How the Sanitary Sewer Works

Urban Wastewater Systems

Every time you flush a toilet, use a sink, do laundry, or take a shower, you create a liquid waste that has to be transported and properly treated. We have to properly manage our wastewater because it contains microorganisms and other dangerous substances that could negatively affect the environment and public health. The City of Madison Sanitary Sewer Utility manages the systems used to convey wastewater from your home until it reaches the Madison Metropolitan Sewage District wastewater treatment plant.

In medieval times, wastewater used to flow down open drains or gutters, creating unsanitary conditions and high mortality rates. Around the 14th and 15th centuries, cities started covering wastewater canals creating the beginning of the evolution of today’s closed sewer system. Some older cities like Milwaukee and Chicago operate on a combined sewer system where stormwater and wastewater mix together in the same, larger capacity pipes. The City of Madison has what is referred to as a “separate sewer system.” This means that stormwater and wastewater run in completely different pipe systems and do not mix.

How the Sanitary Sewer Works

Gravity is a pretty amazing free resource. It also happens to be the main component for how most of your wastewater gets from your house to the sanitary sewer main buried under your street. A sanitary sewer lateral pipe runs from the basement of a building and slopes downward towards the street to connect to a larger sewer main located under the street or in the public right-of-way. 

Modern day sewer lateral pipes are typically made Polyvinyl Chloride (PVC), though older homes may have laterals made of vitrified clay, cast iron, or ductile iron. A lateral is typically 4, 5, or 6 inches in diameter for a single family home and 6 inches on large mixed use buildings.

From the lateral, wastewater flows by gravity to a network of public sewer mains, ranging in size from 8 inches to 36 inches in diameter. Sewer mains flow into progressively larger sewer mains as the pipe network grows and adds more buildings and neighborhoods served by the sewer.  More customers result in more water use and as a result, larger sewer mains are needed. Sewer Access Structures (Manholes) provide workers access to the sewer mains to perform maintenance.  

At low points in the City where pipe contents can no longer flow by gravity due to the topography, a lift station is utilized to pump wastewater to a gravity sewer system. There are 29 lift stations located throughout the City of Madison. 

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