Sustainable valorization of grass biomass via hydrothermal pretreatment for biogas and biofuel co-production

This study investigates the potential of hydrothermal pretreatment to enhance the energy recovery and waste management of Napier grass (Pennisetum purpureum) as a co-substrate in pig farming operations. Pig farms in Thailand generate wastewater rich in organic matter and nutrients, and efficient treatment is essential for meeting environmental discharge standards and reducing greenhouse gas emissions. Two pretreatment strategies were evaluated: a low-temperature hydrothermal fermentative pretreatment (HFP) and a high-temperature thermal hydrolysis pretreatment (THP). Napier grass, characterized by its high lignocellulosic content and rapid growth, was processed at various temperatures (35-100 °C for HFP; 125-200 °C for THP) and reaction times (6-72 h) to produce distinct liquid and solid fractions. Biochemical methane potential (BMP) assays demonstrated that HFP at 35 °C for 12 h yielded the highest overall methane production (184.5 m³ CH₄ per ton of dry biomass), reflecting a 65 % increase in biodegradability relative to untreated grass. Regression analysis further elucidated the interactive effects of temperature and reaction time on methane yields, while mass balance evaluations compared alternative pathways for the valorization of the HFP solids. Notably, converting the solids into biomass pellets delivered a superior net energy output compared to further anaerobic digestion. Overall, the integrated approach of using hydrothermal pretreatment, especially via HFP, not only improves biogas yields but also aligns with sustainable agricultural practices by reducing waste, mitigating environmental impacts, and enhancing the energy efficiency of pig farming systems.