Multi-natural hazard and multi-risk assessment of wastewater treatment plants as critical infrastructures

Wastewater treatment plants (WWTPs) are not only industrial facilities. They are essential infrastructures for the environment. Their correct functioning is very important to protect public health and aquatic ecosystems.In a global context, characterized by an increasing frequency and intensity of natural disasters, these infrastructures are exposed to multiple natural hazards that can generate severe cascading impacts. However, a critical review of the scientific literature reveals that most existing risk assessment approaches focus on single, isolated hazards and frequently overlook WWTPs, focusing instead on other categories of infrastructure or process industries. To address this methodological fragmentation, this research develops a comprehensive, transferable, and scalable framework for the multi-hazard and multi-risk assessment of WWTPs, spanning from the regional territorial scale to the specific plant level. In the first phase of the research, a territorial-scale multi-hazard assessment framework was developed and applied to the region of Sicily (Italy), an area where seismic, geomorphological, hydraulic, volcanic, and tsunami hazards coexist and interact. The methodology integrated official geospatial data and introduced advanced weighting strategies, including a conditional approach (Scenario 3) based on hazard interrelation matrices capable of accounting for the interdependence and cascading effects among different natural phenomena. The application of this framework revealed a highly significant and quantifiable exposure pattern, largely resulting from historical locational choices near urban centers and watercourses: nearly 80% of the regional WWTPs are currently exposed to high or very high multi-hazard conditions. Specifically, the numerical results demonstrated that exactly 58.15% of the regional facilities are located in Class 3 (high hazard) zones, and 21.89% in Class 4 (very high hazard) zones.Building on these territorial findings, the second phase of the research addressed risk at the plant scale by defining a composite risk index integrating hazard, vulnerability, and exposure. The framework distinguished structural and operational vulnerability (assessing parameters such as infrastructure aging, seismic standard compliance, and maintenance practices) from environmental and population exposure. To rigorously test the robustness of this model and identify decision-making priorities, a Global Sensitivity Analysis (GSA) was conducted using the Standardized Regression Coefficients (SRC) method on a sample of 2,200 Monte Carlo simulations. The regression model at the component level showed a high coefficient of determination (R² = 0.94), demonstrating unequivocally that vulnerability is the predominant factor in determining the final risk profile, significantly exceeding the influence of territorial hazard and exposure. This specific finding highlights that overall risk variability is primarily driven by infrastructural robustness and managerial organization, suggesting that mitigation strategies must focus on these internal variables rather than solely on monitoring external, unmodifiable natural threats.Finally, recognizing the growing threat of NaTech events triggered by wildfires in Mediterranean contexts, the research developed a specific operational framework for evaluating wildfire risk in WWTPs. This tool links the external hazard of the surrounding environment (Zones 2 and 3) with the internal vulnerability of the infrastructure (Zone 1). The framework was tested on three diverse case studies, generating distinct, quantifiable semi-quantitative risk profiles. Specifically, the Geraci Siculo WWTP (Italy) exhibited an intermediate and balanced risk profile (Hazard 2.1, Impact 1.8), primarily driven by its mid-slope topographic position surrounded by steep terrain (approx. 30%), dense wildland vegetation, and the high environmental sensitivity of its receiving stream coupled with a lack of technical redundancy. In contrast, the Montcada i Reixac WWTP (Spain) showed the lowest overall risk profile (Hazard 1.6, Impact 1.5) despite serving a large metropolitan population of 360,000 PE. This facility benefited significantly from urban fragmentation, the presence of the Besòs River acting as a natural firebreak, and robust infrastructural redundancy. Finally, the Vallvidrera WWTP (Spain) displayed a uniquely hazard-driven profile (Hazard 2.0, Impact 0.9). Its location within the wildland-urban intermix of a natural park resulted in elevated hazard exposure, but its small size (5,000 PE) and rapid potential recovery times maintained the potential impact at very low levels.Overall, although the proposed framework is semi-quantitative in nature and provides relative risk indicators rather than absolute probabilistic estimates, it represents a clear and useful tool to support decision-making.Ultimately, this research demonstrates that risk is not an absolute value, but the result of a variable combination of external environmental factors and internal management choices.By successfully shifting the paradigm of risk assessment for wastewater treatment plants from a single-hazard perspective to an integrated multi-risk systemic approach, this work places these facilities at the intersection of environmental protection, infrastructure management, and spatial planning.Consequently, it provides a solid methodological basis for setting investment priorities, guiding future research on climate change adaptation, and strengthening the overall resilience of environmental critical infrastructures.