Effects of replacing mineral fertilizers with sewage sludge and biogas digestate on ethanol production of Miscanthus sacchariflorus

The benefits of using sewage sludge and biogas digestate for energy-efficient cultivation of Miscanthus, a promising source of lignocellulosic ethanol, are well documented. However, there is limited knowledge on the effects of these biofertilizers on the conversion of Miscanthus sacchariflorus to ethanol, which has not been studied in this context. This study investigated the effects of different fertilizers (sewage sludge, biogas digestate, and mineral fertilizers) at doses of 100 and 160 kg N ha^-1 on ethanol production from M. sacchariflorus. The fertilization with digestate and mineral fertilizers significantly increased ethanol production and fermentation efficiency compared to the control treatment. Digestate (160 kg N ha^-1) was as productive as mineral fertilizers (100 kg N ha^-1), with corresponding productivities of 2295.2±181.4 and 2276.0±58.83 dm³ A₁₀₀ ha^-1. Moreover, fertilization with mineral fertilizers and digestate alone was sufficient to support yeast fermentation of M. sacchariflorus biomass, eliminating the need for nutrient supplementation and reducing production costs. In addition, sequential hydrolysis and fermentation proved more efficient than simultaneous saccharification and fermentation, resulting in 14% (95% confidence interval (CI): 5-23%) higher ethanol concentrations for digestate and 12% (95% CI: 4-19%) for mineral fertilizer. This study shows that bioethanol can be cost-efficiently produced using M. sacchariflorus fertilized with digestate as a substitute for mineral fertilizers.