A combined sewer is a type of drainage system designed to collect both stormwater and wastewater in a single pipe. This system is commonly used in older urban areas where separate systems for rainwater and sewage were not initially developed. During dry weather, the combined sewer transports both types of water to a treatment facility. However, during heavy rainfall, the volume of water can exceed the system's capacity, leading to combined sewer overflows (CSOs). This can result in untreated sewage being discharged into nearby rivers or lakes, posing environmental and health risks. Upgrading to separate systems is often a goal for modern urban planning.