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Electrochemical Membrane Bioreactors for Sustainable Wastewater Treatment: Principles and Challenges Jinxing Ma1, Zhiwei Wang*,1, Bin Mao2, Junyao Zhang1 and Zhichao Wu1 1

State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, P.R. China; 2China Academy of Urban Planning & Design, Shanghai Branch, Shanghai 200335, P.R. China Abstract: Membrane bioreactor (MBR) is a reliable and promising technology for wastewater treatment and reclamation. Recently, the electrochemical process has been integrated into MBRs, which refers to as an electrochemical membrane bioreactor (EMBR). In this paper, fundamental information of EMBRs, including reactor design and construction materials, is critically reviewed. Comprehensive assessment regarding EMBR performances and their economic and environmental significances is presented based on the recent publications. The existing challenges and future research prospects of EMBRs towards practical applications are also discussed.

Keywords: Electrochemical membrane bioreactor, microbial fuel cell, membrane fouling, wastewater treatment. 1. INTRODUCTION In past decades, many critical problems associated with the insufficient access to the clean and fresh water have been widely acknowledged [1]; up to 2014 there are still 750 million people around the world lacking access to safe water, and diarrhea caused by inadequate drinking water, sanitation, and hand hygiene kills an estimated 2,300 people per day globally [2, 3]. Meanwhile a growing number of emerging contaminants, such as endocrine disrupters and nanoparticles, are entering the water ecosystem due to human activities in both developing and industrialized nations. Public health and environmental concerns on water supplies lead to the stringent discharging and regulatory standards, which, as a result, drives the academic community to develop more effective, low-cost, and reliable technologies to purify the contaminated water. Among the alternatives, membrane bioreactors (MBRs) are definitely illuminating. This technology combines filtration membrane and bioreactors, and has gained worldwide popularity owing to its distinguished advantages over conventional activated sludge (CAS) process, including smaller environmental footprint, higher treatment efficiency and lower sludge production [4-7]. Moreover, MBRs can produce high-quality effluent that is more suitable for irrigation and other industrial applications, which make them ideal for localized and decentralized wastewater treatment, especially in developing countries [1, 5]. In spite of these tempting visions, the practical application of MBRs is still compromised by disadvantages such as higher energy consumption (0.6~1 kW/m3 of *Address correspondence to this author at the State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, P.R. China; Tel: +86-21-65980400; Fax: +86-21-65980400; E-mail: [email protected]

2212-7186/15 $58.00+.00

permeate) compared to other alternatives (e.g., ~0.3 kW/m3 for the CAS process). Membrane fouling that leads to temporal flux losses is another obstacle to the efficient application of MBRs, which can inevitably increase the operational cost by 3~6% due to the regular cleaning in place [8, 9]. Moreover, aeration scouring is a commonly-used approach to alleviating reversible fouling in conventional MBRs, which is, however, energy-intensive and results in the emission of greenhouse gas (GHG). Expectedly, the MBR technology can move forward on condition that the resource and energy in the wastewater are well recovered to partially offset the negative impacts of the treatment process on the environment. Wang et al. from University of Science & Technology of China made an early effort to integrate the bioelectrogenesis process into MBRs, referring to either as a membrane bioelectrochemical reactor or an electrochemical membrane bioreactor (EMBR) [10]. Wang et al. adopted a dual-chamber MBR taking stainless steel mesh with biofilm formed on it as both the cathode and the filtration material, and obtained efficient bioelectricity production (e.g., a maximum power density of 4.35 W/m3 and a current density of 18.32 A/m3) and high removal of organic pollutants and ammonium [10]. Since then, a number of papers regarding this topic have been published, and the findings showed that the development of EMBRs may simultaneously solve the problems that hindered the application of MBRs, such as membrane fouling and waste sludge production [11, 12]. On the other hand, the EMBRs could be alternatively constructed with the introduction of external electric field in conventional MBRs, which primarily aims at improving process performance (e.g., membrane fouling control) [1315]. Despite the growing interest in this area, synthesis of the knowledge of EMBRs is scare. Additionally, evaluation of potential challenges related to this technology is lacking. The objective of this paper was, therefore, to provide a state-of-the-art review of the principles and challenges in EMBRs for sustainable wastewater treatment. Fundamental information of the EMBRs, including reactor design and © 2015 Bentham Science Publishers

Electrochemical Membrane Bioreactors for Sustainable Wastewater Treatment

construction materials, was presented based on the recent publications. Comprehensive assessment regarding EMBR performances and their economic and environmental significances was also carried out. Finally, the existing challenges and future research prospects were discussed. With the increase of research efforts in this field, a critical review and synthesis of obtained research achievements could provide important information to researchers, operators and companies in advancing EMBR technologies.

Current Environmental Engineering, 2015, Vol. 2, No. 1 Resistance

Influent wastewater

e

-

e

Substrate

39

Treated wastewater

-

O2

NO3-

H+

2. PRINCIPLES: DESIGN OF ELECTROCHEMICAL MEMBRANE BIOREACTORS

H 2O

CO2 + H2O

Air

2.1. EMBR Fundamentals As shown in Fig. (1), a typical EMBR is comprised of an anodic chamber and a cathodic chamber. The anodic chamber, where the electrons are driven from the metabolism of influent organic matters to the anode, can be not only constructed according to the design protocols of that in MFCs [16] but developed from the existing unit (e.g., anaerobic zone in the MBR) with the implement of conductive materials [17]. At the cathode side, electron acceptors including oxygen (air and dissolved oxygen) and nitrate/nitrite have been used [10, 18, 19]. A series of proton exchange pathways are employed, such as ion exchange membrane [20, 21], nonwoven cloth [10, 11, 22], and perforated Plexiglas plate [23] to enable the transfer of H+ from the anodic chamber to the cathodic chamber. In an EMBR, the membrane module can be either independent or combined with the cathode, which refers to as filtration biocathode [10, 19]. A brief summary of EMBR components and construction materials is given in Table 1. 2.2. Filtration-biocathode EMBR Systems Typically the filtration biocathode allows dual functions, i.e., reduction of electron accepters and filtration of wastewater. Stainless steel mesh is one of the commonlyused cathode materials in such systems, and biofilm formed

Table 1.

Wastewater

N2

Separator

Fig. (1). Schematic diagram of a typical EMBR.

on the mesh, which refers to as dynamic membrane, serves as both the biocatalyst and filtration material. Figure 2a shows the schematic of a laboratory prototype of the EMBR proposed by Wang et al. [10]. The tubular anodic chamber was placed inside a Plexiglas cylindrical reactor, and the cathodic chamber was constructed by the electrode assembly outside (Fig. 2a). The anodic and cathodic chambers were separated by poly(tetrafluoroethylene)- coated nonwoven cloth supported with a perforated Plexiglas tube. Further attempts were carried out to optimize the reactor design (Fig. 2b, c) as well as to address appropriate cathode materials [12, 22, 29, 30]. Utilization of cheap coarse-pore mesh and biofilm can decrease the capital cost of EMBRs and promote the practical application of these systems. On the other hand, biofilm growth on the coarse-pore mesh is hard to control; an unbalanced thickness at the initial stage of filtration cannot enable satisfying solid-liquid separation, but overdevelopment would significantly decrease the permeability of membrane modules [31, 32]. This can inevitably increase the complexity of system maintenance. As a result, conductive microfiltration/ ultrafiltration membranes, including a variety of materials such as polyester filter cloth modified by pyrrole [33] or PANi (polyaniline)-phytic acid

Summary of EMBR components and construction materials.

Item

Materialsa

Anode

Granular graphite [10, 24], activated carbon [25], activated carbon fiber [23], graphite brush [17, 19, 26], carbon/graphite felt [17, 18, 22, 27], carbon cloth [20, 28], iron plate [29], iron mesh [13]

Electron donors

Acetate [10, 18, 20, 22, 26, 28], sucrose [24, 29], glucose [25, 27], organic matters in domestic/municipal wastewater [17, 19], organic matters in cheese wastewater [20]

Cathode

Stainless steel mesh [10, 22, 29], iron mesh [13], carbon/graphite felt [18, 23, 27], carbon cloth [20, 28], polyester nonwoven coated with multiwalled carbon nanotubes [19], porous nickel-based hollow fiber membrane [26], PANi-PA membrane [24], AQDS/PPY/PT membrane [25]

Electron acceptors

Dissolved oxygen [10, 20, 22-24, 27, 29], air/oxygen [18, 19, 28], nitrate/nitrite [17-19], H+/CO2 [26]

Proton exchange pathways

Plexiglas plate [17, 29], nonwoven cloth [10, 18], non-woven fabrics coated with PTFE [23] or PVDF [22], UF membrane [27], CEM [20, 28], PEM [25]

Membrane modules

Stainless steel mesh [10, 22, 29], nylon mesh [23], graphite felt [18], polyester nonwoven coated with multiwalled carbon nanotubes [19], MF membrane [17, 27], stainless steel mesh-based MF membrane [15], UF membrane [20, 28], porous nickel-based hollow fiber membrane [26], PANi-PA membrane [24], AQDS/PPY/PT membrane [25]

a. PANi-PA, (polyaniline)-phytic acid; AQDS/PPY/PT, anthraquinone-disulphonate/polypyrrole /polyester; UF, ultrafiltration; MF, microfiltration; CEM, cation exchange membrane; PEM, proton exchange membrane.

Anode

Graphite felt cathode

Biocathode

Anode

Ma et al.

Biofilm

Mesh

Current Environmental Engineering, 2015, Vol. 2, No. 1

Anode

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Fig. (2). Schematics of filtration-biocathode EMBRs using stainless steel mesh with biofilm formed on it as both the cathode and filtration material in (a) two-compartment, (b) upflow and (c) three- chamber configurations, and single-compartment EMBRs with a (d) conductive ultrafiltration membrane biocathode or (e) outer cathode made up of graphite felt. (Figures drawn with modifications after Wang et al. [10], Wang et al. [22], Song et al. [29], Malaeb et al. [19], Wang et al. [18], respectively).

(PA) [24], nonwoven cloth loaded with multiwalled carbon nanotubes [19], stainless steel mesh coated with polyvinylidene fluoride [15] and porous nickel-based hollow fiber membranes [26], have been developed. In the studies of Malaeb et al. [19] and Wang et al. [18], the EMBR systems were designed with filtration-biocathode exposed to the air (Fig. 2d, e). The compact configurations resemble that of single-compartment MFC systems, which enable the passive oxygen transfer to the cathodes during the filtration process. Either conductive membrane (Fig. 2d) or coarse-pore filter media (e.g., graphite felt, Fig. 2e) could be employed as biocathodes. It has been reported that these EMBR systems could provide high volume power densities (6.8 ~ 7.6 W/m3) while spare the wastewater aeration that accounts for 40~60% of energy consumption in conventional MBRs [8, 9]. By further excluding oxygen from the systems and “assisting” the potential generated by the bacteria at the anode with applying an additional voltage to the circuit [16],

it is possible to harvest hydrogen at the cathode in an anaerobic EMBR. Katuri et al. developed an anaerobic EMBR that combined a microbial electrolysis cell (MEC) with membrane filtration using electrically conductive, porous, nickel-based hollow-fiber membranes (Ni-HFMs), which was more energy-efficient (0.27 kWh/m3) at an applied voltage 0.7 V than the typical aerobic membrane bioreactors (1~2 kWh/m3) in treating low-strength wastewater [26]. 2.3. Independent-Cathode EMBR Systems Independent-cathode EMBR systems could be developed from MFCs by appending membrane filtration units for efficient solid-liquid separation [20, 27, 28, 34]. Wang et al. designed an EMBR system consisting of a biocathode MFC and a single tubular membrane module (Fig. 3a) [27]. The integration of tubular membrane could improve the performance of the MFC, but fluid recirculation was used in


this case. The energy cost of pumping fluid around was much higher than their power output. One of the effective solutions is to install the membrane modules in cathodic chambers, where aeration providing oxygen to the cathode reaction can be meanwhile used to control membrane fouling. Fig. (3b) shows the schematic of an independentcathode EMBR provided by Li et al. [20]; the reactor was built in a rectangular configuration with a separator of cation exchange membrane (CEM) between the anodic and cathodic compartments. Polyvinylidene fluoride hollow-fiber ultrafiltration membranes were installed inside the cathodic chamber [20]. Moreover, recent studies demonstrated that membrane filtration could be further used to obtain highquality effluent of anolyte in single-chamber, air-cathode MFCs [28, 34, 35]. Granular activated carbon (GAC) fluidization was applied to scour the membrane and to minimize fouling. Ren et al. adopted a second stage anaerobic fluidized bed membrane bioreactor (AFMBR) following wastewater treatment in the MFCs (Fig. 3c) [34]. Another similar design was proposed by Li et al. while the membrane module was submerged in the anodic chamber (Fig. 3d) [28]. The reactor was made of CEM in a cylindrical shape. The anode electrode was a piece of carbon cloth supported by stainless steel mesh along the interior wall, and the cathode consisting of one layer of carbon cloth with a loading rate of 0.05 mgPt/cm2 wrapped the membrane tube and was exposed in the air for passive oxygen transfer [28]. Energy balance analysis showed that, in the absence of energy-intensive air sparging, the energy produced by the EMBR systems (0.0197 and 0.047 kWh/m3) was theoretically sufficient to offset the energy demands for the system operation (0.0186 and 0.046 kWh/m3), although that for larger systems after scale-up would likely change and require further consideration. Another effective approach to construct independentcathode EMBRs is to incorporate bioelectrogenesis process into conventional MBRs for energy production [17, 21, 23], which is likely more suitable for renovation of existing wastewater treatment facilities. Wang et al. proposed a practical MFC-MBR integrated process, in which the aeration tank of an MBR was directly used as the cathode chamber (Fig. 4a) [23]. A carbon felt with the thickness of 0.6 cm was wrapped outside the separator (e.g., nonwoven cloth) of the inserted MFC, and the biofilm developed on the carbon felt subsequently catalyzed the oxygen reduction reaction [23]. In independent-cathode EMBR systems, electron acceptors other than oxygen could be used to improve the process performance on nutrient removal. For example, Fig. (4b) shows the schematic of an EMBR that was developed from the combination of a hollow-fiber MBR and MFC. Probably due to the oxygen gradient in the cathode biofilm on the carbon brushes, there might be some species of electroactive bacteria that could transfer the electrons and utilize the nitrate/nitrite as the terminal electron acceptor for denitrification [21, 36]. As a result, the total nitrogen removal rate of the EMBR was higher compared to that of the conventional MBR operated in parallel. Another design of such system was provided by Ma et al. [17], in which cathodes were installed in the anoxic zones for bioelectrochemical reduction of nitrate (Fig. 4c). This EMBR was developed from a commonly- used MBR and the results clearly demonstrate that the reactor capable of


41

providing superior effluent quality could be incorporated into existing treatment facilities for improving the sustainability of wastewater treatment [17]. 2.4. Other Types of EMBRs As mentioned above, it has been found that the introduction of electrochemical process could enhance the performance of MBRs by suppressing membrane fouling. Consequently, an external electrical field is used in some systems without the bioelectrogenesis process [13, 14, 33]. This technology is usually built by placing the cathode and the counter anode around membrane modules at an appreciable distance [13]. The electric field is expected to expel the charged foulants (e.g., macromolecules and activated sludge) that otherwise would form gel/cake layers on membrane surfaces [14]. If iron anode is used, electrically released iron can facilitate fouling suppression and phosphorus removal [13]. Electro-Fenton process can be also easily integrated into this system for recalcitrant industrial wastewater treatment. In these EMBRs, a range of electrical field densities from 0.2 to 20 V/cm have been adopted to improve the process performance [33]. Moreover, the working electrode (cathode in most cases) can be further combined with the membrane modules (e.g., conductive membrane), improving fouling-control effects while decreasing reactor footprint [15]. The production of peroxide due to the reduction of oxygen near membrane surfaces can chemically clean membrane in situ [15]. Since these EMBRs could be classified as electrolytic cells rather than fuel cells, designing the electrodes and electric field could refer to the reviews on electrocoagulation (EC) or electrolysis [37-39]. 3. PRINCIPLES: PERFORMANCE AND OPERATING CONDITIONS 3.1. Power Production Open circuit voltage (OCV) and internal resistance. For power production, the ideal performance of an EMBR depends on the electrochemical reactions (Eq.1 in Table 2) that occur between the electron donors and final acceptors, i.e., the overall cell electromotive force (emf) [16, 40]. Standard potentials (E0) of commonly-used electron donors and acceptors in EMBRs are summarized in Table 2, and the emf can be calculated using theoretical electrode potentials based on the Nernst equation [16]. Nevertheless, the ideal cell voltage of EMBRs treating municipal wastewater is uncertain because the redox potentials of the terrestrial organic matters are poorly understood and the electrons are transferred to the anode from the organic matters through a complex respiratory chain that varies from microbe to microbe [40, 41]. In an EMBR system, the OCV is always used to present the available cell voltage that can be measured after some time in the absence of current. Due to the electrode overpotentials caused by activation losses, bacterial metabolic losses and concentration losses [16], the OCV is substantially lower than the emf (Eq.2 in Table 2). Low overpotentials in EMBRs can be achieved by adding mediators and catalysts that reduce the energy barriers, improving component design (e.g., increasing electrode

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Ma et al.

(a)

(b) Influent wastewater

Resistance

Substrate

e-

wastewater

Treated wastewater

Resistance

e-

e-

Treated wastewater

e-

H2O

H2O

Pt H+

H+ Influent wastewater

O2

O2 Air

Air

Ultrafiltration membrane

CEM

Treated wastewater

(c) Resistance

Resistance

(d) e-

Treated wastewater

e-

GAC GAC H2O

e-

e-

CEM

H2O O2

H+

O2

Influent wastewater

Influent wastewater

Fig. (3). Schematics of independent-cathode EMBR assembly by integrating membrane filtration in MFCs. (a) A two-compartment MFC equipped with a side-stream tubular membrane module, (b) an EMBR with the installation of ultrafiltration membranes inside the cathodic chamber, (c) an anaerobic fluidized bed membrane bioreactor treating the effluent of single-chamber, air-cathode MFCs, and (d) a fluidized bed membrane bioelectrochemical reactor. (CEM, cation exchange membrane; GAC, granular activated carbon. Figures drawn with modifications after Wang et al. [27], Li et al. [20], Ren et al. [34], Li et al. [28], respectively).

surface area), and enhancing mass transfer. For instance, in EMBRs that utilize oxygen as the final electron acceptor cathode overpotential caused by a lack of oxygen and slow reaction kinetics limits the power density output, and catalysts including Pt [20], anthraquinone-disulphonate [25] and microbes (e.g., Rhodobacter and Hydrogenophaga [43]) and aeration have been employed to decrease the cell potential losses. In general, the OCV values of the EMBRs (i.e., 450~800 mV) [10, 17-19, 21, 23] are comparable to those using defined fermentable or complex substrates as

fuel and oxygen or nitrate/nitrite as electron acceptors [16, 44-47]. Internal resistance is another key factor which influences electricity generation in an EMBR (Eq. (3) in Table 2). In a conventional chemical fuel cell where the ohmic resistance is dominant, internal resistance could be simply considered to consist of ohmic resistance. However, EMBRs are bielectrochemical systems and microbial activities on the heterogeneous electrode surfaces play an important role,



Resistance

Resistance

(b)

(a) e-

43

e-

Treated wastewater

e

e-

-

Treated wastewater

N2

e

-

H2O NO3H2O

Influent wastewater

O2

O2 Influent wastewater

Air

(c) Influent wastewater

Air

Resistance Treated wastewater

e-

e-

e-

N2

N2

NO3-

NO3Air

Perforated plate

Fig. (4). Assembly of bioelectrogenesis process into conventional MBRs for energy production. (a) An oxic MBR with an inserted MFC compartment treating influent wastewater, (b) an EMBR constructed by integration of MFC into hollow-fiber MBR, and (c) an EMBR developed from a commonly-used MBR by introduction of anode and cathodes. (Figures drawn with modifications after Wang et al. [23], Tian et al. [21], Ma et al. [17], respectively).

which are always associated with activation (or polarization, charge transfer) and concentration (diffusion) resistances [48]. Because of the differences in configurations, materials and operational conditions, etc., the internal resistances of EMBRs have been reported to vary from ~(50~80) Ω [18] to ~(300~500) Ω [10, 17, 23, 27, 29]. Therefore, two generalpurpose indicators, i.e., projected internal resistance (rint) and specific internal resistance (ρint), are introduced herein:

rint = Rint Asur

ρint =

Rint Asur l

(4) (5)

where l represents the average distance (m) between the anode and cathode, and Asur the average area of electrode surfaces if applicable. Area of cross section of proton transfer channels (e.g., the separators) can be used as Asur when the area of electrode surface could not be well defined (e.g., carbon brush). As shown in Fig. (5), EMBRs

constructed with fine conductive materials that have higher specific surface areas (e.g., carbon felt and brush) present lower rint (< 10 Ω m2) and ρint (< 50 Ω m), which, on the other hand, increases the costs. According to Eq.5, although it is possible to decrease the internal resistance and to increase power output by shortening the distance between electrodes, adverse effects of this method should be noticed; when oxygen was used as the electron acceptor, its crossover from the cathodic chamber to the anodic chamber would cause the loss of anode potential [49-51]. Power density. In order to compare energy generation from different systems, power output of an EMBR is usually normalized to the working volume of the anodic chamber (Pv, W/m3). Since in many instances the cathode reaction is through to limit overall power generation, especially where filtration biocathode is used [19], the projected surfaces of cathodes can be alternatively utilized to characterize the power density (Pcat, W/m2). As clearly stated in previous review articles [16], power densities of EMBRs over a range

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Table 2.

Ma et al.

Standard potentials (E0) and cell emf and voltage equations. (E0 are shown against the normal hydrogen electrode, at standard conditions (298 K, pH2 = 1 bar [H+] = 1 M)). Reaction

E0(V)a

7 1 1 1 CO2 ( g ) + H + + e− → CH 3COO − + H 2O 8 8 4 4

+0.08

1 1 1 CO2 ( g ) + H + + e− → C6 H 12O6 + H 2O 24 4 4

-0.01

1 1 O2 ( g ) + H + + e− → H 2O 2 4

+1.23

1 1 O2 ( g ) + H + + e− → H 2O2 2 2

+0.70

1 H 2O2 + H + + e− → H 2O 2

+1.78

1 1 1 NO3− + H + + e− → NO2− + H 2O 2 2 2

+0.84

4 1 2 1 NO3− + H + + e− → NO ( g ) + H 2O 3 3 3 3

+0.96

5 1 5 1 NO3− + H + + e− → N 2O ( g ) + H 2O 4 8 8 4

+1.12

6 1 3 1 NO3− + H + + e− → N 2 ( g ) + H 2O 5 10 5 5

+1.25

5 1 3 1 NO3− + H + + e− → NH 4+ + H 2O 4 8 8 8

+0.88

Cell emf

Eemf = Ecat - Ean

Eq.1b

EMBR voltage

Vcell = Eemf - (|Σƞcat| + |Σƞan| + iRΩ)

Eq.2c

Vcell = OCV - iRint

Eq.3d

Anode

Cathode

a. From the data in Morel [42]. b. Ecat, theoretical cathode potential; Ean, theoretical anode potential. c. Vcell, voltage of an EMBR; |Σƞcat|, the overpotential of the cathode; |Σƞan|, the overpotential of the anode; i, the generated current; RΩ, the ohmic resistance of the reactor. d. Rint, the internal resistance of an EMBR.

of current densities can be analyzed and characterized using a potentiostat or a variable resistor box. The maximum power density of an EMBR could be evaluated by the power curve that is calculated from the polarization curve. Typically, the municipal wastewater is of low conductivity (e.g., 1000~2000 μS/cm) and the ohmic resistance is dominant in the internal resistance. As a result, the power curves interpret the power output as a pseudo-symmetrical semi-cycle. The maximum power density can be obtained where the external resistance is equal to the internal resistance [16]. According to prevalent studies, the maximum power density normalized to the working volume of a reactor has been achieved in a single-chamber EMBR (7.6 W/m3, Fig. 2e). 3.2. Treatment Efficiency For wastewater treatment systems, it is important to evaluate their chemical oxygen demand (COD) and nutrient removal efficiencies. With the integration of membrane filtration process, COD removal expected with present

EMBRs can reach up to > 95%, which is sufficient to meet stringent regulatory standards [52]. Moreover, the effluent turbidities of these reactors are always below1 NTU. It has been reported that 5.5~78% of the influent COD could be removed in the anodic chamber [10], and the portion of electrons driven from these organic matters to the anode, i.e., Coulombic efficiency (CE), is crucial for an EMBR that convert substrate into electrical current. In an EMBR treating synthetic wastewater that contains defined fermentable substrates (e.g., acetate), the maximum CE can reach up to 36% [18]. However, this index significantly decreases due to the leakage of electron acceptors (e.g., oxygen) into the anodic chamber as well as the loss of anode potential [23]. As for the cases where real wastewater is used, Coulombic efficiency could be further declined due to the energy loss caused by the fermentation of complex substrates [17, 53]. It is therefore a great challenge to improve the CE of EMBRs substantially prior to practical applications. In EMBRs, of particular significance is that complete biomass retention could be achieved through microfiltration or ultrafiltration, which is important to maintain the structure


30


45

10000

(a)

(b)

Stainless steel and conductive membrane

25 1000

Uint ( m)

rint ( m2)

20

15 Stainless steel

100

10 10 5 Carbon felt and carbon brush

0 0

100

200

300

400

500

1 250

600

Rint ()

300

350

400

450

500

550

Rint ()

Fig. (5). rint and ρint of EMBRs constructed with different materials. (Inverse triangles in Fig. (5a) represent the cathodes made of stainless steel, and those in Fig. (5b) represent the cathodes made of stainless steel and conductive membrane, respectively. Data collected from Li et al. [20], Ma et al. [17], Song et al. [29], Tian et al. [21], Wang et al. [27], Wang et al. [10], Wang et al. [23], Wang et al. [18], respectively).

stability of autotrophic aerobic microorganisms (e.g., AOB and NOB) that have low growth rate. As a result, satisfying nitrification has been achieved in EMBRs [10, 17, 18, 23]. In addition, when nitrate/nitrite is preferred as the electron acceptors, denitrification efficiency of these systems is enhanced due to the contribution of the autotrophic bioelectrochemcial pathway [17, 21, 47]. For instance, our previous study showed that the total nitrogen removal efficiency (78.2%) of an EMBR was slightly higher than that of a control MBR (74.6%) that ran in parallel, in spite of the decrease of organic loading rate of the feed for heterotrophic denitrification [17]. Voltammetric and pyrosequencing analyses revealed that this might be partly attributed to the heterogeneous denitrification fostered by the capture of electrons from the circuit.

negative charge on the membrane surface could increase its energy barrier and lessen the cohesion free energy [54]. In conventional MBRs, a range of external electric gradient from 1 to 6 V/cm has been used to improve their anti-fouling performance [1315], which is partly attributed the electrostatic repulsive force between the negatively-charged foulants and the equally negatively-charged interface. However, in filtration-biocathode EMBRs (Fig. 2) the internal electric field (gradient of ~0.1 V/cm) is generally constructed by connecting the biocathode and the counter anode, and the cathode potential is always positive at the steady state (Table 2). A possible explanation for reduced membrane fouling in these systems might be the accumulation of reduction products (e.g., OH-) at the cathode surfaces due to the limitation of mass transfer, which could subsequently increase the electrostatic repulsive force. Nevertheless, it should be noticed that the efficiency of electric field could be impacted by the charge heterogeneity of the foulants. For example, Huang et al. found that their EMBR was much effective in mitigating polysaccharides-related fouling because proteins contain negatively charged carboxyl groups and positively charged amine groups [15].

3.3. Membrane Fouling Currently membrane fouling is still a major obstacle that hinders the rapid commercialization of MBRs [4, 7]. In EMBRs, membrane fouling alleviation has been achieved due to the introduction of the electrochemical process [11, 33]. According to previous studies, several proposed mechanisms are summarized herein to interpret this phenomenon, which include: •

Electrostatic repulsive force. The surfaces of sludge flocs and soluble foulants (e.g., soluble microbial products) are generally negatively charged, and on the basis of the extended Derjaguin-Landau-VerweryOverbeek (DLVO) theory, an appropriate increase of

•

Generation of oxidants. Another mechanism possibly responsible for the mitigated membrane fouling in EMBRs is the generation of some reactive oxygen species [11, 15]. As shown in Table 2, a number of oxidant (e.g, peroxide, superoxide,

46


hydroxyl radicals) could be formed at the cathode by two-electron reduction of oxygen and a series of subsequent one-electron transfer in the presence of Fe(II) and Fe0 (e.g, stainless steel), which could be insitu used to degrade the membrane foulants. It has been reported that the steady-state concentration of peroxide was ~ 1 mg/L in the catholyte close to the cathode [11, 15]. •

Modification of the rheological and physicochemical properties of the bulk solution. As stated in Section 3.2, a portion of the influent COD could be removed in the anodic chambers of the EMBRs due to the unique microenvironment that allows many reactions that are inherently incapable by strict anaerobic or aerobic technologies to happen [55]. Consequently, sludge reduction has been observed in the membrane zone in previous studies [17, 56]. Among the academic community, it has been well accepted that membrane fouling rate is related to the rheological properties of the mixed liquor [57]. The decrease of mixed liquor suspended solid concentration could reduce the sludge viscosity, which was conducive to maintaining permeate flux [6, 17]. Moreover, with the integration of bioelectrogenesis process, the physicochemical properties of the sludge flocs can be improved. Tian et al. concluded that there were four remarkable effects on the sludge flocs in an EMBR, including reduction of loosely bound extracellular polymeric substances, increase of protein/carbohydrate ratio in the soluble microbial products, homogenization of floc size and inhibition of filamentous bacteria growth [21]. With these effects the sludge in the EMBR had better dewaterability and filterability, and the formed cake layer on the membrane was less compressible with higher filterability [21].

Ma et al.

exoelectrogenesis process might be time-lag [17]. The microbial communities had certain robustness to the deterioration of living conditions, e.g., at low temperature. However, with prolonging of operation at low temperature, microbial activity of the exoelectrogenic community was inhibited and the current output was significantly declined [17]. In long-term operation, temperature threshold is about 15 ºC. 4. ECONOMIC AND ENVIRONMENTAL SIGNIFICANCE Previous studies have collectively demonstrated that the EMBR technology possesses the distinctive advantages, such as energy recovery, wastewater reclamation, membrane fouling alleviation, etc. To better understand its economic and environmental significance, energy consumption in the EMBRs should be well evaluated. Generally, the theoretical energy consumed for recirculation, filtration, aeration and sludge discharge (Ecp, kWh/m3 wastewater) can be calculated using the pump power requirement equation [17, 18, 26], as:

QiγΔhi 1000q ηi i=1 n

Ecp = ∑

(4)

where Qi is the flow rate for recirculation, filtration, aeration and sludge discharge (m3/s), γ is 9800 N/m3, Δhi is the measured hydraulic pressure head loss (m), q is the influent flow rate (m3/h), and ηi is the pump efficiency. If applicable, other energy consumption (e.g., membrane cleaning and sludge treatment) should be also taken into consideration. Meanwhile the electrical energy obtained from an EMBR system, (kWh/m3), could be expressed as:

Eg =

PV V 1000q

or Eg =

Pcat A 1000q

(5) (6)

3.4. Operating Conditions During EMBR operation, optimization of the key factors affecting reactor performance can further improve the power output and contaminant removal efficiency. For wastewater treatment systems, hydraulic retention time (HRT) is very important because this parameter is not only associated with the process performance but involved in the decision of reactor footprint as well as its capital cost. Wang et al. compared the system performance at various HRTs, and found that the current density and the corresponding power density changed slightly at HRTs of 14.5~3.6 h but decreased significantly at a HRT of 1.6 h [18]. A similar pattern was noticed in view of the Coulombic efficiency [18]. Generally, the recommended HRTs in the prevalent studies (e.g., 14.5 h by Wang et al. [18] and 12. 3h by Wang et al. [22] for low-strength wastewater treatment) are similar to those used in conventional MBRs. External resistance also influences the power generation of an EMBR, while the selection of external resistance is depended on the objectives; a low external resistor is preferred to benefit electron transfer by bacteria to electrode [10] and an external resistance equal to the internal resistance will be used to obtain the maximum power output. Moreover, our recent studies have shown that the effect of temperature on

where PV is the average power density normalized to the chamber volume (W/m3), V is the chamber volume (m3), Pcat is the average power density normalized to the cathode surface area (W/m2), and A is the cathode surface area (m2). The theoretical net energy production (En) can be obtained by subtracting energy consumption (Ec) from the electrical energy recovery (Eg) from the EMBRs. Energy balance analyses showed that energy neutral has been theoretically achieved in the insight EMBR systems using air cathodes, where the energy-intensive aeration is avoided [18, 28, 34]. Nevertheless, system scaling-up from these liter-level reactors to the large-scale facilities requires further consideration. Energy balance analysis can be also used to evaluate the economic significances of system renovation with the integration of the bioelectrogenesis process. For example, Ma et al. reported that Ec of their EMBR was decreased by 20% compared to that of the control MBR (0.386 kWh/m3 wastewater) [17]. The results indicated that adopting this renovation in a traditional full-scale MBR plant with a capacity of 20000 m3 wastewater/day would save an energy cost of ~3×105 kWh/year [17].


To date, most studies have been focused on addressing the terminal/local benefits of the EMBR systems, while few of them provide a cradle-to-grave assessment of the environmental significances of the process. For instance, some of the construction materials contain high embodied energy (e.g., 56.7 MJ/kg for stainless steel mesh) and the energy payback period in the EMBRs might excess their service life. As a result, it probably requires more research efforts to analyze the economic and environmental significance of these systems using global models, such as the life cycle assessment (LCA) [58]. 5. CHALLENGES AND PROSPECTS 5.1. Challenges Faced by EMBRs The EMBR technology that simultaneously concerns energy issue and environmental aspects brings new visions towards the ultimate objective of sustainable wastewater management. Nevertheless, in view of the-state-of-the-art technology, there are several challenges to face prior to achieving the envisioned advantages of the hybrid system, as: •

High cost and low power output. In an EMBR, the high cost of construction materials and low power output from municipal wastewater have been and will remain to be the primary obstacles for the commercialization. It has been acknowledged that the fundamental improvement in economics and energy generation of such systems would still heavily rely on further breakthroughs of MFC and membrane filtration technologies.

•

Complexity and operating stability. Generally, the combination of MFC and MBR might increase the complexity of the treatment process (e.g., recirculation and mass transfer in electrodeimmobilized chambers). In long-term operation, it is also very challenging to maintain the functional stability of EMBRs. For example, temporal electrode deterioration due to fouling, corrosion and clogging of electrode materials has been observed in some studies [49, 55, 59].

•

Other challenges, including system scaling-up, electron transfer, collection and storage of the generated electricity, etc.

5.2. Prospects and Outlook As aforementioned, to make EMBRs suitable for practical applications and competitive with the mature wastewater technologies that have been widely used (e.g., conventional MBRs), the performance of EMBRs needs substantial improvements. Because part of the technical bottlenecks in EMBRs have been explored in the individual studies of MFCs and MBRs (e.g., anode/cathode modification, membrane fouling control), a series of review papers are available regarding this topic and the relevant content will not be repeated in this work [16, 40, 55, 60]. Instead, specific comments on system design as well as


47

process monitoring and control for better coordinating the synergies of the hybrid systems are provided herein. Delicate system design has been considered as a fundamental principle enabling high power production and efficient wastewater treatment. Specifically, design optimization allows the decreased distance between electrodes to enhance electricity generation while sparing the adverse effects caused by electron donor/acceptor crossover [34]. The concept of delicate design could be not only applied in new reactor construction but in system renovation. Several artificial models, such as Computational Fluid Dynamics (CFD) model and LCA, could be employed to improve the system design. CFD is a useful tool to model near field flow velocity and surface shear [61], which is helpful to optimize mass transfer in EMBRs and, subsequently, to decrease concentration associated internal resistance. On the other hand, LCA could provide a global view of economic and environmental significances of the EMBRs, especially in compartment construction and material selection and modification [58]. Another hotspot, which is relatively untapped but should be very tempting, is to monitor the process performance and operating stability using the intrinsic bioelectrochemical parameters (e.g., current and electrode potential) [55]. Compared to conventional assays and sensors, these inherent indicators are real-time and more effective, which could also save high cost for measurement. Interpretation and denoising of the signals could be further performed using mathematical methodologies, such as artificial neural network (ANN) [62]. Overall with the worldwide increase of demands in water reclamation and energy recovery, EMBRs, which combine electrochemical process and MBR technology while offering superior-quality effluent, will gain the popularity, although there are still many challenges towards the ultimate success in real-world applications. CONCLUSION The electrochemical process can be synergistically integrated into MBRs, which refers to as EMBRs. Among the academic community, a number of EMBRs using diverse configurations have been established for wastewater treatment. Such integration generates distinctive benefits, including efficient contaminant removal, reduced operating cost, sludge reduction, membrane fouling alleviation, etc. Currently, the major drawbacks of this technology are comprised of high capital cost, low power output, complexity of system design and uncertainness of operating stability. In view of these challenges, future attempts should address the individual technical bottlenecks of MBRs and MFCs, as well as the synergies of the hybrid systems. Optimization of reactor design and monitoring of process performance are also required to achieve the applications of this promising alternative towards sustainable wastewater management. CONFLICT OF INTEREST The authors confirm that this article content has no conflict of interest.

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ACKNOWLEDGEMENTS This work is financially supported by Science & Technology Commission of Shanghai Municipality (12231202100) and the China Scholarship Council (201406260080).

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Received: June 19, 2015

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Accepted: July 27, 2015