Energy Recovery from Sewage Sludge in Ribeirão Preto: A Comparative Analysis Between UASB and Activated Sludge Systems

Energy recovery from sewage sludge represents a sustainable and technically feasible alternative to promote integration between environmental sanitation and renewable energy generation. This study presents a case analysis of the municipality of Ribeirão Preto, São Paulo, focusing on comparisons between two wastewater treatment systems: an Upflow Anaerobic Sludge Blanket (UASB) reactor and a continuous-flow activated sludge system. Using the UASB configuration, we prepared a preliminary design of a treatment plant based on population and effluent generation projections over a 20-year horizon. The estimated sludge and biogas production allowed us to simulate electricity generation then. The comparative economic assessment, which employed Net Present Value (NPV) and Internal Rate of Return (IRR) indicators in accordance with ANEEL Resolution No. 482/2012, showed that the UASB system yields hard superior methane (up to 3235.6 m3/day) and higher electricity generation potential (1839.7 MWh/year) than the activated sludge system (1990 m3/day and 1654.3 MWh/year, respectively). Both systems were economically viable, with a positive NPV, an IRR of up to 16.83%, and payback periods starting in the first cycle. Furthermore, we estimated the cost per cubic meter of generated biomethane, conducted a sensitivity analysis, and assessed the impact on the most important economic indicators, all to identify the advantages and disadvantages of the proposed project and the best use of the generated biogas. This analysis showed that it is possible to recover energy from sewage treatment systems while also reusing sewage sludge for agricultural applications, thereby highlighting additional environmental and economic benefits, particularly in regions with a strong presence of agribusiness, e.g., Ribeirão Preto.