An Integrated Biorefinery for Anaerobic Digestion of Thin Stillage and Microalgae Cultivation for Nutrient Recycling, Bioenergy and Bioproduct Production
Corn grain is one of the main sources for bioethanol production. However, corn bioethanol plants consume a significant amount of energy and water, compete with the food supply chain and may lead to eutrophication as the result of high nitrogen and phosphorous content of waste streams. All these issues challenge the sustainability of bioethanol production from corn grain in terms of energy, water and lands used. In this study, integration of anaerobic digestion-microalgae cultivation with an existing corn-bioethanol-plant is proposed, which can improve the energy balance and reduce the capital and operating cost of the plant. To evaluate the feasibility of the proposed integrated process, thin-stillage was digested in a conventional anaerobic baffled reactor (ABR). The limitations of the conventional ABR were addressed by introducing a novel ABR, in which the chemical oxygen demand (COD) and sulfate removal efficiency and CH4 yield were improved as a result of reduced biomass washout and enhanced phase separation. Furthermore, the effect of operating parameters such as OLR and recycle ratio (RR) on the performance of the novel ABR were studied. According to the results, when the OLR increased from 3.5 to 6 kg COD m-3 d-1, the COD and sulfate removal efficiency and methane yield changed from 92.5±3.2%, 97±1.6% and 305±6 mL CH4 g-1 CODremoved to 78.9±3.4%, 92.9±1.4% and 275±5 mL CH4 g-1 CODremoved at RR of 15. But, reducing the RR from 20 to 10 did not change those parameters significantly. The concentration of nutrients in resulting digestate was reduced by struvite recovery. Various microalgae species in different dilutions of struvite-removed-digestate were grown and C. sorokiniana in two times dilution was selected for cultivation in the photobioreactor. The microalgae biomass concentration reached 1.62±0.11 g/L and the removal efficiencies of nitrogen and phosphorus were 95.3±1% and 78.3±1.1% at the end experiment. The protein, starch and lipid contents of biomass were 37.8±3.4%, 17.8±0.8% and 8.9±0.3%, respectively. The findings for nutrient recovery from anaerobic digestate of thin-stillage in the form of struvite and bioproducts accumulated in microalgal biomass show the potential of integrated biorefinery for improving the sustainability and energy balance of existing corn-bioethanol-plants.