What is meant by a combined sewer?

A combined sewer refers to a system designed to transport both sanitary sewage, which comes from households and businesses, and stormwater, which is runoff from rain or melting snow. This type of sewer system is utilized in many urban areas where space constraints and historical development have led to the merging of these two types of wastewater into one set of pipes.

The importance of this system lies in its ability to efficiently manage water during wet weather conditions. However, during heavy rainfall events, the volume of stormwater can exceed the system's capacity, potentially leading to overflows that discharge untreated sewage into nearby water bodies. This is a critical environmental and public health concern, highlighting the need for upgraded infrastructure in many regions where combined sewers are still in operation.

The other choices reflect different types of sewer systems or statuses that do not accurately capture the dual-functionality of combined sewers. A sewer managing only rainwater does not encompass the sanitary sewage component essential to defining a combined sewer. Similarly, a sewer for only industrial wastewater or a sewer that is no longer in use would not meet the definition of a system designed for both sanitary and stormwater transport.