What is the purpose of a combined sewer?

A combined sewer system is designed to carry both sanitary wastewater, which comes from residential and commercial sources, and stormwater runoff from rain or melting snow. The main purpose of this system is to consolidate these two types of water into a single pipeline for more efficient transport to a wastewater treatment plant.

This dual function helps in maintaining a streamlined infrastructure; during dry weather, the system effectively handles wastewater, and during wet weather or storms, it manages both types of water, which can help prevent flooding and reduce the likelihood of untreated sewage spilling into the environment. However, one must also consider that in heavy rain events, combined sewers can become overwhelmed, potentially leading to combined sewer overflows (CSOs) that can affect water quality in receiving waters.

The design principle of a combined sewer system highlights its role in urban planning and environmental management, where the primary aim is to cope with both types of water while minimizing the risks associated with their discharge.