Enhancing anaerobic digestibility of lignin-rich submerged macrophyte using thermochemical pre-treatment

• Enhancement of hydrolysis and lignocellulose degradation by alkaline pre-treatment.
• Higher delignification from lignin-rich macrophyte than lignin-poor macrophyte.
• Decrease of ferulic acid content from lignocellulose by alkaline pre-treatment.
• Enhancement of CH4 yield from lignin-rich macrophyte by alkaline pre-treatment.
• Inhibition of CH4 recovery at high NaOH dosage due to increase of soluble lignin.

The present study investigated the effect of alkaline thermochemical pre-treatment on anaerobic digestibility of two submerged macrophyte species which have significantly different lignin content. The highest hydrolysis efficiency was achieved at NaOH loading rate of 0.20 g g-TSsubstrate−1, 80 °C, 3.0 h for both species. Alkaline delignification was much conspicuous in lignin-rich macrophyte (Potamogeton maackianus) as compared with lignin-poor macrophyte (Egeria densa). Ferulic acid, which is cross-linked with lignin polymer and polysaccharides, remarkably declined with increase of NaOH loading rate. It suggests that alkali removed lignin–ferulate complex from the surface of polysaccharides. The CH4 yield of pre-treated P. maackianus was 243 mL g-VS−1, which is 51% higher than the un-treated (161 mL g-VS−1). In contrast, the CH4 yield of pre-treated E. densa was 24% higher than the un-treated. These results indicated that alkaline thermochemical pre-treatment could be an effective method for anaerobic digestion of lignin-rich macrophytes. However, it was also suggested that the high NaOH addition to lignin-rich macrophyte possibly inhibit the methane recovery, due to the increase of solubilized lignin in the digestate.