## 3 Results and Discussion  
### Effect of Pretreatment Using Alkaline  
The lignin content of RCS, the lignin removal efficiency and biomass recovery from CS of each NaOH and CaO pretreatment were analyzed and summarized in Table 2.  
As shown in Table 2, the lignin content of RCS was 23.7% \(\pm\) 0.23% before alkaline pretreatment, but it dropped to 13.2% \(\pm\) 0.48% (Treat-CSA), 13.1% \(\pm\) 0.17% (Treat-CSB), 16.1% \(\pm\) 0.22% (Treat-CSC) and 17.8% \(\pm\) 0.87% (Treat-CSD) after pretreatment. However, the lignin removal efficiency declined from 44.5% \(\pm\) 2.11% to 24.8% \(\pm\) 3.83% as the amount of NaOH in the pretreatment decreased. This result indicated that NaOH was effective in degrading lignin in CS. Removal of lignin can improve the accessibility of enzyme or hydrolytic anaerobic microbes during the CS digestion [22; 43].  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & \multicolumn{5}{c}{**NaOH and CaO Loading, g/g CS**} \\ \cline{2-6}
**Dose** & **0** & **0.10 + 0.00** & **0.075 + 0.05** & **0.05 + 0.05** & **0.025 + 0.10** \\ \cline{2-6}  & **RCS** & **Treat-CSA** & **Treat-CSB** & **Treat-CSC** & **Treat-CSD** \\ \hline Lignin, content, \% & 23.7 \(\pm\) 0.23 & 13.2 \(\pm\) 0.48 & 13.1 \(\pm\) 0.17 & 16.1 \(\pm\) 0.22 & 17.8 \(\pm\) 0.87 \\ \hline Removal efficiency, 1 \% & 0.00 \(\pm\) 0.01 & 44.5 \(\pm\) 2.11 & 44.6 \(\pm\) 0.75 & 32.0 \(\pm\) 0.97 & 24.8 \(\pm\) 3.83 \\ \hline CS recovery, 2 \% & 94.9 & 72.0 \(\pm\) 1.14 & 73.1 \(\pm\) 0.08 & 78.3 \(\pm\) 2.01 & 81.6 \(\pm\) 1.21 \\ \hline \hline \end{tabular}  
* Removal Rate = [( lignin of RCS – lignin of treated CS) / lignin of RCS] \(\times\) 100%; 2 CS recovery = (Recovered mass of CS after the treatment / RCS) \(\times\) 100%.  
\end{table}
Table 2: Effect of corn stover (CS) pretreated with NaOH and CaO.  
It is noticeable that CaO was also effective in lignin degradation during the CS pretreatment. The lignin removal efficiency of Treat-CSB reached 44.6% \(\pm\) 0.75%, which was comparable with that of Treat-CSA (44.5% \(\pm\) 2.11%). However, Treat-CSC and Treat-CSD showed less lignin removal efficiencies: 32.0% \(\pm\) 0.97%, 24.8% \(\pm\) 3.83% for Treat-SCS and Treat-CSD, respectively. This is because CaO effectively increased the alkalinity of the reactor system while being added in the pretreatment to obtain more alkalinity. The similar effort of 10%-20% lignin removal rate was reported while Coastal Bermudagrass pretreated with different CaO loadings (0.02-0.20 g of dry biomass) [36]. The lignin removal efficiency for CS decreased from 44.6% \(\pm\) 0.75% to 24.8% \(\pm\) 3.8% as the NaOH dose reduced while the CaO dose increased in the CS pretreatment. This result is due to the poor solubility of Ca(OH)\({}_{2}\), which cannot provide enough alkalinity to decompose lignin of CS [32].  
Biomass recovery is a parameter which determines the total amount of biomass that can be potentially converted to biogas during digestion [33]. The biomass recovery was 72.0% \(\pm\) 1.14%, 73.1% \(\pm\) 0.08%, 78.3% \(\pm\) 2.01% and 81.6% \(\pm\) 1.21% for Treat-CSA, Treat-CSB, Treat-CSC and Treat-CSD, respectively (Table 2). The co-digestion of the NaOH-pretreated CS (Treat-CSA) and swine manure showed the lowest biomass recovery probably because more lignin and hemicellulose in CS was completely degraded during the NaOH pretreatment [34]. However, biomass recovery was obviously increased by replacing some of NaOH with CaO, because CaO inhibits degradation of hemicellulose in the reaction system [2]. CaO enables the formation of calcium linkages within the biomass structure to reduce the solubilization of biomass components under alkaline attack [33; 44].