Introductory Chapter: From Biogas Lab-Scale towards Industrialization

1. Introduction

Production and consumption of food and the exploitation of fossils in the past several decades due to globalization resulted in the depletion of fossil fuels and severe environmental pollution [1, 2]. The emission of greenhouse gases (GHGs) in such an increasing trend causes global warming that devastates aquatic and terrestrial ecosystems. Around 88% of energy worldwide is provided by fossil fuels despite their damage to the environment [3, 4]. In 2019, oil production reached 4484.5 Mt, with natural gas reaching 3989.3 billion cubic meters [5]. Total global coal reserves at the end of 2019 were 1,069,636 million tons (Mt). Being an ancient energy source; coal production showed a slight increase (142.89–167.58 Mt) in the last decade. Carbon emissions in the last ten years increased by 1.1% yearly. According to the British petroleum survey, carbon emissions increased from 29,745.2 Mt to 34,169 Mt in a single decade.

Worldwide energy consumption has increased 17-fold in the last century, and emissions of CO_×, SO_×, and NO_× from fossil-fuel combustion are the primary cause of atmospheric pollution [6] and increased the GHGs [7]. Around 2 Mt of soot and dark particles are released annually only from the world’s largest populations, which is responsible for heating the air and melting the glaciers. Therefore, the glaciers that provide water to south Asian nations are decreasing rapidly due to global warming resulting in catastrophic floods in the region. The fossil fuels beneath the earth’s surface are not evenly distributed, promoting the search for alternative energy sources available globally [8]. Moreover, due to drastic climate changes and energy shortages, approaches to reduce environmental pollution and alternative energy resources are being explored [9, 10, 11].

Renewable energy production and consumption have been increased over time. Among the global renewable energy giants, China contributed 7.9% to the total renewable energy consumption in 2010, while in 2020, this share increased to 24.5% [12]. Besides the depletion of fossil fuels and environmental threats associated with their consumption, modern civilization has produced a tremendous amount of solid organic and inorganic wastes. The global solid waste generation rate was 0.3 Mt per day in 1900, which increased to 3 Mt per day in 2000, and it is supposed to be doubled by 2025. The world’s largest landfills such as Laogang (China), Sudokwon (Seoul), the now-full Jardim Gramacho (Brazil), and Bordo Poniente (Mexico City) receive around 10,000 tons of waste daily [13]. In developed countries, the municipal solid waste generation was reported to be 1.43–2.08 kg/person/day; however, it was 0.3–1.44 kg/person/day for developing countries [14]. Solid waste (municipal solid waste) refers to any garbage, trash, or refuse material, which represents a potential cause of pollution.

The risk of waste production is getting higher day by day, even in developing countries due to the increase in the world’s population and urbanization [15]. Thus, it can be said that waste generation is directly proportional to the rate of population growth. Further, waste organic and inorganic components are equally important as they hold a potential threat to living organisms and the environment [16]. According to Environmental Protection Agency (EPA), solid waste could be hazardous or non-hazardous depending upon its source. It usually consists of everyday items that people throw away. Generally, it is characterized into two major types: trash and garbage/rubbish. Garbage can also refer to food waste or kitchen waste, comprising organic waste, clothing, and food containers. In contrast, trash consists of daily household or other items no longer needed, including furniture, leaves, grass clippings, and junk [17]. Other major waste classes include agricultural waste, bio-medical waste, chemical waste, radioactive waste, construction waste, and e-waste. Waste management is of prime focus worldwide as improper waste disposal has caused severe environmental issues such as air and water pollution, loss of endangered wildlife habitat, disease outbreaks, and climate change. All these have a direct impact on society as well as the world’s economy. To treat waste properly, it is of utmost importance that waste is characterized and collected accordingly. In terms of municipal waste generation, the United States and Canada were the two of the largest per capita waste producers, generating almost 2.58 kg and 2.33 kg daily, respectively [18].

Organic waste has received great attention as it is biodegradable and can be broken down into methane, carbon dioxide, water, and other organic compounds. It could be in the form of food, green waste, or feces. Since the byproducts of organic waste are usually harmless, they can be used on an industrial scale to produce biofuels. Therefore, many countries are consuming waste to generate energy [19]. Organic waste could be the byproduct of various industries such as agriculture, meat, poultry, sugar refineries, and oil industries. The composition of organic waste constantly varies as it is a combination of a variety of compounds. It all depends upon the properties and amount of each component present in organic waste. Therefore, its characterization and segregation are equally crucial in extracting maximum nutrients cost-effectively [20, 21]. Studies have shown the importance of agricultural and livestock waste among organic wastes. With the increase in agro-based industrialization, waste production has been increased up to three folds. These residues are a rich source of biocompounds that can be used for biogas production and manufacturing enzymes, vitamins, antibiotics, and animal feed [22]. Agricultural and livestock waste is always preferred among the various types of organic waste [23, 24]. The waste of slaughterhouses and fallen stocks are also rich in organic compounds that can be converted into valuable biofuels [25]. Each year, more than 2 billion tons of agro-waste are piled up, comprising straw and husk of wheat, rice, and barley. Adding up to this is forest waste (0.2 billion cubic meters), municipal solid waste (1.7 billion tons), industrial waste (approximately 9 billion tons), and animal waste (1.3 billion tons) [26]. If the necessary measurements for waste treatment are not appropriately followed, society, humans, flora, and fauna will face many challenges. With the advancement in science and technology, scientists are focusing on the gross value of waste as the products of these waste treatments are aimed to be environmentally friendly. Organic wastes are considered a potential resource for several applications, including animal feed, raw material in different industries, and feedstocks for biofuel. The R&D for the utilization of various organic waste for biofuels including biodiesel [27], crude bio-oil [28], bioethanol [28], and biogas production [29] developed fast in the past decades due to its lower carbon and GHGs emissions and the reduction of toxic waste from the environment [30]. Among the various methods of using organic waste as an energy source, anaerobic digestion (AD) has gained the most attention.

2. Biogas production

Due to biogas production from organic waste in the last years, there is a relative decrease in greenhouse gas emissions and fossil fuel consumption. Biogas consists of 50–75% methane, 25–50% of carbon-dioxide, 1–2% ammonia, and traces of hydrogen sulfide, oxygen, nitrogen hydrogen, and fermented organic fertilizer [29]. Biogas generation is an economical method since the raw material primarily used is agricultural and food waste. It could also be termed green energy and can be used in boilers for heat generation [31]. The basic phenomenon of biogas is the conversion of solar energy stored in the organic waste into gaseous energy by anaerobic digestion. Therefore, biogas is generated by microorganisms as a byproduct of their metabolism. The total energy level could be calculated by methane quantity [32].

Various process variables affecting biogas production, like the nature of the feedstock and carbon-to-nitrogen ratio, and reactors setup, have been evaluated. Different agricultural residues (wheat stalk, soybean straw, and black gram stalk), food wastes, and animal wastes are suitable for biogas production [31, 33, 34]. In recent years, it has been observed that landfills for waste management had specific side effects on the environment. Previously, biogas plants were established for waste disposals. Nevertheless, this practice has been changed ever since. These plants are now used for energy generation from biomass. For this purpose, many studies have been conducted to evaluate the optimal capacity of waste being converted into energy with greater yields and cost-effective mechanisms [19]. Biogas is used as fuel on the domestic and commercial levels. The production capacity from the installed biogas plants across the globe has been increasing every year.

As a renewable energy, biomethane can be derived from various substrates under anaerobic conditions, including sewage and waste activated sludge, food wastes and vegetable, wastes from forestry, manure from living stocks, agriculture wastes, and wastewater [35]. Biogas derived from organic wastes through AD is suitable to clean energy to fulfill energy demand [36]. AD is commonly considered a reliable and cheap approach for energy recovery and wastes management [37], which minimizes the waste quantity and uncontrolled emissions. Besides, the AD digestates contain nutrients and can serve as a biofertilizer for crops. Biogas might substitute fossil fuels and lower the GHGs emission at households and commercial scale [38]. AD of different feedstocks may have a different biomethane production and obtain more bioenergy to compensate for the net energy utilized during the process.

3. Feedstocks for biogas generation

Most of the biowaste is landfilled, burned, or only reused after composting. However, it can be utilized as a potential source of bioenergy through different practices [10]. A variety of biowaste can be used as substrate in AD to generate clean and renewable energy in the form of biogas and biomethane [39, 40]. Biowaste is mainly composed of 3 major biocomponents, i.e., lipids, proteins, and carbohydrates. Agricultural waste, forest waste, wood residues, fruit and vegetable waste, and municipal sludge contains high content of carbohydrates-based compounds. Protein biowaste is mainly originated from animal sources such as slaughterhouse waste, meat processing industries, and dairy industries. The lipids-based feedstocks are derived from waste oil, oil mills, animal fats from the slaughterhouse, FOG, grease trap waste from sanitation, and wastewater from restaurants. Most of this waste has been applied in AD to generate biogas [7, 41]. Among carbohydrates, lipids, and proteins, the maximum biogas production potential has been reported for lipids. The energy potential of organic wastes and biomass mostly depends on their physiochemical and elemental commotions [42]. Among which volatile solids (VS) and the ratio of carbon to nitrogen (C/N ratio) are the most important as only the organic portion in any waste is attributed as VS, and the carbon is used as food during the microbial process to produce bioenergy [43]. Moisture is another essential aspect for improved degradability of biomasses, especially in agriculture, fruits, and vegetable waste [44]. The COD (chemical oxygen demand) of organic waste material corresponds to the amount of organic substrate available to the microbial community for biogas production [45].

The present book aims to discuss biogas production from different resources and the impact and changes of microbial community during the digestion process. In addition, the possible utilization of biogas byproducts as biofertilizers will be evaluated. Moreover, case studies on biogas production from municipal solid wastes will be presented.