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Generation of bioenergy from industrial waste using microbial fuel cell technology for the sustainable future
Generation of bioenergy from industrial waste using microbial fuel cell technology for the sustainable future
8
K. Senthilkumar and M. Naveen Kumar Department of Chemical Engineering, Kongu Engineering College, Erode, India
8.1
Introduction
Fossil fuels were one of the major sources of energy in recent times. Since availability of fossil fuel reserves is finite, one of the greatest challenges in this century is to manage the global energy demands necessary to maintain the growth of economic and the population. Combustion of fossil fuels such as coal, petroleum, oil, and natural gas for generation of energy and various industrial processes discharges a huge amount of toxic gasses such as CO, CO2, SOx, NOx, and CH4 into atmosphere, which, in turn, causes atmospheric pollution. Majority of the pollution occurs due to the rapid industrialization without any sustainable development. That is environmental resources (natural, cultural, and socioeconomic) are depleted by industry and their activities on environment. Though industrialization is important for the development of our nation, it should not be at the expenses of depletion of natural resources. Any form of industrial wastes discharged into the environment is polluting the entire ecosystem due to its nature of toxicity. It forms the polluting environment in any form of pollution such as air, water, soil, and noise. To overcome all these drawbacks, zero effluent discharge systems (ZEDS) are introduced, but currently, this is not used in many industries because of the high initial cost and also maintenance cost. However, ZEDS concept is profitable for industries because of its capability to recover the freshwater from industrial wastewater, so recovered water would be used for industrial cooling and cleaning purposes. Since the problems on resources should be pointed out for the next-generation development, in this case, we need to develop and implementing new technology for industrial waste treatment. Conventional treatment method has more difficulties in operation. Recently, for industrial waste treatment the most promising and fascinating technology is microbial fuel cell (MFC). Because of its low initial cost, easy operation, and low maintenance cost, MFC plays an important role in producing bioenergy, value-added chemicals, and fuels. The utilization of MFC gives energy in the form of electricity in consistent, fresh, resourceful manners which use Refining Biomass Residues for Sustainable Energy and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-818996-2.00008-9 © 2020 Elsevier Inc. All rights reserved.
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renewable process. Hence, MFCs proves itself as a potential technology for water rescue and conversion of chemical energy into electricity.
8.2
Probable bioenergy from industrial effluents
One of the important energy sources in organic industrial sectors is waste materials such as dregs and by-products. Solid wastes from food industries, such as peelings and scraps chopped from fruits and vegetables, poor-quality food, filtered sludge, pulp, fiber from sugar, and starch extraction wastes, are dumped on nonliving region landfill. Food-processing industries are generating a huge volume of effluent because of various processing operations, such as cleaning, washing, blanching, and precooking. These effluents contain sugars, starches, dissolved and solid organic matters. These industrial wastes may be aerobically/anaerobically degraded to produce several commercial waste-to-energy conversion products such as biogas and ethanol. One of the major polluting industries is the pulp and paper industry, because it consumes more amount of water for various unit operations. This industry-discharged wastewater is heterogeneous in nature, and it contains wood and other raw material related compounds, processed chemicals, etc. The effluent containing black liquor can be used for the generation of bioenergy by using upflow anaerobic sludge blanket (UASB) reactor.
8.3
Environmental effects due to industrial waste
Due to rapid industrialization, the major part of pollution is coming from industries and factories. A range of pollutants are discharged into the environment including the water body, landfilling, and open to atmosphere. All industrial effects are liable for creating a nonconducive environment for living things because of discharge effluent having toxic and highly concentrated contaminants. In addition, it generates problems for living and nonliving things such as habitats, sickness, loss of life, and ecosystem devastation. Due to industrial waste, there are some pollution effects along with their serious consequences. The major effects of industrial pollution are described in the following subsections.
8.3.1 Global warming The major problem of industrial waste is global warming. It is one of the most severe effects of industrial pollution because of uncontrolled industrial activities. Most of the industries release a mixture of greenhouse gases together with carbon dioxide (CO2) and methane (CH4) into the open environment. Released gases are absorbing thermal radiation from the sun; therefore the normal temperature of the earth is increased, leading to global warming.
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8.3.2 Water pollution The untreated industrial pollutants discharged from the industries and factories have prevalent toxic concentration. Generally, these wastes are discharged into the water body and affect the aqua cultures, flora and fauna life cycles, etc. Usage of unsuitable contaminated water and the discharge of untreated industrial wastewater into water bodies mainly create water pollution.
8.3.3 Air pollution The main cause of air pollution is mainly due to the rapid increase in industries/factories. Emissions from various industries have contaminants such as methane (CH4), oxides of nitrogen (NO), and carbon dioxide (CO2). If these gases are highly present in the atmosphere, they repeatedly create several toxic illnesses and dangerous hazards to the environment. Some of the effects of these gases are formation of acid rains, the presence of smog, heightened incidents of respiratory disorders among humans, etc.
8.3.4 Soil pollution Soil pollution occurs due to untreated disposal of industrial wastes into soil; it has high toxic contaminants, which leads to soil pollution. Industrial wastes have different amount of toxic contents and hazardous chemicals such that when deposited in soil, they affect the soil layer strength in the top soil, thus reducing the soil fertility and biological activity of the soil. In addition to that a hazardous effect leads to ecological imbalances, thus making troubles in crop production. Apart from that, it may cause severe health problem for those who are consuming such crops, because soil and crops are contaminated by toxic chemicals and hazardous materials.
8.3.5 Effect on human health Untreated industrial wastes have more amounts of toxic contents and hazardous chemical wastes that are disposed into water bodies or soils. It may cause illness, cancers and human cell poisoning, etc., for example, inorganic arsenic causes the formation of tumors in human health. Therefore untreated industrial wastes are liable for thousands of illnesses and early deaths across the earth.
8.4
Traditional treatments methods on industrial waste
Generally, industrial wastewater treatment includes primary, secondary, and tertiary process that involves large amounts of energy, excessive sludge production, high operational cost, and more bad odors in the treatment of industrial wastewater. A traditional treatment method requires more amount of energy for treating wastewater. In the last three decades, anaerobic treatment processes are finding growing
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application in the treatment of domestic water and industrial effluent. Moreover, due to energy insecurity and worldwide environmental alarms, there is an emergent interest to find out sustainable and clean energy source. Traditional biodegradation processes focus on removing organic pollutants and not on extraction of energy from waste. For overcoming these drawbacks, it is important to develop an alternate technology that will be used to recover precious energy for the sustainable operation.
8.5
Microbial fuel cell technology
MFC is one of the fascinating technologies not only used for the treatment of industrial wastewater but also for the electricity production. The basic principle behind this technology is the chemical energy compounds are converted into electrical energy during decomposition of organic and inorganic matters with the help of catalytic action of microorganisms. There are two categories of MFCs: one is mediator and the other is mediator less. During the 20th century the first MFC was established for transfer of electrons from the bacteria in the cell to the anode using chemical mediator. Most advancement in MFC happened during the years of 1970s called mediator-less type in which the direct electron transfer mechanisms are given more attention, because bacteria have electrochemically active redox capacity to transfer the electrons directly to the anode. After many years, MFC was used for commercial purposes in the 21st century. MFC is like a bioreactor where microorganisms are oxidizing the organic and inorganic matters present in the effluent and produce electricity. Microorganism-generated electrons are transferred to the negative terminal and flow to the positive terminal connected by an external electrical device containing a resistor. The MFC reactor may able to decompose the substrate present in the anode chamber, either batch mode or fed-batch mode; else, the system is considered to be a bio-battery. The generated electrons are to be transferred to the anode electrode by electron transfer mechanisms. Working process of MFCs is the electrons transferred to the cathode compartment through proton exchange membrane (PEM) with protons and oxygen are provided by externally; clean water is the final by-product. Due to the photosynthesis process, biomass comes to the atmosphere. A component of fuel cell was described in Table 8.1. Table 8.1 Basic components of microbial fuel cells. Items
Materials
Anode Cathode Anode chamber Cathode chamber Proton exchange membrane system Electrode catalyst
Graphite, graphite felt, carbon paper, carbon cloth Graphite, graphite felt, carbon paper, carbon cloth, black Glass Glass Ultrex, salt bridge, porcelain septum, solely electrolyte, Nafion Pt, Pt black, MnO2, Fe31, electron mediator immobilized on anode
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8.5.1 Microbial fuel cell configurations and designs Many researchers have been examined for increasing the power output of MFCs, focusing areas such as altering MFC designs, cost-effective electrodes, electrode modification, and membranes for enhancing surface area of the electrode and their actions in order to overcome the difficulties in e2 and H1 transport system. The schematic of MFC is shown in Fig. 8.1. In anode chamber, microorganisms act as electrochemically active bacteria to oxidize the substrates through PEM. These e2 and H1 travel to the cathode through an external electrical device, and it diffused through electrolyte and PEM. With platinum as a catalyst, protons and electrons are joined at the cathodic chamber with oxygen to form H2O. Numerous configurations and shapes of MFCs have been developed to run in batch, fed-batch, or continuous mode operation. A two-staged MFC setup consists of an anode and a cathode compartment separated by a PEM such as Nafion. In recent times, single-chambered MFC (SC-MFC) has gained considerable attention, because cathode is in direct contact with the atmosphere as shown in Fig. 8.2. Since open air-cathode systems can be used in SC-MFCs, they are quite attractive for their increasing power production efficiency. Nevertheless, a lot of SC-MFCs still use Nafion or PEM membrane for separating the electrons and protons. A considerable increase in transfer of oxygen in anodic section in absence of a PEM was reported, whereas two-chambered MFCs can have a wide range of applications even with a relatively low power output. Cathodic denitrification is such a good example. This design has outstanding applications in industrial effluent treatment due to its effortless scale-up, proton transfer problems, etc. Nevertheless, oxygen back diffusion is a dangerous disadvantage that is comparatively higher than in
Figure 8.1 Schematic representation of dual-chambered microbial fuel cell.
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Figure 8.2 Schematic representation of single-chambered microbial fuel cell.
membrane-based used MFCs. It demonstrated successful generation of electricity using an open air biocathode system with biocatalysts (microorganisms).
8.5.2 Advantages of microbial fuel cell G
G
G
G
G
G
G
G
G
G
Direct conversion of biomass (substrate) into bioelectricity. Reduction in sludge generation compared to conventional aerobic-activated sludge processes. Insensitivity to the operational environment. May be used in electrical infrastructure lacking locations. Does not require highly regulated distribution systems. It operates competently at room temperature with secure and good output. Does not require gas treatment. Does not generate greenhouse gasses compared to other treatment processes. Supportable power generation and industrial effluent treatment. Minimum amount of sludge may be produced while treating the wastewater. The treated water may be used for irrigation purposes and industrial-cleaning processes.
8.6
Research prospects in microbial fuel cell technology
8.6.1 Electrodes in microbial fuel cell The performance and cost of MFC are highly dependent on electrode material. To make MFC a cost-effective and ascendible technology, it is important to note that electrode design is the greatest challenge. Recently, the area of motivation is increased in the electrode material, and their configurations of MFCs are studied. From the literature, it is observed that during the earlier period, a number of
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electrodes have been widely used for MFCs. Surface area of the electrode is an important parameter in the generation of electricity. In MFC the electrode materials have different characteristics, and their support materials may have some important characteristics such as good conduction, fine chemical stability, high mechanical strength, and more importantly low cost. Carbon materials and noncorrosive metals are most widely used base materials. However, some specific requirements, such as high biocatalytic activity, high surface roughness, good biocompatibility, and efficient electron transfer between bacteria and electrode surface, are necessary. Electrode surface modification has become an emerging topic of interest in the research field of MFCs. The air-cathode electrode material is fabricated by using a base material, a catalyst, a binder, and a waterproof coating.
8.6.2 Effect of spacing between anode and cathode on power production An important parameter that has a high influence on total power production is spacing between the electrodes. The total power production increased when the spacing between the anode and cathode are at less distance. Ahn et al. (2014) found that maximum power density of 10.9, 8.6, and 7.4 mW/m2 was observed at electrode spacing 20, 24, and 28 cm, respectively, in their research. It is also observed that maximum power density was observed at larger spacing. At external resistance between 900 and 1200 Ω, maximum power density was observed and power density decreased with increase in resistance beyond 1200 Ω.
8.6.3 Effect of electrode surface area on power production Electrode surface area has a significant influence on power production in MFC. The electric power generation in MFCs is predicted to increase with an increase of electrode surface area. Cathode area influences on MFC outputs with hydrogen and by bacteria, whereas anode area does make such impact. Cathodic reaction is a rate-limiting reaction and air-cathode and PEM used in many MFC research. MFC performance may purely depend on cathode reaction, whereas anode reaction may be just pulled forward by the cathode reaction.
8.6.4 Influence of microorganisms in microbial fuel cell The literature evidenced that numerous microorganisms have the capacity to transfer electrons derived from organic matters’ metabolism to the anodic region. The sources of such microorganism are marine sediment, soil, wastewater, and activated sludge. A molecular breeding of bacteria optimized to the MFC environment is also important for the enhancement of the MFC outputs. The anodic electron transfer mechanism in MFC is a key issue in understanding the theory of how MFC’s work. Under anaerobic conditions in soils and sediments, Geobacter belongs to dissimilatory metal reducing microorganisms, which produce biologically useful energy in
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the form of adenosine triphospate. In dissimilarity metal reducing microorganisms, the chemical compounds are involved in the electron transportation. The microorganisms such as Shewanella putrefaciens, Geobacter sulfurreducens, and Geobacter metallireducens transfer electrons to anode by similar method.
8.7
Confront/dispute in microbial fuel cell
The MFC technology has seen wide commercial applications, because they can utilize the same biomass in many cases for energy productions. MFCs are able to convert biomass at low temperature (20 C) and with low substrate concentrations. In order to improve the power density output, new anodophilic microorganisms that enormously improve the electron transport rate are much needed. It is claimed that in MFCs, current flow could increase by four orders of magnitude if Geobacter transports electrons to the anode at the same rate as it does to its natural electron acceptor. In the future, it is possible that an optimized microbial consortium can be obtained to operate an MFC without extraneous mediators while achieving superior mass transfer and electron transfer rates. MFCs can be used in various applications as aforementioned. When MFCs used in wastewater treatment, a large surface area is needed for biofilm to build up on the anode. In some cases, 80% 90% Coulombic efficiency has been achieved; it has little effect on low reaction rate. Although some basic knowledge has been gained in MFC research, there is still a lot to be learned in the scale-up of MFC for large-scale applications. Besides, large-scale application of MFCs has yet to be implemented due to low yields of power and high cost.
8.8
Utilization of microbial fuel cell
8.8.1 Electricity generation MFCs are capable of converting the chemical energy to electrical energy with the aid of microorganisms. In MFCs the substrates oxidization is directly converted into electricity; electricity generation with an electron yield by ferrireducens as high as 80% was reported and higher electron yield up to 89% was also reported (Du et al., 2007). High Coulombic efficiency of 97% was reported during the oxidation of formate with the catalysis of platinum (Pt) black. However, MFC power generation is still very low. To overcome this issue, one sensible method is to make storage of the produced electricity in storage devices and then distribute it to the end users.
8.8.2 Biohydrogen MFCs can be easily modified to produce hydrogen instead of electricity. The protons and the electrons produced by the metabolism of microbes in an MFC are
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thermodynamically unfavorable. It applied an external potential to amplify the cathode potential in a MFC circuit and thus overcame the thermodynamic barrier. Protons and electrons produced by the anolyte reaction are combined at the cathode chamber to form hydrogen. The external potential for an MFC theoretically requires 100 mV, much lower than the 1110 mV required for direct electrolysis of water at neutral pH. This may be due to the fact that some energy comes from the biomass oxidation process in the anodic chamber. In biohydrogen production using MFCs, oxygen is no longer needed in the cathodic chamber. Thus oxygen leak to the anodic chamber is no longer an issue in improved efficiency of MFCs. The main advantage is that hydrogen can be accumulated and stored for future usage. Therefore MFCs provide a renewable hydrogen source that can contribute to the overall hydrogen demand in a hydrogen economy.
8.8.3 Wastewater treatment The important applications of MFCs are treating domestic as well as industrial wastewater. Municipal wastewater contains a multitude of organic compounds that can fuel MFCs. The amount of electric power generated by MFCs during the wastewater treatment process can potentially halve the electricity needed in a conventional treatment process. A hybrid incorporating both electrophiles and anodophilies is especially suitable for wastewater treatment, because more organics can be biodegraded by a variety of organic substances. MFCs using certain microbes have a special ability to remove sulfides as required in wastewater treatment. MFCs can enhance the growth of bioelectrochemically active microbes during wastewater treatment. Continuous flow and single-compartment MFCs and membrane-less MFCs are favored for wastewater treatment due to concerns in scale-up. Sanitary wastes, food-processing wastewater, swine wastewater, and corn stover are all great biomass sources for MFCs because they are rich in organic matters. Up to 80% of the chemical oxygen demand (COD) can be removed in some cases, and a Coulombic efficiency as high as 80% has been reported.
8.8.4 Biosensor Another potential application of the MFC technology is to use it as a sensor for pollutant analysis and in situ process monitoring and control, apart from the aforementioned applications. The correlation between the Coulombic yield of MFCs and the strength of the wastewater makes MFCs possible to serve biological oxygen demand (BOD). A number of works showed a good linear relationship between the Coulombic yield and the strength of the wastewater in a quite wide BOD concentration range. MFC-type of BOD sensors is advantageous over other types of BOD sensor, because they have excellent operational stability, good reproducibility, and accuracy. An MFC-type BOD sensor constructed with the microbes enriched with MFC can be kept operational for over 5 years without extra maintenance.
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8.8.5 Artificial wastewater The wastewater that is artificially produced for research works is called artificial wastewater. It is also called synthetic wastewater. While using the natural wastewater, it should be stored in the cold condition; it shows different characteristics while testing it. So, artificial wastewater is employed for avoiding the change in characteristics of the wastewater and to maintain the same characteristics throughout the research work.
8.9
Conclusion and future prospects of microbial fuel cell
The MFC inoculated with mixed anaerobic sludge demonstrated its effectiveness as a wastewater treatment process along with electricity production, without electrode and membrane. The existence of MFC technology has surfeit of applications in the day-to-day lives, as it is environment friendly and more importantly a green technology. There are more challenges left over for the complete utilization of MFCs, to make it cost effective, to fabricate the innovative MFC bioreactors for industrial effluent treatment. Identification of new microorganisms is essential to treat contaminated effluent with generation of electricity. There is a broad scope for design and development of these reactors as the power density is too low for consumption in various industrial applications. Apart from that, the organism may be genetically altered in order to form high reducing microbial strains with wide range of MFC applications. Future researches are necessary to minimize the internal resistance and corrosion related problems in MFC. MFCs can also have utilization in army applications in order to power up remote surveillance and communication gears for use in unmanned applications. Further, potential researches on optimization of the electricity production from the two-chambered MFC are also necessary. With further attempt, it could be possible to enhance it for the scale-up and commercial applications. In addition, MFC as a continuous reactor may also be studied. Further research toward maintaining aerated condition in the reactor to produce more electricity may be needed. Thus the simultaneous treatment of wastewater along with electricity production might facilitate in compensating the expenses incurred for the industrial effluent management. However, MFC technology is still in initial stages and needs special attention in future research attempts.
References Ahn, Y., Hatzell, M.C., Zhang, F., 2014. J. Power Sources 249, 440 445. Du, Z., Li, H., Gu, T., 2007. Biotechnol. Adv. 25, 464 482.
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Further reading Das, M.P., 2015. Pharma Chem. 7 (11), 8 10. Gautam, R.K., Verma, A., 2019. Microbial electrochemical technology sustainable platform for fuels, chemicals and remediation. Biomass Biofuels Biochem. 451 483. Karuppiah, T., Pugazhendi, A., Subramanian, S., Jamal, M.T., Jeyakumar, R.B., 2018. 3 Biotech 8, 437. Available from: https://doi.org/10.1007/s13205-018-1462-1. Kokabian, B., Ghimire, U., Gnaneswargude, V., 2018. Renew. Energy 122, 354 361. Mehrdadi, N., Nabi-Bidhendi, G., Tashauoei, H.R., 2016. J. Safety Environ. Health Res. 1 (1), 23 26. Nastro, R.A., 2014. Int. J. Performability Eng. 10 (4), 367 376. Naveen Kumar, M., 2017. Int. J. Latest Eng. Res. Appl. 02, 01 09. Prakash, A., 2016. Microbial fuel cells: a source of energy. J. Microb. Biochem. Technol. 8, 247 255. Sevda, S., Sarma, P.J., Mohanty, K., Sreekrishnan, T.R., Pant, D., 2017. Waste to Wealth. Springer, pp. 237 258. Shakunthala, C., Manoj, S., 2017. Int. J. Appn. Innov. Eng. Manag 6 (10), 32 37. Tamboli, E., Satya Eswari, J., 2019. Microbial electrochemical technology. Biomass Biofuels Biochemicals. Elsevier, pp. 407 435. Tharali, A.D., Sain, N., Jabez Osborne, W., 2016. Front. Life Sci. 9 (4), 252 266. Xu, W., Zhang, E., Yu, E.H., 2016. Chem. Eng. Trans. 51, 37 42.
8
K. Senthilkumar and M. Naveen Kumar Department of Chemical Engineering, Kongu Engineering College, Erode, India
8.1
Introduction
Fossil fuels were one of the major sources of energy in recent times. Since availability of fossil fuel reserves is finite, one of the greatest challenges in this century is to manage the global energy demands necessary to maintain the growth of economic and the population. Combustion of fossil fuels such as coal, petroleum, oil, and natural gas for generation of energy and various industrial processes discharges a huge amount of toxic gasses such as CO, CO2, SOx, NOx, and CH4 into atmosphere, which, in turn, causes atmospheric pollution. Majority of the pollution occurs due to the rapid industrialization without any sustainable development. That is environmental resources (natural, cultural, and socioeconomic) are depleted by industry and their activities on environment. Though industrialization is important for the development of our nation, it should not be at the expenses of depletion of natural resources. Any form of industrial wastes discharged into the environment is polluting the entire ecosystem due to its nature of toxicity. It forms the polluting environment in any form of pollution such as air, water, soil, and noise. To overcome all these drawbacks, zero effluent discharge systems (ZEDS) are introduced, but currently, this is not used in many industries because of the high initial cost and also maintenance cost. However, ZEDS concept is profitable for industries because of its capability to recover the freshwater from industrial wastewater, so recovered water would be used for industrial cooling and cleaning purposes. Since the problems on resources should be pointed out for the next-generation development, in this case, we need to develop and implementing new technology for industrial waste treatment. Conventional treatment method has more difficulties in operation. Recently, for industrial waste treatment the most promising and fascinating technology is microbial fuel cell (MFC). Because of its low initial cost, easy operation, and low maintenance cost, MFC plays an important role in producing bioenergy, value-added chemicals, and fuels. The utilization of MFC gives energy in the form of electricity in consistent, fresh, resourceful manners which use Refining Biomass Residues for Sustainable Energy and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-818996-2.00008-9 © 2020 Elsevier Inc. All rights reserved.
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renewable process. Hence, MFCs proves itself as a potential technology for water rescue and conversion of chemical energy into electricity.
8.2
Probable bioenergy from industrial effluents
One of the important energy sources in organic industrial sectors is waste materials such as dregs and by-products. Solid wastes from food industries, such as peelings and scraps chopped from fruits and vegetables, poor-quality food, filtered sludge, pulp, fiber from sugar, and starch extraction wastes, are dumped on nonliving region landfill. Food-processing industries are generating a huge volume of effluent because of various processing operations, such as cleaning, washing, blanching, and precooking. These effluents contain sugars, starches, dissolved and solid organic matters. These industrial wastes may be aerobically/anaerobically degraded to produce several commercial waste-to-energy conversion products such as biogas and ethanol. One of the major polluting industries is the pulp and paper industry, because it consumes more amount of water for various unit operations. This industry-discharged wastewater is heterogeneous in nature, and it contains wood and other raw material related compounds, processed chemicals, etc. The effluent containing black liquor can be used for the generation of bioenergy by using upflow anaerobic sludge blanket (UASB) reactor.
8.3
Environmental effects due to industrial waste
Due to rapid industrialization, the major part of pollution is coming from industries and factories. A range of pollutants are discharged into the environment including the water body, landfilling, and open to atmosphere. All industrial effects are liable for creating a nonconducive environment for living things because of discharge effluent having toxic and highly concentrated contaminants. In addition, it generates problems for living and nonliving things such as habitats, sickness, loss of life, and ecosystem devastation. Due to industrial waste, there are some pollution effects along with their serious consequences. The major effects of industrial pollution are described in the following subsections.
8.3.1 Global warming The major problem of industrial waste is global warming. It is one of the most severe effects of industrial pollution because of uncontrolled industrial activities. Most of the industries release a mixture of greenhouse gases together with carbon dioxide (CO2) and methane (CH4) into the open environment. Released gases are absorbing thermal radiation from the sun; therefore the normal temperature of the earth is increased, leading to global warming.
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8.3.2 Water pollution The untreated industrial pollutants discharged from the industries and factories have prevalent toxic concentration. Generally, these wastes are discharged into the water body and affect the aqua cultures, flora and fauna life cycles, etc. Usage of unsuitable contaminated water and the discharge of untreated industrial wastewater into water bodies mainly create water pollution.
8.3.3 Air pollution The main cause of air pollution is mainly due to the rapid increase in industries/factories. Emissions from various industries have contaminants such as methane (CH4), oxides of nitrogen (NO), and carbon dioxide (CO2). If these gases are highly present in the atmosphere, they repeatedly create several toxic illnesses and dangerous hazards to the environment. Some of the effects of these gases are formation of acid rains, the presence of smog, heightened incidents of respiratory disorders among humans, etc.
8.3.4 Soil pollution Soil pollution occurs due to untreated disposal of industrial wastes into soil; it has high toxic contaminants, which leads to soil pollution. Industrial wastes have different amount of toxic contents and hazardous chemicals such that when deposited in soil, they affect the soil layer strength in the top soil, thus reducing the soil fertility and biological activity of the soil. In addition to that a hazardous effect leads to ecological imbalances, thus making troubles in crop production. Apart from that, it may cause severe health problem for those who are consuming such crops, because soil and crops are contaminated by toxic chemicals and hazardous materials.
8.3.5 Effect on human health Untreated industrial wastes have more amounts of toxic contents and hazardous chemical wastes that are disposed into water bodies or soils. It may cause illness, cancers and human cell poisoning, etc., for example, inorganic arsenic causes the formation of tumors in human health. Therefore untreated industrial wastes are liable for thousands of illnesses and early deaths across the earth.
8.4
Traditional treatments methods on industrial waste
Generally, industrial wastewater treatment includes primary, secondary, and tertiary process that involves large amounts of energy, excessive sludge production, high operational cost, and more bad odors in the treatment of industrial wastewater. A traditional treatment method requires more amount of energy for treating wastewater. In the last three decades, anaerobic treatment processes are finding growing
186
Refining Biomass Residues for Sustainable Energy and Bioproducts
application in the treatment of domestic water and industrial effluent. Moreover, due to energy insecurity and worldwide environmental alarms, there is an emergent interest to find out sustainable and clean energy source. Traditional biodegradation processes focus on removing organic pollutants and not on extraction of energy from waste. For overcoming these drawbacks, it is important to develop an alternate technology that will be used to recover precious energy for the sustainable operation.
8.5
Microbial fuel cell technology
MFC is one of the fascinating technologies not only used for the treatment of industrial wastewater but also for the electricity production. The basic principle behind this technology is the chemical energy compounds are converted into electrical energy during decomposition of organic and inorganic matters with the help of catalytic action of microorganisms. There are two categories of MFCs: one is mediator and the other is mediator less. During the 20th century the first MFC was established for transfer of electrons from the bacteria in the cell to the anode using chemical mediator. Most advancement in MFC happened during the years of 1970s called mediator-less type in which the direct electron transfer mechanisms are given more attention, because bacteria have electrochemically active redox capacity to transfer the electrons directly to the anode. After many years, MFC was used for commercial purposes in the 21st century. MFC is like a bioreactor where microorganisms are oxidizing the organic and inorganic matters present in the effluent and produce electricity. Microorganism-generated electrons are transferred to the negative terminal and flow to the positive terminal connected by an external electrical device containing a resistor. The MFC reactor may able to decompose the substrate present in the anode chamber, either batch mode or fed-batch mode; else, the system is considered to be a bio-battery. The generated electrons are to be transferred to the anode electrode by electron transfer mechanisms. Working process of MFCs is the electrons transferred to the cathode compartment through proton exchange membrane (PEM) with protons and oxygen are provided by externally; clean water is the final by-product. Due to the photosynthesis process, biomass comes to the atmosphere. A component of fuel cell was described in Table 8.1. Table 8.1 Basic components of microbial fuel cells. Items
Materials
Anode Cathode Anode chamber Cathode chamber Proton exchange membrane system Electrode catalyst
Graphite, graphite felt, carbon paper, carbon cloth Graphite, graphite felt, carbon paper, carbon cloth, black Glass Glass Ultrex, salt bridge, porcelain septum, solely electrolyte, Nafion Pt, Pt black, MnO2, Fe31, electron mediator immobilized on anode
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8.5.1 Microbial fuel cell configurations and designs Many researchers have been examined for increasing the power output of MFCs, focusing areas such as altering MFC designs, cost-effective electrodes, electrode modification, and membranes for enhancing surface area of the electrode and their actions in order to overcome the difficulties in e2 and H1 transport system. The schematic of MFC is shown in Fig. 8.1. In anode chamber, microorganisms act as electrochemically active bacteria to oxidize the substrates through PEM. These e2 and H1 travel to the cathode through an external electrical device, and it diffused through electrolyte and PEM. With platinum as a catalyst, protons and electrons are joined at the cathodic chamber with oxygen to form H2O. Numerous configurations and shapes of MFCs have been developed to run in batch, fed-batch, or continuous mode operation. A two-staged MFC setup consists of an anode and a cathode compartment separated by a PEM such as Nafion. In recent times, single-chambered MFC (SC-MFC) has gained considerable attention, because cathode is in direct contact with the atmosphere as shown in Fig. 8.2. Since open air-cathode systems can be used in SC-MFCs, they are quite attractive for their increasing power production efficiency. Nevertheless, a lot of SC-MFCs still use Nafion or PEM membrane for separating the electrons and protons. A considerable increase in transfer of oxygen in anodic section in absence of a PEM was reported, whereas two-chambered MFCs can have a wide range of applications even with a relatively low power output. Cathodic denitrification is such a good example. This design has outstanding applications in industrial effluent treatment due to its effortless scale-up, proton transfer problems, etc. Nevertheless, oxygen back diffusion is a dangerous disadvantage that is comparatively higher than in
Figure 8.1 Schematic representation of dual-chambered microbial fuel cell.
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Figure 8.2 Schematic representation of single-chambered microbial fuel cell.
membrane-based used MFCs. It demonstrated successful generation of electricity using an open air biocathode system with biocatalysts (microorganisms).
8.5.2 Advantages of microbial fuel cell G
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Direct conversion of biomass (substrate) into bioelectricity. Reduction in sludge generation compared to conventional aerobic-activated sludge processes. Insensitivity to the operational environment. May be used in electrical infrastructure lacking locations. Does not require highly regulated distribution systems. It operates competently at room temperature with secure and good output. Does not require gas treatment. Does not generate greenhouse gasses compared to other treatment processes. Supportable power generation and industrial effluent treatment. Minimum amount of sludge may be produced while treating the wastewater. The treated water may be used for irrigation purposes and industrial-cleaning processes.
8.6
Research prospects in microbial fuel cell technology
8.6.1 Electrodes in microbial fuel cell The performance and cost of MFC are highly dependent on electrode material. To make MFC a cost-effective and ascendible technology, it is important to note that electrode design is the greatest challenge. Recently, the area of motivation is increased in the electrode material, and their configurations of MFCs are studied. From the literature, it is observed that during the earlier period, a number of
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electrodes have been widely used for MFCs. Surface area of the electrode is an important parameter in the generation of electricity. In MFC the electrode materials have different characteristics, and their support materials may have some important characteristics such as good conduction, fine chemical stability, high mechanical strength, and more importantly low cost. Carbon materials and noncorrosive metals are most widely used base materials. However, some specific requirements, such as high biocatalytic activity, high surface roughness, good biocompatibility, and efficient electron transfer between bacteria and electrode surface, are necessary. Electrode surface modification has become an emerging topic of interest in the research field of MFCs. The air-cathode electrode material is fabricated by using a base material, a catalyst, a binder, and a waterproof coating.
8.6.2 Effect of spacing between anode and cathode on power production An important parameter that has a high influence on total power production is spacing between the electrodes. The total power production increased when the spacing between the anode and cathode are at less distance. Ahn et al. (2014) found that maximum power density of 10.9, 8.6, and 7.4 mW/m2 was observed at electrode spacing 20, 24, and 28 cm, respectively, in their research. It is also observed that maximum power density was observed at larger spacing. At external resistance between 900 and 1200 Ω, maximum power density was observed and power density decreased with increase in resistance beyond 1200 Ω.
8.6.3 Effect of electrode surface area on power production Electrode surface area has a significant influence on power production in MFC. The electric power generation in MFCs is predicted to increase with an increase of electrode surface area. Cathode area influences on MFC outputs with hydrogen and by bacteria, whereas anode area does make such impact. Cathodic reaction is a rate-limiting reaction and air-cathode and PEM used in many MFC research. MFC performance may purely depend on cathode reaction, whereas anode reaction may be just pulled forward by the cathode reaction.
8.6.4 Influence of microorganisms in microbial fuel cell The literature evidenced that numerous microorganisms have the capacity to transfer electrons derived from organic matters’ metabolism to the anodic region. The sources of such microorganism are marine sediment, soil, wastewater, and activated sludge. A molecular breeding of bacteria optimized to the MFC environment is also important for the enhancement of the MFC outputs. The anodic electron transfer mechanism in MFC is a key issue in understanding the theory of how MFC’s work. Under anaerobic conditions in soils and sediments, Geobacter belongs to dissimilatory metal reducing microorganisms, which produce biologically useful energy in
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the form of adenosine triphospate. In dissimilarity metal reducing microorganisms, the chemical compounds are involved in the electron transportation. The microorganisms such as Shewanella putrefaciens, Geobacter sulfurreducens, and Geobacter metallireducens transfer electrons to anode by similar method.
8.7
Confront/dispute in microbial fuel cell
The MFC technology has seen wide commercial applications, because they can utilize the same biomass in many cases for energy productions. MFCs are able to convert biomass at low temperature (20 C) and with low substrate concentrations. In order to improve the power density output, new anodophilic microorganisms that enormously improve the electron transport rate are much needed. It is claimed that in MFCs, current flow could increase by four orders of magnitude if Geobacter transports electrons to the anode at the same rate as it does to its natural electron acceptor. In the future, it is possible that an optimized microbial consortium can be obtained to operate an MFC without extraneous mediators while achieving superior mass transfer and electron transfer rates. MFCs can be used in various applications as aforementioned. When MFCs used in wastewater treatment, a large surface area is needed for biofilm to build up on the anode. In some cases, 80% 90% Coulombic efficiency has been achieved; it has little effect on low reaction rate. Although some basic knowledge has been gained in MFC research, there is still a lot to be learned in the scale-up of MFC for large-scale applications. Besides, large-scale application of MFCs has yet to be implemented due to low yields of power and high cost.
8.8
Utilization of microbial fuel cell
8.8.1 Electricity generation MFCs are capable of converting the chemical energy to electrical energy with the aid of microorganisms. In MFCs the substrates oxidization is directly converted into electricity; electricity generation with an electron yield by ferrireducens as high as 80% was reported and higher electron yield up to 89% was also reported (Du et al., 2007). High Coulombic efficiency of 97% was reported during the oxidation of formate with the catalysis of platinum (Pt) black. However, MFC power generation is still very low. To overcome this issue, one sensible method is to make storage of the produced electricity in storage devices and then distribute it to the end users.
8.8.2 Biohydrogen MFCs can be easily modified to produce hydrogen instead of electricity. The protons and the electrons produced by the metabolism of microbes in an MFC are
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thermodynamically unfavorable. It applied an external potential to amplify the cathode potential in a MFC circuit and thus overcame the thermodynamic barrier. Protons and electrons produced by the anolyte reaction are combined at the cathode chamber to form hydrogen. The external potential for an MFC theoretically requires 100 mV, much lower than the 1110 mV required for direct electrolysis of water at neutral pH. This may be due to the fact that some energy comes from the biomass oxidation process in the anodic chamber. In biohydrogen production using MFCs, oxygen is no longer needed in the cathodic chamber. Thus oxygen leak to the anodic chamber is no longer an issue in improved efficiency of MFCs. The main advantage is that hydrogen can be accumulated and stored for future usage. Therefore MFCs provide a renewable hydrogen source that can contribute to the overall hydrogen demand in a hydrogen economy.
8.8.3 Wastewater treatment The important applications of MFCs are treating domestic as well as industrial wastewater. Municipal wastewater contains a multitude of organic compounds that can fuel MFCs. The amount of electric power generated by MFCs during the wastewater treatment process can potentially halve the electricity needed in a conventional treatment process. A hybrid incorporating both electrophiles and anodophilies is especially suitable for wastewater treatment, because more organics can be biodegraded by a variety of organic substances. MFCs using certain microbes have a special ability to remove sulfides as required in wastewater treatment. MFCs can enhance the growth of bioelectrochemically active microbes during wastewater treatment. Continuous flow and single-compartment MFCs and membrane-less MFCs are favored for wastewater treatment due to concerns in scale-up. Sanitary wastes, food-processing wastewater, swine wastewater, and corn stover are all great biomass sources for MFCs because they are rich in organic matters. Up to 80% of the chemical oxygen demand (COD) can be removed in some cases, and a Coulombic efficiency as high as 80% has been reported.
8.8.4 Biosensor Another potential application of the MFC technology is to use it as a sensor for pollutant analysis and in situ process monitoring and control, apart from the aforementioned applications. The correlation between the Coulombic yield of MFCs and the strength of the wastewater makes MFCs possible to serve biological oxygen demand (BOD). A number of works showed a good linear relationship between the Coulombic yield and the strength of the wastewater in a quite wide BOD concentration range. MFC-type of BOD sensors is advantageous over other types of BOD sensor, because they have excellent operational stability, good reproducibility, and accuracy. An MFC-type BOD sensor constructed with the microbes enriched with MFC can be kept operational for over 5 years without extra maintenance.
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8.8.5 Artificial wastewater The wastewater that is artificially produced for research works is called artificial wastewater. It is also called synthetic wastewater. While using the natural wastewater, it should be stored in the cold condition; it shows different characteristics while testing it. So, artificial wastewater is employed for avoiding the change in characteristics of the wastewater and to maintain the same characteristics throughout the research work.
8.9
Conclusion and future prospects of microbial fuel cell
The MFC inoculated with mixed anaerobic sludge demonstrated its effectiveness as a wastewater treatment process along with electricity production, without electrode and membrane. The existence of MFC technology has surfeit of applications in the day-to-day lives, as it is environment friendly and more importantly a green technology. There are more challenges left over for the complete utilization of MFCs, to make it cost effective, to fabricate the innovative MFC bioreactors for industrial effluent treatment. Identification of new microorganisms is essential to treat contaminated effluent with generation of electricity. There is a broad scope for design and development of these reactors as the power density is too low for consumption in various industrial applications. Apart from that, the organism may be genetically altered in order to form high reducing microbial strains with wide range of MFC applications. Future researches are necessary to minimize the internal resistance and corrosion related problems in MFC. MFCs can also have utilization in army applications in order to power up remote surveillance and communication gears for use in unmanned applications. Further, potential researches on optimization of the electricity production from the two-chambered MFC are also necessary. With further attempt, it could be possible to enhance it for the scale-up and commercial applications. In addition, MFC as a continuous reactor may also be studied. Further research toward maintaining aerated condition in the reactor to produce more electricity may be needed. Thus the simultaneous treatment of wastewater along with electricity production might facilitate in compensating the expenses incurred for the industrial effluent management. However, MFC technology is still in initial stages and needs special attention in future research attempts.
References Ahn, Y., Hatzell, M.C., Zhang, F., 2014. J. Power Sources 249, 440 445. Du, Z., Li, H., Gu, T., 2007. Biotechnol. Adv. 25, 464 482.
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Further reading Das, M.P., 2015. Pharma Chem. 7 (11), 8 10. Gautam, R.K., Verma, A., 2019. Microbial electrochemical technology sustainable platform for fuels, chemicals and remediation. Biomass Biofuels Biochem. 451 483. Karuppiah, T., Pugazhendi, A., Subramanian, S., Jamal, M.T., Jeyakumar, R.B., 2018. 3 Biotech 8, 437. Available from: https://doi.org/10.1007/s13205-018-1462-1. Kokabian, B., Ghimire, U., Gnaneswargude, V., 2018. Renew. Energy 122, 354 361. Mehrdadi, N., Nabi-Bidhendi, G., Tashauoei, H.R., 2016. J. Safety Environ. Health Res. 1 (1), 23 26. Nastro, R.A., 2014. Int. J. Performability Eng. 10 (4), 367 376. Naveen Kumar, M., 2017. Int. J. Latest Eng. Res. Appl. 02, 01 09. Prakash, A., 2016. Microbial fuel cells: a source of energy. J. Microb. Biochem. Technol. 8, 247 255. Sevda, S., Sarma, P.J., Mohanty, K., Sreekrishnan, T.R., Pant, D., 2017. Waste to Wealth. Springer, pp. 237 258. Shakunthala, C., Manoj, S., 2017. Int. J. Appn. Innov. Eng. Manag 6 (10), 32 37. Tamboli, E., Satya Eswari, J., 2019. Microbial electrochemical technology. Biomass Biofuels Biochemicals. Elsevier, pp. 407 435. Tharali, A.D., Sain, N., Jabez Osborne, W., 2016. Front. Life Sci. 9 (4), 252 266. Xu, W., Zhang, E., Yu, E.H., 2016. Chem. Eng. Trans. 51, 37 42.