**3. BES for wastewater valorization**

#### **3.1. BES for wastewater treatment and power production**

First published studies on MFC and MEC technologies were mainly devoted to further the understanding of how operational parameters (pH, temperature, etc.) affect their performance and to the development of new reactor configurations and new strategies to optimize their figures of merit. Most of these studies were carried out using synthetic media as electrolytes to allow researchers to keep control over substrate composition. Subsequent laboratory tests with real wastewaters served to gain knowledge of the real world potential of MEC and MFC. These studies helped to quantify to which extent reactors performance gets affected by the presence of a real substrate. For instance, MFC fed with actual wastewater produced power densities (normalized to the surface are of the electrodes) in the range of several tens of mW·m−2 (milliwatts per square meter of electrode) [16], which contrast with the hundreds [17] and even thousands of mW·m−2 [18] achievable with synthetic effluents. Despite recent advances in electrode materials and reactor configurations [19], power production in MFC has not improved significantly. Issues such as low conductivity and low buffer capacity are often cited as the main factors that explain the observed poor performances [20]. Unquestionably, MEC has to face similar challenges, although economic feasibility criteria seem to be less stringent [21]. For instance, it has been estimated that the target total internal resistance for MEC technology to be cost effective is 80 mΩ·m−2, while for MFC this target becomes much more restrictive (40 mΩ·m−2) [21]. Moreover, the difference in architecture between a microbial MFC and MEC poses further problems with scaling up. With a pilot scale MFC, aeration to the cathode invites complex issues, as either the cathode cell must be open to the air or there is an added cost of aeration. However with an MEC, the cathode is anaerobic, making the design of a larger system simpler, all of which outlines a more favorable scenario for MEC.

content of WW [26]. This manifests more clearly if we bring to mind that conventional wastewater treatment plants do not only make use of this potential but also demand large amounts of energy. For example, in Spain, wastewater treatment accounts for approximately 1% of the total energy consumed [27]. BES would enable using the wastewater treatment plants not only as facilities for water contamination removal but also as an electrical regulation system, stocking the surplus of energy in the grid as hydrogen or methane [28]. In this sense, redox flow batteries (RFB), which may become a strategic partner in MEC implantation [29], could offer new possibilities to enhance energy management capabilities of MEC. RFB are electrochemical energy storage devices with a quite interesting feature: they allow to decouple power from energy output [30]. Thus, the reducing power obtained in MEC reactors treating waste streams could be stored in an electrolyte solution that would be easily used for electricity generation or in a subsequent electrochemical process. Well-known RFB, such vanadium systems, presents serious environmental pitfalls for its use in massive energy storage applications [31], but recent approaches, like all-organic RFB [32], could open a wider field. Here, the main candidates are quinone-based molecules [33], which, interestingly, have been previously studied in BES as they

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131

Together with urban wastewater treatment facilities, industrial environments represent another clear niche for potential application of MECs. In general, hydrogen production and energy consumption in MEC improve as the organic concentration in the WW increases. In this sense, industrial wastewater, which usually contains high organics concentration, is an ideal waste effluent for MEC. For instance, by feeding a MEC with the effluent of an ethanol-producing reactor, Lue et al. [35] achieved an impressive hydrogen production rate of

in MEC may present significant shortcomings. On the one hand, sometimes their composition is not well balanced and may require some nutrient amendment [36], which may not always result in an economically or environmentally feasible approach. On the other hand, dealing with high organic contraction, wastewater in MEC may become an important challenge as it may favor the proliferation of undesired microorganism that could hinder the activity of the

quent sequestration (mainly in geological or deep sea storage sites) has been put forward as

arouses significant public concern [39]. Another alternative more favorable to public opinion

stock for industrial processes. In this regard, one among many possibilities is the conversion

(most oxidized) form of carbon, and thus, a substantial amount of energy is required to convert it into useful reduced chemicals. For instance, the Sabatier process that converts CO2

into valuable chemicals or fuels. The main issue here is that CO<sup>2</sup>

R·d−1 with an electrical energy efficiency of 287%. Still, the use of industrial effluents

concentration is widely seen as the main driving factor behind

accumulation in the atmosphere [38], although this approach

emissions is the use of this gas as a feed-

is the least energetic

capture at large emission sources (such as power plants) and subse-

may act as intermediates in electron transfer mechanisms [34].

~2 LH2·L−1

electrogenic microorganisms [11].

Increasing atmospheric CO2

a suitable strategy to limit CO2

climate change [37]. CO2

 **valorization**

and that may help to offset the cost of reducing CO<sup>2</sup>

**4. BES for CO2**

of CO2

#### **3.2. BES for wastewater treatment and chemical energy storage**

Due to its ubiquitousness and the enormous volumes produced globally each year [22], urban wastewater is perhaps the most straightforward waste stream for MECs. The first MEC operated on urban waste water (batch with retention times between 30 and 108 h) offered quite promising results in terms of organic contamination removal (almost 100% removal efficiency), although hydrogen recovery was relatively low (~10% of the theoretical maximum) [23]. In a later study, using urban wastewater as fed as well, it was possible to produce hydrogen at a rate of 0.3 LH2·L−1 R·d−1 (liter of hydrogen per liter of reactor per day) in a 100 mL (total volume) continuously operated MEC (hydraulic retention times between 3 and 24 h) [24], and energy consumption figures (~1.5 Wh·g·COD−1 (watts-hour per gram of chemical oxygen demand removal)) were similar to those typically found for conventional wastewater technologies [25]. However, when this same MEC design was replicated to a larger scale (3.3 L), hydrogen production declined significantly (0.01 LH2·L−1 R·d−1) and energy consumption rocketed. In a more recent study performed at a higher scale (130 L) it has been reported energy recoveries of up to 121% with respect to the electrical input [3]. Despite the still important challenges that MECs need to overcome [11], these figures highlight the potential of this technology for exploiting the often untapped energy content of WW [26]. This manifests more clearly if we bring to mind that conventional wastewater treatment plants do not only make use of this potential but also demand large amounts of energy. For example, in Spain, wastewater treatment accounts for approximately 1% of the total energy consumed [27]. BES would enable using the wastewater treatment plants not only as facilities for water contamination removal but also as an electrical regulation system, stocking the surplus of energy in the grid as hydrogen or methane [28]. In this sense, redox flow batteries (RFB), which may become a strategic partner in MEC implantation [29], could offer new possibilities to enhance energy management capabilities of MEC. RFB are electrochemical energy storage devices with a quite interesting feature: they allow to decouple power from energy output [30]. Thus, the reducing power obtained in MEC reactors treating waste streams could be stored in an electrolyte solution that would be easily used for electricity generation or in a subsequent electrochemical process. Well-known RFB, such vanadium systems, presents serious environmental pitfalls for its use in massive energy storage applications [31], but recent approaches, like all-organic RFB [32], could open a wider field. Here, the main candidates are quinone-based molecules [33], which, interestingly, have been previously studied in BES as they may act as intermediates in electron transfer mechanisms [34].

Together with urban wastewater treatment facilities, industrial environments represent another clear niche for potential application of MECs. In general, hydrogen production and energy consumption in MEC improve as the organic concentration in the WW increases. In this sense, industrial wastewater, which usually contains high organics concentration, is an ideal waste effluent for MEC. For instance, by feeding a MEC with the effluent of an ethanol-producing reactor, Lue et al. [35] achieved an impressive hydrogen production rate of ~2 LH2·L−1 R·d−1 with an electrical energy efficiency of 287%. Still, the use of industrial effluents in MEC may present significant shortcomings. On the one hand, sometimes their composition is not well balanced and may require some nutrient amendment [36], which may not always result in an economically or environmentally feasible approach. On the other hand, dealing with high organic contraction, wastewater in MEC may become an important challenge as it may favor the proliferation of undesired microorganism that could hinder the activity of the electrogenic microorganisms [11].

#### **4. BES for CO2 valorization**

**3. BES for wastewater valorization**

130 Energy Systems and Environment

**3.1. BES for wastewater treatment and power production**

**3.2. BES for wastewater treatment and chemical energy storage**

of 0.3 LH2·L−1

nificantly (0.01 LH2·L−1

First published studies on MFC and MEC technologies were mainly devoted to further the understanding of how operational parameters (pH, temperature, etc.) affect their performance and to the development of new reactor configurations and new strategies to optimize their figures of merit. Most of these studies were carried out using synthetic media as electrolytes to allow researchers to keep control over substrate composition. Subsequent laboratory tests with real wastewaters served to gain knowledge of the real world potential of MEC and MFC. These studies helped to quantify to which extent reactors performance gets affected by the presence of a real substrate. For instance, MFC fed with actual wastewater produced power densities (normalized to the surface are of the electrodes) in the range of several tens of mW·m−2 (milliwatts per square meter of electrode) [16], which contrast with the hundreds [17] and even thousands of mW·m−2 [18] achievable with synthetic effluents. Despite recent advances in electrode materials and reactor configurations [19], power production in MFC has not improved significantly. Issues such as low conductivity and low buffer capacity are often cited as the main factors that explain the observed poor performances [20]. Unquestionably, MEC has to face similar challenges, although economic feasibility criteria seem to be less stringent [21]. For instance, it has been estimated that the target total internal resistance for MEC technology to be cost effective is 80 mΩ·m−2, while for MFC this target becomes much more restrictive (40 mΩ·m−2) [21]. Moreover, the difference in architecture between a microbial MFC and MEC poses further problems with scaling up. With a pilot scale MFC, aeration to the cathode invites complex issues, as either the cathode cell must be open to the air or there is an added cost of aeration. However with an MEC, the cathode is anaerobic, making the design of a larger system simpler, all of which outlines a more favorable scenario for MEC.

Due to its ubiquitousness and the enormous volumes produced globally each year [22], urban wastewater is perhaps the most straightforward waste stream for MECs. The first MEC operated on urban waste water (batch with retention times between 30 and 108 h) offered quite promising results in terms of organic contamination removal (almost 100% removal efficiency), although hydrogen recovery was relatively low (~10% of the theoretical maximum) [23]. In a later study, using urban wastewater as fed as well, it was possible to produce hydrogen at a rate

uously operated MEC (hydraulic retention times between 3 and 24 h) [24], and energy consumption figures (~1.5 Wh·g·COD−1 (watts-hour per gram of chemical oxygen demand removal)) were similar to those typically found for conventional wastewater technologies [25]. However, when this same MEC design was replicated to a larger scale (3.3 L), hydrogen production declined sig-

at a higher scale (130 L) it has been reported energy recoveries of up to 121% with respect to the electrical input [3]. Despite the still important challenges that MECs need to overcome [11], these figures highlight the potential of this technology for exploiting the often untapped energy

R·d−1 (liter of hydrogen per liter of reactor per day) in a 100 mL (total volume) contin-

R·d−1) and energy consumption rocketed. In a more recent study performed

Increasing atmospheric CO2 concentration is widely seen as the main driving factor behind climate change [37]. CO2 capture at large emission sources (such as power plants) and subsequent sequestration (mainly in geological or deep sea storage sites) has been put forward as a suitable strategy to limit CO2 accumulation in the atmosphere [38], although this approach arouses significant public concern [39]. Another alternative more favorable to public opinion and that may help to offset the cost of reducing CO<sup>2</sup> emissions is the use of this gas as a feedstock for industrial processes. In this regard, one among many possibilities is the conversion of CO2 into valuable chemicals or fuels. The main issue here is that CO<sup>2</sup> is the least energetic (most oxidized) form of carbon, and thus, a substantial amount of energy is required to convert it into useful reduced chemicals. For instance, the Sabatier process that converts CO2

and H2 into CH4 requires high temperature and pressure [40], which usually implies significant energy requirements. Here, BES can provide a suitable alternative through a bioprocess known as microbial electrosynthesis (MES), which requires much milder conditions. MES is a proof-of-concept technology that converts CO2 into valuable chemicals and fuels within a BES. MES meets the requirements of green chemical technologies as it uses microorganisms as inexpensive and sustainable catalysts, can be operated at ambient conditions, and can be feed with contaminants (CO2 ) for environmental remediation [41]. Later, we review some of the target products that can be obtained through MES, paying special attention to alcohols and methane, as these molecules can be easily used as energy carriers in many applications.

rates of 30.3 LCH4·d−1·m−2 and efficiencies near 100% [50], yielding high grade gas streams with methane content over 95%, thus opening the opportunity for injection in the natural gas grids. Another interesting application of these bioelectromethanogenic systems is biogas upgrading. The biogas produced during anaerobic digestion of wastes usually contains large amounts of

 (typically 30–40%), and so to improve its energy value some kind of refinement is usually required [51]. MES can become an alternative to membrane, absorption, or scrubbing units to

biogas permanently below 10%, showing efficiencies over 80% [51, 52]. Although anaerobic digestion and BES have been traditionally seen as two bio-based technologies competing for the same application niches, here we see that both AD and BES can be integrated to overcome

These studies are revealing that microbial catalyzed reactors are capable of producing a wide spectrum of chemical building blocks, leading to the basis of a renewable chemical platform which might be the future substitute of petroleum-based chemistry. The interaction between distributed electricity grids into delocalized chemical production facilities is extremely attrac-

Plant macronutrients—mainly nitrogen, phosphorus, and potassium—are indispensable elements for the growth of every living beings. However, when supplied to the soil as chemical fertilizers, they can bring about serious environmental issues (e.g., eutrophication of water bodies and greenhouse gases emissions). World population growth and the ever-increasing demand for agricultural products urge the need for an adequate use of fertilizers to avoid not only their undesired environmental impact but also an eventual depletion of the limited mineral deposits, especially phosphate rock [53]. Furthermore, fertilizers production consumes significant amounts of energy, which brings additional environmental and economic concerns. For instance, 3% of total natural gas production in United States is diverted to the fertilizers industry [12], and fertilizers application in China absorbs 4.4% of China's total primary

Nutrients recovery from wastes can prove to be a feasible strategy to tackle both environmental and energy issues simultaneously. On the one hand, it allows to limit the amount of nitrogen and phosphorus discharged into the environment, and on the other hand, it may help to reduce the energy intensity in fertilizers production. There are several technologies available for nutrients recovery from organic wastes, among which struvite precipitation occupies a preeminent position [54]. BES can also offer the possibility of recovering nutrients from waste streams, and thanks to their ability for harnessing the bioenergy present in the organic matter, they also help to offset the energy usage [55]. Most of the research studies in the literature are mainly focused on the use of nitrogen and phosphorus recovery [56], as these two nutrients

studies on using MES for biogas upgrading have been able to keep CO<sup>2</sup>

from the biogas with the advantage of converting it into more methane. Current

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content in the off-

133

CO2

remove CO2

energy use [13].

are usually found in many organic wastes.

some of their inherent limitations.

tive, giving a new extension to the biorefineries concept.

**5. BES for nutrients recovery from waste streams**

*Volatile fatty acids* (*VFA*). VFA are the most studied group of chemicals synthesized in MES These slightly reduced organic compounds can be produced from CO2 by a wide variety of microorganisms following different metabolic pathways. Homoacetogens in particular are responsible for acetate production, and recent studies have reported acetic acid production rates above 0.78 g·L−1·d−1, product titers of up to 13.5 g·L−1, and current-to-acetate conversion efficiencies of 99% [42, 43]. Butyric acid is the second most reported VFA on MES systems. This C4 VFA has mainly been observed as a co-product in acetic acid production systems, although some studies also target this product as main objective [44]. Among the later, production rates of up to 0.16 g·L−1·d−1 and maximum titers of 5.5 g·L−1 at relatively low conversion efficiencies (40%) have been achieved [45]. Other VFAs such as propionic, isobutyric, or medium chain fatty acids have been found as by-products in lower concentrations in acetic/butyric acid-producing MES systems showing low energy efficiency so far [44].

*Alcohols*. Bioethanol is a renewable fuel subjected to strong controversy as its production is related to deforestation and to the rising of food prices. MES offers the opportunity of obtaining ethanol from CO2 -rich streams thus avoiding any concern derived from land-use changes. Although ethanol can be produced directly from CO2 , its production from acetate is thermodynamically and energetically more favorable, providing that the undissociated acid exists in a slightly acidic medium [46]. Still, ethanol production through MES is far from becoming a feasible process yet as titers and efficiencies are relatively low (up to 0.5 g·L−1 and 55%, respectively) [46]. Butanol can also be produced in MES following a similar pattern as in ethanol (butanol begins to appear in butyric acid-producing MES when sufficient reducing agents are present). Butanol has been found as a by-product but not targeted as main product until date, and therefore, the low rates and efficiencies are not representative of its real future potential yet [44]. Other alcohols have been obtained through MES. Arends et al. [47] firstly produced isopropanol in a CO2 -fed continuous MES system, achieving titers up to 0.82 g·L−1. Soussan et al. [48] reported the possibility of producing glycerol when succinate was present together with CO2 in the feed. About 100% CO<sup>2</sup> -glycerol selectivity was achieved with titers from 6.0 to 9.0 mM.

*Methane. Bioelectrochemical power-to-gas.* Methane is a valuable energy carrier and a fuel with low environmental impact compared to other fossil fuels. It can be produced from CO2 through MES by taking advantage of the ability of some microorganisms (e.g., methanogenic archaea) of using a solid surface (cathode) as electron donor. First published experiences reported production rates of 4.5 LCH4·d−1·m−2 and efficiencies up to 80% [49]. More recent studies have succeeded in improving the overall efficiency reaching maximum production rates of 30.3 LCH4·d−1·m−2 and efficiencies near 100% [50], yielding high grade gas streams with methane content over 95%, thus opening the opportunity for injection in the natural gas grids.

Another interesting application of these bioelectromethanogenic systems is biogas upgrading. The biogas produced during anaerobic digestion of wastes usually contains large amounts of CO2 (typically 30–40%), and so to improve its energy value some kind of refinement is usually required [51]. MES can become an alternative to membrane, absorption, or scrubbing units to remove CO2 from the biogas with the advantage of converting it into more methane. Current studies on using MES for biogas upgrading have been able to keep CO<sup>2</sup> content in the offbiogas permanently below 10%, showing efficiencies over 80% [51, 52]. Although anaerobic digestion and BES have been traditionally seen as two bio-based technologies competing for the same application niches, here we see that both AD and BES can be integrated to overcome some of their inherent limitations.

These studies are revealing that microbial catalyzed reactors are capable of producing a wide spectrum of chemical building blocks, leading to the basis of a renewable chemical platform which might be the future substitute of petroleum-based chemistry. The interaction between distributed electricity grids into delocalized chemical production facilities is extremely attractive, giving a new extension to the biorefineries concept.
