Where your wastewater goes after it leaves your house

When you flush a toilet or drain a sink, the wastewater flows through pipes under your street to a municipal treatment plant, not into the ground like a septic tank does. Your city or county owns and operates these pipes and the plant. The system is designed to handle all the wastewater from thousands of homes at once, treating it before releasing it into a river, lake, or ocean.

The journey starts in your home's drain line, which connects to a larger pipe buried under the street called a sewer main. Sewer mains run under most city streets and slope downward to move wastewater by gravity alone — no pumps needed for most of the trip. All the sewer mains in your neighborhood feed into larger trunk lines, which eventually lead to the treatment plant.

This is the opposite of a septic system, where your property handles all treatment on-site. With a municipal sewer, the treatment happens centrally, and your only responsibility is keeping tree roots and grease out of your drain line.

Key Takeaways

  • Municipal sewers use gravity to move wastewater from your home through underground pipes to a central treatment plant operated by your city or county.
  • The treatment plant removes solids, bacteria, and chemicals through multiple stages before the cleaned water is discharged into a river, lake, or ocean.
  • Sewer backups into your home usually happen because of tree roots, grease buildup, or broken pipes in the section you own — the line from your house to the street.
  • Your city maintains the sewer main under the street; you are responsible for the lateral line on your property, which typically costs $3,000 to $25,000 to repair or replace.
  • Stormwater drains are separate from sewer lines in most modern cities, so rainwater does not overwhelm the treatment plant during heavy storms.

The three main stages of wastewater treatment

Primary treatment removes the solids. Wastewater enters a large tank called a clarifier, where it sits long enough for heavy solids to sink to the bottom and grease to float to the top. Mechanical rakes scrape the settled solids — called sludge — toward a drain, and skimmers remove the grease. What flows out is still dirty water, but most of the visible material is gone.

Secondary treatment removes dissolved organic matter and bacteria using microorganisms. The water flows into an aeration basin where air is pumped in, and naturally occurring bacteria consume the remaining organic material. This stage takes several hours. The water then moves to another clarifier to separate the bacteria (which settle out) from the cleaner water above.

Tertiary treatment — also called advanced treatment — removes remaining nutrients, chemicals, and pathogens. This stage is not universal; smaller plants may skip it. Methods include sand filtration, ultraviolet light, or chemical disinfection with chlorine or ozone. The final effluent is tested to meet state and federal standards before it is discharged.

The sludge removed in primary and secondary treatment is not thrown away. It is thickened, digested (broken down further by bacteria), and either dried for use as fertilizer or incinerated. Some plants compost it; others send it to a landfill.

What happens if something goes wrong: backups and overflows

A sewer backup into your home means wastewater is flowing backward through your drain line instead of toward the plant. The most common cause is tree roots growing into cracks in the pipe under your yard. Roots seek moisture and can break through clay or older concrete pipes, creating a blockage. Grease buildup — from cooking oil poured down the drain — is the second leading cause. Grease hardens as it cools and traps other debris.

The section of pipe from your house to the street is your responsibility to maintain and repair. If roots or grease are blocking it, you pay for the cleaning or replacement. Your city maintains the sewer main under the street; if that line is blocked or broken, the city handles the repair at no cost to you.

During heavy rain, some older cities experience combined sewer overflows. These cities use a single pipe for both sewage and stormwater. When rainfall is heavy, the volume exceeds what the treatment plant can handle, and untreated wastewater overflows directly into rivers or streets. Newer cities separate stormwater into its own system, so rain does not mix with sewage.

If you notice sewage backing up into your basement, slow drains throughout the house, or raw sewage in your yard, contact a licensed plumber when ready. They can use a camera to inspect the line and determine whether the blockage is on your property or in the city main.

The difference between sewer and stormwater systems

A sanitary sewer carries wastewater from toilets, sinks, showers, and washing machines. A stormwater drain carries rainwater from gutters, streets, and parking lots. In most modern cities, these are two separate underground pipe systems.

Stormwater drains empty directly into rivers, lakes, or detention ponds without treatment — the assumption is that rain is relatively clean. Sanitary sewers go to the treatment plant. Keeping them separate prevents the treatment plant from being overwhelmed during storms and allows stormwater to be managed independently.

In older cities, a single pipe handled both, which is why heavy rain can cause sewage to back up or overflow. These cities are gradually separating the systems, a costly and long-term project.

On your property, make sure downspouts and gutters drain away from your foundation and into the stormwater system, not into the sanitary sewer. Connecting roof runoff to the sanitary sewer increases the load on the treatment plant and can trigger overflows during storms.

Your responsibilities as a sewer customer

You own and maintain the lateral line — the pipe from your house to the property line or to the sewer main under the street. Your city owns everything beyond that. If your lateral fails, you pay for repair or replacement, which typically costs between $3,000 and $25,000 depending on depth, length, and soil conditions.

To protect your lateral, do not plant trees with aggressive root systems near it. Avoid pouring grease, oil, or food waste down the drain. Do not flush anything except toilet paper and human waste — not paper towels, wipes, feminine hygiene products, or medications. These items do not break down and can clog your line or damage the treatment plant's equipment.

Many cities offer a sewer lateral insurance program or sewer backup insurance through the water utility. These programs cover the cost of repairs if your lateral fails or backs up. The premium is usually $5 to $15 per month. Check with your city water department to see if this is available in your area.

Your sewer bill is typically based on your water usage — the assumption is that most water that enters your home leaves through the sewer. Some cities charge a flat fee instead. The bill covers treatment, maintenance of the mains, and capital improvements to the system.

How sewer systems are maintained and upgraded

Cities inspect sewer mains using CCTV cameras — closed-circuit television cameras mounted on a crawler that moves through the pipe. The video shows cracks, root intrusion, and buildup. Based on the inspection, the city prioritizes repairs.

Common repair methods include pipe bursting, where a new pipe is pulled through the old one, breaking it apart as it goes, and trenchless lining, where a new pipe material is inserted inside the existing pipe without digging. Both methods are less disruptive than traditional excavation but more expensive.

Aging sewer infrastructure is a major challenge in many cities. Pipes installed 50 or more years ago are reaching the end of their lifespan. Replacing them requires significant capital investment, which is why sewer rates have risen in many areas. Some cities are also upgrading to handle climate change — building larger pipes and storage tanks to manage more intense rainfall.

What to do if you suspect a problem with your sewer line

Signs of a failing sewer lateral include multiple slow drains in your home, sewage odors in the yard or basement, patches of unusually green grass (from nutrient-rich sewage), or raw sewage surfacing in your yard. If you notice any of these, contact a licensed plumber with sewer line experience.

The plumber will typically run a camera through your drain to locate the problem. If the issue is on your property, you pay for the repair. If it is in the city main, contact your city's water or public works department to report it — they will investigate and repair it at no cost to you.

Before digging to repair a sewer line, call your local utility locating service (usually called "Call Before You Dig" or "811" in the United States). They will mark the location of underground pipes and cables so the plumber does not hit them.

Frequently Asked Questions

Can I connect my sump pump or downspout to the sanitary sewer?

No. Connecting stormwater to the sanitary sewer overloads the treatment plant and can cause backups and overflows. Stormwater should drain to the stormwater system, a dry well, or a rain garden on your property. Check your local plumbing code; many cities prohibit this connection and fine homeowners who make it.

What should I never flush down the toilet?

Only toilet paper and human waste belong in the sewer. Do not flush wipes (even "flushable" ones), paper towels, feminine hygiene products, medications, grease, or food. These items clog your lateral line and damage treatment plant equipment. Grease hardens in cold pipes and traps other debris; medications can harm aquatic life downstream.

How often should I have my sewer lateral inspected?

If your home is more than 40 years old or you have experienced backups, a camera inspection is worth the cost — typically $300 to $600. It shows whether roots or grease are present before they cause a backup. If your line is in good condition, you do not need routine inspections unless you notice warning signs.

Who pays if a tree root breaks the sewer main under the street?

The city pays to repair the main. However, if the roots came from a tree on your property, the city may bill you for the repair. Some cities have policies against this; others pursue it. Check your city's policy. To avoid the issue, do not plant trees with aggressive root systems near the sewer line.

What is a sewer easement, and why does my deed mention one?

A sewer easement is a legal right for the city to access and maintain the sewer line that runs under your property. It typically extends 10 feet on either side of the pipe. You own the land, but the city can dig it up if needed for repairs. You cannot build a permanent structure over an easement, and you should not plant deep-rooted trees on it.