**4. Materials and Methods**

### *4.1. Substrates and Inoculum of Anaerobic Dry Digestion*

The PM and wood chips were used as the substrate of anaerobic dry digestion. The PM was obtained from the Danzhou Pig Farm (Hainan, China) and stored in a refrigerator at 4 ◦C before the start of the digestion experiment. The wood chips were prepared from the branches of a eucalyptus plant. The branches were crushed to obtain a diameter of less than 6 mm, soaked in biogas slurry, and acclimated for 1 week at room temperature. The inoculum was obtained from anaerobic activated sludge (Shunyi Biogas Plant, Beijing, China) and centrifuged at 10,000 r/min for 30 min. Afterward, the precipitate (inoculum) of the anaerobic activated sludge was acclimated for 1 week at room temperature, and the supernatant was used to soak the wood chips and regulate the total solid content in the anaerobic dry digestion system. The physical and chemical properties of PM, wood chips, RS, and inoculum are shown in Table 1.

### *4.2. Treatment Design and Incubation Experiments*

A 500-mL glass vial was used as an anaerobic dry digestion reactor in this study. Nine treatments were prepared with three replicates each (Table 2). The inoculum accounted for 40% of the digestion material based on the total solid content. The digestion system was replenished with the supernatant of the centrifuged anaerobic activated sludge to 200 g of the total mass, uniformly stirred, placed in an anaerobic dry digestion reactor, and sealed with butyl rubber with an aluminum collecting gas bag. Each anaerobic dry digestion was incubated for 49 days in a constant-temperature incubator at 35 ◦C in the dark.

The wood chips were chemically pretreated with NaOH solution. Specifically, they were separately mixed with distilled water and NaOH solution with a mass fraction of 2% at a solid-to-liquid ratio of 1:10, uniformly stirred, treated at 90 ◦C for 4 h, rinsed with deionized water until neutral, and dried. The biogas slurry was extracted through a reflux operation every 12 h and injected back into the anaerobic dry digestion reactor. In this way, the biogas slurry could be evenly dispersed on the digestion substrate. The wood chips without pretreatment and no reflux operation were also used as controls.

### *4.3. Sample Collection and Analysis*

The biogas generated during digestion was collected ina3L aluminum gas collecting bag. The biogas in the gas collecting bag was obtained regularly by using a 200 mL glass syringe every day to measure the biogas production with a gas flow meter (LMF-1, Cixi Instrument Co., Ltd., Shanghai, China). In addition, a gas sample was periodically collected using a 2 mL syringe for methane determinations.

Total solid (TS) and VS contents were measured in accordance with standard methods [25]. Total carbon (TC) and total nitrogen (TN) contents were determined with a high-temperature automated elemental analyzer (Vario EL cube, Langenselbold, Germany). Cellulose, hemicellulose, and lignin were observed using a semi-automatic fiber analyzer (ANKOM A200i, Longjie Instrument Equipment Co., Ltd., Shanghai, China). Methane concentrations were identified on a gas chromatograph (Agilent 6890, Santa Clara, CA, USA) equipped with a thermal conductivity detector and a flame ionization detector.

Analysis of variance (ANOVA) was performed to compare the di fferences in cumulative methane productions of single and co-digestions from di fferent substrates, and the di fferences in the growth rates of methane production of the anaerobic digestions from WW and RS under di fferent optimizing strategies were determined. ANOVA was conducted with SPSS 20.0 (IBM Corporation Software Group, Somers, NY, USA). Results were considered significant at *p* < 0.05.
