Sewage is the used water that flows out of homes, businesses, and streets into a network of underground pipes. By weight, it is 99.9% water. The remaining 0.1% is the problem: a mix of human waste, food scraps, soap, chemicals, oils, and whatever else washes down a drain or gets carried off a road by rain. That tiny fraction is what makes sewage a health hazard and why entire systems exist to remove it.
What’s Actually in Sewage
The solid portion of sewage, small as it is, contains organic matter (anything that was once living, like food waste and feces), nutrients like nitrogen and phosphorus, household chemicals from cleaning products and personal care items, and suspended particles like sand and grit. In industrial sewage, you can also find heavy metals, solvents, and manufacturing byproducts. The exact makeup varies depending on whether the sewage comes from a residential neighborhood, a factory, or a storm drain.
Raw sewage also carries a wide range of disease-causing organisms. Fecal matter is the primary source of these pathogens, which include bacteria like Salmonella, E. coli, and the organism that causes cholera. Viruses found in untreated sewage include hepatitis A, norovirus, rotavirus, and poliovirus. Parasites such as Giardia and Cryptosporidium are common as well. Even parasitic worms and their eggs, including roundworms and tapeworms, survive in sewage. This is why contact with raw sewage poses serious infection risks.
Three Types of Sewage
Domestic sewage comes from everyday household activities: flushing toilets, running showers, doing laundry, washing dishes. It travels through pipes connected to your home into municipal sewer lines, or into a septic system in more rural areas. This is the largest and most consistent source of sewage in most cities.
Industrial sewage is wastewater generated by manufacturing plants, food processors, breweries, paper mills, oil and gas operations, and mines. It often contains chemicals, heavy metals, and bacteria at much higher concentrations than household wastewater, which means it can strain treatment systems if not pretreated at the source.
Stormwater is rain or snowmelt that runs off roads, parking lots, and rooftops into storm drains. Along the way, it picks up automotive fluids, road salt, fertilizers, animal waste, and trash. In some cities, stormwater flows into the same pipes as domestic sewage. In others, it has its own separate system.
Combined vs. Separate Sewer Systems
Older cities often use combined sewer systems, where household sewage and stormwater travel through the same pipes to a treatment plant. The problem comes during heavy rain: the system can’t handle the volume, so a mix of raw sewage and stormwater overflows directly into rivers, lakes, or coastal waters without treatment. These are called combined sewer overflows, and they’re a major source of water pollution in urban areas.
Newer cities typically have separate systems, with one set of pipes for sewage and another for stormwater. This prevents overflows during storms, but it creates a different issue. Polluted stormwater, carrying heavy metals and chemicals from streets, often discharges directly into waterways without any treatment at all. Neither system is perfect.
How Sewage Gets Treated
Treatment happens in three stages, each removing a different category of contaminants.
The first stage is physical. Sewage flows into large settling tanks where gravity does the work. Heavy solids sink to the bottom and lighter materials like grease float to the surface, where both are skimmed off. Sometimes chemicals called coagulants are added to help clump smaller particles together so they settle faster.
The second stage is biological. The water that exits the settling tanks still contains dissolved organic matter and invisible microorganisms. In aeration tanks, air is pumped through the water to encourage the growth of beneficial bacteria. These bacteria consume the remaining organic material and break down harmful organisms. This is the stage that does the heaviest lifting in terms of cleaning the water.
The third stage adds a final polish. Depending on local requirements, the water may go through chemical disinfection (often with chlorine), ultraviolet light treatment, or advanced filtration to remove any remaining pathogens and fine particles before the water is released into a river, lake, or ocean.
What Happens to the Solids
The solids removed during treatment become a semi-solid, nutrient-rich material called sewage sludge. When this sludge is treated to meet safety standards set by the EPA, it’s referred to as biosolids. Biosolids contain nitrogen, phosphorus, and organic matter that make them useful as a soil conditioner or fertilizer.
They’re applied to agricultural land for growing food, feed, and fiber crops. They’re also used in land reclamation projects at mines and construction sites, on forests and rangelands, and even on parks, golf courses, and home gardens. The treated material can be sold or given away to the public. Before any of this happens, biosolids must meet federal, state, tribal, and local requirements for pathogen reduction and pollutant limits.
Why Untreated Sewage Is an Environmental Problem
Globally, about 47% of all wastewater is released into the environment without any treatment. Only around 41% passes through a treatment plant before being discharged. That gap has measurable consequences.
When raw or poorly treated sewage enters a lake, river, or coastal area, the nitrogen and phosphorus it carries act as fertilizer for algae. Algae populations explode, covering the water surface and blocking sunlight from reaching plants below. As the algae die and sink, bacteria decompose the dead material and consume oxygen in the process. Deep water, cut off from the atmosphere, develops severe oxygen deficits. Fish and other aquatic animals suffocate. This process, called eutrophication, has dramatically altered lakes and rivers worldwide over the past 50 years, driven by the combined effects of sewage discharge, agricultural runoff, and urban development.
Beyond nutrient pollution, untreated sewage introduces the full spectrum of pathogens described above directly into water that communities may use for drinking, swimming, or fishing. It’s one of the leading causes of waterborne disease in regions without adequate sanitation infrastructure.