**1. Introduction**

Biogas, an energy source comprising CH4, CO2, and traces of some gaseous impurities, is generated via biomethanation, i.e., anaerobic digestion of substrates. Irrespective of the substrate, typical biogas is composed of 50–80% CH4, 20–50% CO2, 5–10% of H2, 1–2% of N2, ≈0.3% water vapor, and traces of H2S and H2O(g) [1, 2]. Regardless of their proportions, CO2 and H2S are the major impurities in biogas. Therefore, post-production cleanup processes are required to remove them for optimum performance of the final product. Usually, CO2 is absorbed into hydroxides of

Ca, K, or Ba (Eq. (1)), while CuSO4 removes H2S, FeSO4, Pb(NO3)2, or FeCl3 (Eq. (2)). For CO2 removal, NaOH is an efficient absorbent, although KOH is 27% more effective, using only 125 kWh/Tor CO2 energy [3]. Otherwise, to minimize the cost and avoid additional waste generation, the pristine gas stream could be bubbled through water to remove both gases, albeit with less efficiency [4].

$$\text{Ca(OH)}\_{2(aq)} + \text{CO}\_{2(g)} \xrightarrow{\text{C}\_{2(g)}} \text{CaCO}\_3 + \text{H}\_2\text{O} \tag{1}$$

$$\text{(CH}\_3\text{COO)}\_2\text{Pb}\_{\text{(aq)}} + \text{H}\_2\text{S}\_{\text{(g)}} \xrightarrow{\text{---} \, 2\,\text{CH}\_3\text{COOH}\_{\text{(aq)}} + \text{PbS}\_{\text{(s)}}} \tag{2}$$

### **2. Biodigestion process**

A typical biodigester is made of concrete, metal, or other material that permits anaerobic biomass fermentation [5]. For optimum performance, the operational and ambient conditions must be diligently considered. Several factors that affect biogas production efficiencies include pH, temperature, type and quality of the substrate, mixing speed and consistency organic loading, formation of highly volatile fatty acids, and inadequate alkalinity [6]. The retention (turn-over) time is the period required for organic materials to be decomposed entirely toward achieving maximum biogas yield. Fertilizers and mineralized water are the usual valuable by-products of this process [5].

Research into biogas technology in Africa gained momentum in the last decade. For instance, in Nigeria, biogas production from Bambara nut chaff [6], agricultural waste [7], and abattoir waste [8], and the performance evaluation of a biogas stove for cooking [9] have been reported. Furthermore, biogas generation from co-digested substrates, such as spent grains and rice husk [10], banana and plantain peels [11], pig waste and cassava peels [12], sewage and brewery sludge [13], have also been experimented. In most cases, co-digestion enhances methane yield by ≈60%. Similar studies were carried out in other African countries such as Uganda [14], South Africa [15], Sudan [16], etc.

Generally, plant-based biofuels are environmentally clean energy, with a high potential of lowering fossil fuel consumption to the barest minimum in the near future [17]. Over the past decade, several studies have focused on producing biomethane using lignocellulosic residues of high abundance and low cost [18, 19]. According to Bekkering et al. [20] and Holm-Nielsen et al. [21], biogas can be used as fuel and fuel cells to generate heat, steam, electricity, produce chemicals, upgrade natural gas grids via injection, etc. Elsewhere, Jantsch and Mattiasson [22] discussed how anaerobic digestion could treat wastewater and organic wastes, yielding biogas as a valuable byproduct. The four major sources of biogas production are livestock waste, landfill gas (LFG), activated sludge from wastewater treatment plants, and IIC (industrial, institutional, and commercial waste) [22–24].

Biomethanation occurs in four main steps (**Figure 1**) viz. hydrolysis [23] , acidogenesis [24], acetogenesis [26], and methanogenesis [27]. Methane is the main component of biogas (50–70%). Other components include CO2 (30–40%) and traces of H2S and H2O(g) [28]. The respective equations for the four steps are provided as Eqs. (3)–(6):

$$\text{Hydrolysis} : (\text{CsH}\_{10}\text{O}\_5) + \text{nH}\_2\text{O} \xrightarrow{\text{C}} \text{n(C}\_6\text{H}\_{12}\text{O}\_6) \tag{3}$$

$$\text{Acidogenous} : \text{n}(\text{C}\_6\text{H}\_{12}\text{O}\_6) \xrightarrow{\text{-} \text{3nCH}\_3\text{COOH}} \text{nCH}\_3\text{COOH} \tag{4}$$

*Biogas Generation from Co-Digestion Waste Systems: The Role of Water Hyacinth DOI: http://dx.doi.org/10.5772/intechopen.101568*

**Figure 1.** *Process flow of the degradation of organic material through anaerobic digestion [24, 25].*

$$\text{Acetogenesis} : \text{CH}\_3\text{COOH} \xrightarrow{\text{-}} \text{CH}\_4 + \text{CO}\_2 \tag{5}$$

$$\text{Methanol} \colon \text{CO}\_2 + 4\text{H}\_2 \xrightarrow{\cdot} \text{CH}\_4 + 2\text{H}\_2\text{O} \tag{6}$$

Four major microbial groups are respectively involved: the hydrolyticfermentative bacteria (hydrolyze complex organic compounds into simple ones), fermentative bacteria (convert the simple organic compounds into volatile fatty acids, yielding H2 and CO2), acetogenic bacteria (convert the fatty acids into acetic acid), and methanogenic archaea (produce CH4 either from acetate or from H2 and CO2) [24, 25].
