In what scenario would a 'combined sewer system' be used?

A combined sewer system is designed to collect both stormwater runoff and wastewater in a single piping system. This is beneficial in areas where space for separate systems is limited or where the treatment plant's capacity allows for the handling of both types of water simultaneously. The key characteristic of a combined sewer system is its ability to transport the mixture of regular sewage (from households and industries) and rainwater runoff together to a treatment facility. During periods of heavy rainfall, excess flow may result in combined sewer overflows (CSOs), which pose challenges but are part of the system's design.

The other scenarios describe applications where separation is either necessary or typical, such as separating stormwater from wastewater to prevent pollution or managing only specific types of waste. However, they do not align with the use of a combined sewer system, which inherently involves mixed collections.