Modeling Greenhouse Gas Emissions in a Full-Scale Activated Sludge Unit Based on Benchmark Simulation Model – A Case Study of Isfahan Wastewater Treatment Plant

Background: The objective of this study was to establish a facility-level modeling tool to assess the performance of internal operational strategies and external emissions. Methods: The biological process model in Benchmark Simulation Model No. 2 (BSM2) was upgraded to include two-stage and four-stage nitrification, as well as denitrification processes, to effectively simulate nitrous oxide (N2 O) production. Emissions of carbon dioxide (CO2 ), methane (CH4 ), and N2 O were also incorporated, taking into account digestion, cogeneration, and sludge storage processes. The refined model was utilized in a case study at the Isfahan Wastewater Treatment Plant (WWTP) in Iran to assess various operational strategies and explore potential challenges related to emission management. Results: The simulation results, based on the model’s structural assumptions, indicate a negative impact on greenhouse gas (GHG) emissions associated with regional energy upgrades in the ventilation system and activated sludge sector. For example, variations in the biogenic and non-biogenic CO2 proportions were observed when the total suspended solids (TSS) removal efficiency was altered, decreasing or increasing to 30/70 and 20/80, respectively. While off-site CO2 emissions can be mitigated, this reduction is offset by a significant rise in N2 O emissions. This is particularly concerning given that N2 O exhibits a greenhouse effect nearly 300 times greater than that of CO2 . Conclusion: It should be pointed out that numerous studies are needed to evaluate plant-wide control strategies in WWTPs for informed and effective decision-making regarding performance optimization.