Where your wastewater goes after it leaves your home

A sewer system collects wastewater from your home through underground pipes and carries it to a treatment facility, where it is cleaned before returning to rivers, lakes, or the ocean. Unlike a septic tank, which treats waste on your property, a municipal 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, all sloping downhill toward a central treatment plant.

Gravity does most of the work. Pipes are laid at a slight angle—usually about half an inch per 100 feet—so wastewater flows downhill without needing a pump. In hilly areas, this works naturally. In flat areas, lift stations (small pumping stations buried underground) push the wastewater uphill when necessary, then let it flow downhill again toward the treatment plant.

Key Takeaways

  • Municipal sewer systems use gravity and slope to move wastewater from your home through underground pipes to a central treatment plant.
  • Lift stations pump wastewater uphill in flat areas where gravity alone cannot move it forward.
  • At the treatment plant, wastewater passes through screens, settling tanks, and biological reactors that remove solids and break down contaminants.
  • Treated water is tested for safety before being released into rivers, lakes, or the ocean; solids are either buried in landfills or used as fertilizer.
  • Sewer systems are separate from stormwater drains, which carry rainwater directly to waterways without treatment.

The pipes under your street and how they connect

Your home's drain line—the pipe leaving your toilet, sink, and shower—connects to a lateral, a small pipe usually 4 to 6 inches wide that runs under your yard to the street. The lateral slopes downward and connects to a larger main sewer line running under the street itself. This main line is typically 8 to 12 inches wide and serves an entire block or neighborhood.

Main lines feed into trunk lines, which are much larger—sometimes 2 to 4 feet wide—and carry wastewater from entire neighborhoods to the treatment plant. All of these pipes are made of clay, concrete, cast iron, or PVC plastic, depending on when they were installed. Older systems use clay or cast iron; newer ones use PVC, which lasts longer and resists corrosion.

At regular intervals along the main line, cleanout access points allow city workers to clear blockages or inspect the pipes. If you have a backed-up toilet or slow drain, the problem is usually in your lateral or the main line near your property, and city crews can use these access points to locate and clear it.

What happens at the treatment plant

When wastewater arrives at the treatment plant, it is far from clean. It contains human waste, toilet paper, food scraps, grease, soap, and chemicals from cleaning products. The plant removes these through a series of steps, each designed to separate solids from water and break down harmful bacteria and nutrients.

The first step is screening. Wastewater passes through metal bars or a rotating screen that catches large objects—rags, sticks, plastic bags, and other debris that would clog pipes or damage equipment downstream. These solids are removed and sent to a landfill.

Next comes primary settling. Wastewater flows into large, quiet tanks where it sits for a few hours. Heavy solids sink to the bottom (called sludge), and grease and oils float to the top. Scrapers push the sludge toward a drain at the bottom of the tank, and skimmers remove the grease from the surface. The cleaner water in the middle flows onward.

The biological reactor is where the real cleaning happens. Wastewater is mixed with billions of bacteria and aerated (air is bubbled through it). These bacteria eat the organic matter—the nutrients and contaminants that make wastewater dangerous. This process is similar to what happens in a septic tank, but it is controlled and monitored. The reactor runs for several hours, and the bacteria break down most of the harmful material.

After the reactor, wastewater enters secondary settling tanks, where the bacteria settle out and are either returned to the reactor to keep eating or removed as sludge. The water that leaves these tanks is much cleaner but still contains some dissolved nutrients and microscopic organisms.

Many plants add a final step called disinfection, using chlorine, ultraviolet light, or ozone to kill remaining bacteria and viruses. The treated water is then tested to make sure it meets safety standards before being released into a river, lake, or ocean.

What happens to the sludge and solids

The sludge removed from settling tanks is not yet waste—it still contains water and organic matter. Plants dry it out using large beds or centrifuges, reducing its volume and weight. Once dried, it can be sent to a landfill, burned for energy, or processed into fertilizer for farms and gardens. Some cities use biosolids (treated sludge) on public land or sell it to landscapers. The choice depends on local regulations and what the community decides is safest.

Grease removed from the surface of settling tanks is collected separately. It is often sold to companies that convert it into biodiesel fuel or animal feed, so nothing is wasted.

Stormwater drains are separate from sewer lines

Many people assume rainwater goes into the sewer system, but in most cities it does not. Stormwater drains—the grates you see on sidewalks and in parking lots—carry rainwater directly to rivers, lakes, or streams without any treatment. This is why heavy rain can cause rivers to flood and why pollution from streets (oil, dirt, chemicals) ends up in waterways.

Some older cities use combined sewer systems, where stormwater and wastewater share the same pipes. During heavy rain, these systems can overflow, sending untreated wastewater into rivers. Newer cities separate the two systems to prevent this problem. If you live in an area with combined sewers, your city may have programs to reduce stormwater runoff, such as rain gardens or permeable pavement.

Why sewer systems fail and what causes backups

Sewer backups happen when something blocks the flow of wastewater. The most common cause is tree roots growing into cracks in old pipes, especially clay or cast iron lines. Roots are attracted to moisture and nutrients in wastewater and can eventually break through the pipe wall. Grease buildup is another major cause—when people pour cooking oil down the drain, it cools and hardens inside the pipes, trapping other debris.

Blockages in your lateral (the pipe from your home to the street) are your responsibility to fix. Blockages in the main line are the city's responsibility. If you have a backed-up toilet or slow drain, call your city's public works department or sewer utility to report it. They can send a camera down the main line to see if the problem is on their side of the property line.

Aging pipes also fail. Many sewer systems in North America are 50 to 100 years old and were built to last 50 years. Cracks develop, pipes collapse, and roots invade. Cities are slowly replacing these pipes, but the work is expensive and slow. If you live in an older neighborhood and have frequent backups, ask your city when they plan to replace the main line on your street.

How sewer systems are maintained and funded

Your sewer bill pays for the treatment plant, the pipes, and the workers who maintain them. The bill is usually separate from your water bill, though some cities combine them. Sewer rates vary widely depending on how old the system is, how much treatment is needed, and how much the city invests in repairs and upgrades.

Cities employ sewer maintenance crews who clean pipes, repair breaks, and clear blockages. They use high-pressure water jets to flush out grease and debris, and they use cameras to inspect pipes and find problems before they cause backups. Some cities also have programs to help homeowners replace old laterals or install grease traps to prevent blockages.

Federal and state grants sometimes help pay for major upgrades, especially for combined sewer systems that overflow during rain. However, most funding comes from sewer fees paid by residents and businesses. If your city is planning a major sewer project, you may see a temporary increase in your sewer bill to cover the cost.

Frequently Asked Questions

What should I never put down the drain?

Never pour cooking oil, grease, or fat down the drain—it hardens in pipes and traps other debris. Avoid flushing anything besides toilet paper and human waste, including paper towels, wipes labeled "flushable," feminine hygiene products, and dental floss. Medications, paint, and chemicals should go to a hazardous waste facility, not the sewer.

Can I have a septic tank if I live on a street with a sewer line?

No. If a municipal sewer line runs under your street, you are required by law to connect to it. Septic tanks are only for properties not served by a public sewer system. If you are not sure whether your property is on a sewer line, contact your city's public works or sewer utility department.

Why does my toilet back up after heavy rain?

If you have a combined sewer system, heavy rain can overwhelm the pipes, and wastewater backs up into your home. If you have a separate stormwater system, backups during rain usually mean tree roots or grease are blocking your lateral or the main line. Call your city to report it and ask them to inspect the main line near your property.

How often should I have my sewer lateral cleaned?

If you have no problems, you do not need to clean it. If you have recurring backups, ask a plumber to inspect it with a camera first—they may find tree roots or grease buildup that needs removal. Some older homes benefit from cleaning every few years, but most modern homes never need it if you avoid pouring grease and flushing non-paper items.

What is a grease trap and do I need one?

A grease trap is a small tank installed under your sink that catches grease before it enters the sewer line. Restaurants are required to have them, but homeowners usually do not need them unless they produce large amounts of grease. If you cook frequently with oil or butter, a trap can help prevent blockages in your lateral.