What a sewer system does and where it goes

A municipal sewer system collects wastewater from your home through underground pipes and carries it to a treatment plant, where it is cleaned before being released into rivers, lakes, or the ocean. Unlike a septic tank, which treats waste on your property, a sewer system is a shared network run by your city or county. The pipes under your street connect to larger pipes, which connect to even larger trunk lines that all flow toward a single treatment facility.

When you flush a toilet or drain a sink, that water enters your home's lateral line—the pipe that runs from your house to the public sewer main buried under the street. From there, gravity pulls the wastewater downhill through progressively larger pipes. Most sewer systems rely on slope and gravity alone; pumping stations are used only when terrain makes gravity impossible or when the system needs to move water uphill to reach the treatment plant.

Key Takeaways

  • Municipal sewers use gravity to move wastewater from your home through underground pipes to a central treatment plant.
  • The system includes your home's lateral line, public mains, trunk lines, and the treatment facility—all owned and maintained by the city or county.
  • Treatment plants remove solids, bacteria, and chemicals through screening, settling, biological processes, and sometimes additional filtration before discharge.
  • Blockages in your lateral line are your responsibility to fix, while blockages in the public main are the city's responsibility.
  • Stormwater sewers are separate from sanitary sewers in many cities, so rainwater does not mix with treated wastewater.

The pipes and how wastewater moves through them

Your home's lateral line is typically 4 to 6 inches in diameter and slopes downward at a rate of about 1/4 inch per 10 feet—steep enough for solids to move but not so steep that liquids run ahead and leave solids behind. This line is your responsibility to maintain. Once wastewater reaches the public sewer main under the street, it becomes the city's responsibility.

Public mains are usually 8 to 12 inches in diameter and serve a neighborhood or block. Multiple mains feed into trunk lines, which can be 2 to 10 feet in diameter and carry wastewater from an entire district. All of these pipes are sloped to move water by gravity alone. In flat terrain or where the treatment plant sits at a higher elevation than the surrounding area, the system includes lift stations—essentially large pumps that push wastewater uphill when gravity cannot do the work.

Wastewater moves slowly through these pipes, typically at 2 to 3 feet per second. This speed is intentional: too fast and solids separate from the liquid; too slow and the pipe clogs. The system is designed so that solids settle in the pipes only during low-flow periods, usually at night, and are flushed out when flow increases in the morning.

What happens at the treatment plant

When wastewater arrives at the treatment plant, it enters a series of stages designed to remove solids, bacteria, and chemicals. The first stage is screening, where large objects—rags, sticks, plastic—are caught on a metal grate and removed by hand or mechanical rake. This protects the rest of the equipment from damage.

Next, the wastewater enters a grit chamber, where the flow slows enough for sand and gravel to settle to the bottom. These heavy solids are removed and usually sent to a landfill. The remaining wastewater then moves to a primary clarifier, a large tank where it sits for 1 to 3 hours. During this time, suspended solids sink to the bottom as sludge, and grease and oils float to the top and are skimmed off.

The clarified water then enters the secondary treatment stage, where bacteria and microorganisms consume the remaining organic matter. This happens in an aeration basin, where air is bubbled through the water to feed the microbes, or in a trickling filter, where wastewater is sprayed over rocks colonized by bacteria. The microbes convert dissolved pollutants into more solids, which settle in a second clarifier.

Many plants add a third stage called tertiary treatment, which uses sand filters, ultraviolet light, or chemical disinfection to remove remaining bacteria and nutrients. The treated water—now called effluent—is tested to may support it meets state and federal standards before being discharged into a river, lake, or ocean. The solids removed at each stage are thickened, sometimes digested by bacteria to reduce volume, and then dewatered and disposed of as biosolids or sent to a landfill.

Combined sewers versus separate systems

In older cities, a single pipe carries both sanitary wastewater and stormwater (rainwater from streets and roofs). During heavy rain, the volume can exceed the treatment plant's capacity. When this happens, the system automatically overflows directly into a river or lake without treatment—a necessary safety valve to prevent sewage from backing up into homes and streets.

Newer cities and suburbs typically use separate systems: one set of pipes for sanitary wastewater and another for stormwater. Stormwater is often discharged directly into a waterway after passing through a detention pond or filter to remove sediment and oil. This separation prevents treatment plant overload but means stormwater pollution—oil, sediment, fertilizer—enters waterways untreated.

If you live in a combined sewer area, you may see notices during heavy rain advising against swimming or fishing. These overflow events are tracked by your city's environmental department and are a matter of public record, though they remain a source of water quality problems in many regions.

Your responsibility versus the city's responsibility

The boundary between your responsibility and the city's is the property line. Your lateral line—from your house to the public main—is yours to maintain and repair. If it clogs, roots grow into it, or it cracks, you pay for the fix. A plumber can locate your lateral line using a camera and recommend repair options, which range from snaking out a clog to replacing the entire line.

Everything from the public main onward is the city's responsibility. If the main clogs or breaks, the city repairs it at no cost to you. If you suspect a problem in the public system—sewage backing up into multiple homes, raw sewage in the street, or a persistent smell—contact your city's public works or sewer department. They can send a camera into the main to diagnose the problem.

Some cities charge a sewer fee based on water usage or property size. This fee funds maintenance, repairs, and upgrades to the public system. If you have a septic tank, you typically do not pay a sewer fee, though you may pay a small fee for stormwater management.

Common problems and maintenance

The most common problem in residential laterals is root intrusion. Tree roots seek moisture and can penetrate small cracks in clay or concrete pipes, eventually blocking the line. Symptoms include slow drains throughout the house, gurgling sounds from drains, or sewage backing up into the basement. A plumber can clear roots with a mechanical auger, but if the pipe is severely damaged, replacement may be necessary.

Grease buildup is another frequent issue. Cooking oil, soap, and fat congeal inside pipes and can trap other debris. Pouring grease down the drain is the main cause. Flushing only human waste and toilet paper helps prevent this problem.

In the public system, the most common issue is infiltration—groundwater seeping into cracked pipes through joints or breaks. This increases the volume the treatment plant must handle and can cause overflows during wet weather. Cities address this by replacing old pipes, sealing joints, or lining pipes with epoxy resin to seal cracks without excavation.

How sewer systems are upgraded and paid for

Most municipal sewer systems in the United States were built between 1950 and 1980 and are now aging. Pipes that were designed to last 50 to 75 years are reaching the end of their lifespan. Replacing or rehabilitating these systems is expensive—a single mile of sewer main can cost $500,000 to $2 million to replace, depending on depth and soil conditions.

Cities fund upgrades through sewer fees, bonds, and grants from the state or federal government. The EPA's Clean Water State Revolving Fund provides low-interest loans for treatment plant upgrades and pipe replacement. Many cities are also investing in green infrastructure—rain gardens, permeable pavement, and retention ponds—to reduce stormwater runoff and ease the burden on combined sewers.

If your city is planning major upgrades, you may see notices about sewer assessments or rate increases. These are used to fund necessary repairs and are a normal part of maintaining aging infrastructure.

Frequently Asked Questions

What should I never flush down the toilet?

Never flush anything except human waste and toilet paper. Wipes labeled "flushable," feminine hygiene products, diapers, dental floss, hair, medications, and cooking grease all cause blockages in your lateral line or the public system. These items do not break down in pipes and accumulate in treatment plants, requiring expensive removal by hand.

Why does my basement smell like sewage?

A sewer smell usually means sewer gas is escaping into your home, often through a dry P-trap under a sink or through a cracked lateral line. Check that all drains have water in them—the water creates a seal that blocks gas. If the smell persists, have a plumber inspect your lateral line with a camera to check for cracks or roots.

Can I have a septic tank if my city has a sewer system?

No. If a public sewer main runs under your street, local code requires you to connect to it. Septic tanks are only permitted in areas without municipal sewers. If you are considering a property, ask the city whether a sewer connection is required and what the cost is.

What happens if the treatment plant fails?

Treatment plants have backup systems and are monitored 24/7. If equipment fails, operators switch to backup equipment or reduce flow by closing gates upstream. In rare cases of catastrophic failure, the city may issue a boil-water notice or advise against water contact. These events are reported to the EPA and the state environmental agency.

How often is my sewer line inspected?

Most cities inspect public mains on a 20- to 40-year cycle, though older systems may be inspected more frequently. Your lateral line is not inspected unless you request it or a problem occurs. If your home is more than 40 years old and you have never had the lateral inspected, a camera inspection can reveal cracks or root damage before a blockage occurs.