Sweet Sorghum Irrigated with Treated Domestic Wastewater in the Brazilian Semi-Arid: Agronomic Performance and High-Gravity Bioethanol Production

Sweet sorghum is a promising crop for bioethanol production in semi-arid regions, due to its tolerance to drought and salinity, where conventional energy crops face limitations. This study aimed to evaluate the agronomic performance of sweet sorghum irrigated with treated domestic wastewater (TDW) and its application as a substrate for bioethanol production under high-gravity (HG) and very-high-gravity (VHG) fermentation conditions. Field experiments were conducted in the Brazilian semi-arid using a 5 × 5 full factorial design consisting of five irrigation depths (40–160% crop evapotranspiration, ETc) combined with five potassium fertilization doses (0–80 kg·ha−1), totaling 25 treatments. Agronomic performance, biomass production, and total reducing sugar accumulation were evaluated in both plant cane and ratoon crops. Sweet sorghum juice was subsequently combined with sugarcane molasses and fermented using Saccharomyces cerevisiae in batch and fed-batch processes. Irrigation with TDW associated with moderate potassium fertilization enhanced plant development, biomass yield, and sugar accumulation, particularly at irrigation depths between 100% and 130% of ETc, reaching up to 1908 kg·ha−1 of TRS. Bioethanol production achieved fermentation efficiencies of 91.83% and 84.80% and productivities of 4.63 and 4.21 g·L−1·h−1 under HG and VHG conditions, respectively. These findings indicate that sweet sorghum irrigated with TDW is a promising feedstock for bioethanol production under high-gravity fermentation conditions while supporting the use of alternative water resources in semi-arid environments.