## 4 Enhancement of Energy Recovery (Biogas/Methane Yield) from the Anaerobic Digestion of Rice Straw
### Temperature  
Temperature is one of the most significant parameters affecting activities, survival, and growth of microorganisms in the AD [54]. It also influences the biogas yield and digestate quality [55] in both batch and continuous mode of digestion. Temperature is a crucial parameter to maximize the efficiency of the AD but also in regards to economic input [11]. Generally, there are three different temperature ranges at which anaerobic bacteria can grow. These are: psychrophilic (10-30 \({}^{\circ}\)C), mesophilic (30-40 \({}^{\circ}\)C), and thermophilic (50-60 \({}^{\circ}\)C) [56]. [PERSON] et al. [25] conducted a study in which RS was digested for 36 days at 37 \({}^{\circ}\)C and 55 \({}^{\circ}\)C in the presence of mesophilic and thermophilic inocula, respectively. The cumulative CH\({}_{4}\) yield was 226 L/kg VS at 37\({}^{\circ}\)C (with average 56% CH\({}_{4}\) content) and 281 L/k VS at 55 \({}^{\circ}\)C (average CH\({}_{4}\) content: 67%). At thermophilic conditions, RS produced approximately 25% more CH\({}_{4}\) than at mesophilic conditions. Similar results have been obtained by [PERSON] et al. [54] who co-digested RS with press mud in digesters of 1 m\({}^{3}\) capacity. [PERSON] et al. [54] pointed out that cumulative biogas yield for the duration of 20 days under thermophilic conditions (45-55 \({}^{\circ}\)C) was 34% higher than mesophilic conditions (25-40 \({}^{\circ}\)C). RS mono-digestion has also been evaluated by [PERSON] et al. [57] at mesophilic (35 \({}^{\circ}\)C) and thermophilic conditions (55 \({}^{\circ}\)C) at different TS (7.5% and 20%) in batch tests. Under wet AD, \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Substate**} & \multirow{2}{*}{**Separation Technique**} & \multirow{2}{*}{**AD Mode**} & \multicolumn{3}{c}{**N**} & \multicolumn{3}{c}{**P**} & \multirow{2}{*}{**References**} \\ \cline{5-6}  & & & **LD (\%)** & & & **SD (\%)** & & **LD (\%)** & **SD (\%)** \\ \hline \multirow{3}{*}{Straws (n = 3)} & Centrifuge & Batch & 33.7 \(\pm\) 6.8 & 53.0 \(\pm\) 9.7 & 82.5 \(\pm\) 2.1 & 14.4 \(\pm\) 3.8 & [37] \\  & Centrifuge & Batch & 36.0 \(\pm\) 39.9 & 59.2 \(\pm\) 38.6 & 88.8 \(\pm\) 7.7 & 85.7 \(\pm\) 10.0 & [37] \\  & Screening & Full-scale & 85 & 3 & 90 & 10 & [47] \\ Mixed Substrates (n = 13) & Screw press & Full-scale & 86.8 \(\pm\) 6.2 & 71.3 \(\pm\) 6.2 & 71.6 \(\pm\) 12.0 & [52] \\  & Softing & FASBR & 61.8 \(\pm\) 3.3 & 34.5 \(\pm\) 2.3 & 35.5 \(\pm\) 2.3 & 62.3 \(\pm\) 7.0 & [46] \\ \hline \hline \end{tabular}
**LD**, Liquid digestate; **SD**, Solid digestate; **AD**, Anaerobic digestion; **Straws** (Rice straw, own straw); **Manures** (Swinne manure, cattle manure, chicken manure); **PS**, Pig slurry; **Mixed substrates** (PS, energy crops, commelal, milk whey, agro-industrial residues, cow slurry, molasses, poultry manure); **SM**, Swine manure; **PASBR**, psychrophilic anaerobic sequencing batch reactor.  
\end{table} 
Table 4: Distribution of nutrients into LD and SD. “n” is the number of experiments to calculate the standard deviation of the mean (\(\pm\)). their results showed the CH\({}_{4}\) yield of 120 and 136 L/kg VS at mesophilic and thermophilic conditions, respectively. While CH\({}_{4}\) yield was 123 and 76 L/kg VS at mesophilic and thermophilic conditions, respectively, under dry AD. [PERSON] et al. [42] investigated the effect of increasing temperature from 39 to 50 \({}^{\circ}\)C on the biogas yield from a 300 m\({}^{3}\) digester. The RS was added daily to the digester. The biogas yield increased by approximately 100% (from 200 L/kg TS to 402 L/kg TS) when the temperature of the system increased from 39 to 50\({}^{\circ}\)C. Moreover, the CH\({}_{4}\) content was improved from 50% to 60%. The studies mentioned above investigated the effect of mesophilic and thermophilic conditions on the biogas/methane yield.