**4. Methane production enhancement**

The steam explosion pretreatment that disintegrated the LCC impacted the higher accessibility of the digestion process to convert the cellulose, hemicellulose, and lignin and its derived products into biogas [15]. Those pretreatments were simplified the hydrolysis process, however, to gain the economical factor is necessary to improve the production rate, solid retention time, and hydraulic retention time. The conversion of steam-exploded lignocellulosic biomass into methane was counted heavily on cellulose and hemicellulose as the main conversion source, even though the conversion of lignin-derived products from psychochemical pretreatment also contribute to the amount of methane production. **Figure 2** was described the methane production from lignocellulosic biomass was produced through the simultaneous system from saccharolytic and hydrolytic processes to convert the cellulose and hemicellulose into oligomers and monomers, hydrolytic and dissipotrophic organism as primary anaerobe process, the syntrophic process, acetogenic process, and methanogenic process [54, 55]. The saccharolytic and hydrolytic process initiates the biopolymers

#### **Figure 2.**

*Potential enhancement and low emission of lignocellulosic biomass conversion into methane.*

degradation of polysaccharides such as cellulose and hemicellulose, starch, glycogen, and chitin, also the other common content such as protein, lipids, and nucleic acid. The saccharolytic and hydrolytic degraded those content into oligomers and monomers such as cellobiose, glucose, amino acids, purines, pyrimidines, fatty acids, and glycerol [56]. The cellulose and hemicellulose are commonly converted by cellulolytic microflora from the phylum of Firmicutes commonly *Ruminococcaceae* and *Clostridiaceae* families 17 such as from genus *Clostridium, Ruminococcus, Cellobacterium, Butyrivibrio, Fibrobacter,* and *Acetivibrio* [57–59]. The starch could be degraded by the genus *Thermoanaerobacterium*, *Succinimonas, Ruminobacter, Bacteroides, Prevotella, Bacteroides, Clostridium,* and *Butyrivibrio*. The protein and amino acid are commonly degraded by genus *Syntrophomanas, Bacteroides, Clostridium, Peptostreptococcus Acidaminococcus, Selenomonas, and Fusobacterium.* The xylan and pectin are commonly degraded by genus *Ruminococcus, Lachnospira Bacteroides, Butyrivibrio, Prevotella*, and *Clostridium*. The species from those genera also could degrade the other polymer such as lignin and its derived products especially the species from *Lysinibacillus* and *Paenibacillus.* The hydrolytic and dissipotrops as primary anaerobes process digest the cellobiose, glucose, amino acids, purines, pyrimidines, fatty acids, and glycerol and produce organic acid such as butyrate, succinate, lactate, pyruvate acetate, propionate, and lactate; aromatic compounds; the alcohol form such as ethanol, propanol, butanol, and methanol; carbon dioxide; hydrogen; and also produced volatile fatty acids (VFAs) [59] which dominate the degradation of cellulose. The alcohol form, VFAs, lactate, and succinate continued to degrade into single carbon compounds and hydrogen and acetate through the syntrophic process. The single carbon also could into acetate via homoacetogens process and also could directly form the methane through the hydrogenotrophic methanogens. The methane from acetate formed through the acetoclastic methanogens, however, those process was inactivated in low concentration of acetate and in hightemperature condition, other than that, acetoclastic methanogens could be blocked by the presence of high ammonia and VFAs concentration. That simultaneous system condition directly influences the SRT and HRT that affected the time consumed and energy that affected the production cost.

*Steam Explosion Pretreatment: Biomass Waste Utilization for Methane Production DOI: http://dx.doi.org/10.5772/intechopen.102850*

#### **4.1 Enhancement: Saccharolytic and hydrolytic pathway**

The methane production enhancement could be done by enhancing the simultaneous system from each process such as saccharolytic and hydrolytic, hydrolytic and dissipotrophic, syntrophic, acetogenic, and methanogenic processes. The enhancement process commonly used Biological augmentation by the addition of archaea or bacterial cultures that get high-rate of degradation time and thermophilic condition which could speed up the production rate. The bioaugmentation of the saccharolytic hydrolytic process that converts the cellulose becomes oligomers and monomers was reported in several studies. The bioaugmentation using cellulolytic bacterium from genus *Caldicellulosiruptor* that operate in thermophilic condition i.e., *Caldicellulosiruptor bescii* which focuses on the improvement of hydrolysis process that degraded the carbohydrate content from steam-exploded biomass such as cellulose, hemicellulose, and other lignocellulosic content, and fermented the C5 and C6 sugar on the simultaneous process. The *C. bescii* has a special characteristic that is quite different from other cellulolytic bacteria which has the enzymatic system in a multi-modular pathway, which secreted the individual cellulases and could bind and catalyze multiplied, wherein, this condition will support the indigenous primary anaerobes bacteria synergically [60]. Mulat et al. [61] were applied bioaugmentation for steam-exploded lignocellulosic biomass converted into methane which operated in 62°C, the *C. bescii* was added as bioaugmentation where steamexplosion pretreatment itself enhanced 118% the methane production, and the combination of steam-exploded pretreatment and bioaugmentation was enhanced 140% methane production improvement. The other species cellulolytic microflora from the genus Clostridium such as *Clostridium thermocellum* which operated in a thermophilic condition also has the capability to continuedly form ethanol directly from cellulose, and also accelerates the hydrolysis process and could produce higher H2 that supports the hydrogenotrophic methanogens to produce more methane [62–65]. Other than that, *C. thermocellum* has the special capability to reform non-growth state into sporulation stage and L-phase in stress conditions [66]. The steam explosion and bioaugmentation using *C. thermocellum* were reported to be compared where the steam explosion was enhanced 62% methane production and bioaugmentation was enhanced 12% of methane production [64]. The other report from *C. thermocellum* enhanced the anaerobic digestion of lignocellulosic agricultural residue which resulted in an increase of 39% of methane production [67]. Tsapekos et al. [68] was used *C. thermocellum* and *Melioribacter roseus* as bioaugmentation for lignocellulosic agricultural residue conversion into methane by continuously stirred tank reactor (CSTR) which resulted in 34 and 11% methane production enhancement, respectively. The other species from *Clostridium* such *as Clostridium cellulolyticum* as a bioaugmentation agent for the wheat straw that resulted in 13% of methane production compared to non-bioaugmented [65]. Cetar et al. [69] was reported to trial bioaugmentation agents from various genus such as *Pseudobutyrivibrio* using *Pseudobutyrivibrio xylanivorans, Fibrobacter* using *Fibrobacter succinogenes, Ruminococcus* using *Ruminococcus*, and *flavefaciens* using *Clostridium cellulovorans* to enhance the hydrolysis process of brewery spent grain by comparation using two bioaugmentation agent each treatment that impacted to enhance the biogas production with resulted in 17.8% from *P. xylanivorans* alone, 6.9% from a combination of *P. xylanivorans* and *F. succinogenes,* and 4.9% from a combination of *C. cellulovorans*a and *F. succinogenes*. The other report was described to examine the bioaugmentation that combined with steam explosion using ruminal fungus such as *Pecoramyces sp.* which isolated from goat rumen to enhance the methane production from steam-exploded corn stover [70]*.*
