Anaerobic digestion of thin stillage for energy recovery and water reuse in corn-ethanol plants

Recycling of anaerobically-digested thin stillage within a corn-ethanol plant may result in the accumulation of nutrients of environmental concern in animal feed coproducts and inhibitory organic materials in the fermentation tank. Our focus is on anaerobic digestion of treated (centrifugation and lime addition) thin stillage. Suitability of digestate from anaerobic treatment for reuse as process water was also investigated. Experiments conducted at various inoculum-to-substrate ratios (ISRs) revealed that alkalinity is a critical parameter limiting digestibility of thin stillage. An ISR level of 2 appeared optimal based on high biogas production level (763 mL biogas/g volatile solids added) and organic matter removal (80.6% COD removal). The digester supernatant at this ISR level was found to contain both organic and inorganic constituents at levels that would cause no inhibition to ethanol fermentation. Anaerobic digestion of treated-thin stillage can be expected to improve the water and energy efficiencies of dry grind corn-ethanol plants.