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This paper aims to evaluate the effects of compaction on the anaerobic biodegradability of straw. In the study, compaction tests were carried out at different applied pressures, i.e., 0 (CK), 277 (T1), 555 (T2), and 1109 Pa (T3), respectively. The changes in physicochemical indicators (i.e., pH, VFA, COD, and DHA) of the liquid digestate were monitored. Factor analysis was adopted to analyze biogas production's main factors in the bath Anaerobic digestion (AD) process. Changes in the surface structures and composition of solid digestate were analyzed. The results showed that the maximum gain in biogas production was 298.35mL·g− 1TS for the T2 reactor, significantly higher than that of CK and T3 reactors. The effect of compaction on the physicochemical index of liquid digestate was not significant during the batch-type AD process. The factor analysis results suggested that the major factors affecting biogas production were influenced by the compaction and varied based on the different stages of digestion. Scanning electron microscopy (SEM) showed that the straw surface was damaged as the compaction increases; however, the degree of damage was not significant. This research concluded that compaction on gas production via changing the environment during the bath AD process and proper compaction could positively affect biogas' yield, while excessive compaction will inhibit gas production.

Extrusion, a physical pretreatment, is normally used for the pretreatment of lignocellulosic materials to improve biogas production. Wheat straw (WS) and its solid digestate (WSD) were selected to evaluate the effect of extrusion on the biogas production of lignocellulosic materials with different compositions. The WS and WSD were extruded and then used for biogas production at (37 ± 1)°C. The results showed that the cellulose, hemicellulose, and lignin contents of WS and WSD were 28.37%, 24.60%, 14.35% and 30.20%, 24.73%, 21.58%, respectively, while the cumulative biogas yields of WS and WSD were 178.96 and 56.68 mL/g TSadded (Total solid, TS), respectively, indicating that the biogas yield is proportional to the lignin content of the substrate. After extrusion, the cumulative biogas yields of WS and WSD were increased by 22.55% and 152.33%, respectively, while the cumulative methane yields of WS and WSD were increased by 27.01% and 191.58%, respectively, compared to the unextruded biomass. The results of scanning electron microscope (SEM) and specific surface area (SSA) indicate that the lignocellulose structure in WSD suffered greater destruction and the improvement in the SSA in WSD was 51.89-percentage point higher than that in the WS pretreated by extrusion. The above results indicate that destruction of the lignocellulose structure by extrusion pretreatment differs between lignocellulosic materials, and the lignocellulose structure of materials with higher lignin content is more effectively destroyed. In summary, extrusion is a promising pretreatment technology for lignocellulosic materials, especially for feedstocks with higher lignocellulose content.