Effects of Chitosan Addition on Scum Layer Formation and Biogas Production during Anaerobic Digestion of Corn Straw

This study investigated the regulatory role of chitosan (CTS) in anaerobic digestion (AD) of corn straw (CS) and the related underlying mechanisms. Batch fermentation experiments with a CTS concentration gradient of 0–50 mg/g total suspended solids (TSS) were conducted in conjunction with low-frequency nuclear magnetic resonance (LF-NMR), X-ray diffractometer (XRD), high-throughput sequencing, and metabolic pathway analyses. The results showed that low-concentration CTS (≤ 30 mg/g TSS) enhanced straw particle cohesion, increasing the scum layer volume by 14.81%–28.89%. This structural modification reduced the density of the scum layer and enhanced its porosity, facilitating the penetration of mechanically bound moisture and free moisture to prevent crust formation and improve mass transfer efficiency. At the same time, CTS activated methanogenic metabolism by increasing coenzyme F420 activity and selectively enriching key methanogenic archaea, such as norank_c_Bathyarchaeia. CTS concentrations exceeding 30 mg/g TSS inhibited methane production. At the optimal dosage of 30 mg/g TSS, cumulative methane production increased by 43.3%, from 257.9 ± 15.39 mL/g VS in the control group to 369.82 ± 24.62 mL/g VS. These findings reveal that CTS optimizes AD performance through a dual mechanism involving the modification of the physical structure of CS and the activation of microbial metabolism, providing a theoretical foundation for its application in agricultural waste treatment via AD.