Sewage collection, transmission and treatment systems are among the basic infrastructures of any city that should be designed to have the least vulnerability during crises caused by intentional or unintentional disruptions. Therefore, observing the principles of passive defense and safety in the sewage sector to protect and reduce vulnerability with the aim of continuing the necessary services, protection of sewage facilities and equipment, meeting the needs of the people, protecting people's lives (preventing the spread of microbial and chemical contamination), continuing to manage, and managing the people (preventing discontent and creating chaos) and, consequently, stabilizing the country's top management are essential and inevitable. Methods: This paper identifies the risks to the critical infrastructure of the municipal sewer system with a passive defense approach as the first step in assessing the sustainability of urban sewer systems and the overlooked points in the design and construction of collection, transmission and transmission network systems and provides wastewater treatment. The present research method is a descriptive survey and is a type of applied research and distinguishes between risk or actual risk (Danger) and threat or potential hazard (Hazard); it also examines the status of asset parameters, threat and vulnerability in design, and has developed urban sewage systems. Results: The results of the research indicate that the regulations and methods of designing urban sewage systems in Iran are not sufficient to protect and increase the sustainability and resilience of the aforementioned systems and therefore, the solutions of defense knowledge. Passive can be considered a good complement to existing methods of designing and constructing urban sewer systems.  The research results also show that these systems have been designed or built in the cities of our country, with the least attention to the principles of passive defense. Conclusion: Based on the findings of this study and the nature of urban sewage systems, 48 types of threats and weaknesses and serious vulnerabilities of urban sewage systems were enumerated.