Sewage is the wastewater that flows out of homes, businesses, and industrial buildings through a network of underground pipes. It includes everything that goes down your drains: toilet waste, shower water, kitchen sink runoff, laundry water, and any chemicals or food scraps that come with them. By weight, sewage is 99.9% water. It’s the remaining 0.1% that creates problems and requires treatment: a mix of organic matter, microorganisms (some of them dangerous), and inorganic compounds like metals and chemicals.
What’s Actually in Sewage
That tiny fraction of non-water content is more complex than it sounds. Organic matter includes human waste, food particles, soaps, detergents, and oils. Inorganic compounds include heavy metals, salts, and sand or grit that wash into the system. But the most concerning component is biological: sewage carries a wide range of disease-causing organisms shed by the people whose waste enters the system.
The World Health Organization identifies over 50 pathogens transmitted through inadequate water and sanitation systems, with ten responsible for the greatest health burden worldwide. These include bacteria like E. coli, Salmonella, and the organism that causes cholera (Vibrio cholerae), along with viruses like norovirus, rotavirus, and hepatitis A. A parasite called Cryptosporidium is particularly troublesome because it resists chlorine disinfection and can cause severe illness in people with weakened immune systems. Many of these pathogens cause diarrheal disease, which remains one of the leading causes of death in young children globally.
How Sewage Gets Collected
Most communities in the U.S. use what’s called a separate sewer system. One set of pipes carries wastewater from buildings to a treatment plant. A completely separate set of pipes collects rainwater from street drains, parking lots, and driveways and channels it directly to a local waterway. This keeps the two flows apart so treatment plants aren’t overwhelmed every time it rains.
Some older cities, though, still rely on combined sewer systems where wastewater and stormwater share the same pipes. During dry weather, everything flows to the treatment plant as intended. During heavy rain, the combined volume of sewage and stormwater can exceed the system’s capacity. When that happens, a mix of untreated sewage and rainwater is released directly into rivers, lakes, or coastal waters through designated overflow points. These events, called combined sewer overflows, are a major source of water pollution in the cities that still have these older systems.
How Sewage Gets Treated
Treatment happens in three stages, each targeting a different type of contaminant.
The primary stage is mostly physical. Screens catch large debris like rags and plastic. Heavier particles settle to the bottom of tanks through gravity, while lighter materials like oils and grease float to the surface and get skimmed off. By the end of this stage, the biggest and most visible pollutants are gone, but dissolved organic matter and microscopic contaminants remain.
The secondary stage uses biology to do the heavy lifting. Wastewater is held in tanks where microorganisms feed on the dissolved organic material, breaking it down into carbon dioxide and water. Treatment plants speed this up using techniques like aeration (pumping air into the tanks to help bacteria thrive), filtration through sand or gravel beds, and open ponds where natural processes do the work more slowly. This stage removes the bulk of the pollution.
The tertiary stage is a polishing step. It targets whatever the first two stages left behind: dissolved chemicals, excess nutrients like nitrogen and phosphorus, and remaining microorganisms. Methods include activated carbon filtration, ozone injection, reverse osmosis, and chemical disinfection with chlorine. Not every treatment plant includes a full tertiary stage, but it’s required when treated water will be discharged near sensitive ecosystems or reused.
What Happens to the Leftover Solids
Treatment doesn’t just clean the water. It also concentrates all those removed contaminants into a thick residue called sewage sludge. In the U.S., roughly four million dry metric tons of this sludge are generated each year, and it has to go somewhere.
About 60% of it gets spread on land, either to fertilize crops or improve soil quality. This is the most common disposal route and, when done according to regulations, turns waste into a resource. The remaining sludge is split between landfills (about 25%) and incineration (about 14%). A small fraction is managed through other methods like deep well injection or use as fuel.
Land application has drawn increasing scrutiny, though. More than 65% of microplastics that enter a treatment plant end up concentrated in the sludge rather than passing through with the treated water. When that sludge is spread on farmland, those microplastics enter the soil. Treatment plants were never designed to filter out microplastics, and while they can remove up to 99% from the water itself, the particles simply transfer to the solid waste stream instead of disappearing.
Why Untreated Sewage Is a Global Problem
As of 2020, 44% of all household wastewater generated worldwide was discharged without safe treatment. In many low-income regions, raw sewage flows directly into rivers, lakes, and coastal waters that communities depend on for drinking, bathing, and fishing.
The immediate health risks are the pathogens described above. But untreated sewage also causes ecological damage. When organic matter enters a waterway, bacteria begin breaking it down and consuming oxygen in the process. The more organic waste in the water, the faster oxygen gets depleted. Fish, insects, and other aquatic organisms suffocate as dissolved oxygen drops. In severe cases, entire stretches of river become biologically dead zones where only the hardiest microorganisms survive.
Excess nutrients in sewage, particularly nitrogen and phosphorus from human waste and household detergents, fuel explosive algae growth in lakes and coastal waters. When these algal blooms die and decompose, they strip even more oxygen from the water, compounding the damage. This cycle is one of the most widespread forms of water pollution on the planet, and untreated or undertreated sewage is a primary driver.