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Microbial Fuel Cells for Wastewater Treatment

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Microbial fuel cells (MFCs), which are the bioelectrochemical systems, have been developed rapidly over the past few decades and are considered as a promising technique to obtain renewable resources from wastewater. MFCs can be used to harness electricity from microorganisms during wastewater treatment. This chapter reviews recent literature on MFCs for wastewater treatment. We first introduce the concept of MFCs and summarize the materials and design of MFCs afterward. It shows that through innovative materials and design, the current density of MFCs has been greatly improved during the last decade. Microorganisms play a major role in the electricity production of MFCs and therefore, an in-depth discussion of the microbiology of MFCs was also included in this chapter. Extensive studies on exoelectrogenic bacteria and consortia are beginning to expose the mechanistic and ecological complexities of MFC biofilm communities. Yet, our understanding of electrochemically active microbes is still in its infancy, as the diverse communities have a multitude of undiscovered populations in different MFC applications. Further study is warranted to optimize design, materials, and microbiology to improve electricity recovery from MFCs.
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Book Title Biotechnologies and Biomimetics for Civil Engineering
Series Title
Chapter Title Microbial Fuel Cells for Wastewater Treatment
Copyright Year 2015
Copyright HolderName Springer International Publishing Switzerland
Author Family Name Feng
Particle
Given Name Cuijie
Prefix
Suffix
Division Institute of Urban Environment
Organization Chinese Academy of Sciences
Address 361021, Xiamen, People’s Republic of China
Email
Author Family Name Sharma
Particle
Given Name Subed Chandra Dev
Prefix
Suffix
Division Institute of Urban Environment
Organization Chinese Academy of Sciences
Address 361021, Xiamen, People’s Republic of China
Email
Corresponding Author Family Name Yu
Particle
Given Name Chang-Ping
Prefix
Suffix
Division Institute of Urban Environment
Organization Chinese Academy of Sciences
Address 361021, Xiamen, People’s Republic of China
Email cpyu@iue.ac.cn
Abstract Microbial fuel cells (MFCs), which are the bioelectrochemical systems, have been developed rapidly over
the past few decades and are considered as a promising technique to obtain renewable resources from
wastewater. MFCs can be used to harness electricity from microorganisms during wastewater treatment.
This chapter reviews recent literature on MFCs for wastewater treatment. We first introduce the concept of
MFCs and summarize the materials and design of MFCs afterward. It shows that through innovative
materials and design, the current density of MFCs has been greatly improved during the last decade.
Microorganisms play a major role in the electricity production of MFCs and therefore, an in-depth
discussion of the microbiology of MFCs was also included in this chapter. Extensive studies on
exoelectrogenic bacteria and consortia are beginning to expose the mechanistic and ecological
complexities of MFC biofilm communities. Yet, our understanding of electrochemically active microbes is
still in its infancy, as the diverse communities have a multitude of undiscovered populations in different
MFC applications. Further study is warranted to optimize design, materials, and microbiology to improve
electricity recovery from MFCs.
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1Chapter 18
2Microbial Fuel Cells for Wastewater
3Treatment
4Cuijie Feng, Subed Chandra Dev Sharma and Chang-Ping Yu
5Abstract Microbial fuel cells (MFCs), which are the bioelectrochemical systems,
6have been developed rapidly over the past few decades and are considered as a
7promising technique to obtain renewable resources from wastewater. MFCs can be
8used to harness electricity from microorganisms during wastewater treatment. This
9chapter reviews recent literature on MFCs for wastewater treatment. We first
10 introduce the concept of MFCs and summarize the materials and design of MFCs
11 afterward. It shows that through innovative materials and design, the current
12 density of MFCs has been greatly improved during the last decade. Microorgan-
13 isms play a major role in the electricity production of MFCs and therefore, an in-
14 depth discussion of the microbiology of MFCs was also included in this chapter.
15 Extensive studies on exoelectrogenic bacteria and consortia are beginning to
16 expose the mechanistic and ecological complexities of MFC biofilm communities.
17 Yet, our understanding of electrochemically active microbes is still in its infancy,
18 as the diverse communities have a multitude of undiscovered populations in dif-
19 ferent MFC applications. Further study is warranted to optimize design, materials,
20 and microbiology to improve electricity recovery from MFCs.
21
22 18.1 Introduction
23 Wastewater treatment currently consumes substantial energy about 15 GW
24 (McCarty et al. 2011), or accounts for approximately 3 % of the U.S. electrical
25 energy load (EPA Office of Water 2006), and has similar level to that in other
26 developed countries (Curtis 2010). However, there is abundant potential energy of
27 approximately 17 GW of power (1.5 91011 kWh) contained in domestic,
C. Feng S.C.D. Sharma C.-P. Yu (&)
Institute of Urban Environment, Chinese Academy of Sciences,
361021 Xiamen, People’s Republic of China
e-mail: cpyu@iue.ac.cn
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ÓSpringer International Publishing Switzerland 2015
F. Pacheco Torgal et al. (eds.), Biotechnologies and Biomimetics for Civil Engineering,
DOI 10.1007/978-3-319-09287-4_18
1
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28 industrial, and animal wastewater together (Logan 2004). Thus, capturing part of
29 this energy would provide a new source of electrical power and would also
30 compensate the consumption of energy for wastewater treatment.
31 Recently, microbial fuel cells (MFCs) (Allen and Bennetto 1993; Logan et al.
32 2006; Lovley 2006), which are the bioelectrochemical systems, are generally
33 regarded as a promising and sustainable technology for their direct electrical
34 power production from wastewaters (Rabaey and Verstraete 2005). A conventional
35 MFC consists of a biological anode and a cathode (Fig. 18.1a), where exoelec-
36 trogenic microorganisms could catalyze electrochemical reactions through inter-
37 action with the electrodes (Logan et al. 2006; Rabaey et al. 2007; Clauwaert et al.
38 2008). The electrons available through the metabolism of the electron donors by
39 microorganisms are transferred to the anode and then to the cathode through the
40 circuit; in the cathode, oxidant is reduced with the consumption of protons
41 available through the membrane from the anode (Allen and Bennetto 1993).
42 In terms of potential applications, MFCs and related bioelectrochemical systems
43 can be utilized for renewable energy generation and wastewater treatment,
44 i.e., organic matter elimination and nitrogen removal (Logan and Regan 2006b;
45 Clauwaert et al. 2007; Rozendal et al. 2008; Yu et al. 2011), for the potential
46 production of valuable products, such as hydrogen, methane or hydrogen peroxide
47 (Liu et al. 2005c; Rozendal et al. 2006,2009), for bioremediation of recalcitrant
48 compounds (Catal et al. 2008; Morris and Jin 2008), for desalination (Forrestal et al.
49 2012; Yuan et al. 2012; Feng et al. 2013a), and as biosensors for on-line monitoring
50 of treatment processes (Kim et al. 2007b) and biological oxygen demand or toxic
51 contaminants in wastewater (Kim et al. 2003; Chang et al. 2004; Kim et al. 2007b).
H+
H+
Anode
Cathode
PEM
Influent
Effluent e- e-
AIR
(a) (b)
H+
H+
H+
Anode
Cathode
PEM
Influent
Effluent
Influent
Effluent
H2O
O2
CO2
Organic
matter
e- e-
Fig. 18.1 Schematic diagrams of MFCs: aa two-chamber MFC; ba single-chamber MFC with
open air cathode
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52 Recent investigations have shown that during the last 10 years, the current
53 density of MFCs has been improved by 10,000-fold (Debabov 2008). Power
54 densities of MFCs have increased from less than 1 W/m
3
to over 4000 W/m
3
,
55 which is the highest MFC power density reported up to date (Logan 2008; Bif-
56 finger et al. 2009). Despite their potential applications and continuously improved
57 power, limited maximum power production by these systems impedes commercial
58 applications of bioelectrochemical wastewater treatment, primarily because of
59 high internal resistance including anode limitations and electrochemical losses.
60 Improvements of power generation are also dependent on the materials and design
61 of MFCs and capabilities of the microorganisms. Analysis of the community
62 profiles of exoelectrogenic microbial consortia shows great diversity, ranging from
63 primarily d-Proteobacteria that dominate in sediment MFCs to communities
64 composed of a-, b-, c-ord-Proteobacteria, Firmicutes, and uncharacterized clones
65 in other types of MFCs. Much remains to be discovered about the physiology of
66 these bacteria (collectively referred to as exoelectrogens) capable of exocellular
67 electron transfer.
68 This chapter is intended to provide an overview of recent development and
69 challenges in MFCs with a special focus on the materials, design, and microbi-
70 ology of MFC research. Since microorganisms play a crucial role in the MFCs,
71 comprehensive reviews focused on isolated exoelectrogens that have been iden-
72 tified to produce electricity, their mechanisms of exocellular electron transfer, and
73 the microbial communities found in MFCs. In the end, the prospects for this
74 emerging bioelectrochemical technology were discussed.
75 18.2 History of MFCs
76 Currently, MFCs have been recognized as a promising green technology for the
77 generation of electricity through the microbial oxidation of biodegradable organic
78 matters. The concept of generating electricity by bacteria was introduced more
79 than 100 years ago. The electricity generated by microorganisms was first dem-
80 onstrated in 1911 by Potter, a Professor of Botany Department at the University of
81 Durham (Potter 1911). To examine the electricity producing capability of
82 microorganism, he conducted his experiment using yeast and certain other bacteria
83 in an apparatus consisted of a glass jar containing a porous cylinder. He observed
84 that Saccharomyces cerevisiae and Bacillus coli communis (now called Esche-
85 richia coli) produced electric current when glucose was used as substrate. After
86 that there was no important research on MFCs up to 1966 (Lewis 1966) and most
87 studies on MFCs did not appear until the late twentieth century. However,
88 experiments carried out by researchers used artificial electrochemical mediators to
89 facilitate electron transfer between microbes and electrodes. Thurston and his
90 colleagues used thionine as redox mediator and Proteus vulgaris culture as catalyst
91 in a two-chamber MFC to evaluate coulombic yield from glucose oxidation
92 (Thurston et al. 1985). These chemicals were considered important for obtaining a
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93 higher electron transfer rate and electron recovery between microbial cells and
94 electrodes. In 1999, a breakthrough in MFCs was published by Kim and his
95 colleagues, who showed that exogenous mediators were not necessary to be added
96 to transfer electrons from bacterial cells to electrodes and they developed first
97 mediator-less MFC using a Fe(III)-reducing bacterium, Shewanella putrefaciens
98 IR-1 (Kim et al. 1999). The cell suspension of Shewanella putrefaciens IR-1 was
99 able to generate current without redox mediator in presence of lactate as main
100 carbon source. Another important bacterium Geobacter sulfurreducens can
101 transfer electrons to electrode in the absence of mediators with high current
102 generation (Bond and Lovley 2003) and has become important issue on MFC
103 research. After the discovery of mediator-less MFCs, scientists have become more
104 interested to do research on MFCs, especially in the wastewater treatment because
105 mediator-less MFCs provide a more practical and promising approach to recover
106 electricity from organic waste and wastewater through microbial systems (Liu and
107 Logan 2004; Min and Logan 2004). Presently many research laboratories have
108 been engaged in improving MFC technologies to enhance the electricity produc-
109 tion and efficient removal of wastewater by designing different configurations of
110 MFCs such as single chamber MFC, tubular MFC (Rabaey et al. 2005b), stacked
111 MFC (Aelterman et al. 2006) and also membrane-less MFC (Feng et al. 2013b).
112 The advancement of research on MFCs in future may be the solution of energy
113 scarcity and clean-up of wastewater. Thus, MFCs have received a great deal of
114 attention as a novel green technology for alternative energy generation and
115 wastewater treatment.
116 18.3 Design and Operations of MFCs
117 An appropriate design and architecture is of great significance for improving
118 performance in MFC systems (Du et al. 2007; Pant et al. 2010). The mode of
119 operation and components of a typical two-chamber and a single-chamber MFC
120 are shown in Fig. 18.1.
121 18.3.1 Two-Chamber MFC Systems
122 Traditional two-chamber MFCs consist of an anaerobic anode chamber and an
123 aerobic cathode chamber separated by a proton exchange membrane (PEM) or
124 sometimes a salt bridge, allowing proton transfer from anode to cathode and
125 preventing oxygen diffusion to the anode chamber, as shown in Fig. 18.1a.
126 Regardless of the problems in scale-up, the dual-chamber MFCs have remained
127 the most popular devices for testing microbial activity and optimizing materials.
128 There are a variety of designs and structures occurred based on the principles of
129 two chamber MFC systems, e.g., the widely used and inexpensive H-type MFCs
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130 (Min et al. 2005) and U-shaped MFCs (Milliken and May 2007) (Fig. 18.2a, b). In
131 the H-configuration, the membrane is clamped in the middle of the tubes con-
132 necting the bottle. Although H-shape systems are usually available for basic
133 parameter research, they generate low power densities. This may attribute to high
134 internal resistance and electrode-based losses. Oh and colleagues demonstrated
135 that the power densities had close relationship with the relative sizes (cross sec-
136 tions) of the cathode to that of the anode and the membrane (Oh et al. 2004;Oh
137 and Logan 2006).
138 Ringeisen and colleagues provided a miniature configuration of MFC (Mini-
139 MFC) with a total volume of 1.2 cm
3
(Fig. 18.2c) (Ringeisen et al. 2006). As the
140 result of its specific structure, the mini-MFC maintains a large surface area to
141 volume ratio when graphite felt electrodes were used, enabling high power den-
142 sities to be attained. Min and Logan (2004) designed a Flat Plate MFC (FPMFC) to
143 treat domestic wastewater. The FPMFC was comprised of a single channel formed
144 between two nonconductive (polycarbonate) plates that were separated into two
145 halves by the electrode/PEM assembly (Fig. 18.2d). The anode electrode was a
146 plain porous carbon paper (10 910 cm
2
), while a carbon cloth combining a
147 platinum catalyst (0.5 mg/cm
2
catalyst containing 10 % Pt) servers as cathode
148 electrode. The wastewater was fed into the anode chamber and dry air could pass
149 through the cathode chamber without any catholyte, both in a continuous flow
150 mode. Average power density was obtained at 72 mW/m
2
(Min and Logan 2004).
151 Another reactor design, named upflow MFC (UMFC) working in continuous
152 flow mode, was first tested by He et al. (2005) (Fig. 18.2e). Its configuration was
153 improved by combining the advantages of UASB system, which were operated in
154 continuous mode. Another UMFC with a U-shaped cathode installed inside the
155 anode chamber was developed based on the above configuration (He et al. 2006)
156 (Fig. 18.2f). A U-shaped cathode compartment with a 2 cm diameter was con-
157 structed by gluing two tubes made from PEM into a plastic base connector. In
158 addition to a practical configuration, UMFC achieved promising power outputs
159 with a maximum volumetric power density of 29.2 W/m
3
with an overall internal
160 resistance of 17.3 X(He et al. 2006). They suggested that the main limitation to
161 power generation was the internal resistance. Overall, these systems seem to be
162 more available for practical implementation as they are relatively easy to scale up.
163 18.3.2 Single-Chamber MFC Systems
164 In the single-chamber MFC (SCMFC), the cathode is exposed directly to the air by
165 eliminating the cathodic compartment containing air-sparged solution (Park and
166 Zeikus 2003; Liu and Logan 2004; Liu et al. 2004) (Fig. 18.1b). They typically
167 possess only an anode chamber without the requirement of aeration in the cathode
168 chamber. In comparison with dual chamber system, a SCMFC provides the sim-
169 plified design, increased mass transfer to the cathode, cost savings and an overall
170 decrease in reactor volume.
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(f)
Screw bolt
(c) (d)
Influent
Recirculation
PEM
Effluent
Granular
cathode
Granular
anode
Channel
Screw bolt
PEM
Influent
Air
Effluent
Anode (Carbon paper)
Cathode (Carbon cloth)
Anode
Cathode
Resistance
PEM
(a)
Air
N2
Effluent
Anode
(RVC)
Influent
Recirculation
PEM
(e)
Cathode
(RVC)
Air
PEM
Graphite felt / RVC
PEM
Resistance
AirN2
Anode Cathode
(b)
Fig. 18.2 Schematics of typical two chamber MFCs: aH-type MFC with PEM or salt bridge;
bU-shaped MFC; cMini-MFC; dFlat plate MFC; e, f Up flow MFC with cylindrical shape
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171 Liu et al. (2004) first demonstrated that domestic wastewater could be used as
172 the substrate in MFCs without actively feeding air into a cathode chamber. Their
173 MFC consisted of a single chamber with eight graphite electrodes (anodes) and a
174 single air cathode as shown in Fig. 18.3a. Most importantly, the promising idea of
175 using MFC technology to reduce energy costs in wastewater treatment was initi-
176 ated. A tubular MFC (TMFC), designed by Rebeay and colleagues (Rabaey et al.
177 2005b) was shown in Fig. 18.3b. The TMFC had a wet anode volume of 210 mL
178 and generated a maximum volumetric power of 90 W/m
3
using graphite granules
179 as the anode and a ferricyanide solution in the cathode chamber. A relatively low
180 internal resistance of 4 Xwas achieved by sustaining a short distance between the
181 anode and cathode electrodes and a large PEM surface area. Rabaey et al. (2005b)
182 believed that the use of sustainable open air cathodes was a promising design for
183 practical implementation.
184 It has demonstrated that power output can further be increased in a single-
185 chamber MFC by removing the PEM. Liu et al. (2004) found that there was a
186 significant rise in power density by a factor of approximately 1.9 for glucose and
187 5.2 for wastewater through removing the PEM from a single chamber MFC
188 (Fig. 18.3c). This increase was partly attributed to an enhancement of the proton
189 flux from the anode to the cathode. The lack of a PEM substantially reduce the
190 expenditure on the materials needed to make a MFC and eliminated the disturbing
191 biofouling of membrane. However, substantial oxygen diffusion into the anode
192 chamber in the absence of the PEM could occur to reduce the fraction of electrons
193 recovered as current.
194 18.3.3 Other MFC Configuration
195 Besides the above configurations, a series of variations on these basic designs have
196 emerged in order to achieve different purposes, such as increase of power density,
197 achieving continuous flow or nutrient removal. For example, to increase the
198 overall system voltage, MFCs can be stacked or linked together in series
199 (Aelterman et al. 2006). Another type of MFC, nitrifying and denitrifying MFC for
200 decentralized wastewater treatment was reported by Feng et al. (2013b). Their
201 MFC system was built on the basis of conventional anoxic/oxic wastewater
202 treatment system and achieved the continuous flow mode by using baffle with
203 holes instead of PEM. An integrated photobioelectrochemical system was con-
204 structed by installing a MFC inside an algal bioreactor (Xiao et al. 2012). This
205 system achieves the simultaneous removal of organics and nutrients from a syn-
206 thetic solution, and the production of bioenergy in electricity and algal biomass
207 through bioelectrochemical and microbiological processes.
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208 18.4 Materials
209 MFCs are generally made of three major parts: anode, cathode, and PEM (if
210 present). There are a variety of materials for their construction. Electrode materials
211 play an important role both in the performance and cost of MFCs. A good anode
212 material should have the following properties (Logan et al. 2006;Zhouetal.2011):
213 large surface area; excellent electrical conductivity, strong biocompatibility,
214 chemical stability, appropriate mechanical strength and toughness. Up to now,
215 various materials are used for electrodes including carbon materials, e.g., carbon
216 paper (Liu et al. 2005a), carbon cloth, carbon felt (Chaudhuri and Lovley 2003)
217 and reticulated vitreous carbon (RVC) (He et al. 2005; Rabaey et al. 2005b),
218 graphite materials, e.g., graphite granules and graphite fiber brushes (Aelterman
219 et al. 2006; He et al. 2006; Rinaldi et al. 2008), etc. Since different electrode
220 materials vary obviously in their physical and chemical characteristics, they
221 have impact on microbial attachment, electron transfer, electrode resistance and
222 the rate of electrode surface reaction. Thus, some strategies could be applied to
223 boost the performance in terms of increasing the surface area and the biocom-
224 patibility. The anode materials could be fabricated with C/polyaniline (PANI)
Anode
cover Anode
Sampling
port Cathode
Chamber
(a)
(c)
Granular
anode
Influent
Effluent
Graphite
cathode
PEM
(b)
Resistance
Cathode (carbon/Pt catalyst)
Air
Influent
Effluent
Anode (graphite rods)
PEM
Fig. 18.3 Schematics of typical single-chamber MFCs: aThe first SCMFC for domestic
wastewater treatment; bTubular MFC; ca lab-scale single-chamber MFC
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225 composites, carbon nanofibers, or nitric acid carbon activation (Scott et al. 2007),
226 or integration of carbon nanotubes to PNAI (Qiao et al. 2007), etc.
227 Cathodes are made from the same materials as anodes, and catalysts are usually
228 contained but not necessary. Because oxygen is the terminal electron acceptor in
229 most cases, the high overpotential arising from oxygen reduction reaction causes
230 the noncatalyst cathodes inefficient. Thus, catalysts and/or artificial mediators are
231 generally required to improve performance. They are generally mounted on the
232 cathodes with a binder such as Nafion (perfluorosulfonic acid) or polytetrafluo-
233 roethylene. Pt has become the most popular one (Thurston et al. 1985), but its high
234 cost and reduced activities due to formation of a PtO layer on the electrode surface
235 restrict its practical application. Then non-Pt catalysts including nonabundant
236 metals, e.g., Pd or Ru (Vante and Tributsch 1986; Fernández et al. 2005;
237 Raghuveer et al. 2005) and nonprecious materials, e.g., Fe, Mn and Co (Park and
238 Zeikus 2003) tend to be more appealing. They could exhibit essentially equal or
239 slightly better performance than the more expensive Pt. Among the non-Pt cata-
240 lysts, the most promising CoTMPP and iron (II) phthalocyanine (FePc) (Zhao et al.
241 2005) were proved to be inexpensive and efficient alternatives for MFCs appli-
242 cation. Integration of noncorrosive metals (titanium and nickel) and carbon fibers
243 can be used as cathode materials as well (Hasvold et al. 1997; Zhao et al. 2009).
244 Additionally, catalysts are not required for catholyte cathodes, which use the redox
245 mediators such as ferricyanide (Oh et al. 2004; Venkata Mohan et al. 2008)or
246 permanganate (You et al. 2006). Using them as terminal electron acceptors could
247 result in alternative cathodic reactions and further improve power output to
248 258 W/m
3
(Aelterman et al. 2006). These catholytes seemed to be impractical and
249 unsustainable for practical application owning to the requirement of regeneration
250 of the chemicals. On the basis of the above introduction, a large number of
251 materials have been investigated to improve cathode performance. However, their
252 long-term stability on the cathode should be further evaluated for future
253 application.
254 The PEM is also an important component in the PEM-MFC configuration. It
255 provides a separation between the anode and cathode chambers and allows for
256 transport of positive charges to compensate the electron transport. Currently, the
257 most widely used membrane material is Nafion
TM
(Park and Zeikus 2000; Bond
258 and Lovley 2003), which has set the industry standard for PEM. Its properties have
259 been extensively reviewed (Mauritz and Moore 2004). Obviously, Nafion
TM
was
260 the predominant choices for current MFCs. Nevertheless, it has been recently
261 found that the use of Nafion
TM
leads to some side effects such as pH imbalance
262 and power reduction (Gil et al. 2003; Kim et al. 2004). In addition to Nafion
TM
,
263 polyether ether ketone (PEEK) is a promising polymer being actively studied by
264 the MFC researchers to overcome the drawbacks of Nafion
TM
(Roziere and Jones
265 2003; Mecheri et al. 2006). In fact, membranes can be omitted from the bio-
266 electrochemical configuration. The lack of a PEM could decrease the cost of the
267 materials for a MFC, but substantial oxygen diffusion into the anode chamber in
268 the absence of the PEM could reduce the fraction of electrons recovered as current.
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269 18.5 Isolated Exoelectrogens
270 In nature, there are many microorganisms possessing the ability to transfer elec-
271 trons derived from the metabolism of organic matters to the anode. Microorgan-
272 isms capable of extracellular electron transfer are generally called
273 ‘exoelectrogens’’. These microorganisms attain their required energy by oxidizing
274 organic matter with the release of protons and electrons that are used in MFC to
275 produce electricity. Marine sediment, soil, wastewater, fresh water sediment and
276 activated sludge are rich sources for these microorganisms (Niessen et al. 2006;
277 Zhang et al. 2006). In the beginning, it is considered that only a few types of
278 bacteria are capable of producing electricity and most of them are gram negative
279 Proteobacteria such as Shewanella putrefaciens (Park and Zeikus 2002), Geob-
280 acter sulfurreducens (Bond and Lovley 2003), etc. However, now gram positive
281 bacteria also have been discovered to produce electricity, including Clostridium
282 butyricum within the Firmicutes (Park et al. 2001). The capability to produce
283 electricity generally depends on the nature of bacterial species and their ability to
284 utilize different substrates. Power generation also depends on the optimal growth
285 condition of bacteria, e.g., pH and temperature.
286 Up to now, most of isolated exoelectrogens are bacteria (Table 18.1) and were
287 isolated from different MFCs using large varieties of substrates. Scientists are
288 trying to discover new exoelectrogenic bacteria, which will have the capacity to
289 achieve high power density. The pure strain Geobacter sulfurreducens operated in
290 a two-chamber MFC with PEM and graphite electrode produced electric current
291 density of 65 mA/m
2
using acetate as the substrate (Bond and Lovley 2003). Other
292 pure strains such as Comamonas denitrificans DX-4 and Citrobacter sp. SX-1
293 produced highest power and current density of 35 mW/m
2
and 205 mA/m
2
using
294 acetate and citrate as electron donors in MFC respectively (Xing et al. 2010;Xu
295 and Liu 2011). One scientific report showed that power output in a MFC inocu-
296 lated with a pure culture (Geobacter metallireducens) or a mixed culture
297 (wastewater inoculums) was similar, with 40 ±1mW/m
2
for Geobacter metal-
298 lireducens and 38 ±1mW/m
2
for the wastewater inocula (Min et al. 2005).
299 However, Rhodopseudomonas palustris DX-1, isolated from an air cathode MFC,
300 produced electricity at higher power densities (2720 ±60 mW/m
2
) than mixed
301 culture in the same device using complex substrates including volatile acids, yeast
302 extract and thiosulfate (Xing et al. 2008). In addition, some bacteria require
303 exogenous redox compound to increase maximum power production. For example,
304 Shewanella putrefaciens generated the maximum power density of 10.2 mW/m
2
305 when operated in the absence of exogenous electron acceptors in a single cham-
306 bered MFC, but current production by Shewanella putrefaciens was enhanced 10-
307 folds when an electron mediator, i.e., Mn
4+
or neutral red was incorporated into the
308 graphite anode (Park and Zeikus 2002).
10 C. Feng et al.
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Table 18.1 Some examples of isolated exoelectrogens
Class Microorganism Substrate Mediator Power or current
density
References
a-Proteobacteria Rhodopseudomonas
palustris DX-1
Volatile acids,
yeast extract
Mediator-less 2720 ±60 mW/m
2
(Xing et al. 2008)
Gluconobacter oxydans Glucose 2-hydroxy-1,
4-naphthoquinone
-(Lee et al. 2002)
b-Proteobacteria Comamonas denitrificans
DX-4
Acetate Mediator-less 35 mW/m
2
(Xing et al. 2010)
c-Proteobacteria Shewanella oneidensis Lactate Mediator-less 24 mW/m
2
(Ringeisen et al. 2006)
Citrobacter sp. SX-1 Citrate Mediator-less 205 mA/m
2
(Xu and Liu 2011)
Shewanella putrefaciens Sodium lactate Mn
4+
or neutral red 10.2 mW/m
2
(Park and Zeikus 2002)
Klebsiella pneumoniae Glucose 2-hydroxy-1,
4-naphthoquinone 126.7 ±31.5 mW/m
2
(Rhoads et al. 2005)
Actinobacillus
succinogenes
Glucose Neutral red -(Park and Zeikus 1999)
Proteus mirabilis Glucose Thionin -(Thurston et al. 1985)
Proteus vulgaris Sucrose Thionine -(Bennetto et al. 1985)
Escherichia coli Glucose Neutral red -(Park and Zeikus 2000)
Aeromonas hydrophila Acetate Mediator-less -(Pham et al. 2003)
Pseudomonas
aeruginosa KRP1
Glucose Pyocyanin and phena-zine-1-
carboxamide
-(Rabaey et al. 2005a)
d-Proteobacteria Geobacter
metallireducens
Acetate Mediator-less 40 ±1 mW/m
2
(Min et al. 2005)
Geobacter
sulfurreducens
Acetate Mediator-less 65 mA/m
2
(Bond and Lovley
2003)
Clostridia Clostridium butyricum Glucose Mediator-less -(Park et al. 2001)
18 Microbial Fuel Cells for Wastewater Treatment 11
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309 18.6 Electron Transfer Mechanism of Exoelectrogens
310 The electron transfer mechanism is a key issue to understand the theory of how
311 MFCs work. Numerous investigations were conducted to study how electrons were
312 transferred from microbial cells to anode surface in the MFCs. There are generally
313 two main mechanisms that are direct or mediator-less and indirect or mediated
314 electron transfer (MET).
315 18.6.1 Direct or Mediator-Less Electron Transfer
316 Direct electron transfer (DET) requires a physical contact between the microbial
317 cell membrane or a membrane organelle and the electrode surface. Shewanella
318 putrefaciens (Kim et al. 2002), Geobacter sulferreducens (Bond and Lovley 2003),
319 and Geobacter metallireducens (Min et al. 2005) can effectively transfer electrons
320 directly to electrode across the membrane. Some of DET bacteria transfer elec-
321 trons through direct attachment of cell membrane to anode (Fig. 18.4a), while the
322 rest ones use their pili or nanowires to transfer electrons to anode (Fig. 18.4b).
323 Generally c-type cytochromes associated with bacterial outer membrane and
324 conductive nanowires or pili can be used for DET (Peng et al. 2010).
325 18.6.2 Indirect or Mediated Electron Transfer
326 Although some bacteria can transfer electrons directly, many other microbes need
327 redox-active chemical species or mediators to carry out electron transfer to anode;
328 this type of mechanism is known as indirect or MET. In MET, direct contact
329 between the bacterial cell membrane and the electrode surface is not required, but
330 redox mediator is essential. An electron mediator is a molecule that functions as an
331 electron shuttle between microbes and an electrode. Mediators in oxidized state are
332 easily reduced by capturing electrons from within the bacterial cell membrane or
333 the cytoplasm (Fig. 18.4c). The reduced mediators after passing across the
334 membrane release their electrons to the electrode and become oxidized again in
335 anode chamber and thus are reutilized. Generally chemical mediators are supplied
336 from outside into anode chamber of a MFC. Apart from externally provided
337 mediators, some microorganisms are able to excrete their own mediators such as
338 phenazine, 2-amino-3-carboxy-1,4-naphthoquinone and 2,6-di-tertbutyl-p-benzo-
339 quinone (Rabaey et al. 2005a; Freguia et al. 2009; Deng et al. 2010) that are used
340 to transfer electron from cytoplasm to anode (Fig. 18.4d). In addition, there is
341 another way by which some bacteria, especially fermentative bacteria, produce
342 energy rich reduced metabolites such as H
2
, ethanol or formate, which can be
343 subsequently oxidized to provide electron to anode (Schroder 2007) (Fig. 18.4e).
12 C. Feng et al.
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Substrate CO
2
H
+
Cyt ochrome
Nanowire o r Pili
Substrate CO
2
Anode
e
-
e
-
(a) (b)
Substrat e CO
2
H
2,
Formate H+, CO
2
Anode
e
-
(e)
Med
red
Med
ox
Med
red
Med
ox
Med
red
Med
ox
Sub strate CO
2
Med
red
Med
ox
Med
red
Med
ox
Substrate CO
2
H+
H
+
Anode
e
-
e
-
(d)(c)
Cyt oc hro me
e
-
Fig. 18.4 Electron transfer mechanism of exoelectrogens: direct electron transfer by aattach-
ment of cell membrane or bnanowire; Indirect electron transfer by cexogenous mediators,
dendogenous secondary metabolites or eprimary metabolites
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344 Furthermore, in a synergistic biofilm consortium, it is likely that a nonelectrogenic
345 microbe may secrete mediators that may help the electrogenic microbe to perform
346 better electron transfer.
347 18.7 Microbial Community of Electroactive Biofilms
348 Biofilms more than ten micrometers in thickness are typically formed on the anode
349 surfaces (Bond and Lovley 2003). They contain a complex microbial populations
350 (Kim et al. 2004; Rabaey et al. 2004), apart from the known electrogenic bacteria
351 (Geobacter, Shewanella). Identifying members of the microbial community will
352 be a valuable aid in terms of improving the performance of MFCs and a more
353 comprehensive understanding of the key microbes required for exoelectrogenesis.
354 Up to now, there are many publications associated with microbial communities in
355 MFCs by means of PCR-amplified 16S rRNA gene fragments and sequencing such
356 as denaturing gradient gel electrophoresis (DGGE) (Table 18.2). Analysis of the
357 populations inhabiting such systems demonstrates that microbial communities are
358 phylogenetically diverse in most MFCs. Microbial populations are affected by
359 numerous factors, such as the substrate, cultivation mode, system architectures,
360 anaerobiosis degree, as well as the conditions within the cathode chamber (Logan
361 and Regan 2006a).
362 The composition of substrates has a close relationship with the microbial
363 populations within the anode biofilms and MFC performance, as they serve as
364 carbon (nutrient) and energy source for the microbiological process. Commonly,
365 the carbon sources contain pure compounds (acetate, glucose, lactic acid, etc.)
366 (Chaudhuri and Lovley 2003; Liu et al. 2005b) and a variety of wastewaters
367 (brewery, chocolate, meat packing and paper recycling wastewaters, etc.) (Feng
368 et al. 2008; Huang and Logan 2008). The pure substrate inoculated systems are
369 found to produce more power than those fed with wastewater perhaps as the result
370 of different solution conductivity and buffer capacity (Pant et al. 2010). Based on
371 16S rRNA gene sequences, the dominant community members in the MFCs with
372 pure substrate are more known exoelectrogens (Geobacter sp., Desulfuromonas
373 sp., Rhodopseudomonas sp., etc.) and other bacteria with special function, such as
374 Clostridium sp., which is useful for lignocellulose degradation in cellulose-fed
375 MFCs (Cheng et al. 2011) (Table 18.2).
376 The highest power density of 4.31 W/m
2
was achieved using a mixed culture in
377 a fed-batch MFC and glucose as the substrate in the reactor with a Coulombic
378 efficiency (defined as the fraction of electrons recovered as current versus the
379 maximum possible recovery) of 81 %. The analysis of the population using DGGE
380 showed great phylogenetic diversity, with a complex mixture of bacteria (Firmi-
381 cutes, c-, b-, and a-Proteobacteria). Facultative anaerobic bacteria capable of
382 hydrogen production (Alcaligenes faecalis, Enterococcus gallinarum) were pre-
383 dominant (Rabaey et al. 2004), probably owning to using a fermentable substrate
384 with a mixed culture inocula (Debabov 2008). It was deduced that mediator
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Table 18.2 Summary of dominant microbes present in bacterial community of the anode biofilm
Substrates MFC Technique Dominant community members Power density
(mW/m
2
)
Coulombic
efficiency (%)
References
Acetate Two-
chamber
DGGE Geobacter sulfurreducens N/A 72 (Jung and
Regan 2007)
Single-
chamber
Clone
library
Pelobacter propionicus 835 ±21 20 (Kiely et al.
2011b)
Single-
chamber
DGGE Rhodopseudomonas palustris,Geobacter
sulfurreducens,Pseudomonas alcaligenes 1797 ±10 N/A (Xing et al.
2009)
Two-
chamber
Clone
library
Thauera aromatica,Geobacter sulfurreducens 64.3 72.3 (Chae et al.
2009)
Two-
chamber
Clone
library
Geobacter sulfurreducens,Pelobacter propionicus N/A 50 (Chae et al.
2008)
Ethanol Single-
chamber
Clone
library
Geobacter sulfurreducens,Pelobacter propionicus 820 ±24 11 (Kiely et al.
2011b)
Two-
chamber
RFLP Azoarcus sp., Desulfuromonas sp. 40 ±210 (Kim et al.
2007a)
Lactate Single–
chamber
Clone
library
Pelobacter propionicus,Desulfuromonas sp. 739 ±32.2 20 (Kiely et al.
2011b)
Propionate Two-
chamber
Clone
library
Bacillus sp. 58 36 (Chae et al.
2009)
Butyrate Two-
chamber
DGGE Bacillus sp. N/A 46–67 (Freguia et al.
2010)
Two-
chamber
Clone
library
Dechloromonas sp., Geobacter sp. 51.4 43 (Chae et al.
2009)
Formate Two-
chamber
Clone
library
Paracoccus sp., Geobacter sp. 10 3–11 (Kiely et al.
2010)
DGGE Geobacter sp. N/A 5–6
(continued)
18 Microbial Fuel Cells for Wastewater Treatment 15
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Table 18.2 (continued)
Substrates MFC Technique Dominant community members Power density
(mW/m
2
)
Coulombic
efficiency (%)
References
Two-
chamber
(Ha et al.
2007)
Succinate Single-
chamber
Clone
library
Geobacter sulfurreducens,Pelobacter propionicus 444 ±12.5 16 (Kiely et al.
2011b)
Glucose Two-
chamber
Clone
library
Geobacter
sulfurreducens 156 15 (Chae et al.
2009)
Single-
chamber
DGGE Rhodopseudomonas palustris,Geobacter
sulfurreducens,Clostridium sp. 1000 ±19 N/A (Xing et al.
2009)
Cellulose Single-
chamber
Clone
library
Geobacter sulfurreducens,Clostridium sp. 1070 25–50 (Xing et al.
2009)
Two-
chamber
Clone
library
Clostridiales, Chloroflexi, Rhizobiales,
Methanobacterium 10 N/A (Chae et al.
2009)
Single-
chamber
DGGE Clostridium sp. 331 4 (Wang et al.
2009)
Cysteine Two-
chamber
DGGE Shewanella sp. 39 14 (Logan et al.
2005)
Organic
wastewater
Two-
chamber
DGGE Azoarcus sp, Thauera sp. N/A N/A (Kim et al.
2004)
Dairy manure
wastewater
Single-
chamber
Clone
library
Thauera aromatica,Clostridium sp., Geobacter sp. N/A 12 (Kiely et al.
2011a)
Potato
wastewater
Single-
chamber
Clone
library
Geobacter sp., Pelobacter propionicus N/A 21 (Kiely et al.
2011a)
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385 production accounted for the excellent power generation, as large concentrations
386 of highly colored mediators from this reactor was detected (Logan and Regan
387 2006a).
388 However, there are complicated organic matters in wastewater and complex
389 metabolisms such as fermentation could get involved in MFCs. Molecular char-
390 acterizations of anode community with complex wastewater sources revealed a
391 high diversity of microbial species, dominant with a- (Phung et al. 2004), b- (Kim
392 et al. 2004; Phung et al. 2004), and c-Proteobacteria (Logan et al. 2005). For
393 example, the characterization of anodic community present in a two-chamber
394 MFC treating chocolate wastewater showed a high percentage of b-Proteobacteria
395 (51 %) (Patil et al. 2009). Whereas, microbial communities that developed by
396 MFCs supplied with winery or potato wastewater, were mixed consortia pre-
397 dominated by Geobacter sulfurreducens, representing 44 % and 60 % of 16S
398 rRNA gene clones, respectively (Cusick et al. 2010; Kiely et al. 2011a). Most
399 importantly, large proportion of clones is uncharacterized in these mixed-culture
400 systems, especially with complex wastewater sources. The lower frequency to
401 detect known exoelectrogens implies a greater diversity of this phenotype than
402 presently realized. The significance of potential function of these dominant
403 community members is still unknown.
404 Cultivation mode including fed-batch and continuous flow could affect
405 microbial communities as well. In a continuous flow mode MFC supplied with
406 acetate, the composition of anode community revealed that the most dominant
407 phyla were Proterobacteria (23–33 %), Bacteroidetes (17–40 %) and Chloroflexi
408 (21–30%) on the basis of 454 pyrosequencing technique (Feng et al. 2013b). In an
409 upflow system, a large number of methanogenic archaea in the mixed biomass
410 appeared on the anode based on fluorescence in situ hybridization (He et al. 2005).
411 Literature studies have demonstrated that d-Proteobacteria (50–90 %) were
412 dominant in the anode community of sediment MFC (Bond et al. 2002; Bond and
413 Lovley 2003), while Cytophagales (up to 33 %), Firmicutes (11.6 %), and c-
414 Proteobacteria (9–10 %) were the minor components in the anodophilic consortia
415 (Tender et al. 2002; Holmes et al. 2004).
416 18.8 The MFC’s Full-Scale Applications
417 The development of MFC’s practical application is still in the early stage. To date,
418 most MFCs have been investigated in the bench-scale, generally less than 1 L and
419 produced a maximum potential approximately 0.8 V. Apparently, the power
420 density and MFC configuration have not reached a widely applicable level,
421 remaining the challenging obstacle.
422 Sediment MFCs have been demonstrated at scales effective to be an alternative
423 renewable power source in seawater applications (Bond et al. 2002; Lowy et al.
424 2006; Dewan et al. 2014). According to Fig. 18.5, in principle sediment MFCs
425 consist of two electrodes made of conductive material. The anode is buried under
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426 surface water or marine sediment and cathode is placed in the water above the
427 sediment (Tender et al. 2002; Logan and Regan 2006b; Rezaei et al. 2007). The
428 sedimentary organic carbon (Aller 1994) or sulfate compounds (Rabaey et al.
429 2006) present in the sediment are oxidized by microorganisms growing on the
430 anode surface for production of electricity. There are several attempts to dem-
431 onstrate the availability of sediment MFCs as power source for underwater
432 (Donovan et al. 2013), ground (Donovan et al. 2008), and floating sensors (Nielsen
433 et al. 2007; Tender et al. 2008; Donovan et al. 2011). The first demonstration of
434 scale-up of MFC was used to power a weather buoy embedded with temperature
435 and humidity sensors using two sediment MFCs that generated 24 mW and
436 36 mW (Tender et al. 2008). The sediment MFCs were deployed in the Potomac
437 River, at Washington, DC and Tukerton, NJ, USA. Donovan et al. used sediment
438 MFCs to operate a low-power (11 mW) and a high-power (2500 mW) wireless
439 temperature sensors in a creek at Palouse, WA, USA. (Donovan et al. 2008;2011).
440 The average power generation to power a remote device via a sediment MFC
441 ranges from 3.4 to 36 mW (Dewan et al. 2014). These studies illustrate that MFCs
442 deployed in natural aquatic environment (i.e., rivers, lakes, or oceans) can produce
443 enough energy to operate sensors requiring low power.
444 However, MFCs for wastewater treatment have faced a variety of restrictions in
445 terms of practical implementation. First, the real wastewater contains complex
Anode
Cathode
Sediment
Water
Sensors
O2+4e-+4H+ 4H2O
Organic
matter
CO2, H+
S2- S
e-
Underwater
sensor
Ground
sensor
Floating
sensor
e-
Desulfo.
SO42-
S, H+
H+
+
D. Aceto.
Vcathode-Vanode~0.75 V Open Circuit Potential
1~10 cm
1~10 cm
Fig. 18.5 Schematic representation of fundamental principle of the mediator less sediment
MFCs used to provide energy for on-site sensors. Microorganisms colonizing the anode are most
similar to Desulfuromonas acetoxidans (D. Aceto.), which could oxidize acetate in sediment and
transfer electrons to the anode. Desulfo. represents the species in the Desulfobulbus or
Desulfucapsa genera, which could oxidize anode generated S
0
to SO
42-
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446 organics and diverse microorganisms such as methanogens. This may lead to an
447 inferiority of electroactive biofilm due to methanogenic competition or metabolic
448 diversity. The low ionic strength of real wastewater can limit the power output of
449 MFCs as well (Rozendal et al. 2008). In addition, there are physical constraints
450 with regard to linearly scaling up MFCs. Excessive pressure because of hydrostatic
451 head could require variable permeability to regulate water loss and cathode
452 hydration in the case of permeable membrane. Most importantly, the greatest
453 hindrance lies in the increasing electrical losses and overpotentials with enlarged
454 size (Oh et al. 2010). All of this means that innovative reactor designs are required
455 for practically useful MFCs. As a consequence, after more than two decades of
456 development, in which numerous studies have focused on MFC’s application for
457 wastewater treatment (Habermann and Pommer 1991), successful full-scale
458 application is still relatively rare.
459 In view of the concept of combing MFCs with current wastewater treatment
460 system, several types of MFCs have been proposed. In order to enhance the quality
461 of effluent, Logan (2008) proposed an integrated bioprocess, which combined the
462 post-treatment process, e.g., solids contact (SC) process or membrane bioreactor
463 (MBR) with MFC system (Fig. 18.6a and b). However, performance of post
464 bioreactor can be inhibited due to consumption of most organic matter in the
465 preceding MFC. The MFC can be combined into the existing wastewater treatment
466 facilities as well. Min and Angelidaki (2008) developed a submersible MFC by
467 immersing an air-cathode MFC in an anaerobic reactor. Similarly, Cha et al.
468 (2010) submerged a single chamber MFC into the aeration tank of the activated
469 sludge process to optimize the cell configuration and electrode materials. The
470 submersible MFC can be applied to the anaerobic (or aerobic) facility as an anode
471 (or cathode) chamber without additional constructions (Min and Angelidaki 2008;
472 Cha et al. 2010) (Fig. 18.6c, d). Yu et al. (2011 and Feng et al. (2013b) designed
473 another configuration for decentralized wastewater treatment through immersing
474 the anode into anaerobic tank and the cathode into aerobic tank of the A/O system,
475 respectively (Fig. 18.6e). These types of configuration enable MFCs to be applied
476 to existing wastewater treatment systems.
477 Meanwhile, the work on scaling up MFC for wastewater treatment is moving
478 forward. According to some information on the Internet or public literatures, there
479 are at least two pilot-scale MFCs for wastewater treatment available for practical
480 implementation. The first large-scale test of tubular MFCs was located at Foster’s
481 brewery in Yatala, Queensland (Australia) (http://www.microbialfuelcell.org).
482 This system was constructed by the Advanced Water Management Center of the
483 University of Queensland, led by Jurg Keller and Korneel Rabaey. MFCs consisted
484 of 12 modules with an entire volume of 1 m
3
. The anodes and cathodes are made
485 of carbon fiber based on a brush design. Another pilot-scale multi-anode/cathode
486 MFC (MAC MFC) was developed by researchers of University of Connecticut and
487 their collaborators (Fuss and O’Neill, and Hydroqual Inc.) in the USA. (Jiang et al.
488 2011). The MAC MFC contained 12 anodes/cathodes with a total volume of 20 L.
489 The reactors contain graphite rods as the anode, with Cu-MnO
2
or Co-MnO
2
490 catalyzed carbon cloth cathodes. The systems are treating wastewater, achieving
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491 80 % of contaminant removal at different organic loading rates (0.19–0.66 kg/m
3
/d).
492 The power density of MAC MFC reached 380 mW/m
2
. In addition to the pilot
493 scale MFCs, Ieropoulos et al. (2013) originally exploited a stack of small ceramic
494 MFCs (6.25 mL) fed with real urine to power mobile phone, which was previously
495 considered impossible.
496 Therefore, tremendous efforts should be dedicated in terms of utilizing the
497 voltage from MFCs in the near future. Dewan et al. (2014) pointed out that
498 renewable energy sources tend to be applied to power remote sensors, due to the
499 potential environmental risks and operational cost associated with batteries. More
500 research is also required to focus on assessment of lifetime, reliability and
501 renewability, which are of great significance in the process of promoting the MFCs
502 widespread application.
MFCs Solids
contact
Influent
Effluent
Waste
sludge
MFCs MBR
Influent
Effluent
Waste sludge
Clarifier
Influent
Effluent
Waste
sludge
Anaerobic
tank
Clarifier
MFCs
Air
Influent
Effluent
Waste
sludge
Aerobic
tank
Clarifier
MFCs
Air
Recycled
sludge
Anaerobic
tank
Aerobic
tank Clarifer
Air
Influent
Effluent
(a) (b)
(c) (d)
(e)
Anode
Cathode
Fig. 18.6 Schematic diagram of MFCs combined wastewater treatment process: a, b MFC
combined with a solids contact tank or a MBR; c, d MFC submerged into an anaerobic or aerobic
tank of existing wastewater treatment process; ea decentralized wastewater treatment based on
A/O system
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503 18.9 The Conclusion and Perspective
504 Substantial efforts have been devoted to the development and improvement of
505 MFC technology to reduce its operating cost, and to increase power output
506 although MFC technology has not been widely scaled up for commercial appli-
507 cation. MFC technology covers many distinct scientific disciplines, including the
508 material sciences, microbial ecology, and engineering design. Previous studies
509 have proposed innovative designs of MFC reactors to improve the performance
510 together with reduced capital costs. It has been demonstrated that different elec-
511 trode materials exhibited different behaviors and electrode modification offers a
512 good and effective approach for enhancing the performance. Development of the
513 electrode with excellent proprieties and the reasonable price could be crucial for
514 the practical application. Furthermore, appropriate integration or combination of
515 MFCs with the present wastewater treatment technologies should be taken into
516 consideration.
517 MFCs provide us a model system to study the different microbial populations
518 present in the exoelectrogenic biofilms, and it would be an important research area
519 in understanding how the microbial ecology of electricity producing communities
520 develops and shifts over time. Extensive studies on exoelectrogenic bacteria and
521 consortia begin to expose the mechanistic and ecological complexities of MFC
522 biofilm communities. Yet, our understanding of electrochemically active microbes
523 is still in its infancy, as the diverse communities have a multitude of undiscovered
524 electrochemical capabilities that can be exploited in different MFC applications.
525 Discovery of the potential exoelectrogenic bacteria is important in understanding
526 the function of anodic microbial communities and to improve the electron transfer
527 efficiency of MFCs.
528 References
529 Aelterman P, Rabaey K, Pham HT, Boon N, Verstraete W (2006) Continuous electricity
530 generation at high voltages and currents using stacked microbial fuel cells. Environ Sci
531 Technol 40(10):3388–3394
532 Allen RM, Bennetto HP (1993) Microbial fuel-cells: electricity production from carbohydrates.
533 Appl Biochem Biotechnol 39(1):27–40
534 Aller RC (1994) The sedimentary Mn cycle in long island sound: its role as intermediate oxidant
535 and the influence of bioturbation, O
2
, and Corg flux on diagenetic reaction balances. J Mar
536 Res 52(2):259–295
537 Bennetto H, Delaney G, Mason J, Roller S, Stirling J, Thurston C (1985) The sucrose fuel cell:
538 efficient biomass conversion using a microbial catalyst. Biotechnol Lett 7(10):699–704
539 Biffinger J, Ribbens M, Ringeisen B, Pietron J, Finkel S, Nealson K (2009) Characterization of
540 electrochemically active bacteria utilizing a high-throughput voltage-based screening assay.
541 Biotechnol Bioeng 102(2):436–444
542 Bond DR, Holmes DE, Tender LM, Lovley DR (2002) Electrode-reducing microorganisms that
543 harvest energy from marine sediments. Science 295(5554):483–485
18 Microbial Fuel Cells for Wastewater Treatment 21
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544 Bond DR, Lovley DR (2003) Electricity production by Geobacter sulfurreducens attached to
545 electrodes. Appl Environ Microbiol 69(3):1548–1555
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810 Wang X, Feng Y, Wang H, Qu Y, Yu Y, Ren N, Li N, Wang E, Lee H, Logan BE (2009)
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