What will be the source of energy/fuel in the near future once all the fossils/petroleum are exhausted? The alternative to these is an electrical source of energy. With the focus now shifting to the use of renewable resources for energy or fuel production, the development of new advanced technologies to convert waste into useful energy or fuel becomes even more imminent. What if we combine the two problems and find a single solution? And the solution to this is generating energy in the form of electricity, biofuels, biomethane, biohydrogen, etc. from the waste material. The cost of being dependent on the raw material for generating energy is always high; therefore, a suitably treated waste can serve as a substitute to the conventional raw material to generate energy. Conventional anaerobic digestion (AD) was first demonstrated in the 17th century (the early 1630s) by a Belgian chemist, Jan Baptita Van Helmont. He showed that combustible gases can be obtained by de-composting organic matter. The first sewage plant was built in Bombay in the year 1859. In 1895, England designed a process to recover flammable gases by treating sewage. Two years later, the first biogas plant was set up in Bombay, India. Then, later in the 1930s, the grange waste was used to generate flammable gas to power the street lights of asylum in Bombay. The concept was soon used in the application and in the 1960s, Khadi and Village Industries Commission (KVIC) set up the biogas plant that can be used in rural areas as fuel for cooking and other domestic purposes (Muthudineshkumar & Anand, 2019). Alessandro Volta's experiment in 1776 showed that more amounts of combustible fuel can be produced, using more decaying organic matter. In 1895, England designed the sewage treatment facility and used the by-product generated to light the street lamps in Exeter. Until the early 1960s, China had set up a million biogas plants using the septic tank design as the basis and replaced the dome-shaped tank with a rectangular tank. India followed the same changes and participated in a Biogas Sector Partnership (BSP) along with Nepal and China (Muthudineshkumar & Anand, 2019). With increasing prices of oils and petroleum in the 1980s, the United Kingdom and Europe became interested in the biogas program as an alternative source of energy, which was renewable (Ismail et al., 2014; Wilkinson, 2011). With an increasing demand for energy and the importance of biogas, thus the setup of the first biogas plant in Bombay, India, researchers showed interest and made various modifications to the design of the biogas plant. Among all the plants, Grama Laxmi III was built by Joshbai Patel, which later became a guide for the KVIC floating dome model. The National Biogas and Manure Management program built up to 1,50,000 family-based biogas plants between the years 2009 to 2010 (Davis, 2005; Munasinghe & Khanal, 2010).

Anaerobic digestion is a complex microbiological process in which many anaerobic and facultative bacteria work hand-in-hand/together to break down the complex organic matter into simple forms in anaerobic conditions (Munasinghe & Khanal, 2010, Parkin & Owen, 1986). Initially, the primary objective of anaerobic digestion of wastewater was for the utilization of organic matter, reduction in odor, and conversion of organic matter to methane and carbon dioxide. Thus, the biogas produced is an inexhaustible source of energy that can be utilized in various ways like producing heat, electricity, fuel boilers and furnaces, alternative to fuels for vehicles, and can also be used in households as natural gas pipelines. Today, biogas is cleaned and trace contaminants removed; thus, higher-quality gas is supplied as compressed natural gas (CNG) or liquefied natural gas (LNG). This can be more efficient for the internal combustion of engines and also used for domestic purposes. Anaerobic digestion is greatly used in many technologies, but it has a complex mechanism to understand since the biological factor “microorganisms” are involved, which are affected by slight changes in their environment like temperature, pH, moisture, etc. (Parkin & Owen, 1986; Nasir et al., 2012). The commonly used substrate for anaerobic digestion can be animal manure, food scraps, wastewater treatment solids, and municipal and industrial wastewater residues that are put into a digester to produce biogas (60% methane and 40% carbon dioxide) and digestate. The biogas produced c...