**2. Results**

### *2.1. Daily and Cumulative Methane Production from Di*ff*erent Substrates*

Figure 1 presents the daily and cumulative methane yields with time for each digestion type. The maximum daily CH4 yields of the single-substrate digestions of PM, WW, and RS were 14.2 (day 1), 17 (day 1), and 21.2 mL CH4/g VS (day 2), respectively, during the 49 d of incubation (Figure 1a). After the production peak, the daily methane yield of WW was sharply decreased to approximately 2.6 mL CH4/g VS and then remained constant (Figure 1a). Although the overall trend of daily methane yields of PM and RS dropped observably during the single-substrate digestions, some brief rise and fall during the incubation period were also observed (Figure 1a). After the maximum production peak, the daily methane yield of PM temporarily increased at days 10 to 12, 14 to 17, and 22 to 27. The daily methane yield of RS temporarily increased at days 10 to 15, 24 to 25, and 26 to 27 (Figure 1a). The mean cumulative methane yield of the single-digestion of WW was 175.81 mL CH4/g VS, which was significantly (*p* < 0.05) lower than that of the single-digestion of PM (245.09 mL CH4/g VS) and RS (252.19 mL CH4/g VS) (Figure 1b). The average pH values of the single-digestions of WW, PS, and PM ranged from 7.5 to 8.8, 6.6 to 8.7, and 5.5 to 8.1, respectively (Figure 1c).

**Figure 1.** Daily (**a**) and cumulative (**b**) methane production and pH (**c**) of the single-digestions from different substrates. PM: pig manure, WW: wood waste, RS: rice straw. Different lowercase letters in the inset indicate a significant difference (*p* < 0.05) between the methane yields of different raw materials. For the calculation of methane volume, P and T was 1 standard atmospheric pressure and 25 ◦C, respectively.

### *2.2. E*ff*ects of NaOH Pretreatment on the Daily and Cumulative Methane Production from Di*ff*erent Substrates*

After NaOH pretreatment, the maximum daily methane yields of WW and RS were increased from 17 CH4/g VS to 19.1 mL CH4/g VS (day 2) and from 21.2 CH4/g VS to 22.2 mL CH4/g VS (day 2), respectively (Figure 2a,c). Two mean production peaks were observed for the daily methane yield of WW on days 2 (19.1 mL CH4/g VS) and 25 (6.32 mL CH4/g VS) after NaOH pretreatment (Figure 2a). After the maximum production peak, the daily methane yield of WW temporarily increased at days 23 to 26 (Figure 2a). Three main production peaks were observed for the daily methane yield of RS on days 2 (22.2 mL CH4/g VS), 14 (7.32 mL CH4/g VS), and 30 (6.78 mL CH4/g VS) after NaOH pretreatment (Figure 2a). After the maximum production peak, the daily methane yield of WW temporarily increased at days 8 to 14 and 26 to 30 (Figure 2a). Moreover, the daily methane yields of WW and RS after pretreated with NaOH were significantly (*p* < 0.05) higher than that of the untreated WW and RS, respectively (Figure 2a,c). These results showed that NaOH pretreatment could evidently increase the methane production of WW and RS in the single-substrate digestion process.

**Figure 2.** Daily (**<sup>a</sup>**,**<sup>c</sup>**) and cumulative (**b**,**d**) methane production and pH (**<sup>e</sup>**,**f**) of the single-digestions from WW and RS before and after NaOH pretreatment. PM: pig manure, WW: wood waste, RS: rice straw, WW+NaOH: WW with NaOH pretreatment, RS+NaOH: RS with NaOH pretreatment. Different lowercase letters in the inset indicate a significant difference (*p* < 0.05) between the methane yields of different raw materials.

The mean cumulative methane yield of the single digestion of WW was 243.53 mL CH4/g VS after the NaOH pretreatment, which nearly reached the average cumulative methane yield of the single-digestion of PM (245.09 mL CH4/g VS) (Figure 2b). When pretreated with NaOH, the mean cumulative methane yield of RS was increased from 252.19 to 282.94 mL CH4/g VS (increased by 12.2%), which was remarkably higher than the mean cumulative methane yield of PM (Figure 2d). Moreover, the mean pH values of the single-digestions of WW and RS with NaOH pretreatment ranged from 6.5 to 8.9 and 6.8 to 8.7, respectively (Figure 2e,f).

### *2.3. Daily and Cumulative Methane Production from Di*ff*erent Co-Digestion Types*

Three production peaks (17.8, 6.21, and 5.87 mL CH4/g VS) were observed for the daily methane yield of the co-digestion of WW and PM on days 1, 17, and 29 (Figure 3a). After the maximum production peak, the daily methane yield of the co-digestion of WW and PM temporarily increased at days 6 to 7, 13 to 16, and 22 to 29 (Figure 3a). Four production peaks (23, 9.4, 5.75, and 6.34 mL CH4/g VS) were observed for the daily methane yield of the co-digestion of RS and PM on days 2, 17, 28, and 41 (Figure 3c). After the maximum production peak, the daily methane yield of the co-digestion of RS and PM temporarily increased at days 1 to 2, 11 to 17, 24 to 28, and day 37 to 41 (Figure 3c).

**Figure 3.** Daily (**<sup>a</sup>**,**<sup>c</sup>**) and cumulative (**b**,**d**) methane production and pH (**<sup>e</sup>**,**f**) of the co-digestions from WW and RS with PM. PM: pig manure, WW: wood waste, RS: rice straw, PM + WW: the AcoD of PM and WW, PM + RS: the AcoD of PM and RS. Different lowercase letters in the inset indicate a significant di fference (*p* < 0.05) between the methane yields of di fferent raw materials.

The average cumulative methane yield of the co-digestion of WW and PM was 234.88 mL CH4/g VS, which was increased by 33.6% compared with the single-digestion of WW (Figure 4) and nearly reached the mean cumulative methane yield of the single-digestion of PM (Figure 3b). As shown in Figure 3d, the cumulative methane yield of the co-digestion of RS and PM was remarkably (*p* < 0.05) higher than that of the single-digestions of both RS and PM. The average cumulative methane yield of the co-digestion of RS and PM was 331.12 mL CH4/g VS, which was increased by 31.3% than the single-digestions of RS (Figure 4). The pH values of the co-digestion of PM and WW and the co-digestion of PM and RS ranged from 6.8 to 8.5 and 6.7 to 8.2, respectively (Figure 3e,f).

**Figure 4.** Growth rates of methane yield of the anaerobic digestions from WW and RS under different optimizing strategies. (**a**) NaOH pretreatment, (**b**) AcoD, (**c**) AcoD with NaOH pretreatment. Growth rate = (XT − XO)/XO, XT denotes the mean value of biomethane yield from the treated feedstock, XO denotes the mean value of biomethane yield from the untreated feedstock. Different lowercase letters under the same optimizing strategy indicate a significant difference (*p* < 0.05) between the growth rates of methane production of different feedstocks.

### *2.4. E*ff*ect of NaOH Pretreatment on the Daily and Cumulative Methane Production from Di*ff*erent Co-Digestion Types*

Three production peaks (20.5, 7.31, and 9.11 mL CH4/g VS) were observed for the daily methane yield of the co-digestion of WW (pretreated with NaOH) and PM on days 2, 17, and 28 (Figure 5a). After the maximum production peak, the daily methane yield of the co-digestion of WW (pretreated with NaOH) and PM temporarily increased at days 1 to 2, 14 to 17, and 22 to 28 (Figure 5a). Four production peaks (23.9, 9.14, 7.77, and 6.82 mL CH4/g VS) were observed for the daily methane yield of the co-digestion of RS (pretreated with NaOH) and PM on days 2, 14, 30, and 41 (Figure 5c). After the maximum production peak, the daily methane yield of the co-digestion of RS (pretreated with NaOH) and PM temporarily increased at days 1 to 2, 11 to 14, 25 to 30, and 37 to 41 (Figure 5c).

The average cumulative methane yield of the co-digestion of WW (pretreated with NaOH) and PM was 309.06 mL CH4/g VS, which was increased by 75.8% compared with that of the single-digestion of WW (Figure 4c) and was remarkably (*p* < 0.05) higher than that of the single-digestion of WW (pretreated with NaOH) and the co-digestion of WW and PM (Figure 5b). The average cumulative methane yield of the co-digestion of RS (pretreated with NaOH) and PM was 361.73 mL CH4/g VS, which was increased by 43.4% compared with that of the single-digestion of RS (Figure 4c) and was remarkably (*p* < 0.05) higher than that of the single-digestion of RS (pretreated with NaOH) and the co-digestion of RS and PM (Figure 5d).The pH values of the co-digestion of PM and WW (pretreated with NaOH) and the co-digestion of PM and RS (pretreated with NaOH) ranged from 6.3 to 8.6 and 6.4 to 8.2, respectively (Figure 5e,f).

**Figure 5.** Daily (**<sup>a</sup>**,**<sup>c</sup>**) and cumulative (**b**,**d**) methane production and pH (**<sup>e</sup>**,**f**) of the co-digestions from WW and RS with PM after pretreatment with NaOH. WW + NaOH: WW with NaOH pretreatment, RS + NaOH: RS with NaOH pretreatment, (WW + NaOH) + PM: the AcoD of PM and WW (pretreated with NaOH), (RS + NaOH) + PM: the AcoD of PM and RS (pretreated with NaOH). Different lowercase letters in the inset indicate a significant difference (*p* < 0.05) between the methane yields of different raw materials.
