**3. Biogas feed stocks**

A wide range of biowaste can be used as substrates for biogas production through anaerobic digestion. Significant volumes of lignocellulosic waste are generated from agricultural, municipal, and other activities. The most commonly used materials for biogas production are animal manure and slurry, waste water and sewage sludge, municipal solid waste, and food processing and consumption generated waste [10]. The most used substrates for biogas production are pig and cattle manure, and poultry litter which generally compete with traditional manure [10, 87, 114]. Sewage sludge generated from wastewater treatment plants is the main feedstock for biogas production in places like Gävleborg region and Sweden while the use manure, food waste and industrial waste have been on the rise, although limited. Horse manure was also widely used as a substrate during co-digestion. Policy measures like ban on the landfilling of organic waste and competing applications like manure use can shift the demand for biogas feedstock selection [114, 115].

#### *Biogas Production and Process Control Improvements DOI: http://dx.doi.org/10.5772/intechopen.113061*

The quantity and composition of biogas produced by biomass digestion is a function of depending on the substrate used and process parameters control. Different types of feedstocks can be applied for biogas production ranging from animal wastes, agricultural residues, and energy crops [59]. The various feedstock used have different characteristics and hence biogas potential and hence may require different type and level of pretreatment and process conditions during anaerobic digestion [2, 78]. Feedstock biomass can take the form of a solid, a slurry, or a liquid (either concentrated or diluted). Many different materials can be used as feedstocks, including animal manure, straw, biowaste from homes and businesses, waste from bioethanol and biodiesel production, waste from markets and restaurants, sewage sludge from wastewater treatment plants, and energy crops like maize, silage, grass, sorghum, cereals, and sugar beets. The high lignin concentration of wood makes it unsuitable for anaerobic digestion and biogas production [78, 79, 88, 116].

Several different kinds of biomass garbage can be converted into biogas. Litter from farms, city dumps, and other sources can all contribute to the production of lignocellulosic garbage. Common forms of waste that are utilized as feed stock include animal manure, slurry, sewage sludge, municipal solid waste, food waste, and agricultural waste [10]. The biomass feedstock used to create biogas consists of a wide variety of components, including lipids, proteins, carbohydrates, cellulose, and hemicellulose. Co-substrates are added to alter the organic composition and boost biogas generation. Most co-substrates consist of organic industrial waste, food scraps, and municipal biowaste. Although lipids generate more biogas overall, carbohydrates and proteins convert more quickly [10]. Both biogas potential and yield are affected by the feedstock and co-substrates used in production, in addition to the process parameters [60, 84, 86].

There are many substrates that are suitable for anaerobic digestion, which can be characterized and classified into four main groups, namely energy crops, agricultural residues, food production wastes, and organic wastes [30, 117]. In the European countries, about 70% bio digestion feedstock comes from the agricultural sector like manure and agricultural residues. According to the European Biogas Association, agricultural wastes account for 40–60% in biogas production in Germany, Cyprus, Denmark, France, Italy and Poland [59]. The residual digestate is a feasible material for use as organic fertilizers, with huge optimization potential leading to increased farm production and profitability [117]. The various feedstocks for biogas production are discussed below.

#### **3.1 Crop residues for biogas**

Crop residues, a byproduct of crop husbandry, can be put to several different applications. Combustion, composite manufacture, bio-digestion, and animal feed are just some of the ways that waste materials like these can be repurposed. In terms of surplus agricultural products, India yearly produces around 500 Mt. (million tons) [16]. Nearly 70 million metric tons of organic waste are generated annually in the United States of America. Examples of agricultural waste include spoiled food and water [34]. Agricultural crop residues are a serious concern of farmers and cause significant contributors to GHG generation. Primary agricultural residues remain in fields as byproducts after harvesting cereal grain straws, wheat (Triticum), barley, rice (Oryza), corn stovers, stalks, leaves, etc. On the other hand, secondary agricultural residues are products of agricultural resource processing like bagasse, sunflower husks, nutshells etc. Two sustainability issues associated with the use of agricultural wastes for biogas

production are the potential competition with feeding in animal husbandry and the probable depletion of organic matter in the soil and nutrients in farmlands [59].

GHG, including CH4 and N2O are produced when crop waste is burned on-site. In terms of GHG from crop residues, maize crop residues produce the most (in gigagrams of CO2 equivalent). GHG emissions can be lowered and items like fertilizer can be more profitable if we can find better applications for these wastes [16]. **Figure 4** shows that most carbon dioxide emissions from crop residue combustion are caused by the burning of maize residues as found in [16].

**Figure 4** shows the main crop residues used for biogas production led by maize, rice, wheat, and sugarcane wastes. The potential for making biofuels from cellulosic waste is enormous. This includes materials like bagasse, energy crops, agricultural wastes, and even sewage. Cellulose, hemicellulose, and lignin are the three primary organic components of lignocellulose [10]. Cellulose is the primary structural component that gives plant cell walls their mechanical stability. Macroscopic hemicellulose is composed of repeating polymers of pentoses and hexoses. Three aromatic alcohols (coniferyl alcohol, sinapyl alcohol, and p-coumaryl alcohol) are synthesized within lignin. Composition of lignocellulose differs greatly depending on both the source and the time of year. Cellulose is a linearly connected polymer that contains many β-1,4-glycosidic linkages. Crystalline and amorphous elements coexist throughout the structure [10].

By applying temperatures in the range of 320°C and pressures of 25 MPa, crystalline cellulose can be converted into amorphous cellulose. Cellulose is the most common organic component on Earth, making up more than 25% of plant material. Hemicellulose is an important part of biomass because of the dynamic nature of its structure and the vast variety of polymers it contains, such as pentoses like xylose and arabinose, hexoses like mannose, glucose, and galactose, and sugar/uronic acids like glucuronic, galacturonic, and methylgalacturonic acid. Xylan makes up roughly 90% of the hemicellulose structure, though this value varies with the kind and origin of the feedstock being processed. Hemicellulose requires a wide variety of enzymes in order to be thoroughly degraded into free monomers [10]. Hemicellulose is comprised of multiple sugar units arranged in polymer structures that can be readily hydrolyzed. It possesses shorter side chains and a comparatively lower molecular weight when compared to cellulose. Hemicellulose enhances the overall compactness of the

**Figure 4.** *Leading crop residues in terms of CO2 gigagram equivalent GHG emissions.*

*Biogas Production and Process Control Improvements DOI: http://dx.doi.org/10.5772/intechopen.113061*

cellulose-hemicellulose-lignin network through its interconnection of lignin and cellulose molecules. The solubility range of hemicellulose molecules is typically observed to be between 150 and 180°C, with variations influenced by environmental conditions such as acidity, neutrality, and alkalinity [10, 118–120].

Lignin is a naturally occurring heteropolymer found in the cell wall, composed of three phenylpropane-based units, namely p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are interconnected through linkages. The structure of lignin exhibits resistance to microbial degradation and oxidative stress. Lignin exhibits limited solubility in water, resulting in reduced degradability and hindered biogas production. At approximately 180°C in a neutral setting, the dissolution of lignin and hemicellulose in water occurs. The solubility of lignin in acidic, neutral, or alkaline environments is determined by the presence of the phenylpropane-based unit within its molecular structure. Lignin constitutes approximately 30–60% of the composition of wood, whereas agricultural residues and grasses typically contain lignin in the range of 5–30%. The agricultural produce primarily consists of hemicellulose [120].

The composition and structure of lignin have been found to have a positive impact on the hydrolysis process, resulting in an increase in the generation of biogas. Nevertheless, the presence of higher lignin content in the feedstock has been observed to decrease the efficiency of degradation [10, 120].

#### **3.2 Energy crops**

Energy crops encompass a wide range of plant species that can be classified into two main categories: herbaceous and woody. Herbaceous energy crops include grass, *maize* (*Zea mays* L.), and *raps* (*Brássica nápus* L.), while woody energy crops consist of species such as *willow* (*Salix*), *poplar* (*Populus*), and *oak* (*Quercus*). However, it is important to note that woody crops require a specific delignification pretreatment digestion process to be utilized effectively for energy purposes. Lignin, due to its resistance to degradation in anaerobic digestion, presents a favorable characteristic for gasification, incineration, or composting processes when considering woody substrates [86]. Herbaceous energy crops have many important characteristics making them suitable for anaerobic digestion e.g., high solar energy conversion efficiency leading to high yields, need low agrochemical inputs, and have got low nutrient and water requirement due to their extensive rooting system, to hold onto fertilizers and water, and they also have low moisture content at harvest time. Biogas can be made cost effective and high yielding by using crops with perennial growth habits with low establishment costs and fewer field operations and crops providing high biogas output as a feedstock source [59, 121].

#### **3.3 Animal residues**

Animal waste is a valuable energy resource containing biodegradable nutrients and renewable energy. However, by practice, most animal waste is deposed off, collected in lagoons or left to decompose in the open land causing environmental hazard [37]. Generally, the term "animal waste" refers to feces, urine, dung, or other excrement, urea, digestive emission, urea, or similar substances emitted by farm animals like livestock, poultry, or fish [88, 89, 122]. Animal waste like manure emit methane, ammonia nitrous oxide, volatile organic compounds, hydrogen sulfide, and particulate matter, which can cause serious environmental concerns and health problem [4, 61].

The primary sources of animal waste commonly encountered include dairy shed effluent, mainly comprising of wash water, urine, dung, residual milk, and waste

feed. Additionally, poultry litter, which encompasses water, manure, spilled feed, feathers, and bedding material, is another significant source [21, 60]. Moreover, renderings from slaughterhouses and other byproducts originating from livestock finishing operations contribute to the overall volume of animal waste. Dairy manure and poultry litter are widely recognized as prominent examples of animal wastes that are extensively be used in large-scale biogas energy production [60]. In the past, significant volumes of bovine excrement were subjected to treatment processes and subsequently utilized as a form of agricultural fertilizer, or alternatively, were disposed of through the practice of spreading onto farmland. Nevertheless, numerous countries have implemented rigorous environmental regulations and legislations to regulate the emission of odors, as well as the pollution of surface and groundwater, soil contamination, and nutrient management. These regulations have prompted the adoption of alternative approaches that discourage the disposal of animal manure. Consequently, there is an increasing emphasis on utilizing animal manure in biomass-based production chains, driven by the incentives provided by these regulations [79, 121].

The challenge with poultry waste is high phosphorus (P) run-off to surface water which creates issues like odor and taste problems in drinking water, because of excess algae growth arising from excess phosphorus. For this reason, local land applications of poultry litter are restricted to protect water quality. Poultry waste on the other hand has high methane potential making it an efficient feedstock for biogas production. Slaughterhouses and fish processing factories also generate significant digestible waste for biogas production [78, 121].

#### **3.4 Industrial and process wastes**

Increased demand and consumption of food and industrial produce leads to large scale generation of waste. There is need for proper management of organic waste to minims harm to the environment like as climate change, ecosystem damage, and resource depletion [59]. About one-third of food produced for human consumption globally is discarded as waste, which is approximately 1.3 billion tons of waste per year. Anaerobic digestion of waste prevents the spread of pathogens and environmental degradation arising when waste is carried by to runoff water to basins and drain into the oceans. Anaerobic digestion surpasses thermal treatments technologies for bakery wastes such bread, biscuits, donuts, and pizza dough, as wastes like bakery garbage promotes methane production because it includes easily degradable carbs and vitamins [20, 59].

Implementation of anaerobic technologies started with wastewater treatment hence Industrial wastewater is a major source of aquatic pollution that endangers surrounding environments and ecosystems. Anaerobic digestion of industrial waste is more advantageous than aerobic wastewater treatment since it produces useful biogas, and substantially lowers energy requirement, and generates less sludge that requires disposal Brewery wastewater, cassava (Mánihot esculénta Crantz) starch wastewater, palm oil mill effluent, biodiesel wastewater, and bioethanol wastewater (vinasse) have demonstrated significant biogas potential [59].

#### **3.5 Biogas potential of various feedstocks**

Different biomass feedstock has different biogas potential in terms of both quality and quantity. **Table 2** illustrates gas yields and methane contents for various substrates after 10–20 days of retention at 30°C [37, 61].
