1 Industrial bioelectrochemistry for waste valorization: state of the art and 2 challenges 3 4 Diego Maureiraa, Oscar Romerob**, Andrés Illanesa, Lorena Wilsona, Carminna Ottonea* 5 a 6 School of Biochemical Engineering, Pontificia Universidad Católica de Valparaíso, Avenida Brasil 2085, 7 Valparaíso, Chile 8 b 9 Bioprocess Engineering and Applied Biocatalysis Group, Departament of Chemical, Biological and 10 Enviromental Engineering, Universitat Autònoma de Barcelona, 08193 11 12 * Corresponding author: carminna.ottone@pucv.cl 13 ** Corresponding author: oscarenrique.romero@uab.cat 14 15 16 Abstract 17 Bioelectrochemistry has gained importance in recent years for some of its applications on waste 18 valorization, such as wastewater treatment and carbon dioxide conversion, among others. 19 The aim of this review is to provide an updated overview of the applications of bioelectrochemical systems 20 (BESs) for waste valorization in the industry, identifying current limitations and future perspectives of this 21 technology. BESs are classified according to biorefinery concepts into three different categories: (i) waste 22 to power, (ii) waste to fuel and (iii) waste to chemicals. 23 The main issues related to the scalability of bioelectrochemical systems are discussed, such as electrode 24 construction, the addition of redox mediators and the design parameters of the cells. Among the existing 25 BESs, microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) stand out as the more advanced 26 technologies in terms of implementation and R&D investment. However, there has been little transfer of 27 such achievements to enzymatic electrochemical systems. It is necessary that enzymatic systems learn 28 from the knowledge reached with MFC and MEC to accelerate their development to achieve 29 competitiveness in the short term. 30 31 32 Keywords 33 Bioelectrochemical systems, Carbon dioxide reduction, Enzymes, Electrodes materials, Microbial fuel cell, 34 Wastewater treatment, Carbon-based materials. 35 36 37 38 39 Graphical abstract 40 41 42 Nomenclature 43 ABTS 2,2'-azino-bis(3-ethylbenzo- thazoline-6-sulphonc acid) ADMI American Dye Manufacturer’s Institute AEM Anion exchange membrane AC Anodic chamber AQDS Anthraquinone-1,6-disulfonic acid ATP Adenosín trifosfato BE Benzoquinone BeC Bioelectrolytic cell BES Bioelectrochemical system BesC Bioelectrosynthesis cell BFC Biofuel cell BOD Biochemical oxygen demand CC Cathodic chamber CE Counter electrode Ceff Coulombic efficiency CEM Cation exchange membrane, CO Cobaltocene COD Chemical oxygen demand DET Direct electron transfer E Potential difference EEC Enzymatic electrochemical cell EES Electroenzymatic synthesis EFC Enzymatic fuel cell f Flow rate FDH Formate dehydrogenase HRT Hydraulic retention time I Current i Current density IEM Ion exchange menbrane LCA Life cycle assessment MB Methylene blue MEC Microbial electrolysis cell MES Microbial electrochemical synthesis MET Mediated electron transfer MFC Microbial fuel cell MOF Metal organic frameworks MV Methyl viologen NA Naphthoquinone NAD+ Nicotinamide adenine dinucleotide NADH Nicotinamide adenine dinucleotide (reduced form of NAD+) NER Net energy recovery NPV Net present value NR Neutral red OLR Organic loading rate Ox Oxidized species Pd Power density PEM Polymer exchange membrane PEMFC Proton exchange membrane fuel cell PHA polyhydroxyalkanoate PHB poly (3-hydroxybutyrate) PMS Phenazine methosulfate RE Reference electrode Re Reynolds rotation number Red Reduced species RI Riboflavin RM Redox mediator SHE Standard hydrogen electrode SPI Specific power input STS Stirrer tip speed TLR Technology readiness levels UASB Upflow anaerobic sludge blanket. V-Nase Vanadium nitrogenase VTR Volumetric treatment rate WAS Waste activated sludge WE Working electrode WIPO World Intellectual property organization WtC Waste to chemical WtF Waste to fuel WtP Waste to power 44 45 1. Introduction 46 47 Bioelectrochemistry, which combined electrochemical and biochemical processes, has attracted much 48 attention in the last ten years due to the contribution in terms of catalytic properties that biological agents 49 (enzymes or living cells) can make to meet the sustainability and circular economy principles (de 50 Fouchécour et al., 2022; Al-Sahari et al., 2021; Zhang et al., 2022; Chandraasekhar et al., 2021; Cai et al., 51 2022). The combination of biological systems with electrochemical cells is possible due to the fact that a 52 key part of biological systems are involved in fundamental processes of energy use and capture (Barlett, 53 2008). For instance, redox reactions in microorganisms are involved in the oxidation of carbon substrates 54 to CO2 with the concomitant generation of energy molecules such as ATP; in the case of plants, solar 55 energy is transformed into ATP molecules by means of cascade redox reactions. In both cases, there is a 56 flow of electrons through an electron transport chain, which involves a series of redox proteins and 57 enzymes in a constantly fluctuating state of oxidation and reduction. The application of an external voltage 58 has been proven to yield to a certain degree of manipulation of cellular metabolism. For fundamentals, 59 experimental techniques and applications of bioelectrochemistry systems see Bartlett (2008). 60 Bioelectrochemical systems (BESs) have attracted much attention among researchers because they offer 61 the advantage of combining the treatment of waste effluent with the simultaneous production of a 62 valuable product (Zheng et al., 2020). BESs are able to harness the energy remaining in waste, which would 63 otherwise be thrown away. Thus, BES technologies can be classified, according to the biorefinery 64 concepts, in waste to power (WtP), waste to fuel (WtF) and waste to chemicals (WtC). WtP, WtF and WtC 65 have become popular terms within the valorization of wastes, where the scope is to produce electric 66 energy, fuels and organic compounds of commercial interest, respectively, by using waste as raw material. 67 In this context, wastewater, agro-industrial wastes and greenhouse gases, such as CO2, are the main 68 residues studied with BES technologies. Carbon dioxide is one of the main greenhouse gases, its 69 anthropogenic generation being the main cause of the increase in concentration in the atmosphere. 70 Carbon dioxide reduction is one of the major goals of the climate change battle and finding new 71 alternatives is essential. Among the 17 sustainable development goals proposed by the United Nations, 72 the capture and recycle of CO2 is part of objective number 13: "Take urgent action to combat climate 73 change and its impacts". 74 On the other hand, around 380.000 million m3/year of wastewater (equivalent to 152 million of Olympic 75 swimming pools) are produced worldwide, and this value is expected to increase by ca. 2 % annually (Qadir 76 et al., 2020). Furthermore, according to a study performed by the European Commission, the treatment 77 of wastewater that each individual produces leads to 17 kg/year of dry sludge (European Commission, 78 2017). Thus, it represents a huge problem for big cities due to the high volumes of wastewater that need 79 to be treated and the high amounts of sludge that need to be discarded or processed annually. Some 80 strategies for recovering bioactive compounds from sludge have been proposed (Cristina et al., 2020); 81 however, they are not sufficient to considerably reduce the amount of waste that needs to be discarded. 82 The current treatment of wastewater leads to a high energy expenditure, mainly related to the high 83 requirements of aeration for the activated sludge process, which represents 50% of the energy 84 requirement of the plant (Liu & Tay, 2001). The aerobic consortia require constant aeration since it is at 85 this stage where most of the organic matter present in the wastewater stream is degraded. Paradoxically, 86 the energy potential present in municipal wastewater ranges from 1.16 to 3.09 kWh/m3 (Alsayed et al., 87 2020); thus, finding alternatives to recover this stored energy is crucial to meet the aspirations of a 88 sustainable society. 89 In the last decade, an increasing number of scientific articles related to the development of BESs have 90 been published, as shown in Figure 1. The greatest number of publications are related to biofuel cells, and 91 in particular, microbial fuel cells (MFC), which was the first BES proposed in the literature and is 92 responsible to open opportunities for the development of novel applications of BES, like bioelectrolytic 93 cells (BeC), and most recently bioelectrosynthesis cells (BesC), among others not considered in this review. 94 Although efforts in the scientific community have been focused on the development of BESs for the 95 conversion of wastewater and CO2 into valuable products, their scalability to industrial level remains a 96 challenge. 97 98 99 100 101 102 Figure 1. Evolution of the number of scientific articles related to bioelectrochemical systems in the last 103 decade, divided as biofuel cells (BFCs), bioelectrolytic cells (BeCs) and bioelectrosynthesys cells (BesC). 104 (Source: Scopus, December 2022). 105 106 The first pilot-scale MFC system reported was a modular reactor with a volume of 1 m3 using brewery 107 wastewater as feedstock (Waller & Trabold, 2013). After almost a decade, there are a few companies 108 offering MFC or MEC solutions for wastewater treatment (Jadhav et al., 2022). However, there is still no 109 consensus in the literature about the criteria that need to be taken into consideration for the scale-up of 110 BES reactors. This work aims to provide an updated overview of the applications of BESs for waste 111 valorization in the industry to identify current limitations and future perspectives of the technology. In 112 order to fulfill this objective, this review will be divided in four sections: 1. BESs, where the main parts 113 involved in a bioelectrochemical cell and the mechanism of action will be described; 2. Bioelectrochemical 114 approaches for waste valorization, discussing the wastes used as substrate and the respective 115 commercially valuable products; 3. Consideration related to the scale-up of BES, where the main variables 116 to take into account when scaling up BES to a commercial level will be described; 4. Projection of BES 117 technologies; this final section will give a perspective of BES as a viable commercial alternative to apply in 118 waste valorization. 119 2. Bioelectrochemical systems (BESs) 120 BESs involve the use of microorganisms or enzymes with an electrochemical system to exert influence 121 over the metabolism by affecting the oxidation and reduction ratios. BESs are unique systems capable of 122 converting the chemical energy of organic matter into electricity, hydrogen or chemical products. In Figure 123 2 a typical two-chamber bioelectrochemical cell configuration is depicted, with an ion exchange 124 membrane in the middle as separator. At laboratory scale, the electrochemical cell is made up of a working 125 electrode (WE), a counter electrode (CE) and a reference electrode (RE). The reaction of interest takes 126 place at the WE. The CE allows to close the circuit and the flowing of charges. RE serves for measuring the 127 potential of a single chamber. 128 If the reaction is spontaneous in nature, the flow of electrons will allow the generation of energy; in this 129 case, the electrochemical cell is a galvanic cell. If the reaction is not thermodynamically favorable, the 130 energy needs to be applied from an external source to force the reaction; in this case, the cell is an 131 electrolytic cell (Bajracharya et al., 2017). 132 133 134 Figure 2. Schematic representation of a bioelectrochemical cell applied for waste valorization with the 135 production of energy, hydrogen or organic compounds. IEM: Ion-exchange membrane. The reduced (red) 136 and oxidized species (ox) in each chamber will depend on the bioelectrochemical system (BES) as 137 indicated. 138 BESs make use of an electrochemical cell coupled to a biological catalyst to develop the reaction of 139 interest, where the catalyst is either coupled to the cathodic or to the anodic chamber, depending on 140 whether the reaction of interest is a reduction or an oxidation. In the case of wastewater treatment, the 141 biological catalyst is located in the anodic chamber to oxidize the organic matter, as it occurs in microbial 142 fuel cells (MFCs) or in microbial electrolysis cells (MECs). In the case of CO2 conversion, the biological 143 catalyst is located in the cathodic chamber in order to carry out its reduction (BesC). 144 Although most of the existing literature focuses on the use of microorganisms for the valorization of waste 145 products, the use of enzymes has gained relevance in recent times (Cocuzza et al., 2022; Ottone et al., 146 2021a). The use of isolated enzymes presents clear-cut advantages to whole-cell systems, such as better 147 control of reaction parameters and reaction monitoring, and avoidance of unwanted side reactions, 148 leading to higher production yields (Petroll et al., 2019). Besides, the behavior in terms of reaction kinetics 149 of the enzymatic BES is quite similar to chemical catalysts. However, in microbial systems the metabolic 150 pathways and the synthesis of secondary metabolites need to be considered; however, crucial advances 151 in protein engineering have allowed significant progress in enzyme biocatalysis and its implementation on 152 an industrial scale of production for different products, even from non-conventional substrates (Sakkos 153 et al., 2019). 154 In BES, electron transfer can be divided into two mechanisms: direct or mediated, as depicted in Figure 3. 155 In the direct electron transfer (DET) mechanism, the enzymes or microorganisms are directly wired to the 156 electrode. In the case of enzymes, the DET of electrons occurs for distances between the electrode surface 157 and the redox center of the enzyme below 1.4 nm (Kumar et al., 2017). The DET mechanism in 158 microorganisms occurs when either the microorganism has conductive pili, or conductive 159 materials/proteins are present in their outer membrane (Lovley, 2017; Mohan et al., 2014). As an 160 example, Geobacter and Shewanella are electroactive bacteria in the group of microorganisms with 161 reported DET capabilities (Creasey et al., 2018; Doyle & Marsili, 2018; Hirose et al., 2019; Lovley, 2011; 162 Lovley & Walker, 2019). 163 The mediated electron transfer (MET) mechanism involves the use of redox mediators (RMs). RMs act as 164 intermediate electron carriers between the biological agent and the electrode surface allowing to obtain 165 the final redox reaction at the electrode surface. The possibility exists that the RMs do not establish a 166 direct interaction with the catalyst to generate electron transfer, but interact with a third partner, such 167 as a cofactor; this is called biological transfer of RMs and cofactor. A vast literature could be founded with 168 respect to the use of redox mediators in the electrochemical regeneration of NADH (Gajdzik et al., 2012). 169 Also, it has been observed that some microorganisms can secrete redox mediators; for example, a 170 phenazine-based mediator has been found on MFC inoculated with Pseudomonas sp. (Pham et al., 2008). 171 RM are often used in BES to enhance the electron transfer from microorganisms or enzymes to the 172 working electrode and can be of natural or artificial origin. Their uses in electrochemistry are based on 173 their capacity to accept (reduce) and give (oxidize) electrons to other species (Martinez & Alvarez, 2018). 174 A thorough discussion of redox mediators will be presented in the following sections. 175 176 177 Figure 3. Schematic representation of different mechanisms of electron transfer from electrode to redox 178 species. 179 180 181 3. Bioelectrochemical approaches for waste valorization 182 183 This section gives a brief description of the different BES according to the classification of the biorefinery 184 concepts WtP, WtF and WtC. A schematic diagram of the products and the systems involved is shown in 185 Figure 4. 186 In terms of the different substrates used in BES, the wastewater from agribusiness has great potential 187 since it has a high organic load (chemical oxygen demand (COD)> 1 g/L) and a high degree of 188 biodegradability (biochemical oxygen demand (BOD)/COD> 60%) (Callegari et al., 2018). Therefore, these 189 waste streams are highly attractive to be used in an MFC system. It should be noted that despite the 190 development of current domestic wastewater treatment technologies, they are highly energy-demanding 191 due to the need to stabilize this type of waste, so alternatives that allow energy reduction are extremely 192 attractive. 193 194 Figure 4. Diagram of the products reported in the literature for the valorization of waste using a 195 bioelectrochemical system technology. MEC: microbial electrolysis cell, EEC: enzymatic electrochemical 196 cell, BeC: bioelectrolytic cell, EFC: enzymatic fuel cell, MFC: microbial fuel cell, BFC: biofuel cell, EES: 197 electroenzymatic synthesis, MES: microbial electrochemical synthesis and BesC: bioelectrosynthesis cell. 198 199 3.1 Waste to Power with a biofuel cell (BfC) 200 The use of a biological agents, such as bacteria or enzymes, to produce energy in the form of electricity 201 by means of an electrochemical cell is known as biofuel cells (BfCs). Microbial fuel cells (MFCs) are 202 characterized by having the electroactive bacteria communities in the anode. The microorganisms 203 consume the substrate from the media, growing and forming films on the electrode. Looking for further 204 cell improvements, some researchers have proposed the use of biotic cathodes as well, as an alternative 205 to overcome the limitation regarding oxygen as the ultimate electron acceptor. Among the different 206 alternatives that have been proposed to reduce the overpotential, biocathodes have been highlighted as 207 the most suitable for being cheap and easy to operate (Izadi et al., 2019; Mohamed et al., 2020; Samrat 208 et al., 2018; Sindhuja et al., 2018). 209 In MFCs, an electric current flows through the anode chamber into the cathode chamber producing 210 electricity by energy conversion from the breakdown of chemical bonds in the organic matter (Waller & 211 Trabold, 2013). MFC cannot produce the same level of voltage and current density obtained in classical 212 chemical fuel cells (Muddemann et al., 2019). 213 214 Analogous to MFC, enzymatic fuel cells (EFC) produce electric current by the catalytic oxidation of a fuel. 215 Nevertheless, in contrast to MFC, the exploitation of waste as a substrate for EFC is still an emerging 216 research field. Wastewater containing organic compounds could be potentially used as fuel for EFCs. For 217 instance, alcohol dehydrogenase (Galindo-de-la-Rosa et al., 2018, 2020; Tomassetti et al., 2020) or glucose 218 oxidase (Bahar & Yazici, 2018; Chung et al., 2018; del Torno-de Roman et al., 2018) modified anodes have 219 been studied using alcohol and glucose solutions, respectively. However, having waste valorization in 220 mind, it is necessary to study the interactions that other molecules present in the waste may have with 221 the electrochemical system. In addition, the high selectivity of enzymes works against the system, as they 222 cannot efficiently utilize complex waste streams containing different organic compounds. 223 224 3.2 Waste to Fuel with a bioelectrolytic cell (BeC) 225 226 Hydrogen (H2) is recognized as one of the cleanest energy carriers, having a much higher gravimetric 227 energy density than fossil fuels (Jia et al., 2020). So far, its production comes mainly from the use of fossil 228 fuels (e.g Hydrocarbon catalytic reforming or coal gasification), which leads to huge emissions of CO2 229 (Cormos et al., 2018). 230 An alternative environmentally-friendly technology for producing H2 is the microbial electrolysis cell 231 (MEC). While MFCs produce energy by organic matter oxidation, MEC requires an output source of energy 232 to partially reverse the process to generate H2 from organic matter (Chorbadzhiyska et al., 2011). H2 is 233 obtained at the cathode by the reduction of the protons produced by the degradation of organic matter 234 at the anode (see Figure 2). Usually, an ion exchange membrane separates the anodic and cathodic 235 chambers allowing only the passage of protons. The use of the membrane has the advantage of producing 236 H2 with higher purity than obtained in the counterpart with only one chamber (Kadier et al., 2016). The 237 voltage required for the reaction of H2 production varies between 0.2 and 0.8 V, which is much lower than 238 in a non-biological water electrolysis cell, which requires voltages between 1.23 - 1.8 V (Kadier et al., 239 2016). MECs have been used with a range of substrates similar to those already described for MFC, 240 including wastewater. The organic matter removal from wastewater using MEC ranges between 60 - 90%, 241 similar to that obtained with a MFC (Escapa et al., 2012; Hussain et al., 2018; Liu et al., 2012) 242 In an enzymatic electrolysis cell (EEC), hydrogenases containing the [Ni-Fe] and [Fe-Fe] groups can catalyze 243 the hydrogen reaction in the cathode with similar oxidation rates than in a platinum electrode (Chenevier 244 et al., 2013; Jones et al., 2002; Vignais et al., 2001). The differences between the two groups, [Ni-Fe] and 245 [Fe-Fe], are the catalytic properties and the stability at aerobic conditions. The [Fe-Fe] group exhibits 246 better catalytic properties than the [Ni-Fe] group (Bandyopadhyay et al., 2010), but the first one is strongly 247 sensitive to oxygen (Lu & Koo, 2019). In general, [Ni-Fe]-hydrogenases are isolated from Clostridium and 248 Desulfovibrio bacteria, as well as from some green algae, such as Chlamydomonas (Jugder et al., 2013). 249 250 3.3 Waste to Chemicals with a bioelectrosynthesis cell (BesC) 251 252 Waste to chemical represents the most complex option for chemical synthesis through bioelectrochemical 253 conversion. Two large categories stand out in this field: the conversion of CO2 to carbon compounds and 254 the production of bioplastics from wastewater (Bajracharya et al., 2017). BESs offer a more sustainable 255 alternative than traditional production methods, avoiding the use of fossil fuels, allowing the recycling of 256 CO2 and valorizing wastewater as raw material. 257 Most studies about MES have been focused on the production of methane (Su et al., 2016) and acetate, 258 where the latter represents more than 70 % of all works reported in the last decade (Jourdin & Burdyny, 259 2021). Therefore, the involved mechanism is well-known and the effect of several operational conditions 260 to increase productivity and efficiency has been reported. The productivity of acetate reported varies in 261 a wide range between a few to some thousand g/day/m3 depending on the culture type, strain origin, 262 applied potential, electrode material, and cell configuration, among other main operational parameters 263 (Hengsbach et al., 2022). Recently, the use of mixed cultures has attracted the attention of researchers 264 because it allows to obtain chemicals with higher economic value than acetate, like middle-chain fatty 265 acids (Tahir et al 2021). However, there is no consensus in the scientific community about which strategy, 266 whether mixed or pure culture, has more scalability opportunities since both have their pros and cons 267 (Chu et al., 2021). Other products obtained by MES are biodegradable bioplastics, like 268 polyhydroxyalkanoate (PHA) and poly (3-hydroxybutyrate) (PHB) (Dietrich et al., 2017). Nishio et al (2013) 269 reported that the production rate of PHB was increased by 60 % when an electrically assisted system was 270 used. The latter was attributed to an acceleration of the NAD+/NADH redox cycle, which facilitates 271 glycolytic metabolism and, in turn, PHB production. Other substrates, such as oily coffee waste, have 272 been tested for PHB production by fermentation with Ralstonia eutropha (Bhatia et al., 2018), which 273 illustrates the great potential of BES and the opportunities for waste treatment with the simultaneous 274 production of valuable products. 275 Regarding the enzymatic reduction of CO2, formate dehydrogenase (FDH) and vanadium nitrogenase (V- 276 Nase) have been studied as catalysts for the one-step production of formic acid and methane, respectively 277 (Álvarez et al., 2017; Grasemann & Laurenczy, 2012; Su et al., 2016; Pietricola et al., 2020; Pietricola et al., 278 2021). Also, a cascade multienzyme system has been considered to produce methanol starting from CO2 279 (Liu et al., 2020a; Ren et al., 2020; Zhang et al., 2021). In the reduction of CO2 to formic acid with FDH, the 280 nicotinamide dinucleotide (NAD) acts as RM, which is regenerated in the electrochemical cell (see Figure 281 3). The regeneration of the cofactor is a key step for the integration of enzymes and electrochemistry, 282 since its high cost makes regeneration mandatory for successful industrial implementation (Ottone et al., 283 2021b; Pietricola et al., 2022). 284 The electric wiring of nitrogenases differs from that the one for FDH since nitrogenases are ATP-depending 285 enzymes. Although nitrogenases need ATP as a cofactor, the immobilization of the catalytic subunit of the 286 enzyme on electrodes makes the cofactor dispensable with the consequent economic benefits making 287 the use of nitrogenase-based systems more feasible for large-scale applications. Nitrogenase is made of 288 two main components, a Fe-protein and a bimetal-protein ([Fe-V] or [Fe-Fe]). In the Fe-protein ATP is 289 hydrolyzed to start the reduction cascade reaction which ends up on the [Fe-V] cofactor with the CO2 290 reduction to CH4. In the EES systems, the coupling of the [Fe-V] or [Fe-Fe]-protein unit is done directly to 291 the cathode, leading to a direct transfer of the electrons from the catalytic center of the enzyme to the 292 cathode during the reaction of reduction of CO2 to CH4 (Milton & Minteer, 2019). 293 294 295 4. Considerations related to the scale-up of BES 296 297 BESs have shown great potential for waste recovery, but large-scale processes have not been developed 298 yet. The need to study and identify the key variables in the scaling up of BES is fundamental; therefore, 299 this section presents a compilation of different BES reported in the literature from laboratory to industrial 300 implemented BES. 301 The highest TRL (Technology readiness level) value (TRL 9) was achieved with MFC, MEC and MES systems, 302 whereas the highest TRL reported for the enzymatic system is TRL 3. On the other hand, to the best of the 303 author’s knowledge, no pilot-scale studies have been reported for any of the different enzymatic 304 electrochemical systems. Figure 5 shows a comparison between the number of total publications and 305 patents reported for each BES technology. It can be inferred that there is a strong correlation between 306 the highest TRL number achieved by a specific technology and the corresponding number of publications 307 and patents. 308 309 310 311 Figure 5 Comparison between the number of publications and patents reported for the different BES 312 technologies. Source: Scopus and WIPO, December 2022. EES: enzymatic electrosynthesis; EEC: enzymatic 313 electrolysis cell; EFC: enzymatic fuel cell; MES: microbial electrosynthesis; MEC: microbial electrolysis cell; 314 MFC: microbial fuel cell. 315 316 It is worth noting that only a few commercially available products (TRL 9) have been reported for MFC 317 (L.C.C. Aquacycl; Plant-e; Microorganic Technologies Inc), MEC (Electro-active Technologies Inc) and MES 318 (Cambrian Innovation Inc). By searching in the Scopus database ‘microbial fuel cell’ AND ‘scale-up’, 92 319 articles were found whereas 28 articles were found by searching using ‘microbial electrolysis cell’ AND 320 ‘scale-up’ as keywords. 321 Bird et al. (2022) defined some criteria for identifying when an MFC study can be considered to achieve 322 the pilot-scale level, which can be extrapolated to the different BES technologies. The criteria consider 323 reactor sizes higher than 10 L, the conduction of the reactor both inside and outside of the laboratory, the 324 operation in either batch or continuous mode, operation times longer than one month and the treatment 325 of any waste stream. Using these selection criteria, 30 pilot-scale MFC studies in the period between 2008 326 and 2020 were identified. 327 There is still little information regarding the TRL 9 BES, like electrode material and reactor configuration, 328 which is attributed to safeguarding intellectual property or the competitive edge of the companies. 329 Therefore, to better understand the possibilities regarding the scale-up of a BES, the studies of TRL 4 and 330 TRL 5 available in the literature will be used. Table 1 shows a summary of the different pilot-scale BES 331 published in the last 5 years. In this selection, the only criteria taken into consideration was the size of the 332 reactor (with a working volume bigger than 10 L). The working volume is the effective volume of the liquid 333 inside the reactor, which considers the anolyte and catholyte (if present). The main characteristics of the 334 cell construction like electrode material, reactor volume, separation of the anodic and cathodic chambers 335 by means of a membrane as well as the type of microorganism used, waste treated, operational 336 parameters and the performance of the different cells are indicated. It is interesting to note that more 337 than 70 % of the pilot-scale cells reported in Table 1 consider the use of a membrane for separating the 338 anodic from the cathodic chamber even if one of the biggest issues related to microbial electrochemical 339 cells is the membrane fouling. Nevertheless, the use of ceramic-based membranes, together with carbon- 340 based electrodes highlights the necessity of finding novel cost-effective materials for the construction of 341 the cells. In addition, ceramic materials have the advantage of being mechanically and chemically stable, 342 which allows their regeneration by thermal or chemical treatments. COD removal efficiency and the 343 power density values are typical parameters reported for comparing the performance of different MFCs. 344 However, an important parameter that is hardly reported among the different pilot-scale prototypes is 345 the Net Energy Recovery (NER), which indicates how much energy can be recovered from waste. Only the 346 work published by Babanova et al. (2019) declares this result. This parameter serves as an indicator of 347 how much energy can be used for other purposes different to waste treatment. NER is a useful parameter 348 of comparison between an MFC and other WtP technology. In the case of MECs, there appears to be less 349 consensus on the key parameters to assess performance. For instance, COD removal efficiency is not 350 always reported and an important parameter like the COD to H2 production yield is not calculated. 351 Table 1 Specifications of pilot-scale BES published between 2018 and 2022. 352 Anode Stable Working material phase Volume (projected Cathode material operation Operational BES Waste (L) Membrane area) (projected area) Biological agent Performance time (d) Parameters REF COD removal: 93.52 ceramic graphite felt air %, I: 43.7 mA, Pd: domestic membrane graphite felt cathode (0.706 anaerobic microbial 23.52 mW·m-3, Ceff: fed-batch mode, (Suransh et MFC wastewater 36 (vermiculite) (0.756 m2) m2) consortium 3% 92 HRT: 2-3 d al., 2023) graphite rod (0.003 m2) graphite rod activated sludge with granular (0.003 m2) with from the industrial COD removal: 82 %, semi-continuous dairy graphite bed granular graphite plant treating the Pd: 26.5 W·m-3, Ceff: flow mode, f: (Callegari et MFC wastewater 0.855 CEM (n.a.) bed (n.a.) dairy waste samples 24%, 65 1L·d-1 al., 2018) continuous flow domestic effluent of previous mode, HRT: 12 wastewater carbon fiber carbon air cathode bioelectrochemical h, f: 5.76 L·min- (Rossi et al., MFC 850 no membrane brush (11 m2) (20 m2) systems COD removal: 49 % 100 1 ·m-2 2022) Fimbristylis ferruginea with bacterial community from rhizospheric soils of COD removal: 74.1 316 stainless plants growing at %, ADMI removal: textile dye steel sheets 316 stainless steel textile wastewater 97.32 %, Pd: 197.94 batch mode, (Patel et al., MFC wastewater 30 CEM (0.0126 m2) sheets (0.0126 m2) polluted site mW·m-2 (at 500 Ω) 4 HRT: 2d 2021) COD removal: 81.67 %, Pd: 24.104 batch mode, domestic graphite plate graphite plate mW·m-2, i: 23.844 cycle volume: 20 (Selvaraj et MFC wastewater 43.88 no membrane (0.848 m2) (0.848 m2) macrophyte plants mA·m-2 45 L, total cycles: 3 al., 2023) COD removal: 65%, Ceff: 27%, i: 103 mA·m-2 (at 47 Ω), Pd: graphite fiber 92 mW·m-2, NER: continuous flow swine brushes (1.2 gas diffusion 0.11 kWhr·kg-1COD (at mode, HRT: 4 h, (Babanova et MFC wastewater 110 no membrane m2) cathodes (0.07 m2) lagoon sediment 330 Ω) 170 f: 660 L·d-1 al., 2019) graphite felt coated with CuSn plant secondary Pd. 4.0 mW·m-2, continuous flow (Nath and synthetic ceramic graphite felt on air facing side metabolites treated Ceff: 5% (at 1 Ω), I: mode, f: 150 Ghangrekar, MFC wastewater 125 membrane (0.027 m2) (0.027 m2) anaerobic sludge 40 mA (at 1 Ω) 160 mL·min-1 2020) graphite rod secondary Pd: 10.3 W·m−3, I: continuous flow synthetic (0.022 m2) wastewater sludge 285 mA (at 150 Ω), mode, f: 0.72 - textile with granular graphite rod from the distillery Ceff: 10.67 % (at 150 1.2 L·h-1, HRT: (Nakhate et UASB-MFC wastewater 12.09 no membrane activated (0.011 m2) industry Ω), 46 13.88 h al., 2019) carbon bed COD removal: 71.73 (n.a.) % fed-batch mode, pig slurry digestate from a cycle duration 2- liquid graphite felt graphite felt (0.18 local Wastewater H2 production rate: 5 d, f: 15 L·h-1, E: (San-Martín MEC fraction 16 CEM (0.18 m2) m2) Treatment Plant 0.2 LH2·L-1·d-1 5 1V et al., 2019) ultra-high 316 stainless steel continuous flow molecular flat mesh with microorganism mode, HRT: 5 h, domestic weight graphite felt stainless steel wire from the same f: 150 mL·min-1, (Verbeeck et MEC wastewater 45 polyethylene (1.44 m2) wool (0.6 m2) wastewater H2 purity: 92.8 % 100 E: 0.9 V al., 2018) graphite felt (1.2 m2) ultra-high encased by continuous flow molecular two sheets of 438 grade 1 microorganism mode, HRT: 24 domestic weight stainless steel stainless steel from the same h, f: 75mL·min-1, (Verbeeck et MEC wastewater 135 polyethylene mesh wool (n.a.) wastewater H2 purity: 98.4 % 100 E: 1.2 V al., 2018) continuous flow mode, f: 6 L·d-1, granular activated sludge HRT: 12.6 h, synthetic graphite bed granular graphite (anode), anaerobic COD removal: 56 %, anode potential: (Zeppilli et MEC wastewater 12 AEM (n.a.) bed (n.a.) sludge (cathode) I: 86 mA, Ceff: 13% 140 0.2 V vs. SHE al., 2020) activated sludge i: 0.27 A·m-2, Ceff: (anode), 10%, TOC removal: wastewater 80%, TN removal: collected from the 70%, specific energy continuous flow MEC with domestic graphite felt graphite felt (2.35 denitrification zone consumption of 0.18 mode, HRT: 1d, (Maria et al., denitrification wastewater 150 AEM (2.35 m2) m2) (cathode) kWh·m·L-3 63 E: 1V 2018) COD removal rate continuous flow 316L stainless (AC): 0.83 g·d−1·L−1, mode (AC), f: 45 steel frames electrode pieces acetate production mL·d-1, E: 0.6 - with carbon from previous rate (CC): 0.53 g·d−1 1.2 V, CO2 flow tissue strips bioelectrochemical ·L−1, acetate rate (CC): ~ 1.5 biowaste fixed between 316L stainless systems (anode). maximum mL·s-1, hydrolyzate two stainless steel baskets filled Fermentation licor concentration: 8.3 recirculation (AC), CO2 AEM and CEM steel grills with carbon of different food g·L−1, i: 2 A·m−2, Ceff: flow (CC): 100 (Tian et al., MES (CC) 12.5 frame (0.36 m2) granules (0.36 m2) wastes (cathode) 98.6% 55 L·h-1 2023) batch mode (liquid) with continuous gas Methane production flowing, HRT of (Enzmann rate: 11.7 mmol·d-1, gas: 33 min, E: - and graphite cloth graphite cloth Methanococcus i: 85 mA·m-2, Ceff: 1100 mV vs. Holtmann, MES CO2 50 CEM (0.75 m2) (1.59 m2) maripaludis 100% 3.33 Ag/AgCl 2019) Nomenclature: AC: anodic chamber, ADMI removal: American Dye Manufacturer’s Institute removal efficiency, AEM: anion exchange membrane, BES: bioelectrochemical system, CC: cathodic chamber, Ceff: maximum coulombic efficiency, CEM: cation exchange membrane, COD removal: chemical oxygen demand removal efficiency, E: potential difference, f: flow rate, HRT: hydraulic retention time, I: highest current, i: highest current density, MEC: microbial electrolysis cell, MFC: microbial fuel cell, MES: microbial electrosynthesis, n.a.: not applicable, NER: net energy recovery, Pd: Highest power density, SHE: standard hydrogen electrode, UASB: upflow anaerobic sludge blanket. 353 354 Other issues related to the scale-up of BESs are the electrode construction, the addition of a redox 355 mediator, the configuration of the reactor and the downstream steps. These points will be discussed in 356 detail below. 357 358 359 4.1 Construction of the electrodes. 360 361 A fundamental component in any electrochemical systems, as well as in BESs, is the working electrode. 362 The electrode is responsible for carrying out the reaction of interest, this being the cause of the various 363 studies related to its optimization. Several variables are important for the construction and design of the 364 electrodes, this being particularly relevant for large-scale applications, where size, surface area and cost 365 considerations will determine the feasibility of BES. 366 367 As can be seen in Figure 6, three are the key variables to be considered for the construction of the 368 electrode: chemical composition, shape and morphology, and electrode’s modification. 369 370 Figure 6. Schematic representation of the key variables in electrode manufacturing. 371 The chemical composition is directly related to the base material of the electrode and its chemical 372 structure, giving the electrode its electrochemical characteristic and structural properties but, at the same 373 time, directly influencing the type of interaction of the electrode with the biological catalyst. Shape and 374 morphology are also critical parameters in the electrode design, where the required surface area and the 375 desired final application must be taken into consideration. A third consideration is the possible 376 modifications that can be done to the electrode to improve its properties, such as the increase in the 377 surface area or in conductivity. Similarly to what occurs with other non-biological emerging 378 electrochemical systems (Ottone et al., 2019), the reproducibility of the techniques for electrode 379 construction at a large scale is one of the most important issues in scaling up. 380 As shown in Table 1, carbon-based materials, especially graphite-based, are the most used in scaling-up 381 systems showing a good commercial perspective. Among them, graphite and its derivatives are the most 382 attractive due to their relatively low cost, as shown in Table 2. The advantage of carbon-based materials 383 relies on their lower cost per surface area, since carbon can be easily synthesized in a 3D structure like 384 cloth, felt, and fiber among others. In this table, carbon granules and metal wools are not considered for 385 their irregular shape that difficult the comparison with the same projected area. The data reported in 386 Table 2 consider only the cost of the material used in the construction of the electride. It do not consider 387 the cost of further electrode modifications like activation with functional groups nor thermal and chemical 388 treatments of the base material. 389 390 391 Table 2. Comparison of the cost of different electrode configurations used in BESs considering 1 m2 of 392 projected area. 393 Cost (Dólar) Specific for 1 m2 cost Electrode projected Total surface (USD/total Dimensions Material Specification area area (m2) area) Source Fuel cell 3D Cloth 1900 478 4,0 store Fuel cell 3D Felt 856 66,3 12,9 store Graphite Fuel cell 2D Paper 469 1 468,8 store Sigma 2D Plate 2110 1 2110,0 Aldrich Sigma 3D Nanotubesa 9700 80000 0,1 Aldrich Graphene Sigma 2D Paper 671 1 671,0 Aldrich Glassy Sigma 3D carbon Plate 3519 N.D. N.D. Aldrich Platinum Sigma 2D 99.95% Thin layerb 2854 1 2853,5 Aldrich Stainless Sigma 2D steel Sheet 1001 1 1000,8 Aldrich a 2 Considering a loading of 100 g of nanotubes per m b For a 100 nm deposited thin layer. The cost of the support material is not considered. 394 395 396 397 4.1.1 Chemical composition 398 399 As mentioned before, the chemical composition is directly related to the materials used in the fabrication 400 of the electrodes; here carbon and metal-based groups stand out. Among the family of carbon-based 401 materials, graphite, graphene, and glassy carbon electrodes can be found. Carbon-based materials have 402 some properties that make them attractive for electrochemical applications, such as good electrical 403 conductivity and low resistance, high biocompatibility and chemical stability, resistance to corrosion, large 404 surface area, appropriate mechanical strength and toughness (Dubey & Guruviah, 2019; Thamilselvan et 405 al., 2016). 406 Metal-based electrodes have been widely studied also, and certain metals with good characteristics 407 (conductivity, corrosion resistance, and chemical reactivity) for use in electrochemical cells are 408 recognized. Some of the most used metals for electrochemical applications are copper, platinum, titanium 409 and stainless steel (Terzi et al., 2019). Among them, stainless steel is an excellent material alternative for 410 electrode manufacturing at large scale, with outstanding properties such as corrosion resistance, high 411 electric conductivity and low cost compared to other metals. To increase the surface area, stainless steel 412 foams have been proposed, showing robustness and biocompatibility to form biofilms for MFC 413 applications (Kalathil et al., 2018). 414 Recently, new materials, such as metal organic frameworks (MOFs) and semiconductors (ZnO and TiO2), 415 have emerged with attractive electrochemical and mechanical properties (Ottone et al., 2014a; 2014b; 416 Ottone et al., 2021b). However, these new materials are still under development for industrial use (Jaouen 417 & Morozan, 2014; Liao et al., 2018; Vikrant et al., 2017), so they are not considered in this review. 418 Carbon and metal-based materials are the most used in the fabrication of electrodes, and the literature 419 highlights some of their advantages such as the cost of production, high specific surface area and 420 biocompatibility (Ahn & Logan, 2010; Huong Le et al., 2017; Lin et al., 2019; Liu et al., 2004; Mohanakrishna 421 et al., 2018; Rengaraj et al., 2011; Zhou et al., 2011). 422 Carbon-based electrodes are also attractive from a cost perspective, being significantly cheaper than 423 metals widely used in electrochemistry, such as platinum and copper. This makes carbon-based electrodes 424 extremely attractive for the construction of large-scale BES. Graphite-based electrodes are particularly 425 suitable when considering the design of large-scale BES. Graphite-based electrodes do not need to be 426 supported, can be easily subjected to bulk or surface modifications in order to improve their 427 electrochemical and physical qualities, and are readily available from different commercial suppliers. 428 429 4.1.2 Shape and morphology 430 431 The shape and morphology of the electrode plays an important role in its construction. Based on the 432 forms, they can be categorized as: sheets or plates, foams, nanotubes and fibers, materials based on cross 433 linking or woven units. Different presentations have been developed to increase the specific surface area 434 of the electrode, which is key to achieve better current densities and serve as catalyst support (Artsanti 435 et al., 2017; Debe, 2011; Gude, 2016). 436 Sheets or plates are the usual and basic shape of electrodes, despite the low amounts of catalysts that 437 can be loaded. This limitation is due to the low exposed area, as in MFC (Fu et al., 2012; Li et al., 2009; 438 Zhao et al., 2009). The power densities observed for a MFC using a plate electrode were the lowest when 439 compared to other electrode shapes, such as mesh or cylinder (Fu et al., 2012). The foam electrode 440 structure presents a higher surface area compared with plane plate or sheet, resulting in an improvement 441 on the electrochemical performance (Ali et al., 2014; Liu & Liang, 2000; Montillet et al., 1994; Sen et al., 442 2014). Among the different materials to be used, carbon-based foams have been recognized as 443 exceptional due to their excellent properties, like high electrical conductivity (Degirmenci & Kirca, 2018; 444 Wang et al., 2015). 445 Another of the most widely used forms reported in the literature are nanotubes and fibers, which can be 446 used to build up more complex structures. Nanotubes and fibers are categorized in the same group 447 despite their difference in size, which are about 0.4 nm and in the range of 5 to 10 µm of diameter, 448 respectively (Bhatt & Goe, 2017; Peng et al., 2000). Nanotubes and fibers have been also carbon-based 449 electrodes whose characteristics, like high mechanical strength and ductility, have been highlighted for 450 electrode manufacturing. Moreover, excellent stability and conductivity have been attributed to carbon 451 nanotubes (Zhou et al., 2011). 452 Lastly, materials formed from interlocking or woven units ca be found which are constructed in paper- 453 like, cloth-like and felt-like shapes. Their structure is based on fibers or nanotubes, either interwoven (for 454 cloth) or non-structured (for felt) (Huong Le et al., 2017; Kim et al., 2019), while in paper the fibers are 455 held together in a resin matrix (Radhakrishnan & Haridoss, 2011). Developing a structure based on woven/ 456 interlaced fibers or nanotubes has been found to improve the mechanical properties, in this way being 457 possible to provide strength, stiffness and toughness to the electrodes (Dan-Mallam et al., 2015). 458 459 4.1.3 Electrode modifications 460 461 All the materials, shapes and structures described before can be modified to improve its electrochemical 462 performance. These modifications have been classified as follows: hybrid porous, hybrid surface 463 nanostructures and decoration of electrodes surfaces. The first class, hybrid porous, is based on those 464 primary porous structures which are filled with another material with a high surface area; an example of 465 this is the reticulated vitreous carbon foam, whose micro holes can be filled with nanofibers (Walsh et al., 466 2016). The second class, hybrid surface nanostructure, although defined as “tubes within tubes” is 467 described as an arrangement of surface geometry with micropores filled with microtubes; an example of 468 this technique is the use of nanotubular titanates which were incorporated inside the pores of a wide 469 nanotube array by electrophoretic deposition (Bavykin et al., 2013). The last category is based on the 470 deposition or decoration of nanoparticles on the electrode surface; this can be carried out by combining 471 different techniques, such as redox gel for coating, electrochemical deposition and reduction of metal 472 salts, among others. In the literature, this kind of modification is mainly focused on the deposition of metal 473 on carbon-based materials (Dhibar & Das, 2014; Fattahi et al., 2011; Han et al., 2018; Jeromiyas et al., 474 2019; Li et al., 2013; Quinn et al., 2005; Vaghari et al., 2016; Wang et al., 2007; Xia et al., 2015). More 475 information regarding these modification techniques can be found in Walsh et al., (2016). 476 Carbon-based materials have the best relationship between physical properties, such as specific surface 477 area, and electrochemical properties. The surface area available is extremely important because the 478 material topography influences the bacterial adhesion to the surface or the biofilm formation, and it has 479 been reported that bacteria preferably colonize porous, grooved and braided surfaces (Flint et al., 2000; 480 Whitehead & Verran, 2006). In reactions catalyzed by enzymes, the surface area of the electrode also 481 plays an important role. The available surface area will determine the number of functional groups that 482 can be added and, therefore, the enzyme load immobilized on the electrode (Camelin et al., 2022). An 483 interesting review of some cases of different techniques for protein and enzyme coupling on electrodes 484 has been recently published (Olloqui-Sariego et al., 2021). 485 486 As already mentioned, the most attractive materials for the construction of a bioelectrode are carbon and 487 metal-based, but in terms of their ability to be modified, carbon-based electrodes have an advantage. This 488 makes them extremely attractive to be used in the construction of electrodes for large-scale operation. 489 490 4.2 Addition of redox mediators 491 492 The selection of the BES arrangement will depend on the system to be mounted and the mediator to be 493 used. The use of a free form of RM is recommended in cases where the mediator is cheap and 494 environmentally benign. On the contrary, in those cases where the subsequent waste treatment is not 495 efficient enough to eliminate the RM, their commercial value is high, or poorly soluble in the medium, 496 immobilization will be required to achieve good results (Azhar et al., 2005; Motobar et al., 2021). Kochius 497 et al (2012) described in detail the immobilization techniques available for redox mediators. 498 RM can be used with different purposes depending on what is being targeted: increasing in pollutant 499 removal efficiency or increasing in production efficiency. As an example, Sevda and Sreekrishman (2012) 500 observed that using RMs on MFC for the treatment of sewage and simultaneous generation of energy, a 501 significant improvement on the removal of organic matter was obtained with the use of methylene blue 502 or neutral red, but this good performance was not reflected in terms of energy production. Authors 503 attributed this behavior to an alteration on the cellular metabolism caused by the RM at the level of 504 perturbation in the NADH/NAD+ ratio leading to an imbalance in cofactors which participate in many 505 metabolic reactions (Leropoulos et al., 2005). 506 Tamirat et al., (2020) highlighted some characteristics of RMs that should be met for its selection, such as 507 high reversibility, fast kinetics, high solubility, and negligible side reactions. Table 3 presents a list of the 508 most studied RMs that are compatible with microbial or enzymatic systems. 509 510 Table 3. Redox mediators and some of their characteristics. Redox Solubility in Cost Redox mediator potential * Form Reference water (USD/g)** (vs SHE) Methylene blue -0.1 43.6 g/L 3.12 Free/immobilized Thiele et al., 2008 (MB) Methyl viologen -0.446, - Aulenta et al., 620 g/L 49.8 Free (MV) 0.760 2007 Harrington et al., Neutral red (NR) -0.325 50 g/L 41.7 Free 2015 Field & Brady., Riboflavin (RI) - Poor 17.3 Free 2003 -1.88, - Poor Cobaltocene (CO) 0.94 (vs 54.2 Immobilized Zhao et al., 2015 (<1 g/L) SCE) Phenazine +0.08 200 g/L 47.1 Free Thiele et al., 2008 methosulfate (PMS) Anthraquinone-1,6- Aulenta et al., disulfonic acid -0.184 - - Immobilized 2010 (AQDS) Benzoquinone (BE) +0.28 Poor 4.1 Immobilized Hendler, 1977 Naphthoquinone Poor Meckstroth et al., -0.145 6.7 Immobilized (NA) (3.5 g/L) 1981 2,2'-azino-bis(3- Tsujimura et al., ethylbenzo- 0.670 50 g/L 69 Free/immobilized 2001 thazoline-6-sulphonc acid) (ABTS) * 511 Solubility defined at 25°C and obtained from PubChem data base. ** 512 Data obtained from SigmaAldrich catalogue. 513 514 The studies regarding RM are focusing mostly on the kinetics of different redox mediators in conjunction 515 with different substrates and the analysis of their response at the level of pollutant removal, power 516 generation and/or production of products of interest. An example of the enhancing effect of some RM on 517 BES is the effect of different concentrations of methylene blue (MB) over MFC systems where reported 518 results show that the highest values of energy and electric current generated were observed with 0.3 mM 519 (Rahimnejad et al., 2011) and 0.08 mM (Sevda & Sreekrishnan, 2012). These results highlight the 520 peculiarity of each system and the need to optimize the addition of RM, in order to obtain the best 521 possible results. 522 523 RM plays an important role in BES, and it can be applied in two formats: free form or immobilized on a 524 support. Different studies have shown the use of free RM on BES, but this has a clear disadvantage 525 because its recovery from the reaction solution is difficult or impossible with the corresponding 526 environmental threat (Dai et al., 2016). Although the concentrations of RM used are in the order of µM 527 or mM, the need for their continuous feeding makes them a heavy economic and environmental burden. 528 To solve this, many studies use RMs in an immobilized form, thus, retention and reuse are favored. The 529 use of RM has been shown to have a positive effect on BES, but the use in stoichiometric quantities is not 530 feasible for the operation of continuous systems due to its high commercial price (Hollmann et al., 2006). 531 It has been shown that the systems of RM removal are inefficient and it is known that, even at low 532 concentrations, they can be environmentally hazardous (Azhar et al., 2005; Novotny et al., 2011). 533 Therefore, the alternative of immobilizing RMs has become quite attractive and different immobilization 534 strategies have been reported, such as adsorption, polymerization, entrapment and covalent linkage. 535 More information at this respect can be found in Kochius et al., (2012). 536 537 4.3 Reactor configuration 538 539 The configuration of the electrochemical cell or reactor is also a key issue for the implementation of BES 540 in a commercial application. Different configurations have been proposed, using one, two or three 541 chambers. The main advantage of using one chamber relies on its simplicity; however, as all products are 542 in a unique chamber, the downstream process become more complex. Therefore, the use of two 543 chambers in which the anolyte and catholyte products are separated has been considered a preferable 544 alternative. This two-chamber configuration allows for higher efficiencies since an ion exchange 545 membrane is used to generate the two compartments, thus avoiding reverse reactions, minimizing side 546 reactions and obtaining cathodic products of higher purity (Tiquia-Arashiro et al., 2020). However, one of 547 the disadvantages of this configuration when using wastewater as a substrate is the membrane fouling, 548 i.e., due the accumulation of some of the particulate materials present in the waste stream (Hou et al., 549 2021). 550 The most prominent advances in reactor configuration have been in MFC systems, in which the plate-and- 551 frame reactor proved to be a very convenient and versatile configuration. The plate and frame reactor, as 552 shown in Figure 7, presents a sandwich-style configuration, where the ion exchange plate and the 553 electrodes are trapped between a series of plates and frames. With this type of configuration, scaling up 554 the process is quite easy, by simply adding more modules to increase the cell volume (González-García et 555 al., 2000). The main downside of this configuration is the poor or almost non-existent mixing and 556 homogenization; to solve this, different alternatives have been studied, such as the management of the 557 inlet flow and the use of flow dispensers in the middle of the reaction chambers (Arenas et al., 2020). In 558 addition, an improved design is the inclusion of air cathodes which report positive effects comparing 559 different MFC reactor configurations (Rossi & Logan, 2022). In air cathodes MFC, the oxygen in air can 560 passively flows to the cathode through a hydrophobic layer without need of pumps and thus energy 561 consumption. 562 563 Figure 7. Schematic representation of a plate and frame reactor. Blue and red electrodes represent the 564 anode and cathode respectively. 565 In microbial fermentation processes, there are some criteria that permit the design of large-scale 566 processes leading to the same productivity and product quality as that developed at a laboratory scale 567 (Yang, 2010). Some of these criteria are geometric similarity, such as hydraulic retention time (HRT), 568 specific power input (SPI), Reynolds rotation number (Re) and stirrer tip speed (STS). However, in the 569 scale-up of bioelectrochemical reactors there is no consensus in the research community about which 570 criteria should be taken into consideration. 571 Considering that a large-scale BES will work in continuous mode and the relevance of maintaining the 572 energy requirements, HRT and SPI have been considered for some scale-up BES reactors (Dekker et al., 573 2009; Fitschen et al., 2019; Haavisto et al., 2017; Walter et al., 2016; Ye et al., 2020). Other parameters 574 proposed in the literature are specific current density, electrode spacing and electrode geometry (Cotterill 575 et al., 2017; Jadhav et al., 2021; Janicek et al, 2014; Papillon et al., 2021). On the other hand, Reynolds 576 rotation number or stirrer tip speed have been less proposed, since few MFC and MEC reactors have no 577 agitation (Pan et al., 2019). 578 579 580 5. Challenges and future perspectives for BES technologies 581 582 Among the different BES technologies, MFC and MEC are the most developed systems with the greatest 583 potential for commercial implementation in the near future. In particular, MFC technology is gaining 584 significance for wastewater treatment because of its advantages with respect to the conventional 585 technology of activated sludge which consumes much energy due to the requirement of constant 586 aeration. However, certain aspects still require of improvements, such as reducing the cost of currently 587 expensive electrode materials, proper wastewater pH control, and biofilm growth on the electrode’s 588 surface (Waller & Trabold, 2013). The intensification of the BES processes is a key strategy for achieving 589 the commercialization level of the technology, analogous to what has been proposed for other 590 biochemical processes (Boodhoo et al., 2022). 591 592 The differences between laboratory conditions and relevant environmental conditions need to be taken 593 into consideration for reaching the market. The variation of substrate parameters, including temperature, 594 pH, concentrations and components, is an important issue to have into consideration. Therefore, good 595 design of pilot-scale experiences is necessary for giving useful information for offering attractive BES- 596 based solutions. Interestingly, wastewater treatment in small rural communities, where throughputs and 597 workloads are much lower, rises as a feasible option for introducing BES technology (Nagendranatha 598 Reddy et al., 2018). 599 600 Another important parameter to consider in BES technology is COD removal efficiency. It was reported 601 that the limit of COD removal with MFC is 1.1 kg COD/(m3d), which represents between 60 and 70% of 602 COD removal from the original affluent (de Fouchécour et al., 2022). On the other hand, traditional 603 wastewater treatment (aerobic and anaerobic) leads to COD removal yields over 90% and, most 604 importantly, it is possible to reach a final COD concentration below the disposal standards (Aziz et al., 605 2019), something that it is not possible to reach with current BES technologies. Each region has its own 606 discharge standards; however, the limit of COD concentrations varies between 125 mg/L to 60 mg/L (Deng 607 & Wheatley, 2016; EPA, 2001; E. U. W. Directive, 1991). Since BESs lead to lower COD removal yields than 608 traditional wastewater treatment, it should be highlighted that the purpose of their implementation is to 609 have a higher energy recovery and allow their coupling to another traditional treatment system to reach 610 a COD value below the disposal regulation parameters. Related to COD removal, other reactor design 611 parameters arise, like organic loading rate (OLR) and volumetric treatment rate (VTR), from which 612 hydraulic retention time (HRT) can be obtained (Baeza et al., 2017). Thus, the requirements for waste 613 treatment are a key factor in introducing BES technology as a competitive solution. 614 615 BES seems to be disadvantageous when compared to conventional technologies. For instance, the highest 616 hydrogen production rate reported with a pilot-scale MEC using urban wastewater is 0.004 Nm3/d (Baeza 617 et al., 2017), which is considerably lower than obtained with a polymer exchange membrane (PEM) 618 electrolyzer that arrives at 720 Nm3/h (Dincer et al., 2015). In the case of MFC, the highest power density 619 reported with a pilot-scale reactor is 60 W using a 1000 L reactor for treating municipal wastewater, which 620 is still orders of magnitude lower than that produced with a proton exchange membrane fuel cell (PEMFC), 621 which varies from 1 to 100 kW (Felseghi et al., 2019). Thus, with current materials used in the construction 622 of the cells, the cost of using BES for wastewater treatment is still significantly higher than activated sludge 623 or anaerobic digestion (Rozendal et al., 2008). According to a study performed by Christodoulou, et al. 624 (2017), formic acid is the product that can be produced by means of a MES with the lowest production 625 cost (0.49 £/kg), which is a competitive cost with respect to traditional production systems. However, the 626 current investment costs associated with MES are higher than that of conventional technologies, which 627 represents the most important limitation for investors. A complete study of techno-economic analysis and 628 sustainability through the life cycle assessment (LCA) methodology of different BESs is reported elsewhere 629 (Savla et al., 2021). Thus, MFC and MEC are still far from being able to compete with ongoing technologies 630 in high throughput wastewater treatment plants. However, a feasible option for this technology is 631 wastewater treatment in small rural communities, where throughputs and workloads are much lower 632 (Nagendranatha Reddy et al., 2018). Nevertheless, from an economic point of view, the revenues related 633 to the treatment of the waste are a key factor to consider in the equation that could lead to a positive net 634 present value (NPV) in a techno-economic analysis. 635 636 Moreover, it has been demonstrated that it is possible to use a MEC for the treatment of waste activated 637 sludge (WAS) (Hu et al., 2019), which arises as a promising application of BESs because the accumulation 638 of WAS is a big problem in traditional wastewater treatment plants (Lu et al., 2012). In addition, among 639 the different biological strategies to produce hydrogen, MEC has been defined as the most promising 640 technology due to its better control over hydrogen generation rate and effective conversion of organic 641 feedstock, regardless of thermodynamic limits (Qyyum et al, 2022). The latter suggests that MEC 642 technology will achieve a rapid advance toward maturity. 643 644 645 6. Conclusions 646 647 BES systems have been studied from different perspectives, such as biocatalyst components, 648 electrochemical reactor configuration and electrode construction, among others. However, much more 649 applied research is needed for its commercial application. Such efforts should be primarily focused on 650 electrode manufacture optimization since it is one of the main factors affecting investment costs. For this 651 reason, areas such as material for electrode construction and techniques of electrode modification to 652 ensure a better biocatalyst attachment to the electrodes are critical to be further developed. Although 653 different materials have been studied for electrode fabrication, the high production costs associated to 654 the difficult scale-up of the synthesis procedures for electrode manufacture is a crucial issue that needs 655 to be addressed to become a competitive technology. 656 Among the different BES, the microbial cells stand out as the most promising technology to expand their 657 applications to reach technological significance in the short term because of their advances in technology 658 implementation and R&D investment compared to the enzymatic technologies mentioned. It is necessary 659 that the BES technologies that are in lower TRL values, such as enzymatic electrochemical cells, learn from 660 the knowledge reached with the already commercially available technologies to accelerate their 661 development and thus reach the competitive level in a short period of time. 662 663 7. 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