## 4 Enhancement of Energy Recovery (Biogas/Methane Yield) from the Anaerobic Digestion of Rice Straw
### Inoculum  
The starting of the AD system is a vital and delicate stage to achieve a successful operation [69]. Inoculum contains active microorganisms and is added to the digester to initiate the AD [38; 70]. Usual inoculum is from digested sludge of active biogas plants or organic animal material [71]. A suitable inoculum can improve the biogas yield by increasing the degradation rate, stabilizing the AD, and also shorten the start-up time [72]. Other positive aspects of inoculum are contained N and micronutrients [73]. The micronutrients can enhance the enzyme activity [74] while the extra N can be provided to meet the needs of the methanogens [38]. Therefore, the sources of inoculum, its treatments and the substrate to inoculum ratio (S/I) are important for the AD of RS. The term S/I ratio determines the amounts of substrate and inoculum that should be added in the AD batch digester to prevent the inhibition by avoiding the overlarge portion of the substrate [75]. [PERSON] et al. [38] found that the sources (digested dairy manure, digested swine manure, digested chicken manure, digested municipal sludge, anaerobic granular sludge, and paper mill sludge) of the inoculum can significantly affect the AD of RS. The digested dairy manure resulted in the highest biogas yield of 325 L/kg VS as it provided suitable nutrients and the highest enzyme activities. RS inoculated \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Substrate** & **\(\mathbf{t_{IR}}\) (Days)** & **AD Mode** & **Temperature C\({}^{\circ}\)C** & **C/N** & **CH\({}_{4}\) (Ag VS)** & **CH\({}_{4}\) Enhancement (\%)** & **References** \\ \hline RS & & & & 47.5 & 177.3 & - \\ RS + CoM\({}^{*}\) & 40 & Batch & \(\mathcal{I}\pm 1\) & 19.5 & 181.1 & 2 \\ RS + CoM\({}^{*}\) & & & 22.4 & 196 & 10.5 & \\ RS + CoM\({}^{*}\) & & & 26.6 & 183.5 & 3.5 & \\ \hline RS & & & & 47.5 & 178 & - \\ RS + PM\({}^{*}\) & 35 & Batch & \(\mathcal{I}\pm 1\) & 23.6 & 268.1 & 50 & \\ RS + PM\({}^{*}\) & & & 27.7 & 241.2 & 35.5 & \\ \hline RS & & & & 47.5 & 178 & - \\ RS + CM\({}^{*}\) & 35 & Batch & \(\mathcal{I}\pm 1\) & 14.8 & 240.5 & 35 & \\ RS + CM\({}^{*}\) & & & 17.8 & 250.3 & 40.6 & \\ RS + CM\({}^{*}\) & & & 22.4 & 206.1 & 15.8 & \\ \hline RS & & & & - & 46 & - \\ RS + PM\({}^{*}\) & 90 & Batch & \(\mathcal{I}\pm 1\) & - & 340 & 640 & \\ RS + PWW + PM\({}^{*}\) & & & - & 335 & 628 & \\ RS + PWW + PM\({}^{*}\) & & & - & 302 & 556 & \\ \hline RS & 30 & Batch & 35 & - & - & 26\% more biogas in & \\ RS + MSS & & & - & - & co-digestion & [68] \\ \hline \hline
**AD**, Anaerobic digestion; **RS**, Rice straw; **CoM**, Cow manure; **PM**, Pig manure; **CM**, Chicken manure; **PWW**, Piggery wastewater; **PMS**, Papermill sludge; **MSS**, Municipal sewage sludge; \({}^{\circ}\), Substrates added with different ratios to achieve different C/N; \(\mathbf{t_{IR}}\), Hydraulic retention time. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Effects of co-digestion on the methane yield of RS with other substrates. with other inocula attained biogas yields of 30-280 L/kg VS. In addition, [PERSON] et al. [38] also suggested that a suitable inoculum can also provide N, macronutrients, and micronutrients for the AD. [PERSON] et al. [69] compared five S/I ratios, i.e., 0.25, 0.5, 1.0, 1.5, and 2.0 for anaerobic co-digestion of FW and rice husk using the fresh cow dung as inoculum and the biogas yields were 557, 458, 267, 97 and 71 L/kg VS, respectively. The highest biogas yield was obtained at S/I ratio of 0.25. Higher S/I ratios are attributed to the presence of a higher amount of biodegradable substrate and a lower amount of inoculum. Furthermore, the authors reported an accumulation of volatile fatty acids (VFAs) at higher S/I ratios (1.5 and 2.0), indicating insufficient activity of methanogenic bacteria. In another study, the inoculum was diluted in order to compare the effect of inoculum dilution on the CH\({}_{4}\) yield of pretreated RS at various S/I ratios (2, 4, 6, 8, and 10).