanism known as electro-Fenton (EF), and in this process, the ${(H}_2{O}_2)$ reacts with the iron ${Fe}^{+2}$ ions generated during EC, forming highly oxidizing hydroxyl radicals, and these hydroxyl radicals generated in the electro-Fenton process have a high oxidation potential and are capable of oxidizing recalcitrant organic compounds [106]. However, electro-Fenton generally has a higher removal efficiency of organic contaminants, such as chemical oxygen demand (COD) [107].

e.

Electrocoagulation–Electrooxidation

The hybrid electrocoagulation–electrooxidation process has been shown to improve the removal of COD, TOC, ${{NH}_4}^{+}$, nitrates and phenols [108]. In the EC-EO system, once the required charge load is reached, the electrocoagulation and oxidation of the organic residues by electrooxidation take place [109]. By applying a current, oxidants such as chlorine, hypochlorite, hydrogen peroxide or activated oxygen are generated on the surface of the anode, which break chemical bonds and oxidize organic molecules to carbon dioxide and water [110]. In the biodiesel production industry, their acidified wastewater contains pollutants such as chlorides, TSS, fat oil and COD, using the optimal hybrid electrocoagulation (aluminum electrode pair)–electrooxidation (Ti/${SnO}_2$ anode) process, and the removal efficiency of COD, TSS and oil and fat was 98.9%, 98.2% and 99.8, and the energy consumption was 37.4 KWh/kg COD removed [111].

f.

Electrocoagulation–Ultrasound–UV

The principle of operation of sound, UV and electrocoagulation is based on the application of energy to remove contaminants from water. Sound is used in the sonication technique to generate high-frequency waves that create bubbles in the water, which help break up particles and remove contaminants, and UV is used in the photocatalysis technique to generate hydroxyl radicals that oxidize contaminants [112].

g.

Electrocoagulation–Electrodialysis

In this integrated process, ED uses ion-selective membranes under an electric field to separate ions [113]. Electrocoagulation is used as a stage prior to electrodialysis, and water treated by electrocoagulation is subjected to electrodialysis, and, in this process, unwanted ions and dissolved salts are separated from the water, which helps to further improve water quality by removing ionic contaminants, and the removal efficiency of organic matter, ammonia, chromium and color is found to be high [114]. CE and DE are promising methods because they are environmentally friendly, do not require chemical reagents and generate less sludge [113]. A recent investigation of the hybrid electrocoagulation–electrodialysis process is used to remove chloride ions, turbidity and suspended solids. Optimal conditions in terms of energy consumption and chloride ion separation were obtained, estimated at 7.7 kWh/m and 83%, respectively [115].

h.

Electrocoagulation–Reverse Osmosis (EC-RO)

The hybrid method of electrocoagulation with reverse osmosis is a combination of two technologies for wastewater treatment. In RO, a pressure is applied that generates the movement of the solution of higher solute concentration through a semi-permeable membrane to obtain a low solute solution [116]. Electrocoagulation is applied as a pretreatment prior to reverse osmosis. Over the years, membrane technologies have been employed for different types of wastewaters; however, the membrane loses its performance over time depending on the type of membrane and the type of wastewater [117], as membrane fouling is generated [118]. In contrast, current studies have revealed the effectiveness of the EC process as a pretreatment before RO, as it reduces the amount of organic content and the complex characterization of the wastewater [119].

Table 4. Treatment of distillery wastewater by hybrid electrocoagulation process.

Method	Initial Parameters	Electrode Specifications	Operating Conditions	Treatment Efficiency	R
Electrocoagulation and Electro-Fenton	Turbidity: 34.90 NTU COD: 4750 mg/L	n = 2 Type: Fe Area ≈ 100 cm2 Distance: 3 mm	Electrocoagulation: pH: 4–5.15; current density: 10; 15; 20 mA/cm2; 180 min	Electrocoagulation: COD: 11.1% and 14.3% (15 y 20 mA/cm2)	[120]
			Electro-Fenton: pH: 4–5.15; current density: 30; 40; 50 mA/cm2; 180 min	Electro-Fenton: COD: 92.6%, 88.7%
Electrocoagulation with and without activated carbon of Areca catechu nuts	COD: 18,868.8 mg/L BOD: 11,653.2 mg/L	Monopolar electrodes Type: Fe, Al Dimensions: 104 mm, 25 mm, 6 mm Distance: 28 mm	T: 30 °C; time: 60 min; voltage: 30 V; intensity: 2 A; current density: 182 A/m2 slow agitation	No activated carbon, pH = 5.96 Color: colorless; turbidity: 3.85 NTU; COD: 4618.8 mg/L; BOD: 3565 mg/L With activated carbon, pH = 6.41; Color: colorless; turbidity: 2.7 NTU; COD: 3754.1; BOD: 1199.7 mg/L	[121]
Ozone-assisted electrocoagulation	COD: 80,000–90,000 mg/L BOD: 7000–8000 mg/L	Type: iron Dimensions: 9 cm × 5 cm Area: 45 cm2 Electrode Distance: 1 cm	pH: 2–10; current density: 3 A/dm2; T: 30 °C; O3 generator: 2 g/h; stirring speed: 10,000 rpm; stirring time: 15 min	COD: 83% Color: 100%	[102]
Electrocoagulation with advanced oxidation processes	COD: 8500 mg/L BOD: 3000 mg/L	Type: iron Distance: 2 cm Dimensions: 10 cm × 10 cm × 0.1 cm	pH: 2–10; current density: 0.10–0.20 A/dm2; reaction time: 240 min; O3 generator: 2 g/h	COD: 94% BOD: 92% Color: 100%	[122]
Combination of sound (US), photography process (UV) and electrocoagulation (EC)	BOD: 7000–8000 mg/L COD: 80,000–90,000 mg/L	Type: Fe and Al Dimensions: 0.1 cm × 10 cm × 15 cm Direct current	UV + EC process: Current density: 0.75 A/dm2 Distance: 0.75 cm; pH: 7 US + UV+EC process: Current density: 0.07 + 0.2 A Time: 4 h; Fuente UV: 8–32 W	US + UV + EC process COD: 95.63% Color: 100%	[112]
Electrocoagulation powered by photovoltaic cells	COD:4252 mg/L BOD: 918 mg/L Conductivity: 5.9 mS/cm	Type: Fe and Al Distance: 2 cm Dimension: 50 mm × 60 mm	Reaction time: 60 min Current density: 24.9 mA/cm2	COD: 94.9% Color: 81.3% pH: 7	[17]
Reverse osmosis–electrocoagulation	ph:7.8–8 COD:15,000–16,000 mg/L	Type: stainless steel Area ≈ 0.636 dm2 Distance: 2.2 cm	Current density: 154.32 A/m2 A/dm2 pH: 7.8; time: 135 min	COD: 98% Color: 99.2% TDS: 98.5%	[119]

shows different hybrid electrocoagulation processes in the treatment of distillery wastewater. It has been observed that in most of the coupled processes, the removal corresponding to the chemical oxygen demand is higher than 90%. Likewise, it is also observed that the color removal is close to 100%. This review study evaluates the efficiency of hybrid electrocoagulation processes in relation to conventional electrocoagulation, taking into account the effect of various operating parameters such as current density, electrolysis time, pH, voltage and electrode species. In addition, the contributions of these studied methods will be highlighted to obtain a clearer picture and select the best stillage treatment technology.

6. Conclusions

Electrocoagulation is a versatile, economical and environmentally friendly process that can be used for the treatment of various industrial wastewaters. In order to know the current state of research on CE, an extensive bibliographic search was carried out through the Scopus database. The scientific literature review on the electrocoagulation process confirms that it can be used to reduce chemical oxygen demand, color, turbidity and total dissolved solids in wastewater. Vinasse is a liquid effluent from the distillation process of the sugar and alcohol industry and is a major problem for the sector due to the large quantities produced and its potential effects as an environmental pollutant. The vinasse is mostly discharged into water bodies and soil, resulting in soils and water with a high organic load, acidity and salinity. If this problem is not effectively addressed, it will have negative effects on both crop fields and water bodies. Much research has been conducted on the characterization and treatment processes of stillage. This paper presents the fundamental mechanisms and the various factors that influence the effectiveness of electrocoagulation and its ability to remove pollutants from wastewater, particularly stillage. This study also highlights the latest advances in processes coupled with electrocoagulation for the treatment of stillage.

7. Future Prospective

Electrocoagulation systems have attracted much interest in recent years due to their great versatility, simple handling, low environmental impact, less sludge production and high efficiency in water and wastewater treatment.

Like other treatment techniques, EC also has some disadvantages such as electrode passivation and higher power consumption mainly due to the lower conductivity of some wastewaters. In order to achieve more efficient and economical treatment, other novel EC coupling systems should be investigated to intensify the treatment process, especially for industrial wastewater. For example, the coupling of processes such as EC–electrooxidation, EC–ultrasound, EC–ozone, EC–adsorption, EC–nanofiltration, EC–advanced oxidation processes and EC–electrodialysis for the removal of chemical oxygen demand, turbidity, color and total suspended and dissolved solids could be studied.

EC technology is a very promising technology for the treatment of different types of wastewater. However, the management of process sludge is still a problem. Therefore, urgent research is needed to convert the EC sludge into a valuable resource in order to operate the process sustainably.

The cost of the electrical energy consumed is one of the main drawbacks of applying this technology on an industrial scale. Therefore, it is necessary to develop techniques to apply CE with alternative renewable energy resources, such as solar cells and wind energy as well as hydrogen gas recovery from the CE process, to replace electrical energy which can reduce the operating cost and allow for a sustainable application of the process.

According to research, the sludge formed contains large amounts of metal ions, such as iron and aluminum, as well as other persistent contaminants. Sludge treatment involves enormous challenges.

For the construction of new electrocoagulation systems, studies should focus both on the design of reactors and electrodes and on the development of new energy sources.

Finally, much of the current research is conducted at a laboratory scale in Bach mode. Future research on electrocoagulation technology using continuous and large-scale operations is of utmost importance.