1. Introduction
Globally, over 3.50 × 10
11 m
3 of wastewater is generated annually, with China alone contributing approximately 7.5 × 10
10 m
3 [1],
[2],
[3]. Remarkably, China leads in overall greenhouse gas (GHG) emissions from the wastewater sector, with a staggering 140% increase over the past decade
[4]. Despite this, the wastewater sector’s decarbonization efforts lag behind industries like energy and transportation
[5],
[6],
[7],
[8]. Within wastewater treatment processes, around 3.88 million metric tonnes of carbon are oxidized into CO
2 through energy-intensive aerobic biological treatments
[2]. Presently, wastewater contributes approximately 5% of global non-CO
2 GHG emissions, encompassing CH
4 and N
2O
[9],
[10]. Furthermore, the chemicals utilized in wastewater treatment amplify its environmental repercussions. Remarkably, wastewater remains an underutilized resource, with its contaminated pollutants having the potential to be converted into valuable products as feedstocks for circular manufacturing. This utilization presents an opportunity to address the challenge of resource scarcity currently faced
[1]. Consequently, there is a discernible shift towards wastewater resource recovery, propelled by the urgent demand for advanced technologies to tackle energy and environmental challenges
[1].
Green bio-manufacturing utilizes renewable resources as primary raw materials and leverages biological systems like enzymes, tissues, and living cells to efficiently synthesize commercially valuable molecules, embodying a sustainable production paradigm
[11]. Its versatile applications span various sectors, including chemical, pharmaceutical, agricultural, food, and environmental industries
[12]. Positioned at the forefront of economic and societal transformation, green bio-manufacturing drives greener, more sustainable, and higher-quality development.
Microbial electrochemical wastewater refining stands at the crossroads of wastewater resource recovery and green bio-manufacturing, merging hybrid electrochemical and microbial processes to transform wastewater pollutants into tailored chemical commodities
[1]. Unlike traditional approaches solely focused on contaminant removal, this method prioritizes resource recovery and bio-manufacturing, thereby expanding the spectrum of products beyond those naturally occurring in wastewater. Wastewater emerges as a promising feedstock for green bio-manufacturing, rich in carbon, nitrogen, phosphorus, sulfur, water, and energy
[1],
[13], setting it apart from other raw materials like industrial exhaust gases
[14]. By harnessing the swift kinetics of electrochemical processes and the precision of microbial processes
[15], microbial electrochemical wastewater refining holds the promise of significant benefits
[16]. This approach not only facilitates the conversion of wastewater pollutants into customized chemical products, thereby mitigating carbon emissions
[17], but also creates opportunities to harness wastewater treatment for active and direct CO
2 capture and utilization
[18]. For instance, alternative pathways in microbial electrochemical and phototrophic processes for wastewater treatment have the ability to capture and utilize CO
2 simultaneously. This potential capability could offset the industry’s GHG emissions footprint and position it as a significant contributor to negative carbon emissions globally
[18]. Consequently, microbial electrochemical wastewater refining has the potential to transform wastewater treatment from a carbon-emitting to a carbon-negative process. Its efficacy is typically evaluated through comparative tests, including open circuit control, abiotic control, or the frequently employed electrocatalysts.
While a comprehensive review within the realm of wastewater resource utilization and green biomanufacturing is lacking, microbial electrochemical technologies have already undergone extensive laboratory investigation and found practical applications. Within conventional microbial electrochemical processes, robust microbial catalysts operate under mild reaction conditions to facilitate redox reactions at electrode surfaces, thereby enabling the conversion between electrical and chemical energy
[19]. Notably, the chemical energy inherent in wastewater surpasses nine times the energy required for its treatment
[20], emphasizing its potential as a valuable resource. Consequently, four primary microbial electrochemical technologies have gained traction: microbial fuel cells (MFCs), microbial electrolysis cells (MECs), microbial desalination cells (MDCs), and microbial electrosynthesis (MES). Essentially, the first three technologies rely on the functionality of a bioanode to directly harvest electric energy from the degradation of organic pollutants, while in MES, a biocathode is employed to reduce CO
2 and produce fuels and chemicals.
This article delivers a comprehensive evaluation of the potential of microbial electrochemical wastewater refining, with a primary emphasis on the valorization of aqueous carbon feedstocks inherent in wastewater. Drawing from recent studies, it synthesizes pivotal discoveries over the past five years. The article begins with an exploration of microbial carbon–energy metabolism, which explains the slow reactions observed with traditional inert electrodes. Highlighting the need for a new approach, it underscores the use of active electrodes for CO2 fixation to address sluggish refining processes. Furthermore, it introduces electrode-driven mixotrophy and the method of “binary electron donors” to leverage mixed carbon sources from wastewater, thus overcoming the challenge of low-value refining products. The article also highlights significant applications such as water monitoring, chemical generation, and product separation. Recent advancements in innovative processes and equipment for microbial electrochemical wastewater refining are also discussed.
2. Active electrodes to stimulate the CO2 fixation rate
2.1. Microbial carbon–energy metabolism mechanism using traditional inert electrodes
Conventional MES reactors typically employ the biocathode strategy, where a biofilm is affixed to the cathode. In this setup, microorganisms utilize electrons from the cathode to catalyze the reduction of CO
2, while inert electrodes, commonly used as cathodes, lack the capability to reduce CO
2 themselves
[21]. Theoretically, this technology holds significant promise as an innovative approach to actively mitigate carbon emissions
[22],
[23]. However, it encounters challenges such as a low rate of microbial electrochemical CO
2 fixation
[24].
Of paramount importance is the utilization of mixed cultures to generate thermodynamically constrained end-products (e.g., acetate and methane), which has shown a substantial increase in current density compared to pure cultures
[25]. Nevertheless, it is crucial to acknowledge that much of the research involving pure cultures was primarily geared towards fundamental exploration and conducted in reactors lacking engineering robustness, such as H-type reactors
[26]. For instance, significant insights have emerged from studies focusing on the materials–biology interface. Unexpected findings have indicated enhanced efficiency in utilizing reducing equivalents for CO
2 fixation using
Sporomusa ovata, escalating from less than 80% in chemoautotrophy to over 95% under electroautotrophic conditions
[27].
Despite its improved performance, the application of mixed culture-based MES introduces complexity in elucidating the metabolic mechanisms. Consequently, the underlying microbial carbon–energy metabolism behind the phenomenon of slow reactions using traditional inert electrodes remains unclear, resulting in a lack of theoretical guidance for designing electrodes to expedite the refining process.
Recently, a novel flow-electrode-based MES reactor was constructed, allowing the independent adjustment of the electrode surface area relative to the reactor volume (
Fig. 1(a)), a capability not achievable with commonly used fixed electrodes
[28]. Transcriptional analysis has identified the Wood–Ljungdahl pathway (WLP) and the reductive citric acid cycle (rTCA, albeit partially absent) as the two primary pathways facilitating autotrophic carbon fixation in both flow-electrode-based and conventional carbon felt-based reactors (
Fig. 1(b) [28]).
Remarkably, in flow-electrode-based MES reactors showcasing heightened carbon fixation activity, genes associated with electron transfer via cytochrome were significantly upregulated
[28]. In contrast, conventional carbon felt MES reactors exhibited a prevalence of hydrogenase expression. These findings collectively indicate that the inert electrodes’ inability to surmount the sluggish refining process stems from their operational principles, which entail either direct electron transfer between the electrode and microorganism or electrochemical hydrogen evolution coupled with
in-situ microbial hydrogen utilization.
2.2. Active electrode-based biohybrid CO2 electrolysis
To overcome the challenge posed by inert electrodes impeding the production of refinery products, a novel approach known as active electrode-based biohybrid CO
2 electrolysis has gained increasing attention
[29]. This strategy builds on decades of field experience and is inspired by advancements in electrochemical CO
2 reduction (CO
2RR)
[30],
[31]. The term “active electrode” denotes a cathode capable of reducing CO
2, often achieved using electrocatalysts.
CO
2RR has the capacity to yield 18 distinct gas and liquid products
[32], with C
1 products (CO and formate) being efficiently produced at high partial current densities
[33],
[34]. These C
1 products exhibit a higher maximum CO
2 mitigation potential, in grams of CO
2 per kilowatt hour, compared to C
2+ products
[32]. Furthermore, only C
1 products can be electrosynthesized with a net reduction in emissions
[35]. When formate is the target product, it achieves the maximum benefit in dollars per mole of electron
[36], yet its potential for CO
2 utilization is comparatively lower due to its limited global consumption
[32]. Therefore, the development of a hybrid system is of interest, where formate is initially produced from CO
2RR, followed by upgrading the products in the bioconversion process.
Various microbes have demonstrated the ability to utilize these C
1 chemicals as both carbon and energy sources
[37], positioning them as promising intermediates in a biohybrid CO
2 electrolysis system
[38],
[39]. Electrocatalyst-assisted MES has been successfully achieved by introducing an electrical–biological hybrid cathode (
Fig. 2(a)), with zinc exhibiting the highest acetic acid production rate ((1.23 ± 0.02) g·L
−1·d
−1) among the four metals evaluated
[40]. In a recent study,
Cupriavidus necator cells achieved a polyhydroxybutyrate (PHB) content of 83% of dry cell weight by continuously circulating formate-containing electrolyte between the CO
2 electrolyzer and the gas fermenter
[41]. However, it is noteworthy that utilizing a shared electrolyte for electrocatalysts and microbial catalysts may lead to a significant decrease in current density. Additionally, factors such as the corrosion and toxicity of metals need further evaluation.
To tackle the challenges and limitations associated with using a shared electrolyte for both electrocatalysts and microbial catalysts in hybrid systems, there’s a growing interest in biohybrid CO
2 electrolysis operating under an external mode. This strategy enables the independent optimization of the electrocatalytic and bioconversion processes. For example, when supplied with high-pressure gaseous and liquid CO
2, the Cu/CuO
x catalyst exhibited significantly higher performance compared to operating under ordinary pressure
[42].
Soluble C
2 intermediates, such as acetate and ethanol, possess higher energy and electron-carrying capacities compared to C
1 chemicals. In a demonstration utilizing adaptive
Pseudomonas putida KT2440, ethanol directly generated through CO
2RR was employed for the production of PHBs
[43]. However, this concept demonstration revealed a low ethanol Faradaic efficiency (< 20%). Therefore, when constructing biohybrid CO
2 electrolysis systems using soluble C
2 intermediates, a promising solution involves the tandem use of CO
2RR with carbon monoxide reduction reactions (CORR) and/or the design of advanced electrocatalysts for the selective production of C
2 intermediates.
A tandem catalytic system was devised, employing a covalent organic framework and a metal–organic framework as catalysts for CO
2RR and CORR, respectively
[44]. This configuration yielded an acetate Faradaic efficiency of 51.2% at 410 mA·cm
−2. Additionally, a tandem electrocatalyst utilizing Cu-in-Ag dilute alloy materials was synthesized
[45], achieving an acetate Faradaic efficiency of approximately 85% over 820 h at a current density of 100 mA·cm
−2. Moreover, by leveraging noncovalent interactions
[46], an ethanol Faradaic efficiency of 44.1% and a partial current density of 501.0 mA·cm
−2 were attained. When selectively producing C
2 intermediates, such as through tandem systems or electrocatalysts, the bioconversion process can be leveraged for producing microbial products. For instance, acetate generated from the tandem of CO
2RR with CORR can serve as an intermediate for bioconversion, facilitating the production of long-chain compounds or even food
[47],
[48].
The direct synthesis of pure formic acid and pure acetic acid from CO
2RR, either independently or in tandem with CORR
[44],
[49],
[50], represents a significant milestone in the field
[51]. This breakthrough has the potential to pave the way for advanced synthesis techniques. A recent example of this progress is the development of a biohybrid CO
2 electrolysis system, which utilizes a solid electrolyte and operates in an external mode (
Fig. 2(b))
[52]. This configuration addresses the challenge of biocompatibility arising from dissolved intermediates in high-concentration electrolytes. Moreover, the bioconsumption of formic acid is preferred over formate consumption, as it is a proton-neutral process, whereas the consumption of formate requires the addition of an acid to balance the pH
[24]. Indeed, the potential of solid electrolytes for low-temperature CO
2 valorization has garnered attention in recent study
[16].
2.3. Generation and bioconversion of intermediates in biohybrid CO2 electrolysis
Various approaches were necessary to analyze the mechanisms involved in active electrode-based biohybrid CO
2 electrolysis, with a dual focus on both electrochemical generation and the bioconversion of intermediates.
In-situ Raman spectra have proven invaluable in understanding the reconstruction of electrocatalysts (
Fig. 3(a)), the local microenvironment (
Fig. 3(b)), and the intermediate formation (
Fig. 3(c)) during CO
2RR for pure formic acid generation
[52]. Moreover, the absence of the *OCHO peak at 1400 cm
−1 in
in-situ attenuated total reflectance-infrared (ATR-IR) spectra highlights the critical role of advanced electrocatalysts in enhancing performance (
Fig. 3(d)). Additionally, evidence of formic acid bioconversion into medium-chain fatty acids (MCFAs) has been provided through the use of
13C-nuclear magnetic resonance (NMR,
Fig. 3(e)),
1H-NMR (
Fig. 3(f)), and mass spectrometry (
Fig. 3(g)).
It’s crucial to emphasize that the primary focus of the discussion here lies in utilizing pure formic acid as the intermediary for MCFAs production. Nonetheless, it’s imperative to develop techniques for analyzing the underlying mechanisms when employing various intermediaries, electrocatalysts, aerobic or anaerobic microbes, and intracellular or extracellular products in the biohybrid CO2 electrolysis system.
3. Method of “binary electron donors” to produce high value-added chemicals
3.1. Mechanism of electrode-driven mixotrophic bioconversion
In the realm of microbial electrochemical wastewater refining, the conversion of various waste carbon into fuels and chemicals emerges as a promising strategy to enhance sustainability within society (
Fig. 4(a))
[16]. Numerous wastewater refining technologies exist, primarily focused on recovering bioelectricity, hydrogen, methane, ammonia, and other low-value target products. The emphasis on pursuing high-value-added chemicals is exemplified by comparing methane and acetate generation performance between MES and CO
2RR (
Table 1 [40],
[53],
[54],
[55],
[56],
[57],
[58],
[59],
[60],
[61],
[62]). It becomes evident that MES demonstrates attractive stability, whereas CO
2RR still faces significant challenges in achieving comparable stability. Notably, when gas diffusion electrodes are employed, the current density of CO
2RR surpasses that of MES by one to two orders of magnitude. Furthermore, over the past decade, rapid advancements in CO
2RR, facilitated by the design of sophisticated electrocatalysts and molecular modification approaches, have nearly eradicated the high Faradaic efficiency advantage of MES.
The approach of electrode-driven mixotrophic bioconversion is poised to yield high-value-added chemicals from wastewater refining. Of particular interest is the production of MCFAs, which have garnered increased attention due to their higher value compared to common products like methane and acetate
[63],
[64],
[65]. Mixotrophy represents a microbial growth regime characterized by the simultaneous assimilation of both organic (e.g., sugars) and inorganic (e.g., CO
2) substrates
[66]. To achieve this, mixotrophic MES were implemented by co-feeding glucose and CO
2 [67]. Genome-centric analysis revealed that, under mixotrophic conditions, 27 out of 60 functional microbes with the WLP and/or fatty acid biosynthesis pathways experienced significant enrichment (
Fig. 4(b) [67],
[68]). Despite a slight reduction in carbon metabolic pathways via the WLP and reverse β-oxidation pathways, pre-enrichment of acetogens before establishing mixotrophic conditions resulted in an increased abundance of functional microbes capable of chain elongation through fatty acid biosynthesis.
While no prior attempts have been documented, scaling up mixotrophic MES may encounter challenges related to the low strength of practical wastewater and the arrangement of electrodes, as observed in other microbial electrochemical technologies. Furthermore, effectively addressing the issue of carbon catabolite repression during the simultaneous feeding of organic and inorganic substrates necessitates the development of effective strategies
[66].
3.2. Application “binary electron donors” for chain elongation and electrode-fermentation
Drawing from a mechanism analysis of electrode-driven mixotrophic bioconversion, the concept of “binary electron donors” has been introduced and effectively employed in both chain elongation and electrode-fermentation processes.
The chain elongation process to produce MCFAs was spearheaded utilizing a “binary electron donors” approach, as highlighted in a review paper authored by the Minteer group
[21]. MCFAs, referring to carboxylates with 6–12 carbon atoms, possess more energy and achieve better separability compared to their precursors
[63],
[69]. In fact, the production of MCFAs from waste streams via chain elongation platforms has garnered growing interest in the past five years in environmental engineering
[70],
[71]. The selectivity for caproate production was 80.28%, 32.22%, and 6.97% for the “binary electron donors” (co-supplied with ethanol and electrode as the electron donor), the ethanol electron donor, and the electrode electron donor, respectively
[72]. Furthermore, the “binary electron donors” approach was successfully employed using a CO/CO
2 mixture as the substrate in MES for chain elongation
[73]. The CO-50% test achieved the best performance in producing C4 and C6 carboxylates, and it also increased the relative abundance of
Acetobacterium and
Clostridium.
A thorough examination of the present status and future possibilities of producing MCFAs from organic waste
[63],
[74] and/or CO
2 [68] is available. Remarkably, the concept of electrode-enhanced mixotrophy to enhance MCFAs production has been introduced (
Fig. 4(c)), which depends on the cooperative culture of acetogenic strains and chain elongation bacteria
[68].
Conventional fermentation often leads to a redox imbalance, necessitating simultaneous oxidation and reduction reactions to maintain equilibrium. Addressing this challenge, electro-fermentation has emerged as a method to electrochemically regulate the fermentation process. It employs electrodes either as supplementary electron donors (referred to as cathodic electro-fermentation) or acceptors (known as anodic electro-fermentation)
[22],
[75],
[76]. In cathodic electro-fermentation, the cathode and organic compounds act as the primary and secondary electron donors, respectively (
Fig. 4(d)). The mechanisms driving electro-fermentation for the electrochemical valorization of organic waste have been outlined
[77],
[78],
[79].
The electrode-fermentation technique has demonstrated success in generating various chemicals
[77]. For instance, it has been employed to regulate mixed-culture glucose fermentation
[80], revealing the interactive influences of electrode potential and pH. Furthermore, expanding beyond primary fermentation, electrode-fermentation has been utilized in secondary fermentation processes, such as the chain elongation process, which produces higher-value MCFAs from acetate and ethanol
[81].
4. Novel process and equipment for microbial electrochemical wastewater refining
4.1. Upstream water monitoring
Water monitoring is pivotal as an upstream technology, ensuring the seamless operation of subsequent microbial electrochemical wastewater refining processes. Numerous review papers delve into the realm of microbial electrochemical sensors
[82],
[83], leveraging the electric current output from electrode-biofilm as a signal for water monitoring.
Recent advancements in microbial electrochemical sensors have led to a deeper understanding of microbial–toxic interactions, dynamic output signal modeling, the detection of practical samples with specific requirements, and the design of innovative devices. For example, a novel pulse open-circuit voltammetry method unveiled that toxic formaldehyde primarily affects the intracellular electron generation process rather than the extracellular electron transfer (EET) process
[84]. Understanding the correlation between current output and the metabolism mechanism driven by electrodes is crucial for reducing the time required for biochemical oxygen demand detection
[85]. The response of a microbial electrochemical sensor was utilized to capture dynamic bioavailable carbon uptake at water resource recovery facilities, employing advanced statistical and machine learning methods
[86]. Achieving sensitive detection of biochemical oxygen demand in oxygen-rich environments was made possible through the transition between electrotrophic and heterotrophic respirations of immobilized
Acinetobacter venetiensis RAG-1
[87]. In summary, fundamental research on understanding the EET process has paved the way for leveraging engineering and mathematical approaches to enhance the performance of microbial electrochemical sensors. Furthermore, the integration of synthetic biology and materials engineering is essential for the development of biosensors with improved selectivity in responding to specific chemicals
[88].
4.2. Interfacing current wastewater treatment unit
Advancing the practical application of microbial electrochemical wastewater refining faces several key challenges that demand urgent attention, particularly in identifying interfaces within existing sewage treatment units. One proposed approach involves utilizing anaerobic digestion as an interface, introducing a hybrid process for the high-value utilization of the CO
2 released during wastewater treatment. Biogas typically contains 35%–45% CO
2 and is produced at an annual rate surpassing 7.0 × 10
10 m
3, with a global potential reaching up to 2.0 × 10
11 m
3 per year
[89]. Consequently, global biomethane and biogas production have the potential to fulfill nearly 20% of the global gas demand, with predictions indicating a ninefold increase in demand by 2040 compared to 2018 levels
[90]. Here, a novel process, termed “CO
2 alkaline absorption for biogas upgrading-direct (bi)carbonate electrolysis–syngas fermentation-MCFAs electrochemical recovery” (
Fig. 5(a)), is reinforced through incremental steps. These steps encompass employing a microenvironment engineering approach to construct gas diffusion electrodes to enhance stability
[91], regulating proton flux using various separators
[92], and amending (bi)carbonate solutions with trace surfactants like cetyltrimethylammonium bromide
[93]. Direct (bi)carbonate electrolysis significantly offers a solution to the energy losses incurred in regenerating high-purity CO
2 gas as the feed gas
[92]. In the proposed process, it can align the CO
2 production and consumption rates, along with addressing stability differences between each step. However, there is an urgent call for novel approaches to suppress the hydrogen evolution reaction and enhance selectivity. Moreover, it is essential to enrich functional microorganisms capable of syngas fermentation
[94] and fine-tune the adsorption and dissociation of MCFAs for the generation and separation steps, respectively
[95].
A very recent study introduced a novel hybrid process for MCFAs production, involving tandem acidic CO
2 electrolysis coupled with syngas fermentation (
Fig. 5(b))
[94]. In this study, structural engineering of gas diffusion electrode (GDE) was employed by coating carbon black and graphite for syngas generation with an adjustable composition. Expanding upon this approach, an integrated process could also be proposed for biogas upgrading and the production of high-value-added chemicals by leveraging anaerobic digestion as an interface.
While these hybrid processes are expected to integrate well with anaerobic digestion, extending them to other existing wastewater treatment units poses case-specific challenges. Additionally, ensuring high overall performance remains a concern. For instance, none of these hybrid processes have addressed the recapture and re-utilization of CO2 released during the bioconversion process after the CO2RR steps.
4.3. Novel pathway for sustainable water–carbon–energy nexus
Advancing the practical application of microbial electrochemical wastewater refining poses additional key challenges that require urgent attention, including the development of entirely new processes and equipment.
Combining recent advancements in microbial electrochemistry and electrochemical CO
2 reduction, a novel process and equipment have been devised within the water–carbon–energy nexus. On one hand, employing active electrodes and pure formic acid as an intermediate product, a groundbreaking bio-coupled battery has been established (
Fig. 5(c)), characterized by “ultra-fast charging and long-lasting discharging”
[96]. Notably, this biohybrid battery achieved an astonishing 25 h discharging phase with only a 3 min charging period, with the recovered bioelectricity signal effectively utilized for water monitoring. Additionally, the intermediate product, formic acid, can be employed as a carbon source for denitrification.
4.4. Pilot-scale systems and practical application
Microbial electrochemical wastewater refining has been demonstrated using pilot-scale systems for electricity, hydrogen, methane recovery, and pollutant degradation. Embracing a “modular” approach, the team pioneered the development of a 1000 L modularized MFC system (
Fig. 6(a)), which was the largest volume at the time
[97]. This system enhanced ion concentration at the materials-biology interface through activated carbon adsorption, enabling efficient operation in a municipal wastewater treatment plant (WWTP). Consequently, the system achieved energy recovery from practical municipal wastewater treatment, demonstrating stability over a year and consistently meeting Level A criteria of pollutant discharge standards for municipal wastewater treatment plants in China. Additionally, a pilot microbial electrochemical system with a total volume of 1500 L was developed (
Fig. 6(b)) and operated outdoors in a WWTP
[98]. It utilized a microbial separator based on dynamic biofilm on a low-cost porous matrix and a separate plug-in module architecture integrating 336 pairs of microbial electrochemical system units and 14 separator modules into one wastewater tank. A recent comprehensive review of pilot-scale MFCs emphasized the critical role of separators and cathode configurations in wastewater treatment
[99].
A pilot-scale (1000 L) continuous flow MEC was constructed for hydrogen recovery from treating winery wastewater; however, it primarily yielded methane after 100 days
[100]. In a recent study, nickel–foam cathodes were utilized in a 150 L MEC pilot plant with a cassette configuration
[101], achieving the highest reported hydrogen production at pilot-scale (19.1 L-H
2·m
−2·d
−1 and 0.2 m
3·m
−3·d
−1) when treating real sugar-based industrial wastewater with a reduced solids retention time to suppress methanogenesis. It’s worth noting that achieving higher performance in MEC is attained by feeding industrial wastewater with higher strength compared to municipal wastewater. Therefore, by improving primary treatment techniques to better concentrate organic matter in municipal wastewater, the practical application of MECs can become more efficient.
For methane recovery, a pilot-scale (360 L) anaerobic baffled reactor was developed (
Fig. 6(c))
[102]. By introducing electrodes, a symbiotic relationship between exoelectrogens and short-chain fatty acid-oxidation syntrophs was established. This resulted in enhanced degradation of volatile fatty acids and an increase in methane content from 81.5% to 92.2%. Additionally, a pilot-scale bioelectrochemical coupled anaerobic digestion system with an effective system volume of 5 m
3 was tested for treating membrane manufacturing wastewater
[103].
In a recent breakthrough, a novel cathode-enhanced ecological floating bed coupled with a microbial electrochemical system was implemented for
in-situ remediation on a demonstration scale spanning 2300 m
2 during a year-long operational phase (
Fig. 6(d))
[104]. The treated water consistently met Grade III standards (GB 3838–2002).
The technology readiness level serves as a globally accepted method for assessing the maturity level of a specific technology. Recent developments indicate that MES is rapidly advancing towards technology readiness levels 4 and 5, signifying technological validation and demonstration in relevant environments
[105]. While reports on the production of carbonaceous chemicals other than hydrogen and methane in pilot-scale systems have been scarce, the scaling up of biohybrid CO
2 electrolysis under external mode shows great promise. For instance, several studies have reported on 100 cm
2 cells or electrolyzer stacks for CO
2RR, with operating currents reaching as high as 250 A
[91],
[106],
[107]. Additionally, in the realm of gas fermentation, companies like LanzaTech Global, Inc., USA have achieved industrial-scale ethanol production using
Clostridium autoethanogenum [108]. Moreover, recent advancements include the successful demonstration of a 120 L field pilot scale for the production of acetone and isopropanol
[14].
5. Challenges and perspectives
As highlighted in this paper, the adoption of microbial electrochemical wastewater refining offers a promising pathway for converting energy-intensive, carbon-emitting WWTPs into integrated water resource recovery facilities. By manipulating microbial metabolism, these systems have shown potential for producing various value-added chemicals. Despite significant advancements, such as utilizing active electrodes to boost the CO2 fixation rate, incorporating “binary electron donors” for high-value chemical production, and introducing innovative processes and equipment, substantial challenges remain. Addressing these challenges will require continued and concerted efforts.
To evaluate the performance of microbial electrochemical wastewater refining effectively, a comprehensive set of metrics should be employed
[26]. Initially, the efficacy of microbial catalysts and any newly introduced electrocatalysts must be assessed and documented across diverse experimental conditions, including Faradaic efficiency, overpotential, current density, and stability
[109]. Additionally, the configuration of the cell and the overall performance of the hybrid system should be appraised based on criteria such as energy efficiency, stability, CO
2 conversion efficiency, and the concentration of final products (titer)
[110].
Electrode materials play a crucial role in governing interfacial processes, especially in microbial electrochemical systems, where innovative strategies are increasingly sought after. The strategic design of advanced electrode materials aims to enhance interactions between electrodes and microbes. For example, a single-atom engineering approach has been utilized to enhance electron efficiency by promoting microbe enrichment and facilitating interfacial charge extraction
[56],
[111]. Additionally, a redox-active conjugated oligoelectrolyte has been synthesized to integrate into cell membranes, mimicking the function of endogenous transmembrane proteins in electron transport
[112]. Altering the cathode potential from −0.8 to −1.0 V versus Ag/AgCl shifts the dominant electron transfer pathway from direct to indirect
[113]. However, it’s crucial to recognize that biotic–abiotic electron transfer is a complex process, and beyond electrode potential, other operational parameters require careful consideration in future research
[114].
The involvement of engineered microbes, engineered microbial aggregates, and multi-organism systems is crucial in microbial electrochemical technology. Electroactive microorganisms have the ability to establish outer-surface electrical contacts with other cells, minerals, or soluble molecules
[115],
[116], but their contribution to chemical production requires further investigation. For instance, despite the high conductivity of nanowires, efficient EET in microbial aggregates may be hindered by their complex structure
[117]. Trophic interactions can also influence the spatial organization of biofilm communities within microbial aggregates
[118]. Moreover, by employing synthetic biology-based engineering strategies to engineer microbes
[119], the overall process can be designed to selectively synthesize high-value chemicals through the enhancement of direct interspecific substance and electron transfer
[120]. Innovative and hybrid microbial electrochemical technology-based processes can be developed by drawing inspiration from existing competitive technologies, such as gas fermentation
[14],
[121], and CO
2RR
[105]. Incorporating engineered microbial catalysts into hybrid processes
[122], enables efficient synthesis of high-value chemicals, such as riboflavin
[123]. It’s worth noting that the current focus of research in this field largely revolves around utilizing carbon sources from wastewater. However, future studies should aim to explore the utilization of other elements such as nitrogen, phosphorus, sulfur, among others
[124].
To elucidate the involved mechanisms, operando approaches are essential, as commonly demonstrated in CO
2RR studies
[125]. The introduction of microbial catalysts further underscores the need for nondestructive testing techniques. In addition to
in-situ experimental methods, theoretical calculations offer deeper mechanistic insights into CO
2RR pathways and even biotic–abiotic electron transfer mechanisms
[57],
[126]. When microbial catalysts are employed, the structural parameters of key biomolecules identified from experimental studies, such as the membrane protein cytochrome B, can potentially be utilized for such calculations
[57]. Traditional density functional theory studies are often employed to provide a theoretical basis for existing experimental results or to make predictions for a small number of material structures. By combining machine learning and artificial intelligence techniques, the gap between experimentally observed activity and calculated results can be bridged, accelerating the design of efficient electrocatalysts and materials-biology interfaces
[127].
6. Conclusions
The utilization of microbial electrochemical wastewater refining has become a cornerstone in the interdisciplinary field of wastewater resource recovery and green bio-manufacturing. Significant progress has been achieved in leveraging active electrodes to enhance the CO2 fixation rate, utilizing “binary electron donors” for the production of high-value chemicals, and innovating new processes and equipment. Future studies could advance further through the development of a comprehensive suite of performance metrics, refining advanced electrocatalysts and engineered microbial catalysts, exploring novel hybrid processes, and elucidating mechanisms through the integration of operando and nondestructive approaches complemented with theoretical calculations.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52125001, 52370033, and 31970106).
Compliance with ethics guidelines
Na Chu, Daping Li, Raymond Jianxiong Zeng, Yong Jiang, and Peng Liang declare that they have no conflict of interest or financial conflicts to disclose.
PDF
(2952KB)
