A sewer is an underground pipe that carries wastewater away from your home

A sewer is a pipe or channel buried beneath streets and properties that moves wastewater from homes and buildings to a treatment plant or septic system. The wastewater includes everything that goes down your drains — toilet water, shower water, sink water, and water from washing machines. The sewer system keeps this water out of sight and prevents it from pooling in yards or streets.

Most homes in cities and towns connect to a municipal sewer system, which is owned and operated by your local government or a public utility. If you live in a rural area without municipal service, your home likely has a private septic system instead — a tank buried on your property that holds and breaks down wastewater. Both systems do the same job: they remove wastewater from your home and either treat it or prepare it for treatment.

The word "sewer" can refer to the pipe itself, the network of pipes under a city, or the entire system that moves wastewater. When someone says "the sewer backed up," they mean wastewater is flowing backward into the home or yard instead of away from it.

Key Takeaways

  • A sewer is an underground pipe that carries wastewater from your home to a treatment plant or septic system.
  • Municipal sewers are owned by your city or county and serve multiple homes; septic systems are private tanks on individual properties.
  • Wastewater includes toilet water, shower water, sink water, and laundry water — anything that drains from your home.
  • Sewer backups happen when pipes become blocked or damaged, and they can cause raw sewage to enter your home or yard.
  • Tree roots, grease buildup, and broken pipes are the most common causes of sewer problems in residential areas.

How a municipal sewer system moves water

In a municipal system, wastewater from your home flows through a pipe called a lateral that connects to a larger main sewer line buried under the street. The main line carries wastewater from many homes and buildings toward a treatment plant. The pipes are sloped downward so gravity pulls the water along — no pumps are needed in most cases.

At the treatment plant, the wastewater is cleaned through several stages. Solids are removed first, then the water is treated with chemicals and bacteria to break down harmful material. The cleaned water is released into a river or ocean, and the solids are either buried, burned, or composted. Your local utility handles all of this; you pay a monthly sewer bill that covers the cost.

The entire network — from your home to the treatment plant — is considered part of the sewer system. If something blocks the pipe between your home and the main line, the blockage is usually your responsibility to fix. If the problem is in the main line or at the treatment plant, the city handles it.

How a septic system works instead of a municipal sewer

A septic system is a self-contained wastewater treatment setup on your property. Wastewater from your home flows into a large buried tank, usually made of concrete or plastic. Inside the tank, solids settle to the bottom and bacteria break down the waste. The cleaner liquid, called effluent, flows out of the tank into a drain field — an area of gravel and soil that filters the water further before it soaks into the ground.

Septic systems require regular maintenance. A professional must pump out the tank every three to five years to remove the solids that bacteria cannot break down. If you neglect pumping, solids can overflow into the drain field and cause the system to fail. You also cannot pour grease, chemicals, or non-biodegradable items down the drain, because they will damage the bacteria that do the treatment work.

Septic systems are common in rural areas where municipal sewers do not reach. They cost more to maintain than a municipal sewer connection, but they give you control over your own wastewater treatment.

Common sewer problems and what causes them

Tree roots are the leading cause of sewer damage in residential areas. Roots seek out moisture and can crack pipes or grow into them, blocking the flow of wastewater. This problem is most common in older neighborhoods where large trees have been growing for decades near sewer lines.

Grease buildup happens when cooking oil, butter, or fat is poured down the drain. It hardens inside the pipe and traps other debris, eventually blocking the line. This is one of the easiest problems to prevent — never pour grease down the drain, even if you run hot water after it.

Broken or collapsed pipes occur when pipes age, freeze, or shift due to ground movement. Clay and cast iron pipes, common in homes built before 1980, are especially prone to cracking. A broken pipe allows sewage to leak into the soil and also allows groundwater to seep into the sewer line, which can overwhelm the system during heavy rain.

Flushing non-flushable items — wet wipes, paper towels, feminine hygiene products, or dental floss — can jam pipes. These items do not break down like toilet paper and accumulate in the line or at treatment plants.

Signs your sewer line may have a problem

If your toilet, shower, or sink drains slowly or backs up, the problem may be in your sewer line. Multiple drains backing up at the same time is a stronger sign of a sewer issue than a single slow drain, which is usually a local clog. Raw sewage smell in your yard or basement, soggy patches of grass that stay wet even when it has not rained, or sewage pooling in your yard all point to a sewer line problem.

If you rent, contact your landlord or property manager when ready. If you own your home, call a plumber who can inspect the line with a camera. A camera inspection costs between $300 and $800 and shows exactly where the blockage or damage is. Do not ignore sewer problems — they worsen over time and become more expensive to fix.

The difference between sewer and storm drain systems

Many cities have two separate underground pipe systems: a sanitary sewer that carries wastewater from homes, and a storm drain that carries rainwater from streets and gutters. The sanitary sewer goes to a treatment plant. The storm drain usually empties directly into a river, lake, or ocean without treatment.

Some older cities use a combined sewer system, where both wastewater and rainwater flow through the same pipes. During heavy rain, these pipes can overflow, sending untreated sewage into waterways. This is why some cities ask residents not to water lawns or wash cars during heavy rain — to reduce the amount of water entering the system.

Your home's gutters and downspouts should drain to the storm system, not the sanitary sewer. If they drain to the sanitary sewer, you are adding unnecessary water to the treatment plant, which can cause backups during rain.

What happens if a sewer backs up into your home

A sewer backup means wastewater is flowing backward into your home instead of away from it. This can happen if the main line is blocked, if the treatment plant is overwhelmed, or if your lateral line is damaged. Raw sewage can enter your basement, crawl space, or lowest-level drains first.

If sewage enters your home, do not use any drains or toilets until the problem is fixed. Contact a plumber when ready. If the backup is in the main line or caused by the city's system, the city may be responsible for cleanup costs, but this varies by location. Document the damage with photos and contact your homeowner's insurance — some policies cover sewer backups if you have added that coverage.

To reduce your risk, install a backwater valve in your lowest drain line. This one-way valve allows wastewater to flow out but prevents it from flowing back in. Installation costs $500 to $2,000 depending on your home's layout.

Frequently Asked Questions

What is the difference between a sewer and a septic system?

A sewer is a municipal pipe system that carries wastewater from your home to a treatment plant run by your city. A septic system is a private tank and drain field on your property that treats wastewater on-site. Municipal sewers serve multiple homes; septic systems serve one home or property.

Who is responsible for fixing a broken sewer line?

If the break is in the lateral line between your home and the main sewer line, you are responsible for repair. If the break is in the main line under the street, your city or utility is responsible. A plumber with a camera can determine where the break is located.

Can I pour grease down the drain if I run hot water after it?

No. Hot water may keep grease liquid while it flows down the drain, but it hardens as it cools inside the pipes. Grease buildup is one of the most common causes of sewer blockages. Pour grease into a container and throw it in the trash instead.

How often should a septic tank be pumped?

Most septic tanks should be pumped every three to five years. The exact timing depends on tank size, household size, and how much water you use. A professional can inspect your tank and recommend a pumping schedule based on how much solid material has accumulated.

What should I do if sewage backs up into my home?

Stop using all drains and toilets when ready and call a plumber. Do not enter areas with standing sewage without protective equipment. Contact your homeowner's insurance and take photos of the damage. If the backup is caused by a problem in the city's main line, the city may cover some cleanup costs.