## 4 Enhancement of Energy Recovery (Biogas/Methane Yield) from the Anaerobic Digestion of Rice Straw
### Recycling Liquid Digestate (LD)  
A separation machine (press screws) is usually used to separate the digestate from the digester into LD and SD. The SD is rather simple to handle as it is generally used as fertilizer after composting or even directly. However, the utilization or treatment of LD is a challenge. LD can be used as liquid fertilizer or can be disposed of in the water streams after treatment; however, the most promising option is its recycling to the digester [94]. The recycling of LD can help to improve the moisture content for some high solid substrates [94] and provides a range of nutrients and vitamins needed by the microorganisms [95]. However, the excessive recycling may result in process inhibition by the accumulation of VFAs, ammonia, and nonbiodegradable intermediates [94; 96; 97]. [PERSON] et al. [94] investigated the effect of recycling LD during the AD of MS in the CSTR. They compared three scenarios: No recycling of LD serving as a reference, while in the other two digesters, direct recycling and recycling after aeration carried out. Their results showed that there was \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Substrate**} & \multirow{2}{*}{**Digester Type**} & \multicolumn{2}{c}{**Mixing Mode**} & \multirow{2}{*}{**Mixing Speed**} & \multirow{2}{*}{**OLR**} & \multirow{2}{*}{**Biogas Yield**} & \multirow{2}{*}{**References**} \\  & & & **(Mixing Period [min])** & & & & \\ \hline \multirow{3}{*}{RS} & \multirow{3}{*}{CSTR} & \multirow{3}{*}{Periodic (2 h/5 min)} & 40 rpm & & 299 & \\ \cline{3-7}  & & & 80 rpm & & 370 & \\ \cline{3-7}  & & & 120 rpm & & 332 & \\ \cline{3-7}  & & & 160 rpm & & 327 & \\ \hline \multirow{3}{*}{MS} & \multirow{3}{*}{Batch} & No mixing & - & - & 414 & \\ \cline{3-7}  & & & Once a day & - & - & 491 & \\ \cline{3-7}  & & & Twice a week & - & - & 482 & \\ \hline \multirow{3}{*}{CS} & \multirow{3}{*}{CSTR} & Continuous & & & 427 & \\ \cline{3-7}  & & & (2 h/5 min) & & 459 & \\ \cline{3-7}  & & & (4 h/5 min) & 80 rpm & 1.44 & 430 & \\ \cline{3-7}  & & & (8 h/5 min) & & 420 & \\ \cline{3-7}  & & & (12 h/5 min) & & 418 & \\ \hline \multirow{3}{*}{CS} & \multirow{3}{*}{CSTR} & Continuous & & & 487 & \\ \cline{3-7}  & & & (2 h/5 min) & & 508 & \\ \cline{3-7}  & & & (4 h/5 min) & 80 rpm & 1.78 & 505 & \\ \cline{3-7}  & & & (8 h/5 min) & & 454 & \\ \cline{3-7}  & & & (12 h/5 min) & & 416 & \\ \hline \multirow{3}{*}{CS} & \multirow{3}{*}{CSTR} & Continuous & & & 423 & \\ \cline{3-7}  & & & (2 h/5 min) & & 433 & \\ \cline{3-7}  & & & (4 h/5 min) & 80 rpm & 2.11 & 429 & \\ \cline{3-7}  & & & (8 h/5 min) & & 378 & \\ \cline{3-7}  & & & (12 h/5 min) & & 335 & \\ \hline \hline \end{tabular}  
* **RS**, Rice straw; **MS**, Maize stover; **CS**, Corn stover; **CSTR**, Continuously stirred tank reactor; **rpm**, Revolutions per minute.  
\end{table} 
Table 7: Effects of mixing on the biogas yield in AD of rice and other straw. no significant difference in CH\({}_{4}\) yield, CH\({}_{4}\) content, substrate reduction, or pH in all three modes. This indicates that recycling did not adversely affect system stability. [PERSON] et al. [98] evaluated the effect of the frequency of LD recycling on the biogas yield in the solid-state AD of straw with PS. They compared the impact of different recycling frequencies--once per day, twice per day, and four times per day--with a control system (no recycling). The results suggested that recycling LD had a positive effect on biogas yield when it was more than twice per day. The highest cumulative biogas yield was achieved at a recycling frequency of four times per day. [PERSON] et al. [36] aimed to define and optimize the operational parameters of a farm-scale anaerobic batch digester (13,000 m\({}^{3}\)) using RS and PWW as substrates where recycling of LD was one of the important strategies. Increasing recycling rates of LD (from 0.04 to more than 0.14 m\({}^{3}\)/m\({}^{3}\) straw d\({}^{-1}\)) resulted in increasing biogas yield.