Performance Assessment of Wastewater Treatment Plants : Multi-Objective Analysis Using Plant-Wide Models

Abstract: As the knowledge about anthropogenic impacts of climate change has grown, the awareness of the contributions from treatment of wastewater has widened the scope for wastewater treatment plants (WWTPs). Not only shall ever stricter effluent constraints be met, but also energy efficiency be increased, greenhouse gases mitigated and resources recovered. All under a constant pressure on costs. The main objective of this research has been to develop a plant-wide modelling tool to evaluate the performance of operational strategies for multiple objectives at the plant and for off-site environmental impact. The plant-wide model platform Benchmark Simulation Model no. 2 (BSM2) has been modified to improve the evaluation of energy efficiency and include greenhouse gas emissions. Furthermore, the plant-wide process model has been coupled to a life cycle analysis (LCA) model for evaluation of global environmental impact. For energy evaluation, a dynamic aeration system model has been adapted and implemented. The aeration model includes oxygen transfer efficiency, dynamic pressure in the distribution system and non-linear behaviour of blower performance. To allow for modelling of energy recovery via anaerobic co-digestion the digestion model of BSM2 was updated with a flexible co-digestion model allowing for dynamic co-substrate feeds. A feasible procedure for substrate characterisation was proposed. Emissions of the greenhouse gases CO2, CH4 and N2O were considered. The bioprocess model in BSM2 was updated with two-step nitrification, four-step denitrification and nitrifier denitrification to capture N2O production. Fugitive emissions of the three gases were included from digestion, cogeneration and sludge storage. The models were tested in case studies for the three areas of development: aeration, co-digestion and greenhouse gas production. They failed to reject the hypothesis that dynamic process models are required to assess the highly variable operations of wastewater treatment plants. All parts were combined in a case study of the Käppala WWTP in Lidingö, Sweden, for comparison of operational strategies and evaluation of stricter effluent constraints. The averaged model outputs were exported to an LCA model to include off-site production of input goods and impact of discharged residues and wastes. The results reveal trade-offs between water quality, energy efficiency, greenhouse gas emissions and abiotic depletion of elemental and fossil resources. The developed tool is generally applicable for WWTPs and the simulation results from this type of combined models create a good basis for decision support.