A municipal sewer system is a network of underground pipes, pumping stations, and treatment facilities that collects wastewater from homes and businesses and cleans it before releasing it back into the environment. Most cities and towns operate one, and it handles everything that goes down your drains, toilets, and floor drains. The system relies primarily on gravity to move wastewater toward a central treatment plant, with mechanical pumps stepping in where the terrain doesn’t cooperate.
How Wastewater Moves Through the System
The journey starts at your property. A pipe called a sewer lateral connects your home or business to the larger public sewer main, typically located under the street or alley. These laterals are at least six inches in diameter and run from your building’s plumbing out to the main line. From there, wastewater flows into progressively larger pipes, merging with flow from other properties and neighborhoods, all heading toward the treatment plant.
Gravity does most of the work. Sewer pipes are installed at a slight downhill slope so wastewater flows naturally without any energy input. Along the route, manholes provide access points for inspection and maintenance. They’re placed at every change in pipe direction, pipe size, or slope, and no more than 400 feet apart on smaller lines. Each manhole is at least four feet across on the inside, large enough for a worker to enter.
Where the terrain rises or the pipe would need to be buried unreasonably deep, lift stations (also called pump stations) push wastewater uphill. These are small facilities with a receiving well, pumps, motors, and control systems. Centrifugal pumps are the standard choice, spinning wastewater from a lower collection point up to a higher pipe where gravity can take over again. Every lift station has at least two pumps so service continues if one fails.
Combined vs. Separate Systems
There are two basic designs for municipal sewers. A separate system uses one set of pipes for wastewater from buildings (the sanitary sewer) and a completely different set for rainwater and street runoff (the storm sewer). A combined system carries both in the same pipes.
Many older cities, especially those built before the mid-20th century, still use combined systems. The problem is that during heavy rain, the volume of water can overwhelm the system’s capacity. When that happens, a mix of raw sewage and stormwater overflows directly into rivers or lakes without treatment. Separate systems avoid this issue for sanitary waste, but they come with their own drawback: polluted street runoff containing heavy metals, oil, and debris flows directly into waterways without passing through a treatment plant at all. Neither design is perfect, and the better choice depends on local conditions, costs, and the sensitivity of nearby water bodies.
What Happens at the Treatment Plant
Wastewater treatment typically happens in three stages, each removing a different category of contaminants.
Primary treatment is physical separation. Wastewater sits in large settling basins where heavy solids sink to the bottom as sludge and lighter materials like fats and oils float to the surface. Both layers are skimmed off, and the remaining liquid moves on.
Secondary treatment is biological. Aerobic bacteria, the kind that need oxygen, are introduced to consume dissolved organic material like sugars and fats. Some plants grow these bacteria on fixed filters that water passes through. Others use an “activated sludge” approach, mixing bacteria directly into the wastewater while pumping in air to keep the microorganisms active. This stage breaks down the bulk of the pollution that primary treatment can’t catch.
Tertiary treatment, sometimes called effluent polishing, adds a final layer of cleanup. This is common when treated water will be discharged into a sensitive ecosystem. Methods include sand filtration to remove remaining particles, and biological processes that specifically target nutrients like phosphorus and nitrogen. Specialized bacteria can accumulate phosphorus in their cells, while nitrifying bacteria convert nitrogen compounds into forms that can be safely released. Some systems use lagoons where native plants, algae, and tiny organisms filter the water naturally over time.
What Happens to the Solids
The sludge collected during primary treatment doesn’t just disappear. It goes through its own treatment process, often involving bacterial digestion that breaks down organic material. This digestion can produce methane gas, which some plants capture and use to generate electricity, offsetting the facility’s energy costs.
Once treated to meet federal standards, sludge becomes what the EPA calls “biosolids,” a nutrient-rich material with three main disposal paths: land application, landfilling, or incineration. Land application is the most common beneficial use. About 2.39 million dry metric tons of treated sludge is spread on agricultural land, forests, parks, golf courses, and even home gardens each year in the U.S., acting as a soil conditioner or fertilizer. Smaller quantities go to alternative uses like deep well injection or use as auxiliary fuel.
Your Responsibility as a Property Owner
The city or county owns and maintains the sewer mains under the street, but the sewer lateral connecting your property to that main is typically your responsibility. This is the pipe running from your building out to the public line, and if it cracks, collapses, or gets clogged by tree roots, the repair bill is yours. Some municipalities have started programs to help. Pinellas County in Florida, for example, runs a find-and-fix program that investigates and repairs laterals within designated priority zones at no cost to the homeowner, with rebate programs available for properties outside those zones.
The distinction matters because lateral failures don’t just affect your plumbing. Cracked laterals allow groundwater to seep into the sewer system (called infiltration), which increases the volume the treatment plant has to process and raises costs for everyone.
What Clogs the System
Fats, oils, and grease, known in the industry as FOG, are the most persistent threat to sewer infrastructure. When poured down the drain, these substances cool and solidify inside pipes, gradually building up into thick blockages. Combined with so-called “flushable” wipes that don’t break down, they can form massive clumps called fatbergs that block entire pipe sections.
The consequences are expensive. FOG-related clogs cause burst pipes, sanitary sewer overflows, and backups that can flood private property. Nationally, sewer clogs cost an estimated $1 billion per year. One regional authority in eastern Ohio spends about $85,000 annually just on fatberg removal, which works out to roughly $13 per person per month passed along to ratepayers. If a FOG-related overflow damages your property, you bear the cost of cleanup on your side of the line. The simplest prevention: never pour cooking grease down the drain, and don’t flush wipes regardless of what the packaging says.
Why Sewer Systems Matter for Public Health
Before modern sewage infrastructure, waterborne diseases like cholera, typhoid fever, and dysentery were leading causes of death in cities. Raw sewage contaminated the same water sources people drank from. Municipal sewer systems broke that cycle by physically separating human waste from drinking water supplies and treating wastewater before it re-enters the environment.
In developed countries with established sewer networks, those diseases have been virtually eliminated as endemic threats. Canada, for instance, excludes cholera, typhoid, and amoebiasis from its studies of waterborne intestinal disease because modern water and sewage infrastructure has made local transmission negligible. The infrastructure is so effective that it’s easy to take for granted, but it remains one of the most important public health interventions ever built.