A multi-method risk assessment framework to enhancing the safety of urban underground private sewage treatment systems

• Applied a three-step framework combining 4M, Bow-Tie, and HAZOP for hazard identification. • Analyzed 40 accidents; most risks arose from environmental hazards and equipment failures. • Framework reduced all high and medium–high risks through targeted mitigation measures. • HAZOP identified new process hazards, leading to more focused safety improvements. • Strengthened overall risk management in private underground sewage facilities. Urban underground sewage systems present unique safety challenges due to confined spaces, accumulation of toxic gas, aging infrastructure, and insufficient operational oversight. Despite their increasing prevalence in dense metropolitan areas, systematic risk-evaluation methods tailored to these systems remain limited. This study proposes an integrated, three-step risk-assessment framework that combines 4 M analysis, Bow-Tie analysis, and a Hazard and Operability Study (HAZOP) to identify, visualize, and mitigate risk factors in private underground sewage treatment facilities. Quantitative analysis was carried out on 40 accident cases, and the framework was further validated using a separate real-life methane explosion that occurred in Seoul. Quantitative analysis using a 6 × 5 risk matrix revealed that major risks stemmed from environmental hazards and mechanical failure. The proposed framework eliminated all high- and medium–high-risk elements by redistributing them into lower-risk categories through the implementation of prevention and mitigation barriers. HAZOP analysis confirmed previously unidentified process-level deviations, enhancing the overall system robustness. The proposed methodology provides a structured and replicable approach for improving safety in urban underground infrastructures. It also supports proactive, data-driven risk management strategies that have been demonstrated as feasible in recent studies.