A sewage system is a network of underground pipes, pumps, and treatment facilities that collects wastewater from homes and businesses and cleans it before releasing it back into the environment. Every time you flush a toilet, run a faucet, or drain a bathtub, that water enters the sewage system and begins a journey through collection pipes, biological treatment processes, and disinfection stages that can take hours to complete.
How Wastewater Moves From Your Home
The system starts at your property with a lateral line, a pipe that connects your home’s plumbing to the larger sewer main buried under the street. From there, wastewater flows through progressively larger pipes toward a treatment plant. In densely populated areas, these pipes rely on gravity: they’re laid at a slight downhill angle so water flows naturally without any mechanical help. This is the oldest and most common design for urban sewer networks.
When the terrain is flat or the treatment plant sits at a higher elevation, gravity alone won’t work. In those cases, lift stations (small pumping facilities spaced throughout the network) push wastewater uphill through pressurized pipes called rising mains. In more spread-out communities, each home may have its own small pump station that feeds into a shared low-pressure pipe network, which significantly reduces installation costs compared to large gravity sewers.
Combined vs. Separate Sewer Systems
Not all sewer systems are built the same way. The two main types are separate systems and combined systems, and the difference matters most when it rains.
A separate sewer system uses two independent sets of pipes. One carries sanitary sewage (everything from toilets, sinks, and washing machines) to a treatment plant. The other carries stormwater (rain runoff from streets and rooftops) directly to local streams and rivers, usually with little or no treatment. This keeps the treatment plant from being overwhelmed during storms.
A combined sewer system uses a single pipe for both sanitary sewage and stormwater. In dry weather, everything flows to the treatment plant as intended. The problem comes during heavy rain, when stormwater volume can be nine times greater than the sewage volume. That surge can overwhelm the system’s capacity, causing a mix of raw sewage and rainwater to overflow directly into waterways through emergency discharge points. Many older cities still operate combined systems. In Alexandria, Virginia, for example, about 5% of the city (the historic district) still uses combined sewers, with four permitted overflow outfalls that discharge during storms.
What Happens at the Treatment Plant
Wastewater treatment typically moves through three stages, each one removing a different category of contamination.
Primary Treatment
This is physical separation. Wastewater enters large settling tanks called clarifiers, where gravity pulls heavier solids to the bottom. About half of the suspended organic material settles out during this step, forming what’s called primary sludge. Lighter materials like grease and oils float to the surface and are skimmed off.
Secondary Treatment
This is where biology takes over. The remaining wastewater flows into aeration basins, large tanks where air is continuously bubbled up through the water. Those air bubbles support hundreds of millions of microorganisms, mostly bacteria and protozoa, that feed on dissolved organic waste. These microbes convert organic matter into cell mass, effectively eating the pollution. The process also involves nitrifying and denitrifying bacteria that handle nitrogen compounds. After the microbes have done their work, the water moves to a second set of clarifiers where the microbial clusters settle out.
Tertiary Treatment and Disinfection
The final stage polishes the water to a higher standard. Chemical agents cause remaining suspended particles to clump together so they can be filtered out. Then the water undergoes disinfection, most commonly with chlorine, ultraviolet (UV) light, or ozone. Each method works differently: chlorine chemically destroys pathogens, UV light damages their DNA to prevent reproduction, and ozone oxidizes cell walls. Chlorine tends to be the most effective at eliminating resistant bacteria, though UV and ozone avoid introducing chemical residues into the discharge water.
What Happens to the Leftover Solids
Treatment doesn’t just produce clean water. It also generates large volumes of sludge from both the primary settling and the biological treatment stages. This combined sludge undergoes its own treatment process, typically anaerobic digestion, where bacteria break it down in oxygen-free tanks over a period of weeks. The stabilized end product is called biosolids.
Biosolids can be reused as soil fertilizer for agriculture, applied to land as a form of additional treatment and disposal, or sent to landfills. The treated water itself, now called effluent, is sometimes used for crop irrigation rather than simply being discharged into rivers. The specific disposal method depends on local regulations and the quality of the final product.
Septic Systems: The Decentralized Alternative
Not every property connects to a municipal sewer. In rural areas where centralized systems are unavailable, homes rely on septic systems that treat and dispose of wastewater entirely on-site. A typical septic system includes a buried tank where solids settle and bacteria break down waste, plus a drain field where liquid effluent filters through soil for natural treatment. About one in five U.S. households uses a septic system rather than a public sewer connection.
Why Sewage Systems Matter for Public Health
Before modern sewage infrastructure, waterborne diseases were among the leading causes of death in cities. Untreated human waste contaminates groundwater and surface water used for drinking, bathing, and irrigation. The World Health Organization reports that poor water and sanitation conditions still account for more than one million diarrheal deaths every year worldwide, along with tropical diseases like trachoma and schistosomiasis. Connecting communities to sewers with proper wastewater treatment dramatically reduces these deaths. Modern sewage systems are, in public health terms, one of the most consequential infrastructure investments a city can make.
The Fatberg Problem
Sewage systems face a growing threat from fatbergs: massive clumps of fats, oils, grease, and non-flushable items that solidify inside pipes. Cooking fats, salad dressing, butter, and personal care products combine with “flushable” wipes, facial tissues, and other debris to form blockages that can grow to enormous size. In 2018, crews in Macomb County, Michigan discovered a fatberg 100 feet long, 11 feet wide, and 6 feet tall that caused sewer overflows and millions of dollars in damage. That same year, New York City responded to 2,100 fatberg incidents at a cost of $19 million.
These blockages are responsible for 71% of sewer backups in some cities. U.S. municipalities collectively spend at least $1 billion annually on maintenance to remove clogs before they become full-scale fatbergs. When they’re not caught in time, the results can be severe: a 20-foot fatberg near Baltimore’s Penn Station in 2017 caused two major sewage overflows in two weeks, including one that discharged nearly 1.2 million gallons of raw waste into a local stream. During the COVID-19 pandemic, Des Moines, Iowa reported a 50% increase in fatberg formation, likely driven by more people cooking at home and flushing disinfecting wipes.
Aging Infrastructure and Investment Gaps
Much of the sewage infrastructure in the United States was built decades ago and is showing its age. The American Society of Civil Engineers gave the nation’s wastewater systems a D+ grade in its 2025 Infrastructure Report Card, unchanged since 2021. Drinking water fared only slightly better at C-. Federal funding helps but falls short of the total investment needed to repair, replace, and expand systems across the country. The costs of neglecting this infrastructure show up as sewer overflows, treatment failures, and rate increases passed along to customers.