Microbial electrochemical approaches of carbon dioxide utilization for biogas upgrading

32 Microbial electrochemical approach is an emerging technology for biogas upgrading through carbon 33 dioxide (CO 2 ) reduction and biomethane (or value-added products) production. There are limited 34 literature critically reviewing the latest scientific development on the Bioelectrochemical (BES) based 35 biogas upgrading technology, including CO 2 reduction efficiency, methane (CH 4 ) yields, reactor 36 operating conditions, and electrode material tested in BES reactor. This review analyzes the reported 37 performance and identifies the crucial parameters to be considered for future optimization, which is 38 currently missing. In this review, the performances of BES approach of biogas upgrading under 39 various operating settings in particular fed-batch, continuous mode in connection to the microbial 40 dynamics and cathode materials have been thoroughly scrutinized and discussed. Additionally, other 41 versatile application options associated with BES based biogas upgrading, such as resource recovery, 42 are presented. The three-dimensional electrode materials have shown superior performance in 43 supplying the electrons for the reduction of CO 2 to CH 4 . Most of the studies on the biogas upgrading 44 process conclude hydrogen (H 2 ) mediated electron transfer mechanism in BES biogas upgrading. 45


Graphical Abstract
J o u r n a l P r e -p r o o f
A BES reactor is equipped with an anode for oxidation and a cathode for reduction which are typically divided by an ion-exchange membrane to transport the ions.The anode acts as a terminal electron acceptor where electroactive microorganisms oxidize the organic and inorganic material (Kaur et al., 2021;Logan, 2010).The harvested electrons are transported via an external circuit to the cathode, where they are used to reduce the targeted compound, thereby producing value-added chemicals and J o u r n a l P r e -p r o o f fuels (Logan and Rabaey, 2012).BES systems have been further proven to purify value-added multicarbon organic chemicals and fuels and tested for resource recovery such as nutrients, metal and energy recovery from the waste stream (Aryal et al., 2017;Nancharaiah et al., 2015;Rodríguez Arredondo et al., 2015).BES route of CH4 production utilizing CO2 reveals multiple benefits: (i) efficient conversion of waste CO2 to energy resources (ii) power-to-gas is possible integrating with renewable energy like wind and solar (iii) produced gas could be injected and stored into existing gas grid system (iv) CH4 can be directly utilized for transportation and renewable resources (v) utilization of existing energy infrastructure could save extra investment cost.
The first report on reducing CO2 into CH4 described dates back to 1987 where methanogens utilized the electron from elemental iron as electron donor (Daniels et al., 1987).The terminology 'electromethanogenesis' was reported, where electroactive methanogens catalyzed the CO2 reduction to produce CH4 by utilizing the electron from the cathode or reducing equivalents, e.g.H2 derived from the poised cathode (Cheng et al., 2009).The proof-of-concept of bioelectrochemical upgrading of biogas was demonstrated in 2014as a process attributed to CO2 reduction through direct cathodic electron transfer to electroactive methanogens (Xu et al., 2014).Since then, researchers have widely applied BES system to remove and utilize of CO2 to purify the biogas.
Within the recent five years, researchers have developed several laboratory-scale BES to demonstrate the utilization of CO2 fraction, electromethanogenesis activity, microbial dynamics, reactor design, electron transfer mechanism and resource recovery while purifying biogas to natural gas quality level.
Few reviews have been previously accomplished on biogas upgrading primarily dedicated to physical, chemical, hydrogen mediated microbial and algal-based biological upgrading.However, a critical review of the latest scientific research on the BES-based biogas upgrading technology is still missing.
Therefore, this report has provided novel information with the aim to summarize recent state-of-theart followed by discussions on the electrode materials that transfer electrons to electroactive CO2-J o u r n a l P r e -p r o o f reducing microbe, resource recovery, and future research prospective to overcome the BES technology bottleneck for biogas upgrading.Recent research publications have been selected for review by using biogas upgrading as a keyword, where secondary data collection and analysis were done.

IEM; ion exchange membrane
The most straightforward design so far is the single-chamber bioreactors used without membranes, which have simple reactor architecture and reduced capital costs (Lee et al., 2019).Nevertheless, the issue of oxygen (O2) contamination in the single-chamber system hampers the survival of methanogens.Therefore, most BES studies have been tested in a dual-chamber reactor separated by an ion-exchange membrane that facilitates the transfer of charged species (H + , Na + ) and acts as a separator to stop the crossover of bacterial liquid and O2 from anode to cathode chambers.Figure 1 (A, B, C & D) shows the representations of reactor configurations applied in bioelectrochemical biogas upgrading studies.The most commonly used reactor type is the H-shaped reactor, with two identical bottles or chambers (Fu et al., 2020).In double compartment systems, flat-parallel-plate is also used where parallel plates help to create a uniform electric field across the reactor and avoid electrochemical disturbances due to reactor geometry.However, the volatile fatty acid, VFA (acetate) accumulation and toxicity associated with pH fluctuation are experienced in single and double chamber reactors.In the single-chamber reactor, VFA (propionate and acetate) accumulation was observed that led to pH drop from 7 to 6; thereby, causing acidification which inhibit methanogens activity (Liu et al., 2017).Other operational parameters such as buffering capacity and partial alkalinity could support balancing the pH; however, high concentration causes acidification, resulting in the toxic condition for methanogens (Ahring et al., 1995;Murto et al., 2004).Furthermore, the addition of exogenous hydrogen inside the reactor could promote homoacetogenic activity; thereby, J o u r n a l P r e -p r o o f the accumulation of acetate and other VFA could occur.VFA accumulation reveals high acidogenic and acetogenic activities that cause the kinetic uncoupling between the acid producers (acidogens and acetogens) and acid consuming methanogens for biogas production (Murto et al., 2004).In single chamber, the transport of ions are not limited, and energy losses could be minimized due to no transport limitations.But unwanted oxidation reactions at the anode may also hamper the performance in single chamber.Double chamber reactor may demand slightly high energy input but the unwanted reactions can be minimized by creating only the cathodic condition.
With the aim of the BES up-scaling, three-compartment reactors having an accumulation chamber in-between anolyte and catholyte were also developed (Jin et al., 2017;Krieg et al., 2014;Zeppilli et al., 2017).The three-compartment system facilitated removing excess VFA and ions such as NH4 + , HCO3 -from either side, thereby overcoming the problem associated with VFA accumulation and toxicity as experienced in single and double chamber reactor configuration (Jin et al., 2017;Krieg et al., 2014;Zeppilli et al., 2017).Furthermore, above 90% of CO2 was removed from the biogas with the input of 0.9 kWh electricity per kg CO2 was reported that illustrates the superiority over single or double to multi-compartment configuration (Zeppilli et al., 2017).The electrical energy consumption in three-chamber BES can be invested simultaneously on chemical oxygen demand (COD) removal at the anode, CO2 removal at the cathode, and recovery of ammonium bicarbonate at the accumulation compartment.In another study, the three-compartment configuration was used with a two-side cathode and one anode compartment for the CO2 removal and reduction from biogas.At the same time, the transport of NH4 + from the anode to the cathode recovered nitrogen from AD digestate (see Figure 1c) (Zeppilli et al., 2019b).The two-side cathode in the three-chamber BES configuration showed higher performance by combining CH4 production, CO2 removal and high purity of ammonium recovery than the conventional systems used for each process separately.Three chamber BES technology should be comparable with the commercial biogas upgrading technology such as J o u r n a l P r e -p r o o f water scrubber.However, CH4 production rate of laboratory-scale reactors has not been tested at the demonstration scale to compare the economic feasibility; thus, the technology readiness level (TRL) is low (Aryal et al., 2021a).The multicompartment system are beneficial for multi-task purpose such as recovery of resources, however, the energy requirement in such system could be high.In addition, the reactor configuration may become complex.At the present stage of development of renewable energy technology, energy demand can be fulfilled with renewable sources (Gong et al., 2021).Thus, future research in BES field to incorporate renewable energy are recommended.
A further improvement on CH4 production was achieved by a microbial electrolytic capture, separation and regeneration cell (MESC) reactor consisting of four compartments, e.g.cathode, absorption, regeneration, and anode compartment-separated with bipolar membrane and anion exchange membrane as demonstrated in Figure 1 D (Kokkoli et al., 2018).Such an approach simultaneously treated the domestic wastewater in the anode compartment and CO2 removal at absorption compartment or reduced at the cathode, thereby improving the overall energy and process efficiency.Despite an increase in system complexity, the results from different studies suggest that the multi-compartment reactor configurations of BES in biogas upgrading can offer intrinsic advantages in i) simultaneous wastewater treatment at the anode and biogas upgrading ii) in-situ production of chemicals such as VFA, acetate iii) lowering of possible CH4 escape to the atmosphere while upgrading iv) recovery of CO2, CO3 2− and HCO3 − at regeneration and absorption chamber which can be further utilized v) further possibilities for easy modification (Jin et al., 2017;Kokkoli et al., 2018;Zeppilli et al., 2019bZeppilli et al., , 2017)).The superiority of multi-compartment systems over single and double-compartment has been demonstrated based on the CO2 removal, VFA accumulation, and pH regulations (Zeppilli et al., 2021b); nonetheless, further optimization have not been done yet.The currently tested multicompetent system are far behind for the upscaling due to high energy consumption, low mass transfer rate, difficulty to operate in continuous mode and electrode fouling, J o u r n a l P r e -p r o o f causing the low production rate; to overcome whichs some authors proposed tubular reactor systems.
Tubular reactor are still in early stage to test in BES based biogas upgrading.
Another researcher also elaborated the BES based CH4 enrichment by comparing the single and double chamber configuration (Liu et al., 2017).In contrast to previous studies, CH4 enrichment in double chamber (77% CH4) configuration was more profound than single chamber (56% CH4).The higher CH4 enrichment in a double-chamber is due to the alkalization of catholyte, promoting more CO2 removal from raw biogas.Also, the use of membrane limits the migration of O2 from the anolyte and hence restrict the loss of reducing equivalents at the cathode by maintaining anaerobic environment at the cathode (Rozendal et al., 2008).Furthermore, VFA accumulation in either single or multi-chamber BES reactor negatively affected CH4 upgrading (Liu et al., 2017).In a singlechamber system, propionate and acetate were gradually accumulated; thereby, pH dropped (Liu et al., 2017).In this aspect, ex-situ biogas upgrading would not suffer from the VFA accumulation as high organic loading is not available in BES for ex-situ biogas upgrading; but, it has not been experimentally demonstrated.Therefore, reactor operation mode, in particular, ex-situ, in-situ, batch, and continuous, have significant contributions to conclude reactor set-up.
The H-shaped reactors were frequently used in continuous and batch mode for biogas upgrading as shown in Table 1.To prove the concept of biogas upgrading in the bio-electrochemically assisted system, the authors used an H-type reactor where the membrane separated the anode and cathode compartment then compared it with the single chamber BES reactor in in situ and ex situ mode (Xu et al., 2014).In the case of in situ biogas upgrading, electrodes were directly inserted into the anaerobic digestion to stimulate the simultaneous anaerobic degradation of organic material and CO2 reduction into CH4.In ex situ systems, the biogas collected from AD is passed into the BES reactor, where CO2 from biogas is reduced to CH4 either directly accepting the electron from the electrode or indirectly through H2.A report compared in-situ and ex-situ biogas upgrading in the continuous and J o u r n a l P r e -p r o o f batch mode (Xu et al., 2014).It was claimed that the CO2 reduction rate was higher in in-situ than in ex-situ biogas upgrading based on the current density.The current density illustrated the amount of the charge utilized per unit of electrode for the reduction of CO2 where 0.4 A/m 2 current density was observed in ex-situ, almost half of the current density observed in in-situ 1 A/m 2 ; nevertheless, the charge transfer mechanism was not investigated thoroughly (Xu et al., 2014).Relatively, CO2 gasliquid mass transfer limitations caused lower current densities in the ex-situ system where CO2 produced in independent AD was bubbled into the cathode compartment (Xu et al., 2014).The in situ systems are not subjected to the same challenges of CO2 mass transfer because CO2 here is supplied by organic matter degradation co-occurring in the electrode chamber in the in situ configuration.
Likewise, single chamber in-situ reactor configuration has shown better performance in current density and biogas upgrading due to the availability of more nutrients and active biomass developed on the surface of the electrode (Krieg et al., 2014;Lee et al., 2019;Nogueira et al., 2003;Xu et al., 2014).In situ systems will have multiple anaerobic fermentations involved in addition to CO2 reduction.There could be multiple substrates available in the in situ process in addition to the electricity input.In ex situ, only CO2 reduction is targeted, and electricity remains the only energy source.These observations demonstrated that the operation mode is one of the driving factors for selecting of reactor.Furthermore, fundamental studies such as the investigation of electrode-microbes interaction, electron transfer mechanism, the impact of membranes, and explorations of electrochemical parameters need to perform in in situ vs ex situ to conclude.Additionally, other factors (CO2 utilization, pH, CH4 yield) should be considered before concluding the superiority of in situ over ex-situ.
Various types of membranes have been used in BES reactor, for example, proton exchange  , 2018;Zeppilli et al., 2017).However, the impact of membrane on biogas upgrading has not been investigated yet.
Table 1: Recent progress on microbial electrochemical approaches for biogas upgrading utilizing laboratory-scale reactor

Electrode materials and electron transfer mechanism
Various carbon-based electrodes such as carbon felt, carbon paper, carbon brush, carbon fiber, carbon brush, and reticulated vitreous carbon (RVC) have been employed to create three-dimensional structure for CO2 reduction from biogas as shown in Table 1.The three-dimensional architecture in the electrodes offers a high active surface area to facilitate microbial colonization and electrode interaction, thus possesses maximum electron transfer rate.Of these, carbon felt electrodes are the most common three dimensional electrode materials intensively explored in bioelectrochemical systems, particularly for electromethanogenesis, sensor, MFC and MES application (Table 1) (Geppert et al., 2016).Recently, researchers tested the graphite plate and carbon brush electrode for biogas upgrading, where CH4 formation from CO2 reduction at carbon brush was almost four-fold higher than the graphite plate cathode (Liu et al., 2020) that illustrated the topography of the electrode has a significant impact on CO2 reduction.biogas.Furthermore, direct electron transfer (DET) mechanism by an enriched mixed culture dominated by Methanothrix and Azonexus species were reported for CO2 reduction to CH4 (Yin et al., 2016).Direct flow of electrons to the electromethanogenesis activity could be more efficient than mediated electron transfer mechanism as DET does not involve the limitations pertaining to the redox activities and mass transfer of mediators (Yin et al., 2016).

J o u r n a l P r e -p r o o f
The electrochemical interaction of bacterial cells on the metal-carbon composite electrode further accelerated the hydrogen evolution reaction (HER), and it has been proposed that redox enzymes or biometals can enhance the HER (Aryal et al., 2019;Deutzmann et al., 2015).The study illustrated that the biological conversion of CO2 to CH4 has resulted from the reduction reaction with H2 while upgrading biogas (Bo et al., 2014).Several research reported cathodic H2 formation could be used in biological CO2 reduction and CH4 formation.High H2 production at cathode and simultaneous biogas upgrading was achieved when using metallic electrodes such as Platinum coated titanium woven wire mesh and stainless steel (Blasco-Gómez et al., 2017).While exploring H2 production from metal cathodes, metal-carbon composite electrodes of Cu-Ni and Fe coated onto graphite have also been utilized for biogas upgrading; however, the least insight has been given on the electrode performance (Park et al., 2018).In a related study, the metal-carbon composite electrode has been illustrated as one of the best electrode materials in BES (Gao et al., 2021;Zhou et al., 2020).
As another approach of biogas upgrading, CO2 can also be absorbed or converted to acetate or formate at the cathode.Thereby, separating CO2 from biogas can let the concentrated CH4 at the outlet gas.A report demonstrated microbial CO2 reduction to acetate in a biogas upgrading system by focusing on improving electrode design and operational parameters to utilize the CO2 from synthetic biogas containing 70:30 v/v CH4: CO2 (Jourdin et al., 2016).Briefly, the authors developed multi-walled carbon nanotubes (MWCNT) from electrodeposited reticulated vitreous carbon (RVC) cathode, J o u r n a l P r e -p r o o f which generates multiple layers of micro, meso and macropores to provide high surface area cathodes with three-dimensional architecture to increase the bacteria-material interaction (Jourdin et al., 2016).
The author reported 99% electron recovery to remove CO2 in the form of acetate when applying an applied cathode potential of −1.1 V vs SHE with achieved −200 Am −2 current density.A similar observation was reported when SnO2 nanoparticles were applied as biocathode to upgrade the biogas where CO2 was reduced to formate, stimulating the concentration of the 90% CH4 in off-gas stream (Gao et al., 2021).These studies show that electrode material development and its spatial surface modification are key strategies to optimize the electrode-microbe interactions, thereby reducing CO2 fraction from biogas while upgrading (Elsamadony et al., 2021).
Economically cheap and bioelectrochemically efficient electrode materials are required to reduce the capital expenditure (CAPEX) of the BES reactor.This can support the upscaling the technology.The cost of biogas upgrading by using physiochemical technology varies with CAPEX and operating expense (OPEX) (Angelidaki et al., 2018).A recent study revealed that around € 0.15/m 3 of CH4 was spent when 1000 m 3 /h biogas upgrading installation (conventional) was operated (IREA, 2017).
Nevertheless, the per-unit cost decreased with larger installation capacities.If the plant capacity is high then the overall investment for upgrading will also be high.But when the output quantity is high, then the investment cost per unit output (here per cubic meter methane) will decrease.(Sun et al., 2015) The economic assessment of BES technology is still the less researched; nevertheless, the economic evaluation reports of electricity production in MFC while treating municipality wastewater treatment has been available.It questioned the practical application to compete with conventional WWTP (Batlle-Vilanova et al., 2019).A report compared the different economic scenarios of the biogas upgrading process in BES, including the benefits of anodic chlorine production when combining wastewater treatment and biogas upgrading.The authors highlighted that the multiple purpose use of the BES system could gain an economic advantage in the future; nevertheless, the J o u r n a l P r e -p r o o f BES system has shown the least economic potential in the present scenario (Batlle-Vilanova et al., 2019).Future research has to focus on enhancing CH4 production rates applying cheap renewable energy to compete with traditional commercial biogas upgrading plants.In another study, economic feasibility of BES technology was compared at the different scenarios of cost-benefit assessment for chemical synthesis (acetic acid in particular) in MES technology from CO2 (Christodoulou and Velasquez-Orta, 2016).Additionally, the case for the coupling of MES and AD were also presented considering the CO2 conversions to acids (Christodoulou and Velasquez-Orta, 2016), but CO2 utilization for CH4 production was not explicitly analyzed.Likewise, the sustainability assessment aspect of acetate production from CO2 in BES reported that significant improvement in production rate is essential to compete with fossil-based technology (Gadkari et al., 2021).The BES currently has a low technology readiness level (TRL); therefore, significant development is necessary before competing with current commercial physiochemical biogas upgrading technologies (Aryal et al., 2021b).

Adding value to the biogas upgrading: recovery of resources and ions
BES offers the unique capability to recover resources from wastes.For example, organic and biomass can be converted into electricity in anode, while nutrients and metals can be recovered at cathode (Colombo et al., 2017;Patel et al., 2021).Recently, researchers demonstrated the recovery or removal of nitrogen and phosphorus from the waste stream through nitrification and bioelectrochemical denitrification (Bajracharya et al., 2016a;Zeppilli et al., 2019bZeppilli et al., , 2017)).
Recovering nitrogen from waste is a sustainable approach to maintain nutrient cycling, which could minimize the cost of nitrogen fixation.The bioelectrochemically recovered nitrogen is an excellent source of nutrients that can be utilized as fertilizer in the agriculture field.The nitrogen recovery could be in the form of ammonium.Ammonium ions (NH4 + ) can moves across the ion exchange membranes due to either current-driven or diffusion-driven migration.It has been reported that higher J o u r n a l P r e -p r o o f current density could enhance NH4 + transportation to the cathode compartment due to electricitydriven migration, though the high pH of the catholyte can drive ammonia to escape during the recovery process (Kelly and He, 2013).
In multi-tasking approach of bioelectrochemical biogas upgrading, resource recovery was introduced where nutrients from anode side was recovered while upgrading the biogas at cathode (Zeppilli et al., 2017).The middle accumulation compartment was separated with anode compartment by a CEM, while the cathode compartment was separated by an AEM.The NH4 + ions migrate from anolyte to the accumulation compartment through the proton exchange membrane due to electricity-driven migration.Likewise, CH3COO -and HCO3 -migrate from catholyte to the accumulation chamber through anion exchange membrane.Moreover, the same research group has modified the reactor configuration by placing double cathode system to recover better NH4 + , CH3COO -and HCO3 -without compromising the purity of CH4 enrichment (Zeppilli et al., 2019b(Zeppilli et al., , 2017).Yet another study reported a recovered CO2 using a three-compartment reactor system where CO3 -2 and HCO3 -at cathode compartment migrate through anion exchange membrane to the middle regeneration compartment that allows the recovery of pure CO2 recovery while upgrading biogas (Jin et al., 2017).In the following study, the same research group recovered CO2 in situ and then regenerated it via alkali and acid regeneration while treating wastewater in anode compartment and biogas upgrading at cathode (Kokkoli et al., 2018).It was also reported that the anodic chloride oxidation reaction is 45% less energy demanding compared to water oxidation (Du et al., 2015) .Thus, anodic chlorine production while transforming the CO2 containing effluent (e.g.biogas or wastewater) in the cathode of a BES into CH4 is a better alternative to recover the disinfecting agent (Batlle-Vilanova et al., 2019).Authors further claimed that BES reactor has the potential to compensate current physiochemical biogas upgrading system because it can in-situ generate necessary chemicals, in particular acid and alkaline.

J o u r n a l P r e -p r o o f
The possibility of simultaneous biogas upgrading and resource recovery has been well illustrated via bioelectrochemical approaches.
Another report demonstrated the simultaneous sulfur (S 0 ) recovery from anodic oxidation of H2S and cathodic biogas upgrading from CO2 reduction (Fu et al., 2020).The author proposed applying electron shuttle Fe2 + /Fe3 + at anode to continuously drive the H2S oxidation and energy conservation in the BES reactor.Similarly, an electromethanogenic microbial electrolysis cell (MEC) connected to an ammonia recovery system based on hydrophobic membranes (ARS-HM) was tested to recover ammonia from anodic compartment (NH4 + -N), while upgrading biogas at the cathode (Cerrillo et al., 2021).The recovered ammonia supported to regulate pH value that boosted CH4 production rate almost two folds (Cerrillo et al., 2021).Therefore, BES is a reliable technology for simultaneous biogas purification and resource recovery; still, the profitability of recovered ions have not been compared yet.

Reactor operation modes
BES for biogas upgrading is currently limited to the laboratory scale reactor that has been operated in fed-batch and continuous operational mode, as shown in Table 1.Most of the BES reactors were operated in batch; conversely, continuous operation mode was reported superior over batch and fedbatch mode.The CH4 production rate was increased three times when the BES reactor was shifted from batch mode to continuous mode (Batlle-Vilanova et al., 2015).The continuous and gas recirculation modes are likely to supply substrate CO2 to support overcoming the mass transfer limitations (Bajracharya et al., 2022;Bian et al., 2021).The continuous proton production at the anode resulted in acidification.In contrast, alkalization at cathode due to the reduction reaction (consumption of proton) created the pH gradients, which could deteriorate the biofilm activities of electromethanogens (Torres et al., 2008b).Apart from biofilm stability, a high pH gradient could cause increased power consumption where 59 mV for each unit of pH drop was reported, which J o u r n a l P r e -p r o o f explains the overpotential of the system and energy loss for CO2 reduction (Sleutels et al., 2009).
Therefore, adding an acid or CO2 was proposed as an alternative approach to compensate for potential loss associated with the pH gradient over the cathode and anode compartment (Torres et al., 2008a).
Other operating parameters such as applied voltage, VFAs profile during start-up period and pH may also directly impact on biogas upgrading (Park et al., 2018).The BES system accelerated the CH4 production in AD utilizing the accumulated H2 ions and VFAs degradation during the start-up period (Park et al., 2018).Another report applied 1 V of cell voltage in a single-chamber BES system to observe the CH4 upgrading in different substrate conditions (Lee et al., 2019).The CH4 production rate was doubled compared to the control; nevertheless, the stability of AD and the system performances was found heavily dependent on pH, VFAs profile and applied potential.A similar observation was reported when the applied cell voltage was increased from 0 to 4 V; the maximum CH4 content was reached 97.9% from the initial 60% CH4, thereby reported cell potential dependency for biogas upgrading (Zhou et al., 2020).Not limited to the cell potential of the BES reactor, the mode of electronic operation (potentiostat and galvanostatic) has a significant impact, especially in the multi-compartment reactor.Another group of researchers operated the simultaneous biogas upgrading and ammonium recovery using a three-compartment BES reactor.The author reported improvement in the current draw by galvanostatic operation, promoting CO2 removal by 113 % compared to the potentiostatic condition (Zeppilli et al., 2021a).The polarization control to cathode rather than anode also promotes the CH4 generation while using system to treat chemical oxygen demand at anode and biogas upgrading at cathode (Zeppilli et al., 2019a).

Microbial communities in biocathode for methane enrichment
Microbial biogas upgrading requires the removal and consequent reduction of CO2 to CH4 by adding electron sources such as H2.However, inappropriate H2 addition in AD process could accumulate the J o u r n a l P r e -p r o o f VFA that shifts the microbial dynamics, whereas low concentration favors stable dynamics to reduce CO2 or CH4 formation.Towards such argument, a report compared the effect of H2 addition in in situ biogas upgrading reactor and BES reactor (Tartakovsky et al., 2021).Due to the addition, of exogenous H2, accumulation of 6 g/l acetate was observed in the in situ biogas upgrading reactor.In contrast, acetate accumulation was not observed in the BES reactor, and, thereby, superior performance of the BES reactor was claimed (Tartakovsky et al., 2021).In-situ H2 generation by applying the BES could increase hydrogenotrophic methanogenic activity, thereby modifying the microbial dynamics (Cerrillo et al., 2021;Gao et al., 2021).The poised electrode in the BES contained significantly higher cathode-associated biomass, which was confirmed from protein analysis of biofilms developed at the cathode.The hydrogenotrophic methanogenic species were observed as the main dominating species in the microbial community in 16S rRNA sequences analysis (Bo et al., 2014).
Methanobacterium remained the most abundant species at cathode when the reactor was operated in continuous and batch mode, as shown in Table 1.Microbial community analysis of biogas upgrading cathode reported that the relative dominance of Methanospirillum was increased from 16.0 to 68.4% when electrode potential was increased (Bo et al., 2014).Similarly, molecular biology investigation based on qPCR studies showed that the MEC coupling in AD did not significantly impact acetoclastic methanogens (Bo et al., 2014).Acetoclastic methanogens split acetate into CH4 and CO2.The persistence of acetoclastic methanogens could be due to the availability of acetate; even at autotrophic conditions, CO2 was metabolized by other microbial consortiums (homoacetogens) to acetate which become available for further utilization by acetoclastic methanogens.In contrast, the abundance of hydrogenotrophic methanogens such as Methanomicrobiales and Methanobacteriales was enhanced up to 17.2 folds (Gajaraj et al., 2017).Thus, selective enrichment of hydrogenotrophic methanogens due to hydrogen production via bioelectrochemical pathways can be inferred.

J o u r n a l P r e -p r o o f
Moreover, applying highly negative potential benefits hydrogenotrophic methanogens activity because of the associated in-situ H2 evolution.A report observed enriched species of Azonexus (nitrogen-fixing bacteria) by 42% at cathode and <0.5% in the bulk sludge when -500 mV potential was applied at cathode.Interestingly, Azonexus species dropped by 28% within 9 days when external voltage supply was removed; this shows the microbial dynamics under the selectivity imposed due to externally applied voltage (Liu et al., 2019).In most research, hydrogenotrophic methanogens were dominated due to hydrogen production from the externally applied voltage; however, selectivity of acetoclastic methanogens was not observed during biogas upgrading.

Prospective and Challenges
The prospect of the technology is encouraging as it is a sustainable platform for CO2 utilization and biomethane synthesis, but the following prospective and challenges were identified.

I.
Achieving high coulombic efficiency with low overpotential are the main challenges for the economic operation of biogas upgrading in the bioelectrochemical reactor.To tackle the challenge, BES requires improvements by establishing synergy from other scientific disciplines.Hence research has to be focused on improving CO2 reduction rate by selecting/enriching the microbiome of active species of microbes (Jiang et al., 2019;Kracke et al., 2019).Exotic microbial habitats of chemolithotrophs such as deep saline sediments (Alqahtani et al., 2020(Alqahtani et al., , 2019) ) and underground caves and mines etc. could be explored to find new active microbial species as biocatalysts.In another case, metabolically engineered species can also be developed to enhanced electroactivity and CO2 uptake capacity of the microorganisms.Further studies on the molecular mechanism and metabolic process of CO2 capturing and conversion are required to advance in metabolic engineering attempts.
J o u r n a l P r e -p r o o f II.
Multi-compartment reactor configurations have shown 100% CH4 enrichment (Kokkoli et al., 2018).However, optimal operational parameters must be investigated to achieve high rate biomethane production with simultaneous recovery (ions) and efficient CO2 regeneration.Insitu and ex-situ approaches of biogas upgrading were widely applied where CH4 enrichment up to natural gas quality was not achieved in most of the study; therefore, the synergy of insitu and ex-situ (Hybrid) could support further CO2 utilization into CH4 (Corbellini et al., 2018).At the same time, the reactor design needs to be adapted to ensure an adequate supply of CO2 when the system is scaled up.A high rate of uptake and conversion of CO2 with stable performance and lower cell overpotentials have to be considered when practical applications are developed.The energy supply to produce electrons and H2 has been claimed from renewable sources; however, real integration of renewable electricity and BES reactor is not yet reported.In the future, integration scenarios of BES reactor and renewable electricity should be done rather than commercial H2 gas or commercial power supplies.

III.
The combination of BES technology and fermentation technology was claimed for the valueadded bioprocess utilizing electrical energy (Christodoulou et al., 2017).The utilization of CO2 from biogas as a carbon source in electrochemically assisted microbial fermentation may overcome the logistic and economic challenges.A CO2 utilization from biogas to produce the economically attractive commodities in the bioelectrochemical system, leaving behind the enriched CH4, can attract the developers to build commercial applications. IV.
In addition, the membrane, the reactor design and electrode materials and how they can be arranged in the reactor (surface area/volume ratio) on a pilot or large scale are still unknown.
Membrane materials limit the CAPEX and OPEX of BES while up-scaling technology.
Relatively cheap membranes such as agar-containing membranes could replace costly conventional membranes (Hernández-Flores et al., 2016).The controlling cathode potential J o u r n a l P r e -p r o o f has a significant contribution to operational costs.Further research to control the electric potential losses such as ohmic losses, charge transfer resistance, and pH gradient should be improved for up scaling the technology.
V. A new fermentation platform producing high-value products (sucrose, biofuels, biopolymers, proteins, and enzymes) apart from CH4 can be promising in bioelectrochemical biogas upgrading.Thus, product diversification in bioelectrochemical reduction process could strengthen the technology for the commercially viable application.Nonetheless, downstream recovery of the product is still challenging.

VI.
The CH4 production rate was increased up to 12.5 L CH4/L/d, by using redox flow battery design which was claimed the highest production rate in bioelectrochemical system ; but, CO2 was supplied instead of biogas upgrading (Geppert et al., 2019).That illustrated the reactor design has significant role to optimize the process therefore redox flow battery design can be used for biogas upgrading (Bajracharya et al., 2016b;Geppert et al., 2019).Porous hollow fiber cathode designs have also been shown effective for delivering CO2 immediately at the reduction site of the cathode thereby avoiding mass transfer limitation (Alqahtani et al., 2020(Alqahtani et al., , 2018;;Bian et al., 2018) Biogas upgrading using such electrode in reactor set would be attractive to upgrade biogas.The electron transfer mechanism in microbial electrocatalysis has been poorly known among methanogens.Thus, multidisciplinary knowledge to engineer the electrode, configure the reactor system and energy supply that allows microbial interaction to enhance the electron transfer should be acquired (Singh et al., 2020).

Conclusion
This review provides an overview of research advances in microbial electrochemical approaches for biogas upgrading technology in different operating conditions, in particular in-situ, ex-situ, batch mode, continuous mode.Briefly, reactor configuration, electrode materials used for CO2 reduction to J o u r n a l P r e -p r o o f CH4 have been thoroughly summarized.Additionally, the possibility of integration of bioelectrochemical biogas upgrading with multiple applications, such as nutrient and resource recovery, are presented.The cathode material has significant influences on reactor performances, and the dynamics of the microbial community at biocathode can be controlled with the applied voltage.
Further understanding and studies on the coupling of AD and bioelectrochemical system is important for the advancement to real scale applications.The prospect of the technology is encouraging as it is a sustainable platform for CO2 utilization and biomethane production.

2.Figure 1 :
Figure 1: Various reactor configurations used in microbial electrochemical approaches to membranes (PEM), anion-exchange membranes (AEM) and bipolar membranes, which are different in the limitation of O2 diffusivity from the anode to cathode (Batlle-Vilanova et al., 2019; Kokkoli et J o u r n a l P r e -p r o o f al.

Figure 2 :
Figure 2: Mechanism of electron transfer from biocathode in MES for methane production This work was supported by Regionalt Forskningsfond Vestfold Telemark (RFFVT), for AddCar project and the strategic research plan "Energy and Climate Challenge" of University of South-Eastern Norway.Zhang Y. thanks the Carlsberg Foundation for the Distinguished Fellowships grant (CF18-0084), Denmark J o u r n a l P r e -p r o o f r n a l P r e -p r o o f

Table 1 :
Recent state-of-art for biogas upgrading in laboratory scale BES system Anaerobic osmotic membrane bioreactor, ARS-HM: Ammonia recovery system based on hydrophobic membranes AD: Anaerobic digestion, MWWTP: Municipal Waste Water Treatment Plant, WWTP: Waste Water Treatment Plant, MFC-Microbial Fuel Cell, BES: Bioelectrochemical System, MSW: Municipal Solid waste, MESC: Microbial electrolytic capture, separation and regeneration cell, UASB: Upflow anaerobic sludge blanket digestion, FWTP: Food Waste Treatment Plant, MWCNT: Multiwall carbon nanotube, RVC reticulated vitreous carbon, ng: Not given, Cu: Copper, Ni: Nickel, Pt: Platinum, Fe: Iron, Ti: Titanium J o u r n a l P r e -p r o o f