What are "combined sewers"?

Combined sewers are systems that carry both sewage and stormwater in the same pipe. This design is commonly found in older urban areas where infrastructure was developed before modern regulations and separate systems were put in place. In a combined sewer system, during dry weather, the sewage and stormwater flow together to the wastewater treatment plant. However, during heavy rainfall, the volume of stormwater can exceed the system's capacity, leading to overflows and potential discharge of untreated sewage into nearby waterways. This option accurately reflects the definition and function of combined sewers, making it the correct choice.

The other options describe different types of sewer systems or features: systems designed to separate rainwater and sewage indicate a separate sewer system; those exclusively for stormwater drainage pertain to storm sewers; and systems that utilize gravity for waste movement could refer to any sewer system, but don’t specifically describe combined sewers. Therefore, the description of combined sewers aligns precisely with the correct answer.