## 4 Enhancement of Energy Recovery (Biogas/Methane Yield) from the Anaerobic Digestion of Rice Straw
### Mixing  
Mixing is an important operational factor that determines the performance of the AD. It enhances the mass transfer of organic substrates to microbial biomass due to the homogenization of the content in the digester [83]. Moreover, mixing facilitates the removal of trapped gas bubbles in the digester, \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Substrate**} & \multirow{2}{*}{**Temperature (\({}^{\circ}\)C)**} & **OLR** & **OLR** & **Biogas** & \multirow{2}{*}{**References**} \\  & & **(g VS L\({}^{-1}\) d\({}^{-1}\))** & **(g VS L\({}^{-1}\) d\({}^{-1}\))** & **(g VS L\({}^{-1}\) d\({}^{-1}\))** & **Yield** & **(L/kg VS)** \\ \hline RS + PM & 35 \(\pm\) 0.5 & 0.4, 0.8, 1.3, 1.9, 2.5, & & 2.5\({}^{\rm a}\) & 460\({}^{\rm b}\) & [81] \\  & & and 3.1\({}^{\rm a}\) & & & \\ \hline RS + PM & 37 \(\pm\) 2 & 3.0, 3.6, 4.2, 4.8, 6.0, & 6.0–8.0 & & \\  & & 8.0, and 12.0 & Inhibition by VFAs at 12.0 & 400–450 & [41] \\ \hline RS + CoM & 37 \(\pm\) 2 & 3.0, 3.6, 4.2, 4.8, 6.0, & 6.0 & & \\  & 3.0, 3.6, 4.2, 4.8, 6.0, & Inhibition by VFAs at 12.0 & 380 & [28] \\ \hline RS + CM & 37 \(\pm\) 2 & 3.0, 3.6, 4.2, 4.8, 6.0, & 4.8 & & \\  & 8.0, and 12.0 & Inhibition by FAN at & 380 & [67] \\  & & 6.0–12.0 & & & \\ \hline RS + PM & 55 \(\pm\) 2 & 3.0, 3.6, 4.2, 4.8, 6.0, & 12.0 & 434 & [40] \\  & & 8.0, and 12.0 & No inhibition & 455 & [40] \\ \hline RS + CM & 55 \(\pm\) 2 & 3.0, 3.6, 4.2, 4.8, 6.0, & 4.8 & & \\  & 8.0, and 12.0 & Inhibition by FAN & 422 & [40] \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Effects of OLR on the biogas yield in AD of RS. preventing the sedimentation of heavy particles and establishing uniform temperature [18]. However, intense continuous mixing disrupts the structure of microbial flocks, which disturbs the syntrophic relationship between organisms and leads to a reduction in biogas yield, instability of the digester, and increased vulnerability to shock loadings [83; 84]. Therefore, it is crucial to provide optimal mixing for efficient and stable AD [85]. Generally, gas circulation, mechanical pumping, and mechanical stirring are used for mixing [86]. Mechanical stirring is widely used in process industry operations involving various solid-liquid flows because of its higher homogeneity in practice [87]. Moreover, mechanical agitators are the most efficient mixing device in terms of power consumption [88]. The effect of mixing has been mostly studied for municipal solid waste, biosolids [89; 90], and manures [83; 91]. It is not well understood how mixing affects the AD of agricultural crop residues such as RS [85]. Hence, the optimized mixing parameters for the well-investigated traditionally used substrates could be very different for crop residues, including RS [87]. RS, like other lignocellulosic substrates, has a complex chemical structure. RS also has some specific physical characteristics like low density, high water holding capacity, poor fluidity, and heterogeneity [87; 92]. RS floats on the surface in the digester and does not mix properly, especially when biogas is entrapped in the slurry. Due to poor mixing, the substrate does not get good contact with the microorganisms, thus reducing degradation and, ultimately, lowering biogas yield [92]. [PERSON] et al. [85] conducted batch experiments using RS as a substrate to study the effect of periodic mixing (once a day, once a week, twice a week), continuous mixing (50, 150, and 300 revolutions per minute (rpm)), and no mixing on the CH\({}_{4}\) yield. Their results showed that the differences in CH\({}_{4}\) yield were not statistically significant among the periodic and no mixing conditions, while the total CH\({}_{4}\) yield for the 50, 150, and 300 rpm continuous mixing conditions were 13%, 25%, and 38% lower than for no mixing conditions. [PERSON] et al. [85] also investigated the effect of three modes of continuous mixing during the AD of RS in three continuously stirred tank reactors (CSTR).