**2. Biogas production**

The capacity of biogas plants globally has been on a growth trajectory in many countries because of factors like rising cost of fossil fuels, the easy and affordable supply of biomass feedstock, and growing concerns about emissions and the threat of a global climate emergency. The primary waste sources used in the production of biogas include, but are not limited to, home wastes such food, produce, and fruit as well as animal wastes like dung and bird droppings. Public moist wastes from markets, cafes, restaurants, and food outlets are also frequently used in this procedure, along with biodegradable trash from businesses with high moisture content and degradability. The use of anaerobic digestion to produce biogas acts as a catalyst for improving a

country's energy mix while also significantly advancing the preservation of natural resources and defending the environment [30, 32, 48, 49, 63–65].

Under optimal conditions, a specific group of bacteria can generate biogas through anaerobic processes having a calorific value ranging from 21 to 24 MJ/m3 , thereby establishing it as an environmentally sustainable energy resource [22]. Biodegradation of organic matter generates between 590 and 800 million tons of methane per year, which is then released into the atmosphere without human involvement. Biogas, a byproduct of biomass decomposition, can be recovered and converted into usable energy by use of such devices [6, 22]. The composition of biogas can range from 50 to 70% methane and 30 to 50% carbon dioxide, depending on the substrate employed and the management and regulation of the production process. Nitrogen and hydrogen sulfide are among the extra components that are present. Gas distribution systems can be fitted with enriched biogas from larger plants. The metabolic process that ultimately produces methane and other gases from biomass relies on the action of a suite of bacteria that exists as at least three bacterial communities employed in biogas production systems [22, 66]. The utilization of anaerobic digesters usually occurs in the mesophilic (20–40°C) and thermophilic (above 40°C) temperature ranges [22, 66], depending on the substrate fed into the digester, biogas typically consists of 30–60% methane and 30–50% CO2. Hydrogen gas is also present in biogas although in much smaller amounts.

Large biogas plants can generate biogas for supply into gas supply networks or mains, with a typical heating value of 21–24 MJ/m3 or 6 kWh/m3 . This makes it useful for a wide range of applications, including cooking, heating, lighting, and electricity generating [22, 67, 68]. Over a long period of time, biogas has been used as fuel product anaerobic digestion of waste for sanitation. It is also typical to use biogas plant digested waste as a fertilizer [29, 69].

#### **2.1 Global development of biogas energy resources**

About 0.25% of the world's energy came from biogas in 2011, while biogas accounted for 27% the global biofuel market [35, 70]. Income and standard of living in rural areas can be improved through biogas production and use in two ways by selling electricity derived from to the grid and increasing agricultural productivity by application of organic manure/digestate. Not only may biogas be used for the aforementioned purposes, but it can also be converted into diesel fuel to power appliances in places like houses and classrooms [70, 71]. Biogas can be upgraded to biomethane, and utilized more efficiently as a transportation fuel, a natural gas replacement in commercial and residential settings, and as a feedstock for natural gas distribution networks. Feedstock for greenhouses and a raw ingredient for chemical fuel production, carbon dioxide can be derived from biogas [72]. In 2005, there were around 16 million small household biogas digesters in use worldwide, with China and India accounting for the great majority. A total of 16 million tons of firewood were replaced by biogas in India in 1996, and it met 4% of China's energy requirements at the time. Around 7 million biogas digesters did the same in the USA [70]. The efficiency of using biogas can be greatly increased through cogeneration, which simultaneously produces heat and power. Excess electricity can be sold to the grid to help stabilize the grid and lessen the consequences of global warming by displacing the generation of energy from fossil fuels [47].

Europe has been a leader in the development of biogas technologies from urban trash. More than 14% of Europe's municipal solid waste is converted into biogas by the region's more than 70 active anaerobic digesters [73, 74].
