Energy Recovery from Biogas at Wastewater Treatment Plants in Sorocaba, São Paulo, Brazil: A Comparison Between UASB and Activated Sludge Systems for Electric Energy and H2 Production

This work examined, across a 20-year horizon, how much energy could be recovered from the biogas generated at UASB reactors versus an from an activated sludge process system (ASPS) treating wastewater in Sorocaba (SP, Brazil). A logistic model placed the 2046 Sorocaba population at 1,070,692 inhabitants, corresponding to a total sewage flow of 1916.9 L/s. The UASB reactor required a total volume of 36,819.8 m3 (34 units), producing up to 6524.91 m3/day of biogas and 4241.19 m3 CH4/day, without primary settling, yielding 575.55 kW and 4285.5 MWh/year, whereas the activated sludge system showed substantially higher potential, with 21,330.75 m3/day of biogas (7,785,724 m3/year), 1112.09 kW of installed power, and 8767.70 MWh/year of electricity. These values quantify the gross energy-recovery potential of each configuration, that is, the energy embodied in the recoverable biogas and its conversion to electricity, rather than a plant-wide net-energy balance, since internal process demands, such as aeration in the activated sludge system, are not deducted. Hydrogen output was additionally evaluated by upgrading the biogas to biomethane and then applying steam methane reforming (SMR); here, the activated sludge route achieved a levelized cost of hydrogen (LCOH) of US$ 5.00/kg H2, below the US$ 8.58/kg H2 found for UASB. However, the economic analysis indicated infeasibility for both systems, with a negative NPV in all scenarios (−US$ 1,051,069 for UASB and −US$ 1,904,999 for ASPS, without carbon credits, improving markedly once carbon credits are included) and paybacks of 4.4–7.5 years in the distributed-generation scenario. The industrial wastewater contribution was represented using hypothetical data from a small dairy plant rather than actual discharge records from the Sorocaba system, directly influencing the estimated biogas and energy-recovery potential. Hence, even though lower greenhouse gas emissions and renewable energy use bring clear environmental gains, feasibility remains constrained by capital and operating conditions, indicating that incentive mechanisms are needed to expand the uptake of these technologies within the sanitation sector.