*5.3.1 Anaerobic digestion*

Anaerobic digestion (AD) is the microbial degradation of organic matter in the absence of oxygen to produce mainly biogas (biomethane and carbon dioxide). The conversion of organic matter into biogas is presented in **Figure 7**. It is a series of biochemical reactions where microorganisms anaerobically convert organic materials into products to be finally converted into biogas [2].

Microorganisms break down high molecular mass compounds such as polysaccharides, proteins, fats, cellulose, and hemicellulose into smaller molecular mass

**441**

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

*5.3.1.1 Stages of AD process*

*5.3.2 Fermentation*

*5.3.3 Microbial fuel cell (MFC)*

compounds which are later converted into biogas. The efficiency of AD processes is dependent on the components of the substrate and the activity of microorganisms. AD is a biochemical conversion process that is robust with proven reliable applications. AD has been widely applied in the treatment of organic waste streams, and its development for production of biogas goes far back as the 16th century [55]. The general equation to produce methane from organic matter is given as in Eq. (6).

> 2 24 42 284 284 *n ab ab nab nab CHO n HO CO CH* + −− → −+ + +− (6)

The four main stages involved in the AD of lignocellulosic biomass are hydrolysis, acetogenesis, acidogenesis and methanogenesis. Theses stages occur successively as the product from a preceding stage is utilized in the next stage. Hydrolysis is the first stage in the AD process, and it involves the breakdown of high molecular mass compounds into smaller ones. This stage is followed by acidogenesis, which is the conversion of the smaller molecular compounds into identifiable lower molecular compounds. The acetogenesis stage converts the volatile fatty acids (VFAs) from acidogenesis into acetate, CO2 and H2, where after these intermediate compounds are converted into methane and CO2 with other by-products in trace amounts in methanogenesis stage, with the aid of methanogens. The stages of the AD process

The conversion of biomass to simple sugars with subsequent transformation into alcohol and CO2 with the aid of microorganisms, mainly, yeasts is known as fermentation. This process has been applied for centuries to produce ethanol from sugar crops. It has been used for conversion of lignocellulosic material and algae to ethanol. The by-product from this process after the distillation process are the nonfermentable products which are further directed for use as animal-feed, or as raw feed for the thermochemical conversion. Ethanol produced from the process can be used as fuel for motor vehicles, or as compliment for existing conventional fuels. It has been used as an additive for petrol to improve the octane rating and vehicle emissions reduction in countries such as Australia, Brazil, Sweden, and United States [20]. Anaerobic or dark fermentation of pre-treated lignocellulosic biomass is used in the production of biohydrogen, where the process is like the acidogenic stage in AD. The microorganisms used in this process are mainly hydrogen producing microbes such as *Thermoanaero bacterales, Clostridiaceae* and *Enterobacteriaceae.* This process is gaining interest as the combustion of hydrogen is free of any harmful emissions [56]. The challenge encountered is the low yield of H2 generated, though

there has been research carried out into the upgrading of CH4 into H2.

MFCs employ the activity of micro-organisms to convert pre-treated biomass into bioelectricity that can be fed into an existing electricity grid. The process involves the oxidation of the substrates (cellulosic biomass) at the anode chamber of the cell by microorganisms, (electrode-reducing organisms) to electrons which are transferred to the cathode chamber through a conductive material. In the cathode region, the electrode-oxidizing organisms utilize the electrons for

are affected by various parameters and the biomass chosen.

**Figure 7.** *Anaerobic digestion degradation process.*

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

compounds which are later converted into biogas. The efficiency of AD processes is dependent on the components of the substrate and the activity of microorganisms. AD is a biochemical conversion process that is robust with proven reliable applications. AD has been widely applied in the treatment of organic waste streams, and its development for production of biogas goes far back as the 16th century [55]. The general equation to produce methane from organic matter is given as in Eq. (6).

$$C\_nH\_aO\_b + \left(n - \frac{a}{4} - \frac{b}{2}\right)H\_2O \to \left(\frac{n}{2} - \frac{a}{8} + \frac{b}{4}\right)CO\_2 + \left(\frac{n}{2} + \frac{a}{8} - \frac{b}{4}\right)CH\_4\tag{6}$$

#### *5.3.1.1 Stages of AD process*

*Biotechnological Applications of Biomass*

**5.3 Bio-chemical means**

discussed below.

*5.3.1 Anaerobic digestion*

into products to be finally converted into biogas [2].

usually blended with conventional diesel to be used as fuel for motor vehicles. The oil extracted from the biomass is highly viscous with polyunsaturated characteristics; therefore, transesterification processes which utilize either acids, bases, or enzymes as catalysts, convert the oil into fatty acid methyl esters (FAME) or fatty acid alkyl esters with glycerol as by-product. The esters produced from the transesterification process have lower viscosities and are comparable to conventional fuels [51].

The biochemical conversion process utilizes the metabolic activity of microorganism for conversion of biomass into biofuel and by-products. Biomass conversion is environmentally friendly when compared to thermochemical methods where the residence time for the conversion process to be achieved is longer when compared to thermochemical conversion means. The main biochemical conversion methods are

Anaerobic digestion (AD) is the microbial degradation of organic matter in the absence of oxygen to produce mainly biogas (biomethane and carbon dioxide). The conversion of organic matter into biogas is presented in **Figure 7**. It is a series of biochemical reactions where microorganisms anaerobically convert organic materials

Microorganisms break down high molecular mass compounds such as polysaccharides, proteins, fats, cellulose, and hemicellulose into smaller molecular mass

**440**

**Figure 7.**

*Anaerobic digestion degradation process.*

The four main stages involved in the AD of lignocellulosic biomass are hydrolysis, acetogenesis, acidogenesis and methanogenesis. Theses stages occur successively as the product from a preceding stage is utilized in the next stage. Hydrolysis is the first stage in the AD process, and it involves the breakdown of high molecular mass compounds into smaller ones. This stage is followed by acidogenesis, which is the conversion of the smaller molecular compounds into identifiable lower molecular compounds. The acetogenesis stage converts the volatile fatty acids (VFAs) from acidogenesis into acetate, CO2 and H2, where after these intermediate compounds are converted into methane and CO2 with other by-products in trace amounts in methanogenesis stage, with the aid of methanogens. The stages of the AD process are affected by various parameters and the biomass chosen.

#### *5.3.2 Fermentation*

The conversion of biomass to simple sugars with subsequent transformation into alcohol and CO2 with the aid of microorganisms, mainly, yeasts is known as fermentation. This process has been applied for centuries to produce ethanol from sugar crops. It has been used for conversion of lignocellulosic material and algae to ethanol. The by-product from this process after the distillation process are the nonfermentable products which are further directed for use as animal-feed, or as raw feed for the thermochemical conversion. Ethanol produced from the process can be used as fuel for motor vehicles, or as compliment for existing conventional fuels. It has been used as an additive for petrol to improve the octane rating and vehicle emissions reduction in countries such as Australia, Brazil, Sweden, and United States [20]. Anaerobic or dark fermentation of pre-treated lignocellulosic biomass is used in the production of biohydrogen, where the process is like the acidogenic stage in AD. The microorganisms used in this process are mainly hydrogen producing microbes such as *Thermoanaero bacterales, Clostridiaceae* and *Enterobacteriaceae.* This process is gaining interest as the combustion of hydrogen is free of any harmful emissions [56]. The challenge encountered is the low yield of H2 generated, though there has been research carried out into the upgrading of CH4 into H2.

#### *5.3.3 Microbial fuel cell (MFC)*

MFCs employ the activity of micro-organisms to convert pre-treated biomass into bioelectricity that can be fed into an existing electricity grid. The process involves the oxidation of the substrates (cellulosic biomass) at the anode chamber of the cell by microorganisms, (electrode-reducing organisms) to electrons which are transferred to the cathode chamber through a conductive material. In the cathode region, the electrode-oxidizing organisms utilize the electrons for

reduction of various compounds to other forms (such as CO2 to acetate, nitrate to N2 and O2 to H2O). It is an electrochemical reaction that utilizes microorganisms for catalysis; therefore, it is referred to as a bioelectrochemical process. The anode in an MFC is usually carbon based such as carbon cloth, felt, fiber, rod, and paper, while the cathode is either of the latter coated with platinum [57, 58]. **Figure 8** shows a schematic of the working principles of a microbial fuel cell. The catalyst used may differ based on the application of the fuel cell.
