Combined sewers are systems designed to collect both stormwater and wastewater in a single pipe. This means that rainwater from streets and sewage from homes flow together to a treatment facility. During heavy rainfall, these systems can become overwhelmed, leading to the discharge of untreated water into nearby rivers or lakes. To manage the overflow, many cities use retention basins or combined sewer overflow (CSO) systems. These solutions help to temporarily store excess water until it can be treated properly. While combined sewers can be efficient, they also pose challenges for urban areas, especially during extreme weather events.