Framework for retrofitting municipal wastewater treatment plants for enhanced resilience under failures and inflow variability

Resilience of wastewater treatment plants (WWTPs) is critical for mitigating human impacts on aquatic ecosystems and enhancing environmental sustainability. However, WWTPs are typically designed on the basis of economic considerations, often neglecting system response to failures and disturbances. This work presents a systematic methodology for retrofitting WWTPs that explicitly accounts for both uncertainty in wastewater inflow and expected and unexpected events that may result in equipment failures. The approach integrates process synthesis via the P-graph framework for the generation of plausible retrofitting designs with a systematic method for resilience evaluation based on enumeration of possible failures. This method enables the identification of cost-effective retrofit configurations with built-in redundancy to enhance resilience. The framework is illustrated via a case study that considers extreme weather, such as uneven rainfall, as an event that increases the flow of water. The method results in the evaluation of 1,531 retrofit alternatives and identifies a diverse set of non-dominated solutions offering different compromises between cost and resilience. The identified retrofit alternatives exhibit trade-offs ranging from a resilience six times that of the original design at a cost 47 higher, to a 10% increase in resilience at an additional 3% cost. These results highlight that economically optimal designs may lack sufficient redundancy, while moderate increases in cost can significantly improve resilience. The proposed framework serves as a decision-support tool for systematic retrofitting, enhancing the information available to stakeholders and supporting more sustainable WWTPs by balancing economic performance, tolerance to failures, and practical implementation considerations.