## 3 Results and Discussion
### Anaerobic Co-Digestion. The methane yield in the digestion represents the effect of CS pretreatment. The daily methane yield is presented in Figure 1,b. Figure 1: Methane Yield of Anaerobic co-digestion of swine manure and differently-treated CS, (**a**): Methane yield of co-digestion of swine manure, RCS and CSA; (**b**) methane yield of anaerobic digestion of CSB, CSC and CSD. From Figure 1a, when Treat-CSA was co-digested with swine manure, methane yield increased from 14 mL/d on Day 1 to 42 mL/d on Day 5, and then it gradually decreased to the lowest point probably due to the vigorous acidification phenomenon, which is common in batch-type anaerobic digestion. After that, methane yield sharply increased to 48 mL/d and maintained the high yield for about 30 days. The co-digestion of RCS and swine manure showed relatively lower methane yield than that of Treat-CSA and swine manure in the beginning, only because of the light acidification phenomenon. From Figure 1b, methane yield of the bottles with Treat-CSB and swine manure showed a similar trend to that of the ones with Treat-CSA and swine manure; high methane yield in the beginning of the co-digestion. However, for co-digestion reactors with CS pretreated with more CaO and less NaOH (i.e., Treat-CSC and Treat-CSD), the methane yield was obviously lower than that of the co-digestion with Treat-CSB. In the digestion processes, hydrolytic bacteria can rapidly access the internal cellulose of CS to make more easily biodegradable organics that can be converted to methane in the beginning of digestion [44]. However, the methane yield rapidly decreased to a very low point after the high yield because of the heavy acidification phenomenon producing considerable excess fatty acids. The pH value of the digestion liquid sharply decreased and inhibited the activity of the methane-producing bacteria [45; 46]. In the last stage where little biodegradable biomass was available, a very low methane yield (only about 0-3 mL/d) could be observed. On the other hand, the co-digestion with RCS produced more methane in the last stage (i.e., about 5-21 mL), indicating that cellulose and hemicelluloses in the RCS were slowly hydrolyzed over time [35]. The cumulative amount of methane produced from the anaerobic co-digestion with Treat-CSA, Treat-CSB, Treat-CSC, and Treat-CSD was 987, 1007, 885 and 783 mL, respectively (Figure 2). On the other hand, only 545 mL methane was produced from the co-digestion with RCS, which was only 55% of that from the co-digestion with Treat-CSB. This indicates that addition of alkaline to the pretreatment of feedstock could effectively improve methane yield through decreasing lignin content of CS [6]. The methane yield per gram of VS is an important parameter to estimate the net biogas production of anaerobic digestion. The methane yield was calculated with the methane content and the daily biogas yield of each reactor. After anaerobic digestion for 94 days, the methane yield of the co-digestion with Treat-CSA was 328 mL/g VS, while that of the co-digestion with RCS was only 237 mL/g VS (Figure 3). Methane yields of the co-digestions with alkaline-treated CS were from 5.1% to 38.5% higher than that of the co-digestion with RCS. A similar study was performed by [PERSON] et al. [47], in which the CaO-pretreated _miscanthus_ had been co-digested with cattle manure in a dry leach bed reactor to result in 37% more methane production. This means that alkaline pretreatment of CS is an effective way to result in higher methane production from the co-digestion fed w  
Figure 2: Total methane yield of digestion. the co-digestion of swine manure and Treat-CSB showed higher methane yield of 322 mL/g VS, whereas those with Treat-CSC and Treat-CSD only produced methane yields of 286 and 249 mL/g VS, respectively. When the sugarcane bagasse pretreated with 8.5% Ca(OH)\({}_{2}\) was digested, 47% higher methane yield was obtained comparing to that of the digestion of the untreated [27].