Anaerobic digestion of an invasive aquatic plant Ludwigia grandiflora by thermal hydrolysis pretreatment: Correlations between methane kinetics and substrate composition

Abstract

Ludwigia grandiflora, an emerging aquatic weed, has become a global waterway invader, causing various environmental issues. Anaerobic digestion (AD) of lignocellulosic biomass is a cost-effective treatment method. To enhance methane yield, pretreatment is required to break the rigid lignocellulose structure, which hampers the biodegradability of the biomass. Therefore, in this study, we examined the impact of thermal hydrolysis (TH) pretreatment on L. grandiflora through batch AD tests at temperatures ranging from 150 to 210 °C, with durations of 10 and 30 mins. At 180 °C for 30 mins (severity factor of 3.8), methane yield increased significantly by 142.7 % compared to the untreated condition. Significant methane production differences were observed at temperatures equal to or above 165 °C. Longer retention times, such as 30 mins, were crucial for enhancing methane yield. Additionally, the Modified Gompertz model was a good fit for the AD process, with R2 values ranging from 0.996 to 0.935 and lag phase reduction from 0.273 (untreated) to 0.102-0.000 days for the pretreated conditions. Methane yield and production rates increased from 88.85 mL g-VS−1 and 15.01 mL g-VS−1 day−1 (untreated) to 110.48-206.41 mL g-VS−1 and 30.76-71.33 mL g-VS−1 day−1, respectively. Pearson correlation revealed a positive correlation between cellulose and methane yield enhancement, while lignin, crystallinity index, and pH showed negative correlations. Despite these negative correlations, their impact on the AD process was limited. Overall, changes in lignocellulosic structure and increased cellulose content were the main factors enhancing methane production. Additionally, the study recommends a new cost-effective reactor design for TH pretreatment.