Waste-to-Energy Approaches Towards Zero Waste: Interdisciplinary Methods of Controlling Waste

By Chaudhery Mustansar Hussain

Waste-to-Energy Approaches Towards Zero Waste: Interdisciplinary Methods of Controlling Waste provides a comprehensive overview of the key technologies and approaches to achieve zero waste from energy. The book emphasizes the importance of an integrated approach to waste-to-energy using fundamental concepts and principles, and presents key methods, their applications, and perspectives on future development. The book provides readers with the tools to make key decisions on waste-to-energy projects from zero-waste principles, while incorporating sustainability and life cycle assessments from financial and environmental perspectives.

Waste-to-Energy Approaches Towards Zero Waste: Interdisciplinary Methods of Controlling Waste offers practical guidance on achieving energy with zero waste ideal for researchers and graduate students involved in waste-to-energy and renewable energy, waste remediation, and sustainability.

• Provides an integrated approach for waste-to-energy using zero waste concepts
• Offers decision-making guidance on selecting the most appropriate approach for each project
• Presents the sustainability and life cycle assessment of WTE technologies on financial and environmental grounds

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 9, 2021
• ISBN: 9780323853880

Book preview

Chapter 1

Emerging sustainable opportunities for waste to bioenergy: an overview

Rahul Gautam¹, Jagdeep K. Nayak¹, Achlesh Daverey² and Uttam K. Ghosh¹,    ¹Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee, Roorkee, India,    ²School of Environment and Natural Resources, Doon University, Dehradun, India

Abstract

Energy is an essential building block for human welfare, socioeconomic development, and quality of life. Rapid urbanization, industrialization, and modernization are the major key factors behind the increase of energy demands in the last few decades. Some prominent issues like climate change, global warming, increased greenhouse gases, increasing/fluctuating fossil fuel costs, and high energy demands forced scientific community to explore alternative sources of energy. To meet the extensive pace of population explosion, industrialization, and pollution, the demand for alternative energy sources has gained prominence. The energy crisis and waste generation have catered the route of waste to energy for biofuels. Biofuels are composed of diverse fuels to serve economic and environmental sustainability, reduced dependency on imported petroleum fuels. The organic waste generated from households and industries is potent source for biofuels. Algal and yeast biomasses have achieved significant importance due to their high productivity rate, oil content, and absence of lignin. The lignocellulosic biomass has high productivity and can be used for biorefineries. The biofuels such as bioethanol, biodiesel, biogas, bioelectricity, and biohydrogen from wide range of substrates can serve the energy demand of the globe. The bioenergy for global transportation has further boosted research efforts. Investigating the use of these energy alternative sources is a key priority for most of the countries to meet the pledges to reduce greenhouse gas emissions. This chapter focuses on the different types of biomass transforming technologies for bioenergy from waste. The technology, prospects, and implementation are addressed. The emerging biochemical pathways (i.e., anaerobic digestion, alcoholic fermentation, extraction-transesterification, microbial fuel cell) for biofuels are described. Transesterification to produce biodiesel and waste to bioelectricity in microbial fuel cell are reviewed.

Keywords

Waste to energy; biofuel production; microbial fuel cell; biorefinery; waste management; bioethanol

Contents

Outline

1.1 Introduction 2

1.2 Bioethanol 5

1.2.1 History 6

1.2.2 Substrate suitability for bioethanol 7

1.2.3 Steps in bioethanol production 8

1.2.4 Cost of production 11

1.2.5 Substrates for bioethanol production 12

1.2.6 Uses of bioethanol 12

1.3 Biogas production through anaerobic digestion 13

1.3.1 History of anaerobic digestion 13

1.3.2 Principles of the anaerobic digestion process 16

1.3.3 Factors affecting the anaerobic digestion process 17

1.3.4 Landfill biogas 20

1.3.5 Leachate 20

1.3.6 Codigestion 21

1.3.7 Substrate for biogas production 21

1.3.8 Uses 24

1.4 Biodiesel 25

1.4.1 Why biodiesel? 26

1.4.2 Production process 27

1.4.3 Reaction mechanism 31

1.4.4 Factors affecting biodiesel production 31

1.4.5 Algal biodiesel 32

1.4.6 Oil extraction from microalgae 34

1.4.7 Challenges in microalgae biodiesel production 35

1.4.8 Biodiesel production from yeast 35

1.5 Bioelectricity generation using microbial fuel cell 36

1.5.1 Evolution of microbial fuel cell 37

1.5.2 Principle 37

1.5.3 Factors affecting microbial fuel cell performance 38

1.5.4 Advantages of microbial fuel cell 42

1.6 Conclusion 42

References 43

1.1 Introduction

The continuously increasing world population is expected to rise to 9.7 billion in 2050 and 11 billion by end of the century. India is anticipated to be the most populated country leaving behind China soon (Sharma et al., 2020). Nations worldwide are focussing on boosting their economy, have increased energy consumption (Obileke, Nwokolo, Makaka, Mukumba, & Onyeaka, 2020; Wang, Jiang, Yang, & Ge, 2020). The increased population leads to urbanization, industrialization, and demand for energy resources (Goswami, Pakshirajan, & Pugazhenthi, 2020; Kushwaha, Rani, & Patra, 2020; Umar, Abbas, Mohamad Ibrahim, Ismail, & Rafatullah, 2020). The increase in these sectors stipulates the usage and dependency on finite fossil fuels. This amplified energy demand contributes to greenhouse gas emissions due to fossil fuel usage leads to climate change (Goswami, Kumar, Arul Manikandan, Pakshirajan, & Pugazhenthi, 2019; Goswami, Kumar, Pakshirajan, & Pugazhenthi, 2019; Obileke et al., 2020). The increased fossil fuel usage also caused global warming, melting of polar ice caps, acid rain, ozone layer depletion, etc. (Ali et al., 2020; Goswami et al., 2018; Rizvi, Goswami, & Gupta, 2020; Sharma et al., 2020).

Traditional energy sources, that is, fossil fuels are limited and will be exhausted soon (Amit, Chandra, Ghosh, & Nayak, 2017; Sharma et al., 2020). The uncontrolled CO2 emission has reached 415 ppm in 2020 and is projected to reach 500 ppm by 2050 (CO2.org). Green and renewable energy sources are environmentally friendly alternatives and can minimize CO2 emissions (Goswami, Kumar, Manikandan, Pakshirajan, & Pugazhenthi, 2017). Hydro, wind, solar, biomass, geothermal and hydrogen, and biofuels can be sustainable options (Gupta, Srivastava, Patil, & Yadav, 2020). India consumes around ~9% of the total world’s energy and stands third in the category of highest energy consumers after China (23%) and the United States (17%) (Tan, Lam, Foo, Lim, & Lee, 2020; Wang et al., 2020). The energy crisis in the 1970s develops an interest in biomass as a fuel source. The interest is boosted extensively due to the world’s environmental issues (Rizvi et al., 2020; Yadav et al., 2020). The efforts to develop bioenergy from biomass as alternative sustainable energy have gained momentum (Amit & Ghosh, 2018; Lee et al., 2019; Nayak & Ghosh, 2019). Biomass-derived fuels are environmentally friendly (Ruano et al., 2016; Tan et al., 2020). Algal biomass contains large amounts of lipids suited for biodiesel production, considered third-generation biofuels (Amit & Ghosh, 2018; Lee et al., 2019). The alternative fuels must be economically competitive, environmentally acceptable, and readily available (Balat, 2011).

The increase in living standards and urbanization of the population, leads to surge in the solid waste generation (Kushwaha et al., 2021; Kushwaha, Rani, & Kumar, 2017; Kushwaha, Rani, Kumar, Thomas, et al., 2017; Yaashikaa, Kumar, Saravanan, Varjani, & Ramamurthy, 2020). The volume of municipal solid waste (MSW) is expected to reach 3.40 billion tons/year by 2050 from 2.01 billion tons/year in 2016. The increment of solid waste generation in urban India increases by about 1.3% per year (Ali et al., 2020). This massive urbanization in India resulted in larger domestic and industrial waste generation. The present estimation of daily MSW generation is 160,000 metric tons. Treatment plants and sanitary landfills are MSW management systems but also associated with health risks, thus compelled to waste-to-energy (WTE) routes. The transportation sector is the largest contributor to GHG emissions, and MSW transportation adds to it. The transportation sector majorly releases CO, CO2, SOx, NOx, etc. Seventy percent of global carbon monoxide (CO) and 19% of carbon dioxide (CO2) emissions are attributed to the transportation industry. Around 8 kg of CO2 has been produced by burning one gallon of gasoline. Therefore biofuels have vast applications and requirements owing to renewability and sustainability. They are readily available and biodegradable, aids to development, jobs, reduction of GHG emissions etc. (Mohsenizadeh, Kemal, & Kentel, 2020). Significant innovations in waste management have been achieved for material recovery and energy extraction (Xocaira et al., 2020). The MSW can be divided into two categories, organic and inorganic. It comprises organic waste such as cellulosic materials, food wastes, and plastics, while inorganics include metal, glass, discarded batteries, appliances, disposed medicines, and scrapped vehicle parts (Kushwaha, Rani, Kumar, & Gautam, 2015; Nikku, Deb, Sermyagina, & Puro, 2019). The compositions of the MSW vary according to economic condition, culture, climate, and uses. A massive volume of MSW is required to be treated. The developed nations employ MSW for the generation and production of heat, energy, fertilizers, and solid biofuels (Yaashikaa et al., 2020).

WTE technologies are a significant in development with less environmental pollution, reducing GHGs, creating new jobs, etc. (Khan & Kabir, 2020). The WTE route curbs GHG emissions and can provide a clean environment (Dey, Pal, Kevin, & Das, 2020). Their utilization will limit the harmful emissions of CO2, SOx, NOx, and reduce greenhouse effects (Shuba & Kifle, 2018). Biofuels from renewable sources lead to green energy. This form of energy derived from biomass is economically, ecologically, and environmentally viable (Manmai, Unpaprom, & Ramaraj, 2020). Microalgal bioremediation of the waste materials has proved to be an effective tool for WTE and pollutant removal (Bind et al., 2019; Burger & Abraham, 2019; Chandra, Amit, & Ghosh, 2019; Dutta, Kushwaha, Kalita, Devi, & Bhuyan, 2019).

Microalgae utilize the available organic load of the pollutants as a substrate for renewable biofuel production (Amit & Ghosh, 2018; Nayak & Ghosh, 2019). Microalgal are also being studied to remove the colorants from the wastewater, especially the textile and dyeing industry; they simultaneously produce biomass (Mishra, Nayak, & Maiti, 2020). Produced biofuels should be from readily available biomass, carbon-neutral, environmentally friendly, economically viable, and contribute to sustainability (Balat, 2011; Guo et al., 2013). Shifting to biofuels results into a reduction of dependence on oil imports (Balat, 2011). Biofuels can be produced through fermentation, anaerobic digestion, transetherification, liquefaction, and pyrolysis (Chen, Zhou, Luo, Zhang, & Chen, 2015). There are biochemical and thermochemical processes for biofuels. The United States used corn, Brazil used sugarcane and produced 90% of the world’s bioethanol in 2010. Biofuels can be liquid, solid, or gaseous fuels produced from biomass. A variety of biofuels from biomass includes biogas, bioethanol, biodiesel, biohydrogen, and bioelectricity (Guo, Song, & Buhain, 2015). They can be produced from various feedstock, edible substrates such as corn, sugarcane, and sweet sorghum and nonedible resources like rice straw, wood waste etc.; they are first-generation and second-generation biofuels, respectively (Tajmirriahi, Momayez, & Karimi, 2021).

Biodiesel has been widely accepted in public due to low emissions of pollutants and GHG, and renewability. The studies had suggested that the emission of toxic pollutants such as CO, hydrocarbons, particulate matters, and SO2 can be reduced with the use of biodiesel to an average value of 40% (SO2 could be reduced up to 100%) and CO2 emissions by ~78% as compared to petrodiesel.

Bioethanol, a major bioenergy source, is produced from the different organic substrates such as sugarcane (first generation), lignocellulosic matter (second generation) (Koçar & Civaş, 2013), and microalgal biomass (third generation) (Miranda, Passarinho, & Gouveia, 2012). The advantages are biodegradability and emission of lower pollutants as compared to fossil fuels (Sharma & Singh, 2009). The third-generation bioethanol from microalgae is a future option to meet the upcoming sustainable energy and clean air demand. The major advantages to use microalgae are the sequestration capability for CO2, small cultivation area, high yield, faster growth, easily controllable, and do not compete for land that could be used for food crops (Hassan & Kalam, 2013).

Biogas, by anaerobic digestion from biomass, is prominent and easily accessible bioenergy. This sustainable technology reduces global warming effects (Koçar & Civaş, 2013). In 2010, 8% (331 TWh) of total electricity from renewable sources was generated from biogas worldwide, which is estimated to reach 10% (696 TWh) by 2020 and ~13% (1487 TWh) by 2035 (Giwa et al., 2020). The installed bioenergy capacity of 66 GW in 2010 kept increasing with an annual growth rate of 5% in 2012 and could grow to 270 GW by 2030 (Tajmirriahi et al., 2021). Furthermore, microbial fuel cell (MFC) is a bioelectricity generating device, although less feasible, a lot of improvement is needed (Nayak, Amit, & Ghosh, 2018). The optimization of various parameters and design can play an essential role in improving the productivity of MFCs (Fu, Hung, Wu, Wen, & Su, 2010). The different types of bioenergies focussed in this chapter are Bioethanol, Biogas, Biodiesel and BioElectricity Fig. 1.1.

Figure 1.1 Types of bioenergy and types of biofuels.

1.2 Bioethanol

Ethanol is a primary alcohol, it can be prepared by two major methods, that is, (1) from petroleum and (2) from microbial fermentation of carbohydrates (Sheikh, Al-Bar, & Soliman, 2016). Globally, biomass-based ethanol is gaining attention. The United States and Brazil share 90% of total bioethanol production. Since the United States and Brazil are the global leader producers of corn and sugarcane, respectively, they are the major contributor to bioethanol production. Several other nations like China and India are also utilizing biomass for bioethanol production. In the nations like India, fuel prices are at a peak, and pollution is on top of the priority list; thus, biofuels can provide immediate relief to the low and medium-income groups. India, the third largest energy consumer after China and United States, and significant energy requirements are fulfilled by imported crude oil. India is an emerging economy next to China; the challenge is to support the mounting economy with limited energy reserves (Kumar, Kumar, Kaushik, Sharma, & Mishra, 2010). China, the fourth-largest bioethanol producer and consumer, followed by the United States, Brazil, and the European Union, issued the requirements regarding nationwide use of 10% ethanol by 2020. In Brazil, the bioethanol production industries from sugarcane are well established. Bioethanol can be a partial or full replacement for gasoline in the transportation sector. Bioethanol is produced from organic matter such as sugars, oils, crops, MSW, lignocellulosic materials, and proteins (Achinas & Euverink, 2016). These substrates are renewable, widely available, low cost, and do not compete with the food crops and animal feed. The utilization of crops and wastes such as corn stover, rice husk, wheat husk, and sugarcane bagasse are gaining significant importance worldwide. The biomass conversion to ethanol has four main steps: pretreatment, hydrolysis, fermentation, and distillation Fig. 1.2. A wide range of bacteria is used to for the bioconversion of biomass to bioethanol (Muthaiyan, Limayem, & Ricke, 2011).

Figure 1.2 Flow diagram for bioethanol production.

1.2.1 History

Bioethanol blend with petrol was firstly utilized in 1894 in Germany and France. Brazil is using bioethanol as a fuel since 1925 (Demirbas & Karslioglu, 2007; Lang, Macdonald, & Hill, 2010). Bioethanol is an extensively studied biofuel produced through biochemical reactions, for example, alcoholic fermentation of cellulose, starch etc. and anaerobic digestion of organic matters (Özçimen, Koçer, İnan, & Özer, 2020; Tajmirriahi et al., 2021). Bioethanol from biomass should be cultivated on land devoid of carbon, and by-products should be free from GHG emissions.

Production of ethanol from edible crops will impact food prices, so bioethanol production from waste biomass and less valuable crops like lignocellulose feedstocks grown on marginal lands are focused. Lignocellulosic biomass (LCB) comprises cellulose, hemicellulose, lignin, ash, etc., and lignin is the most complex fraction (Carrillo-Nieves et al., 2019). First-generation bioethanol production consists of two biological transformations, saccharification and fermentation. In contrast, second-generation involves pretreatment, saccharification, fermentation, distillation, and dehydration. The use of LCB requires the removal of the lignin. These wastes are voluminous and slow decomposition, so they are usually burnt in the field, which causes air pollution and toxicity for public health.

1.2.2 Substrate suitability for bioethanol

Bioethanol can be produced from fast-growing, cheap, efficient agro-industrial wastes through hydrolysis and fermentation (Carrillo-Nieves et al., 2019). Mexico, a leading agro-industrial waste generating country, is expected to produce second-generation ethanol from LCB feedstocks to replace fossil fuels (Carrillo-Nieves et al., 2019). For the lignocellulosic products, efficient pretreatment is required to hydrolyze the hemicelluloses to celluloses (Devi, Suhag, Dhaka, & Singh, 2012). The effective and efficient microbes and bioprocessing techniques for lignocellulosic ethanol production should be focused. First-generation biomass is starch based, where as lignocellulose biomass has complex lignin, which requires a high cost for delignification (Ramachandra & Hebbale, 2020).

The bioethanol production is composed of the following steps (1) pretreatment, (2) hydrolysis, and (3) fermentation (Ramachandra & Hebbale, 2020). Pretreatment methods to make the biomass more suitable are characterized by physical, chemical, and biological processes. Pretreatment disrupts the cell constituents and cell wall materials. Physical pretreatment involves size reduction to increase the surface area. Chemical pretreatment is with dilute acid, alkaline, organic, and other solvents, while biological process uses microorganisms for the cell disintegration. The fermentation process involves the conversion of sugars to ethanol by bacteria or yeast. Saccharomyces cerevisiae is the most extensively used microorganisms because it can tolerate high levels of alcohol (Özçimen et al., 2020).

Algal biomass-derived bioethanol is an eco-friendly option, avoiding competition with freshwater, food crops, or cultivable land without any additional nutrients. Macroalgae has a high concentration of polysaccharides and low in lignin, offers low-cost extraction which gives this technology an edge. Algal biomass is higher in photosynthetic efficiency, have higher yield per unit area (Ramachandra & Hebbale, 2020). Algae can be differentiated on the size and morphology, as micro and macroalgae. Microalgal neutral lipids for biodiesel production, while macroalgal carbohydrates are useful for bioethanol production.

Algal bioethanol has advantages of higher octane rating and heat of vaporization. The conventional hydrolysis and fermentation method give rise to ethanol from macroalgal biomass, and it is performed by using baker’s yeast (S. cerevisiae). Also, depending on the seasonal variation in carbohydrate content, mechanical preprocessing (such as drying, milling, and homogenization), physicochemical, and enzymatic pretreatment have been employed.

Fermentation is a bioconversion of carbohydrates into acid or alcohol, mainly carried out in two steps. Firstly, hydrolysis converts polysaccharides into fermentable sugars followed by their conversion to bioethanol using suitable microorganisms. Downstream processing includes purification and concentration of bioethanol (Dave, Selvaraj, Varadavenkatesan, & Vinayagam, 2019). The potential of microalgae was realized in the 1960s as a convincing material for bioeconomy to meet environmental sustainability (Rashid, Lee, & Chang, 2019). Bioenergy production is dependent on factors such as sustainability issues, limited availability of natural resources, socio-economic problems, low technology efficiency, and environmental impacts (Huang, Khan, Perecin, Coelho, & Zhang, 2020). Macroalgal biomass has negligible lignin; thus, no need for delignification during bioprocessing. Macroalgae can be cultivated in larger quantities in the marine environment than microalgae (Dave et al., 2019). The United States, China, Spain, Korea, and Australia are actively researching microalgae for biofuel production and wastewater treatment (Rashid et al., 2019). Adding bioethanol to gasoline raises the octane number (Özçimen et al., 2020). Bioethanol in vehicles can reduce carcinogenic SOx, so curb on acid rains. The complete combustion of bioethanol reduces emissions of pollutants in the atmosphere.

Microalgal bioethanol is very potent and exciting, but challenges are large-scale adaptability and commercialization. The selection of microalgal strain for higher biomass production, high carbohydrate content, easy harvesting techniques, pretreatment procedures, and an efficient fermentation process further needs to be worked. Bioethanol from microalgae includes microalgae cultivation, harvesting, hydrolysis, fermentation, and distillation. The different substrate and methods of bioethanol production are summarized in Table 1.1.

Table 1.1

1.2.3 Steps in bioethanol production

1.2.3.1 Microalgae cultivation

The cultivation of microalgae is done to enhance biomass production to make it sufficient for bioethanol production. The optimized large-scale production makes the process cheaper, abundant, and acceptable. Cultivation is done in a photobioreactor, and for large-scale open ponds are preferred. They require light, CO2, nitrogen, phosphorus, macronutrients Na, K, Ca, K, Mg, P, etc., and micronutrients such as B, Zn, Co, Fe, Mn, etc. The wastewater is nutrient-rich, thus can be used in microalgae production. Optimizing the growth conditions, temperature, varying light, and dark phases, and stresses can enhance the microalgae’s carbohydrate accumulation. Phosphorus limitation and enhanced Ca and Mg have been reported to improve carbohydrate accumulation in Chlorella sp. The flow steps for bioethanol are as follows Fig. 1.2.

1.2.3.2 Harvesting and drying

The effective and economic harvesting methods should be employed. Harvested methods vary according to size and species. Microalgal biomass needs to be separated by flocculation, flotation, or gravity sedimentation methods. Then, thickening by centrifugation and filtration till the 10–20 g/L microalgal concentration. The obtained algal paste is solar dried. After drying, the pretreatment of the dried mass and then saccharification steps are initiated. In the flocculation process, inexpensive, user-friendly, renewable, inorganic, or organic flocculants such as Al2(SO4)3 and Fe2(SO4)3 are used.

1.2.3.3 Pretreatments and saccharification

Pretreatments result in the disruption of cell membranes and releasing the carbohydrate out of the cells. Saccharification breaks the carbohydrates into small fermentable sugars and is transported to the fermenter. Chemical pretreatments use acids (H2SO4, HCl, and HNO3) and bases (NaOH, KOH, and Na2CO3) that are fast, cheap, and easily applicable. Sometimes degradation forms toxic compounds. Microalgae can be exposed to pretreatment and saccharification simultaneously due to their simple cell structure and lack of lignin content.

1.2.3.4 Fermentation and distillation

Fermentation has two methods: (1) separate hydrolysis with fermentation and (2) simultaneous saccharification and fermentation (SSF). Glucose is added with S. cerevisiae to the fermenter to produce bioethanol, and released CO2 gas can be conveyed to the microalgae cultivation system. The unused substrate and other wastes are stored for reuse. Bioethanol production from potato and potato peel waste by S. cerevisiae was evaluated at varying fermentation periods and yeast extract concentration (Sheikh et al., 2016) (Fig. 1.2).

1.2.4 Cost of production

The production cost for fuel depends on substrate availability, connectivity to plant location, purity and end use, plant location, transportation cost, method of pretreatment, production techniques, and fuel’s market price. The majorly used feedstock for bioethanol is sugarcane (Brazil), corn (United States), starch-rich grains (Europe), etc. The inexpensive biomass materials provide stability to supply and price. Enzymatic hydrolysis can result in improvements in fermentation and, thus, reductions in the cost. Cellulosic ethanol has enormous potential but is in the nascent stage, so enhancing efficiency and profitability, technological modification, and adaptability need to be focused (Sheikh et al., 2016). The parameters influencing the bioethanol production process are summurized in Fig. 1.3.

Figure 1.3 Parameters influencing bioethanol production.

1.2.5 Substrates for bioethanol production

Bioethanol can be produced from a wide range of substrates on the basis of regional availability and economic efficiency.

1.2.5.1 Sucrose-containing materials

Sucrose-containing feedstocks are significant choice of fermentation to convert sugar to ethanol, for example, sugarcane, sugar beet, and sugar sorghum.

1.2.5.2 Starchy materials

Starch and similar feedstocks offer a high yield of bioethanol. It involves saccharification followed by fermentation. For example, corn, potato, and sweet potato.

1.2.5.3 Lignocellulosic biomass

The low porosity, high crystallinity, and lignin contents are the hurdles to use LCB for bioethanol production. These feedstocks require pretreatment, for example, steam, acid, and alkali treatments, to make these biomasses more acceptable feedstocks.

1.2.6 Uses of bioethanol

Bioethanol is advantageous over conventional fuels because it is from renewable resources. It reduces carbon load and GHG emissions. It can be blended with gasoline to raise the octane number (Özçimen et al., 2020). Bioethanol in vehicles reduces carcinogenic sulfur oxides (SOx) so curb on acid rains. On complete combustion, it does not give any carbon monoxide, hydrocarbon, and other particles (Table 1.1).

1.3 Biogas production through anaerobic digestion

Bioenergy production is becoming a necessity to counter the increasing demand for energy and environmental concerns (Al-Wahaibi et al., 2020). The nations have different industrial biomass residues. The largest proportion of industrial waste is bagasse, which can be used for biofuel production. The conventional firing of bagasse in boilers produces low-pressure steam (Sathish, Balaji, Shafee, & Mageswaran, 2020). Therefore an alternative method for maximum extraction of energy to attain desired pressure is required.

Anaerobic digestion (AD) is a conversion technology and converts organic matter to energy in the absence of oxygen. The produced gas is a mixture of mainly methane (CH4) and carbon dioxide. AD is one of the oldest technology to treat manure, night soil, sewage sludge, etc. (Mata-Alvarez et al., 2014; Zhang, Su, Baeyens, & Tan, 2014). The composition of biogas depends on various parameters and operational conditions. CH4 and carbon dioxide are the significant components of biogas and traces of hydrogen, hydrogen sulfide, ammonia, and water vapor. CH4 is the only constituent of biogas with significant energy value, while other constituents lower the biogas calorific content. The presence of hydrogen sulfide (H2S) corrodes the anaerobic digester and pipes of the gas distribution (Ruano et al., 2016).

1.3.1 History of anaerobic digestion

Biogas usage is long-aged and was used for heating and other similar uses. The advancement was achieved in 1921 when Guorui Luo popularized and commercialized the technology by constructing a digester of volume 8 m³ and fed it with kitchen waste. Till the 1980s, AD for sewage and agricultural waste, industrial waste, urban waste, etc., was developed faster in China, Germany, Asia, Latin America, and African countries, then taken up by other regions too (Obileke et al., 2020). Small-scale biogas digesters (6–10 m³) for household applications are common in different nations in recent years. The efforts of governments of various countries started subsidy for establishments of the biogas digesters. It has resulted in 30%–100% increment of biogas digesters’ installation from the 1980s to 1990s. In 2007 the Indian government-subsidized over 4 million biogas digesters to the rural areas (Kuo & Dow, 2017; Obileke et al., 2020).

An increase in population and waste generation suggested the need for WTE method. Landfilling contaminates soil and groundwater, and the leachate is also cost-intensive to treat. Incineration aids to air pollution (Rabii, Aldin, Dahman, & Elbeshbishy, 2019). The transport sector requires enormous energy and, in turn, majorly contributes to GHG emissions. These environmental challenges intend to convert biowaste to vehicular fuel in the form of compressed biogas (Fagerström, Al Seadi, Rasi, & Briseid, 2018; Sebola, Tesfagiorgis, & Muzenda, 2014). Biogas processes aim to treat waste, environmental protection, energy extraction, electricity production, heat, and advanced biofuel production. AD digest a variety of organic feedstock and produce biogas and manures. When renewable energy replaces fossil fuels in industrial applications, the process will become significant, environmentally friendly, decarbonized, and sustainable (Fagerström et al., 2018).

The effective management of MSW, industrial effluents, domestic effluents, waste to energy, or waste to resource is needs to be adopted (Berni et al., 2014; Bong et al., 2018). The reduction of waste and generation of the value-added product make these technologies more acceptable (Al-Wahaibi et al., 2020; Lungkhimba, Karki, & Shrestha, 1970). AD is a biological treatment for organic waste in the absence of oxygen, release biogas, and digestate as the main products (Bong et al., 2018). Biogas digesters can be fed with wastes such as night soil (human wastes), agricultural wastes, industrial effluents, etc., to check the environmental contamination and spread of pathogenic diseases. Biogas production has applications in cooking, heating, lighting, etc., in rural areas. It can reduce deforestation, minimize greenhouse emissions, and reduce unpleasant odors and associated diseases (Nasir, Mohd Ghazi, & Omar, 2012). Biogas applications can contribute to rural development, energy generation, job creations, etc. (Obileke et al., 2020; Van, Fujiwara, Tho, Toan, & Minh, 2020). While digestate, coming out of the bioreactor, is nutrient-rich, it can be reused as green fertilizers (Wang, 2014). The digestate consists of slow degradable, stable organic components such as lignin, nitrogen, and phosphorous, and inorganic salts containing phosphate, ammonium, potassium, and other minerals (Fagerström et al., 2018).

The bioenergy production from the biomass feedstock is not trivial. The technology is known to humans for centuries, but the optimization of design and process is still going on (Meegoda, Li, Patel, & Wang, 2018). The biomass availability depends on the local conditions for cultivation, harvest methods, transport, storage, and conversion technologies (Meyer, Ehimen, & Holm-nielsen, 2018; Sandhu & Kaushal, 2019).

Bioenergy harmonizes with public health and interests and improves energy resilience. When digested by AD, food waste is of high energy content, releases stored energy, and produces the digestate for soil amendment (Kuo & Dow, 2017). Biogas production through AD is a renewable energy strategy (Caposciutti, Baccioli, Ferrari, & Desideri, 2020; Fagerström et al., 2018; Kuo & Dow, 2017; Sandhu & Kaushal, 2019; Sebola et al., 2014). It generally takes about 21 days to acclimatize microorganisms and result in biogas production (Lungkhimba et al., 1970; Obileke et al., 2020). Biogas can be used to produce heat, steam, electricity, and vehicle fuel depending upon the usage and energy efficiency of biogas plants (Uçkun Kiran, Stamatelatou, Antonopoulou, & Lyberatos, 2016). AD can extract energy from almost all organic waste types, energy crops (Teixeira Franco, Buffière, & Bayard, 2016). AD treats sewage, industrial effluents, animal manures, agricultural residues, food wastes to produce a mixture of methane and CO2, known as biogas (Lungkhimba et al., 1970; Sathish et al., 2020; Uçkun Kiran et al., 2016). Methane (CH4) presence in the atmosphere is either natural or through various human activities. The decay of natural organic material anaerobically produces abundant CH4; however, human activities produce less abundant, insufficient CH4 to meet significant energy needs (Ruano et al., 2016).

The composition varies according to the feed and operating conditions. The pretreatment helps in the removal of these trace compounds to enhance the yield. The hydrogen sulfide and ammonia has an adverse effect on the process, affecting the efficiency and purity of the biogas (Kuo & Dow, 2017). Various nation’s efforts to tackle huge waste generation, solid waste management, and AD offer promising energy recovery from these wastes (Sebola et al., 2014).

AD has significant environmental benefits; it prevents biomass putrefaction and acidification, reduces air and water pollution, associated diseases, etc. AD also decreases the volume of wastes. AD is affected by factors such as pH, temperature, microorganisms, substrate and inoculum types, and organic loading ratio (Sebola et al., 2014; Uçkun Kiran et al., 2016) Fig. 1.5. Codigestion of different substrates in the AD increases the efficiency (Kuo & Dow, 2017; Rabii et al., 2019). It is superior to AD in solids reductions and enhanced bioenergy production. The codigestion of biodiesel waste, glycerin and MSW resulted in an increase of biogas and methane content by 100% and 120%, respectively (Rabii et al., 2019). Codigestion of rice straw with nitrogen-rich substrates resulted in efficient C/N ratio, nutrient balance, and biomethane yield enhancement. Pretreatment is another effective strategy, which improves the performance of the digester (Shitophyta, 2016).

Figure 1.5 Operational parameters affecting anaerobic digestion.

Several studies have been carried out on the use of rice straw for biogas production. India being an agriculture country, the major population depends on agriculture. It is the third largest paddy producer and generates process residue and field residue as an energy source for biogas digester (Shitophyta, 2016). Bioenergy is estimated to contribute to 50% of the renewable supply goals by the year 2030. Organic fractions of farming, industry, and municipalities contaminate land, water, and air. Biogas through AD offers energy-efficient and environmentally beneficial bioenergy (Merlin Christy, Gopinath, & Divya, 2014).

Organic fractions of various wastes, for example, manures, sewage sludge, organic fraction of MSW, animal manure, biowaste, are biodegradable and contain nutrients and pathogens, so they should not be disposed off in landfills. The organized dumping can contaminate soil, air, and water (Sibiya, Muzenda, & Mbohwa, 2017). Manure in farming practices enhances soil structure, fertility, and biological activity and provides the required nutrients for crops. Chemical fertilizers being cheaper and of high productivity, the use of manure has been decreased. The manures have leaching issues, especially with P and K. The direct discharge of manure into the surface water causes organic degradation and depletes dissolved oxygen. In other countries, treating manure in lagoons before discharged into the surface water bodies is practiced (Lungkhimba et al., 1970; Neshat, Mohammadi, Najafpour, & Lahijani, 2017).

AD takes place in three steps with the help of various microorganisms, that is, hydrolysis, acidogenesis, and methanogenesis (Van et al., 2020). Some of the studies have divided acidogenesis into acidogenesis and acetogenesis (Ruano et al., 2016; Sugumar, Shanmuga Priyan, & Dinesh, 2016; Xu et al., 2019; Zhang et al., 2014). In the hydrolysis step, complex and polymeric substrates such as carbohydrates, lipids, proteins, and nucleic acids are converted to their monomers. Then, acetate, butyrate, hydrogen, carbon dioxide, formate, etc., are produced by acidogenesis. In the final stage, methanogens produce methane. There are two types of methanogens, that is, acetoclastic fed on acetate and hydrogen-utilizing methanogens. The AD with high organic matter, salt, oil and protein content, low C/N ratio, and micronutrient deficiency, inorganics is less prone to biogas production (Negri et al., 2020).

1.3.2 Principles of the anaerobic digestion process

AD is a biochemical process where consecutive interaction of microorganisms with a substrate takes place. The complex biomass is disintegrated into organic polymers and further degraded to form CH4 (Ruano et al., 2016). AD is divided into the following steps Fig. 1.4:

Figure 1.4 Flow diagram of biogas production.

1.3.2.1 Hydrolysis

The organic polymers are hydrolyzed to their monomers by the action of extracellular enzymes. The pretreatment of biomass before hydrolysis makes the hydrolysis more acquiescent to microorganisms and enzymes. Hydrolysis is the slowest and rate-limiting step (Shitophyta, 2016). Microbes in their lag phase of growth acclimatize with the substrate.

(1.1)

1.3.2.2 Acidogenesis

Acidogensis convert the simple monomers and oligomers obtained in the hydrolysis stage into simpler organic compounds. The short-chain fatty acids, acidic functional groups, carbon dioxide, and hydrogen are the major outcomes. This stage produces large amounts of carbon dioxide and hydrogen. The acidogens have a high growth rate and susceptible to pH change.

(1.2)

1.3.2.3 Acetogenesis

The acetogens metabolize the outcomes of the acidogenesis to produce acetate, carbon dioxide, and hydrogen. They take a longer time as compared with acidogensis.

1.3.2.4 Methanogenesis

The two microbial groups are responsible for producing methane and carbon dioxide after acetogenesis. The acetoclastic methanogens transform acetic acid to methane. They are slow growing and sensitive to pH, nutrients, trace element concentrations, etc. The hydrogen-utilizing microbes use hydrogen and carbon dioxide to produce methane, further increasing the methane content. So, the methane content of biogas depends on the substrate and its composition (Fig. 1.4).

(1.3)

(1.4)

1.3.3 Factors affecting the anaerobic digestion process

The microbial consortium in AD responds differently to environmental changes, the substrate, and the operational parameters that influence the AD technology. These parameters must be optimized for efficient biogas generation (Obileke et al., 2020; Ruano et al., 2016; Sebola et al., 2014; Uçkun Kiran et al., 2016). Several other parameters, such as hydraulic retention time (HRT), organic loading of the substrate, C/N ratio, particle size, inoculums, mixing, water content, reactor design, digestibility of the substrate, and feed composition, need to be in an optimal state for performance and the quality of the biogas being produced (Kuo & Dow, 2017; Obileke et al., 2020; Shitophyta, 2016; Uçkun Kiran et al., 2016; Van et al., 2020). Biogas is the desirable product of AD. The parameters influencing biogas’ quality and quantity are discussed below Fig. 1.5.

1.3.3.1 Temperature

The temperature affects the microorganisms, microbial enzymes that characterize the metabolic processes. The higher temperature denatures the proteins, thus the enzymes. Temperature is a major parameter for biogas production. CH4 production has been reported in the range of 0°C–97°C, and the optimum temperature for efficient biogas production is 20°C–35°C. The reaction rate and biogas production rate increase up to a certain temperature, after which it decreases. Thus to maintain a constant temperature, insulations made up of charcoal coating, paddy husk, wood wrapping, etc., are generally used. Cheap and locally available materials are preferred for insulation. Composite made of glass wool, sawdust, clay mud, black cloth, etc., are also employed as insulations (Obileke et al., 2020; Shitophyta, 2016; Sugumar et al., 2016).

1.3.3.2 pH value

pH determines the acidity, alkalinity, or neutrality of the substrate, and it is essential to provide an optimum working environment to microorganisms. Methanogenic bacteria are well suited at pH range of 6.6 to 8.0, while for hydrolysis and acidogenesis, an acidic pH range of 5.5 to 6.5 is preferable. A pH of <6.1 and >8.3 causes a decrease in biogas production. pH also varies according to the substrate used. Carbohydrate rich substrates cause a rapid acidification process and drop of the pH, whereas proteins-rich substrate increases pH level. For acidic waste and industrial waste, pH can be adjusted by adding bases like lime (CaCO3) (Obileke et al., 2020; Shitophyta, 2016; Sugumar et al., 2016).

1.3.3.3 Carbon and nitrogen ratio

C/N ratio is the quantity of carbon to nitrogen available on the substrate. The C/N ratio affects microbial activities and thus on biogas production (Yang et al., 2019). Methanogens prefer a high C/N ratio; the C/N ratio of 20–30 is recommended for AD for biogas production (Obileke et al., 2020; Shitophyta, 2016).

1.3.3.4 Hydraulic retention time

The HRT is the time for which the substrate remains inside the digester. It is a function of the volume of digester and the rate of organic loading. The HRT in mesophilic conditions ranges from 10 to 30 days, while in thermophilic environments decreases to around 10 days. The temperature range of 30°C–35°C, 20–30 days of HRT for biogas production is prefentially reported. The substrate’s nature, microbial consortium, temperature, and pH affect the HRT (Obileke et al., 2020; Sugumar et al., 2016).

1.3.3.5 Particle size

Particle size also plays an important role in AD. It affects the rate of degradation and thus the biogas production rate. The homogenized materials are preferred as they produce higher biogas. Instruments such as a rotary cutter and shredder are used to increase the substrate’s surface area for better availability and interaction with the microbes. While the larger size can create clogging and blocking of the digester (Obileke et al., 2020; Sugumar et al., 2016).

1.3.3.6 Organic loading rate

Organic loading rate (OLR) is defined as the amount of organic waste fed per unit volume of the digester per day. It is an essential parameter, directly affects the biogas production rate. An increase in OLR results in higher biogas production rates but excessive organic loading will lead to process inhibition, thus decreasing gas production. It is a function of the substrate and its concentration, temperature, type of biogas digester, etc. OLRs of 0.5–5 kgVS/m³/day is recommended. According to the thumb rule of AD, 4 kg of fresh cow dung per m³ of biogas digester is required for optimum gas production (Obileke et al., 2020; Sugumar et al., 2016).

1.3.3.7 Inoculum

Inoculum is the initial consortium or the pure culture of the bacterial species transferred to the digester to acclimatize the reactor conditions. Proper inoculums result in higher biogas production. The proper seeding of biodigesters enhances biogas production (Al-Wahaibi et al., 2020; Obileke et al., 2020).

1.3.3.8 Mixing/agitation process

Mixing and agitation attain uniformity of microbes and substrate. The rapid mixing enhances the microbial activities in the digester. Also, the content and size, affect the mixing process. The rate kinetics increases with mixing. Insufficient agitation causes concentration gradients and a decrease in biogas production (Al-Wahaibi et al., 2020; Obileke et al., 2020).

1.3.3.9 Water content

The water content affects the activities of microorganisms and thus the rate of conversion. The lesser and higher water concentration than the optimum causes the decrease in biogas production.

1.3.4 Landfill biogas

Landfill is the waste management system for solid waste. The landfill and gases coming out of it have both positive and negative environmental impacts. The associated gases of the landfill have energy efficiency and can be used for different applications. Nearby colonies and habitat can be affected by landfills and gases. The conversion of biodegradable waste into energy can reduce landfills area (Sebola et al., 2014). Anaerobic reactor is the heart of digestion systems, affected by various parameters such as temperature, pH, and microbes (Van et al.,