**3.1 Anaerobic digestion**

In addition to thermochemical operation, bio-chemical processes such as anaerobic digestion (AD) and fermentation are promising technology as a renewable source of energy products [66, 67]. Regarding human health, environment, economy, and energy conservation issues, AD systems have attracted remarkable attention by the production of bio-methane gas (renewable energy source) through bio-chemical conversion of biodegradable wastes [68]. AD process has occurred in an insufficient O2 atmosphere which prepares suitable conditions for activation of the microorganism to degrade organic matter into biogas [69]. To convert the feedstock to bio-methane, a series of bi-metabolism steps including hydrolysis/acidogenesis, acetogenesis, and methanogenesis occurred in the AD systems reactors [70]. During the first stage, the high molecular weight complex insoluble organic matter is degraded into simple soluble molecules by the extracellular enzymes [69]. During the hydrolysis phase, the organic components of carbohydrate, protein, and lipid polymers are hydrolyzed into simple sugar, amino acid, and long-chain fatty acid respectively [71]. Meanwhile, monosaccharides are produced through hydrolysis of the insoluble compounds

#### *Biomass and Energy Production: Thermochemical Methods DOI: http://dx.doi.org/10.5772/intechopen.102526*

of cellulose and hemicellulose by enzymatic microorganisms (Streptococcus and Enterobacterium) [72]. However, at this step, rigid lignin structure which is resistant to the penetration of microorganisms requires delignification as a pretreatment process to undergo biodegradation [73]. In the next step acidogenic bacterizes such as Clostridium, Peptococcus Anaerobus, Lactobacillus, Psychrobacter, Anaerococcus, Bacteroides, Acetivibrio, Butyrivibrio, Halocella, and Actinomyces (highly active fermenter and the most abundant bacterizes in AD) applied to dissolve and bounded oxygen in the solution and carbon [74, 75]. At the final steps of the process, acetotrophic, hydrogenotrophic and methylotrophic pathways occurred which are the main route of methane production [76]. In the methanogenesis phase, acetic acid and hydrogen that formed in the acetogenesis phase are transformed to biomethane via methanogenic microorganisms while the pH in the system will increase to neutral values within the range of 6.8–8 [71]. The methanogenesis phase effectiveness is very reliant on the balanced relationship between bio-kinetics of microorganisms (Crenarchaeota, Euryarchaeota, etc.) with its growth environment (food supply and accessibility) [77, 78].

Working conditions in AD generally influence the formation of the produced biogas. The degradation process is affected by several factors including operation temperature, carbon to nitrogen (C: N) ratio, pH level, organic loading rate (OLR), Hydraulic retention time (HRT), and stirring [76]. Defining an optimum temperature that causes the stability of the enzymes and co-enzymes activity can have a significant influence on AD and bio-methane production while the efficient AD process is dependent on the optimum temperature [79, 80]. The optimum temperature for digestion process operation of anaerobic microorganisms could be range in psychrophilic (10–30°C), mesophilic (30–40°C), or thermophilic (50–60°C) conditions [81].

Alkalinity or acidity of the substrate is categorized by the important parameter of pH value. The stability of acidogenic activity and methanogenic bacteria is directly influenced by the changes in [82, 83]. Ideally, the optimum pH for acidogenesis and methanogenic stages place in a range from pH 5.5 to 6.5 and from 6.5 to 8.2, respectively [84]. Neutralization is essential in cases of excessively high or low pH during the anaerobic digestion feedstock especially before the plant is fed. The pH is chemically improved by adding the base, such as lime, to the reactor if negligible acidification happens during the AD process [85]. The next effective parameter in the AD process is the ratio of carbon to nitrogen in organic material [86]. A high C: N ratio indicated the low nitrogen sources that are needed to sustain the material supply for digestion. Meanwhile, the low C: N ratio signified the potential of NH4+ inhibition in the digestion process. Ideally, the optimum C: N ratio for the AD process is within the range of 20–35 [87]. The HRT which is defined as the retention period of the substrates inside the digester can vary based on the feedstock composition and digester temperature [88]. High HRT will result in improvement in biogas yields while the lower HTR is interested since decreasing cost of production and enhancement of process efficiency [89, 90]. The OLR also can affect The AD process negatively or positively [91]. Minor OLRs may cause malnutrition of microorganisms and adversely affects the AD while in contrast, a great ORL ratio may cause insufficient resources to support the development of microbial organisms [92, 93]. Temperature condition, characteristics of the substrates, and HRT of the AD operation impacts the OLR behavior and amount [76].

In terms of technological requirements, several types of reactors have been developed that generally can classify into wet or dry reactors based on their total solid contents [94]. In the design and operation of the anaerobic reactor, two parameters of reactor volume to daily flow and OLR have the most important value. The dry types (serve the feedstock with a solid concentration of more than 15%) itself could be categorized into three different types including horizontal plug-flow, vertical plugflow, and non-flow (batch type) [95, 96]. In contrast, the wet digesters are defined to serve the feedstock having a total solid less than 15%value [97].

#### **3.2 Fermentation**

Fermentation is considered as another biochemical technology that can be applied to get energy from biomass. Fermentation defines as a process of central metabolism in which alcohol (for instance ethanol) or acid is produced by an organism through the conversion of carbohydrates, such as starch or sugar. Wines, beers, and ciders are traditionally carried out with fermentation process by using Saccharomyces cerevisiae strains, the most common and commercially available yeast [98, 99]. The utilization of feedstock such as wheat, corn, sugarcane, etc. for biofuel production (first generation biofuel) causes the problem of food security. The use of biomass feedstock (second generation) in bioethanol production solves this matter in many aspects [100]. Depending on fermentation conditions such as temperature, pH, aeration, and nutrient supplementation microorganisms are susceptible to lignocellulosic hydrolysate to produce bio ethanol [101]. Nevertheless, the production of biofuel through fermentation of promising sources (rice straw, wheat straw, corn straw, and sugarcane bagasse) is quite interesting but still meets some technical issues to release the fermentable sugars from lignocellulosic. The problem necessitates a pretreatment process including Physical (mechanical, extrusion, Irradiation), chemical (organosolv, ozonolysis, ionic liquid, acid, and alkali washing) physicochemical, and biological [102].

Several fermentation technologies such as batch and continues and fed-batch modes have been utilized. The complete subtract and highest conversion rate but lower volumetric production can be done through batch mode rather than continuous mode which led to high productivity (due to high dilution ratio and long duration process) and steady residual concentration [103]. Overly, batch fermentation could be applied for high viscous feedstock, while continuous fermentation methods can offer better plant capacity utilization [104]. During the batch and continuum operation, the addition of Indigenous Consortium Streptococcus sp. or enzyme glucoamylase has been reported that helps to fermentation process [105, 106].
