EO is a disinfection technique employed in treating water electrochemically [31] [55] . For small potable water setups, EO has many benefits over conventional chlorine disinfection, including ease of use, environmental protection, and cost-effectiveness [27] [31] [55] . Throughout EO, the electrolysis phenomenon could reduce numerous biological and chemical pollutants through direct oxidation (DO) and indirect oxidation (IO) methods [27] [38] [55] . Also, EO significantly reduces the generation of DBPs because it generally produces lower levels of free chlorine than traditional disinfection techniques [1] [38] .

1) Electrooxidation (EO) pathways

In reducing waterborne pollutants by EO, two significant routes have a crucial contribution: DO and IO [25] [77] . DO happens when water is absorbed on the anode surface and is oxidized to generate ^•OH [77] , which will directly oxidize contaminants in contact with the electrode surface [25] [31] [78] . IO happens after water electrolysis at the anode and cathode [1] , as defined by Equations (2) and (3), respectively:

2H₂O → O₂ + 4H⁺ + 4e⁻ (2)

2H₂O + 2e⁻ → H₂ + 2OH⁻ (3)

EO has a well-known secondary electrolysis phenomenon: the oxidation of Cl⁻ to generate free chlorine, a famous killing agent [27] [55] . Free chlorine generation (hypochlorous acid or hypochlorite ion, following pH) constitutes a similar killing mechanism to traditional method [1] [31] . Additional oxidants (e.g., O₃ or H₂O₂ [79] ) could also be produced [37] [55] . However, concentrations of these oxidants remain lower, and they are not considered as crucial contributors to disinfection relative to free chlorine and ^•OH [1] [37] .

Producing ^•OH presents an additional indirect pollutant reduction pathway for EO, particularly in water without chloride [1] [25] [27] [28] [69] . Hydroxyl radicals break down efficaciously NOM and pathogens [27] [28] [78] .

A supplementary likely IO method is Fenton’s reaction, which is oxidizing Fe²⁺ with H₂O₂ at an acidic pH (<4), leading to the hydroxyl radical’s generation [36] [80] [81] [82] . However, the added metallic salt makes such a reaction unwanted [25] [55] [78] .

2) Electrooxidation (EO) anode

The nature of metals or coated metals utilized as electrodes is crucial to the EO efficacy [28] [55] [77] . For EO, MMOs and BDD are the most frequent electrode kinds employed [1] [25] [83] . MMO electrodes are typically titanium (T_i)-based electrodes and can be better categorized following the metal coating on the electrode surface, such as iridium oxide (IrO₂) or ruthenium oxide (RuO₂) [28] [84] . Adding metal coating elevates the MMO electrode performance by augmenting its surface area, thus enhancing electrolysis yield and reaction kinetics [78] . The anode material dictates the critical route of the EO process [55] [77] [78] . MMO electrodes are dictated mainly by producing free chlorine, besides the coating affecting the chlorine formation rate [28] . On the other hand, BDD electrodes possibly generate ROSs [25] [85] .

3) Electrooxidation (EO) for reducing E. coli

Demobilizing E. coli using EO could be attained by either MMO [28] [38] [55] [69] or BDD [28] [37] [55] anodes. Demobilizing E. coli is mainly affected by CD, water properties, and electrode type [1] . Higher CDs are related to elevated rates of E. coli inactivation [27] [28] [37] [69] . Scientists [38] utilized MMO platinum (Pt) electrodes to determine the effect of specific electrolytes on demobilizing E. coli inactivation. They proved that Cl⁻ has a leading role to inactivation. Most waters carry chloride, and several researchers affirmed that the MMO-Ir/O₂ electrode has the most significant Cl₂ production rate and called it the most performant electrode for killing microorganisms using Cl₂ [28] [86] .

4) Electrooxidation (EO) restriction

EO produces DBPs via oxidation reactions with NOM present water [25] [69] [83] [85] . Retaining NOM by EO needs long application periods or high DCs to reach elimination [1] [83] . MMO and BDD electrodes form DBPs throughout EO treatment [69] [85] . Importantly, through the disinfection of E. coli using MMO electrodes, DBPs were formed, namely total trihalomethanes (TTHMs). Lower CDs produced lower levels of TTHMs, as anticipated, since less free chlorine was formed. Scientists [85] observed 30 mg/L chlorate formation following a 10-min residence period employing a BDD anode. Producing DBPs via EO requires some pretreatment to diminish NOM in the water before EO. As anticipated, lower CD results in lower TTHM values because less Cl₂ is formed. Scientists [85] observed the formation of 30 mg/L of chlorate after a 10-minute residence period at the device employing BDD anode. Producing DBPs via EO requires some pretreatment to reduce NOM in the water prior EO.

Identical to CC, EC has several merits over CC [68] . Chemicals (e.g., Al₂(SO₄)₃・18H₂O and FeCl₃) provoke many troubles in treatment industry [1] . CC could reduce pH and consume alkalinity, leading to more chemical injections to reestablish water neutrality before distribution [87] ; however, EC does not consume alkalinity [63] [88] .

CC has additional trouble dealing with sludge, and EC usually has lower sludge creation [63] [68] [89] [90] . Relative to CC, EC (Figure 1) is also more performant in removing smaller particles thanks to electrophoretic mobility [68] [76] [89] [91] [92] . Such small particles can include dissolved compounds (like hydrophilic acids, a part of NOM, Figure 2) that are not easy to retain by CC [1] [63] [93] . Eliminating hydrophilic acid could happen if pH is acidic (Figure 3), promoting charge neutralization (CN) and precipitation [3] [94] [95] [96] [97] . Further, EC may form H_2(g) at the cathode, inducing an EF phenomenon by pushing flocs to rise to the surface [14] [14] [68] [89] .

A decade ago, we suggested the hydrophilic/hydrophobic ratio as a function of removing the hydrophilic and hydrophobic content by the coagulation process [93] . It is well-established that the coagulation process could reduce dissolved organic carbon by around 30% - 60% by increasing the coagulant dose and optimizing reaction pH, in which large organic molecules with hydrophobic properties were removed preferentially. Furthermore, the literature affirmed that the more excellent removal of UV-absorbing substances indicates that alum coagulation preferentially removed the hydrophobic fraction of the total organic carbon. The hydrophobic fraction needs to be removed entirely without transforming it into hydrophilic fractions by a coagulation process avoiding pre-chlorination/pre-oxidation due to the risk of fragmentation of organic molecules [7] . Determining the exact numerical values of the hydrophilic/hydrophobic ratio for raw water and treated water at different stages of the treatment processes in a water treatment plant would help more focusing on OM control and removal.

Ten years ago, we suggested that the hydrophilic/hydrophobic ratio is a function of the removal of hydrophilic and hydrophobic parts during coagulation [93] . It is known that by augmenting the coagulant injection and optimizing the solution pH, the coagulation method can reduce dissolved OM by about 30-60%, mainly retaining large hydrophobic organic substances. Also, better elimination of UV-absorbing species has been demonstrated showing that Al coagulation mostly retains the hydrophobic portion of OM. The hydrophobic part must be completely eliminated without transforming it into the hydrophilic part through the coagulation technique, averting pre-chlorination stage because of the possible fragmentation of organic substances [7] . Assessing accurate levels for the hydrophilic/hydrophobic ratio of raw and treated water at various steps of the treatment in water treatment factories will help place more significant emphasis on the control and removal of OM.

1) Electrocoagulation (EC) pathways

During the EC process (same as CC), particles are eliminated through physicochemical pathways [1] . The main difference is the in-situ liberation of coagulating metal cations during the EC application [25] [76] . The injected concentration, C (g/L) (Equation (4)), can be calculated according to Faraday’s law (Equation (5)), which determines the dose of metal coagulant ions ([Al³⁺/Fe²⁺]) liberated into the device, according to a certain CD and time [1] [89] :

C = m V (4)

m = I × t × M w z × F (5)

where: m is the amount of dissociated metal (g), I is the electric current (A), t is the residence period (s), M_w is the molecular weight of the metal, z is the number of electrons (for Fe²⁺, z = 2), F is Faraday’s constant (96,485 C/mol), and V is the volume of the treated water (L).

Figure 4 presents numerous Fe and Al species, which could be present depending on the pH and metal level. Identical to CC, the M⁺ ions interact with OH⁻ to generate numerous polymeric hydroxide complexes [25] [65] [68] [98] . Based on the pH, the polymeric hydroxides interact with negatively charged particles (such as NOM and E. coli) via CN [41] [76] [89] [94] . In addition, metal hydroxide precipitate could be instantaneously generated, causing the aggregation of smaller particles via differential settling flocculation [19] [41] . As the pH decreases, CN will have a more considerable role, especially at lower coagulant dosages [41] .

The cathode generates H_2(g) and OH⁻ simultaneously, simultaneously auguring pH during EC [25] [76] [89] . The resulting precipitates will be retained via EF or additional flocculation and sedimentation [11] [25] [89] . Flocculation pathways such as diffusion, advection, or differential settling let EC retain diverse particles from water, comprising pathogens, NOM, and inorganics [17] [18] [19] [65] [67] [99] [100] .

During the EC process, the cathode produces H_2(g) and OH⁻, simultaneously auguring the pH [25] [76] [89] . The resulting precipitate is retained by EF or additional flocculation and sedimentation [11] [25] [89] . Flocculation mechanisms like diffusion, advection, or differential sedimentation enable EC to retain a variety of particles in the water, including pathogens, NOM, and minerals [17] [18] [19] [65] [67] [99] [100] .

2) Electrocoagulation (EC) for removing Natural Organic Matter (NOM)

Researchers [83] discussed the literature on removing NOM (a DBP precursor) using EC, noticing that in many investigations, the NOM removal was more significant than 70% from synthetic solutions and actual water. As a rule,

NOM reductions are identical, even if moderately less employing EC than CC [11] . CDs equal to or less than 10 mA/cm² depicted the more significant elimination of NOM, with more extensive CDs illustrating no more amelioration [101] . Scientists [95] noticed that the electrical charge (applied current × reaction time normalized to the reactor’s volume (C/L)) and the pH were the pivotal variables for eliminating organic substances [1] .

In addition, temperature moderately influenced NOM reduction efficacy, possibly because of the dependence on the production rate of the metal coagulant dose [1] [95] . A crucial parameter needing more estimation is the metal ions speciation in solution, particularly in iron EC [102] . Researchers [22] [103] proved that unique iron species formed throughout EC could retain NOM at varying levels. Fe²⁺ or Fe³⁺ can generate complexes with NOM prior producing metal hydroxides, reducing ideal floc creation [11] [71] [104] .

3) Electrocoagulation (EC) for removing E. coli

Eliminating E. coli using EC could equal or surpass achievements reached in CC with decantation [1] [19] [105] . Scientists [17] discovered that augmenting CD results in a more critical reduction of E. coli, similar to the phenomenon depicted for NOM retention (Figure 5). Electric current (Faraday’s law, Equation (4) is responsible for the level of coagulant produced in EC device, and more significant coagulation formation could generate more flocs [106] for the physical elimination phenomenon (Figure 6) [17] [18] [107] . Increasing the current charge rate by adjusting the application time could also affect the dosage, even at fixed CD, improving the killing of E. coli [108] [109] [110] . Researchers [19] demonstrated enhanced killing of E. coli in synthetic groundwater as the EC iron injection increased. Scientists [18] noted that 2.84?logs of E. coli bacteria were killed utilizing EC run at 2.0 A during 10 min (~110 mg/L Al) in contaminated natural river water, even if a more critical injection could affect the EC performance via forming more considerable sludge mass and wearing the electrodes [25] .

The main elimination pathway of E. coli reduction throughout EC is adsorption via flocculation [111] - [117] . Secondary practicable pathways involve

direct harm to the cell or proteins by passage via the electrical field [18] [91] [92] [118] or reaction with oxidants generated through electrolysis [1] [19] [119] [120] .

Regarding demobilizing E. coli, EO and EC have merits and disadvantages. Deactivating E. coli using EO is exceptionally efficient, even if DBPs may be formed due to reactions between NOM and oxidants [25] [69] [121] [122] . As seen above, EC has been depicted to remove NOM at rates equal to CC and could reduce some E. coli [19] [123] . Consequently, taking advantage of the merits of both techniques in successive applications, identically to that of a traditional multi-barrier treatment factory, could lead to satisfying E. coli killing and suitable NOM retention to meet water quality requirements [1] [39] .

Scientists [39] implemented EC-EO in industrial wastewater, facilitating the degradation of pollutants such as chemical oxygen demand, color, turbidity, and coliforms. They attained 99% elimination of such pollutants, reducing the residence time from 21 hours employing EO alone to 2 hours using EC-EO. Integrating EC with electro-Fenton [80] [124] , which oxidizes Fe²⁺ to form ^•OH, has also depicted efficiency. Scientists [36] established that combining Fe EC and electro-Fenton with BDD electrodes plus air diffusion in domestic wastewater can eliminate numerous microorganisms at a neutral pH. They noted that successive implementation of EC and electro-Fenton (CDs of 20 and 33 mA/cm², respectively) for 30 min led to a more significant reduction than either method alone. Several authors utilized an integrated electrochemical cell, in which EC and EO processes occur in the identical recipient. An integrated electrochemical setup can considerably reduce E. coli concentrations in domestic wastewater, even at CDs below 2 mA/cm² [1] .

Although combined electrochemical techniques have proved efficient E. coli reduction in industrial and domestic wastewater, the use of successive EC-EO to demobilize E. coli in drinking water sources was not noted until the Lynn work (Figure 7) [1] . Lynn [1] focused on assessing the efficacy of a successive EC-EO in reducing E. coli in variable-quality drinking water.

Lynn [1] estimated continuous EC-EO for reducing E. coli in four model drinking waters and demonstrated that EC was not an efficacious pretreatment technique for EO to remove E. coli in groundwaters. Injecting Fe using EC possibly limited ameliorations in groundwater due to the depletion of oxidants and high total remaining Fe concentrations following filtration. Alternately, EO single was sufficient for decreasing E. coli in groundwaters, assuring 4-log and 5-log decrease in the model shallow and model deep aquifer, respectively, utilizing CDs less than those requested for EC. The energy efficiency per order (EEO) of E. coli removal for EO single was less than that of either EC or the continuous procedure [1] .

EC retained 64% of NOM from the model river water, which is considerable because NOM is a DBP precursor. Nonetheless, the retention of NOM by EC did not enhance E. coli removal by EO. Potential reasons for the absence of increased E. coli decrease could be a high final pH, depletion of free chlorine by Fe, or insufficiently implemented EO CD [1] .

More studies utilizing improved EC-EO with pH regulation to deal with surface waters depicted that a lower pH for EC-EO moderately ameliorates E. coli removal. Both surface waters noticed moderate enhancements in retaining NOM at pH 6 or below. Still, the remaining NOM would scavenge oxidants, thus limiting ameliorations in reducing E. coli using EC-EO. At the same time, as additional regulation of NOM removal remains requested, the EEO for E. coli removal using each process enhanced when pH was below 6, proving the interest in pH regulation [1] .

Decreasing pH increased EC’s residual Fe concentrations, which would also deplete free chlorine. However, the residual Fe concentrations following EC-EO decreased, possibly due to the oxidation of Fe(OH)_2(s) to Fe(OH)_3(s), offering more efficacious floc settling. Oxidizing Fe²⁺ could also induce Fenton’s reaction. A moderate correlation was depicted between EC’s remaining Fe levels and E. coli reduction throughout EO (after EC step). Besides, Fe²⁺ was transformed to Fe³⁺ throughout EO, establishing that Fenton-like reactions may take place, leading to improved E. coli demobilization. When decreased initial pH enhanced E. coli demobilization for EO after EC, the difference was unimportant for any case, showing that the EO CD was too low to form the oxidants requested for increased E. coli demobilization [1] .