**7. Comparison of anodic metabolisms in bioelectricity generation by dairy wastewater treatment in microbial fuel cell**

The growing concern about environment safety and rapid depletion of energy reserves have made it imperative to update the waste management methods from the mere waste treatment to a novel prospect of waste to energy [47]. Microbial Fuel Cell is a novel technology for electricity generation from organic matter present in wastewater, treating wastewater simultaneously solves energy crisis and environmental damage issues [48]. To generate electricity, Microbial Fuel Cell (MFC) is a

**Figure 2.**

*(a) Anaerobic anodic metabolism in MFC (adapted from [51]). (b) Aerobic anodic metabolism in MFC (adapted from [51]).*

**107**

*Treatment of Dairy Wastewaters: Evaluating Microbial Fuel Cell Tools and Mechanism*

bio-electro-chemical system that uses bacterial oxidation of biodegradable organics. The development of bio-potential takes place when organic substances get oxidized to electrons and protons through microbial metabolism. The bacteria transport the electrons to the anode via a variety of mechanisms such as electron shuttles or solid conductive matrix. Then electrons get transported to the cathode via circuit externally [45]. The protons from the anode chamber are transferred to the cathode chamber via passing through the proton exchange membrane, where they form water by combining with the electrons and O2 in the presence of a mediator. The potential difference between the bacteria's respiratory metabolism and the electron acceptor creates the voltage and current required to produce electricity [45, 49]. In a study, the MFC system is scaled up, consisting of 40 individual cells that have been constructed and

[50]. Extensive research and scaling up of MFCs will further enable adequate conversion of waste to energy. For long term use, MFCs can be clubbed with the existing technologies for wastewater treatment and electricity generation. MFC uses anaerobic anodic metabolism where it employs bacteria as a substrate for the reduction of COD from wastewaters as shown in **Figure 2(a)**. This technology is further followed by aerobic anodic metabolism where it employs algae as a substrate and under photosynthetic conditions causes the reduction of nitrates and phosphates from wastewaters as shown in **Figure 2(b)**. This combination of treatment with an effective and proper choice of anode material will help in the generation of power followed by the degradation of wastewaters. Further studies need to be carried out for the degradation of antibiotics in such an innovative integrated MFC model for dairy wastewater.

The bacteria transfers the electrons to the anode through different mechanisms, including (i) direct bacterial contact via cytochrome, endogenous redoxactive based self-mediated electron transfer, such as pyocyanin and conductive pili; (ii) artificial electron shuttles or mediator. Electrons then get transported to the cathode by passing via an external circuit, while the protons are passed through from the anode chamber to the cathode chamber by proton exchange membrane or PEM. At the cathode, the concoction of electron, proton, and O2 occurs for the production of water. The potential difference between the respiratory metabolism of bacteria and the electron-acceptor creates the voltage and

The power output of an MFC rests on different aspects including the type of organic content available in wastewater, electron transfer rate from bacteria to the anode, and the membrane ability to carry hydrogen ions [52]. Some micro-organisms are known to transfer electrons to their external environment from their oxidative metabolic pathways, which are called exoelectrogens [53]. Geobacter and Shewanella are the two prime bacterial genera that are known with this ability; the extracellular transportation

of electrons to the electrodes occurs through three different ways namely:

of energy and capable of powering LED panel

*DOI: http://dx.doi.org/10.5772/intechopen.93911*

evaluated which can generate 4.2 W/m3

**8. Degradation mechanism**

current necessary for electrification.

**9. Electron transfer mechanism**

1.Direct transfer of electrons

2.Mediator based electron transfer, and

3.Nanowires based electron transfer.

*Treatment of Dairy Wastewaters: Evaluating Microbial Fuel Cell Tools and Mechanism DOI: http://dx.doi.org/10.5772/intechopen.93911*

bio-electro-chemical system that uses bacterial oxidation of biodegradable organics. The development of bio-potential takes place when organic substances get oxidized to electrons and protons through microbial metabolism. The bacteria transport the electrons to the anode via a variety of mechanisms such as electron shuttles or solid conductive matrix. Then electrons get transported to the cathode via circuit externally [45]. The protons from the anode chamber are transferred to the cathode chamber via passing through the proton exchange membrane, where they form water by combining with the electrons and O2 in the presence of a mediator. The potential difference between the bacteria's respiratory metabolism and the electron acceptor creates the voltage and current required to produce electricity [45, 49]. In a study, the MFC system is scaled up, consisting of 40 individual cells that have been constructed and evaluated which can generate 4.2 W/m3 of energy and capable of powering LED panel [50]. Extensive research and scaling up of MFCs will further enable adequate conversion of waste to energy. For long term use, MFCs can be clubbed with the existing technologies for wastewater treatment and electricity generation. MFC uses anaerobic anodic metabolism where it employs bacteria as a substrate for the reduction of COD from wastewaters as shown in **Figure 2(a)**. This technology is further followed by aerobic anodic metabolism where it employs algae as a substrate and under photosynthetic conditions causes the reduction of nitrates and phosphates from wastewaters as shown in **Figure 2(b)**. This combination of treatment with an effective and proper choice of anode material will help in the generation of power followed by the degradation of wastewaters. Further studies need to be carried out for the degradation of antibiotics in such an innovative integrated MFC model for dairy wastewater.

#### **8. Degradation mechanism**

*Environmental Issues and Sustainable Development*

of carbohydrates present in the substrate, such as alcohol and sugars directly into electrical energy. Currently, efforts have been made towards using MFCs for domestic wastewater treatment and at the same time point, electricity production considering the environmental issues and further reuse of waste [44]. Sewage sludge of anaerobic nature is used to inoculate MFCs, as it is conveniently used from a wastewater treatment plant and it has largely diverse bacterial communities containing electrogenic bacterial strains [45]. MFCs have functional and operational benefits compared with the presently used technologies for producing energy from organic content [46].

**7. Comparison of anodic metabolisms in bioelectricity generation by** 

The growing concern about environment safety and rapid depletion of energy reserves have made it imperative to update the waste management methods from the mere waste treatment to a novel prospect of waste to energy [47]. Microbial Fuel Cell is a novel technology for electricity generation from organic matter present in wastewater, treating wastewater simultaneously solves energy crisis and environmental damage issues [48]. To generate electricity, Microbial Fuel Cell (MFC) is a

*(a) Anaerobic anodic metabolism in MFC (adapted from [51]). (b) Aerobic anodic metabolism in MFC* 

**dairy wastewater treatment in microbial fuel cell**

**106**

**Figure 2.**

*(adapted from [51]).*

The bacteria transfers the electrons to the anode through different mechanisms, including (i) direct bacterial contact via cytochrome, endogenous redoxactive based self-mediated electron transfer, such as pyocyanin and conductive pili; (ii) artificial electron shuttles or mediator. Electrons then get transported to the cathode by passing via an external circuit, while the protons are passed through from the anode chamber to the cathode chamber by proton exchange membrane or PEM. At the cathode, the concoction of electron, proton, and O2 occurs for the production of water. The potential difference between the respiratory metabolism of bacteria and the electron-acceptor creates the voltage and current necessary for electrification.

#### **9. Electron transfer mechanism**

The power output of an MFC rests on different aspects including the type of organic content available in wastewater, electron transfer rate from bacteria to the anode, and the membrane ability to carry hydrogen ions [52]. Some micro-organisms are known to transfer electrons to their external environment from their oxidative metabolic pathways, which are called exoelectrogens [53]. Geobacter and Shewanella are the two prime bacterial genera that are known with this ability; the extracellular transportation of electrons to the electrodes occurs through three different ways namely:


#### **9.1 Direct transfer of electrons**

Geobacter and Shewanella sp. use a direct electron transport mechanism where the electrons are dispatched directly to the electrode surface. The outer C-type cytochrome membrane is associated with the direct dispatch of NADH-produced electrons [54].

#### **9.2 Mediator based electron transfer**

Some species of bacteria like Shewanella and Pseudomonas secrete certain shuttle molecules like flavins, to pass electrons to electrodes via the cell membrane of the bacteria [55, 56].

#### **9.3 Nanowires based electron transfer**

Genera of Geobacter and Shewanella are evident to use conductive auxiliaries for transporting the electrons outside of the cell [57, 58]. Such conductive networks, called nanowires, are cellular outgrowths for as long as 20 μm. These nanowires are claimed to have a substantially higher electrical conductivity than the synthetic metallic nanostructure [59].

### **10. Anode materials for MFCs**

Choosing and designing an anode has a direct effect on the performance parameters which includes the microbial adhesion, transfer of electrons, and oxidation of fuels. An MFC system's achievable power density depends on the selection of an anode that significantly affects the output of an MFC system [60]. As a consequence, achieving higher power density requires the ability to facilitate the improved transfer of electrons from the bacterial cells to the external circuit, thus the anode is of prime importance towards attaining this objective [61]. The electron transfer process necessitates the donation of an electron using extracellular electron transfer (EET) towards the anode surface by the anode respiring bacteria or ARB and, consequently, the current flow in the circuit externally. This mechanism has been interpreted as being similar to transfer electrons to the anode surface from the cell through direct electron transfer mechanism, soluble electron shuttles diffusion, and the transfer of electrons from biofilm via solid component (pili) [61]. Essential features for the anode to attain the best performance include biocompatibility [62–64], corrosion-resistant, low electrical resistance, and high conduction of electricity [62]. The anode must also be of chemically inert in nature that can function in an environment containing diverse biodegradable wastewater composed of variety of organic and inorganic components that are able to react with the anode material causing its deterioration inefficiency.

Lots of anode materials have been used in the last five years to create various anodes for MFCs. The choice of material for the construction of anode, in particular, is significantly influenced by improvement in different MFC system structures. On a particular note, various exotic carbonaceous materials' use is on a hike. This new category includes stainless steel, stainless steel with modified surface, and anodes based on graphene-based carbonaceous anodes. In many recent studies the graphene-based anodes are found very encouraging [65–68]. The grapheme composite anodes have been stated for higher power production [69–71]. Similarly, the use of carbon nanofibers, carbon nanotubes single and multi-walled anodes has also been documented for high-performance MFCs [72]. This chapter has categorized a few

**109**

*Treatment of Dairy Wastewaters: Evaluating Microbial Fuel Cell Tools and Mechanism*

materials are discussed individually in the proceeding sections.

of the recent approaches in the configuration of anode materials dividing them into four vast categories, namely modern carbon-based anodes, carbon-based composite anodes, surface-modified and metal-based anodes, and each of these categorized

In MFC systems, various anode materials based on carbon have been used over the last decade. These include carbon cloth, carbon paper, or sheet or graphite plates and graphite rod. Using carbon-based anode materials has the advantages of cost-effectiveness, biocompatible nature, efficient electrical conductivity, and chemical stability [73]. Due to their potentially high-performance enhancement and excellent properties, these have been recognized as being very useful for building MFCs. Accessible surface area is an essential factor that affects the efficiency of these anode materials [74, 75]. Such anodes comprises of natural or synthetic anode materials which are as follows:

Synthesis of high-efficiency anode components, by using renewable and recyclable components, provides an outstanding ecological solution including both deriving reusable energy from nature and maintaining biodiversity. An interesting example is the layered corrugated carbon anode production from low priced packaging material through carbonization (LLC). It is important to remember that the LCC's 3D surface is normally tunable by differing the height and layers of the flute. A six times increase in the number of layers resulted in a successive rise in current density because of the potential for biofilm formation in wider surface areas. It is evident that the LCC anode has four times the current density as correlated with the graphite felt anode. Natural anode materials prove to be an ideal option for low priced microbial fuel cells due to their 3-dimensional microporous structures, increased electron transfer rate, and high kinetics of the electrogenic bacterial population. A variety of recently produced highly 3-dimensional porous anode material uses LCC as a lowcost high-performance substitute, usually manufactured from carbonized recycled paper [76, 77]. High performance was obtained from the use of 3-dimensional anodes, based on exoelectrogens' 3-dimensional growth. Stronger anode kinetics can be attained by using maximal anode surface area, but the efficiency only rises gradually as the reaction reaches the triple-phase boundary, i.e. lower inner resistance among anode, cathode, and electrolyte. Interestingly, in comparison with the plane graphite electrode, 8 times better performance is seen with carbonized corn stem. However, few benefits of the aforementioned electrode material include increased biocompatibility, less internal resistance, and rougher surface that facilitated linkage to biofilm. A coated rough electrode, constructed from the carbonization of common packaging materials, was observed to be the highest rated anode of all carbon-

based modifications. The current densities achieved were 201 A/m2

higher performance for the construction of MFC.

maximal current density of up to 31 A/m<sup>2</sup>

*10.1.2 Synthetic anode materials*

respectively, from three and six corrugated layers. This is a low-cost material with

It is quite evident that 3-dimensional carbon fiber (non-woven) can achieve a

and blowing the solution. The performance and efficiency of MFCs also depends on the system architecture, based on these 3D materials [78]. Double-sided air cathode reduces the boundaries of mass transfer. The stainless steel frame was

and 391 A/m2

which is prepared by electrospinning

,

*DOI: http://dx.doi.org/10.5772/intechopen.93911*

**10.1 Modern carbon-based anodes**

*10.1.1 Natural anode materials*

of the recent approaches in the configuration of anode materials dividing them into four vast categories, namely modern carbon-based anodes, carbon-based composite anodes, surface-modified and metal-based anodes, and each of these categorized materials are discussed individually in the proceeding sections.
