Technical-economic assessment of bioenergy production from sewage and glycerol co-digestion in a soybean biorefinery

Abstract Anaerobic co-digestion of sewage and glycerol under mesophilic conditions was evaluated for hydrogen and methane production in a soybean biorefinery plant. Seven simulated scenarios were assessed based on scalability and bioenergy production potential, including three single-stage scenarios (with applied organic loading rates of 4.5, 6.0, and 9.0 kg-COD.m⁻ 3 .d⁻ 1 ) and four two-stage scenarios (with applied organic loading rates of 18 and 36 kg-COD.m⁻ 3 .d⁻ 1 for acidogenic reactor and 3.9, 4.8, 6.0 and 11.6 kg-COD.m⁻ 3 .d⁻ 1 for methanogenic reactor). The economic feasibility of these scenarios was examined by determining the net present value (NPV), internal rate of return (IRR), and payback period considering the cost savings from replacing diesel in the boiler with biogas (hydrogen - H 2 / methane - CH 4 / carbon dioxide - CO 2 ) produced in the anaerobic co-digestion process, as well as the reduction in CO 2 emissions due to this substitution. The single- (S1 scenario) and two-stage (S4 scenario) processes with the lowest glycerol: sewage ratio obtained the most favorable overall technical aspects of wastewater treatment for bioenergy production, including reactor volume (794 and 851 m 3 , respectively), organic matter removal efficiencies (75 and 83%, respectively), bioenergy output (168 and 164 kW, respectively), and energy yield (6.06 and 5.14 MJ.kg-COD − 1 , respectively). These improved technical parameters directly enhanced energy recovery and diesel substitution potential, leading to improved economic indicators, including a NPV of USD 640,000, an IRR of 11.8%, and a payback period of 17.3 years for the S1 scenario, and a NPV of USD 146,000, an IRR of 10%, and a payback period of 22.9 years for the S4 scenario.