**4. Biogas production processes**

Generation of biogas gives a multiuse carrier of renewable energy, as methane can be utilized for substituting of nonrenewable source of fuels in both heat and electricity production and as a car fuel. AD of wastes, energy crops, and residues is of growing interest in order to decrease the greenhouse gas emissions and to promote a sustainable development of energy supply [74]. Anaerobic digestion is a technology with proven efficiency, being widely used in the stabilization of industrial wastewater, urban solid waste, animal manure, and sewage sludge [66]. There are many benefits associated with anaerobic digestion technology, which include mass reduction, odor removal, pathogen reduction, less energy use, and more significantly, the energy recovery in the form of methane [75, 76]. The aim of anaerobic digestion process is the production of a methane-rich biogas through biological decomposition of organic matter, in an oxygen-free environment. An aerobic digestion is considered as a low-cost an eco-friendly waste management process, thus it reduces the emission of greenhouse gases. In the meantime, it stabilizes and reduces the wastes. One of the major advantages of an aerobic digestion is its adaptability to deal with a wide range of organic substrates. The produced biogas can be used for power and heat production, or can be upgraded and used as vehicle fuel in the transport sector. In addition, the by-product of AD, the "digestate residue," can be further utilized as a fertilizer on the agricultural land [50]. There are different process types which can be applied for biogas generation, which are classified in dry and wet fermentation systems [21, 77, 78]. Wet digester systems are constantly applied using vertical stirred-tank digester with various stirrer kinds dependent on the source of the feedstock.

Biomass is utilized as substrates for biogas generation as long as it consists of hemicelluloses, cellulose, carbohydrates, proteins, and fats as major constituents. Only powerful lignified organic materials, e.g., wood, are not suitable due to the slowly anaerobic decomposition. The composition of biogas and the methane yield depends on the feedstock type, the digestion system, and the retention time [78]. Maximal gas yields and theoretical methane contents of substrates for biogas production are carbohydrates 790–800 biogas (Nm3 /t TS), 50% CH4 and 50% CO2, carbohydrates only in the form polymers from hexoses, not inulins and single hexoses, raw protein 700 biogas (Nm3 /t TS), (70–71)% CH4 and (29–30)% CO2, and finally raw fat 1200–1250 biogas (Nm3 /t TS), (67–68)% CH4 and (32–33)% CO2 [79].

### **4.1 Biochemical process**

Anaerobic digestion involves bacterial fermentation of organic wastes in the absence of free oxygen. Methane fermentation is a complex process, the fermentation leads to the breakdown of complex biodegradable organics in a four-stage process: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [50, 74, 80].

First stage (hydrolysis process): large protein macromolecules, fats, and carbohydrate polymers (such as cellulose and starch) are broken down through hydrolysis to amino acids, long-chain fatty acids, and sugars.

Second stage (acidogenesis process): the products obtained in first step are then fermented via acidogenesis to form volatile fatty acids, valeric acid, propionic, principally lactic, and butyric. Third stage (acetogenesis): bacteria devour these fermentation products and produce acetic acid, hydrogen, and carbon dioxide. Fourth stage (methanogenic): organisms feed on the hydrogen, acetate, and a few of the carbon dioxide to generate methane [81]. Three biochemical pathways are used by methanogens to achieve this:

$$4\text{CH}\_3\text{COOH} \rightarrow 4\text{CO}\_2 + 4\text{CH}\_4\text{(acetotropic pathway)}\tag{1}$$

$$\text{CH}\_2 + \text{4H}\_2 \rightarrow \text{CH}\_4 + 2\text{H}\_2\text{O} \text{(hydrogenotropic pathway)}\tag{2}$$

$$\text{4CH}\_3\text{OH} + \text{6H}\_2 \rightarrow \text{3CH}\_4 + \text{2H}\_2\text{O} \text{(methylotropic pathway)}\tag{3}$$

Biogas is a multipurpose renewable green energy source, which can be simply used to substitute nonrenewable energy source, in heat and power generation, and as gaseous car fuel. Biomethane can also substitute natural gas as a feedstock for producing chemical materials. The biogas generation during AD provides vital benefits over other bioenergy generation technologies. It is admitted as one of the most energy-efficient and environmentally beneficial technology for generation of bioenergy [82, 83]. Anaerobic digestion is a broadly used technology that provides some benefits over other biofuels generation ways, such as, sustainable biogas production, option for using wastewater and sea water, lower operational costs, maximum biomass utilization, minimum sludge production, lesser energy consumption, and feasibility to recycle nutrients [54, 84, 85].

AD of animal manure provides some socio-economic, environmental, and agricultural benefits via inactivation of pathogens, improved fertilizer quality of manure, and considerable reduction of odors, and last but not least production of biogas generation, as green renewable fuel, for multiple utilizations [58]. The slurry or digestate from the reactor is affluent in ammonium and other nutrients utilized as an organic fertilizer [86, 87]. The European renewable energy directive has set a target to substitute 27–30% of the total energy consumption with renewable energy sources by 2030. It is expected that 14–26% of this renewable energy target could be achieved by biogas from farming and forestry residues [61, 88]. Biogas is presently produced and utilized in Europe. In 2007, Germany was the largest biogas producer in Europe mainly from energy crops, while the UK was the second producer of biogas mainly from landfill sources [50].

There are three common technologies used (in **Figure 2**) to convert biomass to green sustainable products. Thermal approaches that are commonly used to convert biomass into an alternative fuel are: gasification, liquefaction, pyrolysis, and charcoal, while there are two biological approaches that are commonly used to convert biomass into bioenergy: fermentation and anaerobic digestion, as shown in **Figure 2**. This research is going to focus on an anaerobic digestion to produce biogas.

The anaerobic co-digestion is a choice to settle the drawbacks of single substrate digestion system, being the properties of the substrates and chemical composition, the operating parameters (pH, charge rate, temperature, etc.), the bioavailability, biodegradability, and bioaccessibility, significant parameters to be optimized.

Some of raw materials need to be treated to improve the biogas production. In the past, AD was mostly referred to a single substrate/single output process but recently, co-digestion has become a standard technology in agricultural biogas production in many countries [50, 66]. The anaerobic co-digestion is the simultaneous digestion of more than one substrate with complementary characteristics and has become popular as the digestion of several materials can give higher methane yields than those expected when single materials are treated individually [91–93]. Several of the reasons related with the improvement are associated to the combinations of substrates that result in a positive interaction within the system, reducing negative influences of toxic or inhibitory compounds, affecting C/N ratio and reactor stability, supplementing nutrients, and balancing buffer capacity. Additional benefits of using co-digestion techniques including improved balance nutrients, synergistic effect of microorganism, increased load of biodegradable organic matter, and higher biogas yield [50, 82, 94, 95].

### **Figure 2.**

*A schematic of various biomass conversion technologies [5, 89, 90].*
