Optimized Biogas Yield and Safe Digestate Valorization Through Intensified Anaerobic Digestion of Invasive Plant Biomass

Anaerobic digestion (AD) is an environmentally sustainable approach for managing invasive plants species, mitigating pollution, and generating renewable energy. However, the complex structure of these biomasses limits their biodegradability and necessitates pretreatment to enhance methane production. This study explored the biotransformation of two invasive species, Reynoutria japonica and Phragmites australis, harvested across diverse phenological stages. Bioprocess intensification was achieved through a single-stage process using a hydrolytic–methanogenic consortium under thermophilic conditions (55 °C, 25 days). The impact of harvest timing distinct plant fractions (shoot vs. root) on biogas production was meticulously evaluated. Results revealed progressive biogas production. Notably, winter-harvested shoot fractions exhibited the highest methane-rich biogas, achieving 551.12 ± 33.07 mL/g VS for Reynoutria and 401.42 ± 24.09 mL/g VS for Phragmites. The resulting digestate demonstrates a rich composition of essential macronutrients (N-P-K) vital for plant growth, highlighting its potential as a valuable biofertilizer. Significantly, complete inhibition of seed germination was observed, confirming the process’s efficacy in preventing the further propagation of invasive species. This research underscores that thermophilic anaerobic digestion, coupled with hydrolytic treatment, is a significant advancement in the valorization of invasive biomasses, contributing to both renewable energy production and ecological recovery.