A municipal sewer system is a network of underground pipes, pumping stations, and treatment facilities that collects wastewater from homes, businesses, and industrial buildings and carries it to a treatment plant for cleaning before releasing it back into the environment. About 83% of U.S. households are connected to a public sewer system, with most of the remaining 16% relying on private septic tanks.
How Wastewater Moves Through the System
Everything that goes down your drains, toilets, and floor drains flows into a pipe called a sewer lateral, which connects your property to the larger public sewer main running beneath your street. From there, wastewater travels through progressively larger collection pipes toward a treatment plant. Most of this movement happens by gravity: pipes are installed at a slight downward slope so water flows naturally without any mechanical help.
When the terrain doesn’t cooperate and wastewater needs to travel uphill, the system relies on lift stations (also called pump stations). These are underground chambers with a “wet well” that collects incoming sewage. When the water level rises to a set point, sensors trigger pumps that push the wastewater uphill through pressurized pipes called force mains. Once the level drops, the pumps shut off automatically. A single municipal system can have dozens or even hundreds of lift stations depending on the local geography.
Manholes, those round metal covers you see in streets, are access points spaced along the pipe network. Maintenance crews use them to inspect, clean, and repair the system without digging up the road.
Combined vs. Separate Sewer Systems
Not all municipal sewers work the same way. There are two main designs, and which one your city uses has real consequences during heavy rain.
A separate sewer system uses two independent sets of pipes. One carries sanitary sewage (from toilets, sinks, and appliances) to a treatment plant. The other carries stormwater (rain and snowmelt) from street drains directly into local streams and rivers, with little or no treatment. Most newer infrastructure uses this design.
A combined sewer system handles both sanitary sewage and stormwater in a single pipe. During dry weather, everything flows to the treatment plant normally. The problem comes during storms, when stormwater volume can be nine times greater than the raw sewage in the pipe. That surge can overwhelm the system’s capacity. When it does, a mix of untreated sewage and stormwater discharges directly into nearby rivers and streams through designated outfall points. This is called a combined sewer overflow, or CSO.
CSO discharges can contain bacteria, chemicals, sediment, floating debris, and oil. Many older cities in the northeastern and midwestern U.S. still operate combined systems built in the 19th and early 20th centuries, and upgrading them is one of the most expensive infrastructure challenges municipalities face.
What Happens at the Treatment Plant
Once wastewater reaches a treatment plant, it goes through multiple stages of cleaning before it can be safely released into a waterway.
Primary treatment is essentially physical separation. Wastewater sits in large settling tanks where heavy solids sink to the bottom and lighter materials like oils float to the surface. Both are skimmed off. This step removes a significant portion of suspended material but doesn’t address dissolved pollutants.
Secondary treatment is biological. The plant introduces oxygen into the remaining wastewater, which feeds colonies of aerobic microorganisms. These microbes consume dissolved organic matter, breaking it down into carbon dioxide, water, and residual solids. This stage also converts nitrogen compounds into nitrates. By the end of secondary treatment, the water is dramatically cleaner than what entered the plant.
Tertiary treatment goes further by targeting nutrients, primarily nitrogen and phosphorus, that can cause algae blooms and oxygen depletion in waterways. Nitrogen is removed through a process where bacteria convert nitrates into harmless nitrogen gas under low-oxygen conditions. Phosphorus is removed either by adding chemicals that bind to it and form particles that can be filtered out, or through specialized biological processes. Not all plants include tertiary treatment; it depends on local water quality requirements.
Who Owns What: Your Responsibility vs. the City’s
The sewer lateral, the pipe that runs from your building out to the public sewer main in the street, is typically your responsibility as a property owner. This is a detail many homeowners don’t learn until something goes wrong. If that lateral cracks, collapses, or gets clogged by tree roots, the repair bill falls on you, not the city. The public sewer main and everything downstream from the connection point is the municipality’s responsibility.
The exact boundary varies by jurisdiction. Some cities maintain the lateral up to the property line, while others consider the homeowner responsible all the way to the main. Checking with your local public works department clarifies exactly where the line is drawn in your area.
Common Threats to the System
One of the biggest maintenance headaches for municipal sewer operators is fatbergs: massive blockages formed when fats, oils, and grease combine with non-flushable items like wipes and hygiene products inside the pipes. Research from the University of Hawaiʻi found that these deposits are primarily made up of calcium salts of fatty acids rather than simple grease, and that unsaturated fatty acids produce especially sticky deposits that are harder to remove and can corrode concrete pipes. In worst cases, fatbergs cause sewage to back up and overflow out of manholes or into buildings.
Infiltration is another persistent problem. Groundwater seeps into aging pipes through cracks and deteriorating joints, adding volume to the system that doesn’t need treatment and reducing the capacity available for actual sewage. This is especially costly during wet seasons when the extra water can push treatment plants toward their limits.
Regulation and Discharge Standards
Municipal sewer systems in the U.S. operate under the Clean Water Act, which requires any facility discharging treated wastewater into a waterway to hold a federal permit. These permits set specific limits on the volume and character of pollutants a treatment plant can release. Industrial facilities that discharge into a municipal system must also meet pretreatment standards before their wastewater enters the public pipes, preventing heavy metals and toxic chemicals from reaching the treatment plant.
Fees and Costs
If your property is connected to a municipal sewer, you pay a recurring service fee that covers the cost of operating, maintaining, and eventually replacing the collection and treatment infrastructure. These fees are typically billed monthly or quarterly and vary widely depending on where you live and how much water you use. Residential, commercial, and industrial properties are usually charged at different rates.
Connecting a new property to the system involves a one-time capacity fee, which covers your share of the treatment plant’s ability to handle the additional sewage your connection will generate. The amount depends on the size of your connection pipe. Even properties that aren’t connected may owe an annual availability fee simply for being adjacent to a sewer line, since the infrastructure is already in place to serve them. In one Arizona district, for example, that availability fee is $48 per year.