Elyns Journal of Microbes

Microbial Electrochemical Systems for Agro-industrial Wastewater Remediation and Renewable Products Generation: A Review

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Published Date: June 7, 2014

Microbial Electrochemical Systems for Agro-industrial Wastewater Remediation and Renewable Products Generation: A Review

Hongjian Lin1, Xiao Wu2, Bo Hu3 and Jun Zhu4* 

1Postdoctoral Associate, Bioproducts and Biosystems Engineering, University of Minnesota, St. Paul, MN 55108, USA.
2Research Associate, Southern Research and Outreach Center, University of Minnesota, Waseca, MN 56093, USA.
3Assistant Professor, Bioproducts and BiosystemsMinnesota, St. Paul, MN 55108, USA.
4Professor, Biological and Agricultural Engineering, University of Arkansas, Fayetteville, USA.
 
*Corresponding Author: Jun Zhu, Professor, Biological and Agricultural Engineering, University of Arkansas, Fayetteville, AR 72704, USA, Email: junzhu@uark.edu; Ph: 479-575-2883; Fax: 479-575-2846.
 
Citation: Hongjian Lin, Xiao Wu, Bo Hu and Jun Zhu (2014) Microbial Electrochemical Systems for Agro-industrial Wastewater Remediation and Renewable Products Generation: A Review. Arc Micro Biotech 1(1): 20. http://dx.doi.org/10.19104/amb.2014.101

 

Abstract 

 

Microbial electrochemical systems (MESs) that combine the benefits of biological and electrochemical reactors can simultaneously remedy organically contaminated wastewater and recover energy and chemicals from the components of wastewater. This technology, mainly including microbial fuel cells (MFCs) and microbial electrolysis cells (MECs), can be applied to animal wastewater and other agro-industrial wastewater that have high levels of organic matter and nutrients to not only reduce the polluting strength before discharge but also produce renewable, value-added products. This article reviews the recent development of both MFCs and MECs, and discusses ways of further enhancing their performance for bioremediation and production of energy and chemicals from agro-industrial wastewater. 

Keywords: MEC; MFC; Agro-industrial wastewater; Bio energy; Nutrients removal & recovery. 

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Introduction 

 

Microbial electrochemical systems (MESs) represented by microbial fuel cells (MFCs) and microbial electrolysis cells (MECs) are processes that are evolved from the abiotic electrochemical approaches in early years’ studies [1] and that have been applied in waste water treatment for a cleaner environment [2-4]. Take MEC for example, it is a microbial electrochemical or bio electrochemical device [5] that anodically decomposes organic substrates in waste water and cathodically generates hydrogen gas [6,7], with the assistance of an external voltage of typically between 0.3 and 1.0 V and by using protons as the terminal electron accept or it works in a way that has a tremendous energy efficiency, generally larger than 200% based on the electrical power input, for a wide range of substrates via hydrogen evolution reaction (HER) [8]. While for an MFC, bacteria oxidize organic matter at anode followed by electrons traveling around a circuit to the cathode, producing electrical current. At the cathode, an oxidant, such as dissolved oxygen and ferricyanide, is converted to a reduced form, e.g., water and ferrocyanide [9, 10]. From the perspective of energy use, MESs function as a type of fuel cell or regenerative (reverse) fuel cell and can reach high energy efficiency on the substrate basis that is not limited by the Carnot’s efficiency normally seen in internal combustion engines [11]. 

Waste water from agro-industries strongly contributes to environmental pollution. Those waste effluents, e.g., animal manure and food processing waste water, contain high levels of organic matter and nutrients and may require appropriate remediation before discharge. The recovery of energy, fertilizers, and other chemicals will valorize these waste streams [12]. There is recently a trend to utilize MFCs and MECs to mitigate agro-industrial waste water, generate electricity and other products, and achieve high energy efficiency by applying renewable electricity as a power source for MECs [13,14]. Due to these advantages and potential applications of MESs in agro-industrial waste water remediation, electrode materials and catalysts as well as performance optimization for MESs have experienced rapid developments over the years, and there apparently is a need to summarize these progresses and discuss ways of further improving MESs performance with respect to agro-industrial waste water as reactor substrates and influents. 

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Animal Waste water as an Example

 

Agro-industrial waste water with high organic pollutants can include waste effluents from livestock industry(swine and dairy manure slurry) as well as from food and agro-products processing industries, such as brewery waste water, dairy processing waste water, potato processing waste water, sugar-beet processing waste water, olive mill waste water, meat processing waste water, and cheese whey. The high pollution tendency and huge volume of waste water from these industries need be taken care of before discharge. Livestock and poultry industry, an essential component of the agricultural sector worldwide, is used here as an example of the source of agro-industrial waste water. One important trend of the industry in recent two to three decades is that small livestock and poultry farms are being incorporated into larger operations to take advantage of technological innovations and the economics of scale [15]. EPA defines operations as animal feeding operations (AFOs) where animals are kept and raised in confinement without sustained vegetation or crop growth over any portion of the facility. Concentrated animal feeding operations (CAFOs) is further defined based on AFOs, including the following three categories: large operations of AFOs (e.g., operation with over 1000 cattle, or over 2500 growing-finishing pigs, each of which weigh over 55 pounds); medium operations (e.g., between 300-999 cattle, or 750-2499 pigs) which meet specified risk-of-discharge criteria; and small operations which are designated by EPA or the State National Pollutant Discharge Elimination System (NPDES) permitting authority [16]. More and more farms are becoming CAFOs, and the inventories of the most livestock and poultry farms are steadily increasing (Table 1).

 

Table 1. Farms and animal number (in million heads)inventories of major categories of livestock and poultry in US in 1997, 2002, and 2007a
 
a Data are compiled from USDA statistics [17-19]. 

As a result, a tremendous amount of manure and waste water which contains large amounts of nutrients is generated from CAFOs, mainly in the form of liquid or slurry, a mixture of feces, urine, and water. The liquid waste at CAFOs originates from the water used for drinking (excreted as manure and urine), cooling facilities, sanitation, wash water from facilities, and animal waste-disposal systems [15]. In the United States alone, it is estimated to be between 133 and 300 million tons of dry manure per year according to data in 2005 [20], or over 500 million tons per year on a wet basis from the confined animals according to an estimate in 2003 by EPA [16]. The waste water is rich in organic matter and other macro-nutrients needed by crops, mainly nitrogen (N) and phosphorus (p), and the total CAFOs manure N and P as excreted increased to 6,174,812and 1,846,989tons in 2007, respectively [21]. The trend of increasing CAFOs in recent years certainly has increased the opportunity of recovering manure nutrients because CAFOs facilitate manure and waste water collection. On the flip side, this trend may also have resulted in more accumulations of manure that require removal and disposal [22]. 

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Waste water as Resource

 

The increasing amount of wastes by large productions need to be mitigated or treated before disposal otherwise they can cause environmental concerns because inappropriate management and/or treatment of the wastes can lead to severe environmental issues to soil, water, and air [23]. An excessive disposal of livestock manure within localized areas will result in overloading of organic materials, nutrients (N and P), and heavy metals (Cu and Zn) in soil that may leach to ground or surface water, and consequently impair drinking water or cause eutrophication. For swine farms alone, according to the N-based or P-based standards, only 49% or 27% of large farms (>1,000 USDA animal units) found enough land to apply their manure [20]. Besides the pollution caused by manure disposal, the hog housing, manure storage, and even the treatment unit [24] may release air pollutants such as volatile fatty acids and ammonia. Greenhouse gases (GHGs, methane and carbon dioxide) emission, if not captured, may worsen the global warming issues [25]. So appropriate management and treatment of the animal waste becomes an urgent and crucial requirement to sustain the industry as all CAFOs are required to develop and implement nutrient management plans for land application as a condition of NPDES permit [16]. 

On the other hand, animal manure is a valuable resource. If the energy in the manure organic matter, the nutrients (N and P), or even water can be collected and appropriately used, they will substantially contribute to the industry by saving energy, producing fertilizers, and providing irrigation water. For example, assuming an average high heating value (HHV) of dry manure as 16.6 MJ/kg [26,27], the annual manure generated in US contains energy of between 6.15×1011 and 1.39×1012 kWh, equivalent to a total cash value of between $60.9 billion to $137 billion based on the national average electricity price of $0.099/kWh in May 2013 [28]. The total nutrients in manure as excreted can supply 50.3% of N and 102% of P of US demands according to the 2011 fertilizer consumption data [29]; and the CAFOs alone could potentially meet 13.0% and 26.3% of the annual N and P consumption, respectively. 

These facts provide a big drive for developing technologies to harvest energy, nutrients, and water in order to better mitigate and utilize the manure and/or waste water [30,31]. Manure storage and treatment in anaerobic lagoons is usually the first step in AFOs manure handling before it goes to land application, where the solids is liquefied and nutrients are stabilized by hydrolysis and other microbial activities. However, the levels of nutrients and organic matter are still high in the lagoon effluents so other processes are needed to better utilize or treat the lagoon effluents. Since most conventional waste water treatment processes are not suitable or economical [32] for livestock waste water treatment due to its high organic strength (see swine waste water in Table 2 as an example), other potentially feasible technologies are under development for the dual purposes of waste water treatment and energy/products generation, such as anaerobic co-digestion for bio gas production [33], micro algae cultivation [34], struvite precipitation [35,36], and microbial fuel cells [4]. Successful development and application of these technologies may bring tremendous benefits to the animal industry because of their nature of converting waste to renewable energy and resources.

Table 2. Typical characteristics of swine wastewatera
 
aCharacteristics data are calculated and compiled from a comprehensive study on swine wastewater discharged at different pig production stages [37].

The potential application of MESs technology in remediation of animal waste water, food processing waste water, and other agro-industrial waste water is promising, because it recovers electrical power and chemicals (e.g., hydrogen and methane gas and fertilizer) from waste water and may reduce the cost of waste water remediation by simultaneously removing organic matter and some nutrients. Meanwhile, these waste water sources show some properties that favor the use as feed stock for MESs, such as the availability of readily degradable organic matter, strong buffer capacity, and high conductivity. Given the versatile functions of MESs and the huge energy and nutrients values contained in animal and food processing waste water, the potential application of MESs for remediation of the waste water will be very promising.

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Electrode materials  

 

Performance of MESs largely depends on electrode and catalyst materials [38]. Since the first investigation of an MFC in 1911 [39] and an MEC in 2005 [6], some significant developments have been made to optimize their performances in electricity and hydrogen generation. Considering electrical current density (either electrode surface area- or volume-based) as an indicator for efficiency of biochemical reactions in MESs, a higher current density can only be achieved when the electro-active biofilm (or microbial monolayer in some studies)forms well on anode [40], the catalytic efficiencies for both anode and cathode reactions are high [41,42], and the internal resistance of MESs is small [43]. Suitable design and materials release more energy and chemicals from oxidation-reduction reactions, minimize biological and electrochemical losses of energy, and therefore facilitate higher efficiency of MESs. Based on studies mostly focusing on synthetic waste water, this section summarizes some important considerations when choosing MESs components: cathode, anode, catalyst, membrane, and reactor configuration.

Anode

In MESs, electro-active biofilm grow on anode surface, and electrons transfer from the biofilm to anode surface. So some basic requirements on anode materials are that these materials should be biologically compatible, non-corrosive and electrically conductive. Other physical and chemical properties should be met to be qualified anode materials. The Butler-Volmer equation (Equation 1; with only the oxidation proceed) describes the inter-relationship between electrical current (I), exchange current density, and over potential (η_(act,A)) in an electrochemical reaction. It can be seen that a high exchange current density and large electrode surface area are preferred in order to reduce the over potential and generate a large electrical current. So MESs studies are trying to optimize the exchange current density (or minimize activation over potential) and increase relative surface area of electrode materials (Table 3). In large-scale reactors and real applications, the conductivity and cost of the anode materials will become critical constraints as well. Another point worthwhile to note is that anode materials provide a supporting surface for electro-active biofilm (microbial catalyst) attachment for the purpose of organic matter oxidation and electron transfer, so in theory a material shows superior property in MFCs should work well in MECs as well, and vice versa. 

where I is the electric current,joAU is the unit exchange current density of exoelectrogenic biomass, and w is the attached biomass concentration on anode,joAU w=joA is the exchange current density of anode, αA is the electron transfer coefficient of the anodic reaction, and ηact,A is the activation overpotential of anode. 

Carbon-based materials are the most common anode materials (Table 3) in MESs studies, including reticulated vitreous carbon, carbon cloth, carbon paper, carbon mesh, graphite felt, graphite foil, graphite granules, graphite-fiber brushes, graphite rod, carbon nanotubes (CNTs), graphene, and active carbon. With the modification by conductive polymer of polyaniline (PANI) on various anode materials including platinum [44], TiO2 [45], carbon felt [46], and graphene [47], the current and power output of MFCs can be increased by up to orders of magnitude. This can be a result of the enhanced conductivity and extracellular electron transfer between anode and biofilm. Another conductive polymer of polypyrrole (Ppy) [48] shows similar improvement on power generation. CNTs have been identified as a superior anode material in proton exchange membrane fuel cells and enzymatic fuel cells [49], and deposition of CNTs layers on anode surface is also effective in increasing power generation [50]. Graphite-fiber brushes have high specific surface area [51] and are promising MESs anode material especially after ammonia treatment. Graphite granule [52], granular activated carbon [53], and activated carbon nanofiber [54] have high ratio of surface area to volume than typical carbon cloth, and therefore are helpful to improve MFC power generation. An intertwined carbon nano tube-textile [55] increases anode surface area by 10-fold. It shows strong microbe-anode adhesion and lower scharge-transfer resistance at biofilm/anode interface to 30 Ω, with only 10% of the resistance (0.3 kΩ) displayed by carbon cloth at the same cell configuration. Despite better or comparable conductivity, metal- or metal oxide-based anode materials, such as stainless steel [56], titanium [57], and TiO2 [50] generate less power density than carbon-based anodes. Optimization studies about specific anode materials for agro-industrial waste water are lacking; important constituents of the waste water, interactions of anode microbial consortia and waste water, and potentially inhibitive components need special attention when choosing anode materials. 

Cathode and Catalyst

The function of MESs usuallyis defined by and depends on the cathode reaction. For example, oxygen reduction reaction (ORR) occurs on MFCs cathode, while hydrogen evolution reaction (HER) takes place on MECs cathode with the assistance of a small external power supply. Occurrence of these two reactions is cathode and catalyst-dependent, and the activation and concentration over potentials at cathode, especially when current density is high, are considered the dominant energy loss that decrease MESs performance even when Pt catalyst is present [58, 59]. Besides oxygen and proton, there are some studies using the more electrochemically reactive compound of ferricyanide ([Fe(CN)6]3-, or hexacyanoferrate) as intermediate[60,61] or terminal electron acceptor [10,48,52,62-65] in optimization studies, because the reduction of ferricyanide goes efficiently without catalysts and results in small cathode over potential. But utilization of this electron acceptor in MESs is less practical because it has to be expensively regenerated after cathodic reduction [58]. This section focuses on cathode materials and catalysts for ORR and HER. 

Cathode for oxygen reduction reaction: Since MFCs power production is generally limited by the activation and concentration over-potential of the oxygen reduction reaction (ORR) at the cathode, search for effective cathode and catalyst materials has long been undergoing. MES experiments in literature are conducted using different reactor configurations and operational conditions, thus a direct comparison of performance of different electrodes is not appropriate. A material is not necessarily unsuitable for electrode use based solely on a result without control groups; however, electrode materials with steady high performance in different experiments can be concluded as suitable materials.

In MFCs where ORR occurs at cathode and oxygen is aerated to medium in cathode compartment, the aforementioned graphite-based materials for anode can be used as cathode materials without catalyst layer, such as graphite felt [66,67], unpolished graphite [68], and granular graphite [69]. The bio-film formation on graphite cathode surface is later thought to work as catalyst [70]. Recently, based on air-cathode structure[71,72], activated carbon (AC) as catalyst is found to produce a power of 1190, 1214 and 1310 mW/m2 [73-75]. The performance is further improved by treating activated carbon with ammonia, resulting in a power density of 2450 mW/m2 [76]. Carbon nano-tubes(CNTs) [77], nitrogen-doped CNTs [78] and nitrogen-doped carbon nanofibers [79] are evaluated in catalyzing ORR, generating a power density of 147,1600, and 960 and 1900mW/m2, respectively. 

Platinum plate is used as high performance cathode and catalyst [62,64], despite the extremely high cost of this precious metal. So platinum powder bound to cathode support materials, by chemical binders (typically using Nafion) with carbon black and at a load of about 0.5 mg-Pt/cm2, is the most widely used catalyst in MFC studies [72]. A lot of air-cathode based MFCs, one of the optimal cathode structures, adopt this catalyst layer [72]. Alternatively, some other cheaper metal or metal oxide-based cathode are tested, such as golden-covered copper [80], stainless steel [56], and lead dioxide (PbO2)coated on titanium [81]. Lead dioxide (PbO2) coated on titanium performs comparably or better than Pt catalyst for power generation. 

Catalysts that are initially developed for ORR in other electrochemical cells (such as zinc-air cell, oxygen hydrogen fuel cell or a methanol fuel cell) may be used for ORR in MFCs [51,82]. Acomposite electrode consisting of macrocyclic metal complexes for reducing oxygen is highly selectivity for ORR [83]. Using iron(II) phthalocyanine (FePc) and cobalt tetramethylphenylporphyrin (CoTMPP) as catalysts, galvanostatic and potentiostatic experiments confirm that these materials are suitable replacement to Pt [84]. CoTMPP is also an effective catalyst in MFCs experiments, generating a better power density to Pt catalyst at high current density without obvious decrease in power density after repeated use [82]. When some metabolites (e.g., formate and lactate) are present in single chamber MFCs, the performance of pyrolysed-FePc based electrodes is not much affected, but Pt catalyst shows diminished performance [85]. Polypyrrole/carbon black (Ppy/C) composite as a catalyst generates a power density with a value between FePc and Pt (336.6, 401.8, and 575.6 mW/m2), while its cost is 15 times lower than Pt catalyst [86]. 

Cathode for hydrogen evolution reaction:  The hydrogen evolution reaction (HER) in MECs is limited both thermodynamically and kinetically. The extent and rate of HER can be pushed to a higher level by reducing the over-potential via choosing suitable electrode materials [87], and the reaction rate can be further accelerated by using effective catalysts [88]. Studies show that part of limitation on MEC efficiency comes from cathodic over-potential [89,90], and there are various cathode materials and constructions investigated, including carbon paper and cloth, graphite brushes, stainless steel (SS) mesh, brushes, and plate, Ti mesh, MEA, Ni foam, and biocathodes. An ideal cathode should be of a high specific surface area, noncorrosive, cost-effective, conductive and scalable. Wet-proofed (5% to 30% PTFE treated) carbon cloth is among the most extensively studied materials. In MFC studies, carbon cathodes without catalyst generate a decreased current density by a factor of 10 or more [91] compared to those coated with Pt, which could also be expected in MEC reactors; therefore, carbon-based cathodes in almost all MEC studies are coated with catalyst to improve the HER rate [6]. A 30% wet-proofed carbon cloth coated with 0.5 mg Pt/cm2 was tested as a cathode in an MEC for treatment of diluted potato processing waste water (original: pH=6.1; conductivity=5.2 mS/cm; total chemical oxygen demand (TCOD)=7.7 g/L; volatile acidity=0.69 g/L of soluble COD) [92], achieving a hydrogen production rate (Qh) of 0.74 m3/m3-d at an applied voltage (Eap) of 0.9 V. When stainless steel (SS) is used as a cathode material, Pt catalyst is not necessary. A cathode of SS brush (grade 304; 8-11% Ni) produces a Qh =1.7 m3/m3-dwhichis close to 2.0 m3/m3-d for Pt catalyst at the same MEC configuration [88]. By increasing surface area using SS mesh [93], Qhis further improved to 3.3m3/m3-d at Eapof 1.2 V, and 2.1 m3/m3-d at Eap=0.9 V.

Since Pt is a precious metal of high cost and is suspected to cause a gradual process of catalyst poisoning [38,88], alternative catalysts such as Ni and NiOx are compared with conventional Pt for their catalytic performance on cathode of wet-proofed carbon cloth [94]. Results show that at an Eap of 0.9 V, Ni-based catalysts obtained Qh of 0.9-1.3 m3/m3-dvs. 1.6 m3 /m3-d for Pt, hydrogen recovery of 73%-79% vs.75% for Pt. Their catalytic performances are thus comparable to Pt, but at significantly decreased costs from $700/m2-cathode to only $2.82/m2-cathode. Assuming 4.5 ppm of Ni dissolved in each batch of reaction, a replacement of cathode is necessary after about one year operation [94]. Since Ni-based alloys may have comparable or better catalytic capability in HER than single Ni due to synergistic electronic effect, alloys such as Ni-W-P (mass ratio: 73.42%/0.25%:26.33%) and Ni-Ce-P (67.9%:0.14%:31.96%) are made for investigation [95], which in fact outperform the non-alloy catalysts by achieving higher Qh: 1.09 m3/m3-d for Ni-W-P and 0.70 m3/m3-d for Ni-Ce-Pvs. 0.44 m3/m3-d for single Ni at an Eapof 0.9 V. Further analysis showed that a higher Qh is due to a higher cathodic recovery which might be a result of a lower cathodic over-potential. 

A biocathode demonstrates its function as a low cost and self-generating cathodic material in MECs [96]. The catalytic effect in a biocathode is evaluated in a biofilm consisting of electrochemically active microorganisms. The biofilm catalyzing HER has been grown and enriched on the surface of graphite-felt by three-phase startup procedures sequentially: startup of an acetate and hydrogen oxidizing anode, adaptation to hydrogen oxidation, and polarity reversal to a hydrogen producing biocathode and adaptation. This biocathode achieved a Qh of 0.63 m3/m3-d (in a half MEC; Eap not given), which is much higher than 0.08 m3/m3-d produced by the control electrode of graphite-felt without microorganism enrichment. A follow-up study tests similar biocathodes in full MECs at Eap=0.5 V, generating a Qh of 0.03-0.04 m3/m3-d [97]. It is likely that this low hydrogen production rate is due to the two-chambered construction with the presence of a membrane. During a prolonged (1600 h) operation of biocathodes, precipitate of calcium phosphate is suggested present on it by element analysis, thus slowly deteriorating the current density.

The best performed electrode and catalysts materials achieved with synthetic waste water are not necessarily suitable for agro-industrial waste-water, and the performance of MESs fed with the real waste-water are usually decreased. There is an increasing need to investigate electrode and catalyst materials for treating a specific type of agro-industrial waste water.

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Membranes

 

A membrane had been thought necessary part of MESs in early studies. Recent studies shed light on the idea of removing membranes to simplify the reactor design and to reduce the internal resistance [98]. With a“single-chambered” MFC design, membrane removal usually comes as a result of adopting an air-cathode structure [72] that is coated with a PTFE diffusion layer as hydrophobic treatment [99,100]. As discussed aforementioned, air-cathode single-chambered MFCs substantially improve power density of MFCs compared to two-chambered MFCs and currently is the most prevalent reactor design for MFC electricity generation through ORR. Inclusion of a membrane brings about two additional over-potentials (ohmic over-potential, and overpotential due to pH splitting across the membrane), so MESs performance could be impaired due to larger energy loss [101].

The earliest designs of an MECarea cylindrical reactor of 38 mL and an H-type reactor of 400 mL, proposed and used by Liu et al. [6]. These are two-chambered reactors. Electrodes are separated several centimeters apart (0.5 cm and 15 cm for the cylindrical and H-type reactors, respectively) by inserting a proton exchange membrane of Nafion 117 between two anode and cathode chambers. Call and Logan [98] have proposed a single-chambered MEC (28 mL) for the first time and demonstrated that high hydrogen recovery and production rate (Qh) are achievable, with maximums of 96% and 3.12 m3 H2/m3-day, respectively. A similar single-chambered design has been used by Wagner et al. [102] for swine waste water treatment and resulted in a Qh of 1.0 m3 H2/m3-day. Hu et al. [103] make a single-chambered reactor from a wide mouth glass bottle (300 mL) with carbon cloth electrodes. Electrodes are spaced 2 cm apart with a J-cloth in between to prevent short-circuit. This reactor achieves maximum hydrogen recovery of 64% and maximum Qh of 0.69 m3 H2/m3-day. Manuel et al. [104] construct a 50 mL single-chambered MEC reactor and investigated the impact of a Ni-based gas diffusion cathode on hydrogen generation. The reactor is fed on a continuous mode, resulting in a maximum Qh of 4.1 m3 H2/m3-day for 60% Ni catalyst. The authors also conclude that the gas diffusion cathode works better than solid sheet cathodes because they facilitated fast hydrogen removal from the cathode. Ni-based cathodes have higher cathodic efficiency than Pt-based possibly due to better reaction selection for hydrogen evolution reaction.

Types of ion exchange membrane (IEM) in MEC have an effect on its performance when two-chambered or MEA reactors are used. Cheng and Logan [105] evaluate the hydrogen production rate for Nafion (a typical proton exchange membrane; NafionTM 117) and AEM (AMI-7001). Results show that AEM has a higher hydrogen production rate than Nafion within the tested range of applied voltage (Eap: 0.2-0.8 V). The hydrogen production rate of MEC with AEM continuously increased with the increasing applied voltage, while the production rate was slightly reduced afterwards with the Nafion membrane at Eaphigher than 0.6 V. Sleutels et al. [89] observe that MEC with AEM (Fumasep®FAA) produces much higher current density than MEC with CEM (Fumasep®FKE) at the same Eap (1 V), 5.3 A/mvs. 2.3 A/m2. The difference in Qh is also large, 2.1 m3/m3-d (better than some single-chambered reactors) and 0.4 m3/m3-d for AEM and CEM reactors, respectively. The resistance distribution of the two MECs reveals that the internal resistance of AEM and transport resistance of ions through the AEM is much lower than that of CEM. Membrane electrode assembly (MEA) is another design of single-chambered MESs. Rozendal et al. [90] develop a single-chambered reactor (3.3 L) with the MEA supported by a Pt-coated titanium mesh. They found that the hydrogen recoveries of cation and anion exchange membranes (CEM and AEM) MEA are not very different. The hydrogen production rates, Qh, are 0.31 and 0.33 m3 H2/m3-day for CEM and AEM MEA configurations, respectively. Another study uses the J cloth electrode assembly, which places a layer of J cloth between the anode and cathode to prevent short-circuit [106]. Due to high levels of particulates in agro-industrial waste water, membrane fouling is an issue that should be thoroughly evaluated. 

 

Table 3. A summary of typical electrode and catalyst materials of MFCs and MECs, with their manufacturers listed.
 

Note:
1.    c-, short for carbon;
2.    g-, short for graphite;
3.    ss, stainless steel;
4.    CNTs: carbon nanotubes;
5.    PANI: polyaniline;
6.    Ppy: polypyrrole;
7.    BASF Fuel Cell, which bought out the E-Tek Company, does not provide electrode materials currently. 

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Performance of MESs

 

General Comparison to Conventional Processes 

The micro-environments of microbial electrochemical systems (MESs) vary widely because of a wide category of materials, operational conditions, and reactor configurations. For example, MEC anode chamber experiences pH decrease, while the cathode chamber experiences pH increase. This section summarizes the general functions of MESs with comparison to those of the conventional biological remediation technologies such as activated sludge [152] and anaerobic digestion processes. 

Biological waste water treatment processes rely on microbial growth, byproducts generation, and metabolism [153]. Microorganisms resort to enzymes to convert insoluble organic macromolecules in waste water into soluble and small products, which are used as nutrient sources and reducing power for biomass growth and respiration. The accumulated biomass begins to lyse and is then hydrolyzed to the next cycle of lysis-cryptic growth [154]. Through these recycles, the original organic matter are decomposed and gases released to atmosphere as CO2 and CH4; the remaining dead or live microorganisms may be removed from water as sludge; and free energy is liberated from the substrates. Other macronutrients such as nitrogen and sulfur may be released as gaseous components or taken up in sludge. Biomass growth and metabolism is unavoidably an important stage for organic matter removal in MFCs, expressed in the following general macro-chemical reaction(Equation 2) using acetate as a representative substrate [155,156]:


Plankton biomass in a reactor usually forms the attached biomass during operation, either on supporting surfaces or granules. The attached biomass, or the biofilm, retains sludge and reduces biomass washout, and leads to higher overall biomass concentration in the reactor and more efficient organic matter removal. The application of increasing attached biomass to enhance wastewater treatment has been successfully put into use in fixed-bed biofilm reactors (or packed-bed biofilm reactors) [157] and moving-bed biofilm reactors [158,159]. MFC requires a large ratio of the electrode surface areas (especially the cathode [160]) to the reactor volume for better performance, so it may provide ideal supporting surfaces for biofilm formations well, and may consequently facilitate COD and nutrients removal. The electrode surface area can be much more increased with the introduction of packing materials to anode, e.g., granular activated carbon [161,162]. Although there is yet no MFC study reporting the contribution of the attached biomass on organic matter removal, the requirement of large electrode surface area in scale-up MFCs is expected to enhance the role of the attached biomass. 

Besides the metabolism, redox reaction [163] in MFCs consumes substrates in wastewater treatment. Anode-respiring bacteria that feed on organic matter oxide substrates and donate electrons to anode, while the cathode accepts electrons which are eventually used to reduce oxygen gas (or other terminal electron acceptors) at cathode: 

 

The electron motive force (Eemf, in V), or the difference of electrode potentials at equilibrium, indicates the capability of the redox couple to release free energy. The real total power extracted from the redox couple can be indicated as Eemf, consisting of the power extracted from the external circuit (Eext, in a simple circuit loaded only with an external resistor) and the one used to overcome overpotentials (calculated as Heat=IEemf-IEext). The dissipated energy due to overpotential generally includes activation overpotential, mass transfer overpotential, and Ohmic resistance of the medium. 

There are two interrelated functionalities existing in redox reaction: organic matter removal and electricity generation. The rate for organic matter removal is proportional to the total extracted energy (Eemf), while the available electrical power is defined as IEext. For instance, there are many studies on MFC power generation in recent years, which has tremendously promoted the capability of MFC for electricity generation from less than 0.1 mW/m2-anode in 1999 to thousands of mW/m2-anode in 2009 [164]. However, few studies acknowledge the redox reaction for organic matter removal in wastewater treatment as an advantage to biological processes. The fact that the redox reaction diverts the flow of substrate from biomass growth to extracellular heat generation [154] is promising because sludge generation is reduced [165,166] due to this energy dissipation in MFCs. A sludge yield was observed in an MFC of 0.07-0.16 g-VSS/g-COD, while the yield in activated sludge is 0.35-0.45 g-VSS/g-COD, representing a reduction in sludge production by 52% to 82% [152]. Therefore, MFCs demonstrate the capability of reducing the handling cost of sludge by generating a less amount of sludge. 

Electricity Generation via MFCs

Microbial fuel cells (MFCs) are aimed at generating electrical energy and power directly from the oxidation of biodegradable organic matter in wastewater. However, studies for reactor and material optimization for electricity generation, microbial community, and bioelectrochemical mechanisms on the interface of bacteria and electrodes have so far been mostly based on synthetic wastewater consisting of readily degradable organic matter such as short-chain fatty acids (acetate,propionate, butyrate, etc.), glucose [60] or other monosaccharides [167], starch, lactate, and molasses [168]. Some types of real wastewater were also tested for single chamber MFCs, e.g., air-cathode MFCs, including municipal wastewater [169,170], brewery wastewater, cheese whey, liquid dairy and swine manure, etc. 

Table 4 lists the performance of electrical power generation by MFCs from a range of real wastewater in different studies. Most of the studies use single-chamber air-cathode MFCs because this design allows the use of the cheap electron acceptor for cathode reaction, i.e., passive oxygen flow from the air-permeable cathode, to achieve high power density based on electrode surface area. It also saves aeration cost compared to the forced aeration cathode or the aeration implemented in activated sludge processes to which about half of the plant energy cost is attributed. The power densities based on electrode surface area lie in the range of 50 and 500 mW/m2, with the best power density achieved of 483 mW/m2 fed with brewery wastewater supplemented with phosphate buffer solution (PBS) [171]. The power densities based on reactor volume are mainly between 5-20 W/m3. These values are consistently lower than values in studies for reactor optimization with synthetic wastewater, e.g., 4300 mW/m2 and 2.87 W/m3 achieved in a double cloth electrode assembled MFC [125], which presents an explicit challenge for real applications that deserve further research. Another study of MFCs fed the reactors with diluted food-industry wastes of fermented apple juice, wine lees and yogurt wastewater, and generates power densities between 37 and 92 mW/m[172]. Unlike using defined medium, actual waste water is more complex in their organic composition, and may even include inhibitory factors for the bioelectrochemical system operation. For example, it is found that the maximum power density and Coulombic efficiency, CE, of MFCs achieved using waste water are consistently lower than those using acetate solution [9].

 

Table 4(a)

Table 4(b)

Table 4. Electrical power generation by microbial fuel cells fed with real wastewater
 


Biohydrogen Production via MECs

Degradable organic matter (even refractory in some cases) in liquid condition are food for microbes growing and functioning in anodic solution or biofilm as carbon source and electron donor during substrate oxidation [195]. They cover a wide range of chemicals, e.g., volatile fatty acids, glucose, glycerol, cellulose, and complex substrates in wastewater. Some requirements on organic substrates are summarized as [101] cheap, readily degradable, containing nutrients, and suitably conductive. Acetate is a common byproduct of anaerobic digestion, dark fermentation for hydrogen production, and other wastewater treatments [101,196], and it is therefore chosen as a typical substrate for preparing synthetic wastewater for MEC studies by many researchers. Figure 1summarizes the acetate-fed MEC performances on hydrogen production until the year of 2011. Both the hydrogen yield and production rate are experiencing significant improvements, reaching 2.82 mol-H2/mol-acetate and 1.79 L-H2/L-d on average (Table 55) at a reasonable range of Eap (600-1000 mV), respectively. Several studies have almost reached the theoretic maximum of 4 mol-H2/mol-acetate. 

Figure 1. Hydrogen production rate and yield achieved in an acetate-fed MEC. The red point indicates the average values with error bars representing one standard deviation of the samples.

 

Besides acetate, other volatile fatty acids (butyrate, lactate, propionate, and valerate) are tested for hydrogen production in MECs [8]. These acids generated 67%-91% of the theoretical maximum of hydrogen yield, and have production rates between 0.14-1.04 L /L-d. Both values are slightly lower than those of acetic acid, but successfully demonstrate their feasibility of used as MEC influent. MEC studies also use other defined substrates such as glucose [197,198], cellulose [8] and glycerol [198, 199]. The effluent from dark fermentation is used as influent substrate for biohydrogen production through MECs [200,201]. MEC hydrogen production is also tested for treating domestic wastewater which was obtained from the effluent of a primary clarifier of a wastewater treatment plant (WWTP) [9]. The COD removal in the two-chambered MEC ranged between 89% and 97% at Eap of 0.23-0.59 V. This test results in a maximum overall hydrogen production of 0.031 g-H2/g-COD-consumed, which is 24.6% of the theoretical maximum (0.126 g-H2/g-COD-consumed). A low CE, between 5.2% and 44%, is partly attributed to this low hydrogen generation. Full-strength (12,825 mg-COD/L) and diluted swine wastewater as influent for a single-chambered MEC gives an Eap of 500 mV [102], achieving a hydrogen production rate of 0.9-1.0 L-H2/L-d and a rate comparable to the acetate-fed MEC. Meanwhile, 69% to 75% of COD is removed from the full strength wastewater during the 184 h treatment. Part of the COD removal is because of the methane production, which process should be inhibited when pure product of hydrogen gas is the desired product. Using diluted swine wastewater (1,298 mg-COD/L), a two-chambered MEC achieved a hydrogen production rate and COD removal of 0.061 L-H2/L-d and 45%-52%, respectively [202]. Another MEC reactor running on biodiesel wastewater generates hydrogen gas at a rate of 0.14 L/L-d (at 500 mV) and 0.41 L/L-d (at 900 mV), giving a maximum yield of 0.047 g-H2/g-COD [198]. A pilot-scale MEC (1 m3) is tested for winery wastewater treatment and hydrogen gas production [203]. At an Eap of 900 mV, removal of 62% of soluble COD is achieved when the winery waste water has been enriched with volatile fatty acids concentration under raised temperatures. The overall gas production rate reaches 0.19 L/L-d, while most of the gas was methane and carbon dioxide, likely due to the conversion of H2 through hydrogenotrophic methanogens. The study concludes that a prolonged period of MEC operation encourages the growth of hydrogenotrophic methanogens for methane generation. 

Table 5(a)

Table 5(b)

 

Table 5. MEC performances on several common substrates and operational parameters
 

Note: 1. Some of these studies did not aim to improve MEC performances, but to analyze electrochemical properties of reactors. Some data were calculated by the authors from data provided in the original articles. 2.The reference [198] measured CE, rcat, and H2 yield during different lengths of time periods

Other Fuels and ChemicalsProduction via Microbial Electrosynthesis

Microbial electrosynthesis potentially offers a way very different from traditional microbial platforms for producing chemicals from the carbon source of carbonaceous substances. The microbial electrochemical systems used for chemical synthesis are sometimes termed as microbial electrosynthesis cells [206], and can be potentially used for wastewater such as liquid animal manure which has high alkalinity. Besides for hydrogen production, MECs are examined for methane production from carbon dioxide through biocathode via electromethanogenesis [207,208]. From the comparison between the small amount of hydrogen gas generation via abiotic cathode and the much more amount of methane gas generation via biocathode, the biocathode produces methane through a way independent from the abiotic hydrogen gas generation. Based on results of denaturing gradient gel electrophoresis (DGGE), sequencing, and phylogenetic analyses, the populations on cathode are composed of several phylotypes of the Archaea domain, including Methanobacterium palustre, Methanoregula boonei, and Methanospirillum hungatei., and several gram-positive bacteria phylotypes of the Bacteria domain, such as Sedimentibacter hongkongensis, Clostridium sticklandii, andClostridium aminobutyricum [207]. A biocathode inoculated by hydrogenophilic methanogenic culture generates methane gas at a high rate (55 mmol d-1gVSS-1), and the rate is highly dependent on the controlled cathode potentials in the tested range between -650 and-900 mV vs. SHE. It is proposed that the extracellular electron transfer by hydrogenophilic methanogens plays an important role in the bioelectrochemical reduction of carbon dioxide to methane but unclear how this worked [209]. Several mechanisms may explain the process of extracellular electron transfer from cathode to biofilm: in direct electron transfer, c-type cytochromes sometimes together with hydrogenases play a critical role; in mediated electron transfer, some microbial self-excreted chemicals, such as pirroloquinoline quinone (PQQ), can be involved as natural redox mediators [210]. It’s a traditional view that electron transfer in anaerobic aggregates can be realized by shuttle molecules such as hydrogen and formate for methanogenic process [211-213], but the alternative is also becoming accepted that the direct extracellular and interspecies electron transfer can play a role as well [214]; therefore, it seems feasible that biocathode accepts electrons both directly and indirectly for methane generation during microbial electrosynthesis.

Based on these studies of microbial electrosynthesis, other pathways for generating a wide range of chemicals from carbon dioxide have been investigated. It is found that Sporomusa ovate biocathode growing on graphite converts carbon dioxide to acetate and a small amount of 2-oxobutyrate [215]. Some other species are tested for electrosynthesis in a follow-up study: Clostridium ljungdahlii, Clostridium aceticum, and Moorella thermoacetica. Acetate is generally the primary product,and 2-oxobutyrate and formate also are formed. For C. aceticum,2-oxobutyrate is a product equally important as acetate [216]. In a long-term study, an acetate production rate of 17.25 mM day-1 (1.04 g L-1 d-1) and hydrogen production rate of 100 mM day-1 (0.2 g L-1 d-1)are achieved by biocathode dominated by the acetogenic Acetobacterium spp. [217].Carbonaceous substances from MFC anode by degradation of organic matter can be converted to formic acidat 4.27 mg L-1 h-1 through the application of the in situ generated electricity [218]. Feeding acetate as electron acceptor, medium chain fatty acids caproate and caprylate can further be generated by biocathode possibly with concomitantly generated hydrogen as intermediate electron donor [219]. 

Besides choosing or engineering effective microbial strains, modification on cathode provides another way of improving the chemical production rate. Modifications of carbon cloth, e.g.,enhancement with positive-charge surface, increases microbial electrosynthesis rate: functionalization with chitosan or cyanuric chloride enhances acetate production rates by 6 to 7 folds; modification with 3-aminopropyltriethoxysilane and polyaniline both results ina 3-fold increase; and treating carbon cloth with metal particles gives electrosynthesis rates with up to6-fold increase [220].

Ammonia Removalvia Volatilization

A shared property of MFCs and MECs is that the anode is responsible for biological degradation and energy release from organic matter. The anode then transforms part of the chemical energy to electrical energy for utilization on cathode, while the structural and functional variance on the cathode offers versatility of MESs for different purposes. Ammonium is the major form of nitrogen compounds in animal wastewater. It is a weak acid, existing in equilibrium with ammonia, the unprotonated form, which is dissolvable in water. So the equilibrium of ammonia concentration is dictated by the ammonium dissociation constant (Kd= (CNH3*10(-pH))/CNH4+) and the Henry’s law constant (Kh= PNH3(g)/CNH3(l), in atm (mg/L)-1, where PNH3(g)=RTCNH3(g). Ammonium-ammonia dissociation equilibrium and gas-liquid equilibrium are as follows:

The ORR through catalyst at the cathode side of an MFC consumes proton and generates hydroxide. The elevated pH then drives the ammonium ion to form ammonia which can permeate outwards through the gas-diffusion layers of the air-cathode [179,221]. As a result, the total ammoniacal nitrogen (TAN) concentration of waste watercan be reduced. This process at the air-cathode resembles the ammonia removal in membrane contactors [222,223], but could be more effective due to the local pH elevation. This volatilization process doesn’t work well in aqueous cathode where air stripping is necessary to achieve acceptable ammonium removal from reject water [224]. 

In an air-cathode MFC fed with liquid swine manure, TAN concentration decreases by 60% from 188 to 76 mg-N/L after 5 d operation [179]. While the majority of ammonia loss is considered a result of volatilization through the cathode, the presence of ammonia-oxidizing bacteria on cathode also indicates that a small portion of ammonia could be biologically oxidized due to the oxygen diffusion. Nitrite and nitrate level remain low and constant, and the TAN removal follows a pseudo-first order kinetic in air-cathode MFCs fed with liquid swine manure, resulting in a half-life time of 7.8 d [191]. With the optimization of the diffusion layer and structure of the air-cathode, the TAN removal rate for synthetic waste water is substantially improved, with a half-life time reduced to only 2.5 and 0.67 days [191]. Another study treating urine recovers ammonium at a rate of 3.29 g-TAN/m[221]. 

Nitrogen Removal by Nitrification and Denitrification

Another way of removing ammonium is through nitrification followed by denitrification. Nitrification is fulfilled by chemoautotrophic nitrifying bacteria under aerobic condition in two separate reactions: ammonia is oxidized in the first step to nitrite by ammonia-oxidizing bacteria with the assistance of ammonia monooxygenase, and the nitrite is then oxidized in the second step to nitrate by the nitrite-oxidizing bacteria with the help of nitrite oxidoreductase. The growth of nitrifying bacteria is slow since the energy release of the redox couple of nitrite/nitrate is small, so an additional step of nitrifying bacteria enrichment in mineral salt medium in aerobic cathode compartment or on air-cathode is needed. However, it is also observed that nitrifying bacteria naturally grow and function on air-cathode [225] or on electrodes in medium with or without organic substrates [226,227]. 

Denitrifying bacteria can use nitrate as electron acceptor in the absence of oxygen and result in gaseous nitrogen-containing chemicals such as N2, NO, and N2O. These bacteria are either heterotrophs or autotrophs. Among them, hydrogenotrophic denitrifiers are especially of interest because the electron donor, hydrogen, is ubiquitous, and they are more efficient in nitrate removal than bacteria relying on other organic electron donors [228]. To that end, bio-electro-reactors, which generate hydrogen gas during water electrolysis in electrolytic mode, has been devised to provide hydrogen gas for hydrogenotrophic denitrifiers and to simultaneously remove nitrate. As this type of reactors consumes external electrical energy, it is not as sustainable as a reactor that can provide electrons from wasted organics. Given that a potentiostat-poised graphite cathode is able to provide electrons to Geobacter metallireducens for partial denitrification [229], the bio-cathode (biofilm as the catalyst) of MFCs is further conceptualized for nitrate removal [230]. The work leads to the development of a two-chamber MFC with granular graphite as the bio-cathode support, and this MFC is effective to provide electrons to the denitrifying biofilm and decreased nitrate at a rate of 146 g NO3-N m-3 d-1 [231]. Using biocathode grown on carbon cloth, another study removes nitrate at a rate of 2.04 g NO3-N/m-2 d-1, with nitrate being converted to either ammonium or nitrite as the predominant product depending on the applied voltage [232]. By combining the nitrification and denitrification processes (by obligate denitrifiers) in aerated cathode chamber, ammonium removal issuccessfully accomplished using MFCs [233,234].

Phosphorus Accumulation

In a two-chamber MFC with both anode and cathode operatedunder anaerobic condition, the pulverized FePO4, or the grinded digested sewage sludge which naturally contained FePO4, is added to the cathode chamber to serve as electron acceptor [235]. This ferric reduction mobilizes the FePO4 particles and results in ortho-phosphate species, which is then precipitated as struvite crystals as a fertilizer (MgNH4PO4 6H2O) from the cathode effluent after Mg2+ and NH4+ addition. However, immobilized phosphate has not been foundto be present in the form of FePO4 in animal wastewater, so this mobilization capability of MFCs may not find its direct use unless ferric or ferrous was artificially added. 

Another study observes natural struvite precipitation on air-cathode when swine wastewater is used as the substrate [236]. It is proposed that it is the elevated pH in the vicinity of cathode caused by ORR that decreases struvite solubility and promotes crystallization and precipitation [12]. In this reactor, 70-82% of initial total phosphorus is removed, and about 4.6-27% of the total phosphorus is recovered by way of precipitates on the cathode surface, the solids areconfirmed to be mainly consisting of struvite [236]. Phosphorus removal via struvite on cathode is also achieved in single chamber MECs. The struvite precipitation rate of 0.3-0.9 g/m2-h isobtainedwith up to 40% of soluble phosphate removed from the influent [237]. The precipitates on the cathode need to be cleaned regularly if power generation of an MFC is a priority [238]. The principle of elevating pH for precipitation is early proved working in electrolytic cells [239], but the electrolysis process requires substantial external power supplies so is less attractive than MFCs and MECs. 

Sulfur Removal

Sulfur is a widely available nutrient in animal wastewater, and elemental sulfur and sulfur compounds are electrochemically active materials. In anaerobic condition, it is known that sulfate serves as electron acceptor and so is reduced to sulfide by sulfate-reducing bacteria. Sulfide is corrosive and emits to biogas as hydrogen sulfide which is odorous and needs be removed before the biogas can be used in subsequent facilities. Oxidation of sulfide to elemental sulfur takes place on MESs anode either in biotic or abiotic ways; if the process is effective, it can be a useful method for sulfide removal because solid sulfur can potentially be separated from the liquid medium. An anode of granular graphite is found to remove sulfide in synthetic wastewater, despite unclear either via biotic or electrochemical pathways although sulfide-oxidizing species are isolated from anode biofilm [240]. Another study evaluates graphite foil, carbon fiber veil, and activated carbon cloth for electrochemical oxidation of sulfide which has been generated from synthetic wastewater by sulfate-reducing bacteria of Desulfovibrio desulfuricans. Activated carbon cloth outperforms the other two materials for sulfide adsorption and oxidation. Sulfide oxidation on anode surface to solid sulfur is confirmed as a result of abiotic electrochemical oxidation, and the solid sulfur is furthermore proposed as an effective modification over granular graphite for extracellular electron transfer [241]. Studies based on carbon paper anode also confirm the abiotic process of sulfide to elemental sulfur conversion, while further the step of elemental sulfur to sulfate is assisted by microbial oxidation [242]. A lab scale fuel cell is developed based on the abiotic sulfide oxidation for simultaneous electricity generation, which removes aqueous sulfide continuously for 2 months at a rate of 0.62 ± 0.1 kg-S m-3 d-1 [243]. The anode will have to be periodically cleaned by removing deposited elemental sulfur to maintain effective sulfide removal. This process may be applied to anaerobic digestion for sulfide remediation, and was found to substantially decrease hydrogen sulfide level in biogas when electrolytic mode was operated [244]. 

Top ↑

Challenges

 

There are several issues that should be addressed before microbial electrochemical systems (MESs) can be a competitive technique in agro-industrial wastewater remediation, power generation, and fuels and chemicals production. First, although the objectives of the MESs study aims to effectively convert organic contaminants in various types of wastewater to hydrogen and methane gas, there are little amount of literatures using actual agro-industrial waste water for the optimization of electrode and catalyst materials. Some novel materials with confirmed performance in synthetic wastewater have not yet been evaluated in agro-industrial wastewater, not to mention that the materials tested ashigh performance ones in synthetic wastewater aren’t necessarily guaranteed to perform well in agro-industrial wastewater [203]. MESs may run on wastewater of high COD (up to several g-COD/L) as a substrate without diminished performance [102], and the operational concentration range of substrate is wide (from about 100 mg-COD/g) which benefits its steady performance [138,140,245]; however, some contaminants and metabolites of fermentation may bring about inhibition to microbial and chemical catalysts. Some properties of waste water may decrease the performance of MESs (Figure 2), e.g. particulates in wastewater may lead to clogging of electrodes and membrane when it’s present, and the presence of refractory constituents in wastewater makes it necessary to physically or chemically pretreat wastewater or to integrate MESs with other processes [246,247]. 

 

Figure 2. The complex environment of an MFC: opportunities for scientific study and challenges for commercial applications. Cited from [32].

 

 

Second, energy lossesin MESs, mainly through overpotentials, need to be reduced to achieve higher energy and chemical production efficiency. For MFCs, the open circuit potentialis usually observed much less than its theoretical value of equilibrium [101]. Part of energy released from oxidation of substrate by exoelectrogenic bacteria is therefore lost. For the MECs, the same problem of large overpotential occurs, which increases the applied voltage to higher than the equilibrium value and decreases the energy efficiency [196]. Applying an effective anode or cathode catalyst [82], cultivating and genetically modifying the electrochemically active bacterial consortia [101,248], and increasing the surface area of electrodes for microbe attachment [38] or for hydrogen evolution reaction [88] may reduce this overpotential. Concentration overpotential, a term describing the limit of mass transfer process, causes significant energy loss when a large electrical current is present in the system. An important part of concentration overpotential comes from the proton transport; therefore, applying a more strengthened buffer solution may help reduce the concentration overpotential [249, 250]. In addition, the phosphate buffer solution itself works as a homogeneous catalyst for HER [251, 252]. Strategies to reduce internal resistance of the system will reduce this energy loss, e.g., using more conductive materials as electron collect and electrode, elimination of ion exchange membrane in reactor design, increasing the electrolyte concentration [253], rearranging the electrode spacing [103], etc. 

Finally, there are some other issues particularly related to the scale-up of MESsfor practical applications with agro-industrial wastewater, e.g., cost of electrode and catalyst, flow pumping, temperature and pH control, and electrode and cell maintenance [254]. Some experimentsshow that inoculation is more challenging for a scale-up reactor, resulting in a much prolonged startup period [203]. Operational factors, such as a condensed concentration of volatile fatty acids, favorable pH and temperature (30oC), therefore need to be adjusted for a better inoculation. Methanogens could be enriched during the longer period of operation that would significantly consume the hydrogen gas produced, which is not good if hydrogen is the preferred product. Meanwhile, cheaper chemical catalysts for cathode reactions, rather than Pt-based high price catalysts should be evaluated for agro-industrial wastewater in terms of their overpotentials. Only until then can the MES system be used to effectively treat variety of wastewater and produce hydrogen gas and/or electricity as a renewable energy source for beneficial purposes. More studies of scale-up and long-term evaluation of MESs are needed for nutrient recuperation. 

Top ↑

Conclusions 

 

This review indicates that the MESs is a promising technique that can provide an opportunity for remediation of organically contaminated waste water and, at the same time, for hydrogen/methane gas production from organic substrate, other chemicals synthesis (e.g., short and medium chain fatty acids) from dissolved carbonate, electricity production and nutrient accumulation, despite that continuous research effort is needed to clear many technical barriers. The major bottlenecks may lie in the low yields and efficiencies of treatment and by-products production such as biohydrogen/electricity when real wastewater is used, followed by the cost of materials (cathodes, anodes, and chemical catalysts) to build MFCs and MECs as well as their proper maintenance, especially in a large scale. With most available information in the existing literature coming from lab-scale studies using synthetic wastewater, designing and experimenting scale-up reactors fed on real industrial wastewater to further examine the feasibility and practicality of the technique is expected to be the next step towards better understanding the process in order to utilize this advanced technique to protect the environment and reduce our consumption of valuable natural resources in a long run. 

Top ↑

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