Persistent organic pollutants (POPs) and emerging pollutants (EPs) in water are a serious threat to human health and ecological balance since many of them are toxic, carcinogenic and mutagenic [1,2]. In addition, these pollutants are highly recalcitrant and difficult to remove through Biological Oxidation Processes (BOPs). Ttherefore, conventional wastewater treatment plants (WWTPs) are recognized as the main sources of POPs and EPs in the environment [3–5].

In response, several advanced oxidation processes (AOPs) have been introduced, such as Electro-oxidation (EO), UV, ozonation (O₃), Fenton, UV/H₂O₂, UV/O₃, UV/H₂O₂/O₃, and UV/TiO₂ [6,7].

Of the above, EO is the most studied process due to its several advantages such as: environmental compatibility, versatility, energy efficacy, amenability to automation, and high efficacy in destroying POPs and EPs in water, wastewater and sludge [2][8]. EO has been studied along with several effluents, most notably: tannery [9], electrical industry [10], coke production [11], dyes [12] and hospitals [13]. However, the EO process has been criticized for: (1) its high operating costs related to electricity consumption, (2) its potential to form by-products more toxic than the originals, (3) some effluents may require the addition of supporting electrolytes which elevate operating costs and (4) short life of electrodes, due to deposits of organic material on their surface (passivation/ fouling) [14,15]. These disadvantages can be reduced if EO is combined with BOPs, which are cheaper to operate and work well with biodegradable organic matter, nutrients and solids (components that interfere with EO performance). In this sense, EO can be combined with BOPs as pre-treatment or post-treatment [2]. Studies from the 1980s already suggested this combination in order to make EO affordable [16]. For instance, Wang et al. [11] reduced the cost of water treatment from coke production from 116 to 64 kWh/kg COD, when coupling a biofilter to an EO process. However, decision making criteria on where to locate EO in a coupled system is not entirely clear.

This review aims at understanding the potential benefits and drawbacks of different systems combining EO and BOPs, either conventional (e.g. activated sludge) or novel technologies (e.g. organic bed biofiltration); and how they affect the formation of by-products. Additionally, it takes a look at the advances in the scale-up process, its challenges and recommendations for future research.

EO is an alternative to a conventional wastewater treatment to remove the most recalcitrant organic pollutants from complex effluents or to transform them into biodegradable compounds by breaking or fragmenting them until their mineralization or conversion into more easily biodegradable by-products [6,37]. This process occurs in an electrochemical reactor called electrolytic cell, which is composed by a tank, an electrolyte (that could be the same waste-water to be treated), a pair of electrodes, an anode (where the pollutants are expected to be oxidized), a cathode (where commonly heavy metals can be reduced but also organic matter) and a power source (Fig. 2). This process relies on oxidation-reduction reactions that occur in the electrolytic cell to depurate waste effluents [2]. Pollutants can be removed in an EO process following direct and/or indirect oxidation.

Direct oxidation takes place in two stages: (1) the diffusion of the pollutants from the solution to the anode surface and (2) the oxidation of the pollutants on the surface of the anode by direct electron transfer (Fig. 3). Hydroxyl radicals (•OH) are formed by the anodic oxidation of water and physiosorbed at the electrode’s surface [6,38].

Indirect oxidation occurs when a mediator (HClO, HBrO, H₂O₂, H₂S₂O₈) is electrogenerated to carry out the oxidation of pollutants [6]. By-products are formed due to the presence of chloride in wastewater which is oxidized into chlorine (Cl₂) at the anode. Then chlorine reacts with water to form hypochlorous acid (HClO). This compound can react with organic matter and amines to produce trihalomethanes (THMs) and chloramines, respectively (Fig. 3). The resulting synergy could be highly effective in the degradation of many pollutants. Nevertheless, it has the drawback of toxicity (discussed later) [38].

Side reactions could also occur when the current density applied to the EO process is high enough to overcome the oxidation potential of the anode. In this case, oxygen will be produced instead of •OH, thus decreasing the efficacy of the oxidation process. To avoid this, “non-active” electrodes (most commonly BDD and PbO₂) are required to generate the hydroxyl radicals (called “active oxygen” physiosorbed), which in turn assist with the non-selective oxidation of organic compounds, resulting in their total oxidation to CO₂ [38,39]. The electrodes considered “Active” such as Pt, IrO₂, RuO₂, SnO₂ and SbO₂ show low levels of mineralization attributed mainly to a highly active behavior, i.e., low production of •OH [14,40,41].

Due to the simplicity of EO, its application has been evaluated in all kinds of industrial effluents such as: tannery [9], electrical industry [10], coke production [42], dyes [12], landfill leachate [39], olive pomace leachate [43]; and also for other purposes, e.g. disinfection of urban stormwater and secondary effluents [44–46], removal of EPs from hospital effluents [13], among others.

The EO’s main advantage, over other conventional wastewater treatment processes is the conversion of organic compounds into simpler ones and, in theory, up to a final mineralization to CO₂ and H₂O, which would make the addition of a following treatment module unnecessary. In addition, EO (or any other AOPs) results in the only alternative when the BOPs are not able to deliver the desired quality effluent, e.g., in the treatment of leachates [39]. However, since industrial and/or domestic wastewater effluents are often complex matrices that require long retention times and therefore high energy consumption, in the EO process, in order to reach mineralization levels, its widespread use in real scenarios is greatly restricted [2]. It has been concluded then that if this technology is not complemented with a renewable energy source, it is unlikely to be applied as a sole treatment [41]. Nevertheless, there is an alternative: the coupling of EO with BOPs (Table 3) with the main objective of combining the biological degradation of the organic matter with the potential of the EO to oxidize POPs and to degrade organic matter hardly biodegradable until its mineralization in relatively short times [14] (Fig. 4). As already mentioned in the introduction, this option was proposed since the 80s, but now this line of research has gained interest because the regulatory guidelines are becoming stricter; the pressure on water resources is increasing and the vigilance of the authorities to enforce discharge limits is growing. The following section examines and discusses the advantages and disadvantages of coupling an EO process with a biological treatment, both as a pre-treatment and post-treatment, as well as the role of the oxidizing species such as those derived from chlorine.

The background of the EO in terms of wastewater treatment is sufficient to demonstrate the viability of the technology to remove POPs, EPs, organic matter, nutrients and pathogens from all types of waste effluents. Therefore, the logical step to follow is to apply this technology on a larger scale (pilot or full-scale). This section will examine the progress in scaling up EO processes. The methodology details and the problems encountered during the implementation process (for large scale application) will also be presented.

In order to scale conventional biological processes (activated sludge, trickling filters, wetlands, etc.) from results at the laboratory level, it is very important to preserve the dimensional relationships of the corresponding reactor. However, in the case of electrochemical reactors, this criterion usually cannot be met, since the increase in the interelectrode space would result in a high voltage drop and, therefore, an increase in energy costs. Thus it is always better to reduce this distance as much as possible within a certain limit [89,90].

The safest route for the scaling of electrochemical reactors is through the use of multiple laboratoryscale electrochemical cells [91]. According to Anglada et al. [39] and Urtiaga et al. [51] the degradation constant of the pollutant will be similar regardless of the number of cells that are operated simultaneously, while the hydrodynamic conditions are maintained. Thus, the model proposed by Comninellis and Chen. [38] represented in equations 7 and 8 can be applied.

Where j_lim is the limiting current density (A/m²), A (m²) is the area of the anode, V (m³) is the volume of treated wastewater, k_m is the mass transport coefficient in the electrochemical reactor (m/s), COD is the chemical oxygen demand at a given time and F is the Faraday constant (C/mol).

From a series of degradation tests and calculating the mass transport coefficient (k_m), Anglada et al. [39] scaled up a process from laboratory test up 150 times, that is, with a total BDD anode area of 1.05 m² and using several individual cells (area per cell 70 cm²), the results obtained for a single cell satisfactorily described what happened at the pilot level, for the removal of both organic matter and ammonia nitrogen. Since the EO is a non-selective process, the kinetic models apply to any type of molecule that adds COD to the system [92]. In this sense Urtiaga et al. [51] carried out a series of degradation kinetics at laboratory scale using different operating conditions (initial COD 1500 – 3000 mg/L, current density 300, 600 and 1200 A/m²) in order to scale up; COD degradation levels reached in the pilot scale validated the procedure in which the A/V ratio was preserved.

There is a shared opinion among some authors that the safest route for scaling is multiplying the number of cells tested at laboratory scale. This is challenged by others who say that it is likely that the scaling can be performed only by maintaining hydrodynamics of the cell studied at the laboratory level and maintaining the A/V ratio. For instance, Zhu et al. [48] scaled a 24 cm² BDD anode 121 times (2904 cm²) for the removal of phenols. The dimensional relations were maintained in the configuration of the electrode and in the working volume. However, the interelectrode distance and the operating variables (current densities, retention times, conductivity and initial COD) were the same in both scales. It should be noted that the analysis of the kinetic behavior was not performed and only the hydrodynamics and the A/V ratio were maintained. After the scaling up, the results were very similar to those obtained at laboratory scale according to the response surface methodology in which a relatively small standard deviation of 0.2 to 12% was found.

Another way to approach scaling is the one presented by Abou et al. [93], who used a single cell with an effective volume of 19.2 L that contains 49 graphite electrodes with a total area of 0.0126 m², maintaining a distance of 2 cm for each pair of electrodes (Fig. 7). In this way, a large number of electrodes can be installed until the desired A / V ratio is reached. This pilot prototype was evaluated to remove phenols from wastewater from the oil industry, obtaining > 99% efficiency in phenols and 40 to 60% removal of COD.

More recently, Monteil et al. [94] developed a novel pilot-scale reactor to operate in continuous mode. This reactor consists on a cell with a useful volume of 22.5 L in the form of a channel that works with 14 and 28 pairs of electrodes (BDD) (Fig 8). The authors evaluated the effect of the flow (20.4 to 170 mL/min) on the mineralization of the drug hydrochlorothiazide in the solution. The results showed that the flow rate was the most critical parameter: the lower the flow rate, the higher the mineralization percentage. The number of electrodes in both configurations allowed a high mineralization efficiency (> 90%).

The scaling of EO, although successful, it is currently limited to a pilot scale; the main constraint being that, in order to treat a relatively large effluent, several hundreds of small electrolytic cells are required. The technological development should be focused on the creation of large electrodes (such as those used by Zhu et al. [48]) that result in the use of a small number of electrolytic cells to treat high volumes or be focused on the creation of large electrolytic cells with many electrodes (such as those used by [93,94]). On the other hand, BDD electrodes are considered to deliver the best performance in waste-water treatment. However, it is very expensive when compared to DSA^®, carbon or graphite electrodes [92,95]. Therefore, a larger research is needed using these electrodes at a pilot scale.

• Taking into account that DBPs are dangerous substances, all studies related to EO should closely monitor their formation. It takes place while there are low concentrations of Cl⁻ such as 119–144 mg/L, or as high as 4500 mg/L [37,45] and the applied current density is high enough to favor the evolution of Cl⁻ to DBPs.

• There are still insufficient studies that rigorously monitor the formation of DBPs in the final discharge, as well as the optimal COD/Cl⁻ ratios to avoid an increase in the toxicity of effluents treated by EO. There is a limited number of proposals to solve this problem which, along with the high costs, is a great limitation for scaling up.

• There are scarce studies in passivation/fouling focused on long retention times and cleaning/reactivation methods. Efforts are needed in this direction to investigate the feasibility of this technology on a pilot / industrial scale.

• The future developments to scale up the EO process should be aimed at manufacturing cheaper and larger electrodes, in order to decrease the number of electrolytic cells used, thus increasing the treatment capacity to full scale.

EO has proven to be an efficient process to remove recalcitrant pollutants. In addition, its implementation is relatively easy and versatile to treat different effluents. However, in general, it is expensive due to two factors: 1) Long reaction times are required, sometimes hours if the mineralization of POPs and EPs is to be achieved, and 2) the manufacture of some electrodes is expensive.

On the other hand, the unavoidable generation of toxic disinfection and degradation by-products complicates the reuse and / or discharge of an EO-treated effluent. Therefore, various studies recommend that EO be used in combination with a biological process. EO as a pre-treatment is generally applied to increase the biodegradability of an industrial effluent. However, the operation is complicated because: 1) Sufficient substrate must be maintained in the effluent for the subsequent biological process and 2) Generally, there is little control regarding the generation of toxic by-products of disinfection and degradation, which affect the microorganisms of the subsequent biological system. In this sense, the most promising configuration of this coupling seems to be EO as a post-treatment, since solids, organic matter and nutrients are eliminated in the biological stage, thus concentrating the generation of OH radicals from EO to mineralize the more persistent pollutants. By applying EO as a post-treatment, the generation of disinfection by-products such as THMs could also decrease by limiting their precursors such as organic matter or nutrients. An additional benefit is to decrease the formation of a polymeric layer on the surface of the electrodes and therefore their fouling.

So far, the number of studies carried out with EO on an industrial / real scale is limited, although the results obtained on a laboratory scale on a pilot / industrial scale have been successfully replicated. This has been achieved by maintaining the hydrodynamics of the electro-oxidation cell (same A / V ratio) either by using many laboratory-scale cells connected in parallel or by developing larger electrooxidation cells with a pair of electrodes that are also larger or by placing many laboratory-scale electrodes distributed in larger electrolyte cells.